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Comparison of occlusal contacts on mounted dental models to contacts identified on digital 3D models.. Straga, Robert William 2009-04-06

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COMPARISON OF OCCLUSAL CONTACTS ON MOUNTED DENTAL MODELS TO CONTACTS IDENTIFIEDON DIGITAL 3D MODELS USING A NEW VIRTUALALIGNMENT METHODbyRobert William StragaD.D.S., The University of Alberta, 2004A THESIS SUBMITTED IN PARTIAL FULFILMENT OFTHE REQUIREMENT FOR THE DEGREE OFMASTER OF SCIENCEinThe Faculty of Graduate Studies(Dental Science)THE UNIVERSITY OF BRITISH COLUMBIA(Vancouver)April 2009 © Robert William Straga, 2009ABSTRACTAs 3D imaging of dental models becomes more common in clinical dentistry,the need for accurate images will increase.  In order for these 3D models to be ofgreatest benefit, they will need to be aligned to accurately represent the givenpresentation of the individuals they represent.  This is an important step, since theacquisition of digital models results in two unrelated image files.  This studyevaluated a new technique for aligning 3D digital dental models using a 3D scan ofthe anterior teeth in occlusion of articulator mounted models as a “virtual biteregistration.”Three-dimensional digital models of one set of epoxy dental models werecreated using a commercially available 3D laser scanner (Konica Minolta Vivid 910)and Geomagic software.  Ten mountings of these same epoxy models were made,and a 3D scan of the anterior teeth in occlusion was made for each mounting.  The3D digital models were registered to the anterior 3D scan, and virtual occlusalcontacts were recorded and compared to the actual occlusal contacts recorded on theepoxy models using shimstock and articulating film.  Comparison of the newtechnique to the standards was made using sensitivity, specificity, positive predictivevalue and negative predictive value analyses.Specificity was high when using both shimstock and articulating film contactsas standards and digital contacts as tests, 0.97 and 0.98 respectively.  Whencomparing the traditional methods of recording contacts to the new digital techniqueiithe sensitivity with shimstock as the standard was 0.63 and with articulating film asthe standard the sensitivity was 0.54.  Positive predictive value and negativepredictive value of the digital technique compared to shimstock was 0.52 and 0.98respectively.  Compared to articulating paper the values were 0.76 and 0.96respectively.Using a scan of the anterior teeth in occlusion as a virtual bite registrationrepresents an appropriate method for aligning 3D digital dental models in ananatomically correct position.  The technique described may represent a goodtechnique for future comparison of the alignment of digital models to the alignmentfound on articulator mounted models or in the patient regardless of hardware andsoftware being used. iiiTABLE OF CONTENTSABSTRACT........................................................................................................... iiTABLE OF CONTENTS ..................................................................................... ivLIST OF TABLES. .............................................................................................. viLIST OF FIGURES. ........................................................................................... viiACKNOWLEDGMENTS.................................................................................... ix1 INTRODUCTION.............................................................................................. 11.1  Background. ......................................................................................... 11.2  Conventional Dental Models................................................................ 21.3  Articulation of Dental Models. ............................................................ 51.4  Digital Dental Models. ......................................................................... 61.5  Acquisition of Digital 3D Data............................................................. 91.6  Image Registration............................................................................. 131.7  Accuracy. ........................................................................................... 141.8  Occlusal Contacts. ............................................................................. 191.9  Accuracy of Digital Models. .............................................................. 242  STATEMENT OF THE PROBLEM. ............................................................ 303  MATERIALS AND METHODS. ................................................................... 323.1  Virtual Model Acquisition. ................................................................ 323.2  Mounting of Dental Casts. ................................................................. 363.3  Acquisition of Digital Bite Registration. ........................................... 373.4 Alignment of Maxillary and Mandibular Digital Models. ................. 373.5  Detection of Contacts. ........................................................................ 403.6  Independent Examiner Validation. ................................................... 413.7  Data Analysis. .................................................................................... 423.8  Statistical Analysis............................................................................. 434  RESULTS. ....................................................................................................... 455  DISCUSSION . ................................................................................................ 516  FUTURE DIRECTIONS. ............................................................................... 62iv7  CONCLUSIONS. ............................................................................................ 63REFERENCES.................................................................................................... 64vLIST OF TABLESTABLE 1 Number of contacts present by three recording methods.............................46TABLE 2 Comparison of occlusal contacts calculated using virtual models and twotraditional techniques..................................................................................47TABLE 3 Percent examiner agreement (positive)........................................................48viLIST OF FIGURESFIGURE 1 Set-up for acquiring 3D digital images of dental casts.  The 3D scanner (a)was angled down to eliminate undercuts and to adequately image theocclusal surface of the stone dental model that was placed on the rotatingstage (b), both the scanner and stage were controlled by a personal computer(c) which also contained the software for 3D image manipulation andregistration.................................................................................................33FIGURE 2 Each individual scan only captured a portion of the full dental cast due tothe fact that laser scanners can only image the surfaces facing the lens andlaser source.  Areas hidden from the lens or laser result in holes in the image(a-d).  Global registration results in all four images being aligned withrespect to each other and gives the appearance of a complete 3D model (e)..............................................................................................................34FIGURE 3 Screen capture from Geomagic showing part of the articulator captured bythe scan.  The articulator is clearly not part of the dental cast and is deletedduring as one of the first steps in registration...............................................35FIGURE 4 Example of a complete maxillary and mandibular digital 3D mode.l..............36FIGURE 5 3D scan of the anterior teeth in occlusion (a) acted as the virtual biteregistration.  The complete maxillary digital model was registered with thecorresponding maxillary teeth captured by the anterior scan (b).  The stepwas repeated for the complete mandibular digital model (c).  This resulted inthe digital models being in the correct relationship with respect to each other...........................................................................................................38FIGURE 6 Two views of 3D digital maxillary and mandibular aligned casts.  Sincemountings were arbitrary there was not always good intercuspation of teeth,but this was not important since the key measurement was similaritybetween occlusal contacts marked digitally and those marked on the actualcast.s............................................................................................................39FIGURE 7 Representative pairing of epoxy (a) and corresponding digital model (b). Contacts are indicated by arrows on both images.  Alignment of the digitalmodels resulted in four out of the five contacts being visible digitally.  Theonly contact not indicated on the digital model that is present on the epoxymodel is the 15 buccal cusp tip (c).  This pairing represent good agreementbetween actual and digital contacts..............................................................41viiFIGURE 8 Anatomic contact regions.  Contacts were defined qualitatively based onlocation on tooth anatomy.  Region and tooth number identified contacts(arrow).  B, Buccal; C, Central; L, Lingual; M, Mesial................................43FIGURE 9 Comparison of the new test to the gold standard.  In the present study thedigital contacts were always the test and either shimstock or articulatingfilm marks were considered the standard.  Sensitivity = a/a + c, specificity =d/b + d, positive predictive value (PPV) = a/a + b, negative predictive value(NPV) = d/c + d.........................................................................................44FIGURE 10 Photo of maxillary cast with contacts indicated by arrows (a).  Digital castof the same mounting showing only one contact present (arrow) (b).  Samedigital cast after being rotated about an axis representing the articulatorhinge axis.  All true contacts (arrows) are now present with no false positive(c)...............................................................................................................50viiiACKNOWLEDGMENTSI would like to thank Dr. Alan Hannam for giving me the opportunity toexplore an aspect of dentistry that my periodontal training would otherwise not haveexposed me to.  Because digital technology can often be a mystery to those notexposed to the inner-workings of it, the opportunity that I received from Dr. Hannamto work with 3D imaging hardware and software will no doubt benefit my clinicalcareer if, for no other reason than the familiarity that I developed with this emergingaspect of dentistry. ix1 INTRODUCTION1.1  BackgroundClinical dentistry involves the gathering of a great deal of information abouteach patient in order to diagnose problems and formulate a treatment plan that willresult in the most benefit to the patient.  Much of this information is gatheredthrough the clinical examination, radiographs, and record taking.  As with mostother professions there is a continued increase in the use of digitaltechnologies within dentistry in order to improve diagnosis and treatment andshorten required time involved (Calberson, Hommez, & DeMoor, 2008)  Use ofcomputers for patient booking and billing is almost universal.  Use of digitalradiography is also becoming very commonplace.  As dental offices become morecomputerized, other opportunities for use of digital technologies become available. Because much of the digital technology that is entering dental offices is replacingolder traditional methods of carrying out some process, the new technology must becompared to the old in order to know if implementing it is beneficial or simply forthe sake of technology.  A practitioner who is considering switching over to a newdigital technology must ask the following: What will the cost be initially and overthe long term?  Is the new technology easier or harder to use than the old? Does thenew technology give any additional information that will allow better patient care? Is the new technology at least as accurate or more accurate than the currenttechnology, or in other words, is the new technology sufficiently accurate?  An1example is digital radiography.  The cost may involve sensors, scanners, software,computer systems, and monitors but also the eliminated cost of film, chemicals, wetdeveloper, and possibly dedicated darkroom space.  The digital radiography systemmay also require retraining of staff and involves a certain degree of learning curveboth in acquiring and viewing the image, depending on the computer literacy of thestaff being trained (Farman, Levato, Gane, & Scarfe, 2008; Ramamurthy, Canning,Scheetz, & Farman, 2006).  Additional information gained from the new technologycan be a key point when deciding to switch.  With digital radiographs, softwaremanipulation programs can correct poorly exposed images and calculatechanges over time through subtraction radiography software which cannot be donewith film (Reddy & Jeffcoat, 1999; Wenzel, Warrer, & Karring, 1992).  Finally, theinformation gained from the new technology must be true.  Using the old techniqueas a gold standard, the line pair resolution of the digital system can be evaluated.  Ifthe digital sensor has a higher resolution than the human eye can detect, which isaround 11 lp/mm, (Kunzel, Scherkowski, Willers, & Becker, 2003)  the degree towhich this will be beneficial to diagnosis must be considered. 1.2  Conventional Dental ModelsThe thought process above can be used with any new technology introducedin dentistry, and should be, in order to prevent new technology from beingintroduced just for the sake of technology.  One technology that is gaining2momentum in dentistry is the use of digital models in place of traditional stonemodels (Birnbaum & Aaronson, 2008; Christensen, 2008).  Dental models are usedfor many purposes such as treatment planning (study models), surgical planning,fabrication of fixed and removable prostheses, fabrication of night guards, andtreatment assessment (such as in orthodontics) (Beuer, Schweiger, & Edelhoff,2008; Hajeer, Millett, Ayoub, & Siebert, 2004b; Lauren & McIntyre, 2008; Lin,Zhang, Chen, & Wang, 2006; Rekow, Erdman, Riley, & Klamecki, 1991).  Theaccuracy needed for each of these purposes varies and so do the new technologiesbeing used, but regardless of this, they all need to be compared to the currentstandard being used. The first step in acquiring dental models is taking an intraoralimpression of the teeth and adjacent tissue.  This can be accomplished using varioustraditional materials such as reversible or irreversible hydrocolloid, polyether,polyvinylsiloxane materials, and can be done using stock metal or plastic traysor rigid custom trays.  The choice of which technique and materials are used is oftenbased on the accuracy needed in the final result.  The American Dental Association(ADA)  has stated that the elastomeric materials used for impressions for fabricationof precision castings must be able to reproduce fine detail of 25ìm or less (RevisedAmerican, 1977).  This level of accuracy may not be needed if the impression isbeing used for a different purpose but is a good best case scenario standard to use tocompare emerging technology.  The gypsum die materials used to pour up theimpression into a usable stone model have less ability to reproduce fine detail than3the impression material itself, so they are the limiting factor in the process ofcreating stone models (Donovan & Chee, 2004).  The ADA specification for finedetail replication for gypsum die material is 50ìm (Donovan & Chee, 2004).  Inaddition to these quantifiable accuracies in specific materials, the technique usedwill also affect the accuracy, as will the type of impression tray used.  Carrotte,Johnson, and Winstanley (1998) found that when comparing impressions of singlecrown and three unit bridge preparations using a polyvinylsiloxane impressionmaterial, the least discrepancy was found when a rigid impression tray was used(50ìm) and the greatest discrepancy was with a flexible impression tray (180-210ìm).  Set impression materials can also change over time.  Alginate, forexample, can imbibe (gain) or lose water depending on the environment it is storedin and the length of time it is stored (Peutzfeldt & Asmussen, 1989; Sedda,Casarotto, Raustia, & Borracchini, 2008).  This property will further affect theaccuracy of the final stone models and must be kept in mind when working with thematerials or reading literature that use these materials.  As mentioned above, theneed for a certain degree of accuracy is dependent on the purpose for the dentalcasts.  Generally, alginate is used for study models and for orthodontic purposesbecause of the ease of use, low cost and adequate accuracy.  Various alginates wereshown to have accuracy between 44-180ìm compared to elastomeric materialswhich showed accuracy between 39-130ìm with the elastomeric materials being 4statistically more accurate as a whole compared to the alginates.(Peutzfeldt &Asmussen,1989). 1.3  Articulation of Dental ModelsDental models that will be used for the fabrication of a fixed or removableprosthesis or other dental appliance will often need to be mounted on an articulatorin order to reproduce as closely as possible the correct maxillo-mandibularrelationship.  This process can introduce additional error in the system (Breeding,Dixon, & Kinderknecht, 1994).  The first aspect that can intoduce additional error isthe interocclusal record.  If the maxillary and mandibular cast have goodintercuspation and have a tripod of adequately spaced contacts, the most accuratetechnique for mounting the casts is by hand articulation without an interocclusalrecord (Dixon, 2000; Squier, 2004).  No interocclusal record material hasdemonstrated absolute dimensional accuracy, and most have dimensional changeover time (Freilich, Altieri, & Wahle, 1992).  In addition, the interocclusal record,regardless of material, may have differences in surface detail compared to the stonemodels that were obtained from various impression materials.  These differencesmay add to the error in mounting of casts.(Breeding et al., 1994). 51.4  Digital Dental ModelsAll dental materials used for the reproduction of the dental arch have inherentlimits to accuracy and resolution, but the majority of error introduced in traditionaltechniques is from the inappropriate use of the materials and techniques (Donovan &Chee, 2004).  Despite all these steps that can and do introduce error into the finaloutcome, the use of impressions and stone models for fabrication of precise fixedrestorations has shown to have very good long-term success for the life of the finalprosthesis (Goodacre, Bernal, Rungcharassaeng, & Kan, 2003; Palmqvist & Swartz,1993).  With an established level of success and acceptance in the dental profession,what is the benefit of trying to change this system to a digital one?  There are manyadvantages to digital casts over traditional stone models.  There is a decreased needfor storage space.  A model box used to store four models takes up about the spacetaken up by a modern one Terabyte  external hard drive which could hold up to100,000 models (assuming 10 Megabytes/model), which is a reasonable sizedepending on the file format used (in this study models saved as .obj files requiredjust under 10mb/cast).  This physical storage space can be completely eliminated bystoring the data offsite and accessing it through an internet or local area networkconnection (Farman, et al., 2008).   Another advantage of digital casts is that they donot change.  Once the file is saved it can be accessed an infinite number oftimes without affecting the original data.  It can also be easily manipulated withoutlosing the original data.  The process can be non-destructive.  Stone models, on the6other hand, can be broken or the occlusal surfaces can be worn by repeatedlybringing the maxillary and mandibular model in contact.  No adjustments can bedone on the original models in a non-destructive way, but instead, additionalimpressions must be taken of the original models and poured up.  Digital filescan just as easily be shared between two dentists in the same office as with a dentistin another country.  This can be a benefit if several practitioners are involved in asingle case and all want to base decisions on the same information.   Despite the advantages of digital casts, there are still some disadvantages. The biggest disadvantage is the lack of tactile sensation.  In treatment planning casesmany dentist have grown accustomed to manually handling casts tocheck articulation or to do bench top adjustments to the teeth.  This may only be arelative disadvantage of digital models since hard copies of the digital models can beproduced when needed, although the cost is currently much greater than impressingand pouring up traditional duplicate models.  Issues related to accuracy of digitalmodels compared to traditional stone models are still being investigated but mayrepresent a disadvantage depending on the system used for digitizing and thepurpose of the digitized cast (Bell, Ayoub, & Siebert, 2003; Brusco, Andreetto,Lucchese, Carmignato, & Cortelazzo, 2007; Delong, Heinzen, Hodges, Ko, &Douglas, 2003; Kuroda, Motohashi, Tominaga, & Iwata, 1996).  Other concerns thatpractitioners may have include loss of data, systems crashes, and privacy issuesrelated to security of digital records.  These are valid concerns, but other digital7systems have demonstrated that they are of limited threat when properlyaddressed. (Brian & Williamson, 2007).  These advantages and disadvantages of digital models cannot be simplyconsidered on their own but must be taken in context of the final purpose of thedental model.  Digital models being used solely for linear measurements for spaceanalysis that lack tactile sensation may not be an issue to the dentist compared tomodels that a dentist wants to use to do an ideal wax-up. Similarly, a model that issolely used as a record of the initial presentation of an individual will have asignificantly different requirement in resolution and accuracy compared to a modelbeing used for fabrication of a fixed prosthesis.  Therefore, in analyzing thetechnologies available, they must be compared to techniques that represent thecurrent standard accepted in the specific discipline.  As mentioned, the ADArequires 25ìm resolution for elastomeric impression materials used for castrestorations and 50ìm resolution for the stone in which those impressions are pouredup in.  Therefore, 50ìm is the very best resolution required by traditional methodsfor 3-dimensional (3D) replication of dental structures and would represent a goodgoal for digital methods.  Although this would be a good goal, it is obviously morethan is required in some disciplines; for example, orthodontics, which has shownthat in a Bolton analysis by trained individuals there is a final range of ±2.2mm inoutcomes (Shellhart, Lange, Kluemper, Hicks, & Kaplan, 1995).  This wouldrepresent a needed accuracy in the millimetre range compared to the micrometre8range in order to have an acceptable outcome.   Similarly, it is known that humanscan only detect occlusal discrepancies of greater than 20ìm between teeth, sodemanding an accuracy greater than this is probably unreasonable from a clinicalstandpoint (Karlsson & Molin, 1995)  These arguments represent one of the weaknesses in digital imaging used indentistry.  There are no established standards or ranges of what accuracy isacceptable for most clinical uses. Nevertheless, digital 3-dimensional imaging is agrowing area of clinical dentistry.1.5  Acquisition of Digital 3D DataPresently there are established clinical tools for acquiring and manipulating3D images in use in orthodontics, prosthodontics , restorative dentistry, implantdentistry, and radiology.  Examples of these are Orthocad, Procera, Cerec, Simplant,Nobelguide, and I-cat, and recently two new intraoral 3-D scanners, iTero and 3MESPE Lava scanner, have been released with the ability to image the entire archintra-orally.  When considering the process of 3D imaging and modeling, one mustunderstand the steps required to produce a complete 3D model and also be aware ofthe different technologies that can accomplish this goal.  The first step is imageacquisition.  Acquiring a 3D image requires some type of image capture devicecapable of imaging in more than two dimensions.   There are currently several majorclasses of such devices available commercially. 9Computed tomography (CT) is one type or modality used for 3D imagecapture.  CT uses ionizing radiation to build, in layers, a 3D image (White &Pharoah, 2008).  There are variations of this technology such as cone beam CT(CBCT) and micro CT which are used for specific purposes.  Cone beam CT is usedmost frequently in dentistry because of the lower radiation exposure to the patientand the small window that is used in this type of imaging which is adequate to imagethe jaws compared to the more extensive standard medical CT scanners (Chau &Fung, 2009; White & Pharoah, 2008).  Cone beam CT scans vary in radiationexposure from one manufacturer to another but range from as low as 12-477ìSvdepending on the field of view compared to over 2200ìSv from a medical CT scanof the mandible and maxilla (White, 2008).  In comparison, a digital panoramicradiograph is between 6-7ìSv (Howerton & Mora, 2008).  Computed tomographyproduces true 3D images, which is an advantage over some other scanningtechnologies available.  Disadvantages of CBCT technology are the radiationexposure to a patient, the relatively low resolution (except the micro CT), and theinability to image metal restorations which can result in streak artefacts that canobstruct the surrounding anatomy.  (Howerton & Mora, 2008) The accuracy of linearmeasurements of CBCT scans of skulls compared to measurements from the actualskulls has been shown to be within 1mm in absolute value, or a percentagedifference of less then 5% for most measurements (Brown, Scarfe, Scheetz, Silveira,& Farman, 2009).  10Another group of scanners that can also produce true 3D images are touchprobe scanners or contact profilers.  These use a spherical tipped stylus that is placedin contact with the object being scanned as the object is rotated in order to record allsurfaces of the object.  Contact scanners tend to be highly accurate but the resolutionof complex free-form surfaces is limited by the size of the stylus tip, which tends tobe 0.1mm or greater.  An example of this type of scanner is the Nobel BiocareProcera scanner, which is used for scanning crown preparations.  Due to the size ofthe stylus tip, crown preparations must meet certain design criteria or else theycannot be properly scanned.  This fact eliminates the possibility of scanning fulldental arches, which can have acute angles and complex anatomy, with thismachine.Non-contact scanners use light instead of a contact stylus to determine thesurface anatomy of an object.  They come in two general varieties - structured whitelight or laser light.  Laser scanners use a line of laser light that sweeps across anobject while a camera records the shape of the reflected laser light.  Throughtriangulation the three dimensional location of each surface point is calculated.(Bernardini & Rushmeier, 2002; Lane & Harrell, 2008)  This can be done because afixed, known angle exists between the laser source and the capture lens, and anydeviation or deflection of the straight line of laser light represents a surface variationof the object.  Structured light scanners use a similar technique, except that insteadof a line of laser light they use a pattern of white light projected on the image and11use triangulation, interferometry, phase shifting, Moiré fringe patterns or, several ofthese, to determine surface points. (Bell et al., 2003; Brusco et al., 2007; Hajeer,Millett, Ayoub, & Siebert, 2004a; Kuroda et al.,1996)  A new intraoral 3D scanner ,iTero (Cadent Inc., Carlstadt, NJ) uses similarprinciples to that used in confocal microscopy to create a 3D image.  Briefly, itprojects 100,000 beams of parallel laser light through a filter onto an object surfacewhich then reflects the light back.  Only objects at the right focal distance willreflect light that will pass back through the filter, all other reflected light will beblocked (Garg, 2008; Henkel, 2007).  In addition, 3M has released a new intraoralscanner that uses a novel structured light technique developed at MIT (Rohaly &Hart, 2000).  Both scanners have been in development for several years, but thecommercial products are both still very new, so very little information is currentlyavailable.The advantage of non-contact scanners is that they do not need to touch thesurface of the image they are capturing, which is a benefit if intraoral scanning is thegoal or if the object has fine details that are smaller than a touch probe scanner.  Inaddition, no radiation is involved and they are much faster than contact scanners(Brusco et al., 2007).  One disadvantage is that non-contact scanners do not producea true 3D image but instead are often called 2.5D scanners because shadowingoccurs when undercuts are present or anywhere that the surface is hidden from thelaser source or the lens.(Xiaoguang Lu, Jain, & Colbry, 2006)  To overcome this12limitation, multiple scans must be taken at different angles in order to sample theentire surface and also to ensure that there is enough overlap in each scan foraligning each image into the complete 3D model (Bernardini & Rushmeier, 2002;Levoy, 1999).  Alignment or registration of the multiple scans is a necessary step in the process of creating a complete 3D image with this type of scanner (Lane &Harrell, 2008).1.6  Image RegistrationRegistration is the alignment of two or more 3D surfaces based on similarityof the overlapping surfaces or by means of aligning common fiducial markers and isgenerally carried out in two broad steps - pairwise registration of scans followed byregistration of all scans, or global registration.  The first step is carried out in twosteps - rough and fine alignment.   Rough alignment is often done by manuallyselecting one or more corresponding points on pairs of images and allowing thesoftware to rotate and translate the two images until the points are as closely alignedas possible(Brusco et al., 2007).  Alternately, this process can be done completelymanually by identifying surface markers or surface features and through trial anderror, moving the images until they appeared aligned.  Software advances andincreased computational power has made this time-consuming manual methodalmost completely obsolete.  A completely automatic technique for rough alignmentalso exists, and uses specific forms of 3D images called spin images.  As long as13pairs of scans have at least 30% overlap and the surfaces are characterized byadequate geometric features they will have similar spin images (Johnson & Hebert,1999).  These can be rotated and translated by the software until the spin images are aligned, which results in a good alignment of the actual image if the same rotationand translation is applied.Fine registration is accomplished by the iterative closest point algorithm orone of its variants (Besl & McKay, 1992; Kapoutsis, Vavoulidis, & Pitas,1999;Rusinkiewicz & Levoy, 2001).  The general idea of this technique is to find a set ofmatching points on the overlapping surface of two scans and minimize the distancebetween each of these points.  Once all scans have been registered or aligned theyare merged into one surface in order to decrease the file size and simplify furthermanipulation of the model (Delong et al., 2003).  If this final merged image hasholes these can be filled at this time by the software but will not be a representationof true surface of the object (Brusco et al., 2007)  1.7  Accuracy As with any recording technique or device, the accuracy, precision, andresolution must be acceptable for the application it is used for.  Accuracy is definedas how well a measured value represents the truth, precision is the repeatability ofthe measurement system, and resolution is the degree of detail visible in an image(Brosky, Major, DeLong, & Hodges, 2003; Persson, Andersson, Oden, &14Sandborgh-Englund, 2008)  Resolution is the number of pixels per unit area, pointsper unit area, or in CT scans the size of voxel (volume pixel) used.  These values are given by most manufacturers and depend on the number of pixels present on thesensor.  They increase as sensor size or density increases.  Accuracy is a much more difficult measure to define when describing non-contact 3D scanners and free form shapes because there is no established standardfor measuring accuracy on these machines.  The only standard has been establishedfor contact scanners using the substitution method in which repeated measures arecarried out on calibrated objects and measurements are compared to the calibrateddata (Brusco et al., 2007; Savio, De Chiffre, & Scmitt, 2007).  Metrologicalstandards for optical scanners and free-form shapes is still an open research area(Brusco et al., 2007).  In addition to the measuring device, error can come from themeasuring strategy, the item being measured, the environment (such as ambientlight), the operator, and other sources (Brusco et al., 2007). Determining the accuracy of 3-D scans of complete dental arches has beenpublished extensively in the orthodontic literature (Bell et al., 2003; Hildebrand,Palomo, J., Palomo, L., Sivik, & Hans, 2008; Kuroda et al., 1996; Okunami et al.,2007; Quimby, Vig, Rashid, & Firestone, 2004; Santoro, Galkin, Teredesai, Nicolay,& Cangialosi, 2003; Zilberman, Huggare, & Parikakis, 2003).  The generaltechnique that is used in this literature is to measure linear distances between pointsand compare the results from the digital model to those from the actual stone model. 15Because orthodontics is concerned with space availability, and traditionallymeasurements on dental casts involve the measurement of tooth width and archlength, this technique gives a sufficient measure of accuracy for orthodonticpurposes.  A brief review of these studies and their findings follows.  Many of these studies have used digital models produced from a commercialprovider, Orthocad (Cadent Inc, Carlstadt, NJ), in order to determine the accuracy ofthat particular system and to determine if it is a valid alternative to stone models. Error can come from several sources along the line when producing digital dentalcasts, such as impression material, impression technique, methods and materials forpouring up the stone models, scanning system, etc.  In studies using Orthocadservices only the total error can be roughly determined because Orthocad does notrelease detailed information on either the process of producing the cast or thescanning method (Quimby et al., 2004)  In addition, the final digital cast can only beviewed in the company’s proprietary software, which is mainly aimed at linearmeasurements and not measuring free-form surfaces.  Despite these limitations moststudies come to similar conclusions that digital casts produced by Orthocad areadequate substitutes for stone models in orthodontics.  Zilberman et al. (2003) compared individual tooth widths on an originaldentoform and stone model of the dentoform using digital calipers to the individualtooth widths measured in Orthocad software.  They found the highest correlationbetween the original dentoform teeth and the stone models (R=0.929-0.998) and16lower correlation between dentoform and computer models (R=0.784-0.976) andstone and computer models (R=0.763-0.975) but no statistically significantdifference between any measuring method.  They concluded that measuring stonecasts with digital calipers is better than using Orthocad but that Orthocad isclinically acceptable. Instead of dentoform teeth which can be removed and measured, Santoro etal. (2003) used actual patients to carry out a very similar study.  Seventy-six patientswere involved and tooth width measurements were compared between stone modelsand Orthocad digital models.  They found statistically significant differences between the two methods of measuring for most teeth measured.  The digital modelsalways measured smaller than the stone models with a mean difference rangingbetween 0.16-0.38mm per tooth.  The authors conclude that although the differenceis statistically significant, the magnitude of the difference does not seem to beclinically significant.  One difference between these two studies that may havecontributed to the different findings was the impression material used.  Zilberman etal. (2003) used polyvinylsiloxane whereas Santoro et al. (2003) used alginate.  Thismay play a role because the impressions were mailed to Orthocad for pouring up. This likely results in a delay of greater than 12 hours before pouring up of themodels, which may affect the water content of the alginate impressions and thereforetheir size (Alcan, Ceylanoglu, & Baysal, 2009; Sedda et al., 2008).17If individual teeth widths are different between stone and digital models, evenif by a very small amount, this may result in significant differences in total archlength or total space needed.  Instead of individual teeth, Quimby et al. (2004)measured arch lengths and space required, in addition to several cross archmeasurements, to compared dentoform and stone models to digital models producedby Orthocad.  They found statistically significant differences for arch length, spacerequired, and all cross-arch measurement when comparing stone to digital models. Measurements made on the digital models were larger than those made on the stonemodels, and the difference was generally less than 1mm except for maxillary spacerequired and mandibular space available which were 2.23mm and 2.88mm greateron the digital models respectively.  The question arises whether or not thesestatistically significant differences are clinically significant.  The authors concludedthat it was questionable if the measured differences would lead to a significantlydifferent treatment outcome.Other authors have taken a similar approach as those mentioned above, todetermine accuracy of digital casts for orthodontic purposes, but have used variousin-lab scanning systems versus a scanning service. (Bell et al., 2003; Kuroda et al.,1996).  Bell et al. (2003) used a structured light non-contact scanner and in-housesoftware for model reconstruction.  Instead of measuring tooth widths or arch length,they placed 6 points along the arch and made 15 measurements between thesevarious points.  They found no statistically significant difference between the18measurements made on the stone and the digital models.  The differences rangedbetween 0.16-0.38mm.  This value, the authors felt, would not be clinically significant and that digital models offer a valid alternative to long-term storage ofstone models.Instead of making linear measurements to determine accuracy of digitalmodels, some authors have used interocclusal contact points as a surrogate marker ofhow accurate digital models are (Delong, Knorr, Anderson, Hodges, & Pintado,2007; Delong, Ko, Anderson, Hodges, & Douglas, 2002a; Maruyama, Nakamura,Hayashi, & Kato, 2006).  In general, these authors have compared contact pointsmarked on mounted stone models, using some form of articulating ribbon orshimstock, to the contacts that appeared on digital models after aligning them. Whenever a new technology is developed and tested, the results need to becompared to the established gold standard.  This becomes a little difficult whenusing contact points because of the discrepancy between techniques and materials.1.8  Occlusal ContactsMarking occlusal contacts is a very common procedure in clinical dentistry. It is used to diagnose occlusal interferences, check for appropriate height of newrestorations, and as an initial screening for most new patients.  The procedure ismost commonly accomplished using thin, inked paper or film, which is placed onthe occlusal surface of a patient’s teeth.  When the patient bites or grinds the teeth19together, coloured marks are left where the contact or where there are areas of nearcontact.  More recently, various types of computer controlled devices (T-Scansystem, Sentek Corp, Boston, Mass.) have been developed for recording occlusalcontact forces.  Whenever new technology is developed for detecting occlusal contacts, itshould be compared to the existing “gold standard” to establish validity, accuracy,etc.  This usually means that the new technique is compared to occlusal film orpaper, although this has not been established as a “gold standard” due to the varyingthicknesses, inks, and plasticity of the marking films and papers and because of thevarious operator techniques in obtaining occlusal markings.    Marking occlusal contacts is not a precise science.  Although there are manytypes of articulating film and paper ranging in thickness from 8ìm up to 200ìmwhich results in differences in results between brands and types, there has also beenshown to be significant differences in results with the same material on the samecasts.  Millstein and Maya (2001) found that between brands there was as much as a9mm  difference in surface area marked on the same tooth (2.16±0.56 mm  vs2 211.16±2.57mm ).  In addition to surface area, they found the number of contact areas2varied from a mean of 1.24/tooth up to 6.68/tooth for the same tooth using differentbrands of articulating paper or film.  These results may be expected because of thedifferences between brands, but even within the same film or paper type the authorsfound significant differences in surface area marked as well as number of contact20areas/tooth.  The results of Saad, G. Weiner, Ehrenberg, and S. Weiner (2007)support Millstein and Maya’s (2001) findings that the number of occlusal contactsrecorded clinically depend greatly on the type of articulating paper or film beingused.  Saraço lu and Özpinar (2002) also compared various types of articulatingpaper and film as well as the T-Scan system and found that all lost sensitivity aftermultiple uses, or, stated differently, showed fewer occlusal markings for the sameteeth after each additional use.  They concluded that the most accurate material wasthe one that resulted in the most occlusal markings.  There is no evidence in theirstudy that in fact more markings represented greater accuracy than fewer markingsbecause there was no standard that all others were being compared to.  One findingthey reported that is of clinical value is that there was a significant decrease inocclusal markings in wet conditions compared to dry conditions for all the materialsthey tested except for the T-Scan system.  Gazit, Fitzig, and Lieberman (1986) concluded that neither a novel photo-occlusion technique nor the standard colour marking technique was reproducible. They marked occlusal contacts in subjects at two separate times, one month apart. Although the new technique they used was reported to be more reproducible,between 20-50% of markings were only seen at one of the two time points.  Thisthey attributed to the non-standardized biting of the subjects as well as natural 21changes over time and stated that marking on articulated models may avoid theseerrors. Both accuracy and validity are difficult to assess for occlusal markingsbecause there is no test that is known to give an accepted true value (Delong et al.,2007).  Because most studies use mounted casts as the experimental subjects, onecomparison to assess validity would be to compare the markings made on mountedcasts to those made in the actual patient.  Once again, there are a few variables thatmust be considered when making this comparison, such as the quality of the castsand the accuracy of the mounting.  In a recent study, the authors compared the markings obtained from occlusalfilm and the T-Scan system on mounted casts to the markings made in the actualpatients.  They found that the quantity of marks was lower on the articulated castscompared to in the mouth but that the location of the marks present were similar(Cabral, Andrade, Buarque, Landulpho, & Buarque, 2006).  Unfortunately, statisticalsignificance was not reported for the difference in quantity of markings.  Severalrecent studies have examined the role that force plays on the quantity and quality ofocclusal markings. (Carey, Craig, Kerstein, and Radke, 2007; Saad et al., 2007). Both groups found that the size of the occlusal mark is not directly related to theforce of closing, although Carey et al. (2007) found that there was a non-linearrelationship between force and size of occlusal marks in some instances. Both 22authors emphasized that from their results equal sized markings on adjacent teeth donot necessarily represent similar occlusal forces. Until a gold standard for marking occlusal contacts is established dentists willcontinue to use a variety of articulating papers and techniques for marking occlusionin both clinical dentistry and dental research.  There are very few studies reportingon the validity, accuracy, precision, and repeatability of occlusal markingtechniques.  More research is necessary in order to establish a gold standard so thatwhen new technology for marking occlusal contacts is developed, both researchersand clinical dentists will be able to critically evaluate the value of the newtechnology.  Despite this shortcoming, the use of occlusal contacts as a surrogatemeasure for accuracy of digital casts may be justified just as the use of contacts ontraditional mounted casts acts as a measure of mounting accuracy.  Just as articulator mounted casts must reproduce the occlusal contacts notedclinically, so too digital casts must be able to reproduce the clinical contacts.  Notonly do occlusal contacts act as a method for verification of the mounting of stonemodels and digital models, but the occlusal contacts are an important clinical record(Harrel, Nunn, & Hallmon, 2006; Kim, Oh, Misch, & Wang, 2005) and can play arole in diagnosis and treatment planning in various disciplines in dentistry.231.9  Accuracy of Digital ModelsDelong et al. (2002a, 2003, 2007) have published several papers that all use avery similar experimental design to test the accuracy of aligned digital models bycomparing occlusal contacts.  Briefly, stone dental casts are scanned using a non-contact 3D scanner and digital models are created using commercial software. Standard interocclusal records are taken on the articulator mounted casts and theseare also scanned.  Using in-house software, the maxillary and/or mandibular digitalmodels are aligned with the scanned bite record, and a limit is set whereby any twopoints on the mandibular and maxillary model that are within a given distance areconsidered contact points.  Alternately, the bite record alone can be used todetermine contact points using the same technique, where any point on the maxillaryand mandibular surface are considered a contact point if they are within a givendistance.  Delong et al., (2002a) compared digital contacts from the digitized bite recordand the aligned digital models to actual mounted models and found that regardless ofalignment technique, the resultant specificity and sensitivity was adequate forclinical requirements - 0.95-0.98 and 0.76-0.89 respectively.  Also, digital dentalcasts could produce contacts equivalent to those noted on mounted stone models. Although they compared several alignment procedures, some automatic and onemanual, all aligned casts were manually “refined to correct for penetration...or forseparation of the 2 virtual surfaces.” (Delong et al., 2002a, p. 626)24In a separate study, the same group again compared occlusal contacts onaligned digital casts, scanned bite records, and using a technique of transilluminationof bite records to the contacts determined on mounted stone models that usingshimstock.  They allowed for a separation of up to 0.350mm to be considered acontact in the two digital methods, casts and bite record.  They found that aligneddigital casts, scanned bite record, and transillumination, all had better agreementthan when any of those were compared to shimstock, although agreement betweenall methods was greater than 80% (Delong et al., 2007).Although this direction of research may validate the use of digital modelsfrom a generic or specific source for use in clinical dentistry, it does not address theissue of absolute accuracy of the final digital model.  The manufacturers of mostcommercially available 3-D scanners give a value for accuracy, but this refers to asingle scan under ideal conditions (Delong et al., 2006).  In order to create acomplete digital model of a dental cast, several scans are required, as is softwarereconstruction of those scans into a complete 3D model.  Each of these steps willintroduce additional error into the final outcome (Brusco et al., 2007; Delong et al.,2003; Hirogaki, Sohmura, Satoh, Takahashi, & Takada, 2001).           A couple of groups have attempted to measure the overall surface accuracy offree-form dental models acquired from non-contact scanners and apply this to theaccuracy required in dental applications (Brusco et al., 2007; Delong et al., 2003) . With regards to full arch models, the most stringent accuracy requirement in dental25practice is likely for interocclusal contacts, since patients are sensitive to a change of0.020mm in their occlusal anatomy. (Delong et al., 2003; Karlsson and Molin,1995).  DeLong et al. (2003) used a calibrated standard (7 steel ball bearingspositioned in a steel arch) from which impressions were taken and stone modelsmade.  Three-dimensional scans were made of the stone models as well as the vinylpolysiloxane impressions and were compared to a mathematical model of thestandard, which was produced by a calibration service with the aid of a coordinatemeasuring machine.  Creation of the digital models required 20 individual scans ofthe stone models or impressions to be filtered, aligned, and merged into one finalobject.  Accuracy after each step was determined.  They found the single scanaccuracy of their system to be 0.018mm and final accuracy after processing to be0.013mm±0.003mm for the scanned impression and 0.024±0.002mm for thescanned stone models.  The alignment step created the greatest improvement inaccuracy throughout the process, and the other processing steps had minimal effecton accuracy.  The accuracy was deemed adequate for dental uses includingaccurately determining the location of occlusal contacts (Delong et al., 2003).Brusco et al. (2007) followed a slightly different direction in determining theaccuracy of a dental cast scanning system.  One advantage of their system was that itwas completely automated, in other words no human input was required for theprocessing of individual scans into the final 3-D model.  This saves both time andthe need for specially trained personnel for the digitization of dental casts.  Their26entire system, both hardware and software, was produced in-house and is notavailable commercially, as compared to most similar studies.  Accuracy wasdetermined using the substitution method (Savio, Hansen, & De Chiffre, 2002)where a calibrated block was glued to a dental cast for scanning and measurementswere made from this block.  They found that positioning of the object, ambient light,and calibration of the imaging system all resulted in changes in accuracy.  Inaddition, as the number of voxels increased, the standard deviation, decreased withlittle change in mean error values (no change in accuracy), but, as the number ofscans increased, the mean error increased.  This means that in order to produce adigital model of the greatest precision and accuracy, the smallest number of scanspossible and the greatest resolution should be used.  The final models built using 11scans had a mean error of 0.0175±0.228mm.  The mean error is comparable to theresults of Delong (2003), but the standard deviation is much greater.  These authorsfelt that this result was well within the range needed for use in orthodontics (Bruscoet al., 2007)The volume of dental literature that is addressing the overall accuracy ofdental casts is very limited, as is the determination of what accuracy is needed fordental applications.  This is understandable, since the standards for determiningaccuracy of free-form surfaces using non-contact scanners is still an open researcharea in the engineering field, and until there are accepted standards, the dentalresearch will continue to use surrogates such as the linear measurements used in27orthodontics or comparing the location of occlusal contacts.  These surrogatemeasurements are not inherently bad for determining accuracy of digital models forclinical applications because they allow the clinician to understand the accuracy in areal life measurement that they are familiar with from clinical experience.Production of digital models in various forms is continuing to increase indentistry despite the limitations and questions regarding accuracy.  Although digitalmodels alone are useful in various aspects of dentistry, such as measuring toothwidths and arch length as is done in orthodontics or as an initial record of toothlocation, the full advantage of digital models will only be reached when an accuratestatic and dynamic relationship between maxillary and mandibular teeth can beincorporated into the digital process.  This is especially true for any prosthetic work,since, if the occlusal relationship is not accurate, any prosthetic work done by adental technician will be off (Henkel, 2007).  When stone dental models aremounted in an articulator, tooth contacts can be determined in both excursive andprotrusive movements as well as maximal intercuspation or centric relationdepending on the mounting.  The advantage that this offers is that dental restorationand prostheses can be fabricated and tested in a dynamic environment that will limitor eliminate the amount of intraoral adjustment of the restoration.  Most articulatorsare limited because they do not replicate exactly the jaw motion of a patient butinstead use average values that can approximate jaw movement.  Despite thislimitation articulator use is essential for major dental reconstruction.  A digital28system that can incorporate 3D models of a patient’s teeth into a dynamic model ofthe patient’s jaw motion may allow for even more accurate occlusal relationships inthe final restorations than is possible with current articulator mounted casts. Evaluating this type of system for accuracy will be a significant challenge toresearchers due to a lack of gold standard comparisons.  The first step in validatingthis type of model will be validating the static relationship of the maxillary andmandibular models in a pre-determined starting point.292  STATEMENT OF THE PROBLEMAs 3-D imaging becomes more common in clinical dentistry, the need foraccurate and high resolution images will increase.  In order for these highly accurate3-D models to be of greatest benefit, they will need to be aligned accurately torepresent the given presentation of the individuals they are acquired from.  This isespecially true for 3-D dental casts, since the acquisition of maxillary andmandibular models results in two unrelated image files.  This is in contrast to CBCTimages of the maxilla and mandible that, although of lower resolution, are alignedanatomically correctly because of the nature of the scan.  Unlike traditional modelsthat can often be positioned to accurately represent the static relationship betweenmaxillary and mandibular teeth based on tactile feedback, this is not possible withdigital models.  Therefore, a simple, accurate, and reproducible technique foraligning digital models is necessary.  It is hypothesized that 3D maxillary andmandibular digital models can be aligned using a 3D image of the anterior teeth inocclusion.  This alignment will result in the visualization on the digital models of theocclusal contacts as seen on the articulator mounted casts   The purposes of this study are: 1) Demonstration of a technique for 3D digital acquisition of stone dentalmodels using a commercially available 3D scanner. 302) Alignment of the maxillary and mandibular models in an anatomicallycorrect position relative to each other with a “virtual bite registration” instead of atraditional interocclusal record.  3) Validation of the method by comparing the reproducibility of occlusal contacts between the digital models and the actual occlusal contacts recorded bytraditional techniques on the mounted dental models.313  MATERIALS AND METHODS3.1  Virtual Model AcquisitionThroughout the study a single set of generic epoxy dental casts was used(Denar).  A Minolta VIVID 910 non-contact laser scanner (Konica Minolta Sensing,Ramsey, NJ) was used for scanning the dental casts and for scanning the anteriorteeth with the models in occlusion.  The Minolta VIVID 910 has a reported accuracyin each axis of X±0.22mm, Y±0.16mm, and Z±0.10mm.  All scanning was doneusing the tele lens at a distance between 600-750mm on the fine scanning setting,which takes 2.5 seconds/scan.  Cast acquisition was accomplished by placing one ofthe casts on a rotating stage controlled by the same computer that controlled thescanner, occlusal surface up (Figure 1).  The scanner was angled at 45 degrees downfrom the horizontal in order to eliminate undercuts.  Four separate images wereacquired by the scanner with the stage rotating 90 degrees between each scan.  Thisresulted in four images that were roughly aligned due to the fact that the softwarerotated each image the same angle that the stage rotated, but in the oppositedirection.  The goal was to limit the number of scans necessary to have completeimaging of the occlusal surfaces.  Brusco et al. (2007) showed that the error in thecompleted model was greater with an increased number of scans.  Since only theocclusal surface was of concern, additional scans to correct holes that resultedbecause of undercuts hidden from view of the scanner were not done.  Incidentally, 32using the method described resulted in very few and only small voids, mostcommonly in the interproximal region at the gingival margin.After acquisition of the four scans, further image handling was carried out inGeomagic Studio software (Geomagic, Research Triangle Park, NC).  By using therotating stage, the four images were roughly aligned so no further manual alignmentwas needed (Figure 2).  Instead they were finely aligned using the global registrationfunction in the software.  This was accomplished by selecting all four images andthen allowing the software algorithm to calculate a translation and rotation that Figure 1.  Set-up for acquiring 3D digital images of dental casts.  The 3Dscanner (a) was angled down to eliminate undercuts and to adequately image theocclusal surface of the stone dental model that was placed on the rotating stage(b), both the scanner and stage were controlled by a personal computer (c) whichalso contained the software for 3D image manipulation and registration. 33Figure 2. Each individual scan only captured a portion of the full dental castdue to the fact that laser scanners can only image the surfaces facing the lens andlaser source.  Areas hidden from the lens or laser result in holes in the image (a-d).  Global registration results in all four images being aligned with respect toeach other and gives the appearance of a complete 3D model (e).34resulted in the least difference between overlapping regions.  Once all four imageswere aligned manual cleaning was carried out which involved erasing areas of eachscan that were clearly not part of the epoxy model.  This can be either surroundingsurfaces that were captured or shadowing artifact from the scanner (Figure 3). Another global registration was carried out prior to merging the four images. Merging created a single complete model out of the four scans.  After merging, holesmay be present in the models as was described previously.  These holes could befilled by the software for esthetic purposes, but in the case of this study, as long asthe holes were not present on the occlusal surface, they were be left open.  If holeswere noted on the occlusal surface new scans were necessary to capture this region. Figure 3.  Screen capture from Geomagic showing part ofthe articulator captured by the scan.  The articulator isclearly not part of the dental cast and is deleted during asone of the first steps in registration.35 One weakness of all structured light scanners, as well as laser scanners, is thelimited ability to correctly scan shiny surfaces (Bernardini & Rushmeier, 2002). Epoxy models have a slight sheen compared to standard stone dental models and itwas found that this resulted in detectable artefact in the images (Wheeler, Sato, &Ikeuchi, 1998).  In order to avoid this, a washable matte spray, Spotcheck SKD-S2Developer (Magnaflux, Glenview, IL), was used on the surface of the epoxy modelsprior to scanning and on the anterior views prior to scanning.  These sprayed modelswere the only ones used in the final analysis (Figure 4).  Only one set of virtualmodels was used for all mountings (below).3.2  Mounting of Dental CastsThe epoxy models were mounted on a Denar articulator using snow whitestone and allowed to set for 30 minutes.  Ten arbitrary mountings were performed Figure 4. Example of a complete maxillary and mandibular digital 3D model.36and each time only the maxillary model was remounted.  The anterior view of eachmounting was scanned as described below. 3.3  Acquisition of Digital Bite RegistrationIn order to align standard stone dental models, an interocclusal bite record isused such as a wax wafer, vinyl polysiloxane registration material, etc.  In this studythe aim was to eliminate that step since it represents an additional clinical step(additional time) and may introduce additional error (Breeding et al., 1994).  Instead,an image of the anterior maxillary and mandibular casts was acquired while inocclusion (virtual bite registration).  This was accomplished by placing the epoxymodels in a Denar articulator and aligning the facial surfaces of the anterior teethparallel to the scanner’s lens.  Two identical images were acquired, registered usingthe global registration function, and merged into a single image.  Two scans wereused instead of one in order to create a merged file which eased subsequent steps(see below).  3.4 Alignment of Maxillary and Mandibular Digital ModelsAlignment of the virtual maxillary and mandibular models to the virtual biteregistration was accomplished in Geomagic software (Figure 5).  This alignment wascarried out in two steps for the maxillary and two steps for the mandibular virtualmodels.  Because the initial alignment of the virtual models and virtual bite 37registration may be significantly off, a coarse manual alignment needed to be carriedout first.  In the software this was accomplished by the manual alignment function. The two images to be aligned were first chosen and aligned in two separate windowsso that the images viewed on the monitor were at roughly the same angle.  The nextstep was selection of one or more points on the first image and the correspondingpoints on the second image in the other window.  The software then roughly alignedthose points as well as the matching surfaces on the images.  If there was goodoverlap between the two images then selecting a single point was sufficient toFigure 5. 3D scan of the anterior teeth in occlusion (a) acted as thevirtual bite registration.  The complete maxillary digital model wasregistered with the corresponding maxillary teeth captured by theanterior scan (b).  The step was repeated for the complete mandibulardigital model (c).  This resulted in the digital models being in thecorrect relationship with respect to each other.38roughly align the models.  This was carried out separately for the maxillary andmandibular virtual models using the virtual bite registration as the fixed image. After this was accomplished, the images could then be aligned using the globalregistration function.  This was again done in separate steps for the maxillary andmandibular models by “pinning” (Geomagic function) the virtual bite registration inplace so that the only translation and rotation was of the virtual models.  These finalaligned images were not merged as was performed in creation of the final modelsbut were left as separate images (Figure 6).Figure 6. Two views of 3D digital maxillary and mandibular aligned casts.  Sincemountings were arbitrary there was not always good intercuspation of teeth, but this wasnot important since the key measurement was similarity between occlusal contactsmarked digitally and those marked on the actual casts. 39 3.5  Detection of ContactsUnlike other studies that calculated contacts as regions of the maxillary andmandibular casts that were within a certain proximity, in the present study contactswere identified as areas where the two virtual models were actually in contact(Delong et al., 2007; Delong et al., 2002a) .  These areas could be visualized bycutting the top of the maxillary virtual model and the bottom of the mandibularmodel off and looking at the inside occlusal surface of the virtual models.  Bymaking the two models contrasting colours these contacts were readily visible.Actual contacts were determined on the articulated models using a new pieceof Accufilm II articulating film (Parkell Products Inc., Farmingdale, NY) which hasa thickness of 21um for each mounting and firmly tapping the maxillary modelagainst the mandibular model three times.  Marked contacts were then checked with8ìm shimstock (Hanel Shimstock, Almore International Inc., Portland, OR).  Digitalphotographs were taken of the marked models using an Olympus Evolt E-300 with a50mm Olympus macro lens and ring flash. (Olympus Imaging America, CenterValley, PA)For the analysis, no refinement of the alignment of the models was performedeven in cases that showed no occlusal contacts. This was because one of the goals ofthe study was to determine how predictably the virtual models would replicate theactual clinical situation without a knowledge of the clinical situation beforehand(Figure 7).403.6  Independent Examiner ValidationTwo additional experienced dental clinicians reviewed the photographs of themarked dental casts and the images of the occluding digital models in order to verifythe findings of the principal investigator.  Each one recorded which regions theythought represented clinical contacts from photos of the marked epoxy casts as wellas from the images of the digital casts.  The examiners also compared eachcorresponding epoxy and digital set of images to determine which contacts theyconsidered, in their clinical judgement, to be coincident between the articulatingpaper markings and the digital contacts.  This data was recorded on a standard formFigure 7. Representative pairing of epoxy (a) and corresponding digital model (b). Contacts are indicated by arrows on both images.  Alignment of the digital modelsresulted in four out of the five contacts being visible digitally.  The only contact notindicated on the digital model that is present on the epoxy model is the 15 buccal cusp tip(c).  This pairing represent good agreement between actual and digital contacts.41for both examiners.  All examiners were blind to the results of the other examiners atthe time of recording.3.7  Data AnalysisThe location of all occlusal contacts noted using articulating film andshimstock on the actual mounted models were recorded, as were all the digitallyproduced contacts. Sensitivity and specificity were calculated twice, first usingshimstock as the standard, and then using articulating paper as the standard. Contacts were considered similar based on location and clinical judgement. Location of contacts was determined by dividing the occlusal surfaces on the archinto 56 regions as demonstrated by Delong et al. (2002a) (Figure 8).  Eachmaxillary/mandibular contact pair was recorded as a single contact for analysispurposes since using two values will artificially increase the number of data pointsdespite the fact that each point in a contact pair is not independent.  423.8  Statistical AnalysisVirtual contacts from the digital models were compared to the standardshimstock and articulating paper contacts according to sensitivity, specificity,positive predictive value, and negative predictive value (Figure 9).  All data fromeach group (digital, shimstock, or articulating paper) was combined to give a singlevalue comparing total digital contacts to shimstock and articulating paper.  This wasdone because the models in each series were identical, and the only difference wasFigure 8. Anatomic contact regions.  Contacts were defined qualitativelybased on location on tooth anatomy.  Region and tooth number identifiedcontacts (arrow).  B, Buccal; C, Central; L, Lingual; M, Mesial.43the mounting.  Contact location was based on anatomic regions as described andused by Delong et al (2002a, 2007).   Percent positive agreement between examinersfor each set of images was calculated from the data gathered from the additionalindependent examiners.                                                                                                                                               positive (occlusal               negative (occlusal                        contact truly exists)          contact truly not present)positive (occlusal contact appears on new test - digitalalignment) true positives         a      false positives bnegative (occlusal contact does not appearon new test - digitalalignment)         c         false negatives d   true negativesFigure 9.  Comparison of the new test to the gold standard.  In the present study thedigital contacts were always the test and either shimstock or articulating film marks wereconsidered the standard.  Sensitivity = a/a + c, specificity = d/b + d, positive predictivevalue (PPV) = a/a + b, negative predictive value (NPV) = d/c + d. 444 RESULTSThe mean number of contacts per model pair recorded by shimstock,articulating paper, and digital, were 2.7±1.34, 4.9±1.58 and 3.3±2.53 respectively. The total number of contacts for each recording technique and each mounting pair,as well as the number of contacts that were considered coincident between thevarious recording techniques are shown in table 1.   45CastMountingPairShimstock(total numberof contactsrecorded bymethod)Articulatingpaper (totalnumber ofcontactsrecorded bymethod)Digital (totalnumber ofcontactsrecorded bymethod)Number ofcontactscoincidentbetween allthreemethodsNumber ofcontactscoincidentbetweendigital andarticulatingpaper onlyNumber ofcontactscoincidentbetweenshimstockandarticulatingpaper onlyNumber ofcontactsunique toonly onerecordingmethod111110002364112433663104427924045241101261410112755440108462202292510122104543012TOTALS2749331781022Table 1.  The table demonstrates the number of contacts present on each cast mounting pair and the number of contacts that wereconsidered identical between recording methods.  Since each mandibular contact had a coinciding maxillary contact, eachmandibular/maxillary pair was considered as a single contact. There is no column for coincident contacts between digital and shimstockmethods only because all shimstock contacts were also identified with articulating paper. 46Using shimstock as the standard and digital markings as the test thespecificity was 97%, the sensitivity was 63% and the negative predictive value andpositive predictive value were 98% and 52% respectively.  When articulating papermarkings were used as the standard and digital contacts as the test the correspondingvalues were 98%, 54%, 96% and 76% respectively. (Table 2)Standard Specificity Sensitivity PPV NPVShimstock         0.97 0.63          0.52 0.98Articulatingpaper 0.98 0.54 0.76 0.96Table 2.  Comparison of occlusal contacts determined using virtual models and twotraditional techniques.  The table shows the specificity, sensitivity, positivepredictive value (PPV), and negative predictive value (NPV) of the digital contactmethod when using the two standards (shimstock and articulating paper) used in thepresent study.  Digital contacts were always used as the test for calculations. Overall positive examiner agreement for articulating paper marked casts was83%, for digital casts was 86%, and for coincident contacts between actual anddigital casts was 85%.  Pair wise agreement between examiners is shown in table 3.47Pairings ArticulatingpaperDigital CoincidentA-B 84% 89% 89%A-C 94% 89% 92%B-C 93% 94% 92%Table 3. Percent (positive) examiner agreement.  The table shows the pair-wisepercent positive agreement for the three examiners for the two contact methods used. In addition percent positive agreement between examiners for the contactsconsidered to be coincident between the two marking methods is shown.  The totalnumber of contacts for all cast pairs in each marking method were combined tocalculate percent agreement. After analyzing the data it was noted that some mountings had very goodagreement of occlusal contacts between the epoxy and digital models while otherdigital models showed almost no contacts.  It was decided to test whether or notbetter agreement could be obtained by closing the digital models through a path ofrotation representing the motion of the articulator.  This step was only carried out onone set of mountings where the digital model showed poor agreement with theepoxy model due to lack of contacts present digitally.  This step was similar to themanual adjustments made in other studies (Delong et al. 2007; Delong et al., 2002a),except instead of simply moving the mandibular cast perpendicular to the maxillarycast, a rotational axis was created and the mandibular cast was rotated into themaxillary cast in an attempt to simulate a jaw closing motion.  The axis wasdetermined by using a coordinate measuring machine (MicroScribe-3DX) and48marking points at three locations on the mandibular teeth and at the two points alongthe hinge axis of the articulator.  These points were imported into Rhino 3Dmodeling software and aligned with the mandibular cast, which was already alignedwith the maxillary cast from the original registrations.  An axis of rotation wascreated through the two points from the articulator hinge axis, and the mandibularcast was rotated through various known angles.  The original alignment hadproduced only a single virtual contact compared to the five contacts achieved witharticulating film.  A rotation of only 0.15 degrees achieved all five contacts on thevirtual casts with no false positive contacts.  This amount of rotation represented atranslation of just over 0.2mm at the anterior teeth and approximately 0.14mm at theposterior teeth (Figure 10). 49Figure 10. Maxillary cast with contacts indicated by arrows (a).  Digitalcast of same mounting showing only one contact present (arrow) (b).  Samedigital cast after being rotated through axis representing articulator hingeaxis.  All true contacts (arrows) are now present with no false positives (c). 505  DISCUSSION Acquisition of digital models is the first step in creating a virtual patient, andthis can be accomplished with a variety of scanners.  The accuracy and resolution ofthe scanner chosen will affect any results further down the line.  In this study, thescanner used was a Konica Minolta 910 with a stated accuracy in the X, Y, and Z axesof ±0.22mm, ±0.16mm, and ±0.10mm respectively.  Delong et al. (2003, 2007, 2002a)used a Comet 100 optical digitizing system with a stated accuracy of ±0.040mm. These accuracies are for a single scan under ideal conditions, so the accuracy of thefinal 3-D model would also depend on the number of scans required to produce thefull model, since increasing the number of scans results in a decrease in overallaccuracy of the model (Brusco et al., 2007).  Regardless of the number of scans, oneof the limitations in this study was the accuracy of the scanner used.  Since themanufacturer’s stated accuracy is for a single scan, the creation of a complete 3-Dmodel from four scans will result in even less accuracy, and therefore it can beassumed that the average point accuracy for the final models in the present study isless than 0.22mm.  Although for linear measurements this value may be adequate, itcould potentially represent either large interocclusal gaps between the maxillary andmandibular teeth when in fact a contact should be present, or conversely, pass throughbetween the maxillary and mandibular teeth when in fact no contact is present. Shimstock contacts represent gaps between the maxillary and mandibular teeth of 8ìmor less, and articulating film represent gaps of 21ìm or less.  These values are at least51an order of magnitude finer than the accuracy of the scanner used in the present study. Despite this large discrepancy, the digital contacts correlated moderately well withboth the shimstock and the articulating film contacts.  Use of a higher accuracyscanner may have resulted in improved values in this study while still maintaining thesame protocol.  Konica Minolta has recently released a new 3-D laser scanner (KonicaMinolta Range 7) with a stated accuracy of 40ìm in all three axes and a precision of4ìm.  This represents an accuracy improvement of roughly five times over the scannerused in the present study.False negative values will directly affect the sensitivity value of a test.  Foracceptance as a diagnostic test, sensitivity should be >0.70 and specificity >0.90(Delong et al., 2002a).  Sensitivity values in this study fell just short the value neededfor a test to be considered clinically  acceptable, although the sensitivity when usingshimstock as the standard did approach the needed value.  This differs from the resultsof Delong et al. (2002a) that found sensitivities for different alignment protocols torange between 0.76 and 0.89.Despite the fact that the specificity values in the present study reached levelsthat are considered acceptable for new clinical tests, the value of this result in thepresent study is limited.  This is because specificity represents the ability of the test toidentify true negatives.  Since the number of contacts present on each model pair wasvery small, ranging from one to nine, the total number of regions without contacts wasalways much greater than those with contacts.  This results in the numerator being52consistently large in comparison to the denominator in the calculation of specificity asshown in figure 9.     There are several reasons that the results in the present study were just short ofthe necessary sensitivity value whereas the results of Delong et al. (2002a) did surpassa sensitivity of 0.70.  The first being the accuracy of the scanner used as mentionedpreviously.  The second is related to the way contacts were recorded.   Comparison ofthe digital contacts was made to two different standards since in occlusal markingthere is no universally accepted gold standard (Delong et al., 2002a).  Shimstock andarticulating film were both used as standards since both are commonly used in clinicaldentistry.  The shimstock thickness was 8ìm and the thickness of the articulatingpaper was 21ìm.  Since the greatest accuracy of the scanner was 0.1mm in the Z axisand this represents 5X the thickness of the articulating paper, it is possible that areasof the actual model that should be within the limits to be marked with articulatingpaper are lost during the digitization process, and the result is a false negative on thealigned digital models.  This type of error would be expected to be even greater whencomparing to shimstock since it is less than 1/10th the thickness of the upper end ofaccuracy of the scanner.  This was not found in this study since the sensitivity wasactually slightly greater (0.63) when shimstock was used as the standard compared toarticulating paper (0.54).  The reason for this may be due to how contacts were determined on the digitalmodels.  Unlike other studies (Delong et al., 2007; Delong et al., 2002a) and software53(Orthocad) that use a tolerance range to determine contact areas, in this study a contactwas only recorded digitally if the actual 3D maxillary and mandibular surfaces cameinto contact.  This technique has not been used previously, and there are somelimitations to it but also some benefits.  If the maxillary and mandibular surfaces areeven within 1ìm but not contacting no contact will show up on the digital model butclinically if only 1ìm space exists between teeth, either intraorally or mounted on anarticulator, they will hold an 8ìm thick piece of shimstock, and therefore the digitalmodel will show a false negative.   In one study, Delong et al. (2002a) used a range of 0.050mm of separation todetermine occlusal contacts.  This meant that any areas of the maxillary andmandibular digital models that were within 0.050mm of each other were marked ascontacts.  This value was chosen because it was slightly larger than the accuracy of thescanner being used.   In another study, they used a value of 0.350mm as the tolerancerange (Delong et al., 2007).  This second value that was used seems clinicallyinappropriate since it is over 40 times the thickness of a piece of shimstock.  Allowing for a range of separation in the present study would likely haveresulted in a greater number of contacts recorded on the digital casts, but it is notknown if these would have been true contacts or false positive contacts.  Also, in atrue clinical situation, allowing for a tolerance range is justified since, unlike stonemodels, teeth are not rigidly positioned.  Due to the periodontal ligament attachment,teeth can move both horizontally and vertically in the socket.  This movement ranges54from 25-100ìm axially and up to 200ìm in a horizontal direction for healthy teeth(Kim et al., 2005).  The advantage of not using a tolerance range, especially if shimstock is beingused as the standard to compare to, is the decrease of false positives that can becreated by using an inappropriately large tolerance range.  Future studies couldovercome this problem by using a range of tolerances from zero, as was used in thisstudy, up to some arbitrary value, and compare the results in order to determine, in the context of their system, what the ideal range of tolerance would be for determiningcontacts.        One of the goals of the method used in this study was to limit the amount ofoperator input necessary to output results.   An additional goal was that the methodwould be able to predict the contacts without prior knowledge of location or number. Both these goals were achieved to varying degrees.  Some operator input is necessarywith any system, whether that be simply placing the stone model in a scanner andpushing a button or manually scanning each view of the casts, aligning the scans, andorienting the maxillary and mandibular casts.  In the method described here, theoperator is required during each step to push a button, but the software carries out thevital tasks.  This is an important point because a digitizing system that requires skilledoperators will increase the cost to the dentist and limit the use of the system within adental practice.  In addition, a digital technique that requires significant user input anddecision making will introduce bias into the final outcome, a system that is automatic55limits the user bias and provides for a more standardized output.  A major differencebetween the present study and that of Delong et al. (2007, 2002a) is the elimination ofany manual adjustments to the alignment of the casts.  Delong et al. (2007, 2002a)manually refined the alignment of the maxillary and mandibular casts after automaticalignment in order to correct for separation or excessive penetration beyond thetolerance range used by moving the mandibular virtual cast perpendicular to themaxillary cast.  In addition, the positions of the contacts were visible on the 3-Dmodels, whereas in this study the contact points were not visible on the 3-D modelsbecause only one set of scanned models was used for all 10 mountings and they werescanned before any mounting or occlusal marking was carried out.  The use of automatic alignment and a single set of unmarked casts eliminatedthe bias that may have been introduced if manual adjustments were made.  It alsoprovides a system that can be carried out by anyone with basic knowledge of thehardware and software used.  In this study all the tests were carried out by a singleinvestigator, who also developed the method.  An interesting and valuable test wouldbe for someone unfamiliar with the system to be trained in the basic steps necessary tocreate and align 3-D models to see if the outcome is dependent on the skill orknowledge of the operator.  Ideally, the more automatic the method is, the less theresult will depend on the operator, and therefore, the more consistent the results willbe between operators.56For this reason the additional testing using the created path of rotationdetermined from the articulator was only carried out on a single model set as a pointof interest and as a test to see whether or not manual manipulation would improve theresults in this method.  Although this test was not carried out on all the mountings, itdoes demonstrate within the methods described here that if manual adjustments arecarried out on the automatically aligned digital models, the comparison between actualand digital contacts may improve.  The reasons that this procedure was not carried outon all the models are varied.  First, not all models had poor agreement betweenarticulating film and digital contacts.  In fact, some mounting sets had perfectagreement after automatic alignment, and therefore, any manual adjustments could nothave improved the agreement.  The second reason for only one application of thistechnique relates to the first in that it increases the technique sensitivity of the method,and since it may only be needed on some models, the decision to make manualadjustments introduces bias into the method.  The decision to do this step was onlymade after comparison of contacts between the epoxy and the digital models and onlybecause the actual contact points were known to be different from those representeddigitally.  In a clinical situation where only digital models are obtained with, forexample an intraoral 3D scanner, and there is no recording made of the actual occlusalcontacts for comparison, the clinician would not know if manual adjustment ofdigitally aligned models was necessary or not.  Therefore, adding it as a step withinthe methods when one is assessing accuracy of digital model alignment may result in57improved results but represents a step which is clinically inappropriate.  Additionally,although it is a trivial step to determine the hinge axis on articulator mounted casts, itis not trivial in the human.The use of virtual dental models leads to the idea of a “virtual articulator”where the aligned casts can be “mounted” and moved to represent the patient’smovement just as is done on a traditional articulator.  The static alignment of dentalcasts has been demonstrated by several methods by a variety of authors, including this one, and appears to be quite reliable.  Introduction of dynamic capabilities to themodels involves several new challenges that have been approached in different ways.  Mandibular movements involve both translations and rotations, and the easiestway to incorporate these movements into a dynamic virtual model is to program themin.  This approach could truly be called a “virtual articulator” because the geometriesand constraints of an actual articulator are simply programmed into a softwarepackage into which the 3-D virtual dental models can be “mounted”(Maruyama et al.,2006).   The advantage of this type of system is the ability to visualize contact pathsand locations during dynamic processes such as excursive and protrusive movements. This system could also be used for automatic designing of interference-freerestorations.  Limitations to this type of system are similar to the limitations with astandard articulator.  Most notably, the settings are somewhat arbitrary and will notexactly match the movements of the patient.  In addition, the virtual dental modelsneed to be aligned with respect to each other statically before being inserted into the58virtual articulator.  This is where the process described in the present study could beapplied to this type of virtual articulator.  Just as stone models need to be aligned withrespect to each other using a bite registration or hand articulation, digital models needto be aligned with respect to each other in a static relationship before being mountedin a virtual articulator.  If the static relationship is not established and accuratebetween the digital models, any dynamic relationship that is produces on a virtualarticulator will also be inaccurate. Various systems that allow for 3-dimensional recording of the patients actualjaw movements have been developed (Bisler, Bockholt, Kardass, Suchan, & Voss,2002; Bisler, Bockholt, &Voss, 2002; Fang & Kuo, 2008; Gartner & Kordass, 2003). The most reported system uses the Jaw Motion Analyzer from the Zebris company torecord patient jaw movements (Bisler et al., 2002; Bisler et al., 2002; Gartner &Kordass, 2003).  This system uses ultrasound to measure the position of three trackingsensors which are attached to the lower jaw.  The position of these trackers is alsoused for the alignment of the virtual dental models into the dynamic path.  As with thepreset virtual articulator, this system allows for visualization of dynamic occlusalcontact paths in any jaw movement.  The advantage is that the movement of the modelrepresents the patient’s actual movements.  Although this system has been describedin several papers, the accuracy of the system has not been reported, nor has the clinicalpracticality of the system.  59As mentioned above, any articulating system, whether bench top or digital,requires an accurate static relationship between the maxillary and mandibular modelsprior to mounting in order for the dynamic relationship produced by the articulator tobe accurate.  The technique described in the present study outlines a technique thatrequires minimal user input, uses commercial 3D imaging hardware and software thatexports files in common multi-platform formats, and achieves near clinical sensitivitydespite the limited accuracy of the scanner for statically aligning digital models.  An important limitation that is often mentioned in regards to this area of dentalresearch is the rigidity of the system. (Delong et al., 2002a;  Maruyama et al., 2006). Just as stone dental models and mechanical articulators are rigid, so too are the 3-Dvirtual models used in all the studies.  When using virtual models for orthodontictreatment planning, the rigidity of the system is not a concern but if the goal isocclusal assessment or fabrication of a fixed restoration, then the lack of toothmovement and jaw flexure could affect the results (Delong, Ko, Olson, Hodges, &Douglas, 2002b; Korioth & Hannam, 1994).Despite the current limitations with 3D digital imaging, it continues to increasein use in clinical dentistry.  Any digital system that will involve articulation of teethwill require accurate static alignment of the maxillary and mandibular casts as one ofthe primary steps.  This step will be necessary if the digital models are used forfabrication of machined restorations, recording static occlusal contacts, or measuringdynamic contacts between teeth.   The method describe in the present study can act as60a blueprint for a formal method for testing any new digital system that provides astatic or dynamic relationship between digital maxillary and mandibular models.  It is perceivable that it will be possible to incorporate into the “virtualarticulator” individual viscoelastic properties for each tooth that would be measuredclinically, as well as a measure of the flexure of the mandible.  Add to this collisionproperties that would not allow the maxillary cast to penetrate the mandibular cast, butinstead allow for displacement of teeth depending on their individual viscoelasticproperties, and provide recordings of forces experienced by the teeth and the resultwould be more appropriately called a “virtual patient” instead of a “virtualarticulator.” 616  FUTURE DIRECTIONSSeveral directions of research could follow from the present study.  Initially,the most beneficial direction would be to try to obtain results with clinicallyacceptable sensitivity.  The sensitivity results in the present study fell just below thelevel considered acceptable for new clinical tests.  Performing the identical procedurewith the higher accuracy 3D scanner, such as the Konica Minolta Range 7, may resultin acceptable sensitivity levels.  An additional important step would be to determinethe level of reproducibility between different operators provided by the presentmethod.  For this method to be useful clinically, it should provide similar resultsregardless of the individual operating the system.Once the method is considered clinically consistent between operators andconsistently achieves sensitivity levels that are clinically acceptable the next step indeveloping the method would be to incorporate movement of the digital casts thatrepresents the patient’s own movements in order to visualized dynamic occlusalcontacts.627  CONCLUSIONSThe present study was a demonstration of acquisition and alignment of digital3D dental models.  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