Dentistry, Faculty of
DSpace
UBCV
Shivaraman, Visalakshi
Master of Science - MSc
University of British Columbia
Objective: To compare the marginal fit and internal adaptation of novel additively manufactured (3D printed) ceramic filled hybrid composite definitive crowns and milled monolithic zirconia milled crowns. Materials & Methods: A single die STL file was used to create a congruent base with 6 points of reference using Ivoclar’s CAM 5 software and milled. Fourteen samples were manufactured for each group (n=14): milled zirconia (MI) and printed (PR). The e.max ZirCAD crowns were milled and sintered using a fast-sintering protocol. The VarseoSmile Crown Plus crowns were printed using a Sprintray 3D DLP printer. The crowns of both groups were ‘cemented’ using ultra-light body PVS to replicate the misfit space using a standard load. Dual scan protocol was used to digitize the misfit space and saved as STL files. Geomagic Control X 2020 was used to assess the misfit of the marginal gap (MG), Occlusal surface (OC) and axial surface (AX) of each sample in both groups. A statistical software program (IBM SPSS Statistics) was used to calculate the means and standard deviations of the internal (AX and OC) and marginal discrepancies (MG) for each group. Normality distribution was confirmed and assumption of equality of variances was tested using Levenè’s test. Since the data was not normally distributed and Levenè’s test was significant, a non-parametric Mann-Whitney Test was used to analyze the data. Results: The mean marginal gap (MG) between the two groups was not found to be statistically significant (p>0.05). The mean discrepancy in the internal adaptation at the axial surface (AX) and occlusal surface (OC) was found to be higher in the PR group compared to the MI group. This difference was statistically significant for both AX and OC measurements. Discussion and Conclusion: 3D printed VarseoSmile Crown Plus crowns have similar marginal fit when compared to milled e.max ZirCAD zirconia crowns. However, the milled e.max ZirCAD zirconia crowns showed improved internal adaptation when compared to the 3D printed VarseoSmile Crown Plus crowns.
IN VITRO COMPARISON OF MARGINAL AND INTERNAL FIT OF 3D PRINTED DEFINITIVE CROWNS AND CAD CAM MILLED CROWNS by Visalakshi Shivaraman BDS, Maharashtra University of Health Sciences, 2017 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Craniofacial Science) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) April 2023 © VISALAKSHI SHIVARAMAN, 2023 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, a thesis entitled: IN VITRO COMPARISON OF MARGINAL AND INTERNAL FIT OF 3D PRINTED DEFINITIVE CROWNS AND CAD CAM MILLED CROWNS submitted by Dr. Visalakshi Shivaraman in partial fulfillment of the requirements for the degree of Master of Science in Craniofacial Science Examining Committee: Dr. Vincent Lee, Assistant Professor of Teaching, Department of Oral Health Sciences, UBC Supervisor Dr. Nesrine Mostafa, Clinical Associate Professor, Department of Oral Health Sciences, UBC Supervisory Committee Member Dr. Ross Bryant, Assistant Professor, Department of Oral Health Sciences, UBC Supervisory Committee Member Dr. Bingshuang Zou, Associate Professor, Department of Oral Health Sciences, UBC Supervisory Committee Member iii Abstract Objective: To compare the marginal fit and internal adaptation of novel additively manufactured (3D printed) ceramic filled hybrid composite definitive crowns and milled monolithic zirconia milled crowns. Materials & Methods: A single die STL file was used to create a congruent base with 6 points of reference using Ivoclar’s CAM 5 software and milled. Fourteen samples were manufactured for each group (n=14): milled zirconia (MI) and printed (PR). The e.max ZirCAD crowns were milled and sintered using a fast-sintering protocol. The VarseoSmile Crown Plus crowns were printed using a Sprintray 3D DLP printer. The crowns of both groups were ‘cemented’ using ultra-light body PVS to replicate the misfit space using a standard load. Dual scan protocol was used to digitize the misfit space and saved as STL files. Geomagic Control X 2020 was used to assess the misfit of the marginal gap (MG), Occlusal surface (OC) and axial surface (AX) of each sample in both groups. A statistical software program (IBM SPSS Statistics) was used to calculate the means and standard deviations of the internal (AX and OC) and marginal discrepancies (MG) for each group. Normality distribution was confirmed and assumption of equality of variances was tested using Levenè’s test. Since the data was not normally distributed and Levenè’s test was significant, a non-parametric Mann-Whitney Test was used to analyze the data. Results: The mean marginal gap (MG) between the two groups was not found to be statistically significant (p>0.05). The mean discrepancy in the internal adaptation at the axial surface (AX) and occlusal surface (OC) was found to be higher in the PR group compared to the MI group. This difference was statistically significant for both AX and OC measurements. iv Discussion and Conclusion: 3D printed VarseoSmile Crown Plus crowns have similar marginal fit when compared to milled e.max ZirCAD zirconia crowns. However, the milled e.max ZirCAD zirconia crowns showed improved internal adaptation when compared to the 3D printed VarseoSmile Crown Plus crowns. v Lay Summary Digital processes such as intraoral scanning, desktop scanning, CAD CAM milling and 3D printing are clinically acceptable and routinely used ways to produce a single dental crown. Amongst other factors, marginal gap of permanent crowns affects its clinical longevity. Monolithic zirconia is a commonly used material to mill prostheses owing to its desirable mechanical properties, aesthetic finish, and biocompatibility. Zirconia has a wide range of applications in dentistry, especially dental crowns with excellent marginal adaptation. A novel resin composite material from Bego for 3D printing of permanent crowns has been tested in this study comparing it to milled monolithic zirconia crowns. The marginal fit was tested to check if it is clinically acceptable and how it compares to milled zirconia crowns. The result concluded that crowns tested in both groups have similar marginal fit. vi Preface This research project was done under the direct supervision of Dr. Vincent Lee. Committee members include Dr. Nesrine Mostafa and Dr. Ross Bryant. The research idea was conceptualized by Dr. Visalakshi Shivaraman and Dr. Vincent Lee. Provision of zirconia puck for milling and resin for 3D printing, digital design and milling of the master die, milling, printing and post-processing of samples of both groups was done by Mr. Angus Barrie (CLT) from BC Perio-BC Digital Design Studio. The author performed the dual scanning, testing, subsequent data collection, measurements, and statistical analysis independently. The study did not require approval by the UBC Research Ethics Board since it did not involve the use of human subjects, animals, or bio-hazardous materials. vii Table of Contents Abstract .......................................................................................................................................... iii Lay Summary: ................................................................................................................................ v Preface ........................................................................................................................................... vi Table of Contents .......................................................................................................................... vii List of Tables .................................................................................................................................. x List of Figures ................................................................................................................................ xi List of Abbreviations ................................................................................................................... xiii Acknowledgements ..................................................................................................................... xiv Dedication ..................................................................................................................................... xv Chapter 1: Introduction ................................................................................................................... 1 1.1 Digital CAD/CAM Workflow .............................................................................................. 1 1.2 Subtractive Manufacturing ................................................................................................... 2 1.3 Monolithic Zirconia .............................................................................................................. 3 1.3.1 Crystalline structure and phase transformation of zirconia ........................................... 3 1.3.2 Multilayered Monolithic Zirconia Crowns .................................................................... 5 1.4 Additive manufacturing ........................................................................................................ 6 1.5 Ceramic filled hybrid composite .......................................................................................... 7 1.6 Crown Fit .............................................................................................................................. 8 1.6.1 Terminology .................................................................................................................. 8 viii 1.6.2 Acceptable Marginal gap ............................................................................................. 10 1.6.3 Methods to measure marginal gap ............................................................................... 12 1.6.4 Factors affecting marginal fit ....................................................................................... 17 Impact of Number of Measurements/ Sample Size .............................................................. 17 Influence of Method of Fabrication ...................................................................................... 17 1.7 Summary of available literature and study rationale .......................................................... 18 Chapter 2: Research Question and Specific Aim ......................................................................... 19 2.1 Research Question .............................................................................................................. 19 2.2 Definitive Aim .................................................................................................................... 19 2.3 Research Hypothesis ........................................................................................................... 19 Chapter 3: Materials and Methods ................................................................................................ 20 3.1 Design and fabrication of the master die and crowns ......................................................... 20 3.1.1 Design and milling of master die ................................................................................. 20 3.1.2 Sample Preparation ................................................................................................... 23 3.1.3 3D printing of ceramic filled hybrid crowns ............................................................... 25 3.2 Sample testing and Data acquisition ................................................................................... 26 3.2.1 Dual Scan Technique ................................................................................................... 26 3.2.2 Sample testing .............................................................................................................. 27 3.3 Digital measurements using Geomagic .............................................................................. 31 3.4 Statistical Analysis ............................................................................................................. 33 3.4.1 Sample size calculation ............................................................................................... 33 3.4.2 Statistical Tests ............................................................................................................ 34 ix Chapter 4: Results ......................................................................................................................... 35 Chapter 5: Discussion ................................................................................................................... 39 Chapter 6: Conclusions ................................................................................................................. 43 Chapter 7: Limitations, clinical implications, and future direction .............................................. 44 Bibliography ................................................................................................................................. 46 x List of Tables Table 1 United States Public Health (USPHS) criteria for marginal integrity. ............................ 13 Table 2 CAD parameters for crown design .................................................................................. 23 Table 3 Comparison of mean marginal discrepancy between printed (PR-Group A) and milled (MI-Group B) respectively at margin of crown (MG) in mm ...................................................... 36 Table 4 Internal discrepancy between printed method (PR-Group A) and Milled method (MI-Group B) respectively at the mid portion or axial surface of the crown in mm ........................... 36 Table 5 Comparison of internal discrepancy between Printed method (PR -Group A) and Milled method (MI-Group B) respectively at top part of crown (OC) in mm. ........................................ 37 xi List of Figures Figure 1 Complete digital workflow with an optional conventional workflow for fabrication of a single crown. .................................................................................................................................. 1 Figure 2 Fit definitions according to Holmes et al. ....................................................................... 9 Figure 3 Master zirconia die with 6 reference points on the base ................................................ 21 Figure 4 A. Half arch PMMA model with removable die B. Zirconia die with congruent base designed and merged with the original model C. Margin identification of the new zirconia die D. STL file of the resultant milled zirconia master die E. Design of final crown ............................. 22 Figure 5 Representative picture of e.max ZirCAD Prime Shade A1 multilayered monolithic zirconia with one milled and pre-sintered crown. ........................................................................ 24 Figure 6 Milled monolithic zirconia sample after sintering without finishing and polishing ....... 24 Figure 7 BEGO VarseoSmile Crown Plus bottle shade A1 - photopolymerizable resin .............. 25 Figure 8 Printed ceramic filled hybrid crown without finishing and polishing ............................ 26 Figure 9 A. Ney Surveyor with abutment fixed to the platform B. Putty holding the die in a standard position on the removable scanner plate ........................................................................ 28 Figure 10 Standard Load on the Ney surveyor for testing ............................................................ 29 Figure 11 Sample held in place with a standard load using surveying arm .................................. 30 Figure 12 PVS misfit replica on zirconia master die .................................................................... 31 Figure 13 Three regions corresponding to marginal, axial, and occlusal surface created on the reference die. ................................................................................................................................ 32 Figure 14 A. MG misfit B. OC misfit C. AX misfit D. Overall 3D misfit with tolerance color E. Overall 3D misfit without tolerance color. The color-coded map shows the amount of discrepancy at every part of the crown. Green represents values within the tolerance limit of 120 µm. ............................................................................................................................................... 33 xii Figure 15 Showing range and outliers in each group for each measured entity, MG, AX and OC. ..................................................................................................................................................... 38 xiii List of Abbreviations 3DP 3D printed AX Axial gap AMD Absolute Marginal Discrepancy CAD Computer-aided design CAM Computer-aided manufacturing DST Dual-scan technique IG Internal gap IQR interquartile range MG Marginal gap MI Milled μm Micrometer MZ Monolithic zirconia OC Occlusal gap PR Printed PVS Polyvinylsiloxane STD Standard deviation STL Standard Tesellation Language USPHS United States Public Health Service VSCP VarseoSmile Crown Plus VST VarseoSmile Temp Y-TZP Yttrium-Tetragonal Zirconia Polycrystals xiv Acknowledgements I would like to offer my sincere gratitude to: Dr. Vincent Lee for his guidance and patience through this process. Thank you for holding me to higher standards, teaching me the value of working independently. Dr. Nesrine Mostafa for rising to the occasion majestically by providing timely help. For her invaluable information on the research process, resourcefulness in training and understanding the measurement software and answering my endless queries from inception. Dr. Ross Bryant who with his thought-provoking questions fostered in me critical thinking and the qualities of a researcher. He taught me patience, broadened my perspective, and penetrated my intellect with his pearls of wisdom at every interaction. Mr. Angus Barrie at BC Digital Dental Studio for his time and sharing his vast, rich experience with me. For providing the right materials for the research, his expertise in digital dental technologies, creation of the samples and allowing me to utilize his lab to carry out the testing and scans myself. He taught me practicality and the value of simplifying a process to optimize the output. Josh Hines at Rapid 3D for Geomagic Control X 2020 training. Nikolas Krstic from ASDA for help with Statistics. The staff and lab technicians at BC Perio, West Broadway for their warmth and hospitality. xv Dedication To my parents Mekala Shivaraman and Shivaraman Srinivasan, for tirelessly supporting me through years of education and making innumerable personal sacrifices so I could have a better life. For standing up for me when I couldn’t, instilling values in me which kept me grounded and not letting any hardship stand in the way of my academic success. Your prayers and blessings have helped me sail through even the direst of situations. To Anu, my brother, for never failing to lift my spirits and brining a smile to my face. Without his encouragement and motivation, my higher education would never been possible. To my grandparents in their time and after, for their love, support, and prayers. I hope I made you proud. To Adhi, you are my true north. Thank you for your patience through my mental and emotional roller coasters. Your love gave me hope and made me a better person in the face of adversity. To Vivek, Prerana, Sakshi, Mai, Rav, Gina and Faisal for being true friends. Thank you for always having my back 1 Chapter 1: Introduction 1.1 Digital CAD/CAM Workflow A complete digital dentistry workflow is gaining momentum to fabricate indirect restorations, including crowns, owing to the availability of open systems of design software, intraoral scanners, milling machines, and 3D printers. (1) We are no longer bound to only one system, and seamless integration of conventional as well as digital processes has also improved workflows for both the dentist and the lab technician (Figure 1). To this effect, significant development of materials has taken place in the last decade from machinable partially sintered blanks and discs to printable hybrid resin-ceramic materials. Newer materials are being introduced to meet the demands of the digital dentistry evolution. (2) Figure 1 Complete digital workflow with an optional conventional workflow for fabrication of a single crown. 2 1.2 Subtractive Manufacturing Subtractive manufacturing is the process of creating a desired geometry by systematically removing material from a preformed standard-shaped block or disc. With the advent of digital dentistry, CAD/CAM has gained momentum to facilitate this process and can be an entirely digital workflow starting with intraoral scans (data acquisition) to designing, and to milling, or it can be created as a last step after an analog workflow up to the stage of creation of a wax-up. Subtractive manufacturing can be accomplished by different types of milling units: 3-axis, 4-axis, and 5-axis. The 4-axis and 5-axis milling units are most often used in dentistry for subtractive manufacturing of a dental crown. The 4-axis milling machines work in X, Y, and Z axes representing the three-dimensional coordinates of a manufacturing space. The machines also have an additional A-axis of rotation along the X-axis. This means that the bur in the milling unit also rotates around an A-axis in addition to linear movement in the X-axis. Examples of 4-axis chairside milling units are the Dentsply Sirona MC XL and Planmeca Planmill 40S. 5-axis milling machines are generally industrial-level units used most in a dental lab. These milling machines also work in the three spatial axes: X, Y, and Z; however, they can rotate along each of the X, Y, and Z, representing the A, B, and C axes respectively. 5-axis milling units can produce more complex anatomical features with higher accuracy than 4-axis milling machines. (3) The first dental lab 5-axis milling machine to be introduced was the Programill One by Ivoclar Digital. Other examples of dental 5-axis milling machines include inLab MCX5 by Dentsply Sirona, PrograMill PM7 by Ivoclar Vivadent and CeraMill micro-IC by Amann Girrbach. 3 1.3 Monolithic Zirconia Zirconium dioxide, when made into a single block or disc, is commonly referred to as monolithic zirconia in dentistry. Monolithic zirconia is gaining popularity as a material for the computer aided design/computer aided manufacturing (CAD/CAM) milling of single crowns owing to its desirable mechanical properties, biocompatibility, and aesthetic appearance. (4) The complete digital workflow for the manufacture of a ceramic or resin crown has been illustrated in Figure 1. The availability and wide application of high precision dental intraoral scanners have made it an acceptable choice for taking an intraoral impression. The file format is STL-Standard Tessellation Language, which is compatible with most CAD/CAM software for the wax-up, designing, and final milling or printing of the desired crown. The materials available for milling come as either un-sintered or partially sintered blocks/discs which need to be heat sintered after milling to achieve its final colour and strength. The reproduction of the intraoral scans as a physical model can happen via 3D printing of a resin cast. The die can either be made on this cast individually or virtually via a CAD software. A virtual wax-up of the crown is made within the CAD software and once approved, the final crown design file can be exported for manufacturing. Physical resin models are generally printed at this stage to be delivered along with the milled or printed crowns to the clinician. If an analog impression has been taken, the impression can be poured in dental stone as per the conventional workflow and then scanned using a desktop scanner at the lab. The scanned cast, which is now represented as a digital file, can be used to follow the rest of the steps in the digital workflow, as illustrated in Figure 1. 1.3.1 Crystalline structure and phase transformation of zirconia Zirconia as a material exists in three forms: the monoclinic phase (m), which is stable at room temperature up to 1170 °C; above which it converts to the tetragonal phase (t), which is stable up 4 to 2370 °C; and then transforms to the cubic phase (c), that has its melting point at 2680 °C. At the corresponding temperatures, zirconia converts between the three phases. This two-way phase transformation is accompanied by major changes in mechanical properties, with greater strength at the tetragonal phase, followed by the cubic phase. For dental prostheses, stabilizing zirconia at the tetragonal phase is desirable. However, when cooled to room temperature (inducing the tetragonal to monoclinic phase transformation), it is associated with a 4-5% volumetric expansion that causes severe cracking, ultimately leading to catastrophic failure of the material. Since the most stable phase at room temperature has the least desirable mechanical properties, the addition of dopants is necessitated to stabilize the tetragonal phase despite cooling to room temperature, and so optimize the mechanical properties. (5) This is done through the addition of yttrium oxide (Y2O3), calcium oxide (CaO), or magnesium oxide (MgO) into the ZrO2 crystal structure to form partially or fully stabilized zirconia. (6) The quantity of dopant in molar concentration used in a zirconia is abbreviated as, for example, 3Y-TZP for 3 mol% Y2O3, 4Y-TZP as 4 mol% Y2O3, or 5Y-TZP as 5 mol% Y2O3. (6) At room temperature, this maintains improved mechanical, thermal, and electrical properties and allows for dental applications. The strength of each of the phases decreases with increase in yttria concentration. While the opaquest 3 mol% is the strongest with approximately 85-90% of it being in the tetragonal phase, 5 mol% has approximately 50% in the cubic phase rendering it weaker. The increase in concentration of yttrium from 3 to 5 also results in increased translucency, which allows for more aesthetic restorations that mimic the complex translucent nature of the natural tooth structure. 5 A smaller grain size of at least 100 nm also lends higher translucency to 5Y-TZP. (7) Indeed, there are numerous factors that can affect the opacity of dental zirconia including a high refractive index (2.20), contrast in refractive indices of zirconia matrix and second phase due to impurities (sintering aids), larger particle sizes than wavelengths of visible light spectrum (380-780 nm), and flaws or pore-refractive indices. (8) To reduce the opacity, several approaches have been adopted. These include replacing tetragonal crystals with cubic zirconia particles and increasing the yttria concentration to reduce light scattering at the grain boundary. (9) Advanced techniques have recently been devised to lower the opacity of zirconia by keeping its grain size small, ideally under 100 nm, because the impact of pores is considerably less when the grain size decreases below approximately a micron. (10) The disadvantage of both these approaches is the increase in brittleness and reduction in strength attributed to the increase in cubic zirconia. (10) 1.3.2 Multilayered Monolithic Zirconia Crowns Multilayered monolithic zirconia crowns with different combinations of 3Y-TZP, 4Y-TZP, and 5Y-TZP have been introduced. The need to create a multilayered monolithic zirconia block suitable for milling dental crowns was primarily for improved aesthetics. Alternatively, zirconia materials (discs, blocks, blanks) developed for CAD CAM milling of dental crowns, which do not have this gradual transition in translucency inherently built into the material, may need a layer of veneering ceramic on the facial surface of the crown for improved aesthetics. However, this veneering ceramic introduces a layer of weakness in the dental crown. (11) Chipping of the ceramic layer has been reported as the most common mechanical failure for ceramic layered zirconia crowns. (12) This renders the crown unaesthetic, and it requires repair or replacement, which can be frustrating to both the patient and the clinician. Crowns milled from multilayered monolithic zirconia affords the clinician a more natural-looking dental crown (without introducing a layer of potential mechanical failure) when a gradual increase in translucency is 6 needed from the cervical to the incisal third of the tooth. The first manufacturer to introduce this concept of multilayered zirconia was Katana (Kuraray Noritake, Japan) who introduced a partially sintered block with different translucencies at the incisal, middle, and cervical portions. Katana named these three gradations of translucency as Ultra Translucent Multi Layered (UTML), Super Translucent Multi-Layered (STML), and High Translucent Multi Layered (HTML). (13–15) Ivoclar Vivadent has introduced e.max ZirCAD Prime and e.max ZirCAD Prime Aesthetic with a combination of 3Y-TZP at the coronal third and a gradual transition in translucency to 5Y-TZP at the incisal third. (14) The superior flexural strength and aesthetic outcomes offered by these materials without the drawback of chipping of veneering ceramics makes it a good material of choice. (15) 1.4 Additive manufacturing Additive manufacturing is the technique of adding material layer by layer to create a solid three-dimensional object. It can be done in dentistry either via resins or metals. The most used methods for printing resins in dentistry are stereolithography (SLA) and digital light processing (DLP). These two types of printer methods are used to print study casts, custom trays, surgical guides, clear aligners, and temporary crowns. Both SLA and DLP printing techniques use photopolymerizable resin and light to create models, but they differ in how they operate. SLA printers employ a laser beam that is deflected by a rotating mirror to trace each layer on the resin, resulting in higher resolution and finer detail. On the other hand, DLP printers use light projection to cure an entire layer of the liquid resin at the same time. This method is faster than SLA printing and can print larger parts or multiple models simultaneously but with low resolution. An example of an SLA printer is the Form 2 by FormLabs, and an example of a DLP printer is SprintRay Pro 95 by SprintRay Inc. The materials for printing of crowns are typically 7 resins that do not require sintering, like zirconia, but instead require curing with UV light and cleaning with alcohol. 1.5 Ceramic filled hybrid composite Recently, BEGO GmbH & Co. KG introduced a material, VarseoSmile Crown Plus (VSCP), that was intended for the printing of definitive dental crowns. The desirable properties of a definitive crown are a high flexural strength, optimum surface hardness, low modulus of elasticity, high fracture toughness and optimum marginal fit. There are 2 studies in the literature which tested the mechanical properties of the BEGO VarseoSmile Crown Plus. (16,17) The modulus of elasticity was noted as 4.37-4.69 GPa, and Vicker’s Microhardness as 28.13 HV. The flexural strength of the material is lower than dentin and lowest when compared to machinable composite resin blocks. (16,25,26) The second study looked at the flexural strength and color stability of VSCP under different simulated use conditions – aging in thermal cycles, distilled water, coffee, and UV light when compared to 6 other printable and machinable composite hybrids mostly for temporary use. Perhaps not surprisingly, the outcomes for this definitive version of VSCP were highest in every group except Telio CAD Temp when compared to the other temporary crown materials. (17) Although none of the studies looked at the marginal gap of the crowns thus printed, other studies that looked at the factors influencing marginal fit of 3D printed temporary crowns, found that the orientation of the layers while printing should be along the long axis of application of tensile forces and not across as the flexural strength was found to be higher in the former. (18,19) 8 The exact composition of VSCP is not readily available; however, in comparison a printable temporary version of the crown material introduced by BEGO, VarseoSmile Temp (VST), is reported to have the same filler content as VSCP. The manufacturer classifies the definitive material as a ceramic-filled hybrid resin. Chemically, it is an esterification product of 4.4-isopropylidiphenol, ethoxylated and 2-methyl- prop-2enoic acid. The matrix consists of >50-<75 % wt. Bis-EMA. Silanized dental glass as a filler, methyl benzoylformate, diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide are the additional materials. The total content of inorganic fillers (particle size 0.7 μm) is 30–50 % by mass. (16,17) Interestingly, the chemical composition of both VST and VSCP appear identical. (20) They are both methacrylic acid ester-based materials with the same filler content and manufacturing instructions. However, a study published in October 2022 by Wuersching et al. (20) examined the biological cell response of surrounding tissue to several printable resins and found that VST resulted in more cytotoxicity than VSCP. While outside of the scope of this project, it may indicate that while the chemical constituents may appear similar the materials may indeed be different. (20,21) 1.6 Crown Fit 1.6.1 Terminology Assessment of crown fit is described by the measurement of the marginal and internal gaps of crown restorations. Holmes et al. (22) and Sorenson et al. (23) were among the first to define what marginal gap means. Several studies because of a lack of clarity on the terminology, have either measured only the vertical or only the horizontal marginal gap. Holmes et al. proposed a precise terminology to describe the fit of dental restorations (22) according to Figure 2 as follows: 9 • Internal gap (IG) – it is the perpendicular distance from the internal surface of the crown to the axial wall of the preparation. • Marginal gap (MG) -it is the perpendicular distance from the internal surface of the crown to the axial wall of the preparation at the margin of the crown. • Overextended margin is the perpendicular distance from MG to the crown margin. • Underextended margin is the vertical distance from MG to the cavo-surface angle of the tooth. • Absolute marginal discrepancy (AMD) can be defined as the angular combination of MG and the extension error (over-extension or under-extension), which represents the distance from the preparation finish line to the restoration margin. It reflects the total crown misfit at that point (vertically and horizontally). Figure 2 Fit definitions according to Holmes et al. 10 1.6.2 Acceptable Marginal gap Unacceptable marginal gap (MG) of crowns causes a nidus for bacterial deposition, plaque accumulation, gingival inflammation, microleakage, secondary caries, and discoloration. (24,25) Improper fit of a crown to its abutment can also cause mechanical failures like chipping of crown margins. Open crown margins can also result in the dissolution of luting cement, leading to leakage of oral fluids and, consequently secondary caries. (26) Clinical outcomes studies indicate marginal integrity, mechanical properties, and structural durability to be the major influential factors for the longevity of indirect full coverage crowns. Overall, the type of manufacturing process and variations within each process such as different build angles for additively manufactured and different sintering protocols for subtractively manufactured crowns can influence the marginal fit. (27,28) Furthermore, it is difficult to discern the acceptable MG in literature. The American Dental Association (ADA) provides certain specifications as a means of ensuring quality and standards for every instrument and material used in dentistry. Clinically, the ADA specification 8 states that the luting cement thickness should be 25 µm when utilizing a type I luting agent (glass ionomer, zinc phosphate, and polycarboxylate cement) and 40 µm when utilizing a type II luting agent (glass ionomer-resinous hybrid and resin cement). (29) Most related studies have been performed on cast crowns rather than ceramic crowns. The clinically acceptable MG from the various studies conducted on cast crowns has been suggested to be between 50 µm to 120 µm. While McLean et al. (24) evaluated over 1000 crowns over a 5-year period and discerned that an acceptable MG of 80 µm is clinically impossible to achieve, they stated the clinically acceptable values after luting the crown in place should be less than 150 µm. Fransson et al. were close to this value at 120 µm in their research. (29) Based on the available evidence, several in vitro 11 studies considered MG up to 120 μm to be clinically acceptable for cast and ceramic restorations. For CAD/CAM milled ceramic crowns, there is a lack of conclusive acceptable values for margin fit; however, margin gaps have been reported within a range of 7.5-206 µm according to a recent systematic review encompassing 183 studies. (30) The potential reason for this wide range is attributed to the different CAD/CAM materials used, the fabrication methods and protocols followed for each, the stage at which analysis was performed, and the different modes employed to measure the gap. A review of the literature to evaluate acceptable marginal gaps for additively manufactured crowns showed varying results. (27,31,32) Although build angle was found to be the biggest influential factor on the overall fit of printed crowns, the optimum angle according to Alharabi et al. (18) is 120° while it was 150°-180° for Ryu et al. (19). Comparing the marginal fit of provisional crowns directly fabricated chairside, milled and 3D printed crowns, Wu et al. in 2021 (31) found the least marginal gap to be in the direct chairside fabrication technique while Chaturvedi et al. (33) found it to be the least in the 3D printed samples. Alharbi et al. (32) stated that the 3D-printed crowns have a lower MG and better internal fit than milled crowns although both were clinically acceptable. (32) Additionally, the type of manufacturing technique influences the accuracy and marginal adaptation more than the type of the finish line. (32) 12 1.6.3 Methods to measure marginal gap Clinical methods for measuring marginal gap The clinical fit of dental restorations can be assessed chairside using a mouth mirror and a dental explorer. In-vivo studies have patient and technique-related challenges such as tooth preparation, impression, and cementation technique sensitivity, in addition to saliva contamination and patient compliance. (34) Challenges with in-vivo measurements include a lack of unobstructed access all around the tooth and the inability of explorers to measure marginal defects in microns. Nonetheless, a combination of clinical probing and radiographic assessment chairside can help discern an open margin or overhang. In the literature, most in-vivo clinical studies have evaluated the marginal discrepancy of fixed dental prostheses using standardized criteria such as that mentioned in Table 1 United States Public Health (USPHS) criteria for marginal integrity. These criteria are based on surface, color, anatomic form, and marginal integrity. There are 4 possible ratings for marginal integrity: Alpha, Bravo, Charlie, and Delta. Restorations that fall in the Alpha and Bravo categories are considered acceptable. Alpha means that the restoration is excellent and meets all the clinical standards, while Bravo indicates that the restoration has minor errors but is clinically acceptable. Charlie refers to a visible mechanical complication such as a crack that can be confirmed by an explorer. Delta considers absolute failures such as mobility or fracture of the tooth or restoration and secondary caries contiguous with the margin of the crown. Both Charlie and Delta are indicative of a need for replacement of the restoration. (35) 13 Table 1 United States Public Health (USPHS) criteria for marginal integrity. Alpha (A) No visible evidence of crevice along the margins; no catch or penetration of the explorer. Bravo (B) Visible evidence of crevice and/or catch of the explorer, no penetration of the explorer. Charlie (C) Visible evidence of crevice and penetration of the explorer. Delta (D) Mobile or fracture restoration OR caries contiguous with the margin or the restoration OR tooth structure fractured. In-vitro methods for measuring marginal gap In-vitro experimentation facilitates a standardized setup and provides a readily controlled testing environment. There are different techniques, both destructive and non-destructive to measure the marginal fit of indirect restorations as follows: (30,36,37) • Direct view technique (non-destructive) -The most popular non-destructive and repeatable approach for measuring MG between the crown and the die is done under a microscope, usually an optical one. In comparison to other techniques, this method is less expensive and time-consuming. The drawbacks of this method include the inability to reliably locate reference measurement spots and repeat measurements from the same angle. Another drawback is projection error, which implies that the measurement obtained could have been from the angle the specimen was placed at during measurement instead of the angle it was tested at originally. (38,39) 14 • Cross-sectioning after cementation (destructive) – The SEM measurement done in this method is a disruptive process in which crowns are cut into sections and marginal and/or internal gaps are examined under a microscope in both the horizontal and vertical axes. It does not, however, permit repeated evaluation of the same specimens both before and after cementation or before clinical placement. Additionally, the selection of the sectioning plane reduces access to other planes, and so limits the number of measurements available to the quantity of measurements possible. This will not necessarily capture the largest MG discrepancy around the entire periphery of the crown thus not representing the overall fit of the restoration. (30) • Silicone Replica technique, (non-destructive) - The silicone replica technique is another method similar to the cross-sectioning technique that uses light body PVS in lieu of luting cement. The replica is sectioned the same as the previous procedure and measured at different sites. This method being easy and cheap has been used in several studies. (40) Light body PVS has the disadvantage of tending to tear upon removal from the crown. It can also make identifying the crown margins and finish lines challenging. The measurements provide values in two dimensions which is not an accurate representation of the misfit. (41) • Profilometry (non-destructive)- Profilometry produces a sharp focus on a monitor by showing the crowns on each die in the same focal plane. (41) However, the cement thickness of the cement layer at the marginal areas can only be assessed indirectly, and 15 when doing sequential analysis, extra caution should be used while repositioning the samples to prevent inconsistent re-profiling. (42) • Micro-computed tomography (micro-CT)- Using micro-CT, the technique provides 3D volumetric datasets that can be analyzed to provide quantitative measurements of internal and marginal fit of dental crowns. It enables the comparison of the marginal and internal fit of a sample before and after using various testing variables. (43,44) Micro-CT is a non-destructive technique. (51,52) One limitation of its application is the scanning of metallic objects such as copings and frameworks. Given the radiologic nature of micro-CT, its use in -vivo is also limited by the ALARA principle which dictates that radiation exposure should be as low as reasonably achievable. This precludes the researcher from exposure to an excessive amount of unwanted radiation. This can be reduced by factoring in time, dosage, and duration of exposure. • Optical coherence tomography- This technology, which uses coherent light to capture high-resolution 2D and 3D images in optical scattering media in real-time, is non-destructive and non-radiological. (45) In vivo research frequently makes use of it. However, it has trouble gauging sizable, thick restorations or materials that are optically opaque. (46) Triple scan and Dual Scan techniques (40,47–49) • Triple scan technique: This method is an alternative internal 3D fit method that registers point clouds of three scans in STL format; the crown, the abutment, and the 16 crown on the abutment. The last complex scan is used to reposition the abutment and crown surfaces using the initial alignment and best-fit algorithm. 3D and 2D compare is applied after the inversion of surfaces. Being repeatable and reproducible, it allows for trend analysis in terms of accuracy of fit. The difficulty arises with scanning the intaglio surface of the crown since a probe tip attached to a scanner is required which is not optical or routinely used in dental practice. There is a tendency for negative values to be obtained owing to the scanner identifying surfaces as smaller than their true dimensions. (56) The implications of this are incorrect quantitative data for fit measurements of the specimen since it cannot be compared to the scan data of the die that was obtained. • Dual scan technique The dual-scan technique was introduced by Lee et al. (49) to overcome the miscalculations that may arise while aligning the 3 digital scans in the triple-scan technique. Analysis can be done semi-qualitatively using color-coded maps as well as quantitatively in microns using data points. It produces more accurate calculations since it superimposes only two digital scans. It can be sectioned in an unlimited number of directions in virtual space without destruction of the samples and the virtual cement space is stored permanently in contrast to the conventional replica technique. Measurements can be made before and after cementation. This method is especially valuable when the research is conducted with a small sample size since there is an unlimited number of points distributed all over the three-dimensional surface both on the margin and internal surface of the crown and abutment. (51) Potential sources of error are spraying the abutment before scanning, layer of lubricant to act as a separator, and 17 tearing of the ultra-light body PVS material while removal of the crown from the abutment. (52,53) 1.6.4 Factors affecting marginal fit Impact of Number of Measurements/ Sample Size Groten et al. determined that a minimum of fifty measurements are required to determine marginal gap (i.e., within ±5 μm variability for arithmetic means). Groten et al. concluded that at least 20 measurements per sample could be accepted based on the required precision level. (54) Gassino et al. contradicted Groten’s study and stated 18 measurements would suffice for abutments prepared in the lab with crowns fabricated on them versus 90 measurements for abutment teeth prepared intraorally and crowns designed and made for those teeth. (39) Contrepois et al. reported that the optimum number of measurement sites required to evaluate marginal fit ranges between 18 and 50 points, (38) while Nawafleh et al. pointed out that most authors have used 4 to 12 points. (30) All the studies reviewed in the literature had a small sample size. To produce accurate measurements with statistically small standard deviations, several points had to be chosen and measured to compensate for the lack of a bigger sample size. With the advent of digital measuring procedures, we have the capability of measuring several locations, including three-dimensional measurements as opposed to two-dimensional measurements traditionally reported in the literature. Influence of Method of Fabrication Several studies have compared the accuracy of metal ceramic, metal, and all ceramic restorations, either single crowns or FDPs made with conventional and milling techniques. In the last 5 years, studies about the accuracy of models manufactured via additive manufacturing have been published. (47,55) Wu et al., Alharbi et al., and Ryu et al. have published work on the 18 accuracy of 3D printed provisional crowns according to finish lines and build angles. Their studies found that the optimum build angle is 120 degrees, though another study found a range of 150-180 degrees to be optimal, whereas the type (or design) of finish line did not have a significant influence on the outcomes. (18,19,32) 1.7 Summary of available literature and study rationale There is a gap in the literature to clearly discern which, if either, of the two methods, additive or subtractive manufacturing produces better adapted crowns – and how the methods interact with the influence of various crown materials. There is currently a new US FDA approved material available for 3D printing of definitive dental crowns manufactured by BEGO called Varseo SmileCrown Plus (VSCP). This material is yet to be tested for marginal fit of dental crowns produced from it. There is no existing literature available that tells us about the fit of crowns produced from this material. The few studies carried out on this resin tested for mechanical properties (18,32,33) and biocompatibility (20). It is important to analyze the clinical relevance of the discrepancy obtained from both materials and both methods of manufacture. The recently introduced monolithic multilayered zirconia also lacks studies to evaluate the marginal fit of those crowns. Through this study, we will thus be adding more data to the existing literature in terms of comparing marginal fit of crowns produced by the two manufacturing methods and comparing it to previous similar studies. 19 Chapter 2: Research Question and Specific Aim 2.1 Research Question Which of the two processes 3D printing or CAD/CAM milling produces definitive crowns with better marginal fit and internal fit? 2.2 Definitive Aim • To assess if the marginal fit and internal fit of 3D printed definitive ceramic filled hybrid resin crowns is comparable to or better than CAD/CAM milled multilayered monolithic zirconia crowns. 2.3 Research Hypothesis • Null Hypothesis 1 (H1): There is no statistically significant difference between the marginal fit of definitive 3D printed crowns and CAD/CAM milled crowns. • Null Hypothesis 2 (H2): There is a no statistically significant difference between the internal fit of definitive 3D printed crowns and CAD/CAM milled crowns. 20 Chapter 3: Materials and Methods 3.1 Design and fabrication of the master die and crowns 3.1.1 Design and milling of master die A solid PMMA resin printed half-arch model of quadrant 1 was fabricated containing a removable resin die of tooth #15 prepared with a deep chamfer margin. The resin model with the removable die was scanned first using a desktop scanner (3Shape, E4) which has an accuracy of 4 microns as per ISO standard 12836 and a scanning speed of 9 seconds for a full arch model. The die was removed from the model and scanned by itself creating the “die file”. This single die file was then modified to add the design of a congruent base with 6 points projecting from the base. These points were created for reproducibility, reliability, and ease of alignment on the measurement software between the original die file (STL file of master zirconia die) and each test sample file consisting of the PVS misfit space replica and the master zirconia die (identified in Geomagic as the Reference scan). The die was designed using the Ivoclar CAM 5 software. The resultant design was used to mill the master zirconia die (Sagemax zirconia-NexxZr T) in the Ivoclar PM7 mill (Figure 3). A copy was made of the original PMMA resin model without the die on the CAM software and when the software prompted for the die to be scanned, the newly milled zirconia die with the base was scanned in the system thus merging the two files (Figure 4). 21 Figure 3 Master zirconia die with 6 reference points on the base 22 Figure 4 A. Half arch PMMA model with removable die B. Zirconia die with congruent base designed and merged with the original model C. Margin identification of the new zirconia die D. STL file of the resultant milled zirconia master die E. Design of final crown A B D C E 23 3.1.2 Sample Preparation All samples were produced from a single virtual crown designed on the STL file of the master zirconia die using the CAM software (3Shape Dental System 2021) with standard parameters according to the manufacturer’s instructions (Table 2 CAD parameters for crown designFrom the multilayered monolithic zirconia disc as illustrated in Figure 5 (e.max ZirCAD Prime Shade A1 -98.5-16mm), the zirconia samples (Figure 6) were milled using a 5-axis milling machine (Ivoclar PrograMill PM7). Subsequently, they were sintered using a fast-sintering protocol for Ivoclar e.max ZirCAD Prime single crowns as per manufacturer’s instructions. (Programat S1 1600 software version v 6.0-Preset Program 3). Table 2 CAD parameters for crown design Parameter Values in microns (µm) Cement gap 40 Extra Cement gap 70 Distance to margin line 800 Smooth distance 200 Drill radius 700 Drill compensatory offset 710 24 Figure 5 Representative picture of e.max ZirCAD Prime Shade A1 multilayered monolithic zirconia with one milled and pre-sintered crown. Figure 6 Milled monolithic zirconia sample after sintering without finishing and polishing 25 3.1.3 3D printing of ceramic filled hybrid crowns The same STL file of the crown designed on 3Shape Design 2021 was imported into the SprintRay RayWare software to print the samples using the DLP 3D printer (SprintRay Pro 95). The samples were oriented with their occlusal surface parallel to the build platform. Supports were automatically generated in the SprintRay RayWare software. The print layer thickness was set to 50 µm. The lift speed of the platform was 50 mm/hr. The resin tank was filled with a freshly opened bottle of Bego Varseo Smile Crown plus in shade A1 (Figure 7). Samples were pre-cleaned in a 96% unheated ethanol ultrasonic bath for 3 minutes followed by thorough cleaning for 2 minutes in a fresh 96% unheated ultrasonic ethanol bath (SprintRay ProWash/Dry). As the next step, the samples were cured in a UV LED curing chamber (SprintRay ProCure) for 2 cycles of 20 minutes each at 20 °C (68 °F). The supports were left in place until the time for sample testing at which point, they were removed using side cutters (Figure 8). No sandblasting, finishing, or polishing was done for any of the samples prior to testing to preserve uniformity between the MI and PR groups. Figure 7 BEGO VarseoSmile Crown Plus bottle shade A1 - photopolymerizable resin 26 Figure 8 Printed ceramic filled hybrid crown without finishing and polishing 3.2 Sample testing and Data acquisition 3.2.1 Dual Scan Technique The Dual Scan technique represents a combination of scanning the abutment followed by the replica of the intaglio surface of the crown on the abutment. The STL files of both scans are then merged to identify the discrepancy in the fit of every crown after considering the cement space already incorporated in the design stage. 27 3.2.2 Sample testing The master die was fixated on the centre of the removable platform of the desktop scanner (3Shape Desktop scanner E4) using putty for stability. Two horizontal lines were created using a high-speed turbine handpiece on the buccal surface of the congruent base projection of the zirconia die as additional references for the Geomagic Control X software and for repeatability and correct orientation while testing each sample. This die was scanned once using the desktop scanner (3Shape E4 Desktop Scanner) and the file was saved in STL format as the master reference die. A Ney Surveyor was used as the setup for sample testing. The surveying table held the removable scan platform with the master zirconia die in a standard position. It was locked into place by the model clamp lock nut (Figure 9). The intaglio surface contour of every MI and PR crown was recorded using ultra-light body PVS (Aquasil Ultra LV, Dentsply Sirona, Germany) impression material. The samples were lightly coated using a new micro brush for every sample, with a small amount of petroleum jelly which acted as a lubricant. This was air distributed before taking the PVS impression of the misfit space (cement space and potential areas of misfit). A new tip was used in the impression gun for every sample. A standard load was applied to the surveying arm using a full one-litre water bottle fixed on a plastic platform with putty. It was applied at the time of seating every crown on the die to fully seat the samples and hold them in place till complete set by means of the screw to lock the spindle ( Figure 10). The crown was held in place on the die using a standard load for 7 minutes each (Figure 11). After setting, the crown was removed carefully with a gentle rocking motion in the mesiodistal direction using crown removal forceps, leaving a thin layer of the PVS impression material on the surfaces of the die ( 28 Figure 12). The exposed surface of the impression material (representing the intaglio surface of the misfit space) was scanned with the die still in the same position on the removable platform using the same scanner as before and saved as an individual STL file. This process was repeated for all the samples. In conjunction with the earlier reference scan, this scan essentially recorded the thickness of impression material representing a replica of the cement space (or misfit space), for both the internal fit as well as the marginal fit of the crown. Figure 9 A. Ney Surveyor with abutment fixed to the platform B. Putty holding the die in a standard position on the removable scanner plate. 29 Figure 10 Standard Load on the Ney surveyor for testing 30 Figure 11 Sample held in place with a standard load using surveying arm. 31 Figure 12 PVS misfit replica on zirconia master die 3.3 Digital measurements using Geomagic The STL files of both the master reference die and each of the PVS replicas was imported into the 3D compare software Control X 2020 (Geomagic, GmbH, Stuttgart, Germany). The scans were aligned using initial fit and best fit alignment wherein nearly 56,000 data points were selected automatically by the software. These data points were limited to the reference points on the congruent base apical to the chamfer margin. Each reference scan was divided on the crown portion of the die, into 3 regions: occlusal (OC), axial (AX), and marginal (MG). This was done to accurately differentiate the discrepancies according to the region; to reflect the internal fit (OC and AX) and marginal fit (MG). Thus, multiple automated measurements within each region were used to compare the internal and marginal fit of the crowns relative to the reference die scan (Figure 13). The positive deviation of the internal fit was calculated as a mean value for 32 each of the samples in both groups. A 3D compare tool was used to generate a color-coded map representing the deviations as determined by the software and 5 additional locations without a predefined preference were selected to act as comparison points at each of the three created regions. (Figure 14 A-E). The tolerance level was set to 0.12 mm meaning the color map showed all values within tolerance levels to be green, whereas values exceeding 0.12 mm were reflected depending on the amount of misfit qualitatively as either light blue (0- 0.12 mm), yellow (0.12-0.4 mm), orange (0.4-0.8 mm) or red (0.8-1 mm). Each sample individually had quantitative values generated automatically by the software in a table that reflected the color-coded map. Figure E represents the misfit values in blue between 0-0.12 mm which is without the tolerance color setting of green. The same image is shown in Fig 14 D with tolerance color thereby rendering the blue colors green (having the same values within the tolerance range). Figure 13 Three regions corresponding to marginal, axial, and occlusal surface created on the reference die. 33 Figure 14 A. MG misfit B. OC misfit C. AX misfit D. Overall 3D misfit with tolerance color E. Overall 3D misfit without tolerance color. The color-coded map shows the amount of discrepancy at every part of the crown. Green represents values within the tolerance limit of 120 µm. 3.4 Statistical Analysis 3.4.1 Sample size calculation Power analysis, based on the pilot study, indicated that a sample size of 14 was required to detect significant mean differences of 40 μm between the groups in the presence of a 5 μm standard deviation. In this power analysis, α was set to 0.05, β was set to 0.20, to allow for 80% power. For the milled zirconia (MI) group, 14 zirconia crowns (e.max ZirCAD Prime) were milled A B C D E 34 (n=14). For the printed group (PR), 14 crowns were printed in the VSCP material in a SprintRay Pro95 3D printer (n=14). 3.4.2 Statistical Tests A statistical software (IBM SPSS Statistics 27) was used for the statistical analysis. The milled (MI) and printed (PR) groups were checked for the assumption of normality. The measured values of MG, AX, and OC of both groups were both skewed and kurtotic. This was confirmed using the Kolmogorov-Smirnov and Shapiro-Wilk test. The results showed that the distribution was not normal. This was further confirmed visually using boxplots. Homogeneity of variance was not fulfilled as shown by Levenè’s test (p<0.05). Given that two out of three assumptions (normality and equality of variances) were not fulfilled in addition to a small sample size (n<30) data were assessed by the non-parametric Mann-Whitney U Test. It compares the mean ranks of two groups. The descriptive statistics value of the median and interquartile range (IQR) along with U and p-values for significance were reported. 35 Chapter 4: Results All 28 samples were analyzed for volume discrepancy at three regions: margins (MG), axial (AX), and occlusal (OC) surfaces (Figure 13). The samples from the printed (PR) and milled (MI) groups were tested using descriptive statistics which yielded the mean, median, standard deviation (SD), and 95% confidence interval (CI). The mean marginal discrepancy for the printed crowns (PR) was numerically lower than that of the milled crowns (MI). However, the difference was not statistically significant. The measured values in the PR group for MG were 94 ± 44 μm ( 36 Table 3 Comparison of mean marginal discrepancy between printed (PR) and milled (MI) respectively at margin of crown (MG) in mm, at AX 59 ±20 μm (Table 4 Internal discrepancy between printed method (PR) and Milled method (MI) respectively at the mid portion or axial surface (AX) of the crown in mm, at OC 73±94 μm (Table 5 Comparison of internal discrepancy between Printed method (PR) and Milled method (MI) respectively at top part of crown (OC) in mm.Error! Reference source not found.. The measured values for the MI group at MG 97±19 μm (Error! Reference source not found.), at AX 36±34 μm (Error! Reference source not found.) and at the OC 52±77 μm (Error! Reference source not found.). There was no statistically significant difference in the mean marginal gap between MI and PR groups, but the difference was significant for the internal fit at the axial and occlusal surfaces between both groups, the mean discrepancy being larger in the PR group. 37 Table 3 Comparison of mean marginal discrepancy between printed (PR) and milled (MI) respectively at margin of crown (MG) in mm Margin of crown (MG) Mean SD Mean Difference (SE) Mann Whitney ‘U’ test P value, Significance Printed method 0.094 0.044 p =0.946 0.002 U = 96.5 Milled (NS) (0.012) method 0.097 0.019 p>0.05 – no statistically significant difference (NS) Table 4 Internal discrepancy between printed method (PR) and Milled method (MI) respectively at the mid portion or axial surface (AX) of the crown in mm Middle of crown (AX) Mean SD Mean Difference (SE) Mann Whitney ‘U’ test P value, Significance Printed method 0.059 0.02 0.022 U = 36.0 p = 0.004* Milled (0.01) method 0.036 0.034 *p< 0.05 – statistically significant difference 38 Table 5 Comparison of internal discrepancy between Printed method (PR) and Milled method (MI) respectively at top part of crown (OC) in mm. Top part of crown (OC) Mean SD Mean Difference (SE) Mann Whitney ‘U’ test P value Significance Printed method 0.073 0.094 0.021 U = 40.0 p = 0.007* Milled (0.032) method 0.052 0.077 *p<0.05 – Statistically significant difference The box plot as shown in Figure 15 tells us that the samples in the Printed group for MG are normally distributed with no outliers. In contrast, the data for the MI group is positively skewed with one outlier. This means the data has more values between the 25th to 50th percentile. The data distribution for OC and AX for both groups, PR and MI are negatively skewed with multiple outliers indicating high variability in the internal fit. Outliers are of significance when they are more than 1.5 times the interquartile range (IQR) or the median value represented by the horizontal black line in the box plot. They tend to affect the overall result of the data tested. In Figure 15 the outlier for the Milled MG group is not of significance since it is less than 1.5 times the IQR. However, Milled OC values have three outliers of significance which could have influenced the 39 result. The single outlier for the Printed OC group represents an anomaly one sample also indicating either an error in testing or that the sample itself was faulty. Figure 15 Showing range and outliers in each group for each measured entity, MG, AX and OC. Marginal and internal misfit 40 Chapter 5: Discussion The first null hypothesis (H1) was accepted since there was no statistically significant difference between the marginal fit of the MI and PR groups. The second null hypothesis (H2) was rejected as there was a statistically significant difference in the internal fit between the PR group as compared to the MI group, the PR group having the higher misfit value. The marginal fit of PR group was like the MI group. Both are within the clinically acceptable value of 120 μm. As per a systematic review of the marginal fit of single unit milled zirconia crowns done by Ahmed et al. in 2019 (55), the marginal fit is influenced by several factors, especially finish lines. According to the study mentioned, marginal fit of milled zirconia crowns was found to be better when a chamfer finish line was used. Hence, this study was designed with the master zirconia die prepared with a chamfer finish line. A significant influence on the fit of a restoration is the type of zirconia used, the process used to manufacture it, the sintering protocol; fast or slow, and the method employed to quantitatively analyze the discrepancy. (55) More specifically, the marginal fit can be affected by the type of manufacturing process used and changes within each process, such as variable build angles for additively created crowns and varied sintering protocols for subtractively made crowns. (27,28) As was previously mentioned, determining the permissible marginal gap in literature can be challenging. According to different research done on cast crowns, the clinically appropriate MG should be between 50 μm and 120 μm. (24,29) While McLean and colleagues assessed more than 1000 crowns over the course of five years and determined that it is clinically impossible to reach an acceptable MG of 80 μm, they indicated that the values after cementing the crown in place should be less than 150 μm. At 120 μm, research by Fransson et al. was rather close to this 41 value. (29) For CAD/CAM milled ceramic crowns, misfit was found to be between 7.5-206 μm according to a recent systematic review by Nawafleh et al. encompassing 183 studies. (30) In the current study, mean MG values for both PR and MI groups were found to be just under 100 μm. The largest mean value for MG was 115.8 μm and 171.6 μm for MI and PR groups respectively. The large mean value for the PR groups appears to be an outlier as shown in Figure 15. This finding relates to a significant deviation from the average mean values obtained for MG in both groups, which could be an indication of bigger variations in marginal fit from one crown to another in the PR group. The outliers in the PR group are also apparent from Figure 15 described in the previous chapter. Unlike Holmes et al., who described discrepancies only at the cavosurface margin, this study looks at the mean discrepancy over the entire prepared chamfer finish line from the cavosurface margin to the junction between the axial wall and the finish line. This is an important distinguishing point to be noted by the reader. (36,65) The results of a survey of the literature to determine permissible marginal gaps for additively produced crowns were mixed. (27,31,32) While build angle was discovered to have the greatest impact on the general fit of printed crowns, Alharbi et al. and Ryu et al. determined that the ideal build angle for better fitting crowns is 120°. (19,32) In this study, all the crowns in the PR group have been printed only according to a single build angle of 90°. This could potentially have influenced the marginal fit of the PR group. Perhaps, if a build angle of 120° had been followed in the protocol, our results might have potentially differed. Alharbi et al in 2018 (32) and Chaturvedi et al. in 2020 (33) found the marginal gap to be the least in the 3D printed samples when comparing provisional chairside directly produced, milled, and 3D printed crowns. Both authors tested provisional 3D printing materials. The current study is the first to report the marginal fit of crowns produced from a material meant for definitive 3D printed crowns, the 42 Bego VarseoSmile Crown plus (VSCP). The findings of this study, like previous studies, also show the marginal fit of 3D-printed crowns to be numerically better as per mean values (though not statistically so) than the milled crowns. The measurement software, Geomagic Control X 2020, picked more than 20,000 points in a three-dimensional space at the margins, mid axial, and occlusal surfaces. These points are much more than those proposed by Gassino et al, Groten et al, and Anadioti et al. (1,39,54) Having more points of measurement ensures that areas that might have a higher discrepancy are not omitted. A potential source of error in the software which was noted at the time of conducting the pilot study was the superimposition of scans identifying values tending to zero on the occlusal surface. This either indicates excellent fit of the crown to the die or potentially a lack of flow of light body PVS to those areas on the occlusal surface, rendering that area of the reference die and test crown the same. This could be because of the well-established fact that low viscosity cements do not allow complete seating of a dental crown. In theory, assuming for the moment that PVS material behaves somewhat like a dental cement, a larger cement space could have contributed better to the seating of the crown and consequently led to a smaller marginal discrepancy and larger internal discrepancy. Counterintuitively, an alternative explanation of cause and effect might be reversed. A slightly larger marginal gap could have allowed greater PVS escape during seating resulting in lower internal fit discrepancy as seen in the MI group. Including the pilot samples, each group had a total of 22 samples manufactured. Out of the total 22 samples, 15 samples were manufactured for the final testing instead of a sample size of 14 to compensate for any manufacturing defects. Interestingly, one sample from the PR group had shrunk so significantly that it could not be placed on the die to be tested, even without PVS. This could be because of a greater shrinkage during the post-processing stage. As per the quality 43 assurance protocol that crowns must meet in order to be used or tested, this sample was discarded from the PR group bringing the total number of samples tested and measured to 14. In the MI group, one sample was found to have a chipped margin and was likewise eliminated from the testing process based on the quality assurance requirements. Care was taken to standardize the scanners, sintering protocols, amount and duration of load application as well as setting time of PVS, printer settings, duration of wash and cure, and Geomagic calculation of data. However, several variables in both manufacturing processes, including calibration, scanning protocol, design software, milling machines, type of printer used, postprocessing, and shrinkage after final firing or curing of the restoration, can affect the marginal and internal accuracy of a restoration. 44 Chapter 6: Conclusions • 3D printing of crowns with VSCP produces mean marginal gaps that are similar to those of milled e.Max ZirCAD zirconia crowns over the entire surface of the prepared chamfer margin. • The internal fit of MI group was found to be better than that of PR group. 45 Chapter 7: Limitations, clinical implications, and future direction Drawbacks and limitations of the study: Some factors which could have introduced errors are the processes involved in producing the initial scan of the physical abutment and milling it into another master zirconia die. As per ISO standard 12836, the 3Shape E4 scanner has an accuracy of 4 µm, assuming this error in addition to the post milling shrinkage of a zirconia die adds another layer of inaccuracy. The CAD file designed on the zirconia die would have eliminated this first layer of inaccuracy. Following this, the spraying of an anti-reflective coating on the zirconia master die also could have potentially introduced an error between 20-40 µm. Lastly, every sample was coated on the intaglio surface with a thin layer of separating medium accounting for another discrepancy. The large variation in values between the samples of the printed group with multiple outliers could be attributed to the handling and post processing of the material which could cause shrinkage unevenly leading to poorer fit on the internal surface and better fit at the margins. It could also inherently be a property of the material which is being tested for the first time that yields different results in the samples all printed at the same time under the same conditions. While measuring the mean marginal gap values, this study looked at the MG on the entire prepared chamfer margin, from the cavosurface margin to the junction of the prepared margin and axial wall, unlike Holmes et al who described MG only at the cavosurface margin of the die. Clinical implications and future direction: No clinical implications can be drawn directly from this study as it simulates clinical conditions and only looks at one of the parameters used to judge clinical acceptability. The definitive 3D printed VSCP crowns are potentially acceptable based solely on the fit of the crowns. However, 46 future studies on durability, longevity outcomes and mechanical properties are necessary to comment further on clinical implications and clinical acceptability. 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Thesis/Dissertation
eng
Craniofacial Science
Vancouver : University of British Columbia Library
Attribution-NoDerivatives 4.0 International
Graduate
Lee, Vincent
In vitro comparison of marginal and internal fit of 3D printed definitive crowns and CAD CAM milled crowns
Text