- Email: [email protected]
Flexural strength of denture base acrylic resins processed by conventional and CAD-CAM methods
RESEARCH AND EDUCATION
Flexural strength of denture base acrylic resins processed by conventional and CAD-CAM methods Brian C. Aguirre, DDS, MS,a Jenn-Hwan Chen, DMD, MS,b Elias D. Kontogiorgos, DDS, PhD,c David F. Murchison, DDS, MMS,d and William W. Nagy, DDSe Polymethyl methacrylate (PMMA) ABSTRACT was introduced in 1936 and reStatement of problem. High flexural strength is one of the desirable properties for denture base mains the denture base material resins, yet only few studies have evaluated the physical properties of newer denture bases such as 1,2 of choice. It is a colorless polycomputer-aided design and computer aided manufacturing (CAD-CAM) milled products. mer of methyl methacrylate, and its high mechanical strength, Purpose. The purpose of this in vitro study was to compare the flexural strength of 3 different types of denture base resins: compression molded, injection molded, and CAD-CAM milled. modulus of elasticity, low water Material and methods. Three groups (n=10) of acrylic denture base resins were tested: injection solubility, and dimensional stamolded, compression molded, and CAD-CAM milled resin. ISO-compliant, rectangular specimens bility make it a suitable denture were fabricated (64×10×3.3 mm) (n=30). Specimens were stored in water for 1 week, and 1,2 base material. Denture resins flexural strength was measured by using a 3-point bend test until failure. The Student t test should be biocompatible, esthetic, was used to evaluate differences in the flexural strength and modulus of elasticity among cleansable, and easily repairable; specimen groups. The Bonferroni formula was used to set significance at a=.017 to account for adhere to denture teeth; and have multiple comparisons among the 3 groups. adequate physical and mechaniResults. The flexural strength of the CAD-CAM milled group was significantly higher than that of cal properties.1 These products the other 2 groups (P<.001), while the strength of the compression molded group was significantly should have adequate strength greater than that of the injection molded group (P<.001). The flexural modulus of the CAD-CAM and toughness to endure forces group was significantly higher than that of the other 2 groups (P<.001). generated during function while Conclusions. CAD-CAM milled denture bases may be a useful alternative to conventionally also being dimensionally stable processed denture bases in situations where increased resistance to flexural strength is under varying thermal condi- needed. (J Prosthet Dent 2019;-:---) tions.2 High flexural strength is essential because of the uneven force distribution the process.1,2 Alternatively, chemically activated or autopobase will endure under load and as the alveolar ridge lymerized denture base resins do not require thermal irregularly resorbs.3 Hence, it should be able to resist energy but instead use tertiary amines to activate benzoyl plastic deformation and fatigue resistance under repeated peroxide.1,2 Dentures made with autopolymerized resins 4 loads. have lower mechanical properties than those made with Different categories of denture PMMA are currently heat-activated resins because of excess residual monoavailable. In heat-activated resins, used in compression mer.5 Although light-activated resin exhibits higher or injection molded methods, thermal energy activates flexural strength than heat-activated PMMA, it also benzoyl peroxide, which initiates the polymerization shows brittleness and greater variability because of the This research is a partial fulfillment of the requirements for the degree of master of science at Texas A&M University. a Former Graduate Prosthodontics Resident, Department of Restorative Sciences, Texas A&M University College of Dentistry, Dallas, Texas. b Clinical Assistant Professor, Department of Restorative Sciences, Texas A&M University College of Dentistry, Dallas, Texas. c Professor, Department of Restorative Sciences, Texas A&M University College of Dentistry, Dallas, Texas. d Adjunct Professor, Department of Diagnostic Sciences, Texas A&M University College of Dentistry, Dallas, Texas. e Associate Professor, Department of Restorative Sciences, Texas A&M University College of Dentistry, Dallas, Texas.
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Clinical Implications The CAD-CAM milled denture resin demonstrated higher initial flexural strength than the conventionally processed or injection molded products. If future studies confirm that this higher strength is maintained after fatigue testing and cyclic loading, the milled denture bases may be a useful alternative when heavier functional loads are anticipated or for patients who have experienced multiple denture fractures.
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demonstrates greater flexural strength than does the compression molded resin.28 The purpose of this in vitro study was to compare the flexural strength and flexural modulus of 3 different types of denture resins: CAD-CAM milled and compression or injection molded. The research hypothesis was that the flexural strength of the CAD-CAM milled product would be significantly different from that of the other two, based on its higher density and greater degree of conversion. MATERIAL AND METHODS
difficulty in obtaining dense specimens.3 Since the turn of the century, computer-aided design and computer aided manufacturing (CAD-CAM) has evolved to include the fabrication of complete dentures.6,7 The production of the resin puck includes high pressure and heat, resulting in a condensed acrylic resin and minimized shrinkage, porosity, or free monomer.8 These bases are milled from prepolymerized resin pucks, which promise superior strength and fit and reduced bacterial adhesion.9 The incidence of denture fracture has been reported as high as 63% in the first 3 years of service.10 More fractures occur in maxilla prostheses, in an approximate ratio of 2:1, than in mandibular prostheses.11,12 Stress distribution within dentures has been investigated by using finite element modeling, photoelastic analysis, strain gauges, and holography.13-16 However, clinically, maxillary dentures show higher tensile stresses in the incisor region, and midline fractures are reported as the most common fracture site.17,18 Occlusal contacts, diastemas, palatal tori, and sharp frenal notches create localized stress areas, adding to failures in denture base integrity.10,16,19,20 Because denture bases undergo repeated flexing during mastication over several years, dentures may fracture or crack intraorally while in use because of fatigue failure.21 Flexural strength, or transverse strength, represents the highest bending stress experienced within a material at its moment of fracture. A high flexural strength is required to prevent catastrophic failure under load, which is paramount for the success of a denture.22-24 According to American Dental Association Standard No. 139, in accordance with ISO 20795-1 for denture base polymers, a flexural strength test (also known as 3-point flexural test), which examines the stiffness and resistance of the tested materials, is frequently used to measure the flexural strength of denture base resins.25-27 Furthermore, incompletely polymerized acrylic resins have inferior mechanical properties.27 The flexural strength of injection molded resin is shown to be higher than that of compression molded resin.27 Moreover, the CAD-CAM resin THE JOURNAL OF PROSTHETIC DENTISTRY
Ten rectangular specimens (in accordance with ISO 20795-1; 64×10×3.3 mm) of a prepolymerized denture base material were fabricated by a commercial dental CAD-CAM laboratory facility (Vertex PMMA, AvaDent Original shade; Global Dental Science). A polyvinyl siloxane putty (Express STD; 3M ESPE) matrix was created from these specimens. Pink base plate wax (Modern Materials Baseplate Wax; Kulzer GmbH) was dripped into the matrices to form wax duplicates. From these wax forms, 2 groups of PMMA acrylic denture base resins were processed (n=10). The denture base groups were divided with respect to their method of processing and resin composition into the groups compression, injection, and milled. The rectangular specimens were standardized for finishing and polishing treatment and completed by 1 operator (B.C.A.). After surface treatment, the specimens were stored in distilled water at 37 ±1 C for 1 week before testing. Flexural strength and flexural modulus were obtained from a bench 3-point bend test by using a universal testing machine (5567 Universal Testing Machine; Instron Ltd) with a span length of 50 mm and a crosshead speed of 5 mm/min. For fabrication of the compression molded specimens, wax duplicates were flasked and invested according to the manufacturer instructions in ISO Type 3 dental stone (Microstone; Whip Mix Corp). The flask was heated in a boil out solution (Patterson Boil Out Solution; Patterson Dental) for 8 minutes and separated, and the wax was flushed out by using the boil out solution. The final flush was completed by using clean water, and the flask halves were allowed to cool to room temperature. Liquid tin foil substitute (Al-Cote; Dentsply Sirona) was applied to the stone and allowed to dry. Denture base resin (Lucitone 199; Dentsply Sirona) was proportionally hand-mixed according to manufacturer instructions. The mixture was allowed to reach packing consistency and was condensed into the mold by using finger pressure, and the flask was closed by using a pneumatic flask press (Coe-Bilt; Nevin) under a 27 kN load. The flask was then placed in a spring clamp and deposited into a water polymerization unit (Hanau Curing Unit; Hanau Engineering Company Inc) for 9 hours at 73 C, followed by Aguirre et al
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30 minutes in boiling water (100 C), per manufacturer instructions. Bench cooling of the processed flask was allowed for 30 minutes, and then the flask was immersed in 21 C water for 15 minutes before deflasking. For injection molded specimens, wax duplicates were flasked and invested according to manufacturer instructions in ISO Type 3 dental stone (Microstone; Whip Mix Corp). The flask was heated in a boil out solution (Patterson Boil Out Solution; Patterson Dental) for 8 minutes and separated, and the wax was flushed by using the boil out solution. The final flush used clean water, and the flask halves were allowed to cool to room temperature. Liquid tin foil substitute (Separating Fluid; Ivoclar Vivadent AG) was then applied to the stone and allowed to dry. Premeasured capsules of resin and monomer (SR Ivocap High Impact; Ivoclar Vivadent AG) were combined in a commercial mixer (Cap Vibrator; Ivoclar Vivadent AG) for 5 minutes. The flask halves were joined in a clamping frame under 29 kN force. The contents of the mixed capsule were inserted into the flask, and the pressure injection apparatus (SR Ivocap System; Ivoclar Vivadent AG) was attached. The pressure apparatus was connected to a compressed air supply (600 kPa) to allow the plunger to descend and inject material into the mold for 5 minutes on the bench. The assembly was then submerged and polymerized in boiling water (100 C) for 35 minutes according to the manufacturer instructions. The assembly was then removed and immediately placed in cold water, maintaining pressure, for 30 minutes, after which the specimens were deflasked. Prepolymerized CAD-CAM specimens milled to the desired dimensions of a commonly used, high-impact denture base material (Vertex PMMA, AvaDent Original; Global Dental Science) were supplied from a commercial CAD-CAM dental laboratory (AvaDent Digital Dental Solutions; Global Dental Science). The finish of the specimen surface was requested to be the same as the condition for definitive removable prosthesis placement. After processing, all specimens were evaluated to ensure the absence of voids or gross irregularities under ×3.5 optical magnification. The specimens were then finished with wet 220-grit to 600-grit silicon carbide paper (Wetordry; 3M ESPE) to a final dimension of 63×10×3.3 mm as measured by using digital calipers (Digital Caliper 01407A; Neiko) at 5 points to ±0.03 mm. Before the 3-point bend test, all specimens were stored in distilled water at 37 ±1 C for 1 week. The specimens were tested by using a 3-point bend test as per guidelines of ISO 20795-1 for denture base polymers. Each specimen was placed on circular support beams with a 50-mm span. A load cell was applied with the upper anvil assembly of a universal testing machine Aguirre et al
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Table 1. Means ±standard deviations for performed tests according to group Flexural Strength (MPa) Group
Flexural Modulus (MPa)
Mean ±SD
Mean ±SD
86.7 ±7.1
2121.3 ±176.6
Injection Compression
116.6 ±3.1
2918.4 ±106.3
Milled
146.6 ±6.6
3816.7 ±44.3
SD, standard deviation.
Table 2. Student t test (t value) and significance (P) for performed tests according to group Flexural Strength (MPa)
Flexural Modulus (MPa)
Group
t Value
P
t Value
P
Injection versus compression
−12.21
<.001
−11.78
<.001
Injection versus milled
−19.29
<.001
−27.81
<.001
Compression versus milled
−12.58
<.001
−24.67
<.001
(5567 Universal Testing Machine; Instron Ltd) to the center of the specimens at a crosshead speed of 5 mm/ min until fracture. The moment of fracture was designated as the moment the applied load dropped to zero. Data were recorded by using a software program (Bluehill v1.5; Instron Ltd). The maximum load exerted at failure was recorded in Newtons (N). Flexural strength (Fs) and flexural modulus (Fm) were then calculated from the following equations25: Flexural strength ðMPaÞ=3PL=2bd2
(1)
Flexural modulus ðMPaÞ=PL3 4Ybd3 ; where P=maximum load (N), b=specimen width (10 mm), L=span length (50 mm), d=specimen thickness (3 mm), and Y=recorded deflection when the load is applied at the middle of the beam. The data were analyzed by using a statistical software program (IBM SPSS Statistics, v25.0; IBM Corp). The Student t test was used to evaluate differences in the flexural strength and modulus of elasticity of the specimen groups (compression, injected, and milled). The Bonferroni formula was used to set significance at a=.017 to account for multiple comparisons among the 3 groups. RESULTS The flexural strength and flexural modulus of the 3 tested groups are listed in Table 1. The Student t test results for flexural strengths and flexural modulus are listed in Table 2, which show statistically significant differences among the 3 groups (P<.001). The milled and compression groups fractured, showing minimal-to-no plastic deformation. In contrast, the injection group displayed pronounced deformation, which can be seen on the stress-strain curve graph THE JOURNAL OF PROSTHETIC DENTISTRY
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Injection 0 –20
Load (N)
–40 –60 –80 –100 –120 –18
–17
–16
–15
–14
–13
–12
–11
–10
–9
–8
–7
–6
–5
–4
–3
–2
–1
0
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Extension (mm) Figure 1. Representative examples of four stress-strain curves of the injection group. Vertical variable: load (N). Horizontal variable: extension (mm) =recorded deflection when load applied at middle of beam.
Figure 2. Reapproximated specimens after test. A, CAD-CAM milled. B, Injection molded.
(Fig. 1). This finding was visualized by reapproximating the specimens after fracture (Fig. 2). DISCUSSION The test results support the research hypothesis that the flexural strength of the CAD-CAM milled product would be significantly different from that of injection and compression molded resins. Subjecting a denture base to a 3-point bend test simulates its ability to succeed intraorally under high functional loads during mastication and parafunction.3,27,29 Previous testing has used this test to evaluate the suitability of novel denture base materials.3,27,29 According to the international standards for polymer materials and ISO 20795-1 for denture base polymers, the 3-point flexural test is a THE JOURNAL OF PROSTHETIC DENTISTRY
common method for measuring flexural properties.25 The standard states that acrylic resins should achieve no less than 65 MPa.25 By using that criterion, all groups in the present study are suitable for clinical use. The mean flexural strength of the CAD-CAM resin in the present study is comparable with that of another study.29 Flexural strength is one of the major determinants of the mechanical properties of acrylic resin and is affected by the degree of polymerization achieved.27 When acrylic resin strengths are compared, those with a lower degree of conversion exhibit inferior mechanical properties.27 The higher flexural strength values of CAD-CAM specimens may be attributed to a higher degree of conversion.30 The CAD-CAM resins are milled from solid, prepolymerized pucks, and it is reasonable to assume that the pucks are polymerized by using Aguirre et al
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equipment capable of providing greater polymerization potential than available with conventional processing methods. In the present study, the compression molded group showed higher flexural strength than others.29 Also, these data disagree with a previous study in which injection molded resins demonstrated a higher flexural strength than did a compression-molded type.27 Both observed differences in these results could be explained by the different polymerization methods used. A long processing time was used in the present study, whereas a short polymerizing cycle was used in the previous study.2 Another possible explanation is that different brands of heat-activated PMMA were used in the studies. Flexural modulus demonstrates a material’s rigidity.27 The results from the present study showed that all 3 tested denture resins met the standards for ISO-20795-1, that is, the flexural modulus of processed resins should not be under 2 GPa.27 A noteworthy finding revealed that the injectable resin underwent more permanent deformation (lower flexural modulus) before fracture than did the other 2 denture base types. Clinically, this finding may lead to the subclinical deformation of the denture rather than fracturing under load. Both CADCAM and compression molded resins displayed minimal-to-no deformation before fracture (higher flexural modulus). Although the flexural strength of the injection specimens was the lowest and demonstrated significant plastic deformation before failure, the product remains a suitable denture base material. A limitation of the present study is that following the ISO 3-point flexural test, cyclic loading, and thermocycling could have provided further information on the fatigue resistance of the material tested. Also, the specimens tested did not reflect the shape of an actual denture. A previous study reported thermocycled (5000 cycles) specimens of Lucitone 199 displayed significantly lower flexural strength than specimens that were not thermocycled.28 This result is most likely because of the effect that absorbed water has on the physical properties of processed polymers; this causes structural changes as water molecules enter and interfere with the polymer chains, acting as plasticizers.31 The specimens in the present study were submerged in deionized water for 1 week to simulate the aqueous intraoral environment. In addition, cyclic loading can be used to simulate stress fatigue within the specimens to mimic intraoral conditions.28 However, a previous study showed no significant difference in flexural strength between artificially fatigued heat-activated specimens that had undergone cyclic loading (10 000 cycles) versus a control group that had not.3 Although the improved flexural strength and flexural modulus for CAD-CAM resin are promising findings, future studies to evaluate other mechanical properties Aguirre et al
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such as the flexural fatigue, impact strength, and fracture toughness of denture resins are recommended. CONCLUSIONS Within the limits of this in vitro study, the following conclusions were drawn: 1. CAD-CAM milled denture resin exhibited higher flexural strength than the 2 conventional processing methods: compression and injection molded resins 2. The injection molded resin displayed pronounced deformation before fracture with a lower flexural modulus, whereas the CAD-CAM milled and compression molded resins fractured with minimalto-no plastic deformation. 3. If future studies confirm that this higher strength is maintained after fatigue testing and cyclic loading, the milled denture bases may be a useful alternative when heavier functional loads are anticipated or for patients who have experienced multiple denture fractures. REFERENCES 1. Zarb GA, Fenton AH. Prosthodontic treatment for edentulous patients. 13th ed. St. Louis: Mosby/Elsevier; 2012. p. 133, 136, 140. 2. Anusavice KJ, Shen C, Rawls R. Phillips’ science of dental materials. 12th ed. Philadelphia: Saunders/Elsevier; 2012. p. 94, 97, 480, 481, 483. 3. Diaz-Arnold AM, Vargas MA, Shaull KL, Laffoon JE, Qian F. Flexural and fatigue strengths of denture base resin. J Prosthet Dent 2008;100:47-51. 4. Jagger DC, Harrison A, Jandt KD. The reinforcement of dentures. J Oral Rehabil 1999;26:185-94. 5. Bates JF, Stafford GD, Huggett R, Handley RW. Current status of pour type denture base resins. J Dent 1977;5:177-89. 6. Choi JE, Ng TE, Leong CKY, Kim H, Li P, Waddell JN. Adhesive evaluation of three types of resilient denture liners bonded to heat-polymerized, autopolymerized, or CAD-CAM acrylic denture bases. J Prosthet Dent 2018;120: 699-705. 7. Miyazaki T, Hotta Y, Kunii J, Kuriyama S, Tamaki Y. A review of dental CAD/ CAM: current status and future perspectives from 20 years of experience. Dent Mater J 2009;28:44-56. 8. Infante L, Yilmaz B, McGlumphy E, Finger I. Fabricating complete dentures with CAD/CAM technology. J Prosthet Dent 2014;111:351-5. 9. Bidra AS, Taylor TD, Agar JR. Computer-aided technology for fabricating complete dentures: systematic review of historical background, current status, and future perspectives. J Prosthet Dent 2013;109:361-6. 10. Hargreaves AS. The prevalence of fractured dentures. A survey. Br Dent J 1969;126:451-5. 11. Beyli MS, von Fraunhofer JA. An analysis of causes of fracture of acrylic resin dentures. J Prosthet Dent 1981;46:238-41. 12. Darbar UR, Huggett R, Harrison A. Denture fractureea survey. Br Dent J 1994;176:342-5. 13. Craig RG, Farah JW, el-Tahawi HM. Three-dimensional photoelastic stress analysis of maxillary complete dentures. J Prosthet Dent 1974;31:122-9. 14. Dirtoft BI, Jansson JF, Abramson NH. Using holography for measurement of in vivo deformation in a complete maxillary denture. J Prosthet Dent 1985;54: 843-6. 15. Cheng YY, Cheung WL, Chow TW. Strain analysis of maxillary complete denture with three-dimensional finite element method. J Prosthet Dent 2010;103:309-18. 16. Rees JS, Huggett R, Harrison A. Finite element analysis of the stressconcentrating effect of fraenal notches in complete dentures. Int J Prosthodont 1990;3:238-40. 17. Kydd WL. The comminuting efficiency of varied occlusal tooth form and the associated deformation of the complete denture base. J Am Dent Assoc 1960;61:465-71. 18. Kim SH, Watts DC. The effect of reinforcement with woven E-glass fibers on the impact strength of complete dentures fabricated with high-impact acrylic resin. J Prosthet Dent 2004;91:274-80. 19. Ates M, Cilingir A, Sulun T, Sunbuloglu E, Bozdag E. The effect of occlusal contact localization on the stress distribution in complete maxillary denture. J Oral Rehabil 2006;33:509-13.
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20. Cilingir A, Bilhan H, Baysal G, Sunbuloglu E, Bozdag E. The impact of frenulum height on strains in maxillary denture bases. J Adv Prosthodont 2013;5:409-15. 21. Kelly E. Fatigue failure in denture base polymers. J Prosthet Dent 1969;21: 257-66. 22. Oku JI. Impact properties of acrylic denture base resin. Part 1. A new method for determination of impact properties. Dent Mater J 1988;7: 166-73. 23. Neihart TR, Li SH, Flinton RJ. Measuring fracture toughness of high-impact poly(methyl methacrylate) with the short rod method. J Prosthet Dent 1988;60:249-53. 24. Zappini G, Kammann A, Wachter W. Comparison of fracture tests of denture base materials. J Prosthet Dent 2003;90:578-85. 25. ADA. ANSI/ADA Standard No. 139 (ISO 20795-1), Denture Base Polymers. American Dental Association; 2013. Available at: https://webstore.ansi.org/ Standards/ISO/ISO207952013?gclid=EAIaIQobChMI5eGNoJ_64AIVT57 ACh0YsAsyEAAYAiAAEgKeaPD_BwE. 26. Abdulwahhab SS. High-impact strength acrylic denture base material processed by autoclave. J Prosthodont Res 2013;57:288-93. 27. Gharechahi J, Asadzadeh N, Shahabian F, Gharechahi M. Flexural strength of acrylic resin denture bases processed by two different methods. J Dent Res Dent Clin Dent Prospects 2014;8:148-52.
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28. Machado AL, Puckett AD, Breeding LC, Wady AF, Vergani CE. Effect of thermocycling on the flexural and impact strength of urethane-based and high-impact denture base resins. Gerodontology 2012;29:318-23. 29. Al-Dwairi ZN, Tahboub KY, Baba NZ, Goodacre CJ. A Comparison of the Flexural and Impact Strengths and Flexural Modulus of CAD/CAM and Conventional Heat-Cured Polymethyl Methacrylate (PMMA). J Prosthodont 2018 June 13. doi: 10.111/jopr.12926. 30. Steinmassl PA, Wiedemair V, Huck C. Do CAD/CAM dentures really release less monomer than conventional dentures? Clin Oral Investig 2017;21: 1697-705. 31. Tuna SH, Keyf F, Gumus HO, Uzun C. The evaluation of water sorption/ solubility on various acrylic resins. Eur J Dent 2008;2:191-7. Corresponding author: Dr Jenn-Hwan Chen 3302 Gaston Avenue, Room 733-1 Dallas, TX 75246 Email: [email protected] Copyright © 2019 by the Editorial Council for The Journal of Prosthetic Dentistry. https://doi.org/10.1016/j.prosdent.2019.03.010
Aguirre et al
Flexural strength of denture base acrylic resins processed by conventional and CAD-CAM methods Brian C. Aguirre, DDS, MS,a Jenn-Hwan Chen, DMD, MS,b Elias D. Kontogiorgos, DDS, PhD,c David F. Murchison, DDS, MMS,d and William W. Nagy, DDSe Polymethyl methacrylate (PMMA) ABSTRACT was introduced in 1936 and reStatement of problem. High flexural strength is one of the desirable properties for denture base mains the denture base material resins, yet only few studies have evaluated the physical properties of newer denture bases such as 1,2 of choice. It is a colorless polycomputer-aided design and computer aided manufacturing (CAD-CAM) milled products. mer of methyl methacrylate, and its high mechanical strength, Purpose. The purpose of this in vitro study was to compare the flexural strength of 3 different types of denture base resins: compression molded, injection molded, and CAD-CAM milled. modulus of elasticity, low water Material and methods. Three groups (n=10) of acrylic denture base resins were tested: injection solubility, and dimensional stamolded, compression molded, and CAD-CAM milled resin. ISO-compliant, rectangular specimens bility make it a suitable denture were fabricated (64×10×3.3 mm) (n=30). Specimens were stored in water for 1 week, and 1,2 base material. Denture resins flexural strength was measured by using a 3-point bend test until failure. The Student t test should be biocompatible, esthetic, was used to evaluate differences in the flexural strength and modulus of elasticity among cleansable, and easily repairable; specimen groups. The Bonferroni formula was used to set significance at a=.017 to account for adhere to denture teeth; and have multiple comparisons among the 3 groups. adequate physical and mechaniResults. The flexural strength of the CAD-CAM milled group was significantly higher than that of cal properties.1 These products the other 2 groups (P<.001), while the strength of the compression molded group was significantly should have adequate strength greater than that of the injection molded group (P<.001). The flexural modulus of the CAD-CAM and toughness to endure forces group was significantly higher than that of the other 2 groups (P<.001). generated during function while Conclusions. CAD-CAM milled denture bases may be a useful alternative to conventionally also being dimensionally stable processed denture bases in situations where increased resistance to flexural strength is under varying thermal condi- needed. (J Prosthet Dent 2019;-:---) tions.2 High flexural strength is essential because of the uneven force distribution the process.1,2 Alternatively, chemically activated or autopobase will endure under load and as the alveolar ridge lymerized denture base resins do not require thermal irregularly resorbs.3 Hence, it should be able to resist energy but instead use tertiary amines to activate benzoyl plastic deformation and fatigue resistance under repeated peroxide.1,2 Dentures made with autopolymerized resins 4 loads. have lower mechanical properties than those made with Different categories of denture PMMA are currently heat-activated resins because of excess residual monoavailable. In heat-activated resins, used in compression mer.5 Although light-activated resin exhibits higher or injection molded methods, thermal energy activates flexural strength than heat-activated PMMA, it also benzoyl peroxide, which initiates the polymerization shows brittleness and greater variability because of the This research is a partial fulfillment of the requirements for the degree of master of science at Texas A&M University. a Former Graduate Prosthodontics Resident, Department of Restorative Sciences, Texas A&M University College of Dentistry, Dallas, Texas. b Clinical Assistant Professor, Department of Restorative Sciences, Texas A&M University College of Dentistry, Dallas, Texas. c Professor, Department of Restorative Sciences, Texas A&M University College of Dentistry, Dallas, Texas. d Adjunct Professor, Department of Diagnostic Sciences, Texas A&M University College of Dentistry, Dallas, Texas. e Associate Professor, Department of Restorative Sciences, Texas A&M University College of Dentistry, Dallas, Texas.
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Clinical Implications The CAD-CAM milled denture resin demonstrated higher initial flexural strength than the conventionally processed or injection molded products. If future studies confirm that this higher strength is maintained after fatigue testing and cyclic loading, the milled denture bases may be a useful alternative when heavier functional loads are anticipated or for patients who have experienced multiple denture fractures.
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demonstrates greater flexural strength than does the compression molded resin.28 The purpose of this in vitro study was to compare the flexural strength and flexural modulus of 3 different types of denture resins: CAD-CAM milled and compression or injection molded. The research hypothesis was that the flexural strength of the CAD-CAM milled product would be significantly different from that of the other two, based on its higher density and greater degree of conversion. MATERIAL AND METHODS
difficulty in obtaining dense specimens.3 Since the turn of the century, computer-aided design and computer aided manufacturing (CAD-CAM) has evolved to include the fabrication of complete dentures.6,7 The production of the resin puck includes high pressure and heat, resulting in a condensed acrylic resin and minimized shrinkage, porosity, or free monomer.8 These bases are milled from prepolymerized resin pucks, which promise superior strength and fit and reduced bacterial adhesion.9 The incidence of denture fracture has been reported as high as 63% in the first 3 years of service.10 More fractures occur in maxilla prostheses, in an approximate ratio of 2:1, than in mandibular prostheses.11,12 Stress distribution within dentures has been investigated by using finite element modeling, photoelastic analysis, strain gauges, and holography.13-16 However, clinically, maxillary dentures show higher tensile stresses in the incisor region, and midline fractures are reported as the most common fracture site.17,18 Occlusal contacts, diastemas, palatal tori, and sharp frenal notches create localized stress areas, adding to failures in denture base integrity.10,16,19,20 Because denture bases undergo repeated flexing during mastication over several years, dentures may fracture or crack intraorally while in use because of fatigue failure.21 Flexural strength, or transverse strength, represents the highest bending stress experienced within a material at its moment of fracture. A high flexural strength is required to prevent catastrophic failure under load, which is paramount for the success of a denture.22-24 According to American Dental Association Standard No. 139, in accordance with ISO 20795-1 for denture base polymers, a flexural strength test (also known as 3-point flexural test), which examines the stiffness and resistance of the tested materials, is frequently used to measure the flexural strength of denture base resins.25-27 Furthermore, incompletely polymerized acrylic resins have inferior mechanical properties.27 The flexural strength of injection molded resin is shown to be higher than that of compression molded resin.27 Moreover, the CAD-CAM resin THE JOURNAL OF PROSTHETIC DENTISTRY
Ten rectangular specimens (in accordance with ISO 20795-1; 64×10×3.3 mm) of a prepolymerized denture base material were fabricated by a commercial dental CAD-CAM laboratory facility (Vertex PMMA, AvaDent Original shade; Global Dental Science). A polyvinyl siloxane putty (Express STD; 3M ESPE) matrix was created from these specimens. Pink base plate wax (Modern Materials Baseplate Wax; Kulzer GmbH) was dripped into the matrices to form wax duplicates. From these wax forms, 2 groups of PMMA acrylic denture base resins were processed (n=10). The denture base groups were divided with respect to their method of processing and resin composition into the groups compression, injection, and milled. The rectangular specimens were standardized for finishing and polishing treatment and completed by 1 operator (B.C.A.). After surface treatment, the specimens were stored in distilled water at 37 ±1 C for 1 week before testing. Flexural strength and flexural modulus were obtained from a bench 3-point bend test by using a universal testing machine (5567 Universal Testing Machine; Instron Ltd) with a span length of 50 mm and a crosshead speed of 5 mm/min. For fabrication of the compression molded specimens, wax duplicates were flasked and invested according to the manufacturer instructions in ISO Type 3 dental stone (Microstone; Whip Mix Corp). The flask was heated in a boil out solution (Patterson Boil Out Solution; Patterson Dental) for 8 minutes and separated, and the wax was flushed out by using the boil out solution. The final flush was completed by using clean water, and the flask halves were allowed to cool to room temperature. Liquid tin foil substitute (Al-Cote; Dentsply Sirona) was applied to the stone and allowed to dry. Denture base resin (Lucitone 199; Dentsply Sirona) was proportionally hand-mixed according to manufacturer instructions. The mixture was allowed to reach packing consistency and was condensed into the mold by using finger pressure, and the flask was closed by using a pneumatic flask press (Coe-Bilt; Nevin) under a 27 kN load. The flask was then placed in a spring clamp and deposited into a water polymerization unit (Hanau Curing Unit; Hanau Engineering Company Inc) for 9 hours at 73 C, followed by Aguirre et al
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30 minutes in boiling water (100 C), per manufacturer instructions. Bench cooling of the processed flask was allowed for 30 minutes, and then the flask was immersed in 21 C water for 15 minutes before deflasking. For injection molded specimens, wax duplicates were flasked and invested according to manufacturer instructions in ISO Type 3 dental stone (Microstone; Whip Mix Corp). The flask was heated in a boil out solution (Patterson Boil Out Solution; Patterson Dental) for 8 minutes and separated, and the wax was flushed by using the boil out solution. The final flush used clean water, and the flask halves were allowed to cool to room temperature. Liquid tin foil substitute (Separating Fluid; Ivoclar Vivadent AG) was then applied to the stone and allowed to dry. Premeasured capsules of resin and monomer (SR Ivocap High Impact; Ivoclar Vivadent AG) were combined in a commercial mixer (Cap Vibrator; Ivoclar Vivadent AG) for 5 minutes. The flask halves were joined in a clamping frame under 29 kN force. The contents of the mixed capsule were inserted into the flask, and the pressure injection apparatus (SR Ivocap System; Ivoclar Vivadent AG) was attached. The pressure apparatus was connected to a compressed air supply (600 kPa) to allow the plunger to descend and inject material into the mold for 5 minutes on the bench. The assembly was then submerged and polymerized in boiling water (100 C) for 35 minutes according to the manufacturer instructions. The assembly was then removed and immediately placed in cold water, maintaining pressure, for 30 minutes, after which the specimens were deflasked. Prepolymerized CAD-CAM specimens milled to the desired dimensions of a commonly used, high-impact denture base material (Vertex PMMA, AvaDent Original; Global Dental Science) were supplied from a commercial CAD-CAM dental laboratory (AvaDent Digital Dental Solutions; Global Dental Science). The finish of the specimen surface was requested to be the same as the condition for definitive removable prosthesis placement. After processing, all specimens were evaluated to ensure the absence of voids or gross irregularities under ×3.5 optical magnification. The specimens were then finished with wet 220-grit to 600-grit silicon carbide paper (Wetordry; 3M ESPE) to a final dimension of 63×10×3.3 mm as measured by using digital calipers (Digital Caliper 01407A; Neiko) at 5 points to ±0.03 mm. Before the 3-point bend test, all specimens were stored in distilled water at 37 ±1 C for 1 week. The specimens were tested by using a 3-point bend test as per guidelines of ISO 20795-1 for denture base polymers. Each specimen was placed on circular support beams with a 50-mm span. A load cell was applied with the upper anvil assembly of a universal testing machine Aguirre et al
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Table 1. Means ±standard deviations for performed tests according to group Flexural Strength (MPa) Group
Flexural Modulus (MPa)
Mean ±SD
Mean ±SD
86.7 ±7.1
2121.3 ±176.6
Injection Compression
116.6 ±3.1
2918.4 ±106.3
Milled
146.6 ±6.6
3816.7 ±44.3
SD, standard deviation.
Table 2. Student t test (t value) and significance (P) for performed tests according to group Flexural Strength (MPa)
Flexural Modulus (MPa)
Group
t Value
P
t Value
P
Injection versus compression
−12.21
<.001
−11.78
<.001
Injection versus milled
−19.29
<.001
−27.81
<.001
Compression versus milled
−12.58
<.001
−24.67
<.001
(5567 Universal Testing Machine; Instron Ltd) to the center of the specimens at a crosshead speed of 5 mm/ min until fracture. The moment of fracture was designated as the moment the applied load dropped to zero. Data were recorded by using a software program (Bluehill v1.5; Instron Ltd). The maximum load exerted at failure was recorded in Newtons (N). Flexural strength (Fs) and flexural modulus (Fm) were then calculated from the following equations25: Flexural strength ðMPaÞ=3PL=2bd2
(1)
Flexural modulus ðMPaÞ=PL3 4Ybd3 ; where P=maximum load (N), b=specimen width (10 mm), L=span length (50 mm), d=specimen thickness (3 mm), and Y=recorded deflection when the load is applied at the middle of the beam. The data were analyzed by using a statistical software program (IBM SPSS Statistics, v25.0; IBM Corp). The Student t test was used to evaluate differences in the flexural strength and modulus of elasticity of the specimen groups (compression, injected, and milled). The Bonferroni formula was used to set significance at a=.017 to account for multiple comparisons among the 3 groups. RESULTS The flexural strength and flexural modulus of the 3 tested groups are listed in Table 1. The Student t test results for flexural strengths and flexural modulus are listed in Table 2, which show statistically significant differences among the 3 groups (P<.001). The milled and compression groups fractured, showing minimal-to-no plastic deformation. In contrast, the injection group displayed pronounced deformation, which can be seen on the stress-strain curve graph THE JOURNAL OF PROSTHETIC DENTISTRY
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Injection 0 –20
Load (N)
–40 –60 –80 –100 –120 –18
–17
–16
–15
–14
–13
–12
–11
–10
–9
–8
–7
–6
–5
–4
–3
–2
–1
0
1
2
Extension (mm) Figure 1. Representative examples of four stress-strain curves of the injection group. Vertical variable: load (N). Horizontal variable: extension (mm) =recorded deflection when load applied at middle of beam.
Figure 2. Reapproximated specimens after test. A, CAD-CAM milled. B, Injection molded.
(Fig. 1). This finding was visualized by reapproximating the specimens after fracture (Fig. 2). DISCUSSION The test results support the research hypothesis that the flexural strength of the CAD-CAM milled product would be significantly different from that of injection and compression molded resins. Subjecting a denture base to a 3-point bend test simulates its ability to succeed intraorally under high functional loads during mastication and parafunction.3,27,29 Previous testing has used this test to evaluate the suitability of novel denture base materials.3,27,29 According to the international standards for polymer materials and ISO 20795-1 for denture base polymers, the 3-point flexural test is a THE JOURNAL OF PROSTHETIC DENTISTRY
common method for measuring flexural properties.25 The standard states that acrylic resins should achieve no less than 65 MPa.25 By using that criterion, all groups in the present study are suitable for clinical use. The mean flexural strength of the CAD-CAM resin in the present study is comparable with that of another study.29 Flexural strength is one of the major determinants of the mechanical properties of acrylic resin and is affected by the degree of polymerization achieved.27 When acrylic resin strengths are compared, those with a lower degree of conversion exhibit inferior mechanical properties.27 The higher flexural strength values of CAD-CAM specimens may be attributed to a higher degree of conversion.30 The CAD-CAM resins are milled from solid, prepolymerized pucks, and it is reasonable to assume that the pucks are polymerized by using Aguirre et al
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equipment capable of providing greater polymerization potential than available with conventional processing methods. In the present study, the compression molded group showed higher flexural strength than others.29 Also, these data disagree with a previous study in which injection molded resins demonstrated a higher flexural strength than did a compression-molded type.27 Both observed differences in these results could be explained by the different polymerization methods used. A long processing time was used in the present study, whereas a short polymerizing cycle was used in the previous study.2 Another possible explanation is that different brands of heat-activated PMMA were used in the studies. Flexural modulus demonstrates a material’s rigidity.27 The results from the present study showed that all 3 tested denture resins met the standards for ISO-20795-1, that is, the flexural modulus of processed resins should not be under 2 GPa.27 A noteworthy finding revealed that the injectable resin underwent more permanent deformation (lower flexural modulus) before fracture than did the other 2 denture base types. Clinically, this finding may lead to the subclinical deformation of the denture rather than fracturing under load. Both CADCAM and compression molded resins displayed minimal-to-no deformation before fracture (higher flexural modulus). Although the flexural strength of the injection specimens was the lowest and demonstrated significant plastic deformation before failure, the product remains a suitable denture base material. A limitation of the present study is that following the ISO 3-point flexural test, cyclic loading, and thermocycling could have provided further information on the fatigue resistance of the material tested. Also, the specimens tested did not reflect the shape of an actual denture. A previous study reported thermocycled (5000 cycles) specimens of Lucitone 199 displayed significantly lower flexural strength than specimens that were not thermocycled.28 This result is most likely because of the effect that absorbed water has on the physical properties of processed polymers; this causes structural changes as water molecules enter and interfere with the polymer chains, acting as plasticizers.31 The specimens in the present study were submerged in deionized water for 1 week to simulate the aqueous intraoral environment. In addition, cyclic loading can be used to simulate stress fatigue within the specimens to mimic intraoral conditions.28 However, a previous study showed no significant difference in flexural strength between artificially fatigued heat-activated specimens that had undergone cyclic loading (10 000 cycles) versus a control group that had not.3 Although the improved flexural strength and flexural modulus for CAD-CAM resin are promising findings, future studies to evaluate other mechanical properties Aguirre et al
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such as the flexural fatigue, impact strength, and fracture toughness of denture resins are recommended. CONCLUSIONS Within the limits of this in vitro study, the following conclusions were drawn: 1. CAD-CAM milled denture resin exhibited higher flexural strength than the 2 conventional processing methods: compression and injection molded resins 2. The injection molded resin displayed pronounced deformation before fracture with a lower flexural modulus, whereas the CAD-CAM milled and compression molded resins fractured with minimalto-no plastic deformation. 3. If future studies confirm that this higher strength is maintained after fatigue testing and cyclic loading, the milled denture bases may be a useful alternative when heavier functional loads are anticipated or for patients who have experienced multiple denture fractures. REFERENCES 1. Zarb GA, Fenton AH. Prosthodontic treatment for edentulous patients. 13th ed. St. Louis: Mosby/Elsevier; 2012. p. 133, 136, 140. 2. Anusavice KJ, Shen C, Rawls R. Phillips’ science of dental materials. 12th ed. Philadelphia: Saunders/Elsevier; 2012. p. 94, 97, 480, 481, 483. 3. Diaz-Arnold AM, Vargas MA, Shaull KL, Laffoon JE, Qian F. Flexural and fatigue strengths of denture base resin. J Prosthet Dent 2008;100:47-51. 4. Jagger DC, Harrison A, Jandt KD. The reinforcement of dentures. J Oral Rehabil 1999;26:185-94. 5. Bates JF, Stafford GD, Huggett R, Handley RW. Current status of pour type denture base resins. J Dent 1977;5:177-89. 6. Choi JE, Ng TE, Leong CKY, Kim H, Li P, Waddell JN. Adhesive evaluation of three types of resilient denture liners bonded to heat-polymerized, autopolymerized, or CAD-CAM acrylic denture bases. J Prosthet Dent 2018;120: 699-705. 7. Miyazaki T, Hotta Y, Kunii J, Kuriyama S, Tamaki Y. A review of dental CAD/ CAM: current status and future perspectives from 20 years of experience. Dent Mater J 2009;28:44-56. 8. Infante L, Yilmaz B, McGlumphy E, Finger I. Fabricating complete dentures with CAD/CAM technology. J Prosthet Dent 2014;111:351-5. 9. Bidra AS, Taylor TD, Agar JR. Computer-aided technology for fabricating complete dentures: systematic review of historical background, current status, and future perspectives. J Prosthet Dent 2013;109:361-6. 10. Hargreaves AS. The prevalence of fractured dentures. A survey. Br Dent J 1969;126:451-5. 11. Beyli MS, von Fraunhofer JA. An analysis of causes of fracture of acrylic resin dentures. J Prosthet Dent 1981;46:238-41. 12. Darbar UR, Huggett R, Harrison A. Denture fractureea survey. Br Dent J 1994;176:342-5. 13. Craig RG, Farah JW, el-Tahawi HM. Three-dimensional photoelastic stress analysis of maxillary complete dentures. J Prosthet Dent 1974;31:122-9. 14. Dirtoft BI, Jansson JF, Abramson NH. Using holography for measurement of in vivo deformation in a complete maxillary denture. J Prosthet Dent 1985;54: 843-6. 15. Cheng YY, Cheung WL, Chow TW. Strain analysis of maxillary complete denture with three-dimensional finite element method. J Prosthet Dent 2010;103:309-18. 16. Rees JS, Huggett R, Harrison A. Finite element analysis of the stressconcentrating effect of fraenal notches in complete dentures. Int J Prosthodont 1990;3:238-40. 17. Kydd WL. The comminuting efficiency of varied occlusal tooth form and the associated deformation of the complete denture base. J Am Dent Assoc 1960;61:465-71. 18. Kim SH, Watts DC. The effect of reinforcement with woven E-glass fibers on the impact strength of complete dentures fabricated with high-impact acrylic resin. J Prosthet Dent 2004;91:274-80. 19. Ates M, Cilingir A, Sulun T, Sunbuloglu E, Bozdag E. The effect of occlusal contact localization on the stress distribution in complete maxillary denture. J Oral Rehabil 2006;33:509-13.
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20. Cilingir A, Bilhan H, Baysal G, Sunbuloglu E, Bozdag E. The impact of frenulum height on strains in maxillary denture bases. J Adv Prosthodont 2013;5:409-15. 21. Kelly E. Fatigue failure in denture base polymers. J Prosthet Dent 1969;21: 257-66. 22. Oku JI. Impact properties of acrylic denture base resin. Part 1. A new method for determination of impact properties. Dent Mater J 1988;7: 166-73. 23. Neihart TR, Li SH, Flinton RJ. Measuring fracture toughness of high-impact poly(methyl methacrylate) with the short rod method. J Prosthet Dent 1988;60:249-53. 24. Zappini G, Kammann A, Wachter W. Comparison of fracture tests of denture base materials. J Prosthet Dent 2003;90:578-85. 25. ADA. ANSI/ADA Standard No. 139 (ISO 20795-1), Denture Base Polymers. American Dental Association; 2013. Available at: https://webstore.ansi.org/ Standards/ISO/ISO207952013?gclid=EAIaIQobChMI5eGNoJ_64AIVT57 ACh0YsAsyEAAYAiAAEgKeaPD_BwE. 26. Abdulwahhab SS. High-impact strength acrylic denture base material processed by autoclave. J Prosthodont Res 2013;57:288-93. 27. Gharechahi J, Asadzadeh N, Shahabian F, Gharechahi M. Flexural strength of acrylic resin denture bases processed by two different methods. J Dent Res Dent Clin Dent Prospects 2014;8:148-52.
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28. Machado AL, Puckett AD, Breeding LC, Wady AF, Vergani CE. Effect of thermocycling on the flexural and impact strength of urethane-based and high-impact denture base resins. Gerodontology 2012;29:318-23. 29. Al-Dwairi ZN, Tahboub KY, Baba NZ, Goodacre CJ. A Comparison of the Flexural and Impact Strengths and Flexural Modulus of CAD/CAM and Conventional Heat-Cured Polymethyl Methacrylate (PMMA). J Prosthodont 2018 June 13. doi: 10.111/jopr.12926. 30. Steinmassl PA, Wiedemair V, Huck C. Do CAD/CAM dentures really release less monomer than conventional dentures? Clin Oral Investig 2017;21: 1697-705. 31. Tuna SH, Keyf F, Gumus HO, Uzun C. The evaluation of water sorption/ solubility on various acrylic resins. Eur J Dent 2008;2:191-7. Corresponding author: Dr Jenn-Hwan Chen 3302 Gaston Avenue, Room 733-1 Dallas, TX 75246 Email: [email protected] Copyright © 2019 by the Editorial Council for The Journal of Prosthetic Dentistry. https://doi.org/10.1016/j.prosdent.2019.03.010
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