Dentistry, Faculty of
DSpace
UBCV
Mann, Ravdeep Singh
Master of Science - MSc
University of British Columbia
Purpose: The aim of this study was to determine KIC of a conventional denture base material, using the notchless triangular prism (NTP) specimen KIC test, and compare it with that of CAD/CAM and 3D-printed denture base materials after 7 d and 90 d storage in 37 °C water. Materials and methods: Lucitone 199 (C), Lucitone 199 CAD (M) and Lucitone Digital Print (P) (Dentsply International Inc., York, PA) were used to fabricate NTP specimens (40/group). Samples were stored in 37 °C water for 7 d (20/group) and 90 d (20/group) and were conditioned, according to ISO 20795-1. For testing, samples were secured in custom-made jigs, replicating the chevron-notch short rod specimen configuration. The test assembly was loaded in tension (0.1 mm/min) until crack arrest or failure. The maximum-recorded load was used to calculate KIC. Two-way ANOVA, followed by Scheffé multiple mean comparisons (α = 0.05), independent t-tests and Weibull statistics were used to analyze the results. Light and scanning electron microscopy were used to characterize fractured surfaces. Results: Crack arrest was observed in all test specimens. The analyses of the results have shown that the three tested materials had significantly different KIC at 7 d and 90 d, with the same ranking, i.e., P > C > M (p < 0.005). The analyses of the results have also shown that ageing in 37 ºC water for 90 d resulted in a significant decrease in KIC in the C and M groups (p < 0.001). Significant crazing was observed in the 3D-printed specimens, which resulted in them having significantly larger work of fracture values (~8 KJ/m2 vs ~3 KJ/m2). Conclusion: The tested 3D-printed denture base material had significantly higher KIC, exhibited crazing, had a higher absorbed energy before fracture and was stable under ageing conditions, suggesting that it could be more resistant to crack propagation than the tested conventional and milled materials. The tested milled denture base showed the least resistance to crack propagation, with the lowest KIC values both at 7 d and 90 d. Water storage for 90 d significantly decreased KIC of the tested conventional and milled materials.
FRACTURE TOUGHNESS OF CONVENTIONAL, MILLED AND PRINTED DENTURE BASES by Ravdeep Singh Mann 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) January 2022 © Ravdeep Singh Mann, 2022 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, a thesis entitled: Fracture toughness of Conventional, Milled and Printed denture bases submitted by Ravdeep Singh Mann in partial fulfillment of the requirements for the degree of Master of Science in Craniofacial Science Examining Committee: Dr. N. Dorin Ruse, Professor, Department of Oral Biological & Medical Sciences, UBC Supervisor Dr. Nesrine Mostafa, Assistant Professor, Department of Oral Health Sciences, UBC Supervisory Committee Member Dr. Tom Troczynski, Professor, Department of Materials Engineering, UBC Supervisory Committee Member Dr. Chris Wyatt, Professor, Department of Oral Health Sciences, UBC Additional Examiner iii Abstract Purpose: The aim of this study was to determine KIC of a conventional denture base material, using the notchless triangular prism (NTP) specimen KIC test, and compare it with that of CAD/CAM and 3D-printed denture base materials after 7 d and 90 d storage in 37 °C water. Materials and methods: Lucitone 199 (C), Lucitone 199 CAD (M) and Lucitone Digital Print (P) (Dentsply International Inc., York, PA) were used to fabricate NTP specimens (40/group). Samples were stored in 37 °C water for 7 d (20/group) and 90 d (20/group) and were conditioned, according to ISO 20795-1. For testing, samples were secured in custom-made jigs, replicating the chevron-notch short rod specimen configuration. The test assembly was loaded in tension (0.1 mm/min) until crack arrest or failure. The maximum-recorded load was used to calculate KIC. Two-way ANOVA, followed by Scheffé multiple mean comparisons (α = 0.05), independent t-tests and Weibull statistics were used to analyze the results. Light and scanning electron microscopy were used to characterize fractured surfaces. Results: Crack arrest was observed in all test specimens. The analyses of the results have shown that the three tested materials had significantly different KIC at 7 d and 90 d, with the same ranking, i.e., P > C > M (p < 0.005). The analyses of the results have also shown that ageing in 37 ºC water for 90 d resulted in a significant decrease in KIC in the C and M groups (p < 0.001). Significant crazing was observed in the 3D-printed specimens, which resulted in them having significantly larger work of fracture values (~8 KJ/m2 vs ~3 KJ/m2). iv Conclusion: The tested 3D-printed denture base material had significantly higher KIC, exhibited crazing, had a higher absorbed energy before fracture and was stable under ageing conditions, suggesting that it could be more resistant to crack propagation than the tested conventional and milled materials. The tested milled denture base showed the least resistance to crack propagation, with the lowest KIC values both at 7 d and 90 d. Water storage for 90 d significantly decreased KIC of the tested conventional and milled materials. v Lay Summary The ease and efficiency afforded by CAD/CAM technology in other fields of dentistry has led to its adoption in the manufacturing of removable prostheses. The most commonly used removable prostheses are denture bases. The technology shift and its application in manufacturing of denture bases has led to development of novel denture base materials. These materials are being commercialized as having better mechanical and physical properties, along with improved biocompatibility. However, the available evidence and independent assessment of these materials is lacking. The long-term behavior of the properties of these materials has also not been assessed. To help clinicians in their selection of a suitable denture base material, the aim of the study was to assess the short and long-term ability of three different denture base materials to resist crack propagation. vi Preface All components of this project, including research, sample calculations, sample preparation and testing, statistical analysis, and dissertation writing, were completed by Ravdeep Singh Mann without collaboration, under the guidance and expertise of my supervisor, Dr. N. Dorin Ruse, UBC Faculty of Dentistry. Dr. Nesrine Mostafa, UBC Faculty of Dentistry and Dr. Tom Troczynski, UBC Department of Materials Engineering were part of the supervisory research committee. This was an in-vitro study with involvement of no human or animal subjects, and no biohazardous materials, hence no Ethics Board approval was required. vii Table of Contents Abstract ........................................................................................................................................ iii Lay Summary ................................................................................................................................ v Preface .......................................................................................................................................... vi List of Tables ................................................................................................................................ xi List of Figures ............................................................................................................................. xii List of Equations ........................................................................................................................ xiv List of Abbreviations .................................................................................................................. xv List of Symbols .......................................................................................................................... xvii Acknowledgements .................................................................................................................. xviii Dedication ................................................................................................................................... xix Chapter 1: Introduction ............................................................................................................... 1 1.1 History of Denture Bases ...................................................................................................... 2 1.2 Classification of Denture Bases ............................................................................................ 3 1.3 Poly(Methyl Methacrylate) as Denture Base Material ......................................................... 5 1.3.1 Poly(methyl methacrylate) ............................................................................................ 5 1.4 Denture Fabrication Techniques ........................................................................................... 8 1.4.1 Conventional Techniques .............................................................................................. 8 viii 1.4.2 Comparison of Compression Molding with Injection Molding Technique ................ 10 1.4.3 Digital Dentures (CAD/CAM) .................................................................................... 11 1.5 Comparison between Conventional, Milled and 3 D Printed Denture Bases ..................... 16 1.5.1 Retention ...................................................................................................................... 16 1.5.2 Accuracy and Reproducibility ..................................................................................... 17 1.5.3 Residual Monomer Content ......................................................................................... 18 1.5.4 Physical and Mechanical Properties ............................................................................ 19 1.5.5 Color Stability .............................................................................................................. 20 1.5.6 Number of Appointments ............................................................................................ 21 1.5.7 Cost .............................................................................................................................. 22 1.5.8 Candida Colonization .................................................................................................. 23 1.6 Effect of Water Storage on Denture Bases ......................................................................... 26 1.7 Fractures of Denture Bases ................................................................................................. 26 1.8 Fracture Toughness of Denture Bases ................................................................................ 28 1.9 Notchless triangular prism (NTP) specimen KIC test ......................................................... 29 Chapter 2: Research Question and Specific Aim .................................................................... 31 2.1 Research questions ............................................................................................................. 31 2.2 Aim ..................................................................................................................................... 31 2.3 Null Hypotheses (H0) .......................................................................................................... 31 Chapter 3: Materials and Methods ........................................................................................... 32 3.1 Test Materials: .................................................................................................................... 32 3.1.1 Lucitone 199 (C) Denture Base ................................................................................... 32 ix 3.1.2 Lucitone 199 CAD (M) Denture Base ......................................................................... 33 3.1.3 Lucitone Digital Print (P) Denture Base ...................................................................... 33 3.2 Processing of denture bases for sample preparation ........................................................... 34 3.2.1 Lucitone 199 Denture base resin group (C) ................................................................. 34 3.2.2 Lucitone 199 CAD Denture Base Disc Group (M) ..................................................... 36 3.2.3 Lucitone 3D Print Denture Group (P): ........................................................................ 37 3.3 Sample Size Calculation ..................................................................................................... 39 3.4 KIC Test ............................................................................................................................... 40 3.5 Specimen Preparation ......................................................................................................... 40 3.6 Water storage and conditioning .......................................................................................... 42 3.7 Notchless Triangular Prism (NTP) Specimen Fracture Toughness Test ............................ 43 3.8 Statistical Analysis ............................................................................................................. 45 3.9 Scanning Electron Microscopy ........................................................................................... 46 Chapter 4: Results ...................................................................................................................... 47 4.1 Fracture Toughness (KIC) ................................................................................................... 47 4.2 Crack Propagation and Arrest ............................................................................................. 50 4.3 Work of Fracture ................................................................................................................ 58 4.4 SEM Analysis ..................................................................................................................... 59 4.5 Weibull Statistics ................................................................................................................ 63 Chapter 5: Discussion ................................................................................................................. 65 x 5.2 Could the notchless triangular prism (NTP specimen KIC test be a better alternative to the SENB test currently recommended by ISO for the determination of fracture toughness of denture base materials? ............................................................................................................. 69 5.3 Clinical Significance ........................................................................................................... 70 5.4 Limitations and Future Considerations ............................................................................... 70 Chapter 6: Conclusion ................................................................................................................ 71 Bibliography ................................................................................................................................ 72 xi List of Tables Table 1: Classification of denture bases (ISO 20795-1) ................................................................. 3 Table 2: Classification according to method of processing ............................................................ 4 Table 3: Summary Table Comparing Conventional Denture with CAD-Milled and CAD-3D Printed Dentures ........................................................................................................................... 24 Table 4: Materials ........................................................................................................................ 32 Table 5: Lucitone 199 (C) Denture Base Composition ................................................................ 32 Table 6: Lucitone 199 CAD (M) Denture Base Composition ...................................................... 33 Table 7: Lucitone Digital Print (P) Denture Base ........................................................................ 34 Table 8: Results (Mean ± SD)# ..................................................................................................... 47 Table 9: Work of Fracture (WOF) (in KJ/m2) .............................................................................. 58 Table 10: Weibull Modulus (m) and Characteristics Weibull Fracture Toughness (KIC) ............ 63 xii List of Figures Figure 1: Chemical formula of methyl methacrylate and poly(methyl methacrylate) ................... 6 Figure 2: Success Injection System (Dentsply International, Inc.) ................................................ 9 Figure 3: CAD/Milled Denture Base Processing (51) .................................................................. 12 Figure 4: 3D Printed dentures using SLA technology .................................................................. 15 Figure 5: NTP testing assembly (135) .......................................................................................... 30 Figure 6(a), (b): Finished acrylic bars fabricated by the (C) injection molding method .............. 36 Figure 7: Lucitone 199 CAD Disc ................................................................................................ 36 Figure 8: CAD file designed using AutoDesk Fusion 360 Software ........................................... 37 Figure 9: STL file for printed samples ......................................................................................... 38 Figure 10 (a), (b): Printed triangular prisms ................................................................................. 38 Figure 11: Custom Cutting Jig ...................................................................................................... 41 Figure 12: Isomet saw ................................................................................................................... 41 Figure 13: Custom Grinding Jig ................................................................................................... 42 Figure 14: Samples being aged in an Incubator ............................................................................ 42 Figure 15: NTP specimen custom holder ..................................................................................... 44 Figure 16 (a), (b): Custom Mounting Jig ...................................................................................... 44 Figure 17: Test Assembly ............................................................................................................. 45 Figure 18: Box Plot ....................................................................................................................... 48 Figure 19: Scheffé post hoc KIC 7 d ............................................................................................. 49 Figure 20: Scheffé post hoc KIC 90 d ........................................................................................... 49 Figure 21: Load/Displacement graph for C group at 7 d and 90 d ............................................... 52 Figure 22 (a), (b): Crack Propagation and arrest in C specimens at 7 d and 90 d ........................ 53 xiii Figure 23: Load/Displacement graph for M group at 7 d and 90 d .............................................. 54 Figure 24 (a), (b): Crack Propagation and arrest in M specimens at 7 d and 90 .......................... 55 Figure 25: Load/Displacement graph for P group at 7 d and 90 d ............................................... 56 Figure 26 (a), (b): Crack Propagation and arrest in P specimens at 7 d and 90 d ........................ 57 Figure 27 (a), (b): SEM images - C 7 d at 25x (a) and 150x (b) magnification ........................... 60 Figure 28 (a), (b): SEM images - C 90 d at 25x (a) and 150x (b) magnification ......................... 60 Figure 29 (a), (b): SEM images - M 7 d at 25x (a) and 150x (b) magnification .......................... 61 Figure 30 (a), (b): SEM images - M 90 d at 25x (a) and 150x (b) magnification ........................ 61 Figure 31(a), (b): SEM images - P 7 d at 25x (a) and 150x (b) magnification ............................. 62 Figure 32 (a), (b): SEM images - P 90 d at 25x (a) and 150x (b) magnification .......................... 62 Figure 33: Weibull Plots for the tested materials ......................................................................... 64 xiv List of Equations Equation 1: Lehr’s Rule of Thumb ............................................................................................... 39 Equation 2: Two-parameter Weibull statistics ............................................................................. 39 Equation 3: Calculation of KIC ..................................................................................................... 43 Equation 4: Weibull formula ........................................................................................................ 45 Equation 5: Calculation of WOF .................................................................................................. 58 Equation 6: Requirement to ensure plane strain conditions ......................................................... 67 xv List of Abbreviations 2D Two-dimensional 3D Three-dimensional C Conventional denture base CAD/CAM Computer-aided design/Computer-aided manufacturing CNC Computer numerical control CNSR Chevron-notched short rod FDA Food and Drug Administration ISO International Organization for Standardization IUPAC International Union of Pure and Applied Chemistry J Joule kN Kilo Newton M Milled denture base MPa Mega Pascal N Newton NTP Notchless triangular prism PMMA Poly(methyl methacrylate) P Printed denture base SA Surface Area SD Standard deviation SEM Scanning electron microscopy SENB Single-edge notched beam SiC Silicon carbide SPSS Statistical Package for the Social Sciences xvi STL Standard Tessellation Language Wc Weibull characteristic value WOF Work of fracture wt Weight xvii List of Symbols a Crack length 𝛼 Significance value B Specimen thickness 𝛽 Power D Specimen diameter K Stress intensity factor KIC Fracture toughness (critical stress intensity factor in Mode I loading) m Weibull modulus n Number of samples ln Natural logarithm Pmax Maximum load Pf Probability of failure Tg Glass transition temperature μ Mean 𝜇𝑚 Micrometer W Specimen length Y*min Dimensionless stress intensity coefficient factor minimum Δ Standardized difference 𝜎y Yield strength xviii Acknowledgements “The mediocre teacher tells. The good teacher explains. The superior teacher demonstrates. The great teacher inspires.” – William Ward My sincere gratitude goes to Dr. N. Dorin Ruse, who has accepted me under his wings. He is an inspiring teacher who has guided me throughout this journey and taught many valuable lessons, both professional and personal. He has always taken time from his busy schedule to address my concerns. I am forever indebted for his unconditional support, encouragement, and constructive feedback that has enabled me to accomplish my goals. I cannot thank him enough for everything. I would also like to thank my committee members, Dr. Nesrine Mostafa and Dr. Tom Troczynski, who have offered their constant support and critical feedback during our committee meetings. Your insightful comments have allowed me to complete this project with much ease. I would also like to thank my fellow residents and my Prosthodontics faculty members for their constant support and encouragement throughout this journey. It has been a pleasure working with you all. Lastly, thanks to my family members who have been my pillar of strength and support throughout these years. xix Dedication To my lovely wife Kiran and my family who have always believed and supported me throughout this journey. To all the amazing teachers who have inspired, supported, guided, and helped shape me into the person I am today. 1 Chapter 1: Introduction Complete dentures are the minimally invasive and cost-effective option for full mouth rehabilitation of edentulous patients.(1) It has been projected that by 2050, 8.6 million individuals (~2.5 % of the population) in US will suffer from edentulism (2) and a significantly greater percentage in developing countries.(3) Historically, various materials have been used in the fabrication of dentures, including bone, ivory, wood, vulcanite, metals and polymers.(4) Developed in 1937, poly(methyl methacrylate) (PMMA) has ever since been used as denture base material. The polymer has been modified numerous times to improve its mechanical and chemical properties as well its working properties, to adapt to various denture fabrication techniques.(5) Various fabrication methods are available for complete dentures. The main goal of each of these fabrication methods is to produce denture prostheses with excellent mucosal adaptation, providing good stability, support and retention.(6) The three popular conventional denture fabrication techniques are compression molding, fluid resin pour and injection molding. The compression molding technique, so called “Pack and press”, has been the most widely used technique. The pour technique, as it is less time consuming, has increased in usage. However, some undesirable features of this technique are the movement of teeth during processing, poor tooth bond with the acrylic denture base and air entrapment.(4)(6) There has been continuous improvement in the fabrication techniques to eliminate the drawbacks associated with conventional techniques and to enhance the properties of the denture base materials. The introduction of computer-aided design/computer-aided manufacturing (CAD/CAM) technology in dentistry, in the early 1980s, has revolutionized the dental field.(7) CAD/CAM has been able to overcome and eliminate some of the problems associated with 2 conventional denture fabrication.(8) Various CAD/CAM prosthetic systems have been introduced into the market, such as AvaDent, Dentca, Weiland, Ceramill and Baltic Denture system.(9) 1.1 History of Denture Bases Denture base materials have undergone a paradigm shift over the last 150 years.(10) Before the start of the 19th century, dentures were primarily made from wood, ivory, bone, porcelain and even the currently used metal alloys.(11) However, high cost, patient dissatisfaction, requirement for precise technique and skilled dentists shifted the focus from these materials towards plastic materials.(12) Plastic materials were not only economical but could also render a greater degree of precisions.(12) Vulcanized rubber (Vulcanite) was introduced in the middle of the 19th century as a denture base material. This material eventually shifted the focus towards polymers as denture base materials.(13) In the 1930’s, PMMA was introduced as dental base material due to significantly improved physical, chemical, biological, esthetic and handling properties.(5) PMMA was introduced as injection molded material by Imperial Chemical Industries Ltd. in 1935. Kulzer and Co. patented the dough moulding process and marketed a denture base material called Paladon, supplied as powdered PMMA and liquid monomer (methyl methacrylate). When mixed, the powder - liquid forms a dough which is packed into the dental mould and polymerized using heat activation.(11)(14) Polymers, such as polystyrene, poly(acrylic acid), epoxy resins, polysulphones, polycarbonates, polyamides, urethane dimethacrylate were tested and used as denture base materials, but none could match the excellent properties exhibited by PMMA.(14)(15)(16) Since then, PMMA has dominated as denture base material for denture fabrication.(11) Chemical modifications have been 3 done over the years to improve the mechanical and physical properties, while still retaining its flexibility to be used as a suitable denture base material.(5) Digital (CAD/CAM) technology has been developed for the fabrication of complete dentures to overcome and eliminate some of the problems associated with conventional denture fabrication.(17)(18) Various modified PMMA based polymers, such as IvoBase CAD, Lucitone 199 CAD, NextDent 3D+, Lucitone 199 Digital Print, are commercially available to be used with CAD/ CAM systems.(9) 1.2 Classification of Denture Bases Denture bases can be classified according to the activation of the polymer material and the processing technique. Table 1 summarizes the classification of denture base materials according to the International Organization for Standardization (ISO) document 20795-1 (19) Table 1: Classification of denture bases (ISO 20795-1) Type 1 Class 1 Heat-polymerizable materials (Powder and liquid) Class 2 Heat-polymerizable materials (Plastic Cake) Type 2 Class 1 Auto polymerizable (Powder and Liquid Class 2 Auto polymerizable (Powder and Liquid for pour type resin Type 3 Thermoplastic blank or powder Type 4 Light activated materials Type 5 Microwave cured material 4 Table 2 summarizes the classification of denture base materials according to the method of processing. Table 2: Classification according to method of processing The American Dental Association classifies denture base polymers as:(20) Conventional Compression Molding Conventional Injection Molding Conventional Fluid Resin technique Digital CAD and milled (Subtractive) Digital CAD and Printed (Additive) 5 1.3 Poly(Methyl Methacrylate) as Denture Base Material Poly(methyl methacrylate) (PMMA) is a commonly used polymer in dentistry for the fabrication of denture bases, orthodontic retainers, and denture teeth. As a denture base material, it has been traditionally available as a powder-liquid system.(21) The powder composition is mainly PMMA in which additives, such as pigments, filler particles and synthetic fillers, are added to enhance the esthetic and mechanical properties. The liquid contains methyl methacrylate monomer with the addition of cross-linking and inhibiting agents.(21)(22) The material gained popularity as a denture base material due to its biocompatibility, repairability, good esthetics, ease of manipulation and cost effectiveness.(5) In spite of these advantages, PMMA has disadvantages, such as low thermal conductivity (0.167-0.25 W/m.K), high-water sorption (~17 µg/mm3), poor impact (0.02 kJ/m) and flexural strength (~100 MPa).(23) Several investigations have been conducted on this material to overcome these disadvantages and improves its physical and mechanical properties. The addition of plasticizers, reinforced filler particles, bio-composites and even the modification of the processing conditions (pressure packing, high pressure-high temperature processing) have been attempted.(24)(25) 1.3.1 Poly(methyl methacrylate) PMMA is a synthetic polymer of methacrylate monomer, with IUPAC name of poly [1-(methoxy carbonyl)-1methyl ethylene] and poly (methyl 2-methyl-propenoate), from hydrocarbon and ester standpoints respectively (Fig. 1).(26) PMMA is a thermoplastic material, exhibiting optical transparency and is widely used due to its high impact strength, easy handling, and processing.(27) Apart from its industrial applications, it has been widely used in biomedical devices.(28) 6 Figure 1: Chemical formula of methyl methacrylate and poly(methyl methacrylate) [Source: https://www.dreamstime.com/methyl-methacrylate-mma-poly-methyl-methacrylate-pmma-molecule-methyl-methacrylate-monomer-production-pmma-image189692430] 1.3.1.1 Physical Properties of PMMA With regards to its physical properties, PMMA is a colorless, amorphous polymer with a glass transition temperature (Tg) of 100 °C to 130 °C and density of 1.20 g/cm3. The melting temperature of the polymer is 130 °C, with a linear shrinkage of (0.003-0.0065) cm/cm.(29) PMMA has high thermal stability in the range of -70 °C to 100 °C and high resistance to sunshine exposure, rendering it biocompatible.(26) The denture bases should have high thermal conductivity, in order for patients to be able to sense the temperature of the food/drinks. However, PMMA has low thermal conductivity (0.23 W/m∙K), which can affect the ability of the patient to sense these changes in temperature.(30) The polymerization shrinkage of the material can result in inaccuracies during denture fabrication.(4) However, PMMA has low polymerization shrinkage compared to other polymers and, therefore, it is still used as a denture bases.(31) PMMA also exhibits low color stability, becoming discolored over long periods of time in the oral environment. Various pigments can be added to PMMA to mask the discoloration and provide adequate esthetics.(32)(33) 7 1.3.1.2 Mechanical Properties of PMMA Good mechanical properties are an important requisite for denture base materials to ensure good functional performance, since they are subjected to complex masticatory forces in the mouth.(32) With respect to mechanical properties, PMMA has Young’s modulus of ~3 GPa.(29)(34) Studies have reported good flexural strength (~66 -78 MPa) of PMMA denture bases.(26) However, long-term water storage has led to decreased flexural strength. Fracture toughness is an important properties for PMMA to act as denture base material.(26) Studies have reported the fracture toughness of PMMA denture bases to be in the range of (1.84 ± 0.33) MPa·m1/2 to (2.11 ± 0.29) MPa·m1/2. (35)(36) The wear resistance of the PMMA is considerably lower than that of metals and ceramics.(37) 1.3.1.3 Biological Properties of PMMA If properly processed, PMMA has low residual monomer content, therefore good biocompatibility.(38) Any residual monomer remaining can cause allergies, mucosal inflammation, and cytotoxicity.(38) The residual monomer content can be decreased by increasing the degree of polymerization using the recommended processing cycle for PMMA.(38)(39) 1.3.1.4 Chemical properties of PMMA The chemical resistance is reasonable, PMMA is not affected by most aqueous solutions; however, it is highly soluble in aromatic hydrocarbons, esters, ketones and chlorinated solutions.(26)(29) 8 1.4 Denture Fabrication Techniques 1.4.1 Conventional Techniques 1.4.1.1 Compression Molding Conventional compression molding technique is the most widely used technique to fabricate denture bases and is used in conjunction with heat-activated materials. In this technique, an accurate wax trial denture is prepared and packed into a mold. The wax is then eliminated using hot water and detergent and a separating medium, such as sodium or ammonium alginate, is applied. The polymer and monomer are mixed in a ratio of 2:1.(4) Once the material reaches the doughy stage, the flask is closed, using gradual incremental pressure, which helps in distributing the resin throughout the mold space. Once the flask is fully closed, the polyethylene packing sheet is removed, along with excess flash material. The process is repeated till no flash is evident; the flask is closed for the last time under pressure. The flask is then transferred to the flask carrier under pressure. The polymerization reaction is activated by immersing the flask in a hot water bath at a set temperature. The curing can be done either by using a long or a short cycle.(4) 1.4.1.2 Injection Molding Injection molding technique was introduced in 1942 by Pryor to overcome the problem of polymerization shrinkage by forcing the resin into the processing flask.(40) Ivoclar was the first company to introduce the commercially available injection molding system (SR Ivocap) in the 1970s, (41) with Dentsply following the suit with their Success injection system (Fig. 2).(42) In these systems, capsules containing pre-proportioned polymer and monomer are used. A special flask with a sprue is used to inject the PMMA into the mold cavity using a piston at a set pressure 9 (8 MPa). The pressure is maintained as the material is slowly injected.(43) Curing is completed in the water bath using the long or short curing cycle.(4) Figure 2: Success Injection System (Dentsply International, Inc.) [Source: https://www.pattersondental.com/en-CA] 1.4.1.3 Fluid Resin Technique (Pour) The fluid resin technique (pour) uses a pourable, auto-polymerizing (chemically-activated) resin. This technique utilizes powder and liquid, which when mixed together in proper ratio yield a low viscosity resin. Wax trial dentures are invested in hydrocolloidal material, and the wax is eliminated using a hot water bath. The low viscosity acrylic is poured through sprue channels, under pressure. The flask assembly is placed in a pressure pot at ambient temperature and the resin is allowed to polymerize for 30-45 mins. The resin used in this technique is chemically activated and no use of external energy is needed. Dentures fabricated with this technique are inferior to those processed by compression molding or injection molding techniques.(4) 10 1.4.2 Comparison of Compression Molding with Injection Molding Technique Compression molding technique has been associated with numerous disadvantages. Polymerization shrinkage and the dimensional changes associated with it have been much higher than those associated with injection molding.(44) This could lead to problems with adaptation between the denture bases and the oral mucosa, compromising stability and retention.(45) It has also been reported that the compression molding technique can lead to occlusal discrepancies, necessitating a lab remount.(46) Compression molding technique can also lead to increased alteration in the incisal pin opening, leading to time consuming occlusal adjustments and disfigurement of the occlusal anatomy of the artificial teeth.(42) The injection molding technique claims to eliminate these errors and produce dentures that require minimal adjustments.(42) It has been reported that the injection molding technique produces more accurate denture bases with no difference in the laboratory working times.(42)(47) The occlusal adjustments are also minimal for denture produced by injection molding technique.(42) The deformation of the denture bases has also been minimal with injection molding technique compared to that of denture bases processed with the compression molding technique.(43) A study by Clements et al. has also reported increase in mechanical properties of denture teeth processed using the injection molding technique.(41) They also reported that the amount of residual monomer remaining with injection molding technique is less than that in dentures processed with compression molding technique.(41) 11 1.4.3 Digital Dentures (CAD/CAM) 1.4.3.1 CAD/Milled Milling is based on subtractive manufacturing, in which a finished product is obtained from a large blank with the use of computer numeric controlled (CNC) machines. The process involves sequencing, milling, tool movements through a series of commands that dictate the CNC machine into a specific direction. Varying sizes of burs are incorporated into the machine and the CAM software controls the bur(s) movement on the desired surface. CNC machines used in dentistry have multi-axis to facilitate the milling of dental prostheses in three dimensions.(48) Depending on the movement of the milling burs (𝑥-, 𝑦- and 𝑧- axes) and additional rotational axes, these can be divided into 3-, 4- (one additional rotational axis around x- axis) or 5-(two additional rotational axes around x-axis and y- or z- axis) axes milling machines.(49) 3-axis milling machines are the most widely used with the bur moving according to calculated path in 3 axes. The advantage of 3-axis milling machining is that they render a faster milling time and require minimal computation. However, these cannot produce convergence, divergence or highly defined features and surfaces. In addition, the restricted movement of the bur does not allow the production of large prostheses. 4-axis milling machines and its additional rotational axis around the x-axis (A-axis) allows for the milling of larger frameworks and prostheses. The 5-axis milling units allow for two rotational axes of the milling bur or blank. Complex geometries and smooth external surfaces can be produced using these machines.(48)(49) Acrylic denture bases with complex anatomies and smooth surfaces can be produced using 5-axes milling machines.(50) 12 Final functional impressions are taken, either conventionally or digitally. Once the impressions and prescriptions are transferred into the software, the gingival color base and the teeth set-up is chosen, and CAD is completed according to the protocols. Pre-polymerized disks (”pucks”) are then milled using 5-axis CNC machines fitted with suitable burs (Fig. 3).(51) The teeth are attached to the denture bases using bonding agent and the denture is finished and polished according to conventional protocols.(52) Figure 3: CAD/Milled Denture Base Processing (51) 1.4.3.2 3D Printing 3D printing, is an additive manufacturing process in which an object is created by adding multiple layers of a material to create a three-dimensional object. The process has been widely used in many fields, such as aerospace, engineering, medicine, and architecture. In recent years, in field of dentistry, its use in combination with CAD technology has been revolutionary.(53)(54) 13 Discretization and sequential stacking are two basic concepts for 3D printing. The CAD complex 3D object has it layers sliced and changed to 2D layers by discretization.(54) The multiple sliced layers have certain layer thickness with information about the contours and the design of the object to be printed.(53) Subsequently, sequential stacking deposits the material in pre-determined form and areas, with layers firmly stuck to the previous, creating the desired 3D model.(54) There are many rapid prototyping systems available, such as stereolithography (SLA), fused deposition modeling (FDM), selective laser sintering (SLS), inkjet based three-dimensional printing(3DP), laminated object manufacturing (LOM), laser engineered net shaping (LENS).(54) SLA, FDM, SLS and 3DP have been widely adopted for use in dentistry.(55) Stereolithography (SLA): SLA was the first rapid prototyping system, developed by Charles Hull in 1980’s.(56) The process involves building the object layer by layer on the build platform by tracing a beam of ultraviolet (UV) light on top surface of the photosensitive liquid resin. The UV light causes the solidification of the layer of the polymer resin. Subsequent layers adhere to the first solidified layer as the platform is lowered to the next layer thickness. The tracing of the layers continues as the platform is lowered and the solidified layer is coated with new self-adhering resin until the fabrication of the completed object.(54) Power of the light source, the composition and chemistry of the resin and amount of photo initiators, all effect the kinetics and the depth of the polymerization.(53) This process is widely used in dentistry for the fabrication of surgical guides, denture bases, maxillofacial prosthesis, and study models. The finished objects have high accuracy, good details, good surface finish and good mechanical strength. The disadvantages of this process include expensive equipment, wet handling, and additional post curing.(54) 14 Selective Laser Sintering (SLS): This process involves the fusion of powdered material, with a CO2 laser, to produce a 3D object. The measured powdered material is delivered incrementally through the cylinder, with the specific layer thickness, and is spread across the build platform. The laser traces the design on the surface of the powdered material, raising the temperature of the powder particles to their point of melting and ultimately forming a solid object. Modulation of the laser beam is done just as to melt the powder in the geometrically defined areas of the object. The temperature of the powder is maintained below its melting temperature but high enough to cause sintering.(54) Once the first layer is fused, additional layers are deposited by the moving roller on the build platform.(55) The process repeats until the completion of the object.(54) This process requires no post curing cycles. This process is used for the fabrication of metal substructures, ceramics and thermoplastic composite objects.(55)(57) Fused Deposition Modeling (FDM): In this prototyping technique, a thermoplastic material is extruded layer by layer through a temperature-controlled nozzle. The thermoplastic material is heated to convert it to a semi-liquid state, allowing it to flow. The thermoplastic material solidifies immediately after it is ejected from the nozzle head, bonding to the layer below with precision. The system deposits one layer on top of the other on the build platform, operating in XYZ axes. Any overhangs or supporting structures can be removed by cutting and polishing to achieve the desired object geometry.(54)(55) This process is usually used to manufacture medical grade bone models, surgical stents, etc., using a single manufacturing process. No post cure cycle is needed in this rapid prototyping process.(54) Three-Dimensional Printing (3DP): The process involves dispensing a measured amount of powder from the supply chamber. The supplied material is distributed and compressed using rollers over the fabrication chamber. A two dimensional pattern of liquid adhesive is jetted onto 15 the powdered layer, bonding to the powder below to form an object layer. The layers are subsequently added as the build platform moves incrementally and next powdered layer is spread and bonded. The layer-by-layer process continues until the whole object is completed. Any unbounded powder is removed with heat treatment. This technique provides fast fabrication with minimal material cost. However, the finished object lacks resolution, surface finish and adequate strength.(54) 3D Print Denture Fabrication 3-D denture printing technology utilizes digital light processing technology (Stereolithography), in which CAD data builds up a designed structure exposing the photopolymerizable resin liquid layers to ultraviolet light. Computer aided design image is projected through a slit, using a micro-mirror device, followed by projection of ultraviolet light onto the surface of liquid resin. Ultraviolet light polymerizes the layers of photopolymerizable resin leading to bottom up stacking of layers of certain thickness to form a 3-D object followed by a post curing cycle (Fig. 4).(58) Figure 4: 3D Printed dentures using SLA technology [Source- https://www.3dprintingmedia.network/carbon-dentsply-sirona-3d-printed-denture-workflow/] 16 1.5 Comparison between Conventional, Milled and 3 D Printed Denture Bases 1.5.1 Retention Lack of sufficient retention in the prosthesis is one of the most frequent complaints by denture wearers.(59) The retention of complete dentures is dependent of various factors, such as good adaptation to denture bearing area, border seal, surface tension, viscosity and film thickness of saliva.(60) Processing errors during conventional denture fabrication can lead to linear distortion in the denture bases in the range of 0.45 % to 0.9 %.(6) This has negative impact on fit by decreasing proper adaptation to tissues and ultimately reducing the retentive forces.(61) Digital denture fabrication process is an automated process and is able to accommodate error better than the manual processing techniques.(62) Compared with conventional denture bases, significantly higher reproducibility and tissue adaptation has been reported for CAD/CAM milled dentures. CAD/CAM had the smallest dimensional distortion, almost zero, and was the most reproducible processing technique.(6) Conventional techniques lead to volumetric changes during processing due to shrinkage.(6) The milled dentures, on the other hand, are processed from fully polymerized, higher density pucks, and show no distortion and better adaptation.(63)(64) There are only few studies comparing the adaptation of 3-D printed dentures with that of milled dentures. Chen et al. have reported that 3-D printed dentures have comparable tissue adaptation to that of milled dentures.(65) It has also been reported that digital dentures produce adequate mucosal compression, required for retention, without inducing inflammation of the tissues and are much more acceptable clinically than conventional dentures.(66) It has been found that 3-D printed dentures had better tissue adaptation than milled.(65) 3-D printed denture fabrication was able to reproduce residual ridge 17 irregularities better than milled denture fabrication, thereby resulting in better tissue adaptation and retention. The surface adaptation of 3-D printed denture has been within 100 µm accuracy.(58) 1.5.2 Accuracy and Reproducibility During the fabrication of dentures using the heat curing method, there is tendency of the palatal aspect to move towards the cast and for the borders to lift.(67) It has been reported that as the denture lifts from the model, there is increased dimensional changes between the denture and working cast.(68) The fabrication process with conventional fabrication techniques involves time consuming procedures, such as waxing, investing, and wax elimination, each incorporating some errors, leading to decreased accuracy.(69) In addition, residual stress incorporated during the packing is released during the removal, causing the finished denture base to shrink, resulting in the displacement of the denture teeth, leading to inaccuracies in the occlusion and poor fit. In case of maxillary dentures, the maximum amount of shrinkage occurs in the center of the palate.(70) CAD/CAM processing eliminates uncontrollable manual processing errors.(62) Therefore, the CAD/CAM fabrication process is more accurate as compared to conventional procedures.(6) However, a study comparing the deformation of denture bases and the influence of arch shape and palatal vault on shrinkage have found that CAD/CAM milled dentures are equally well fitting to dentures processed by injection molding and better than those processed by compression molding techniques.(71) In 3-D printed dentures, additive technology is employed and the denture base is layered in thin resin layers with no lifting from the model as the process is digitally controlled.(70) The accuracy of the light cured resin layered with additive technology is superior to that obtained by conventional heat-curing methods.(70)(31) The mean value of discrepancy measured in the mid- 18 palatal suture area between the maxillary denture and the working cast was reported to be lowest in the 3-D printed group, followed by milled and injection molding groups.(72) 3-D printed digital denture offer added advantages, such as the ability to bypass deep undercuts, to obtain finer reproduction details, be faster, have lower anisotropy as well as lower material costs.(73) The accuracy of printed dentures is influenced by factors such as intensity of light, orientation and print angle, printing software, number of layers, supporting structure, postprocessing techniques, shrinkage between layers and offset between teeth and denture recesses.(72) Different 3-D print resins have their own activation ranges and require different light wavelength and exposure time for polymerization. Compatibility of resin material with all available 3-D printers is also an issue.(72) 1.5.3 Residual Monomer Content The presence of remaining unreacted residual monomer is highly undesirable as it can compromise the mechanical and physical properties of the denture bases as well their biocompatibility.(74) Residual monomers are likely to leach into the surrounding tissues, leading to cytotoxicity and possible responsibility for allergic reactions. Burning mouth sensation, denture stomatitis, tissue edema, ulceration of the oral mucosa are some of the symptoms associated with residual monomer leaching into oral cavity.(74)(75) The amount of leached residual monomer is directly proportional to the concentration of remaining monomer within the processed resin. Hence, increasing the degree of conversion of residual monomer is desired, which depends upon the curing methods, temperature, pressure application and processing time.(74)(76)(77) 19 Conventionally fabricated denture bases have been shown to release residual monomer into the surrounding environment, with significantly higher amounts released from chemically-cured than from heat-cured dentures, associated with the degree of polymerization achieved.(78) Digitally milled dentures are fabricated by milling from polymethylmethacrylate pucks that have been pre-polymerized under high pressure and temperature.(79) These conditions lead to higher degree of monomer conversion as well as formation of longer polymer chains, leading to lower residual monomer content.(80) It has also been hypothesized that since CAD/CAM processes eliminate the human impact on processing, there have been greater standardization of the processes involved, leading to higher degree of polymerization, lower porosity and increased mechanical properties.(74) A study comparing the CAD/CAM denture bases with conventional denture also reports that higher material thickness of the pre-polymerized PMMA puck might also prevent the leaching of the residual monomer from the center, which often is milled as the denture surface. The results of the same study reported that lower monomer release from CAD/CAM was statistically significant when compared to auto-polymerizing conventional dentures.(74) No available literature is available comparing the release of residual monomer from printed denture bases over long term. 1.5.4 Physical and Mechanical Properties Dentures can break upon impact, so it is necessary for them to have a higher impact strength.(81) Better mechanical properties may also allow for dentures to be fabricated without need for any additional strengthening through metal or fiber reinforcement. High modulus of elasticity can prevent crack propagation.(82) Dentures can then be fabricated with minimal thickness without incidents of fracture and thereby increasing patient comfort. Patients will experience more natural 20 speech and feel as the denture requires less volume within the oral cavity.(83) The resins materials used for conventional and digital denture bases are chemically similar, however they differ in the process of production. The mechanical properties of digital dentures are distinctly different from those of conventional dentures.(83) Digital dentures possess increased impact strength, ultimate strength and higher modulus of elasticity.(79)(81)(84) These can be explained as CAD/CAM PMMA pucks are manufactured under high pressure and temperature resulting in minimal shrinkage, porosity and free residual monomer.(79)(81) However, results of another study reported higher flexural strength for CAD/CAM milled dentures but lower for 3D printed dentures.(81) Surface hardness of the CAD/CAM dentures has also been reported to be higher than that of conventional dentures, preventing mechanical damage, retention of plaque and stains and increasing life of the dentures.(81)(85) 1.5.5 Color Stability Color stability is an important clinical feature of denture bases. To create an esthetic appearance, color and appearance matching to the underlying tissues is required of denture base materials.(86) Color change is a major cause of patient dissatisfaction with prosthesis and reason for replacement.(87) Aged and worn dentures show changes in color, leading to unaesthetic appearance.(88) Water sorption, physical and chemical changes, accumulation of stains due to exposure to foods and beverages (coffee, spices, wine etc.) and increased surface roughness can affect the color of the denture bases.(89) Red wine has been shown to have greater effect on the color due to its acidic pH as well as the alcohol content. Alcohol can lead to softening of the material, changing the surface smoothness and causing expansion of polymer chains, leading to higher absorption of pigments.(90)(91) Resin composition and method of polymerization can also influence color stability.(92) Conventional denture bases processing is dependent upon the 21 technician for proper material mixing ratio, processing time, which can affect color stability.(93) On the contrary, CAD/CAM processing relies on industrially pre-polymerized pucks, which have better mechanical properties, less porosity, less residual monomer, less water sorption and show less wear. All these factors lead to a better color stability in CAD/CAM dentures.(94) A study has also reported that the greatest susceptibility to color change is the interface between the prosthetic teeth and the denture base due to effects of polymerization shrinkage.(95) Stains and microorganisms can penetrate this interface leading to discoloration.(95) 1.5.6 Number of Appointments Difference in the number of appointments has been reported for dentures fabricated via digital and conventional processing. There are various commercially available companies, such as Weiland (Ivolcar), AvaDent, Dentca and Ceramill. Each of these companies have their own protocol for digital denture fabrication, appointments ranging from 3 appointments (Weiland (Ivolcar), AvaDent, Dentca) to 4 appointments (Ceramill), which is still less than the 5 appointments required for conventional dentures. It has been reported that digital dentures require a mean 2.39 appointments to delivery compared to 5 appointments for conventional denture, with no influence of operator experience level.(96) Another study reported that digital dentures can be delivered in 2 appointments even by students in predoctoral setting under faculty supervision.(97) A significant reduction in clinical time by 3.5 hours compared to conventional dental fabrication has also been reported.(97) This reduction in clinical time can be utilized to ensure good outcome through good laboratory communication and provide more effective treatment.(96)(98) Misfit range for CAD/CAM denture bases was lower than for those of conventional technique. Therefore, it can be postulated that there will be less need for postinsertion appointments for 22 CAD/CAM dentures will also be less. Bidra et al reported patient centered and clinical results of 2 appointment CAD/CAM denture bases and found that an average of 3.3 denture adjustments were needed for CAD/CAM denture bases.(98) Another study, however, reported no significant difference in the number of appointment for conventional or digital denture bases.(99) 1.5.7 Cost There have been studies supporting that the number of appointments required for digital dentures are less than that for conventional dentures.(96)(97)(98) However, none of these studies have quantitatively determined the cost of each processing technique. A study by Srinivasan et al. compared the cost of digital denture fabrication with conventional denture and found that the overall cost of digital denture is approximately 50 % less than that of conventional denture.(100) They have also reported that in spite of higher clinical material cost initially, the laboratory costs are significantly reduced.(100) During their cost minimization estimation, an estimated ten-year profit of about $150,000 was determined within a university setting with digital dentures over conventional dentures.(100) However, the initial cost of milling units is higher than that of the 3-D printers and more suitable for commercial manufacturing rather than for small dental laboratories. The amount of energy consumption is also significantly higher. Subtractive milling is less environmentally friendly as it leads to material waste, thereby contributing to environmental pollution. Despite the high cost, small desktop and intraoral scanners can be afforded by individual dentists and small laboratories. These scanners can be connected to digitally print in house 3-D dentures, thereby avoiding delays in delivery and shipping costs.(101) 23 1.5.8 Candida Colonization Colonization by Candida in denture wearers has been established as one of the predisposing factors for denture stomatitis.(102) The adhesion of Candida to the denture occurs through the adhesion of biofilm to the denture and also on surface irregularities present on the denture.(103) Conventional processing techniques result in surface porosity due to air entrapment, uncontrolled temperature, evaporation of monomer, inadequate pressure and residual monomer content.(104) This can result in porosity and roughness and can lead to adherence of Candida species.(105) Since CAD/CAM dentures are processed from pre-polymerized pucks, there is less porosity and a better surface finish, leading to decreased adherence of Candida.(106) Bidra et al. also reported decrease adhesion of Candida with CAD/CAM denture surface due to decreased porosity.(98) Conventional denture bases also need to be stored in water as hydration negates the effect of polymerization shrinkage and establishes equilibrium with residual monomer content.(4) CAD/CAM denture experience less distortion, hence water storage can be avoided. A recent systematic review has found that Candida growth is slower on the surface of dry dentures.(107) 24 Table 3: Summary Table Comparing Conventional Denture with CAD-Milled and CAD-3D Printed Dentures Outcome Conventional CAD- Milled CAD-3D Printed Retention Low High High Accuracy & Reproducibility Low High High Residual monomer release High Low Low Tooth movement and Occlusal discrepancies High Low Low Color Stability Low High High Patient satisfaction Low High High Esthetics High Low Low, but higher than milled Number of appointments More (5) Less (3) Less (3) Post insertion appointment More Less Less Clinical Time More Less Less Cost High but with low initial material cost Low with high initial material cost Low with high initial material cost 25 Outcome Conventional CAD- Milled CAD-3D Printed Mechanical Properties Inferior to CAD/CAM milled but superior to printed Superior to Conventional Inferior to both milled and conventional Chances of Fractures High Low Low Electronic storage and archive Absent Present Data be Electronically stored and used at later date Present Data be Electronically stored and used at later date Denture adhesive Increases retention Decreases retention Decreases retention Biocompatibility Low High High Special equipment No special equipment required Required Required Processing Manual Automation Automation Tooth Bonding to denture base Chemical during the processing. No special bonding agent needed. Special bonding agent needed to bond teeth with denture base recess Special bonding agent needed to bond teeth with denture base recess Duplication Need more clinical and laboratory steps Can be done in 1 appointment Can be done in 1 appointment Deep Undercuts reproduction Easy Difficult Very easy 26 Outcome Conventional CAD- Milled CAD-3D Printed Material Waste High High Low Operator skills/experience High skills and experience needed to build good dentures Not required Not required 1.6 Effect of Water Storage on Denture Bases Water sorption is one of the disadvantages associated with PMMA material being used as denture base materials. Water molecules can easily penetrate the PMMA polymer chains and act as plasticizer, causing expansion and dimensional instability.(108) The dimensional stability of the PMMA material is related to its solubility as well.(18) ISO 20795-1 recommends that the water sorption and solubility of the denture base material to be less than 32 µg/mm3 and 1.6 / µg/mm3, respectively.(19) Most commercially available denture materials fulfil these minimum requirements.(19) Water sorption also effects the color stability of the denture bases. Water sorption and solubility are directly affected by the residual monomer content.(32) The higher the residual monomer content, the higher is the water sorption and solubility of the denture base materials. Since the residual content of the CAD/CAM and 3-D printed denture material is less than that of conventionally fabricated denture bases, the dimensional changes in the former are lesser than those of the latter.(74) 1.7 Fractures of Denture Bases Denture bases do not fracture just from the obvious direct trauma/fall, but also due to the flexural fatigue that sets in over repeated use.(109)(110) A denture base is flexed millions of times during 27 its use. Maxillary denture flex as much as 1.5 mm, while mandibular denture flex in a range of 1.5 mm to 3.6 mm during the masticatory cycle, with the maximum in the midline area.(109) The average fracture rate in removable dentures is as high as 68 %.(111)(112) Clinical surveys have shown that the most common fractures in denture bases are midline fractures.(111) Occlusal forces lead to the development of microscopic breaks or cracks in the denture base. Under repeated stress, this crack continues to grow, eventually leading to the catastrophic fracture of the denture base.(113) Sharp frenal notch, diastema and presence of tori can increase the incidence of the fractures by introducing stress concentrators.(111)(114) Finite element stress analysis investigations have found that longer and sharper frenal notches can result in higher concentration of stress, thereby in higher chances of fracture initiating from these areas.(114)(115)(116). Notch geometry has been shown to play an important role in the nature of stress occurring in denture bases. A shallow, smooth notch has higher tensile forces acting than a deep and sharp frenal notch.(111) PMMA has been widely and efficiently used as the denture base materials since decades. However, the material has poor impact strength and low fatigue resistance, thereby fracturing often.(117) 28 1.8 Fracture Toughness of Denture Bases Denture bases are subjected to various stresses while functioning in the mouth.(118) The presence of surface flaws on the denture bases can act as stress concentrators leading to crack propagation and ultimate failure of the denture bases.(119) A denture base material should be able to resist crack propagation and withstand surface flaws to produce a strong and fracture resistance denture, enhancing longevity and clinical performance.(120) Fracture mechanics methodology can be used to measure the resistance of denture base to crack propagation.(121) Fracture toughness (KIC) is an intrinsic material property, characterizing the ability of a material to resist unstable crack propagation, caused by internal flaws, under applied force (tension).(122) Hence, requirement of adequate KIC is an important requirement for denture bases.(122) Chevron notched short road (CNSR) specimen (123), tapered cleavage specimen (124), single edged notched beam (SENB) specimen (125) and notchless triangular prism (NTP) specimen (126) KIC tests have been used to determine KIC of denture base materials. 1.8.1 Appropriateness of KIC testing in denture bases: Clinical failure of the denture bases is a slow process; therefore, KIC testing is more appropriate than impact strength testing. Impact strength is dependent of geometry and loading conditions, while KIC is independent of these factors and dependent only on the tested material.(127)(128) Thus, to assess the ability of a material to resist fracture, KIC is a reliable quantitative method.(129) 29 1.8.2 Challenges to conducting KIC in denture bases: Studies have been performed on polymers to predict the degree of stress intensity at the flaw tip, which lead to crack initiation.(130) Due to the heterogeneity of the available denture base materials, KIC values differ for individual denture bases materials.(131) Molecular weight and degree of cross-linking in denture base materials can also affect KIC. Depending on the method of fabrication, each type exhibits different degree of brittleness, due to differences in crosslinking, molecular weight, viscoelasticity and chemical composition of the matrix.(118)(132) Application of fracture mechanics is quite complex for denture bases, as crazing and development of plastic zone can occur around the crack tip.(133) 1.9 Notchless triangular prism (NTP) specimen KIC test CNSR test, introduced by Barker, has been a commonly used method to determine KIC of brittle materials.(123) The method was subsequently modified to accommodate small samples of dental relevance.(134) However, there are several disadvantages of this method as it is much more technique sensitive, difficult to control with small brittle material sample and error prone in characterization of adhesive interfaces.(126) In 1996, Ruse et al. developed and validated a novel method to determine KIC of various dental materials, as well as of bonded interfaces, while retaining the overall CNSR geometry (Fig. 5).(126) The NTP specimen KIC test is detailed in the materials and methods section (Pg. 44). There are several advantages of this methods, as the specimen preparation is simple and reproducible, allows testing of materials with KIC < 1 MPa∙m½, can be applied to various materials 30 and interfaces, provides controlled and stable testing conditions, and results in values comparative to those obtained by CNSR testing.(126) Figure 5: NTP testing assembly (135) 31 Chapter 2: Research Question and Specific Aim 2.1 Research questions • Do newly developed CAD/CAM [Milled (M)] and 3D-printed (P) denture base materials have improved fracture toughness (KIC) compared to conventionally processed (C), counterparts? • Does long-term water storage effect KIC of the denture bases? 2.2 Aim The aim of this project was to use the NTP specimen KIC test (126) to determine KIC of a conventionally fabricated denture base material and compare it with that of CAD/CAM and 3D-printed materials. KIC was determined after storing the samples in 37 °C water for 7 d and 90 d. The guidelines of ISO 20795-1 (19) were followed for the ageing and conditioning of the samples prior to testing. 2.3 Null Hypotheses (H0) • There is no difference between KIC of the three different denture base materials investigated. • There is no effect of long-term storage in 37 °C water on the KIC of the three different denture base materials investigated. 32 Chapter 3: Materials and Methods 3.1 Test Materials: For this study, the test materials were kindly donated by Dentsply International Inc. (Dentsply International Inc., York, PA), Table 4 lists the materials used in this study. Table 4: Materials Materials Lot Manufacturer Lucitone 199 Denture Base Resin (C) 015822 Dentsply International Inc., York, PA Lucitone 199 CAD (M) 180222 Lucitone Digital Print (P) NA 3.1.1 Lucitone 199 (C) Denture Base Lucitone 199 Denture base resin has been a Dentsply Inc. proprietary resin since the 1980s. Available in different shades and translucency, it has been extensively used by laboratories and clinicians for denture base fabrication. The materials come in an inactivated powder and liquid form, which, when mixed together in a ratio of 2:1, results in a homogenous polymer resin to be used as denture base material. Table 5 lists the components and their composition. Table 5: Lucitone 199 (C) Denture Base Composition Component Composition Weight % Powder Poly(methyl methacrylate) (90-100) % Liquid Methyl methacrylate (80-100) % Ethylene dimethacrylate (1-2) % 33 3.1.2 Lucitone 199 CAD (M) Denture Base Lucitone 199 discs are used in CAD/CAM digital denture base fabrication. Available as pre-polymerized 98.5 mm diameter discs of various heights (20 mm, 25 mm, 30 mm, or 35 mm), enabling the laboratories to mill them directly into denture bases. They maintain dimensional stability under high-speed milling and offer a clean, safer way to fabricate denture bases to which acrylic teeth can be bonded or milled together. Table 6 shows the chemical composition of these discs. Table 6: Lucitone 199 CAD (M) Denture Base Composition Components Weight (Wt) % Polymer blend * [mainly poly(methylmethacrylate)] (>99) % * The exact concentration of components is withheld by Dentsply as trade secret. 3.1.3 Lucitone Digital Print (P) Denture Base Lucitone Digital Print is a novel light-cure resin material developed by Dentsply. It has received FDA approval in 2019 for the fabrication of digital denture bases. Fabrication of prosthesis using this material requires a CAD system and an additive 3D printer. Carbon 3D Digital printers have been optimized for this resin material and require wash and light cure post processing. Similar to Lucitone 199 discs, it requires chemical bonding to adhere denture teeth to the base. Table 7 lists the chemical composition of the resin. 34 Table 7: Lucitone Digital Print (P) Denture Base Components Weight (Wt) % Urethane Methacrylate (40-50) % Methacrylate Monomer (40-50) % Acrylate monomer (1-5) % Photoinitiator (1.5) % 3.2 Processing of denture bases for sample preparation 3.2.1 Lucitone 199 Denture base resin group (C): Horizontal bars measuring [(65x20x10) mm] were prepared in the lab by stacking layers of baseplate wax (Dentsply TruWax, York, PA). The horizontal wax bars were finished, polished and made ready for investing in the plaster. Commercially available flasks were used with the SUCCESS injection system after application of petroleum jelly for easy release. Type 3 dental stone (Microstone, Whipmix, Louisville, KY) was mixed in manufacturer specified water: powder (40 ml: 140 g) ratio, poured into the dental flask and the wax bars were embedded in it. Any undercuts present were eliminated, and Success Sprue Wax was used to build the injection sprue on one side of the investment flask. Separating medium (Petroleum jelly) was applied after the stone had set. The upper half of the flask was attached to the lower half. Type 3 dental stone (Microstone, Whipmix, Louisville, KY) was mixed and poured to cover the remaining top half of the flask and allowed to set completely. The injection flask was placed in boiling water for (6 to 10) min to soften and eliminate the wax. The flask was opened after complete wax boil-out, removing and discarding the remaining wax. 35 The residual cavity and the flasks were flushed and cleaned thoroughly to remove any remaining wax residues. The cavity and the margins of the flask were verified for intimate contact. Two coats of alginate separating media was applied to the dental stone before the acrylic was packed into the flask. The injection flask was attached to the Success injection system using metal injection insert, as per instructions. As per manufacturer instructions, Lucitone 199 powder (17 g) was mixed with the liquid monomer (8 ml) and stirred for 15 sec. in order for the powder particles to be completely wetted. Once the material reached “soft pack” stage, it was loaded into the injection cartridge. The flask was placed into the injection unit and positioned using the “O” ring and tightened. Once the flask was secured and aligned vertically, the activation switch of the injection unit was turned on and acrylic material was injected into the flask. A pressing device with piston was attached to the injection socket and kept in assembly for a couple of minutes. The flask was allowed to cool on bench top for 30 min to avoid porosities. The flask was put in a water bath for heat curing, using the long cycle at temperature of 73 °C for 9 h, followed by bench-top cooling for 30 min. The flask was unscrewed, the top half of the investment flask was separated from the bottom half and the processed acrylic bar was removed from the flask. The excess flash from the bar was removed and final finishing and polished was done using acrylic burs and wheels. Fig. 6(a) and (b) show the finished acrylic bars fabricated by the conventional injection molding method. The NTP specimens were prepared from these bars by cutting, finishing and polishing to required dimensions. 36 Figure 6(a), (b): Finished acrylic bars fabricated by the (C) injection molding method 3.2.2 Lucitone 199 CAD Denture Base Disc Group (M): The pre-polymerized [(98.5x35) mm] disc (Fig. 7) was cut, using a handsaw, into blocks that were further processed by grinding and polishing into NTP specimens of required dimensions. Figure 7: Lucitone 199 CAD Disc a b 37 3.2.3 Lucitone 3D Print Denture Group (P): Autodesk Fusion 360 (San Rafael, CA), which is a cloud-based CAD software, was used to design oversized NTP of dimensions (10x10x10x40) mm (Fig. 8). The CAD was subsequently converted into a standard tessellation language (STL) file, which is the native file for the stereolithography printing (Fig. 9). The STL file was sent to Carbon M2 printer (Redwood City, CA) for the 3D printing of NTP. The orientation of the prism with respect to the build platform was 0°. The XY (2D) resolution of the printer was 75 𝜇m (default setup by manufacturer) and Z resolution was set at 50 𝜇m. Lucitone 3D print resin was used as tank material. The printed NTP were given a wash cycle in ethyl alcohol and post processed using a light curing unit to fully cure to final stage. These oversized NTP (Fig. 10 a and b) were further processed by grinding and polishing into NTP specimens of required dimensions. Figure 8: CAD file designed using AutoDesk Fusion 360 Software 38 Figure 9: STL file for printed samples Figure 10 (a), (b): Printed triangular prisms a b 39 3.3 Sample Size Calculation A power analysis (a = 0.05 and b = 80 %), according to Lehr’s equation (Equation 1) (Rule of Thumb) (131) was performed to determine the required sample size (n). A 20 % difference between the means of two groups was considered to be clinically relevant. n = 16∆! ∆= 𝜇"#$!𝜎 = 𝛿𝜎 Equation 1: Lehr’s Rule of Thumb In Equation 1, D is the standardized difference in units of the standard deviation, is the treatment difference to be detected. The mean and standard deviation from the study by Lee et al. (35) were used in the calculation. The following values were used to calculate the sample size. Mean = 1.84 SD = 0.33 ∆ = 1.12 The calculated sample size was n = 13 per group. Denture bases are brittle material and the presence of intrinsic defects of different sizes can affect KIC. Since these intrinsic defects are not normally distributed in the material and are more likely to result in material failure under a given stress, a two-parameter Weibull statistics [equation 2: (136)] was used to analyze the results. Pf = 1 − 𝑒𝑥𝑝 9− :%"##%"#𝔲 % "#& ; '< Equation 2: Two-parameter Weibull statistics 40 As recommended by Quinn and Quinn (136), a sample size of 20 would produce least bias in a Weibull analysis. Therefore, a sample size of n=20 for each test group was chosen. 3.4 KIC Test The notchless triangular prism (NTP) fracture toughness (KIC) (126) test was used to determine the KIC of conventional (C), CAD/Milled (M) and CAD/3D-printed (P) denture base materials. 3.5 Specimen Preparation Forty NTP specimens [(6x6x6x12) mm] were prepared for each group. For conventional (C) test groups, the prepared bars were polished on each side using silicon carbide abrasive disk of grit size 600 (Buehler, Lake Bluff, IL) to limit finishing/polishing at later stages. The bars were subsequently adhered using double-sided tape, to a custom fabricated jig (Fig. 11) that allow 60° angle cutting of specimen, resulting in a prism. The whole assembly was transferred to the Isomet low speed saw (Buehler, Lake Bluff, IL) and the prism specimens were cut from the bar using a diamond impregnated blade (MK Diamond Products, Inc., Torrance, USA) under constant irrigation (Fig. 12). The prisms obtained were ground, finished and polished using custom finishing/polishing jig under constant irrigation using 320/600-grit SiC paper on a Metaserv wheel grinder (Buehler, Lake Bluff, USA) to their final dimensions. For Milled (M) and Printed (P) test groups, the previously prepared triangular prisms were mounted in a custom finishing and polishing jig (Fig. 13). The samples for both these groups were finished and polished using the same methods as for the conventional test group samples, using SiC abrasive discs. 41 A digital caliper was used to ascertain the dimensions of the specimens at various intervals until the final dimensions were vobtained [(6x6x6x14) mm]. The length of the prism specimens was kept at 14 mm, 2 mm longer than the required length for the custom specimen testing holder. This increased length has no effect on the testing methodology. However, it allowed for proper holding of the specimen in the custom specimen holder and prevented the sample from slipping out of the holder. Figure 11: Custom Cutting Jig Figure 12: Isomet saw 42 Figure 13: Custom Grinding Jig 3.6 Water storage and conditioning The prepared 120 NTP samples were aged in a (37 ± 1) °C water bath (Isotemp Incubator Model 630D (Fisher Scientific, Ottawa, Canada) (Fig. 14). Half of the samples (n = 20/group) were tested after 7 d of storage and the other half (n = 20/group) after 90 d of storage. Prior to testing, the samples were moved to a (23 ± 1) °C water bath and conditioned for (60 ± 15) min, as per ISO 20795-1.(19) Figure 14: Samples being aged in an Incubator 43 3.7 Notchless Triangular Prism (NTP) Specimen Fracture Toughness Test Each specimen was carefully viewed under a light microscope (Olympus, Tokyo, Japan) to check for irregularities and to identify a flawless edge on which to create a crack initiation point. The prisms were secured in one half of the custom specimen holder (Fig. 15). A ~0.1 mm deep defect, to act as a crack initiation point, was introduced on the flawless edge using a 200 𝜇m thick surgical GEM stainless-steel blade (Ted Pella Inc., Redding,CA) and verified under 6X optical magnification. The side opposing the initiated defect was marked using a sharpie point to identify the direction of crack propagation, later to be used for fractographic analysis under scanning electron microscope. A custom mounting jig was used to secure the two halves together, utilizing a 200 𝜇m thick spacer that kept the two halves of the specimen holder apart, replicating the CNSR specimen configuration [Fig 16 (a),(b)]. The test assembly was secured in custom designed grips, attached to an Instron 4301 (Instron; Canton, MA), equipped with a 1 kN Instron load cell and controlled by Bluehill 2 software (Instron, Norwood, USA) (Fig. 17). The test assembly was loaded in tension at a crosshead speed of 0.1 mm/min until crack arrest or failure. The maximum recorded load was used to calculate KIC, using the equation below:(126) 𝐾() = 𝑌'*+∗ 𝑃'-.𝐷𝑊//! Equation 3: Calculation of KIC In Equation 3, Y*min = minimum value of the dimensionless stress intensity factor coefficient (28 for the NTP test), D = specimen diameter (12 mm), and W = specimen length (10.4 mm). 44 Bubsey et al. reported the Y*min value for specimens in which length/diameter (W/D) ratio was between 1.5 to 2 and the initial crack length (a0) to specimen length (W) ratio (a0 ) was in range of 0.2-0.5.(137) In NTP testing, a0 is 0.5 and the W/D ratio is 0.88. Since ratio of W/D is outside the range reported by Bubsey et al., Ruse et al. extrapolated and calculated the value of Y*min to be 28 for the NTP testing.(126)(137) Figure 15: NTP specimen custom holder Figure 16 (a), (b): Custom Mounting Jig 16a 16b 45 Figure 17: Test Assembly 3.8 Statistical Analysis The collected KIC data was assessed for normality and analyzed by two-way ANOVA followed by Scheffé multiple mean comparisons (𝛼 = 0.05) (SPSS v27 software; IBM, Armonk, NY). For between mean comparisons at 7 d and 90 d, independent student t-tests were performed. Weibull statistics was also used to analyze the results, to determine the characteristic Weibull KIC and the Weibull modulus (m). The equation 4 below, results in the graphical representation of these Weibull parameters. Equation 4: Weibull formula The analysis of these graphs provides us with the parameters attributed to different tested material. The slope of the linear trend line provides us the Weibull modulus (m), which indicates the 46 reliability of the tested material. The steeper the slope, the higher the Weibull modulus (m) and the higher is the reliability of the tested material. Characteristic Weibull value (KIC) was determined as the value corresponding to the failure probability value Pf = 63.2%. 3.9 Scanning Electron Microscopy Since crack arrest was recorded for all test specimens, they were broken into halves by hand for fractographic analysis. The two halves of all fractured samples were examined under a light microscope to characterize the site of crack initiation, surface texture, presence/absence of defects and failure mode. One representative sample from each test group was selected (closest to the mean KIC) for fractographic analysis with a scanning electron microscope (SEM) (Hitachi, S-3000N; Hitachi, Japan). Samples were mounted on SEM studs and coated with gold in an Edwards S150A sputter coater (Edwards Vacuum; Crawley, UK). Photomicrographs were recorded for each half at various magnifications (25x, 150x, 500x). 47 Chapter 4: Results 4.1 Fracture Toughness (KIC) Twenty samples from each group were tested at both 7 d and 90 d, meeting the minimum sample size requirement for performing Weibull statistics. For each test group and storage time, the results are summarized in the Table 8 and presented as Box plots in Fig. 18: Table 8: Results (Mean ± SD)# Material Group KIC (MPa·m1/2) 7 d 90 d Lucitone 199 Denture base resin (C) 2.09 ± 0.13 b* 1.82 ± 0.19 b* Lucitone 199 CAD (M) 1.96 ± 0.14 b* 1.59 ± 0.27 c* Lucitone Digital Print (P) 2.23 ± 0.11 a 2.24 ± 0.17 a # Identical small letter superscript identifies no significant differences between the test groups at the same storage time; Asterisks identify significant differences between 7 d and 90 d groups. The box plots show the median, minimum and maximum range of KIC for each test group after 7 d and 90 d (Fig. 18). The plots show that P groups have higher KIC than C and M groups. Wider distributions were noted in all the test groups after 90 d storage in 37 °C water. 48 Figure 18: Box Plot Analysis of the results by two-way ANOVA followed by Scheffé post hoc tests, showed that the three tested materials had significantly different KIC at both 7 d (Fig. 19) and 90 d (Fig. 20), with the same ranking, i.e., P > C > M (p < 0.005). Independent student t-test for between mean comparisons at 7 d and 90 d, have shown that ageing in 37 ºC water for 90 d resulted in a significant decrease in KIC in the C and M groups (p < 0.001). 49 Figure 19: Scheffé post hoc KIC 7 d Figure 20: Scheffé post hoc KIC 90 d 50 4.2 Crack Propagation and Arrest Fig. 21, 23 and 25 show representative 7 d and 90 d load-displacement curves for specimens in groups C, M, and P, respectively. The curves of groups C (Fig. 21) and M (Fig 23) look similar to each other, with the 7 d slopes being much steeper than the 90 d ones. The 7 d and 90 d slopes of the P group (Fig. 25) are similar to each other and look similar to the 90 d C and M curves. Upon examination of the 7 d and 90 d cracked surfaces under a high-resolution macro lens, a smooth fracture was noticed for C (Fig. 22) and M (Fig. 24) denture base specimens, with crack arrest. No permanent deformation was seen in the specimens and the morphology was indicative of brittle fracture. Calibrated analyses of the cracked surfaces showed that the crack propagated much farther (4.38 mm) in 7 d specimens (Fig. 22 a) compared to at the 90 d specimens (3.43 mm) (Fig. 22 b) C group. Similar results were seen for the M group (Fig 25 a and b), with crack arrest occurring much farther (3.81 mm) at 7 d than at 90 d (3.34 mm). Distinct failure zones in the C and M groups are seen as shown by pointed arrows in the Figs 22 a, b and 24 a, b. The highest load values were recorded in P group (Fig. 25), followed by C group, and M group displaying the lowest values. The 7 d P group showed a distinct load-displacement behavior: increase in load up to ~ 100 N, followed by a plateau stage where minimal changes in load occurred with changes in displacement. Once sufficient energy was absorbed, the plateau was followed by a subsequent increase in load before a sudden drop, corresponding to failure by crack arrest (Fig. 25). The 90 d P specimens did not exhibit this behavior. Both 7 d and 90 d P specimens showed crazing (broken double arrows), with a teardrop pattern at the crack tip before the arrest occurred (pointed arrows) (Figs. 26 a and b). There was also large permanent plastic deformation visible in these specimens. Two distinct teardrop patterns were seen 51 in the 7 d specimen while only a single teardrop was seen in 90 d; both were followed by a zone of crazing. Another interesting finding was that the zone of crazing was much larger in the 90 d specimens than in the 7 d specimens. However, the crack arrest in both 7 d and 90 d specimens occurred at similar distances (4.19 mm and 4.21 mm). 52 Figure 21: Load/Displacement graph for C group at 7 d and 90 d 0501001502002500 1 2Load (N)Displacement (mm)Conv 7 d Conv 90 d 53 Figure 22 (a), (b): Crack Propagation and arrest in C specimens at 7 d and 90 d 22a 22a 22b 22b 54 Figure 23: Load/Displacement graph for M group at 7 d and 90 d 0501001502002500 1 2Load (N)Displacement (mm)Milled 7 d Milled 90 d 55 Figure 24 (a), (b): Crack Propagation and arrest in M specimens at 7 d and 90 24a 24a 24b 24b 56 Figure 25: Load/Displacement graph for P group at 7 d and 90 d 0501001502002500 1 2Load (N)Displacement (mm)Print 7 d Print 90 d 57 Figure 26 (a), (b): Crack Propagation and arrest in P specimens at 7 d and 90 d 26a 26a 26b 26b 58 4.3 Work of Fracture Work of fracture (WOF) is the work per unit area, i.e., the total energy required for crack propagation divided by the cross-sectional area of the crack. Table 9 summarizes the WOF for the specimen groups and was determined by calculating the area under the load displacement graphs (in J) (Figs. 21, 23, 25) and dividing it by twice the cross sectional area (in m2) of the fractured surfaces [Figs. 22, 24, 26 (b)]. WOF = U/ 2SA Equation 5: Calculation of WOF Where U = energy (in J) calculated from the area under the load displacement curves, SA = surface area (in m2) of the fractured surfaces. Table 9: Work of Fracture (WOF) (in KJ/m2) Material (Group) WOF (in KJ/m2) 7 d 90 d Lucitone 199 Denture base resin (C) ~ 2.3 ~ 1.7 Lucitone 199 CAD (M) ~ 2.7 ~ 1.8 Lucitone Digital Print (P) ~ 8.7 ~ 6.3 59 4.4 SEM Analysis Figs. 27-32 show SEM images recorded under 25x and 150x magnification, revealing various zones on the fractured surfaces: the zone of crack initiation and slow crack propagation, followed by the zone of rapid crack propagation, clearly discernable at 150x magnification and marked on the figures. The zone of crack initiation was marked by the presence of vertical striations on the surface, and it was present in all groups. The immediate relatively smooth zone, from where cracks radiate outwards from the flaw (crack initiation), was the zone of slow crack propagation, also called the mirror. Immediately following the mirror, the presence of hackle lines marks the area of rapid crack propagation. The materials underwent crack propagation and ultimately crack arrest, distinct from the hackle lines. The crack arrest was well defined for 7 d C and P groups, but not for the others. All specimens, except for the 7 d P specimen group, showed single “mirror” immediately adjacent to the crack initiation. Two zones of slow crack propagation were seen in the 7 d P specimen, one immediate to the crack initiation and one after the initial rapid crack propagation has occurred. The zone of slow crack propagation was clearly seen as a plateau on the load displacement graph (Fig. 25 pg. 58), after an initial rapid crack propagation zone. The hackle lines for the C and M specimen group were similar to those seen in brittle material fracture. For the P specimen group, circular cracks radiate outward from the flaw, with the advancing crack front creating hyperbolic markings on the fractured surface, similar to those seen in polymers. 60 Figure 27 (a), (b): SEM images - C 7 d at 25x (a) and 150x (b) magnification Figure 28 (a), (b): SEM images - C 90 d at 25x (a) and 150x (b) magnification Crack arrest Crack Initiation Crack Initiation Zone of rapid crack propagation/ hackle lines Zone of slow crack propagation/mirror Zone of slow crack propagation/mirror Zone of rapid crack propagation/ hackle lines Crack arrest a b a b 61 Figure 29 (a), (b): SEM images - M 7 d at 25x (a) and 150x (b) magnification Figure 30 (a), (b): SEM images - M 90 d at 25x (a) and 150x (b) magnification Crack Initiation Crack Initiation Crack arrest Crack arrest Zone of slow crack propagation/mirror Zone of slow crack propagation/mirror Zone of rapid crack propagation/ hackle lines Zone of rapid crack propagation/ hackle lines a b a b 62 Figure 31(a), (b): SEM images - P 7 d at 25x (a) and 150x (b) magnification Figure 32 (a), (b): SEM images - P 90 d at 25x (a) and 150x (b) magnification Zone of rapid crack propagation/ hackle lines Zone of rapid crack propagation/ hackle lines Zone of slow crack propagation/mirror Zone of slow crack propagation/mirror Crack arrest Crack arrest Crack Initiation Crack Initiation a a b b 63 4.5 Weibull Statistics Table 10 summarizes the results of the Weibull statistics analysis while Fig. 33 shows the Weibull plots. Table 10: Weibull Modulus (m) and Characteristics Weibull Fracture Toughness (KIC) Material Group m KIC (MPa·m1/2) 7 d 90 d 7 d 90 d Lucitone 199 Denture base resin (C) 19.81 11.38 2.14 1.90 Lucitone 199 CAD (M) 17.01 6.58 2.02 1.71 Lucitone Digital Print (P) 23.94 16.01 2.28 2.31 From the Table 9, it should be noted that at 7 d, P group showed the highest reliability (m = 23.94) while the C (m =19.81) and M (m = 17.01) groups had similar reliabilities. At 90 d, the Weibull modulus (m) of all the three groups was lower compared to that at 7 d, with M (m= 6.58) group exhibiting lower reliability than C (m= 11.38) and P (m= 16.01) groups. Overall, M group had the lowest reliability at both 7 d and 90 d. 64 Figure 33: Weibull Plots for the tested materials 0.00 0.20 0.40 0.60 0.80 1.00-4.000-3.000-2.000-1.0000.0001.0002.000KIC (in MPa·m1/2)Pfln(KIC)ln(ln(1/(1-Pf)))7 d conv 90 d conv 7 d CAD 90 d CAD 7 d Print 90 d Print1.22 1.49 1.82 2.25 2.711.00.9990.9340.6320.3080.1260.0480.018 65 Chapter 5: Discussion The results of this study showed significant difference in KIC between C, M and P denture bases at 7 d and 90 d, thus rejecting the null hypothesis (H01). The results obtained for the C group, Lucitone 199, are in accordance with those reported by previous studies.(35)(36) A study by Pacquet et al. reported similar KIC values [(2.11 ± 0.29) MPa·m1/2] for milled denture base to our study.(138) As far as these authors know, there are no published results for the KIC of printed denture base materials. The C and M groups showed similar KIC. The loading pattern occurred similarly in both groups, with the load increasing continuously up to the point where the specimens failed with crack arrest (Fig. 21 and 23). The characterization of the tested C and M specimens under the light microscope revealed a fine fracture line on the surface of the specimens up to the point of crack arrest and no permanent deformation of the specimens was noted (Fig. 22 and 24). The distance from the crack initiation to the crack arrest/failure was much greater at 7 d compared to 90 d in both C and M groups (Figs. 22 a, b and 24 a, b). This can explain the failure occurring at lower loads and therefore lower KIC values at 90 d compared to 7 d in C and M groups. In the 7 d P group, however, the load increased continuously up to ~100 N, at which stage, rather than decreasing sharply, it only dropped slightly, was followed by a plateau, then by an increase and finally by a sharp drop (Fig. 25). The characterization of the tested specimens under the light and scanning electron microscope revealed a significant crazing zone around the propagating crack, up to the crack arrest point (Figs. 26, 31 a). It is likely that at the site of crazing, microcrack bridging took place, allowing the load to be transmitted to the rest of the material, thus creating a 66 crack resistance zone. The plateau may be the area shown by a teardrop (Fig. 26 a) where significant resistance is offered by the material to the crack propagation (also seen on SEM Fig. 31(a); “mirror” area). Additional energy was absorbed by the material without any changes in load. Once the amount of energy was sufficient to overcome this zone, the material started to take more load before crack arrest occurred. We reported the KIC of this group based on the highest load recorded prior to the plateau [(2.23 ± 0.11) MPa·m1/2]. However, if the maximum load recorded during the testing were to have been considered as a point of crack initiation/propagation, KIC would have been [(4.04 ± 0.51) MPa·m1/2], which is significantly higher than that of the other groups. Moreover, the area under the load displacement curve for this group, which represents the energy (in J) absorbed by the material during testing, was significantly higher in 7 d P group (~ 8.7 KJ/m2) than that of any other groups tested. (Table 9, pg. 59) In contrast, the plateau was not seen in 90 d P group (Fig. 25), even though the crazing around the propagating crack was still present, as was the plastic deformation of the samples (Fig 26 b). The teardrop pattern was also seen in the 90 d specimen, much closer to the area of crack arrest as compared with the 7 d P specimen. The absence of the plateau in the load displacement graph and the presence of zone of resistance (teardrop) closer to the crack arrest indicate that less resistance was offered by the material to crack propagation. The maximum recorded KIC in 90 d was [(2.24 ± 0.17) MPa·m1/2] and the energy absorbed by the 90 d specimen was ~ 6.3 KJ/m2. The difference in the behaviour between the 7 d and 90 d samples could be due to monomer/component leaching or polymer degradation. Experimentally, in mode I loading, it has been determined that the ratio of plastic zone to the specimen thickness has to be ~ 0.025 in order to ensure pain strain conditions. This implies that the specimen thickness (B) and crack size (a) of the specimen has to be equal or greater than 2.5 67 times the square of the fracture toughness (KIC) to yield stress (𝜎1) ratio, as described in the following equation:(139) Equation 6: Requirement to ensure plane strain conditions Based on the available reported values for KIC and sy of conventional and milled denture base materials, the NTP specimen dimensions [(6X6X6x12) mm)] used in this study satisfy the above requirement. In view of the significant plastic deformation and crazing observed for the printed denture base material tested, it is possible that the specimen dimensions were inadequate to secure plain strain conditions. Due to the lack of data regarding KIC and sy of the printed denture bases, we could not determine the required sample dimensions to ensure plain strain conditions. Therefore, the KIC values reported for printed denture base should be considered tentative. The difference in crack propagation between the C or M and the P group could be due to the material chemistry and the degree of cross-linking within the polymers. Highly cross-linked polymers would exhibit significantly less crazing and plastic deformation. Urethanes form lightly cross-linked thermoset polymers, with flexible and rubbery characteristics, being more flexible than the methacrylates over wide temperature ranges. Urethane methacrylate is the main component in P denture bases, while C and M denture bases are based on polymethacrylates. The difference in material composition may play an important role in the difference in KIC between the materials tested.(118) 68 Another likely reason for the differences recorded in KIC may be due to the difference in the polymerization pathway of these materials. The layered, laser-induced polymerization of the P materials is very likely leading to a completely different final structure in these materials. The significant crazing and plastic deformation observed in the 7 d specimens could well be the consequence of the polymerization protocol. Further, spectroscopic, differential scanning calorimetric and dynamic mechanical analyses of the different denture base materials could lead to a better understanding of their structure and properties and may reveal differences in the degree of polymerization and crosslinking between them. The second null hypothesis (H02) that water storage does not affect the KIC of the denture bases has also been rejected. The results from this study showed that KIC of the C and M denture bases decreased after 90 d of storage in 37 °C. The results of our study are opposite to those by Finoti et al., who reported that KIC of the denture base materials increased after long term water storage.(140) However, the materials tested were not the same. Another study by Wady et. al showed that water has varying effect on the fracture resistance to denture bases.(141) No decrease in the KIC was detected in P denture base group. Two processes could explain the decrease in KIC upon storage in water: water sorption/desorption and release of residual monomer. Water sorption/desorption is a time dependent process that could have a profound effect on the KIC of denture bases. Water molecules could lead to the displacement of polymer chains, the reduction in intermolecular forces and secondary bonds between the acrylate groups, thereby increasing crack propagation and decreasing KIC.(142)(143) A study by Balkenhol et al. has shown that methacrylate-based materials have lower mechanical properties due to weakening of the polymer matrix as a result of water sorption.(144) Water also has a plasticizing effect, thereby decreasing the brittleness of the material. The release of residual 69 monomers can induce changes in the material composition and structure, affecting mechanical properties and, possibly, reducing the ability to withstand crack propagation. Another process that may have contributed to the decreased KIC of the specimens after water storage is the loss of plasticizer(s) included in the composition to increase flexibility and the ability to absorb higher energy before failure. Over long-term water storage, the release of plasticizer into water can contribute to the decrease in KIC.(140)(143) Since, printed denture base has undergone complete polymerization during the post processing stage, it can be deduced that no release of the monomer or plasticizer occurred in the printed denture group. Therefore, no change in the KIC values were seen in printed denture base group at 7 d and 90 d. 5.2 Could the notchless triangular prism (NTP specimen KIC test be a better alternative to the SENB test currently recommended by ISO for the determination of fracture toughness of denture base materials? Finally, considering the good correlation of the results obtained in this study with those obtained by using other test procedures, the small sample volume (~180 mm3) compared to ISO recommended SENB sample volume of (~1250 mm3 ), reduced surface flaws and the ease of sample fabrication, the NTP specimen KIC test may be a viable alternative to the single edge notched beam (SENB) test currently recommended by ISO for the determination of KIC of denture base materials. 70 5.3 Clinical Significance The results of this study showed that all the tested denture base materials surpassed the 1.9 MPa·m1/2 norm set by the International Organization for Standardization (ISO) for KIC.(19) The printed denture base material tested in this study had the highest fracture toughness (~2.2 MPa·m1/2), the highest reliability (m = 24) and the highest work of fracture (~8 KJ/m2), implying a better ability to resist fracture. The longevity of a denture base has a socio-economic impact on the quality of life of the elderly, saving clinical appointment times and millions of dollars in cost. 5.4 Limitations and Future Considerations The study assessed KIC of a printed denture base printed at 0º plane. Since the printing can be done at varying angulations, such as 45º, 90º etc., we do not know the effect the orientation of the layer/ direction of the print has on KIC. Future spectroscopic, differential scanning calorimetric and dynamic mechanical analyses of the different denture base materials could lead to a better understanding of the property/structure relationship. Other properties (such as flexural strength, impact strength etc.) need to be investigated. Only one material was tested for each group. More materials from different manufacturers need to be compared. 71 Chapter 6: Conclusion The printed denture base material tested in this study exhibited significantly higher KIC (~2.2 MPa·m1/2), absorbed higher energy before fracture (~8 KJ/m2) and exhibited stability under ageing conditions, suggesting that it could be more resistant to crack propagation than the conventional and milled materials tested. Of the three test groups, milled denture bases showed least resistance to crack propagation with lowest KIC values both at 7 d and 90 d. Weibull statistics also showed that P group exhibited the highest reliability at 7 d (m = 24), followed by C (m = 19) and M (m = 17) groups. The reliability decreased after 90 d for all groups, with M group exhibiting the lowest reliability (m = 6). Water storage for 90 d significantly decreased KIC of C and M groups, with no effect on the KIC of the printed denture bases. 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Thesis/Dissertation
eng
Craniofacial Science
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Ruse, N. Dorin
Fracture toughness of conventional, milled and printed denture bases
Text