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LaserGauge : development of a device for automatic measurement of bore depth in bone during surgery Demsey, Daniel 2017

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					LaserGauge:	Development	of	a	device	for	automatic	measurement	of	drilled	bore	depth	in	bone	during	surgery	by	Daniel	Demsey		A	THESIS	SUBMITTED	IN	PARTIAL	FULFILLMENT	OF	THE	REQUIREMENTS	FOR	THE	DEGREE	OF		MASTER	OF	APPLIED	SCIENCE	in	The	Faculty	of	Graduate	and	Postdoctoral	Studies	(Biomedical	Engineering)		THE	UNIVERSITY	OF	BRITISH	COLUMBIA	(Vancouver)	July	2017	©	Daniel	Demsey,	2017				 		 ii	Abstract	Purpose:		This	thesis	comprised	two	main	phases.		Initial	work	focused	on	clarifying	the	need	and	use	case	for	a	novel	device	to	measure	drilled	bore	depth	in	bone	during	osteosynthesis	surgery.		Next,	I	demonstrated	the	feasibility	and	reliability	of	an	optical	sensing	device	for	automatic	measurement	of	drilled	bore	depth	in	bone	during	surgery	compared	with	conventional	methods.	Methods:		I	completed	a	structured	Needs	Assessment	followed	by	an	Engineering	Design	process	to	develop	a	series	of	prototypes	using	laser	displacement	sensors	mounted	on	a	surgical	drill	to	determine	drilled	bore	depth	in	bone.		In	all	versions	of	the	prototypes	bore	depth	was	computed	based	on	a	characteristic	pattern	of	drilling	velocity	in	bicortical	bone.		Prototypes	consisted	of	one	or	more	laser	displacement	sensors	sending	displacement	and	time	data	to	a	microprocessor	and	then	a	personal	computer.		After	data	filtering	with	a	second	order	Butterworth	filter	velocity	and	acceleration	were	calculated	using	differentiation	and	double	differentiation.		Characteristic	spikes	in	velocity	and	acceleration	indicated	cortical	breach	and	allowed	identification	of	bore	depth.		Exploratory	experiments	were	done	with	multiple	sensor	arrangement	concepts	in	porcine	long	bones,	and	more	rigorous	final	evaluation	experiments	were	done	with	the	lead	designs	in	pig	hind	limbs	with	comparison	to	CT	scan	as	‘gold	standard’.	Results:		In	exploratory	experiments	a	design	involving	two	laser	displacement	sensors	angled	towards	the	drilling	axis	measuring	distance	from	a	mock	drill	guide	performed	better	than	alternative	designs.		This	design	in	final	evaluation	experiments	showed	superior	performance	to	the	conventional	depth	gauge	under	three	clinically	relevant	drilling	conditions	(standard		 iii	deviation	0.70	mm	vs.	1.38	mm,	0.86	mm	vs.	3.79	mm,	0.80	mm	vs.	3.19	mm).		A	positive	bias	was	present	in	all	drilling	conditions.	Conclusions:		An	optical	sensing	device	can	be	used	to	measure	bore	depth	in	bone	during	surgery.			 		 iv	Lay	Summary		 This	thesis	describes	the	development	of	a	new	instrument	for	use	in	surgery	on	the	bony	skeleton.		The	purpose	of	the	instrument	is	to	measure	the	depth	of	bore	drilled	in	bone,	to	allow	the	surgeon	to	make	appropriate	choices	on	the	size	of	screw	or	other	implant.		The	device	makes	use	of	a	laser-based	displacement	measurement	sensor.			 		 v	Preface		 All	the	work	described	in	this	thesis	is	that	of	the	author,	with	the	exception	of	that	in	the	section	labelled	‘Prior	Work.’		The	Needs	Assessment,	Design,	and	Experimental	sections	were	carried	out	by	the	author.		Juan	Pablo	Gomez	Arrunategui,	a	Master’s	student	from	the	Surgical	Technologies	Lab	assisted	with	the	experiments	and	developed	the	Arduino	code	and	the	Matlab	code	for	the	data	storage.		Juan	also	assisted	in	in	the	electrical	components	of	the	prototype	section.		 No	part	of	this	thesis	has	been	published	at	the	time	of	this	writing,	however	a	manuscript	is	in	preparation.																											 vi	Table	of	Contents				Abstract	...............................................................................................................................	ii					Lay	Summary	......................................................................................................................	iv					Preface	................................................................................................................................	v					Table	of	Contents	................................................................................................................	vi					List	of	Tables	.......................................................................................................................	ix					List	of	Figures	.......................................................................................................................	x					Acknowledgements	...........................................................................................................	xii					Dedication	........................................................................................................................	xiii	1	Introduction	........................................................................................................................	1	1.1	Overview	....................................................................................................................................	1	1.2	Prior	Work	..................................................................................................................................	4	1.3	Background	and	Literature	Review	.............................................................................................	5	1.3.1	Bone	Drilling	in	Clinical	Practice	.................................................................................................	5	1.3.2	Bone	Anatomy	and	Screw	Placement	.........................................................................................	7	1.3.3	Alternatives	to	Surgical	Depth	Gauge	.........................................................................................	8	1.3.3	Research	on	Surgical	Bone	Drilling	...........................................................................................	10	1.3.4	Porcine	Surgical	Modelling	.......................................................................................................	13	1.4	Thesis	Overview	.......................................................................................................................	14	2	Needs	Assessment	..............................................................................................................	15	2.1	Overview	..................................................................................................................................	15	2.2	Prior	Work	................................................................................................................................	15	2.3	Needs	Assessment	....................................................................................................................	16	2.3.1	Interviews	with	Practicing	Surgeons	.........................................................................................	16	2.3.2	OR	Observations	.......................................................................................................................	17	2.3.4	Updated	Needs	Statement	.......................................................................................................	18	2.4	Context	Identification	...............................................................................................................	19	2.4.1	Clinical	Uses	of	the	Depth	Gauge	..............................................................................................	19	2.4.2	Surgical	Drills	............................................................................................................................	20	2.4.3	Surgical	Screws	.........................................................................................................................	23	2.4.4	Surgical	Exposure	Dimensional	Analysis	...................................................................................	25	2.4.5	Fixation	Construct	Analysis	.......................................................................................................	28	2.4.6	Current	Surgical	Depth	Gauge	..................................................................................................	31	2.5	Problem	Statement	..................................................................................................................	31	3	Design	................................................................................................................................	33	3.1	Overview	..................................................................................................................................	33	3.2	Problem	Statement	..................................................................................................................	33	3.3	Concept	....................................................................................................................................	33	3.4	Design	Specifications	................................................................................................................	35	3.5	Functional	Structural	Decomposition	........................................................................................	36	3.6	Sensor	......................................................................................................................................	37		 vii	3.6.1	Concept	Selection	.....................................................................................................................	37	3.6.2	Optical	Sensing	.........................................................................................................................	37	3.6.3	Laser	Triangulation	...................................................................................................................	38	3.6.4	Sensor	Orientation	....................................................................................................................	39	3.7	Microprocessor	.........................................................................................................................	41	3.8	Algorithm	.................................................................................................................................	41	3.9	User	Interface	...........................................................................................................................	42	3.10	Mounting	Assembly	................................................................................................................	42	3.11	Power	Supply	..........................................................................................................................	42	3.12	Housing	..................................................................................................................................	43	3.13	Prototyping	............................................................................................................................	43	3.13.1	Single	sensor,	parallel	to	drilling	axis,	measuring	from	tissue	................................................	43	3.13.2	Dual	sensor,	parallel	to	drilling	axis,	measuring	from	tissue	..................................................	46	3.13.3	Dual	sensor,	angled	towards	drill	bit,	measuring	from	tissue	................................................	47	3.13.4	Dual	sensor,	angled	towards	drill	bit,	measuring	from	drill	guide	..........................................	48	3.14	ENG	PHYS	Design	Project	........................................................................................................	49	3.15	Design	Summary	.....................................................................................................................	50	4	Materials	and	Methods	......................................................................................................	51	4.1	Overview	..................................................................................................................................	51	4.2	Animal	Models	.........................................................................................................................	51	4.2.1	Chicken	Long	Bone	....................................................................................................................	51	4.2.2	Porcine	Long	Bone	....................................................................................................................	51	4.2.3	Porcine	Hind	Limb	.....................................................................................................................	52	4.3	Exploratory	Experiments	..........................................................................................................	53	4.4	Evaluation	of	Final	Concept	......................................................................................................	55	5	Results	................................................................................................................................	59	5.1	Overview	..................................................................................................................................	59	5.2	Final	Evaluation	Results	............................................................................................................	59	6	Discussion	..........................................................................................................................	64	6.1	Significance	of	Design	...............................................................................................................	64	6.2	Sensor	Concepts	.......................................................................................................................	64	6.2.1	Single	vs.	Dual	Sensors	..............................................................................................................	64	6.2.2	Parallel	vs.	Angled	Sensors	.......................................................................................................	67	6.2.3	Tissue	vs.	Drill	Guide	Reference	................................................................................................	68	6.3	Drilling	Conditions	....................................................................................................................	69	6.3.1	Condition	SD	.............................................................................................................................	69	6.3.2	Condition	AD	.............................................................................................................................	69	6.3.3	Condition	SM	............................................................................................................................	70	6.4	Final	Concept	Selection	.............................................................................................................	70	6.5	Bore	Depth	‘Gold	Standard’	......................................................................................................	70	6.5.1	Bone	Geometry	.........................................................................................................................	70	6.5.2	Measurement	Methods	............................................................................................................	71	6.5.3	Interpretation	of	Measurement	by	Surgeon	............................................................................	72	6.5.4	Required	Precision	....................................................................................................................	72	6.6	Additional	Considerations	........................................................................................................	73	6.7	Existing	Literature	.....................................................................................................................	73		 viii	6.8	Limitations	...............................................................................................................................	73	7	Conclusion	..........................................................................................................................	75	7.1	Future	Work	.............................................................................................................................	75	References	............................................................................................................................	77	Appendix	A	–	Incision	Dimensional	Analysis	..........................................................................	80	A.1	Overview	..................................................................................................................................	80	A.2	Data	Tables	..............................................................................................................................	80	Appendix	B	–	Fixation	Construct	Analysis	..............................................................................	82	Appendix	C	–	Exploratory	Experiments	..................................................................................	83	C.1	Overview	..................................................................................................................................	83	C.2	Single	parallel	sensor	measuring	from	tissue	............................................................................	83	C.3	Paired	parallel	sensors	measuring	from	tissue	..........................................................................	85	C.4	Paired	angled	sensors	measuring	from	tissue	...........................................................................	86	C.5	Paired	angled	sensors	measuring	from	drill	guide	.....................................................................	87	Appendix	D	–	Final	Evaluation	Experiments	...........................................................................	88	Appendix	E	–	Sterilization	Approval	Process	..........................................................................	89	E.1	Overview	..................................................................................................................................	89	E.2	Approval	Process	......................................................................................................................	89	E.3	Sterilization	Methods	...............................................................................................................	89	E.3.1	Autoclave	Steam	Sterilization	...................................................................................................	89	E.3.2	Ethylene	Oxide	(ETO)	Sterilization	............................................................................................	90	E.3.3	Chlorine	Dioxide	(CD)	Gas	Sterilization	.....................................................................................	90	E.3.4	Vaporized	Hydrogen	Peroxide	(VHP)	Sterilization	....................................................................	91	E.3.5	Hydrogen	Peroxide	Plasma	Sterilization	...................................................................................	91	E.3.6	Gamma	Ray	Sterilization	...........................................................................................................	91	E.3.7	Electron	Beam	Sterilization	.......................................................................................................	92			 		 ix	List	of	Tables	Table	1	–	Dimensional	Analysis	Summary	....................................................................................	27	Table	2	–	Summary	of	common	optical	distance	sensing	method	properties	.............................	38	Table	3	–	Laser	displacement	sensors	available	from	KEYENCE	...................................................	39	Table	4	–	Results	from	exploratory	experiments	.........................................................................	54	Table	5–	Final	evaluation	data	summary	.....................................................................................	60	Table	6	–	Mean	error	and	SD	for	the	dual	laser	prototype	compared	with	conventional	depth	.	61			 		 x	List	of	Figures	Figure	1	-	Intellisense	Drill	(McGinley	Orthopedics,	USA).		An	advanced	surgical	drill	with	integrated	sensors	including	bore	depth	measurement	........................................................	2	Figure	2	-	The	conventional	depth	gauge	.......................................................................................	6	Figure	3	-	Long	bone	anatomy	........................................................................................................	7	Figure	4	-	SMARTdrill	(SMD	Inc.,	USA)	............................................................................................	9	Figure	5–	Typical	orthopedic	surgical	exposure	with	plates	and	screws	in	situ	...........................	18	Figure	6	–	Distribution	of	surgical	cases	using	the	depth	gauge	between	different	services	at	VGH	......................................................................................................................................	20	Figure	7	–	Pistol	type	drill	.............................................................................................................	21	Figure	8	–	Pencil	type	drill	............................................................................................................	21	Figure	9	–	Small	Bone	Drill	Market	USA	2015	..............................................................................	22	Figure	10	–	Medium	Bone	Drill	Market	USA	2015	.......................................................................	22	Figure	11	–	Mandible	Screws	from	Stryker	..................................................................................	23	Figure	12	–	Upper	Face	Screws	from	Stryker	...............................................................................	24	Figure	13	-	Orthopedic	trauma	screws	from	Stryker	....................................................................	24	Figure	14	–	Surgical	exposure	dimensional	analysis.		Incision	dimensions	are	indicated	in	green,	and	bone	exposure	dimensions	are	indicated	in	purple.	.....................................................	26	Figure	15	-	Example	of	incision	marking	image	............................................................................	27	Figure	16	–	Plot	of	the	Incision	Dimensional	Analysis	..................................................................	28	Figure	17	-	Condition	SD	(straight	diaphysis),	perpendicular	drilling	to	long	axis	in	diaphyseal	bone.		Perpendicular	entry	and	exit	of	screw	relative	to	bone	surface.	..............................	29	Figure	18	-	Condition	AD	(angled	diaphysis),	angled	drilling	to	long	axis	in	diaphyseal	bone.		Angled	entry	and	exit	of	screw	relative	to	bone	surface	......................................................	29	Figure	19	-	Condition	SM	(straight	metaphysis),	perpendicular	drilling	to	long	axis	in	metaphyseal	bone,	angled	entry	and	exit	relative	to	bone	surface	.....................................	30	Figure	20	-	Condition	AM	(angled	metaphysis),	angled	drilling	to	long	axis	in	metaphyseal	bone,	perpendicular	entry	and	angled	exit	relative	to	bone	surface	.............................................	30	Figure	21	-	Screw	conditions	in	Construct	Analysis	......................................................................	31	Figure	22	-	Relationship	between	displacement	measured	and	bore	depth	...............................	34	Figure	23	-	Graph	of	measured	displacement	and	computed	velocity,	acceleration	during	bore	drilling	...................................................................................................................................	35	Figure	24	–	Device	architecture	diagram	.....................................................................................	37	Figure	25	-	Displacement	sensor	collinear	with	drilling	axis,	measuring	from	tissue	surface	......	40	Figure	26	-	Displacement	sensor	angled	relative	to	drilling	axis,	measuring	from	tissue	surface	40	Figure	27	-	Displacement	sensor,	collinear	or	angled,	measuring	from	fixed	surface	such	as	drill	guide	.....................................................................................................................................	41	Figure	28	–	Single	parallel	sensor	prototype	................................................................................	44	Figure	29	–	Data	and	software	flow	chart	....................................................................................	45	Figure	30	–	Prototype	circuit	diagram	..........................................................................................	45	Figure	31	-	Periodogram	for	displacement	...................................................................................	46	Figure	33	–	Dual	parallel	sensor	prototype	..................................................................................	47		 xi	Figure	34	–	Correction	for	beam	angle	.........................................................................................	48	Figure	35	–	Dual	sensor,	angled	towards	drill	bit	.........................................................................	48	Figure	36	–	ENG	PHYS	student	design	concept	............................................................................	50	Figure	37	-	Porcine	femur	in	drilling	test	......................................................................................	52	Figure	38	-	Porcine	hindlimb	model	.............................................................................................	53	Figure	39	–	Summary	of	exploratory	experiments	.......................................................................	54	Figure	40	-	Drilling	conditions	tested	...........................................................................................	57	Figure	41	-	Cumulative	frequency	distribution	for	Condition	SD	.................................................	62	Figure	42	-	Cumulative	frequency	distribution	for	Condition	AD	.................................................	62	Figure	43	-	Cumulative	frequency	distribution	for	Condition	SM	................................................	63	Figure	44	–	Bland	Altman	plots	for	the	LaserGauge	and	the	conventional	depth	gauge	comparing	to	CT	scan	as	‘gold	standard’	..............................................................................	63	Figure	45	–	Effect	of	drill	tilt	on	sensor	readings	..........................................................................	65	Figure	46	–	Effect	of	change	of	drill	trajectory	on	sensor	readings	..............................................	66	Figure	47	–	Rotation	and	irregular	bone	surface	..........................................................................	66	Figure	48	-	Two	sensors	on	opposite	sides	of	drilling	axis	correct	for	tilt	error	...........................	67	Figure	49	–	Drill	guide	constraints	................................................................................................	68	Figure	50	-	Condition	SD	...............................................................................................................	69	Figure	51	-	Condition	AD	..............................................................................................................	69	Figure	52	-	Condition	SM	..............................................................................................................	70	Figure	53	–	Measurement	variations	in	bone	bore,	shown	in	cross	section.		Three	arrows	indicated	three	different	measurements	of	bore	depth.	.....................................................	71				 		 xii	Acknowledgements		 I’d	like	to	acknowledge	my	supervisors,	Dr.	Antony	Hodgson	and	Dr.	Nick	Carr	for	their	guidance	and	support	during	my	graduate	studies.		Both	were	extremely	generous	with	their	time	and	their	experience.		I’d	like	to	thank	Dr.	Mu	Chiao	for	sitting	on	my	thesis	committee.		 For	salary	support	I’d	like	to	thank	the	University	of	British	Columbia	Clinician	Investigator	Program,	and	more	specifically	Dr.	Sian	Spacey	and	Tessa	Feuchuk.		For	research	funds	and	freedom	from	clinical	responsibilities	I’d	like	to	thank	the	Division	of	Plastic	Surgery,	particularly	Dr.	Mark	Hill	and	Dr.	Alex	Seal.		 Juan	Pablo	Gomez	Arrunategui	was	indispensable	in	helping	me	with	the	electrical	component	of	the	prototype	development	and	with	conducting	the	experiments.		Dr.	Pierre	Guy	provided	immense	assistance	with	the	Needs	Assessment	and	the	Final	Evaluation	experiments.		I’d	like	to	thank	Masashi	Karasawa	for	his	help	with	producing	concept	drawings,	and	the	members	of	the	Surgical	Technologies	Laboratory	for	their	support	and	hospitality.		For	facilities	support	thanks	go	to	the	Centre	for	Hip	Health	and	Mobility	and	the	University	of	British	Columbia.		 Most	importantly	I’d	like	to	thank	my	family	for	their	support	during	completion	of	my	thesis,	which	turned	out	to	be	a	more	challenging	and	more	rewarding	experience	than	any	of	us	imagined.		 		 xiii	Dedication		To	Dad.	 1		1	Introduction	1.1	Overview		 Surgeons	who	perform	orthopedic,	plastic,	and	oral/maxillofacial	surgery	routinely	need	to	measure	the	depth	of	holes	drilled	in	bone.		Measuring	the	drilled	bore	depth	accurately	is	necessary	to	select	the	appropriate	screw	to	use	in	osteosynthesis	surgery.		The	current	methodology	uses	an	instrument	called	the	‘depth	gauge’,	which	consists	of	a	hooked	wire	and	a	sliding	component	marked	with	graduations.		Evidence	shows	that	the	current	method	results	in	placing	screws	of	incorrect	length	9%	of	the	time	in	fixation	of	distal	radius	fractures	(Ozer	and	Toker	2011).		Incorrect	screw	length	has	significant	clinical	consequences	such	as	rupture	of	adjacent	tendons	(Caruso,	Vitali,	and	del	Prete	2015;	Maschke	et	al.	2007).		Previous	interviews	with	practicing	surgeons	have	also	shown	that	the	instrument	is	frustrating	and	time	consuming	to	use.		Drill	guides	and	drill	bits	with	graduated	aspects	have	attempted	to	address	this,	but	are	likely	to	be	inaccurate.		Advanced	surgical	drills	with	bore	depth	measurement	features	are	under	development	(“Smart	Drill	–	Prevent	Plunge,	Measure	and	Control	Depth,	Determine	Bone	Density”	2017),	but	these	methods	are	not	compatible	with	the	existing	stock	of	surgical	drills,	and	thus	require	a	significant	additional	capital	investment.		There	could	also	be		 2	significant	ongoing	costs	in	the	form	of	specialized	consumables	(eg,	device-specific	drill	bits).	Figure	1	-	Intellisense	Drill	(McGinley	Orthopedics,	USA).		An	advanced	surgical	drill	with	integrated	sensors	including	bore	depth	measurement	My	overall	research	objective	is	therefore	to	develop	a	reliable	and	inexpensive	replacement	for	the	current	bore	depth	measurement	method	that	can	be	retrofitted	to	existing	surgical	drills.		The	design	process	I	used	to	achieve	this	goal	involved	assessing	the	use	cases	for	such	a	measurement	device,	determining	design	specifications,	extending	and	modifying	an	existing	design	concept,	and	building	and	testing	several	prototypes	in	increasingly	realistic	use	scenarios.		Previous	research	work	in	our	lab	and	elsewhere	has	introduced	sensors	into	the	drilling	process	to	detect	progress	of	the	drill	bit	through	tissue	layers,	typically	to	identify	the	point	where	the	drill	bit	breaks	though	the	far	bone	cortex	(Brett	et	al.	1995;	Taylor	et	al.	2010;	Coulson	et	al.	2013;	Benedetto	Allotta,	Giacalone,	and	Rinaldi	1997;	B.	Allotta	et	al.	1996;	Colla	and	Allotta	1998;	Ong	and	Bouazza-Marouf	1998;	Hsu,	Lee,	and	Lin	2001;	Wen-Yo	Lee	and	Shih	2006;	W.-Y.	Lee,	Shih,	and	Lee	2004;	Louredo,	Díaz,	and	Gil	2012;	Louredo,	Diaz,	and	Gil	2012).			 3	Measurements	of	force,	torque,	and	displacement	have	all	been	used.		Most	had	the	primary	purpose	of	attempting	to	stop	drilling	automatically	to	prevent	the	drill	bit	from	plunging	into	adjacent	tissues.		However,	these	designs	did	not	explicitly	measure	the	depth	of	the	drilled	bore	and	so	could	not	guide	screw	selection.		Additionally,	they	relied	on	sensors	and	actuators	within	the	drill	transmission	itself,	and	thus	required	a	new	drilling	device.		At	the	time	that	we	began	this	project,	we	were	unaware	of	any	devices	focused	on	the	task	of	accurately	measuring	the	bore	depth,	so	we	initially	targeted	this	functionality.	Previous	work	in	our	lab	demonstrated	that	a	single	sensor	design,	using	a	linear	potentiometer,	could	be	mounted	on	an	existing	drill	and	provide	an	accurate	measure	of	drilled	bore	depth	in	animal	tissue	(Cavers	et	al.	2016).		Limitations	of	this	design	include	use	of	a	mechanical	arm	that	intrudes	into	the	surgical	field,	leading	to	challenges	in	developing	a	mechanical	system	that	is	compatible	with	multiple	models	of	surgical	drill.	Because	of	the	limitations	we	identified	related	to	the	mechanical	design	I	evaluated	the	possibility	of	using	a	laser-ranging	device	for	this	purpose.		I	hypothesized	that	such	a	device	would	simplify	the	overall	device	design,	avoid	mechanical	interference	with	the	surgical	site,	and	allow	for	better	compatibility	with	existing	drill	models.		Relatively	recently,	we	have	become	aware	of	two	related	devices	that	are	similar	in	concept	to	what	we	proposed	and	investigated	here:		(1)	the	AutoGauge	–	a	device	invented	at	the	AO	Foundation	in	Switzerland	that	uses	a	laser	displacement	sensor	measuring	displacement	from	a	modified	drill	guide	to	compute	bore	depth,	and	(2)	a	patent	application	by	McGinley	on	a	device	using	a	single	sensor	to	detect	the	leading	edge	of	an	instrument	passing	through	materials	of	variable	properties.		While	these	devices	will	certainly	affect	our	ability	to	seek	a	patent	on	the	system	presented	in		 4	this	thesis,	neither	of	these	devices	have	had	their	performance	reported	on	in	the	published	literature.		My	specific	research	objectives	are	therefore	to:	1. Perform	a	systematic	needs	assessment	for	a	bore	depth	measurement	device	based	on	a	laser-sensing	principle	2. Develop	design	specifications	for	the	device	3. Develop	multiple	concepts	for	a	laser	sensor	device	4. Perform	feasibility	testing	of	device	concepts	5. Perform	accuracy	and	reliability	testing	of	leading	concepts	in	surgically	relevant	simulated	scenarios		 We	used	commercially	available	laser	displacement	sensors	for	the	concept	prototypes.		The	concepts	tested	included	using	single	vs.	multiple	sensors,	aligning	the	sensor	with	the	drill	axis	or	at	an	angle	to	it,	and	measuring	the	displacement	from	the	tissue	vs.	an	instrument	surface.	1.2	Prior	Work	Our	lab	previously	developed	a	prototype	that	incorporated	a	mechanical	displacement	sensor	attached	to	a	surgical	drill.	The	breakthrough	point	was	identified	by	analyzing	the	depth	vs	time	trajectory	and	observing	the	point	where	the	drill	rapidly	accelerated	after	breaching	the	distal	cortex.		The	change	in	displacement	between	the	initial	and	exit	positions	was	taken	to	be	the	bore	depth.		Previous	investigators	found	that	this	system	could	accurately	compute	the	bore	depth	in	an	animal	model	based	on	the	depth	vs	time	trajectory	when	compared	with	digital	calipers	(mean	error	0.5mm,	standard	deviation	0.5mm)(Cavers	et	al.	2016).	However,		 5	the	use	of	a	linear	sliding	mechanism	would	complicate	use	of	the	device	in	to	the	operating	room.		The	arm	would	need	to	extend	into	the	surgical	incision	and	make	contact	with	the	tissue,	increasing	the	profile	of	the	device	and	obstructing	the	surgeon’s	field	of	view.		Furthermore,	the	device	was	initially	tested	in	a	limited	range	of	settings	with	animal	bones,	so	the	relevance	to	surgical	applications	was	insufficiently	examined.		For	these	reasons,	we	decided	to	explore	a	laser-based	alternative	approach	and	evaluate	it	in	more	surgically-relevant	scenarios.		Before	explaining	our	design	process	in	detail,	we	begin	by	reviewing	the	context	of	the	targeted	surgical	tasks.	1.3	Background	and	Literature	Review	1.3.1	Bone	Drilling	in	Clinical	Practice	Fractures	of	the	human	skeleton	are	common	and	frequently	treated	with	operative	fracture	reduction	and	placement	of	mechanical	hardware	(osteosynthesis).		This	facilitates	healing	and	allows	early	mobilization.		The	most	common	osteosynthesis	method	involves	placing	plates	and	screws	into	the	fracture	segments	to	stabilize	them	in	appropriate	alignment.		The	surgeon	must	choose	screws	of	appropriate	length	–	too	short	and	there	is	a	theoretical	risk	of	a	weak	fixation	construct	(“Oxford	Textbook	of	Trauma	and	Orthopaedics	-	Oxford	Medicine”	2017);	too	long	and	there	is	risk	of	damage	to	tissue	structures	adjacent	to	the	bone	(Caruso,	Vitali,	and	del	Prete	2015;	Maschke	et	al.	2007).		 A	depth	gauge	is	commonly	used	to	measure	bore	depth	in	bicortical	bone	osteosynthesis.		This	is	a	thin	wire	instrument	with	a	hook	on	one	end	and	measurement	gradations	along	its	length.		The	instrument	is	placed	through	the	drilled	bore,	hooked	on	the		 6	far	cortex,	and	gently	retracted.		The	depth	of	the	bore	is	measured	from	the	gradations	marked	on	the	instrument.		Focus	sessions	and	individual	interviews	we	conducted	with	surgeons	who	use	this	instrument	identified	that	it	is	‘frustrating,	time	consuming,	and	inaccurate	to	use.’		Measuring	bore	depth	using	the	current	instrument	results	in	placing	screws	that	are	too	long	9%	of	the	time	in	certain	operations	(Ozer	and	Toker	2011).		Screws	that	are	too	long	can	lead	to	complications	such	as	tendon	rupture,	which	has	been	discussed	in	case	reports(Schnur	and	Chang	2000;	Caruso,	Vitali,	and	del	Prete	2015).				Figure	2	-	The	conventional	depth	gauge		 Intraoperative	fluoroscopy	is	generally	performed	to	confirm	adequate	fracture	reduction	once	plates	and	screws	have	been	placed.		Cadaver	studies	have	shown	that	the	standard	views	obtained	will	often	not	show	that	a	screw	is	the	wrong	size,	as	a	screw	has	to	be	an	average	of	2.5-6.5mm	too	long	before	being	identifiable	on	intra-op	X-ray	(Maschke	et	al.	2007).		This	suggests	that	many	wrong-sized	screws	go	unrecognized.		Because	incorrect	screw	sizing	is	often	missed,	it	is	especially	important	to	select	the	correct	screw	size	initially.				 7	1.3.2	Bone	Anatomy	and	Screw	Placement		 The	long	bones	of	the	human	skeleton	have	three	sections:	the	epiphysis,	the	metaphysis,	and	the	diaphysis	(Netter	2014).		The	main	length	of	the	bone	shaft	is	the	diaphysis	which	has	outer	layer	of	dense	cortical	bone.		This	surrounds	the	medullary	canal,	which	contains	soft	bone	marrow.		The	epiphysis	at	the	proximal	and	distal	bone	ends	consists	of	a	thinner	outer	layer	of	cortical	bone	surrounding	a	middle	section	of	spongy,	cancellous	bone.		The	metaphysis	section	transitions	between	the	properties	of	the	other	two	sections.		Cortical	bone	has	superior	material	strength	to	cancellous	bone(Reilly	and	Burstein	1975;	Carter,	Schwab,	and	Spengler	1980).		The	metaphysis	section	transitions	between	the	properties	of	the	other	two	sections.			Figure	3	-	Long	bone	anatomy		 Screws	placed	during	osteosynthesis	surgery	should	have	purchase	in	the	strong	cortical	portion	of	the	bone	(“Oxford	Textbook	of	Trauma	and	Orthopaedics	-	Oxford	Medicine”	2017).			 8	This	means	having	the	screw	pass	through	the	cortex	on	both	sides	of	the	bone	(bicortical)	when	putting	fixation	in	the	diaphyseal	component.		The	external	surface	of	the	metaphysis	and	epiphysis	also	has	a	cortical	layer	(Rho,	Kuhn-Spearing,	and	Zioupos	1998),	and	screws	placed	in	these	portions	of	the	bone	should	also	engage	this	cortical	layer.		1.3.3	Alternatives	to	Surgical	Depth	Gauge		 Review	of	the	patents	related	to	the	surgical	depth	gauges	found	two	advanced	surgical	drills	with	integrated	depth	measurement,	in	addition	to	other	features.		Cost	information	is	not	publically	available	for	the	devices	described	below	but	is	assumed	to	be	significant.		A	standard	surgical	drill	costs	approximately	$20	000	USD.		 The	Intellisense	Drill	(McGinley	Orthopedics,	USA)	(“IntelliSense	Bone	Drill	With	Auto	Depth	Measurement	and	Edge	Detection	FDA	Cleared	(VIDEO)	|”	2015)includes	integrated	sensors	to	determine	drilled	depth	in	bone	(Figure	1).		A	combination	of	a	force	and	displacement	sensors	are	used	to	measure	the	drill	position	and	the	force	of	the	tissue	on	the	drill,	allowing	the	type	of	tissue	at	the	drill	bit	tip	to	be	determined.		It	also	has	a	programmable	‘stop’	function	to	prevent	the	drill	plunging	once	the	bone	has	been	breached.		It	can	be	integrated	into	standard	operative	techniques,	but	requires	replacement	of	the	existing	drill	hand	piece.		 The	SMARTdrill	(SMD	Inc,	USA)	(“Smart	Drill	–	Prevent	Plunge,	Measure	and	Control	Depth,	Determine	Bone	Density”	2017)	also	integrates	sensors	into	a	drill	hand	piece.		It	uses	a	combination	of	force	and	displacement	sensors,	and	includes	a	moveable	drive	assembly	in	the	hand	piece	to	allow	mechanical	control	of	drilling	depth.		Its	features	include	depth		 9	measurement,	programmable	drilling	depth,	measurement	of	bone	density,	and	measurement	of	drill	bit	tip	performance.		It	requires	replacement	of	the	entire	drill	hand	piece.		 			Figure	4	-	SMARTdrill	(SMD	Inc.,	USA)	AO	Biomedical	Engineering	has	recently	developed	an	optical	sensor	based	device	(the	AutoGauge)	that	attaches	to	an	existing	surgical	drill,	and	measurements	movement	of	the	drill	relative	to	a	custom	drill	guide.		This	has	been	presented	at	a	major	orthopedics	conference	(DKOU	2016).		There	is	currently	no	published	data	on	the	efficacy	of	this	device.		My	supervisor,	Dr.	Hodgson	and	I	had	a	conversation	with	the	lead	on	this	project,	Dr.	Peter	Varga.		Dr.	Varga	shared	that	his	team	was	working	on	a	concept	using	optical	measurement	of	drill	displacement	to	compute	drilled	bore	depth	in	surgical	bone	drilling.		No	information	on	their	design	has	been	published	in	the	scientific	literature	or	was	available	in	the	patent	search	process.				 10	1.3.3	Research	on	Surgical	Bone	Drilling	Most	of	the	previous	research	on	bone	drilling	was	focused	on	mathematical	modelling	of	the	material	properties	of	the	process.		The	more	general	case	of	surgical	bone	drilling	was	described	using	finite	element	analysis	by	(Basiaga,	Paszenda,	and	Szewczenko	2010).		Qi	et	al	compared	rigid-plastic	with	rigid	elastic	models	in	their	finite	element	analysis	and	found	similar	results	under	high	speed	drilling	conditions	(Qi,	Wang,	and	Meng	2014).		Tuijoft	compared	the	thrust	forces	required	for	a	constant	drill	feed	rate	between	cortical	and	cancellous	bone	drilling,	finding	significant	differences	(10-110	N	vs.	3-65	N,	respectively)	(Tuijthof,	Frühwirt,	and	Kment	2013).		This	modelling	research	was	less	relevant	to	our	objective	of	measuring	the	drilled	bore	depth	in	bone.	More	relevant	to	our	objectives	is	research	done	on	creating	an	automatic	surgical	drill,	as	this	would	require	detection	of	layer	transition.		Many	research	groups	have	attempted	to	automate	the	drilling	process	so	that	the	depth	of	the	drilling	is	not	guided	purely	by	the	surgeon’s	intuition	of	where	the	drill	bit	is.		This	is	in	contrast	to	our	previous	work	which	used	the	detecting	of	drilling	tissue	level	to	compute	depth	of	drilled	bore,	but	which	was	not	meant	to	automate	the	drilling	process	(Cavers	et	al.	2016).	 		 A	research	group	in	the	UK	developed	a	micro	drill	for	use	in	middle	ear	surgery	that	could	detect	drill	bit	breakthrough	by	analysis	of	drilling	torque	and	axial	load	on	the	drill	bit.		At	that	point	their	device	automatically	stopped	drilling,	preventing	drill	bit	plunge	(Brett	et	al.	1995).		The	anatomy	of	this	region	is	extremely	sensitive	and	thus	precise	control	of	drilling	depth	is	essential.		They	further	refined	their	concept	and	developed	a	robotic	surgical	drill	for	use	in	the	OR	for	preparing	cochleostomies	(Taylor	et	al.	2010)	and	initiated	clinical	trials		 11	(Coulson	et	al.	2013).		The	animal	component	of	their	2013	trial	showed	1/20	average	velocity	of	adjacent	tissue	when	compared	with	conventional	methods,	demonstrating	that	their	device	was	able	to	more	accurately	detect	the	breach	of	the	far	bone	surface	and	halt	the	drill	than	the	conventional	method.			A	separate	group	in	Italy	studied	the	problem	of	surgical	bone	drilling	in	long	bones.		They	first	developed	and	validated	a	theoretical	model	for	twist	drilling	of	surgical	long	bones	(B.	Allotta	et	al.	1996).		In	this	model	they	developed	an	algorithm	to	detect	breakthrough	between	cortical	bone	and	trabecular	tissue	or	cortical	bone	and	soft	tissue	based	on	a	threshold	axial	force.		This	algorithm	was	included	in	a	mechatronic	drilling	tool	that	stopped	automatically	at	breach	of	cortical	bone,	again	preventing	drill	bit	plunge	(Benedetto	Allotta,	Giacalone,	and	Rinaldi	1997).		The	group	then	applied	wavelet	transform	analysis	to	their	previous	experimental	data	and	were	able	to	detect	drill	bit	breakthrough	with	a	more	computationally	efficient	approach	(Colla	and	Allotta	1998).		 Ong	et	al	identified	that	previous	approaches	to	breakthrough	detection	relied	on	relatively	homogenous	bone	density	and	ignored	the	influence	of	system	compliance	(Ong	and	Bouazza-Marouf	1998).		They	showed	that	system	compliance	can	vary	significantly	at	different	anatomic	bone	locations,	and	result	in	different	force	profiles.		To	account	for	these	factors	they	developed	a	new	detection	algorithm	incorporating	a	Kalman	filter	and	analysis	of	force	difference	between	successive	samples	and	drill	bit	rotation	speed	that	functioned	in	a	more	realistic	range	of	settings.		 Recognizing	that	previous	automatic	drilling	systems	required	replacing	the	whole	drill	hand	piece	at	significant	expense,	Hsu	et	al	developed	a	modular	system	that	incorporated	an		 12	existing	drill	(Hsu,	Lee,	and	Lin	2001).		Their	system	analyzed	the	voltage	draw	of	the	drill,	with	high	loads	associated	with	drilling	cortical	bone	and	low	loads	with	trabecular	bone	or	soft	tissue.		Similar	to	previous	work	this	system	automatically	stopped	drilling	at	breakthrough	(Hsu,	Lee,	and	Lin	2001).		This	work	suggests	that	adding	a	sensor(s)	to	an	existing	surgical	drill	could	detect	layer	transition	without	replacing	the	whole	drill	handpiece.		 Returning	to	systems	requiring	a	new	surgical	drill,	Lee	et	al	developed	a	system	that	allowed	for	a	variable	feed	rate	of	the	drill	assembly	–	previous	systems	kept	the	forward	motion	of	the	drill	constant.		This	allowed	for	a	faster	drilling	process.		The	feed	rate	of	the	drill	was	controlled	by	dual	force	feedback	signaling	from	measurement	of	drilling	force	and	torque.		Their	algorithm	was	based	on	measurement	of	drilling	torque	trend,	threshold	thrust	force,	and	feed	rate	(W.-Y.	Lee,	Shih,	and	Lee	2004).		Subsequently	they	included	a	similar	control	algorithm	in	a	three	axis	surgical	robot	for	orthopedic	drilling	(Wen-Yo	Lee	and	Shih	2006).	Louredo	et	al	took	a	different	approach	in	developing	an	automatic	drilling	system.		They	developed	a	breakthrough	detection	algorithm	based	purely	on	measurements	of	the	drill	bit	position	that	demonstrated	superior	performance	to	previous	methods	involving	measurement	of	force	and	torque	(Louredo,	Diaz,	and	Gil	2012).		This	algorithm	used	the	difference	between	an	ideal	drill	position	based	on	a	set	translational	speed	and	a	measured	real	position,	with	a	control	scheme	that	aimed	to	minimize	the	position	error.		As	the	drill	passed	through	the	cortical	bone	the	position	error	would	increase,	and	as	it	neared	the	end	of	the	cortex	there	would	be	less	resistance	to	drilling	and	the	system	would	accelerate	to	minimize	the	error.		This	acceleration	was	detected	by	the	algorithm	which	would	stop	the	drilling	process.		This	algorithm	was	included	in	their	mechatronic	bone	drilling	system,	dubbed		 13	the	DRIBON	(Louredo,	Díaz,	and	Gil	2012).		This	work	demonstrated	that	measurement	of	displacement	could	be	used	to	detect	layer	transition	in	bone	drilling.		 Bone	drilling	is	a	significant	part	of	dentistry	in	placing	oral	implants	to	replace	lost	teeth.		Quest	et	al	developed	a	system	using	lasers	to	ablate	the	bone,	and	a	laser	triangulation	sensor	to	intermittently	measure	the	depth	of	bore,	as	part	of	an	automated	‘drilling’	system	(Quest,	Gayer,	and	Hering	2012).		This	study	demonstrated	that	optical	sensing	could	be	done	from	the	surface	of	bone	during	a	drilling	process.		 To	our	knowledge,	no	previous	studies	have	specifically	looked	at	trying	to	determine	the	drilled	bore	depth	in	bone	automatically.	1.3.4	Porcine	Surgical	Modelling		 Surgery	has	been	simulated	in	a	variety	of	ways,	including	using	human	cadavers,	synthetic	tissue	substitutes,	and	animal	models.		Since	it	is	relatively	expensive	to	use	human	cadaver	specimens,	we	would	like	to	consider	using	animal	models	in	this	study	to	assess	the	performance	of	our	laser-based	depth	gauge.	Pigs	have	a	long	history	of	use	in	surgical	experiments	as	they	are	relatively	similar	to	humans	in	anatomy	(Swindle,	Smith,	and	Hepburn	1988).		In	experimental	comparison	of	bone	mineral	density	and	mechanical	strength	between	human	bone	and	various	animal	models,	pig	and	dog	were	found	to	be	the	most	similar	to	humans	(Aerssens	et	al.	1998).	Pigs	have	been	used	in	a	variety	of	ways	for	orthopedic	surgical	education.		They	have	been	used	in	the	simulation	of	osteosynthesis	surgery	(Leong	et	al.	2008),	arthroscopy	(Mattos	e	Dinato,	de	Faria	Freitas,	and	Iutaka	2010),	and	flexor	tendon	surgery	(Smith	et	al.	2005).		We	therefore	feel	that	it	would	be	appropriate	to	use	porcine	bone	models	to	assess	the	performance	of	our	system.		 14		1.4	Thesis	Overview		 In	the	remaining	chapters,	we	present	our	device	and	associated	experiments.		Chapter	2	presents	the	Needs	Assessment	process,	which	specified	the	requirements	for	the	device	to	be	built.		Chapter	3	overviews	the	engineering	design	process	that	led	to	our	final	concept	of	dual	angled	sensors	measuring	from	a	modified	depth	gauge.		Chapter	4	details	the	experimental	protocol	developed	to	evaluate	the	repeatability	of	the	design	in	surgically	relevant	scenarios.		Chapter	5	presents	the	results	obtained	from	these	experiments	and	Chapter	6	contains	the	discussion.		Chapter	7	offers	our	conclusions	and	guidance	for	future	work.		 		 15	2	Needs	Assessment	2.1	Overview		 This	chapter	describes	the	Needs	Assessment	for	the	design	of	a	new	laser-based	depth	gauge	for	use	in	surgery	on	the	bony	skeleton.		The	current	method	for	measuring	bone	bore	depth	is	described,	along	with	its	limitations	and	the	clinical	context	of	these	measurements.		Surgeon	interviews	and	OR	observations	are	reported.		I	describe	the	currently	used	surgical	drills	and	the	market	share	for	the	major	vendors.		I	also	analyze	the	commonly	used	surgical	incisions	and	hardware	fixation	constructs	used	in	osteosynthesis	surgery.		This	information	is	synthesized	to	develop	a	Problem	Statement	that	guided	my	engineering	design.	2.2	Prior	Work		 A	focus	group	of	the	UBC	Division	of	Plastic	Surgery	and	the	UBC	Biomedical	Engineering	‘Engineers	in	Scrubs’	was	held	in	2012	to	identify	problems	in	current	surgical	practice	that	could	benefit	from	the	application	of	engineering	design	principles.		The	author	co-ordinated	this	activity	from	the	clinical	side,	and	approximately	twenty	staff	surgeons	and	ten	trainees	attended.		The	current	method	of	measuring	drilled	bore	depth	in	bone	during	hand	and	craniofacial	surgery	was	identified	as	an	area	for	improvement.		 A	team	of	engineering	students	conducted	a	preliminary	needs	assessment	and	came	up	with	the	following	problem	statement:	“Detecting	correct	drilled	depth	is	time	consuming	and	incorrect	measurement	leads	to	surgical	complications”.		 16		 	 They	then	progressed	to	concept	generation,	selection,	and	an	initial	round	of	prototyping.		In	the	following	section,	the	Needs	Assessment	is	repeated	and	expanded	to	provide	a	more	thorough	basis	for	my	design.	2.3	Needs	Assessment	2.3.1	Interviews	with	Practicing	Surgeons		 	 I	had	informal	conversations	with	practicing	surgeons	who	use	the	conventional	depth	gauge	in	their	practice.		I	expanded	the	scope	of	the	Needs	Assessment	to	include	orthopedic	surgeons	as	well	as	plastic	surgeons	based	on	my	understanding	of	surgical	practice.		The	surgeons	interviewed	were	Dr.	Pierre	Guy	in	orthopedics,	and	Dr.	Nick	Carr,	Dr.	Alex	Seal,	Dr.	Jim	Boyle,	and	Dr.	Erin	Brown	in	plastic	surgery.		100%	of	the	surgeons	interviewed	expressed	dissatisfaction	with	the	conventional	depth	gauge.		They	said	it	was	difficult	to	use,	time	consuming,	inaccurate,	and	at	times	unusable	due	to	limited	surgical	exposure	(particularly	in	oral	surgery).		Notably,	specific	incidences	of	wasted	screws	or	inaccurate	measurements	resulting	in	need	for	re-operation,	alluded	to	in	the	previous	needs	assessment	(and	the	literature	reported	in	Chapter	1?),	were	not	reported.		Surgeons	expressed	a	significant	emotional	aversion	to	the	current	device.		All	expressed	interest	in	an	alternative	method	of	measuring	drilled	bone	bore	depth	that	is	more	user-friendly.				 	 The	plastic	surgeons	interviewed	described	using	a	range	of	screw	sizes	from	5-15mm	in	length.		Dr.	Guy	stated	that	the	longest	screw	used	in	orthopedic	trauma	practice	was	85mm.		 17	2.3.2	OR	Observations		 	 I	went	to	the	OR	to	observe	the	depth	gauge	in	use	in	both	orthopedic	trauma	and	plastic	surgery	contexts.		This	confirmed	the	previous	understanding	of	the	use	of	the	device.			 In	practice,	the	bone	is	surgically	exposed	at	the	site	of	fracture	or	osteotomy.		An	appropriate	fixation	plate	is	selected,	and	the	location	of	the	drilled	bore	for	screw	placement	is	selected	to	match	the	selected	plate.		The	surgical	drill	is	used	to	bore	through	the	bone,	generally	bicortically	and	nominally	perpendicular	to	the	long	axis	of	the	bone,	though	different	orientations	may	be	used	in	selected	circumstances.		The	surgeon	notes	the	feeling	of	the	drill	passing	through	the	cortex	(slow	forward	movement	of	the	drill	when	passing	through	the	cortex,	and	rapid	acceleration	at	transition	to	medulla	or	adjacent	soft	tissue)	and	knows	the	bore	is	complete	when	the	second	cortex	has	been	breached.		The	drill	is	removed	and	the	depth	gauge	is	passed	through	the	bore,	and	then	pressed	against	the	side	of	the	bore	and	pulled	back	to	‘hook’	the	tip	of	the	gauge	on	the	far	cortex.		With	the	tip	secure,	a	measurement	can	be	read	from	the	graduated	aspect	of	the	gauge.		This	measurement	is	used	to	select	the	appropriate	screw.		Fluoroscopy	is	used	intraoperatively	to	assess	the	adequacy	of	fracture	fixation,	but	not	explicitly	to	confirm	the	correct	screw	choice	has	been	made.		Occasionally	an	incorrect	screw	length	is	noted	intraoperatively	with	these	images	and	may	be	corrected	with	screw	replacement,	but	this	is	due	to	chance	rather	than	planning.		Routine	fluoroscopy	has	been	shown	to	be	inaccurate	in	detecting	wrong-sized	screws	during	routine	surgery	(Ozer	and	Toker	2011)	and	replacing	a	screw	with	a	new	one	has	been	shown	to	decrease	final	construct	pull-out	strength	(Matityahu	et	al.	2013).		 18		Figure	5–	Typical	orthopedic	surgical	exposure	with	plates	and	screws	in	situ		 	 I	observed	a	total	of	19	uses	of	the	depth	gauge	during	two	surgeries.		In	three	instances	the	surgeon	struggled	with	the	depth	gauge.	This	was	due	to	difficulty	in	getting	the	wire	component	to	follow	the	drilled	bore	path	and	hook	on	the	far	cortex.		This	resulted	in	the	process	taking	more	time	than	usual.		No	screws	were	wasted	as	a	result	of	the	device	malfunctioning.		No	screws	needed	to	be	removed	or	replaced	during	the	use	of	the	depth	gauge.		There	was	no	obvious	expense	or	extra	utilization	of	resources	associated	with	use	of	the	current	depth	gauge.		Theoretically	there	is	a	chance	that	incorrect	screw	selection	based	on	the	measurements	of	the	conventional	depth	gauge	will	result	in	adverse	outcomes	for	patients.		There	is	opinion	among	surgeons	that	screws	that	are	too	short	will	result	in	a	suboptimal	fracture	fixation	construct,	but	there	is	no	evidence	this	has	a	clinical	effect.			2.3.4	Updated	Needs	Statement		 	 I	synthesized	the	information	from	the	previous	work,	and	the	confirmatory	conversations	with	surgeons	and	observations	made	in	the	OR	to	draft	a	new	‘Needs	Statement.’		 19		“There	is	a	need	for	a	device	to	measure	drilled	bore	depth	in	bone	during	surgery	that	is	more	accurate,	less	time	consuming,	and	less	frustrating	for	the	surgeon	than	the	current	depth	gauge	approach.”	2.4	Context	Identification	2.4.1	Clinical	Uses	of	the	Depth	Gauge		 	 I	assessed	the	use	cases	of	the	conventional	surgical	depth	gauge	based	on	my	knowledge	of	surgical	technique	from	previous	training	and	review	of	appropriate	surgical	textbooks.		This	assessment	was	corroborated	in	conversation	with	senior	orthopedic	(Dr.	Pierre	Guy)	and	plastic	surgeons	(Dr.	Nick	Carr	and	Dr	Kevin	Bush).		The	depth	gauge	is	needed	in	any	surgical	case	where	components	of	bone	are	to	be	attached	using	plates	and	screws,	and	when	the	length	of	screw	is	to	match	the	length	of	available	bone.		Use	cases	of	the	depth	gauge	are	summarized	below.		Areas	of	orthopedic	surgery	not	included,	because	the	depth	gauge	is	not	used,	are	arthroplasty,	spine	surgery,	and	pelvis	surgery.	Orthopedic	Surgery:	upper	and	lower	extremity	trauma	Plastic	Surgery:	hand	trauma,	craniofacial	trauma,	osseous	reconstruction	Oral	and	Maxillofacial	Surgery:	orthognathic	surgery	Other:	Veterinary	surgery,	rib	and	sternal	fixation		 	 I	obtained	information	from	the	Vancouver	General	Hospital	(VGH)	Business	Manager,	Meilan	Robson,	about	the	total	number	of	these	cases	performed	at	VGH	per	year	to	estimate	the	size	of	the	market	for	depth	gauges.		VGH	is	a	quaternary	care	acute	hospital	with	approximately	700	beds	and	23	000	inpatient	and	outpatient	surgical	cases	per	year.		In	the		 20	2015/2016	fiscal	year	1087	surgical	cases	would	likely	have	used	the	depth	gauge.		The	distribution	among	different	surgical	services	is	shown	in	the	figure	below,	with	orthopedic	trauma	being	the	largest	user	of	the	depth	gauge.		Figure	6	–	Distribution	of	surgical	cases	using	the	depth	gauge	between	different	services	at	VGH	2.4.2	Surgical	Drills		 Surgical	drills	come	in	two	main	types:	pistol	type	and	pencil	type.	Orthopedic	surgeons	exclusively	use	pistol	type	drills,	while	oralmaxillofacial	(OMF)	surgeons	tend	to	use	pencil	type.		Plastic	surgeons	generally	use	pistol	type	drills,	though	some	will	use	pencil	type	for	operations	on	the	facial	skeleton	as	a	matter	of	preference.		There	is	no	clear	superiority	of	one	type	over	another,	and	different	use	patterns	tend	to	reflect	the	predominantly	used	device	where	the	surgeon	trained.		Orthopedic	trauma	surgeons,	which	as	shown	in	the	above	section	perform	most	of	the	operations	involving	the	depth	gauge,	use	exclusively	pistol	type	surgical	drills.		188627272VGH	Cases	2015/2016	using	Depth	GaugeOMF Ortho	Trauma Plastics	 21		Figure	7	–	Pistol	type	drill			 			 Drills	are	classified	by	size	and	power	as	small	bone,	medium	bone,	and	large	bone.		Large	bone	drills	are	used	for	orthopedic	reconstructive	procedures	that	do	not	require	use	of	the	depth	gauge,	so	they	will	be	omitted	from	further	analysis.		In	North	America	the	majority	of	surgical	drills	are	sold	by	Stryker	and	Linvatec	(personal	communication	with	Stryker	Sales	Representative	Darren	Harris).		Market	size	and	share	for	small	and	medium	bone	surgical	drills	are	shown	in	the	figures	below.		Figure	8	–	Pencil	type	drill		 22		Figure	9	–	Small	Bone	Drill	Market	USA	2015		Figure	10	–	Medium	Bone	Drill	Market	USA	2015	Stryker	67%Linvatec/Hall14%Other19%Small	Bone	Surgical	Drill	Market	(US$108.3M)Stryker	 Linvatec/Hall OtherStryker68%Linvatec/Hall16%Synthes13%Other	2%zimmer	1%Medium	Bone	Surgical	Drill	Market	(US$68.1M)Stryker Linvatec/Hall Synthes Other	 zimmer		 23		 Stryker	and	Linvatec	combined	account	for	more	than	80%	of	both	the	small	and	medium	bone	market.	2.4.3	Surgical	Screws		 Surgical	screws	come	in	standard	lengths	with	the	instrument	sets	for	the	various	indications	(oral	maxillofacial,	orthopedic	trauma).		Shorter	screws	increase	in	increments	of	1	mm,	and	the	increments	get	larger	as	the	screws	get	longer.		A	selection	of	different	screw	lengths	available	for	different	indications	are	shown	in	the	tables	below	(Stryker	product	catalogues).		Figure	11	–	Mandible	Screws	from	Stryker			 24		Figure	12	–	Upper	Face	Screws	from	Stryker		Figure	13	-	Orthopedic	trauma	screws	from	Stryker		 25		 	2.4.4	Surgical	Exposure	Dimensional	Analysis		 Because	Orthopedic	trauma	surgery	of	the	appendicular	skeleton	represents	the	largest	usage	for	depth	measurement,	I	concentrated	on	assessing	these	applications.		To	characterize	the	geometry	of	these	use	case	scenarios,	I	conducted	a	dimensional	analysis	of	surgical	approaches	to	the	appendicular	skeleton	commonly	performed	in	orthopedic	surgery.		My	goal	is	to	use	this	knowledge	of	the	surgical	exposure	geometry	in	evaluating	potential	configurations	of	the	laser-sensing	device	on	the	surgical	drill	(eg,	establishing	the	optimal	position	and	orientation	of	the	sensor).		 Accurate	dimensions	of	the	incisions	used	in	orthopedic	surgery	have	not,	to	my	knowledge,	been	analyzed	or	reported	in	the	orthopedic	literature.		Surgery	may	be	described	with	reference	measurements	to	anatomic	landmarks,	but	the	dimensions	of	the	final	incisions	or	the	area	of	bone	exposed	have	not	been	described.		However	accurate	images	of	the	surgical	exposures	are	available	in	surgical	textbooks,	and	I	used	these	as	the	basis	for	this	investigation.		 I	used	the	‘Atlas	of	Surgical	Exposures	of	the	Upper	and	Lower	Extremity’	by	Tubiana	et	call	(Martin	Dunitz	Ltd.	2000).		This	textbook	provides	detailed	illustrations	of	the	incisions	and	exposures	used	in	orthopedic	surgery,	as	well	as	visual	references	to	the	size	of	the	incisions	on	the	human	body.		I	reviewed	all	images	in	the	text,	and	selected	those	used	in	orthopedic	trauma	surgery	and	likely	to	require	use	of	the	depth	gauge	(n	=	28).		This	knowledge	comes	from	my	training	in	surgery,	as	well	as	conversation	with	staff	orthopedic	surgeon	Dr.	Pierre	Guy.		 26		 I	analyzed	the	selected	images	for	dimensionless	proportions	using	Adobe	Photoshop	(Adobe,	USA).		I	took	measurements	of	the	length	and	width	of	the	incision,	as	well	as	the	length	and	width	of	the	exposed	bone	(see	Figure	14).		An	estimated	‘true’	length	of	incision	was	obtained	by	reviewing	the	indicated	incision	marking	in	the	text	and	then	measuring	the	corresponding	length	on	a	human	volunteer.		The	proportion	between	this	value	and	the	dimensionless	incision	length	from	the	Photoshop	analysis	was	used	to	compute	a	correction	factor,	which	could	then	be	used	to	calculate	the	predicted	values	for	the	other	exposure	dimensions.				Figure	14	–	Surgical	exposure	dimensional	analysis.		Incision	dimensions	are	indicated	in	green,	and	bone	exposure	dimensions	are	indicated	in	purple.		 27		Figure	15	-	Example	of	incision	marking	image	A	summary	of	the	data	is	presented	in	Table	1.		Data	is	presented	in	Appendix	A.			Length	(cm)	Std.	Dev.	Width	(cm)	Std.	Dev.	Incision	 18.5	 5.3	 8.8	 3.3	Bone	 11.4	 4.1	 1.9	 0.8	Table	1	–	Dimensional	Analysis	Summary		 28		Figure	16	–	Plot	of	the	Incision	Dimensional	Analysis	2.4.5	Fixation	Construct	Analysis		 I	also	performed	a	detailed	descriptive	analysis	of	the	physical	arrangement	of	the	bone,	plates,	and	screws	in	fracture	fixation	constructs	to	clarify	the	use	case	environment	for	bore	depth	drilling.		The	AO	Surgical	Reference(Colton	et	al.	2017)	is	an	online	resource	that	includes	instructions	and	schematics	for	performing	orthopedic	trauma	surgery.		I	reviewed	all	the	described	procedures,	and	identified	those	that	involve	use	of	the	depth	gauge.		A	total	of	63	different	fixation	constructs	with	520	screws	were	relevant	based	on	my	prior	review	of	the	use	case,	discussions	with	practicing	surgeons,	and	my	surgery	training.		 The	conditions	of	each	screw	were	classified	based	on	section	of	bone	(diaphyseal	or	metaphyseal)	and	orientation	of	the	screw	relative	to	the	bone	surface	(perpendicular	or	angled)	at	entry	and	exit	from	the	bone.		Unicortical	screws	were	noted	in	the	construct	analysis	but	only	bicortical	screws	were	included	in	determining	construct	types,	as	depth	measurement	is	principally	relevant	when	using	bicortical	screws.		I	used	this	information	to	024681012141618200 5 10 15 20 25 30 35Length	(cm)Width	(cm)Incision	Dimensional	AnalysisIncisionBone	 29	classify	the	screw-bone	relationship	into	four	types,	depicted	in	the	figures	below.		Data	tables	are	available	in	the	Appendix	B.		Figure	17	-	Condition	SD	(straight	diaphysis),	perpendicular	drilling	to	long	axis	in	diaphyseal	bone.		Perpendicular	entry	and	exit	of	screw	relative	to	bone	surface.		Figure	18	-	Condition	AD	(angled	diaphysis),	angled	drilling	to	long	axis	in	diaphyseal	bone.		Angled	entry	and	exit	of	screw	relative	to	bone	surface		 30		Figure	19	-	Condition	SM	(straight	metaphysis),	perpendicular	drilling	to	long	axis	in	metaphyseal	bone,	angled	entry	and	exit	relative	to	bone	surface		Figure	20	-	Condition	AM	(angled	metaphysis),	angled	drilling	to	long	axis	in	metaphyseal	bone,	perpendicular	entry	and	angled	exit	relative	to	bone	surface		 The	relative	number	of	screws	classified	into	each	conditions	is	shown	in	Figure	21.		 31		Figure	21	-	Screw	conditions	in	Construct	Analysis	2.4.6	Current	Surgical	Depth	Gauge		 The	standard	form	of	the	surgical	depth	gauge	has	not	changed	significantly	since	that	described	by	Gunther	in	1948	(Gunther	and	Keyes	1948).		The	basic	design	is	a	thin	wire	shaft	of	known	length	with	a	hooked	end	that	is	passed	through	the	drilled	bore.		The	hooked	end	engages	with	the	far	cortex	as	the	gauge	is	retracted,	and	gradations	on	the	length	of	the	gauge	shaft	are	used	to	measure	the	depth	of	the	instrument	and	the	bore.		 It	can	be	difficult	to	engage	the	hooked	end	of	the	gauge	with	the	far	cortex,	resulting	in	delays	in	obtaining	a	measurement.		The	passage	of	the	instrument	is	not	visualized	(a	‘blind’	procedure)	so	it	can	be	challenging	to	orient	the	instrument.	2.5	Problem	Statement		 Information	from	the	past	work,	the	new	Needs	Assessment	process,	and	the	Context	Identification	results	were	synthesized	to	develop	a	problem	statement	to	guide	our	design:	40612 619Fixation	Construct	Screw	ConditionsCondition	SDCondition	ADCondition	SMCondition	AM	 32		 The	objective	is	to	develop	a	device	that	will	measure	the	depth	of	drilled	bore	in	bone	during	surgery	reliably	to	an	accuracy	of	1mm,	is	compatible	with	the	most	commonly	used	pistol	type	surgical	drills,	does	not	require	a	separate	measurement	step,	and	does	not	add	significantly	to	the	cost	per	surgical	case.				 		 33	3	Design	3.1	Overview	 		 In	this	section	I	outline	the	design	process	taken	in	developing	a	replacement	for	the	conventional	depth	gauge	in	bone	surgery.		I	decided	to	use	the	same	measurement	concept	as	the	prior	work,	with	continuous	displacement	measurement	of	the	drill	relative	to	the	bone	as	the	analyzed	signal.		I	did	a	Functional	Structural	Decomposition	to	identify	the	necessary	components	for	the	final	device.		All	the	necessary	components	for	a	final	device	are	discussed	in	the	initial	design	phases,	and	then	a	more	limited	number	are	taken	forward	to	feasibility	and	reliability	testing.		The	Prototyping	section	describes	the	prototypes	developed	to	test	the	different	arrangements	of	laser	sensors	for	displacement	measurement	that	are	tested	in	the	Experimental	section.	3.2	Problem	Statement	The	objective	is	to	develop	a	device	that	will	measure	the	depth	of	drilled	bore	in	bone	during	surgery	reliably	to	an	accuracy	of	1mm,	is	compatible	with	the	most	commonly	used	pistol	type	surgical	drills,	does	not	require	a	separate	measurement	step,	and	does	not	add	significantly	to	the	cost	per	surgical	case.			3.3	Concept	The	forward	movement	of	the	drill	along	its	drilling	axis,	relative	to	a	fixed	surface,	is	the	same	as	the	forward	movement	of	the	drill	bit	as	it	bores	through	the	bone.		Therefore,	the	change	in	displacement	of	the	drill	relative	to	a	fixed	surface	in	front	of	it	between	the	point	of		 34	drilling	initiation	and	that	of	breach	of	the	far	cortex	is	the	same	as	the	depth	of	the	drilled	bore.		This	is	shown	in	Figure	22.		Figure	22	-	Relationship	between	displacement	measured	and	bore	depth	𝐵𝑜𝑟𝑒	𝐷𝑒𝑝𝑡ℎ	 = 𝑑,-./01	./23-4	52-6.7 − 𝑑909396: 	Differentiation	and	double	differentiation	of	the	displacement	signal	gives	the	velocity	and	acceleration,	respectively.		Drilling	through	the	cortical	bone	is	essentially	uniform	at	low	velocity,	followed	by	a	rapid	acceleration	when	the	cortex	is	breached.		This	acceleration	happens	twice	in	bicortical	drilling,	first	in	the	transition	between	the	cortex	and	the	medullary	canal	and	again	when	breaching	the	second	cortex	into	the	adjacent	soft	tissue.		The	time	point	of	rapid	increase	in	velocity	and	acceleration	therefore	corresponds	to	the	cortex	breach,	and	can	be	used	to	identify	the	displacement	at	that	point.		This	allows	computation	of	the	drilled	bore	depth.	A	typical	displacement,	velocity,	and	acceleration	tracing	is	shown	in	Figure	23.	!"#"$"%& !'()*#+	)*-$(.	/-(%)01234	5467ℎ	 35			 	 	 	 	 Time	(msec)	Figure	23	-	Graph	of	measured	displacement	and	computed	velocity,	acceleration	during	bore	drilling	This	concept	was	shown	to	be	effective	(Cavers	et	al.	2016)	and	has	advantages	related	to	the	design	objectives	outlined	below	–	specifically	that	it	supports	compatibility	with	existing	surgical	drills.			3.4	Design	Specifications	The	following	key	specifications	arising	from	the	need	identification	process	are	labeled	with	an	(R)	if	it	represents	a	requirement,	and	with	(EC)	if	it	represents	an	evaluation	criterion.	Functional	• Device	provides	measurement	of	drilled	bore	in	bone	to	an	accuracy	of	+/-	1mm	(R)	• Device	will	measure	holes	up	to	10cm	depth	(R)	Economic	• Device	is	inexpensive	(<$300	/case	used)	(EC)	(d0) Breach	1	(d1) Breach	2	(d2)Cortex	1 Cortex	2	 36	Durability	• Device	is	compatible	with	sterile	operating	conditions	(R)	Ergonomic	• Device	is	compatible	with	right	or	left	handed	use	(R)	• Use	of	device	is	appealing	to	surgeons,	as	defined	by	user	experience	study	(EC)	Input/Output	Constraints	• Device	is	compatible	with	currently	used	surgical	drills	in	North	America	(EC)	Regulatory	• Device	can	be	approved	through	FDA	510k	regulatory	process	(R)		3.5	Functional	Structural	Decomposition		 A	block	diagram	of	the	device	architecture	is	shown	in	Figure	24.		The	sensor,	drill	mount	assembly,	and	computer	system	were	present	in	basic	form	in	the	previously	developed	design.		These	components	require	further	development	to	produce	a	final	device.		Additionally,	a	user	interface,	battery	power	supply,	and	a	housing	compatible	with	the	sterile	operating	room	environment	are	required	for	the	final	form	of	the	device.			 37		Figure	24	–	Device	architecture	diagram	3.6	Sensor	3.6.1	Concept	Selection		Prior	work	used	a	mechanical	approach	to	measure	displacement	of	the	drill	relative	to	the	bone,	with	a	sliding	measurement	arm	and	a	linear	potentiometer.		To	improve	this	design,	I	decided	to	explore	optical	distance	sensing	for	measurement	of	the	displacement.		Removing	the	need	for	a	mechanical	arm	reduces	the	intrusion	of	the	device	into	the	operative	field,	improving	the	user	experience.		It	is	also	less	dependent	on	the	geometry	of	the	surgical	drill,	which	could	be	helpful	in	achieving	compatibility	with	multiple	drill	models.	3.6.2	Optical	Sensing		 Multiple	methods	of	optical	sensing	are	described	in	the	literature	and	used	in	engineering	practice.		These	include	intensity-based	sensors,	triangulation	sensors,	time	of	Functional	Structural	DecompositionUser	InterfaceComputer	SystemDrill	Mount	AssemblySensorPower	SourceHousing	 38	flight	sensors,	confocal	sensors,	and	interferometric	sensors	(Berkovic	and	Shafir	2012).		A	summary	of	the	range	and	resolutions	of	the	commonly	used	techniques	in	commercial	use	is	shown	in	Table	2.		Based	on	the	target	measurement	range	(up	to	10cm	of	drilling	depth)	and	desired	resolution	(precision	to	within	1mm)	laser	triangulation	is	most	appropriate	for	the	device.	Method	 Range	 Resolution	 Notes	Intensity-based	 5-50	mm	 1	µm	 Sensitive	to	change	in	surface	conditions	Triangulation	 10	mm	–	1	m	 Several	µm	 Resolution	can	change	across	working	range	Time	of	Flight	 Long	(km	+)	 1	cm	 RADAR	Confocal	 Several	mm	 Sub	µm	 Used	to	build	confocal	microscopes	Table	2	–	Summary	of	common	optical	distance	sensing	method	properties	3.6.3	Laser	Triangulation		 Laser	triangulation	sensors	are	used	frequently	in	industry	as	part	of	manufacturing	systems.		They	can	provide	continuous,	high	resolution	measurements	of	the	displacement	of	manufactured	parts	from	machine	tools	and	robots.		Off	the	shelf	devices	exist	for	different	measurement	ranges	and	resolutions.			 39		Table	3	–	Laser	displacement	sensors	available	from	KEYENCE	3.6.4	Sensor	Orientation		 Multiple	orientations	of	the	displacement	sensor	relative	to	the	drilling	axis	are	theoretically	compatible	with	the	measurement	of	drilled	bore	based	on	the	displacement	signal.		The	core	idea	is	illustrated	in	Figure	25	–	here,	the	laser	beam	is	directed	parallel	to	the	drilling	axis.		As	long	as	the	drill	remains	in	the	same	orientation	throughout	the	drilling	cycle,	the	laser	spot	on	the	anatomy	will	remain	in	the	same	place,	and	the	resulting	depth	estimate	will	be	accurate.		However,	if	the	drill	tilts	or	the	point	where	the	laser	hits	moves	relative	to	the	bone	during	drilling,	an	error	can	be	introduced.	To	compensate	for	the	tilting	effects,	we	could	mount	displacement	sensors	on	opposite	sides	of	the	drill	(see	Figure	25)	and	average	the	resulting	measurements	from	the	different	sensors,	which	would	significantly	reduce	the	sensitivity	of	the	measurement	to	tilt,	though	the	system’s	Sensor headsModel IL-030 IL-065 IL-100 IL-300 IL-600 IL-2000AppearanceReference distance 30 mm 1.18" 65 mm 2.56" 100 mm 3.94" 300 mm 11.81" 600 mm 23.62" 2000 mm 78.74"Measurement range 20 to 45 mm 0.79" to 1.77" 55 to 105 mm 2.17" to 4.13" 75 to 130 mm 2.95" to 5.12" 160 to 450 mm6.30" to 17.72" 200 to 1000 mm  7.84" to 39.37" 1000 to 3500 mm39.37" to 137.80" Light sourceRed semiconductor laser, wavelength: 655 nm (visible light)Laser class Class 1 (FDA (CDRH) Part1040.10) 1.Class 1 (IEC 60825-1)Class 2 (FDA (CDRH) Part1040.10) 1.Class 2 (IEC 60825-1)Output 220 µW 560 µWSpot diameter (at standard distance) Approx. 200 × 750 μm Approx. 550 × 1750 μm Approx. 400 × 1350 μm Approx. ø0.5 mmø0.02" Approx. ø1.6 mmø0.06" Approx. 1400 x 7000 μm Linearity 2. 3. ±0.1% of F.S.(25 to 35 mm 0.98" to 1.38")±0.1% of F.S.(55 to 75 mm 2.17" to 2.95")±0.15% of F.S.(80 to 120 mm 3.15" to 4.72")±0.25% of F.S.(160 to 440 mm 6.30" to 17.32")±0.25% of F.S. (200 to 600 mm 7.84" to 23.62") ±0.5% of F.S. (200 to 1000 mm 7.84" to 39.37")±0.16% of F.S. (1000 to 3500 mm39.37" to 137.80")Repeatability 4. 1 μm 2 μm 4 μm 30 μm 50 μm 100 μmSampling rate 0.33/1/2/5 ms (4 levels available)Operation status indicators Laser emission warning indicator: Green LED, Analog range indicator: Orange LED, Reference distance indicator: Red/Green LEDTemperature characteristics 3. 0.05% of F.S./°C 0.06% of F.S./°C 0.06% of F.S./°C 0.08% of F.S./°C 0.016% of F.S./°CEnvironmentalresistanceEnclosure rating IP67Ambient light 5. Incandescent lamp: 5000 lux Incandescent lamp: 7500 lux Incandescent lamp: 5000 lux Incandescent lamp: 10000 luxAmbient temperature -10 to +50°C 14 to 122°F  (No condensation or freezing)Relative humidity 35 to 85% RH (No condensation)Vibration 10 to 55 Hz Double amplitude 1.5 mm 0.06" XYZ each axis: 2 hoursPollution degree 3Material Housing material: PBT, Metal parts: SUS304, Packing: NBR, Lens cover: Glass, Cable: PVCWeight Approx. 60g Approx. 75g Approx. 135g Approx. 350g1. The laser classification for FDA (CDRH) is implemented based on IEC 60825-1 in accordance with the requirements of Laser Notice No.50.2. Value when measuring the KEYENCE standard target (white diffuse object).3. F.S. of each model is as follows. IL-030: ±5 mm ±0.20" IL-065: ±10 mm  ±0.39" IL-100: ±20 mm  ±0.79" IL-300: ±140 mm  ±5.51" IL-600: ±400 mm ±15.75"4. Value when measuring the KEYENCE standard target (white diffuse object) at the reference distance, sampling rate: 1 ms, and average number of times: 128. For the IL-300/IL-600, the sampling rate is 2 ms.5. Value when the sampling rate is set to 2 ms or 5 ms.Amplifier unitModel IL-1000 IL-1500 IL-1050 IL-1550AppearanceType DIN-rail mount Panel mount DIN-rail mount Panel mountMain unit/expansion unit Main unit Expansion unitHead compatibility CompatibleDisplayMinimum  displayable unit IL-030: 1 μm, IL-065/IL-100: 2 μm, IL-300: 10 μm, IL-600: 50 μm, IL-2000: 100 μm 3.94" Display range IL-030/IL-065/IL-100: ±99.999 mm to ±99 mm (4 levels selectable), IL-300/IL-600: ±999.99 mm to ±999 mm (3 levels selectable), IL-2000: ±9999.9 mm to ±9999 m (2 levels selectable)Display rate Approx. 10 times/sec.Analog voltage output 1. ±5 V, 1 to 5 V, 0 to 5 V  Output impedance 100 ΩNoneAnalog current output 1. 4 to 20 mA  Maximum load resistance of 350 ΩControl input 2.Bank switch inputNon-voltage inputZero-shift inputStop emission inputTiming inputReset inputControl output 3.Judgement output Open collector output (NPN, PNP changeover possible/N.O., N.C. changeover possible)Alarm output Open collector output (NPN, PNP changeover possible/N.C.)CurrentPower voltage 4. 10 to 30 VDC ripple (P-P) 10% included, Class 2 Supplied by main unitPower consumption 2300 mW or less (at 30 V: 77 mA or less) 2500 mW or less (at 30 V: 84 mA or less) 2000 mW or less (at 30 V: 67 mA or less) 2200 mW or less (at 30 V: 74 mA or less)Environmental resistanceAmbient humidity -10 to +50°C 14 to 122°F  (No condensation or freezing)Ambient temperature 35 to 85% RH (No condensation)Vibration 10 to 55 Hz Double amplitude 1.5 mm 0.06" XYZ each axis: 2 hoursPollution degree 2Material Case / Front sheet: Polycarbonate; Key tops: Polyacetel; Cable: PVCWeight (including attachments) Approx. 150g Approx. 170g Approx. 140g Approx. 160g1. Select and use one of ±5 V, 1 to 5 V, 0 to 5 V or 4 to 20 mA.2. Assign an input of your choice to the 4 external input lines before using.3. – The NPN open collector rated output is: 50 mA max./ch (20 mA when adding an expansion unit) less than 30 V, residual voltage less than 1 V (less than 1.5 V when adding over 6 units including the main unit)– The PNP open collector rated output is: 50 mA max./ch (20 mA/ch when adding expansion units), less than power voltage, and less than 2 V residual voltage (less than 2.5 V when adding over 6 units including the main unit) 4. If there are over 6 additional expansion units, please use a power voltage of 20 to 30 V.SPECIFICATIONS12	 40	depth	measurement	would	remain	vulnerable	to	relative	movement	between	the	laser	spots	and	the	bone.	The	laser	beam	could	also	be	angled,	as	shown	in	Figure	26,	to	reduce	the	distance	between	the	drill	axis	and	the	laser	spot,	though	this	would	mean	that	the	laser	spot	would	translate	laterally	during	drilling;		if	the	surface	were	not	perpendicular	to	the	drill	axis,	this	would	produce	a	depth	measurement	error.	In	practice,	the	majority	of	drilling	is	done	using	a	drill	guide,	as	shown	in	Figure	27.		By	adding	a	protrusion	to	the	guide	that	the	laser	spot	could	land	on,	we	could	ensure	that	the	spot	always	remains	in	the	same	position,	which	would	significantly	reduce	sensitivity	to	drill	tilt.		Figure	25	-	Displacement	sensor	collinear	with	drilling	axis,	measuring	from	tissue	surface		Figure	26	-	Displacement	sensor	angled	relative	to	drilling	axis,	measuring	from	tissue	surface		 41		Figure	27	-	Displacement	sensor,	collinear	or	angled,	measuring	from	fixed	surface	such	as	drill	guide	3.7	Microprocessor		 The	analogue	output	signal	of	the	displacement	sensor	is	sent	to	a	microprocessor	unit	(see	Prototyping,	below).		In	the	final	design	the	microprocessor	will	compute	the	bore	depth	based	on	an	algorithm	and	output	the	value	to	the	display	component	of	the	user	interface	–	a	‘standalone’	device.		The	microprocessor	must	communicate	with	the	user	interface.		This	must	allow	for	both	display	of	computed	bore	depth	and	for	device	control.		For	testing	purposes,	we	do	not	need	the	information	to	be	processed	in	real	time,	so	we	are	currently	storing	the	data	on	a	data	card	and	processing	it	post	facto.	3.8	Algorithm		 In	the	final	form	the	algorithm	would	run	on	the	device	microprocessor	and	compute	the	drilled	bore	depth	by	detecting	cortical	breach	based	on	the	spike	in	velocity	and/or	acceleration.		The	algorithm	must	be	able	to	account	for	noise	in	the	signal	and	the	measured	values	associated	with	routine	surgical	use.	 Most	likely	the	algorithm	would	detect	when	the	drill	exceeded	a	threshold	for	velocity	and/or	acceleration	and	identify	that	point	as	the	cortical	breach.		 42	3.9	User	Interface		 A	device	that	functions	in	real-time	would	need	a	display	that	shows	the	computed	bore	depth	to	the	surgeon	after	drilling	the	bore	in	the	bone.		The	system	would	have	to	be	activated	in	some	way	to	begin	measuring	displacement,	either	through	a	controller	or	by	some	passive	detection	system.		 In	my	experiments	I	developed	prototypes	that	required	post	processing,	and	did	not	go	through	the	detailed	steps	to	determining	what	components	would	be	necessary	for	a	real-time	design.		3.10	Mounting	Assembly		 The	device	is	to	be	compatible	with	existing	surgical	drill	stock.		Therefore,	the	mounting	assembly	must	allow	secure	attachment	of	the	device	to	existing	drills	with	variable	geometry.		It	must	be	secure	enough	that	the	alignment	of	the	sensor	is	not	disrupted	during	surgical	use,	and	the	mounting	step	must	be	simple	and	quick.		Based	on	the	analysis	of	existing	drills	performed	in	the	‘Needs	Assessment’	the	preliminary	concept	for	this	function	is	a	‘clamp’	method	that	affixes	the	device	to	the	superior	aspect	of	the	drill	body.	3.11	Power	Supply	 		 The	device	must	be	battery	powered,	as	a	cord	would	not	be	compatible	with	operating	room	sterile	technique.		There	must	be	sufficient	energy	storage	for	use	for	an	entire	surgical	case,	which	typically	lasts	90	minutes	but	in	more	complicated	circumstances	could	last	in	the	range	of	240-300	minutes.		A	typical	case	will	involve	measuring	5-10	bore	depths,	but	20	bore	depths	is	possible.		 43	3.12	Housing		 The	internal	electrical	components	of	the	device	must	be	encased	in	a	housing	that	can	be	sterilized	for	use	in	the	operating	room.		The	housing	thus	must	be	water	and	vapor-proof,	both	for	the	sterilization	process	and	for	the	surgical	environment,	where	the	device	will	be	exposed	to	blood	and	aqueous	fluids.		The	housing	must	either	protect	the	electrical	components	from	the	sterilization	process,	or	create	a	barrier	between	the	operating	environment	and	non-sterile	electrical	components	inserted	into	the	housing	at	the	time	of	surgery.		A	reusable	or	disposable	housing	design	are	both	possible.	3.13	Prototyping		 The	objective	of	the	prototyping	phase	of	the	project	was	to	demonstrate	the	feasibility	and	reliability	of	laser	triangulation	in	the	displacement	sensing	concept	for	automatic	measurement	of	drilled	bore	in	bone.		I	tested	multiple	concepts	of	sensor	number	and	orientation.		These	early	prototypes	required	data	post-processing	and	analysis	on	a	computer,	and	did	not	address	user	interface	or	sterilization	issues.		Four	sensor	arrangements	were	developed	and	tested.		 Pilot	testing	was	done	on	each	of	the	prototypes	to	assess	performance	and	drive	further	design	efforts.		The	results	of	these	experiments	are	summarized	in	Chapter	4.		A	discussion	on	the	performance	of	each	sensor	arrangement	is	presented	in	Chapter	5.	3.13.1	Single	sensor,	parallel	to	drilling	axis,	measuring	from	tissue	The	initial	sensor	used	in	the	design	was	a	KEYENCE	IL-300	laser	displacement	sensor.		It	has	a	reference	distance	of	300mm	and	a	range	of	160	–	450	mm.		Measurement	repeatability	is	30	µm	and	the	sampling	rate	is	controllable	between	0.33	and	5	ms.		There	are	multiple		 44	outputs	for	electronic	integration	of	the	sensor,	and	we	used	the	0-5	V	analogue	output.		This	sensor	was	chosen	because	it	had	an	appropriate	measurement	range,	but	its	resolution	was	better	than	we	required	and	was	quite	costly	(~$2500),	so	we	would	not	likely	use	this	specific	device	in	a	future	commercial	system.		Nonetheless,	we	deemed	it	suitable	for	evaluating	the	fundamental	concept.			The	sensor	was	mounted	on	an	aluminum	frame,	which	included	an	adjustable	C-clamp	for	drill	attachment.		The	assembly	was	mounted	on	a	ConMed	MPower2	medium	size	pistol	type	surgical	drill,	with	the	device	positioned	superior	to	the	drill	body	(see	Figure	28).		Figure	28	–	Single	parallel	sensor	prototype		 In	our	early	stage	prototype	the	microprocessor	output	time	and	displacement	values	via	USB	to	a	personal	computer	running	MATLAB	(Mathworks,	USA).		Initially	the	device	used	an	Arduino	UNO	microprocessor	(10	bit	ADC),	and	later	I	switched	to	an	Arduino	DUE	(12	bit	ADC)	for	better	resolution(Arduino,	Italy).		The	flow	diagram	for	data	and	the	accompanying	software	is	shown	in	Figure	29.			A	basic	circuit	diagram	of	the	prototype	is	shown	in	Figure	30.			 45		Figure	29	–	Data	and	software	flow	chart		Figure	30	–	Prototype	circuit	diagram	In	the	early	stage	prototype,	we	used	MATLAB	code	for	data	filtering	and	visualization,	with	manual	interpretation.		The	code	was	the	time	limiting	step	in	the	process,	and	allowed	for	a	sampling	frequency	of	100	Hz.		Initial	data	was	quite	noisy,	so	a	signal	processing	step	was	added	using	MATLAB’s	Signal	Processing	Toolbox.	Laser	Sensor	Measures	DistanceArduino	collects	the	sensor	readings	(from	analog)Arduino	sends	info	over	serial	port	to	CPUData	analysis	and	extractionArduino	code	for	information	retrievalMatlab code	for	processing,	visualization,	storageMatlab code	for	data	filtering,	visualization,	analysisFlow	of	Data SoftwareLaser	SensorKEYENCE	IL-300orPanasonic HG-C1400GROUND 10	V	DCAnalog	OUT0-5VAnalog	GROUNDVoltage	Divider Analog	OUT0-3.3V+-Arduino	DUE• analog	input• analog	to	digital	conversionCPUSerial	OUTPUT	 46	Butterworth	filters	are	effective	in	filtering	noise	from	human	kinematic	movements(Winter,	Sidwall,	and	Hobson	1974).		We	chose	a	second	order	Butterworth	filter	with	a	cutoff	frequency	of	10	Hz.		Cutoff	frequency	was	selected	based	on	Fast	Fourier	Transform	Analysis	and	review	of	the	resulting	periodogram.		A	sample	periodogram	is	shown	in	Figure	31.		Figure	31	-	Periodogram	for	displacement		 Bore	depth	was	determined	by	graphical	interpretation	of	the	filtered	displacement	data.		Figures	showed	a	typical	initial	period	of	minimal	change	in	displacement	associated	with	drilling	of	the	first	cortex,	followed	by	a	rapid	change	associated	with	breaching	the	cortex	and	passing	through	the	medulla.		There	was	then	another	period	of	minimal	change	associated	with	drilling	the	second	cortex,	and	another	rapid	change	associated	with	breaching	the	second	cortex.		This	is	shown	in	Figure	23.		3.13.2	Dual	sensor,	parallel	to	drilling	axis,	measuring	from	tissue		 To	address	the	sensitivities	to	tilt	found	in	the	single	collinear	design	presented	above,	I	added	a	second	laser	displacement	sensor	on	the	opposite	side	of	the	drilling	axis	to	the	first.			 47	Rather	than	simply	replicating	the	expensive	Keyence	sensor,	I	chose	instead	to	use	a	Panasonic	HG-C1400	sensor	($600).		It	has	a	reference	distance	of	400	mm,	a	measurement	range	of	200	–	600	mm,	a	repeatability	of	300	–	800	µm	(range	dependent;		better	at	the	low	end	of	the	range	where	we	expect	to	be	making	our	measurements),	and	a	controllable	sampling	rate	between	1.5	–	10	ms.		These	performance	parameters	are	expected	to	be	acceptable	for	our	application.		Computing	and	software	was	the	same	as	the	previous	prototype.		The	measured	bore	depth	was	taken	as	the	average	of	the	values	determined	by	the	two	sensors.		The	prototype	is	shown	in	Figure	33.		Figure	32	–	Dual	parallel	sensor	prototype	3.13.3	Dual	sensor,	angled	towards	drill	bit,	measuring	from	tissue		 Reliability	issues	were	still	present	in	the	dual	sensor	parallel	design,	so	I	made	the	further	modification	of	angling	the	beams	towards	the	drilling	axis.		This	would	limit	the	effect	of	rotation	or	tilt	of	the	drill	on	the	displacement	measures	of	the	sensors	(see	Discussion),	as	well	as	make	it	more	feasible	to	measure	from	the	surface	of	the	bone	rather	than	adjacent	soft	tissue.		Computation	and	software	were	the	same	as	in	the	previous	prototype,	though	a	correction	factor	had	to	be	applied	to	the	displacement	measurement	due	to	beam	angle.			 48	Determination	of	the	correction	factor	is	shown	in	Figure	34.		The	prototype	is	shown	in	Figure	35.		Figure	33	–	Correction	for	beam	angle		Figure	34	–	Dual	sensor,	angled	towards	drill	bit	3.13.4	Dual	sensor,	angled	towards	drill	bit,	measuring	from	drill	guide		 Previous	prototypes	took	displacement	measurements	from	the	tissue,	either	bone	or	muscle.		This	is	an	irregular	surface,	and	vibration	or	rotation	of	the	drill	hand	piece	could	result	in	fluctuations	in	measured	displacement	not	associated	with	forward	motion	of	the	drill.		⍬⍬dmeasureddactual!"#$%"& = cos +×!-."/%0.1	 49	Fluctuations	would	be	due	to	the	beam	moving	over	curved	bone	or	tissue	surfaces,	or	encountering	tissue	debris.		To	minimize	this	source	of	error	I	tested	the	previous	dual	angled	sensor	prototype	with	a	simulated	drill	guide.		A	drill	guide	is	used	in	many	situations	in	orthopedic	surgery	to	give	more	control	of	drill	trajectory,	and	a	feature	could	easily	be	added	to	provide	a	smooth,	constant	surface	for	displacement	measurement.	In	our	pilot	studies,	the	drill	guide	was	kept	‘flush’	and	fixed	relative	to	the	bone	during	use	(see	Discussion).		I	simulated	this	by	using	a	4	cm	x	10	cm	x	1.5	cm	piece	of	aluminum,	with	a	drilled	hole	the	same	size	as	the	orthopedic	drill	bit.		The	bit	was	passed	through	this	hole	prior	to	drilling,	and	then	the	drill	guide	was	seated	snuggly	against	the	bone	surface.		The	drill	guide	remained	fixed	relative	to	the	bone,	and	the	sensors	were	oriented	to	read	displacement	from	its	surface.		In	this	way	the	displacement	was	measured	from	a	regular	surface	and	was	less	sensitive	to	vibratory	or	rotational	movements	of	the	drill.	3.14	ENG	PHYS	Design	Project		 While	working	on	this	project,	I	supervised	an	Engineering	Physics	student	group	on	a	4th	year	design	project	related	to	the	problem	of	measuring	drilled	bore	depth	in	bone	during	surgery.		They	developed	an	alternative	optical	approach	to	measuring	drill	bit	displacement,	by	shining	a	‘sheet’	of	light	on	or	adjacent	to	the	drill	bit	itself.		A	linear	camera	was	directed	so	as	to	respond	to	light	in	a	plane	that	intersected	the	sheet	of	projected	light	along	a	line.		This	camera	would	respond	to	the	illuminated	point	where	this	intersection	line	encountered	the	bone	(typically	immediately	adjacent	to	the	drill	bit).		An	image	subtraction	technique	was	used	to	continuously	determine	the	depth	of	the	drill	bit,	allowing	drilled	bore	depth	to	be	determined	with	a	similar	approach	to	that	described	above.		Although	the	students	presented		 50	promising	initial	results,	their	prototype	was	not	sufficiently	developed	to	be	evaluated	in	the	experiments	described	in	Chapter	4,	though	we	believe	that	this	technique	could	offer	a	lower-cost	approach	(as	compared	with	laser	range-finding)	to	achieving	accurate	bore	depth	measurement	and	we	plan	to	evaluate	their	design	in	greater	detail	in	future.		Figure	35	–	ENG	PHYS	student	design	concept	3.15	Design	Summary	The	results	for	the	pilot	experiments	for	the	different	sensor	concepts	are	presented	in	the	next	chapter.	It	is	not	a	major	limitation	for	a	design	to	require	a	modified	drill	guide	as	they	are	commonly	used	in	plastic	and	orthopedic	surgery.		 		 51	4	Materials	and	Methods	4.1	Overview		 This	chapter	explains	the	experiments	performed	to	assess	the	feasibility	and	reliability	of	a	laser	displacement	sensor	in	developing	a	device	for	automatic	measurement	of	drilled	bore	in	bone	during	surgery.		The	concepts	discussed	in	the	Design	chapter	were	tested	in	different	animal	models.		Exploratory	experiments	were	done	to	quickly	assess	the	feasibility	of	the	concepts,	and	are	discussed	here	briefly.		The	concepts	that	performed	best	were	assessed	in	the	more	formal	Final	Evaluation	Experiments,	which	constitute	most	of	the	discussion.	4.2	Animal	Models	I	used	different	animal	models	at	different	phases	of	the	project.		They	varied	in	terms	of	cost	and	surgical	fidelity.	4.2.1	Chicken	Long	Bone		 Initial	tests	were	performed	using	chicken	thighs	obtained	from	the	grocery	store.		The	advantages	of	this	model	are	low	cost	and	convenience.		The	chicken	femur	is	similar	in	size	to	the	metacarpal	bones	of	the	human	hand.		Disadvantages	of	this	model	include	the	small	size	of	the	overall	specimen	compared	with	a	human,	and	the	relatively	thin	cortex	of	the	avian	long	bone.	4.2.2	Porcine	Long	Bone		 The	porcine	bone	as	a	model	has	the	advantages	of	being	from	a	mammal	of	similar	size	and	structure	to	humans.		Porcine	tissues	are	used	in	a	range	of	surgical	simulations	(Swindle,	Smith,	and	Hepburn	1988).	The	porcine	femur	has	a	similar	diameter	to	that	found	in	the		 52	human.		Experiments	were	done	using	bare	porcine	femurs	obtained	from	a	local	butcher	(Pete’s	Meats).		These	had	the	advantages	of	a	similar	bone	quality	to	human	long	bones.		However	the	total	length	of	the	specimen	was	significantly	smaller	than	a	human	extremity.		Additionally,	the	lack	of	surrounding	soft	tissues	reduced	the	fidelity	of	the	model	in	terms	of	how	well	it	could	simulate	a	typical	surgical	exposure.		Figure	36	-	Porcine	femur	in	drilling	test	4.2.3	Porcine	Hind	Limb		 On	several	occasions,	I	obtained	total	porcine	hind	limbs	from	a	local	butcher.		I	performed	a	surgical	exposure	of	the	femur	and	tibia	to	create	a	situation	similar	to	a	surgical	approach	on	a	human	limb.		The	incision	dimensions	were	also	very	close	to	those	obtained	from	the	dimensional	analysis	performed	on	orthopedic	surgical	exposures.		This	provided	a	high-fidelity	animal	model	for	orthopedic	surgery	of	the	appendicular	skeleton.		 53		Figure	37	-	Porcine	hindlimb	model	4.3	Exploratory	Experiments		 I	performed	a	series	of	exploratory	experiments	to	test	first	the	feasibility	and	then	the	reliability	of	the	different	sensor	configuration	concepts	described	in	the	previous	chapter.		These	pilot	studies	were	all	single	user	studies.		The	feasibility	testing	was	done	in	the	porcine	bone	model,	and	reliability	testing	in	the	porcine	hindlimb	model.		A	summary	of	the	experiments	and	results	is	shown	in	Figure	39	and	Table	4,	with	complete	details	are	presented	in	Appendix	D.		Briefly,	I	drilled	bicortical	holes	using	the	surgical	drill	with	the	attached	sensor	prototype,	and	computed	drilled	bore	depth	from	the	measured	displacement	traces	using	the	method	previously	discussed	(Chapter	3).		I	compared	these	values	to	those	obtained	using	the	conventional	depth	gauge	and	digital	calipers,	the	latter	of	which	served	as	the	‘gold	standard’	for	these	experiments.		 54		Figure	38	–	Summary	of	exploratory	experiments	Sensor	Number,	Orientation,	Measurement	target	single,	parallel,	tissue	single,	parallel,	tissue	dual,	parallel,	tissue	dual,		angled,	tissue	single,	angled,		drill	guide	Animal	Model	 pig	bone	 pig	leg	 pig	bone	 pig	bone	 pig	bone	n	 9	 25	 20	 7	 11	Sensor	Error	(mm)	 2.05	 2.82	 0.43	 1.59	 0.02	Sensor	SD	(mm)	 0.67	 3.21	 1.1	 0.36	 0.74	Depth	Gauge	Error	(mm)	 1.55	 0.71	 -1.15	 -0.69	 -0.6	Depth	Gauge	SD	(mm)	 0.83	 2.69	 0.62	 0.815	 1.29	Table	4	–	Results	from	exploratory	experiments			Is	a	single,	parallel	optical	sensor	feasible?Pig	BoneMean	error	2.05mmSD	0.67mmIs	a	single,	parallel	optical	sensor	reliable?Pig	LegMean	error	2.82mmSD	3.21mmAre	paired,	parallel	optical	sensors	feasible?Pig	Bone	Mean	error	0.43mmSD	1.1mmAre	paired,	angled	optical	sensors	feasible?Is	single	angled	optical	sensor	reading	from	drill	guide	feasible?Pig	Bone	Mean	error	1.59mmSD	0.36mmPig	Bone	Mean	error	0.02mmSD	0.74mm	 55		 As	shown,	most	of	the	concepts	resulted	in	a	positive	mean	error	from	the	gold	standard	bore	depth	measurement.		This	represents	a	bias	in	using	this	form	of	sensing	(eg,	potentially	due	to	partial	exiting	of	the	drill	bit	prior	to	the	breakthrough	pulse	happening)	and	could	potentially	be	corrected	for	in	the	interpretation	and/or	computation	step.		More	important	is	the	variance	of	the	error,	which	I	took	as	the	principal	evaluation	criterion.		A	discussion	of	the	performance	of	each	sensor	arrangement	and	possible	explanations	are	presented	in	Chapter	6.		Based	on	these	results,	I	decided	to	use	both	single	and	paired	angled	sensors	reading	from	the	drill	guide	in	the	final	evaluation	experiments.	4.4	Evaluation	of	Final	Concept	Purpose:	To	evaluate	the	performance	of	the	lead	sensor	concepts	under	a	broader,	more	surgically	relevant	range	of	expected	drilling	conditions,	with	multiple	surgeon	users.			Designs	Tested:	1)	single	angled	laser	displacement	sensor	measuring	from	mock	drill	guide	and	2)	dual	angled	laser	displacement	sensor	measuring	from	mock	drill	guide.	Drilling	conditions:	information	from	the	fixation	construct	analysis	(see	Needs	Assessment)	was	used	to	select	the	drilling	conditions	for	the	experiment.		Condition	AM	was	not	tested,	as	it	is	geometrically	similar	to	SD	and	SM.	a. Condition	SD	-	perpendicular	diaphyseal	drilling.	b. Condition	SD	-	angled	diaphyseal	drilling.	c. Condition	SM	-	perpendicular	metaphyseal	drilling.	Operators:	i. Daniel	Demsey	MD,	plastic	surgery	resident	(year	4	of	training)	ii. Pierre	Guy	MD,	staff	orthopedic	surgeon		 56	Methods:		All	experiments	were	conducted	in	the	Biomedical	Engineering	Laboratory	at	the	Centre	for	Hip	Health	and	Mobility	located	on	the	Vancouver	General	Hospital	campus,	part	of	the	University	of	British	Columbia.		Appropriate	specimen	storage	and	handling	protocols	were	followed	at	all	times.		Ethics	approval	was	not	required	as	only	deceased	animal	tissue	was	used,	and	only	members	of	the	research	team	were	involved	in	the	experiments.		Biosafety	approval	was	obtained.		 I	used	fresh	porcine	hind	limbs	for	these	experiments.		In	these	experiments,	I	performed	a	surgical	exposure	of	the	femur	and	tibia	of	each	hind	limb	specimen.		I	used	suture	ligature	and	muscle	tissue	removal	to	simulate	appropriate	retraction	of	tissues	for	bone	exposure.		I	marked	the	transition	between	diaphysis	and	metaphysis	with	a	marking	pen	for	data	collection,	and	placed	the	specimens	on	surgical	drapes	on	a	table	at	surgical	working	height.		The	LaserGauge	prototype	was	mounted	on	a	Conmed	MPower2	surgical	drill,	a	medium	sized	battery	powered	pistol	type	drill.		I	used	3.5mm	diameter	orthopedic	surgical	drill	bits	from	Stryker	in	these	experiments.		The	prototype	sent	data	signals	to	an	Arduino	Due	microprocessor,	which	transmitted	data	to	a	Matlab	code	run	on	a	personal	computer.		Data	files	were	saved	for	later	analysis.		 The	experiment	consisted	of	drilling	a	series	of	bicortical	holes	in	the	long	bones	(tibia	and	femur).		Standard	surgical	technique	was	used.		A	piece	of	1”	thick	aluminum	with	a	predrilled	hole	the	diameter	of	the	drill	bit	was	used	to	represent	the	drill	guide.		The	drill	bit	was	passed	through	the	hole	in	the	drill	guide	and	seated	on	the	bone,	and	then	the	drill	guide	was	set	flush	against	the	bone	surface	with	gentle	pressure	from	the	surgeon’s	non-dominant	hand.		The	displacement	sensors	measured	from	the	surface	of	the	guide	simulator.		One		 57	difference	from	normal	surgical	practice	is	that	this	approximation	of	a	drill	guide	did	not	perform	the	normal	function	of	stabilizing	the	drill	entry	point,	as	it	did	not	have	the	metal	‘teeth’	present	in	a	real	drill	guide.		 The	two	surgeons	alternated	drilling	the	holes.		The	drilling	condition	of	each	hole	to	be	drilled	(SD,	AD,	or	SM	–	see	Figure	40)	was	selected	in	advance.		Drilling	conditions	were	evenly	distributed	between	bone	type	(femur	or	tibia)	and	surgeon.		After	each	hole	was	drilled	the	data	plot	was	visually	inspected	to	confirm	an	expected	appearance	based	on	previous	experiments.		Figure	39	-	Drilling	conditions	tested		 Once	all	holes	had	been	drilled,	I	measured	the	depth	of	each	hole	using	the	conventional	depth	gauge.		It	was	used	in	the	usual	fashion,	and	if	three	attempts	failed	to	obtain	a	measurement,	the	value	was	recorded	as	a	‘failure’.		Measurements	were	recorded	in	an	Excel	spreadsheet.		After	these	measurements	were	obtained	I	dissected	the	long	bones	free	from	the	soft	tissues.		I	then	used	digital	calipers	to	measure	the	hole	depth.		Specimens	were	then	sealed	in	plastic	for	CT	imaging.		The	bones	were	imaged	using	High	Resolution	Peripheral	Quantitative	Computer	Tomography	(HR-pqCT)	in	an	XTreme-CT	model	device	(Sanco).		Two	bones	were	imaged	at	once	to	save	costs.		Image	files	were	saved	in	DICOM	format	and	loaded	into	MIMICS	software	(Materialise,	Belgium)	for	analysis.		The	entry	and	exit	points	of	each	hole	Condition	SDCondition	ADCondition	SM	 58	were	identified	in	the	CT	volumes	twice	on	the	proximal	and	distal	aspect	of	the	bore	and	the	results	averaged	to	produce	the	final	‘gold	standard’	value	of	hole	depth.		 A	total	of	125	drilling	attempts	were	made	in	4	porcine	hindlimbs.		95	holes	were	suitable	for	analysis,	with	30	holes	rejected	for	the	following	reasons:	overlap	with	adjacent	holes	(n=3),	drill	bit	exit	into	joint	space	(n=11),	technical	problems	with	the	drill	(drill	bit	loosening	or	drained	battery)	(n=3),	and	technical	problems	with	the	prototype	apparatus	(clamp	coming	loose,	sensor	shifting	relative	to	drill)(n=13).		Of	the	95	holes	included	in	the	analysis,	28	holes	were	from	Condition	SD,	22	holes	from	Condition	AD,	and	36	holes	from	condition	SM.		Each	drilled	hole	depth	was	analyzed	using	both	a	single	sensor	reading	and	the	dual	sensor	reading.			 		 59	5	Results	5.1	Overview		 This	chapter	presents	the	results	of	the	Final	Evaluation	experiments,	which	evaluated	the	dual	angled	laser	sensor	design	measuring	from	a	drill	guide,	and	then	discusses	the	interpretation	of	the	results,	along	with	implications	for	clinical	application.		5.2	Final	Evaluation	Results	Mean	errors	and	standard	deviations	for	the	two	sensors	treated	individually	as	well	as	in	combination	are	shown	in	Table	5,	separated	by	user	and	drilling	condition.		The	combined	sensor	tended	to	have	less	variability	than	either	sensor	individually,	and	was	more	consistent	than	either	sensor	individually	across	the	multiple	tests.		However,	each	single	sensor	met	the	design	criteria	of	having	less	variability	than	the	conventional	depth	gauge	under	all	drilling	conditions.		A	positive	bias	in	the	measurement	(ie,	over-estimation	of	hole	depth)	was	present.		An	explanation	for	the	bias	could	be	that	the	tip	of	the	drill	emerges	from	the	bone	prior	to	the	edges	of	the	cortex	giving	way	and	allowing	the	bit	to	plunge	through,	resulting	in	a	‘delay’	of	the	acceleration	spike.		Curiously,	the	reading	from	sensor	1	seemed	to	be	consistently	more	biased	than	that	from	sensor	2	across	all	conditions	and	users,	which	is	not	directly	compatible	with	the	above	hypothesis.					 60				 In	terms	of	variability,	the	data	from	the	two	sensors	treated	individually	is	relatively	similar	to	one	another,	so	for	the	balance	of	our	analysis,	we	consider	only	the	data	for	the	combined	sensors.		Statistical	comparison	of	the	variability	between	users	for	each	drilling	condition	showed	no	significant	differences	(F	test	for	unequal	variances,	p<0.05).		Based	on	this,	the	data	for	the	two	users	was	combined	and	is	presented	in	Table	6.						 	User	1	(DD)	 		 		 		User	2	(PG)	 		 		 			 Sensor	1	 Sensor	2	 Combined	Depth	Gauge	 Sensor	1	 Sensor	2	 Combined	Depth	Gauge	Condition	SD	 		 	  		 		 	  		Mean	Error	(mm)	 2.56	 0.10	 1.33	 0.10	 2.72	 -0.01	 1.36	 -0.28	SD	 0.65	 0.81	 0.66	 1.16	 0.88	 0.77	 0.78	 1.66	n	 16	 16	 16	 16	 12	 12	 12	 12	Condition	AD	 		 		 		 		 		 		 		 		Mean	Error	(mm)	 2.77	 0.37	 1.57	 1.68	 3.01	 0.48	 1.74	 3.04	SD	 0.86	 1.05	 0.87	 4.49	 0.78	 1.07	 0.87	 3.00	n	 11	 11	 11	 10	 11	 11	 11	 10	Condition	SM	 		 		 		 		 		 		 		 		Mean	Error	(mm)	 3.49	 0.33	 1.91	 1.01	 3.65	 0.91	 2.28	 1.91	SD	 0.97	 0.84	 0.75	 3.06	 1.31	 1.16	 0.81	 3.34	n	 16	 16	 16	 14	 20	 20	 20	 16	Table	5–	Final	evaluation	data	summary		 61			Dual	Laser	Sensor	Depth	Gauge	Statistically	Significant	Condition	SD	 		 		 		Mean	error	(mm)	 1.34	 -0.06	 		SD	(mm)	 0.70	 1.38	 *	n	 28	 28	 				 		 		 		Condition	AD	 		 		 		Mean	error	(mm)	 1.66	 2.36	 		SD	(mm)	 0.86	 3.79	 *	n	 22	 20	 				 		 		 		Condition	SM	 		 		 		Mean	error	(mm)	 2.11	 1.51	 		SD	(mm)	 0.80	 3.19	 *	n	 36	 30	 		Table	6	–	Mean	error	and	SD	for	the	dual	laser	prototype	compared	with	conventional	depth		 I	compared	the	variances	for	each	drilling	condition	using	the	dual	laser	sensor	and	the	depth	gauge	and	found	the	laser	sensor	to	have	significantly	less	variability	under	all	conditions	(F	test	for	unequal	variance,	p<0.05).		Interestingly,	the	depth	gauge	showed	markedly	higher	variability	for	the	two	conditions	with	angles	on	either	or	both	of	the	entry	and	exit	points,	whereas	there	was	no	noticeable	difference	amongst	these	conditions	for	the	laser-based	sensor.	The	mean	error	for	the	prototype	and	the	conventional	method	is	shown	graphically	in	Cumulative	Frequency	Distribution	and	Bland	Altman	plots	below.		Detailed	experimental	results	are	present	in	Appendix	E.		 62		Figure	40	-	Cumulative	frequency	distribution	for	Condition	SD		Figure	41	-	Cumulative	frequency	distribution	for	Condition	AD	00.10.20.30.40.50.60.70.80.91-4 -2 0 2 4Cumulative	FrequencyMean	Error	(mm)Condition	SDLaser	SensorDepth	Gauge00.10.20.30.40.50.60.70.80.91-5 0 5 10 15Cumulative	FrequencyMean	Error	(mm)Condition	ADLaser	SensorDepth	Gauge	 63		Figure	42	-	Cumulative	frequency	distribution	for	Condition	SM		Figure	43	–	Bland	Altman	plots	for	the	LaserGauge	and	the	conventional	depth	gauge	comparing	to	CT	scan	as	‘gold	standard’	The	Cumulative	Frequency	Distribution	and	Bland	Altman	plots	demonstrate	that	there	was	a	notable	positive	bias	in	the	measurements	made	by	the	LaserGauge,	and	a	significantly	lower	variability	in	mean	error	than	the	conventional	gauge	(including	condition	SD).		 	00.10.20.30.40.50.60.70.80.91-5 0 5 10 15Cumulative	FrequencyMean	Error	(mm)Condition	SMLaser	SensorDepth	Gauge-6 -4 -2 0246810120 5 10 15 20 25 30 35 40Condition	SD	- Laser	Sensor-6 -4 -2 0246810120 10 20 30 40Condition	SD	- Depth	Gauge-6 -4 -2 0246810120 10 20 30 40Condition	AD	- Laser	Sensor-6 -4 -2 0246810120 10 20 30 40Condition	AD	- Depth	Gauge-6 -4 -2 0246810120 10 20 30 40 50Condition	SM	- Laser	Sensor-6 -4 -2 0246810120 10 20 30 40 50Condition	SM	- Depth	Gauge	 64	6	Discussion	6.1	Significance	of	Design		 Using	an	optical	sensor	mounted	on	an	existing	surgical	drill	to	measure	bore	depth	in	bone	has	not	been	previously	described	in	the	literature.		Drills	with	automatic	bore	depth	measurement	capabilities	have	been	described,	but	all	require	replacement	of	the	existing	drill	hand	piece(Louredo,	Díaz,	and	Gil	2012;	Ong	and	Bouazza-Marouf	1998;	Benedetto	Allotta,	Giacalone,	and	Rinaldi	1997;	Wen-Yo	Lee	and	Shih	2006).		The	advantage	of	our	approach	is	that	it	requires	minimal	change	from	existing	surgical	practice	and	uses	existing	surgical	equipment.		I	have	demonstrated	that	a	design	using	laser	triangulation	is	feasible	in	multiple	forms,	and	that	a	paired	sensor	device	measuring	from	a	modified	drill	guide	more	reliable	than	the	conventional	depth	gauge	in	the	laboratory	setting.			6.2	Sensor	Concepts	6.2.1	Single	vs.	Dual	Sensors		 The	single,	parallel	sensor	performed	better	than	the	conventional	depth	gauge	in	the	feasibility	testing	in	pig	bone	models	(mean	error	2.05mm	SD	0.67mm	vs	mean	error	0.83mm	SD	1.55mm),	providing	an	initial	indication	that	the	optical	sensor	concept	could	potentially	work	for	automatic	measurement	of	drilled	bore	in	bone	under	sufficiently	controlled	conditions.		However,	it	did	not	perform	as	well	in	the	reliability	testing	compared	with	the	conventional	method	in	the	more	realistic	pig	leg	model	(mean	error	2.82mm	SD	3.21mm	vs.	0.71mm	SD	2.69mm).		 65		 Using	a	single	sensor	reading	from	tissue	relies	on	an	assumption	of	perfect	(or	near	perfect)	forward	motion	of	the	drill	along	the	drilling	axis.		In	surgery,	the	drill	is	used	free-hand	so	this	may	not	be	a	valid	assumption.		The	effect	of	the	drill	tilting	about	the	contact	point	of	the	drill	bit	on	the	bone	is	shown	in	Figure	45,	and	may	be	a	source	of	the	error	found	in	the	feasibility	testing.	Depending	on	the	direction	the	drill	tilts,	the	measured	displacement	of	the	sensor	from	the	bone	will	increase	or	decrease	without	any	progress	in	bore	drilling,	resulting	in	inaccuracies	in	the	final	computed	bore	depth	(a	change	in	indicated	displacement	not	due	to	drilling).		Figure	44	–	Effect	of	drill	tilt	on	sensor	readings		 The	orientation	of	the	drill	could	also	change	once	the	first	cortex	is	breached,	as	the	medulla	is	essentially	hollow	and	does	not	present	significant	resistance	to	movement	of	the	drill	bit	perpendicular	to	the	drilling	axis.		This	is	shown	in	Figure	46.		The	resulting	effect	on	displacement	measurement	is	similar	to	that	seen	in	the	‘tilt’	phenomenon	described	above.	Tilt	Radius Error	 66		Figure	45	–	Effect	of	change	of	drill	trajectory	on	sensor	readings		 The	drill	vibrates	during	operation,	and	this	could	result	in	movement	in	a	radial	direction	about	the	drilling	axis.		As	the	surface	of	the	tissue	is	not	flat,	this	will	again	result	in	changes	in	measured	displacement	not	associated	with	forward	drill	movement	and	error	in	the	computed	bore	depth	(see	Figure	47).		Figure	46	–	Rotation	and	irregular	bone	surface	The	addition	of	a	second	sensor	on	the	opposite	side	of	the	drilling	axis	and	averaging	the	bore	depths	computed	with	each	sensor	individually	should	reduce	the	effect	of	these	errors.		Generally,	tilt	resulting	in	a	decrease	in	one	displacement	measurement	should	result	in	an	equal	increase	in	displacement	measurement	in	the	sensor	on	the	opposite	side,	and	vice	Tilt	ErrorIrregular	Bone	SurfaceDrilling	AxisRotation	TrajectoryWikipedia	Commons	 67	versa.		This	is	shown	in	Figure	48.		Errors	associated	with	rotational	movement	of	the	drill	are	likely	to	decrease	due	to	statistical	effects	of	multiple	measurements	(regression	to	mean).		Figure	47	-	Two	sensors	on	opposite	sides	of	drilling	axis	correct	for	tilt	error	6.2.2	Parallel	vs.	Angled	Sensors		 Placing	the	sensor	in	a	collinear	orientation	to	the	drilling	axis	leads	to	a	simple	design	and	doesn’t	require	mathematical	correction	of	the	measured	displacement.		However,	given	the	geometry	of	the	drill	and	mounting	system,	there	is	a	limit	to	how	much	the	offset	between	the	drilling	axis	and	the	sensor	beam	can	be	reduced.		In	my	prototypes,	the	minimum	offset	is	approximately	3	cm.		Reducing	this	distance	would	reduce	the	effect	of	tilt	error	described	above.				 By	angling	the	sensor	towards	the	drilling	axis,	the	distance	between	the	point	of	drilling	and	the	point	from	which	displacement	can	be	measured	is	reduced.		Implementing	this	change	Error	1Error	2!""#"1 = −!""#"2	 68	in	the	prototype	design	improved	the	performance	relative	to	the	collinear	sensor	(mean	error	1.59mm	SD	0.36mm	vs.	0.43mm	SD	1.1mm).		There	were	not	enough	data	points	for	this	difference	to	reach	statistical	significance.	6.2.3	Tissue	vs.	Drill	Guide	Reference		 Drill	guides	are	routinely	used	in	surgical	practice	to	allow	for	better	control	of	the	drill	bit	trajectory(“Oxford	Textbook	of	Trauma	and	Orthopaedics	-	Oxford	Medicine”	2017).		Adding	a	feature	with	a	machined,	uniform	surface	could	improve	the	performance	of	the	prototype	in	multiple	ways.		First,	the	uniform	surface	would	prevent	errors	due	to	rotational	vibration	of	the	drill,	as	slight	rotational	movements	of	the	measured	point	would	not	significantly	change	the	measured	displacement.		Second,	the	orientation	of	the	drill	bit	would	be	constrained	relative	to	the	drill	guide	(Figure	49)	so	that	tilt	or	movement	of	the	drill	perpendicular	to	the	drilling	axis	would	not	be	possible.		This	would	prevent	the	tilt-associated	error	described	above.				Figure	48	–	Drill	guide	constraints	BoneDrill	GuideDrill	BitSensor	Beam	 69	6.3	Drilling	Conditions	6.3.1	Condition	SD		 The	conventional	depth	gauge	performed	best	in	the	SD	condition	when	compared	with	the	other	conditions.		This	is	intuitive	as	the	depth	gauge	should	obtain	the	same	measurement	regardless	of	which	position	around	the	circumference	of	the	hole	it	hooks	to.		Figure	49	-	Condition	SD	6.3.2	Condition	AD		 The	greatest	discrepancy	between	prototype	and	conventional	depth	gauge	was	when	drilling	in	Condition	AD.		The	prototype	performed	significantly	better	under	these	conditions.		The	angled	bore	does	not	allow	for	perpendicular	contact	of	the	gauge	against	the	bone	cortex	which	could	explain	the	accuracy	issues.		It	could	also	be	due	to	variations	in	the	measured	value	depending	on	which	side	of	the	bone	the	instrument	engages	with.		Figure	50	-	Condition	AD	Condition	SDCondition	AD	 70	6.3.3	Condition	SM		 The	measurements	made	by	the	conventional	depth	gauge	also	showed	significant	variability	in	the	straight	metaphyseal	condition.		The	prototype	showed	consistent	performance	with	the	other	conditions.		Figure	51	-	Condition	SM	6.4	Final	Concept	Selection		 Based	on	the	experimental	results,	an	angled	beam	measuring	from	the	drill	guide	surface	performed	the	best.		Two	sensors	had	better	performance	than	one,	but	the	single	sensor	still	functioned	better	than	the	conventional	depth	gauge.		Based	on	the	results	presented	here,	our	recommended	design	is	the	angled	dual	sensor	measuring	from	the	drill	guide.		However,	depending	on	final	design	costs,	a	single	sensor	version	could	be	acceptable	based	on	experimental	results.	6.5	Bore	Depth	‘Gold	Standard’	6.5.1	Bone	Geometry		 Animal	and	human	bones	have	an	irregular	geometry,	making	the	definition	of	‘true’	bore	depth	difficult	to	define	or	measure.		The	measurement	can	vary	depending	on	whether	it	Condition	SM	 71	is	taken	at	the	sides	of	the	drilled	bore	or	the	centroid.		This	is	shown	in	Figure	53.		The	three	arrows	show	different	‘depths’	of	the	bore	when	measured	at	different	points.		Figure	52	–	Measurement	variations	in	bone	bore,	shown	in	cross	section.		Three	arrows	indicated	three	different	measurements	of	bore	depth.	6.5.2	Measurement	Methods		 For	the	purposes	of	my	experiments	we	used	a	CT	scan	based	‘gold	standard’	measurement	of	the	drilled	bore	depth	in	the	final	evaluation.		To	obtain	this,	I	performed	high	resolution	CT	scan	of	the	bones	after	drilling,	and	measured	the	depth	of	the	drilled	bore	from	the	3D	reconstructions	in	MIMICS	software.		The	measurement	of	the	bore	was	taken	on	two	sides	of	the	drilled	bore,	and	an	average	was	taken	of	these	two	values	to	give	the	‘gold	standard’	value.		In	Figure	53	this	would	be	the	average	of	the	orange	and	the	green	measurement.		Significant	differences	were	noted	between	the	measurements	of	depth	on	the	opposing	side	of	the	bore	on	the	CT	scan	(mean	difference	0.46mm,	SD	1.42mm).		Averaging		 72	these	values	would	logically	produce	a	closer	estimate	of	the	true	bore	depth,	but	this	also	emphasizes	the	intrinsic	ambiguity	as	to	what	is	the	‘true’	depth	of	the	drilled	bore.			6.5.3	Interpretation	of	Measurement	by	Surgeon		 The	significance	of	the	bore	depth	measurement	is	the	effect	it	has	on	the	choice	of	screw	length.		Choosing	a	longer	screw	means	a	better	chance	of	full	thread	engagement	in	all	the	available	cortical	bone,	resulting	in	a	stronger	fixation	construct.		However,	this	means	a	greater	chance	of	the	screw	extending	beyond	the	far	bone	cortex	and	potentially	impinging	on	and	damaging	adjacent	sensitive	structures.		Choosing	a	shorter	screw	lowers	this	chance	but	also	is	more	likely	to	result	in	weaker	fixation	construct.		 In	an	interview,	Dr.	Guy,	a	staff	orthopedic	surgeon,	stated	that	the	interpretation	of	the	measurement	and	resulting	screw	selection	depends	highly	on	the	anatomic	location.		The	surgeon	will	choose	a	shorter	screw	in	areas	where	complications	of	long	screws	are	frequent	such	as	the	distal	radius.	6.5.4	Required	Precision		 For	evaluation	purposes	I	set	an	initial	target	precision	of	+/-	1	mm	relative	to	true	bore	depth.		As	I	performed	the	evaluation	experiments	it	became	apparent	that	there	was	uncertainty	as	to	what	is	the	true	‘gold	standard’	for	bore	depth,	and	that	the	conventional	depth	gauge,	which	has	a	clinically	accepted	precision,	was	not	close	to	meeting	my	original	target.		The	criteria	for	acceptable	performance	was	then	revised	to	be	less	variance	in	repeated	measurement	than	the	conventional	gauge.		Only	in	the	smallest	sizes	(<10	mm	length)	of	screws	is	a	precision	of	1	mm	clinically	necessary,	as	small	screws	increase	in	size	by	an	increment	of	1	mm.		In	screws	>	10	mm	in	length,	the	increments	become	3	mm	and	then	5		 73	mm,	making	measurement	precision	less	critical.		Testing	in	a	more	representative	simulation,	with	placement	of	screws	based	on	the	bore	depths	estimated	by	our	device	could	better	delineate	the	clinical	precision	sensitivity.		Exceeding	the	performance	of	the	current	instrument	is	a	reasonable	indication	of	design	success.	6.6	Additional	Considerations		 Medical	devices	require	FDA	approval	prior	to	use.		Devices	are	classified	based	on	the	risk	they	present	to	patients,	and	the	regulatory	pathway	varies	for	the	different	classes	(Health	2017).		The	cost	difference	between	the	regulatory	pathways	is	considerable,	and	so	it	can	be	an	important	design	consideration	to	target	a	specific	regulatory	pathway.		The	LaserGauge	is	likely	to	be	identified	as	a	Class	II	device,	and	the	ideal	regulatory	process	would	be	through	the	510(k)	Exception.		This	would	be	an	important	factor	moving	forward	with	the	design.	6.7	Existing	Literature		 The	published	engineering	and	medical	literature	do	not,	to	my	knowledge,	contain	any	research	focused	on	the	methods	of	measuring	bore	depth	in	bone	during	surgery.		This	has	been	a	considered	a	solved	problem	for	some	time,	so	it	is	understandable	that	this	has	not	been	looked	at	rigorously	prior	to	our	work.	6.8	Limitations		 Limitations	of	my	experimental	work	include	being	performed	in	deceased	animal	tissue,	which	could	have	somewhat	different	mechanical	properties	than	live	human	tissue	and	thereby	affect	the	breakthrough	dynamics,	though	we	regard	this	as	relatively	unlikely.		In	the	current	version	of	the	system,	the	measurements	required	post-processing	to	determine	a	bore		 74	depth	and	were	done	using	a	process	that	involved	human	interaction	rather	than	an	automatic	algorithm;		such	an	automatic	analysis	technique	would	be	required	to	implement	a	clinically-usable	device.		Finally,	the	measurements	were	not	used	to	select	screw	length	for	a	fixation	construct,	so	the	rate	of	incorrect	screw	placement	could	not	be	compared	between	our	device	and	the	conventional	depth	gauge.			 		 75	7	Conclusion		 My	research	objective	was	to	determine	whether	optical	displacement	sensors	could	be	used	to	measure	drilled	bore	depth	in	bone	during	surgery.		I	was	able	to	show	that	this	concept	is	feasible	and	reliable	in	a	simulated	surgical	setting.		A	prototype	based	on	two	laser	displacement	sensors,	angled	towards	the	drilling	axis	and	measuring	from	a	custom	drill	guide,	displayed	markedly	less	variability	in	measurement	error	than	the	conventional	depth	gauge	in	a	range	of	clinically	relevant	simulated	drilling	conditions.			The	problem	of	measuring	drilled	bore	depth	during	surgery	was	identified	by	practicing	surgeons,	and	the	solution	has	been	developed	with	them	and	their	patients	in	mind.		This	approach	has	not	been	previously	described	in	the	published	academic	literature.		There	is	a	patent	for	a	similar	concept,	and	we	have	spoken	with	a	research	group	(AO)	that	is	developing	a	similar	idea.		However	neither	of	these	groups	have	described	successful	validation	of	their	design,	nor	have	they	explored	the	range	of	optical	sensor	arrangements	described	in	this	thesis.		A	device	using	this	concept	has	the	potential	to	improve	current	surgical	practice,	resulting	in	fewer	complications	secondary	to	incorrect	size	screw	placement.		It	could	also	significantly	improve	the	user	experience	for	practicing	surgeons.	7.1	Future	Work		 Multiple	design	milestones	must	be	achieved	prior	to	completion	of	a	‘works	like’	prototype.			In	the	first	stage,	the	device	must	be	made	to	function	automatically	and	in	a	stand-alone	configuration.		To	do	this,	an	algorithm	needs	to	be	developed	and	tested	to	automatically	determine	the	drilled	bore	depth	without	user	interpretation	at	the	post		 76	processing	phase.		This	algorithm	may	be	developed	in	MATLAB,	but	ultimately	must	be	converted	to	C++	or	a	similar	language	to	run	on	the	prototype’s	microprocessor.		A	user	interface	with	displays	and	controls	is	also	required.	Lastly,	the	prototype	must	also	be	converted	to	a	battery-based	power	supply.				 A	drill	guide	with	a	feature	to	support	displacement	measurement	must	be	designed	and	produced,	and	must	ultimately	be	compatible	with	a	range	of	drill	bit	diameters.		This	would	likely	be	done	with	a	single	guide	with	exchangeable	‘sizing’	components.		 After	the	‘works-like’	prototype	is	complete,	a	usability	test	would	be	performed	in	both	animal	and	cadaver	tissue.		A	screw	and	plate	fixation	construct	would	be	placed	using	the	device’s	measurements,	and	afterwards	the	adequacy	of	the	construct	would	be	assessed,	likely	both	by	mechanical	loading	and	by	inspection	following	dissection.		User	experience	metrics	would	also	be	analyzed	through	a	post-use	survey.		If	indications	are	strongly	positive	at	this	point,	we	would	likely	begin	consultations	with	potential	commercialization	partners.		 In	order	to	move	forward	to	a	clinical	testing	phase,	a	housing	and	sterilization	protocol	would	also	need	to	be	developed.		This	is	an	expensive	step	that	would	involve	contracting	a	third-party	organization	to	validate	the	sterilization	protocol.		The	main	challenge	is	likely	to	be	how	to	protect	the	sensitive	electronic	components	from	the	harsh	sterilization	conditions	(see	Appendix	F).		 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Meng.	2014.	“3D	Finite	Element	Modeling	and	Analysis	of	Dynamic	Force	in	Bone	Drilling	for	Orthopedic	Surgery:	3D	FEA	OF	BONE	DRILLING	FOR	ORTHOPEDIC	SURGERY.”	International	Journal	for	Numerical	Methods	in	Biomedical	Engineering	30	(9):	845–56.	doi:10.1002/cnm.2631.	Quest,	D.,	C.	Gayer,	and	P.	Hering.	2012.	“Depth	Measurements	of	Drilled	Holes	in	Bone	by	Laser	Triangulation	for	the	Field	of	Oral	Implantology.”	Journal	of	Applied	Physics	111	(1):	13106.	doi:10.1063/1.3676219.	Reilly,	Donald	T.,	and	Albert	H.	Burstein.	1975.	“The	Elastic	and	Ultimate	Properties	of	Compact	Bone	Tissue.”	Journal	of	Biomechanics	8	(6):	393IN9397–396IN11405.	Rho,	Jae-Young,	Liisa	Kuhn-Spearing,	and	Peter	Zioupos.	1998.	“Mechanical	Properties	and	the	Hierarchical	Structure	of	Bone.”	Medical	Engineering	&	Physics	20	(2):	92–102.	Schnur,	D.	P.,	and	B.	Chang.	2000.	“Extensor	Tendon	Rupture	after	Internal	Fixation	of	a	Distal	Radius	Fracture	Using	a	Dorsally	Placed	AO/ASIF	Titanium	Pi	Plate.	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Kment.	2013.	“Influence	of	Tool	Geometry	on	Drilling	Performance	of	Cortical	and	Trabecular	Bone.”	Medical	Engineering	&	Physics	35	(8):	1165–72.	doi:10.1016/j.medengphy.2012.12.004.	Winter,	David	A.,	H.	Grant	Sidwall,	and	Douglas	A.	Hobson.	1974.	“Measurement	and	Reduction	of	Noise	in	Kinematics	of	Locomotion.”	Journal	of	Biomechanics	7	(2):	157–59.	doi:10.1016/0021-9290(74)90056-6.				 		 80	Appendix	A	–	Incision	Dimensional	Analysis		A.1	Overview		 This	appendix	contains	the	data	tables	for	the	Incision	Dimensional	Analysis.		This	information	came	from	analysis	of	the	images	from	the	textbook	‘Atlas	of	Surgical	Exposures	of	the	Upper	and	Lower	Extremity’	by	Tubiana	et	call	(Martin	Dunitz	Ltd.	2000).		Reference	measurements	were	performed	on	a	human	volunteer.	A.2	Data	Tables		Surgical	Exposure	Dimensional	Analysis	 	      n=29	 Image	(Dimensionless)	 	  Human	(measured)	 	 *predicted	Image	 IncisionL	IncisionW	 BoneL	 BoneW	 refIncision	 correctionF	 IncisionW*	 BoneL*	 BoneW*	Femur1	 14.5	 4.9	 8.1	 1.2	 30	 2.068965517	 10.13793103	 16.75862069	 2.482758621	Femur2	 12	 7	 6.6	 1.6	 26	 2.166666667	 15.16666667	 14.3	 3.466666667	Femur3	 11.7	 7.3	 7	 1.8	 20	 1.709401709	 12.47863248	 11.96581197	 3.076923077	Femur4	 12.7	 5.5	 6.8	 1.2	 23	 1.811023622	 9.960629921	 12.31496063	 2.173228346	Femur5	 12.5	 5.4	 6.4	 1.2	 18	 1.44	 7.776	 9.216	 1.728	Femur6	 12.6	 6.8	 4.7	 1	 22	 1.746031746	 11.87301587	 8.206349206	 1.746031746	Fibula1	 11.6	 5.5	 7.8	 0.5	 16	 1.379310345	 7.586206897	 10.75862069	 0.689655172	Fibula2	 13.6	 4	 9	 0.5	 16	 1.176470588	 4.705882353	 10.58823529	 0.588235294	Fibula3	 13.3	 6	 8.1	 1	 18	 1.353383459	 8.120300752	 10.96240602	 1.353383459	Humerus1	 9.8	 5.1	 5.7	 0.9	 15	 1.530612245	 7.806122449	 8.724489796	 1.37755102	Humerus2	 7.7	 4.9	 4.4	 1	 27	 3.506493506	 17.18181818	 15.42857143	 3.506493506	Humerus3	 8.1	 5.5	 5.9	 1.2	 10	 1.234567901	 6.790123457	 7.283950617	 1.481481481	Humerus4	 7.8	 5.6	 5.9	 1.4	 12	 1.538461538	 8.615384615	 9.076923077	 2.153846154	Humerus5	 10.8	 4.7	 6.9	 1.2	 20	 1.851851852	 8.703703704	 12.77777778	 2.222222222	Humerus6	 8.1	 5.3	 4.9	 0.8	 25	 3.086419753	 16.35802469	 15.12345679	 2.469135802	Humerus7	 9.9	 4.2	 3.9	 1.1	 18	 1.818181818	 7.636363636	 7.090909091	 2	Radius1	 12	 4.8	 10.4	 1	 16	 1.333333333	 6.4	 13.86666667	 1.333333333	Radius2	 7.5	 3.5	 3.9	 1	 12	 1.6	 5.6	 6.24	 1.6	Radius3	 7.1	 3.7	 3.4	 0.7	 18	 2.535211268	 9.38028169	 8.61971831	 1.774647887	Radius4	 7.1	 3.2	 5.5	 0.9	 10	 1.408450704	 4.507042254	 7.746478873	 1.267605634	Tibia1	 8.5	 5.5	 6.2	 2.1	 12	 1.411764706	 7.764705882	 8.752941176	 2.964705882	Tibia2	 11.3	 4.7	 8.7	 1.3	 17	 1.504424779	 7.07079646	 13.08849558	 1.955752212		 81	Tibia3	 13	 4.9	 10.3	 1.2	 25	 1.923076923	 9.423076923	 19.80769231	 2.307692308	Tibia4	 12.5	 3.3	 9.9	 0.8	 16	 1.28	 4.224	 12.672	 1.024	Tibia5	 11.4	 4.6	 5	 1.4	 16	 1.403508772	 6.456140351	 7.01754386	 1.964912281	Tibia6	 12.8	 6.5	 4.8	 1.9	 22	 1.71875	 11.171875	 8.25	 3.265625	Ulna1	 9.6	 3.8	 4.6	 0.6	 13	 1.354166667	 5.145833333	 6.229166667	 0.8125	Ulna2	 11	 2.9	 9.6	 0.9	 26	 2.363636364	 6.854545455	 22.69090909	 2.127272727	Ulna3	 6.3	 3.1	 5	 0.5	 18	 2.857142857	 8.857142857	 14.28571429	 1.428571429		     Incision	L	 	 Incision	W	 Bone	L	 Bone	W		    average	 18.51724138	 	 8.75007748	 11.37394517	 1.942835561		    std	dev	 5.342689863	 	 3.319535161	 4.062818408	 0.790394069		    n	28	 	    		 		 82	Appendix	B	–	Fixation	Construct	Analysis		Link	to	supplemental	digital	content:		https://docs.google.com/spreadsheets/d/1PYT9uhjF7J148YfpH9ZxR7MgD-YAyK8gWsQcRNRl4Nw/edit?usp=sharing			 		 83	Appendix	C	–	Exploratory	Experiments	C.1	Overview		 This	section	includes	summary	figures	and	data	tables	from	the	exploratory	experiments	with	different	sensor	arrangements	in	different	animal	models.	C.2	Single	parallel	sensor	measuring	from	tissue					00.10.20.30.40.50.60.70.80.91-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5Cumulative	Frequency	DistributionError	wrt	Digital	Calipers	(mm)Test	Results	with	Chicken	Bone	(n=21)mean	=	0.22mmSD	=	0.86mm00.10.20.30.40.50.60.70.80.91-2 -1 0 1 2 3 4Cumulative	Frequency	DistributionError	wrt	Digital	Calipers	(mm)Test	Results	for	Pig	Bone	(n=9)	Optical	SensorDepth	Gaugemean	=	2.05mmSD	=	0.67mmmean	=	0.83mmSD	=	1.55mm	 84																											00.20.40.60.811.2-5 0 5 10 15Cumulative	Frequency	DistributionError	wrt	Calipers	(mm)Test	Results	with	Pig	Leg	(n=25)Depth	GaugeOptical	Sensormean	=	2.82mmSD	=	3.21mmmean	=	0.71mmSD	=	2.69mm	 85	C.3	Paired	parallel	sensors	measuring	from	tissue				 Mean	Error	 Std	Dev	Depth	Gauge	(n=19)	 -1.15	 0.62	Sensor	1	(n=18)	 1.54	 2.13	Sensor	2	(n=18)	 -0.69	 1.22	Sensor	Average	(n=18)	 0.43	 1.1																	00.10.20.30.40.50.60.70.80.91-4 -3 -2 -1 0 1 2 3 4 5 6 7Cumulative	FrequencyError	(mm)Mk	5	in	Pig	BoneSensor	1Sensor	2Sensor	AverageDepth	Gauge	 86	C.4	Paired	angled	sensors	measuring	from	tissue				 Mean	Error	(mm)	 SD	(mm)	Depth	Gauge	(n=15)	 -0.69	 0.815	Sensor	1	(n=7)	 2.79	 0.7	Sensor	2	(n=11)	 0.53	 0.8	Combined	(n=7)	 1.59	 0.36																00.10.20.30.40.50.60.70.80.91-3 -2 -1 0 1 2 3 4Cumulative	Frequency	DistributionError	wrt	Digital	Caliper	(mm)2	Angled	Sensors	in	Pig	BoneDepth	GaugeSensor1Sensor2Combined	 87	C.5	Paired	angled	sensors	measuring	from	drill	guide				 Mean	Error	(mm)	 SD	(mm)	Depth	Gauge	 -0.6	 1.29	Sensor	2	 0.02	 0.74			 	00.10.20.30.40.50.60.70.80.91-4 -3 -2 -1 0 1 2 3Cumulative	Frequency	DistributionMean	Error	(mm)Angled	Sensor,	Drill	Guide,	Pig	BoneDepth	GaugeLaser	 88	Appendix	D	–	Final	Evaluation	Experiments		Link	to	supplemental	digital	content:		https://docs.google.com/spreadsheets/d/16ylK64joPV4DD9s1Sd_gsZgFF03ArgQOHMUZB-O5-oE/edit?usp=sharing			 		 89	Appendix	E	–	Sterilization	Approval	Process	E.1	Overview		 The	following	information	comes	from	an	interview	with	Janet	Bristair,	Co-Ordinator	of	the	Reprocessing	Department	Improvement	Program	at	VGH.		She	would	be	involved	in	the	process	of	bringing	an	experimental	device	into	the	OR.	E.2	Approval	Process		 Independent	of	the	clinical	research	ethics	process,	a	sterilization	approval	process	needs	to	be	followed	prior	to	bringing	a	device	into	the	operating	room.		The	process	is	as	involved	for	an	experimental	device	as	it	would	be	for	a	commercial	device.		It	requires	contracting	a	third	party	laboratory	to	develop	and	validate	a	sterilization	protocol	for	the	device,	as	well	as	a	detailed	Standard	Operating	Procedure.		The	SOP	and	the	certification	of	the	sterilization	protocol	need	to	be	provided	to	the	reprocessing	department	of	the	hospital	prior	to	the	device	being	allowed	into	the	OR.				 For	the	laboratory	I	contacted	(Alfamed	Consulting,	Winnipeg	MB)	three	complete	prototypes	would	be	required	to	perform	the	validation	studies.		Estimated	costs	for	the	process	are	between	$20	000	-	$30	000.		A	timeline	of	approximately	3	months	would	also	be	needed.			E.3	Sterilization	Methods	E.3.1	Autoclave	Steam	Sterilization	Method:	Batch	process	with	steam	at	121oC	–	148oC	pressurized	at	1-3.5	atm.		These	conditions	kill	all	bacteria/viruses	through	protein	denaturation	etc	(cooking)		 90	Duration:	15-60	min	Indications:	Most	common/inexpensive	sterilization	process.		Suitable	for	most	surgical	instruments.		Temperatures	above	125oC	can	damage	semiconductors,	and	will	likely	damage	embedded	batteries.		Significant	moisture	involved,	which	could	also	damage	components.	E.3.2	Ethylene	Oxide	(ETO)	Sterilization	Method:	Batch	process	with	device/instruments	placed	in	sealed	container.		Process	involves	five	stages:	evacuation	and	humidification,	gas	introduction,	exposure,	evacuation,	and	air	washes.		Max	temp	of	60oC.		ETO	reacts	with	DNA,	amino	acids,	and	proteins	to	prevent	microbial	replication.	Duration:	8-12	hours	Indications:	First	method	available	for	heat/moisture	sensitive	devices.		Can	work	with	semiconductors,	but	vacuum	phase	can	affect	embedded	batteries	E.3.3	Chlorine	Dioxide	(CD)	Gas	Sterilization	Method:	Batch	process	with	device/instruments	placed	in	sealed	container.		Process	involves	five	stages:	preconditioning	with	humidification,	conditioning,	generation	and	delivery	of	chlorine	dioxide	gas,	exposure,	and	aeration).		Max	temp	of	40oC.		Gas	acts	as	an	oxidizing	agent,	which	damages	cell	membranes,	and	results	in	cell	lysis	and	death.			Duration:	2.5	hours	Indications:	Best	process	for	devices	that	are	temperature	and	moisture	sensitive.		 91	E.3.4	Vaporized	Hydrogen	Peroxide	(VHP)	Sterilization	Method:		Batch	process	with	three	stages:	conditioning	including	vacuum	generation,	H2O2	injection,	and	aeration.		Max	temp	of	50oC.		Mechanism	is	not	completely	understood,	but	involves	production	of	reactive	oxygen	molecules.	Duration:		1.5	hours	Indications:		Suitable	for	some	moisture/temperature	sensitive	devices,	however	the	vacuum	can	damage	embedded	batteries.	E.3.5	Hydrogen	Peroxide	Plasma	Sterilization	Method:		Batch	process	with	four	stages	(vacuum	generation,	H2O2	injection,	diffusion,	and	plasma	discharge).		Max	temp	of	65oC,	with	13.56	MHz	radiofrequency	energy	at	200	–	400W.		Functions	by	free	radical	and	reactive	species	generation	by	hydrogen	peroxide	gas	and	the	plasma	phase.	Duration:	1-3	hours	Indications:		Similar	low	temperature/pressure/moisture	as	VHP,	but	also	has	less	vacuum	generation.		However	the	high	levels	of	RF	energy	can	damage	semiconductors	in	electronic	components.	E.3.6	Gamma	Ray	Sterilization	Method:		Continuous	process	where	device	is	moved	on	a	conveyor	belt	in	proximity	to	cobalt	60,	which	emits	radiation	at	between	1.17	MeV	and	1.33	MeV.		The	radiation	produces	powerful	oxidizing	and	reducing	agents	which	result	in	degradation	of	essential	cell	components	such	as	enzymes	and	DNA.	Duration:		Variable		 92	Indications:	No	temperature,	moisture,	or	pressure	associated	damage	with	radiation	sterilization.		However	the	radiation	degrades	semiconductors,	causing	them	to	malfunction.	E.3.7	Electron	Beam	Sterilization	Methods:	Continuous	process	where	device	is	moved	on	a	conveyor	belt	in	proximity	to	an	electron	beam	generator	(similar	to	a	cathode	ray	tube,	but	more	powerful).		Energy	levels	required	for	sterilization	are	between	5	-10	MeV.		Radiation	produces	free	radicals,	which	react	with	DNA	and	result	in	cell	death.				Duration:		Variable	Indications:		Same	as	gamma,	but	does	not	produce/require	nuclear	materials.		Electron	beam	will	build	up	charge	on	electronic	components	and	cause	damage	to	semiconductors.															

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