- Email: [email protected]
Wear of resin teeth opposing zirconia
RESEARCH AND EDUCATION
Wear of resin teeth opposing zirconia Jonathan Esquivel, DDS,a Nathaniel C. Lawson, DMD, PhD,b Edwin Kee, MCDT,c Karen Bruggers, DDS, MS,d and Markus B. Blatz, DMD, PhDe The placement of 4 or more ABSTRACT implants has become a comStatement of problem. The use of dissimilar materials for opposing complete-mouth mon procedure for restoring implant-supported prosthesis has become popular, especially when one arch is made from an edentulous arch with fixed anatomical contour zirconia. However, the amount of wear zirconia causes on resin and other 1-3 dental prostheses. A titadenture tooth materials is largely unknown. nium bar supporting acrylic Purpose. The purpose of this in vitro study was to determine the volumetric wear of 4 commercially resin and denture teeth is available resin materials used for denture teeth in complete-arch implant-supported prostheses the most prevalent prosthetic opposed by zirconia. design for complete-arch Material and methods. A total of 32 maxillary central incisor denture teeth were evaluated (n=8): implant-supported prosthedouble crosslinked polymethyl methacrylate (PMMA) (DCL), nanohybrid composite resin (PHO), and 3 4 ses. Box et al reported that computer-aided design and computer-aided manufacturing (CAD-CAM)-fabricated teeth made the most common prosthetic from crosslinked PMMA (TEL) and acrylate polymer (ZCAD). Antagonist cone-shaped specimens complication with metalwere milled from zirconia. Specimens were mounted in acrylic resin, polished to a flat surface by using 1200-grit SiC paper, and stored in water (37 C for 24 hours) before exposure in a custom acrylic resin complete-arch dual-axis wear simulator for 200 000 cycles with a vertical load of 20 N, a horizontal slide of 2 implant-supported prostheses mm, and a frequency of 1 Hz. Volumetric wear was measured by using a noncontact was the wear of posterior profilometer and a superimposition software program. Data were analyzed with a 1-way analysis denture teeth. Monolithic zirof variance (ANOVA) and the Tukey Honestly Significant Difference (HSD) post hoc test (a=.05). conia complete-arch implantResults. A statistically significant difference in volumetric wear was found between groups (P<.001), supported prostheses are with PHO (4.3 ±1.0 mm3)
a
Assistant Professor, Department of Prosthodontics, School of Dentistry, Louisiana State University, New Orleans, La. Assistant Professor, Department of Clinical and Community Sciences, School of Dentistry, University of Alabama at Birmingham, Birmingham, Ala. Associate Professor of Clinical Prosthodontics, School of Dentistry, Louisiana State University, New Orleans, La. d Assistant Professor and Department Chair, Department of Prosthodontics, School of Dentistry, Louisiana State University, New Orleans, La. e Professor of Restorative Dentistry, Chairman, Department of Preventive and Restorative Sciences and Assistant Dean for Digital Innovation and Professional Development, University of Pennsylvania School of Dental Medicine, Philadelphia, Pa. b c
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Clinical Implications Proper selection of resin denture teeth to oppose zirconia complete-arch implant-supported prostheses is essential to limit abrasion wear.
Different materials may be used for complete-arch implant-supported prostheses in opposing arches. Even though the survival of implants is not affected by the number of arches restored,8 a patient restored with implant-supported prostheses in both arches may experience more mechanical complications than a patient restored with an implant-supported prosthesis in a single arch. Gonzales and Triplett9 reported complications with layered zirconia complete-arch implant-supported prostheses that opposed similar layered zirconia completearch implant-supported prostheses or a natural dentition. When the same layered zirconia complete-arch implant-supported prostheses were opposing metalacrylic resin complete-arch implant-supported prostheses, all the complications occurred with the denture teeth in the opposing arch.9 In a 1-year clinical report of 2 patients with monolithic zirconia complete-arch implantsupported prostheses opposing natural dentition or nanohybrid-composite resin teeth, Cardelli et al10 reported that the wear of the nanohybrid-composite resin teeth was comparable with the wear of enamel opposing a zirconia complete-arch implant-supported prostheses. In addition, the nanohybrid composite resin teeth had more wear than the opposing zirconia prostheses.10 As wear of denture teeth is a concern for metal-acrylic resin complete-arch implant-supported prostheses and wear of the opposing prostheses is a concern for monolithic zirconia implant-supported prostheses, the effect of zirconia on the abrasion wear of opposing denture teeth needs further evaluation. The wear of zirconia has been studied extensively.7,11-13 When zirconia opposes natural tooth structures, it causes less wear on enamel than feldspathic porcelain because of its small-particle grain size and a surface that remains smooth throughout the wear process, while feldspathic porcelain fractures and becomes rougher.7 When zirconia opposes polymeric materials, it causes significantly more wear to composite resins than it does to enamel.14,15 The abrasion wear resistance of different resin-based materials has been reported,16-22 with different testing protocols used to evaluate their resistance to wear and on how these test conditions relate to intraoral use.23-25 While wear behavior of different antagonist and denture tooth materials has been tested, data on the wear of denture teeth caused by zirconia are limited.26-29 Milled polymethyl methacrylate (PMMA) has become a preferred material for interim THE JOURNAL OF PROSTHETIC DENTISTRY
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implant-supported prostheses because of improved physical properties when compared with conventional PMMA.30,31 Although its abrasion wear resistance to opposing zirconia has not been extensively studied, it is an essential factor in the long-term clinical behavior and survival of these restorations. The purpose of this in vitro study was to measure and compare the volumetric wear of different artificial resin tooth materials opposed by zirconia. The null hypothesis was that no difference in volumetric wear would be found among the materials tested. MATERIAL AND METHODS Thirty-two maxillary central incisor denture teeth were evaluated (n=8): DCL (Blueline DCL; Ivoclar Vivadent AG), PHO (SR Phonares II; Ivoclar Vivadent AG), TEL (Telio CAD; Ivoclar Vivadent AG), and ZCAD (ZCAD Temp Esthetic; Harvest Dental Products LLC.). Groups DCL and PHO were prefabricated denture teeth, while TEL and ZCAD were designed by using a computeraided design and computer-aided manufacturing (CADCAM) software program (exocad Valletta 2.2; exocad GmbH) and milled in a 5-axis milling unit (Tizian cut 5 smart; Schutz Dental GmbH). The denture teeth specimens were mounted in the center of brass holders with autopolymerizing acrylic resin (Coldpac; Yates Motloid) with the labial surfaces exposed. The specimens were polished by using a series of SiC abrasive paper (320, 600, and 1200 grit) (CarbiMet; Buehler) under water spray for 1 minute per paper and a rotational polishing device (No: 233-0-1997; Buehler). The specimens were stored in distilled water at 37 C in an incubator for 24 hours. Antagonists were designed in a cone shape (Cerec InLab 18.1; Dentsply Sirona) and milled from zirconia (Katana LT; Kuraray Noritake Dental Inc) in a 5-axis mill (inLab MC X5; Dentsply Sirona). The specimens were sintered in a furnace (inFire HTC Speed; Dentsply Sirona) following the manufacturer’s recommended sintering schedule. The zirconia cusps were mounted in a custom dual-axis mastication simulator (Fig. 1). The test device and parameters have been described in detail in a previous publication.32 A vertical load of 20 N and a horizontal slide of 2 mm were applied for 200 000 cycles at a frequency of 1 Hz. A 33% glycerin solution (Glycerine; Sigma Aldrich) was used as a lubricant. The specimens were scanned before and after wear testing by using a noncontact 3D surface measurement instrument (PROSCAN 2000; Scantron Industrial Products Ltd). A 4×4-mm area of the specimen was scanned with a 20-mm resolution. The before and after scans were superimposed to determine volumetric material loss (ProForm Software; Scantron Industrial Products Ltd). Representative specimens were examined by digital light microscopy (VHX600; Keyence Corp). One-way analysis Esquivel et al
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Figure 1. Zirconia cusp mounted on screw for testing. Table 1. Mean ±standard deviation volumetric wear values of tested specimens Composition
Mean Volumetric Wear (mm3)
n
Group
8
DCL
Cross-linked PMMA
8
ZCAD
Cross-linked acrylate polymer (CAD CAM)
14.3 ±0.8
8
TEL
Cross-linked PMMA (CAD CAM)
11.9 ±2.0
8
PHO
Nano-hybrid composite resin
17.3 ±1.0
4.3 ±1.0
Statistically significant differences found in volumetric wear among all groups (P<.05).
of variance (ANOVA) and the Tukey Honestly Significant Difference (HSD) post hoc test were applied to analyze the data (a=.05). RESULTS The volumetric wear of the 4 materials tested are presented in Table 1. The 1-way ANOVA indicates significant differences among materials (P<.001). The Tukey post hoc analysis ordered materials into statistically distinct groups (average volumetric wear ±standard deviation): PHO (4.3 ±1.0 mm3)
materials was rejected. PHO, a nanohybrid composite resin material, demonstrated higher wear resistance to the opposing zirconia cusps than the other groups. The milled PMMA groups TEL and ZCAD had higher wear resistance than group DCL, which had the least resistance to wear. Some of the differences in the performance of the teeth may be because of their microstructure. The teeth in group PHO are described as a nanohybrid composite resin. More specifically, this material is composed of a urethane dimethacrylate matrix reinforced with inorganic fillers. The fillers in this nanohybrid composite resin affect the wear resistance of the denture teeth. The harder filler particles protect the softer resin matrix during the wear process. Inspection of the worn surfaces of the PHO denture teeth revealed fine scratches traversing in the same direction as the path of the zirconia antagonists (Fig. 3A). These scratches suggest that the denture teeth underwent abrasive wear. In this mechanism of wear, irregularities on the surface of the zirconia antagonist, called asperities, plow through the softer denture tooth surface. The filler particles in the nanohybrid composite resin were harder than the surrounding resin matrix and, therefore, were not as easily abraded by the asperities on the zirconia antagonist. Previous studies of composite resins confirm the role of filler particles on wear resistance. The denture teeth with unfilled polymer (TEL, ZCAD, and DCL) had a wear scar that is slightly differently shaped than the one in the PHO group (Fig. 3B-D). The head of the wear scar (the area that was first impacted by the antagonist) was wider than the tail of the scar. This wider area of first antagonist impact likely experienced fatigue wear. Fatigue wear occurs when microcracks form below the surface of a material and eventually coalesce to free a small fragment of the material. Materials with a lower elastic modulus, such as unfilled resins, are susceptible to fatigue wear. The tail end of these wear scars demonstrates the scratches associated with abrasive wear occurring when the antagonist performed its horizontal motion.21 Despite the differences in volumetric wear observed for the 3 unfilled polymers, no observable differences were found in the wear scar that would imply different mechanisms of wear were occurring. Additionally, the manufacturers of these materials do not provide compositional information regarding their products that would explain the differences in measured volumetric wear. The wear of cross-linked PMMA and nanohybrid composite resin denture teeth has been evaluated.16,20,28 The outcome of the present study in which the resin composite material was more wear resistant than the unfilled polymer materials is consistent with the outcomes of earlier investigations. Ghazal and Kern19 reported that composite resin denture teeth have better THE JOURNAL OF PROSTHETIC DENTISTRY
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Figure 2. Scan of representative specimens (20-mm resolution; ProForm software; Scantron Industrial Products Ltd). A, Group PHO. B, Group TEL. C, Group ZCAD. D, Group DCL. DCL, double crosslinked polymethyl methacrylate; PHO, nanohybrid composite resin; TEL, computer-aided design and computer-aided manufactured from crosslinked PMMA; ZCAD, computer-aided design and computer-aided manufactured acrylate polymer.
wear resistance than acrylic resin teeth when opposing a steatite ceramic ball. Similar results were found in a study comparing conventional acrylic teeth, cross-linked acrylic teeth, and composite resin teeth opposing a silicon nitride ceramic ball.27 In contrast with the present study, Munshi et al16 reported that nanohybrid composite resin teeth exhibited more wear than various types of PMMA denture teeth.16 In their study, the different types of denture tooth materials were abraded against the same materials. This variation in testing methodology explains the different results. A previous study29 reported that the type of antagonist material had a significant effect on the wear properties of different denture teeth. A ceramic antagonist discriminated better among different types of denture teeth than metal or polymer antagonists. However, studies with zirconia as an antagonist material in wear tests of denture teeth are lacking. Intraoral wear is complex and related to both the composition of denture teeth and the patient’s oral habits. A patient’s sex and intraoral clinical factors such as antagonist material, mechanical factors, neuromuscular force, intraoral pH, as well erosion and corrosion, have also been described as influencing parameters.18,23 Replicating these THE JOURNAL OF PROSTHETIC DENTISTRY
clinical factors in vitro is challenging; however, the results of these studies can only serve as a guide because the applied in vitro test arrangement and parameters cannot fully replicate the intraoral conditions. One advantage of the current wear device is that the load was applied with a nondampened load unlike the previous iteration of this wear device in which the load was dampened with a spring. As there are no periodontal ligaments associated with opposing implant restorations, a nondampened load should better replicate the clinical situation. In the present study, a 2-body wear test was used to evaluate the wear behavior of different resin materials for complete-arch prostheses opposing zirconia, simulating dissimilar materials for complete-arch implant-supported prostheses. A 33% glycerin solution served as a lubricant because of its similarity to saliva and to help wash away worn material particles.7 The concentration of glycerin was chosen to replicate the viscosity of stimulated saliva, acting as a boundary lubricant.7 The ability to control intraoral factors that vary among patients makes in vitro 2-body wear testing a valid model to measure antagonist wear without involving an intermediate medium that may influence the results.16,23,24 Esquivel et al
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Figure 3. Digital light microscopy of representative specimen (VHX600; Keyence). Original magnification ×15. A, Group PHO. B, Group TEL. C, Group ZCAD. D, Group DCL. DCL, double crosslinked polymethyl methacrylate; PHO, nanohybrid composite resin; TEL, computer-aided design and computeraided manufactured from cross-linked PMMA; ZCAD, computer-aided design and computer-aided manufactured acrylate polymer.
The 200 000 cycles applied in this study represent approximately 9.6 months of clinical use.25 These types of correlations of wear cycles and clinical service are based on correlating the volumetric loss with a reference restorative material in a laboratory device with clinical wear measurements. Therefore, this estimation is likely flawed. A more useful observation is that the PHO teeth had 3 to 4 times less wear than all other materials. Assuming a linear wear rate, these teeth would last 3 to 4 times longer than the others. An increased wear rate of denture teeth represents a clinical and financial disadvantage, leading to an earlier remake. As denture teeth wear more progressively, the patient will lose vertical dimension of occlusion (VDO). This loss of VDO may lead to mechanical failures such as the fracture of anterior denture teeth, the prosthetic substructures, or functional and esthetic alterations. Limitations of the present study include that only 4 different resin materials were tested from the great range of available materials. Furthermore, the creation of a flat surface on the anterior teeth tested removed part of the outside enamel layer of the denture teeth in groups PHO and DCL and may have altered their Esquivel et al
abrasion wear resistance. This procedure could not be avoided, however, because of the configuration of the wear testing device. In general, in vitro studies can only assess limited parameters and may not be able to fully represent clinical reality and conditions. Some of these parameters may include the effects of temperature and pH cycling on the wear behavior of the material. However, limiting influencing parameters allows for a focused assessment of the materials and research question at hand. Further studies are suggested to better understand the behavior of materials exposed to function and abrasion wear. Such future studies should incorporate simulated aging parameters such as temperature and pH cycling. CONCLUSIONS Based on the findings of this in vitro study, the following conclusions were drawn: 1. Denture teeth made from different materials demonstrated significantly different volumetric substance loss when subjected to occlusal wear by zirconia antagonists. THE JOURNAL OF PROSTHETIC DENTISTRY
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2. Nanohybrid composite resin denture teeth had the least amount of volumetric substance loss. REFERENCES 1. Brånemark PI, Adell R, Breine U, Hansson BO, Lindström J, Ohlsson A. Intra-osseous anchorage of dental prostheses. I. Experimental studies. Scand J Plast Reconstr Surg 1969;3:81-100. 2. Zarb GA, Zarb FL. Tissue integrated dental prostheses. Quintessence Int 1985;16:39-42. 3. Sadowsky SJ. Treatment considerations for maxillary implant overdentures: a systematic review. J Prosthet Dent 2007;97:340-8. 4. Box VH, Sukotjo C, Knoernschild KL, Campbell SD, Afshari FS. Patientreported and clinical outcomes of implant-supported fixed complete dental prostheses: a comparison of metal-acrylic, milled zirconia, and retrievable crown prostheses. J Oral Implantol 2018;44:51-61. 5. Bidra AS, Tischler M, Patch C. Survival of 2039 complete arch fixed implantsupported zirconia prostheses: a retrospective study. J Prosthet Dent 2018;119:220-4. 6. Park JH, Park S, Lee K, Yun KD, Lim HP. Antagonist wear of three CAD/ CAM anatomic contour zirconia ceramics. J Prosthet Dent 2014;111:20-9. 7. Janyavula S, Lawson N, Cakir D, Beck P, Ramp LC, Burgess JO. The wear of polished and glazed zirconia against enamel. J Prosthet Dent 2013;109:22-9. 8. Maló P, Araújo Nobre MD, Lopes A, Rodrigues R. Double full-arch versus single full-arch, four implant-supported rehabilitations: a retrospective, 5-year cohort study. J Prosthodont 2015;24:263-70. 9. Gonzalez J, Triplett RG. Complications and clinical considerations of the implant-retained zirconia complete-arch prosthesis with various opposing dentitions. Int J Oral Maxillofac Implants 2017;32:864-9. 10. Cardelli P, Manobianco FP, Serafini N, Murmura G, Beuer F. Full-arch, implant-supported monolithic zirconia rehabilitations: pilot clinical evaluation of wear against natural or composite teeth. J Prosthodont 2016;25: 629-33. 11. Preis V, Behr M, Handel G, Schneider-Feyrer S, Hahnel S, Rosentritt M. Wear performance of dental ceramics after grinding and polishing treatments. J Mech Behav Biomed Mater 2012;10:13-22. 12. Passos SP, Torrealba Y, Major P, Linke B, Flores-Mir C, Nychka JA. In vitro wear behavior of zirconia opposing enamel: a systematic review. J Prosthodont 2014;23:593-601. 13. Gou M, Chen H, Kang J, Wang H. Antagonist enamel wear of toothsupported monolithic zirconia posterior crowns in vivo: a systematic review. J Prosthet Dent 2019;121:598-603. 14. Pereira GKR, Dutra DM, Werner A, Prochnow C, Valandro LF, Kleverlaan CJ. Effect of zirconia polycrystal and stainless steel on the wear of resin composites, dentin and enamel. J Mech Behav Biomed Mater 2019;91:287-93. 15. Ludovichetti FS, Trindade FZ, Werner A, Kleverlaan CJ, Fonseca RG. Wear resistance and abrasiveness of CAD-CAM monolithic materials. J Prosthet Dent 2018;120:318.e1-8. 16. Munshi N, Rosenblum M, Jiang S, Flinton R. In vitro wear resistance of nanohybrid composite denture teeth. J Prosthodont 2017;26:224-9.
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17. Kamonwanon P, Yodmongkol S, Chantarachindawong R, Thaweeboon S, Thaweeboon B, Srikhirin T. Wear resistance of a modified polymethyl methacrylate artificial tooth compared to five commercially available artificial tooth materials. J Prosthet Dent 2015;114:286-92. 18. Stober T, Henninger M, Schmitter M, Pritsch M, Rammelsberg P. Three-body wear of resin denture teeth with and without nanofillers. J Prosthet Dent 2010;103:108-17. 19. Ghazal M, Kern M. Wear of denture teeth and their human enamel antagonists. Quintessence Int 2010;41:157-63. 20. Hirano S, May KB, Wagner WC, Hacker CH. In vitro wear of resin denture teeth. J Prosthet Dent 1998;79:152-5. 21. Lawson NC, Burgess JO. Wear of nanofilled dental composites at varying filler concentrations. J Biomed Mater Res B Appl Biomater 2015;103: 424-9. 22. Preis V, Hahnel S, Behr M, Rosentritt M. Contact wear of artificial denture teeth. J Prosthodont Res 2018;62:252-7. 23. Stober T, Lutz T, Gilde H, Rammelsberg P. Wear of resin denture teeth by two-body contact. Dent Mater 2006;22:243-9. 24. Wassell RW, McCabe JF, Walls AW. Wear characteristics in a two-body wear test. Dent Mater 1994;10:269-74. 25. DeLong R, Douglas WH. An artificial oral environment for testing dental materials. IEEE Trans Biomed Eng 1991;38:339-45. 26. Ghazal M, Yang B, Ludwig K, Kern M. Two-body wear of resin and ceramic denture teeth in comparison to human enamel. Dent Mater 2008;24:502-7. 27. Hao Z, Yin H, Wang L, Meng Y. Wear behavior of seven artificial resin teeth assessed with three-dimensional measurements. J Prosthet Dent 2014;112: 1507-12. 28. Stober T, Bermejo JL, Rues S, Rammelsberg P. Wear of resin denture teeth in partial removable dental prostheses. J Prosthodont Res 2019. doi: 10.1016/ j.jpor.2019.04.004. [Epub ahead of print]. 29. Hahnel S, Behr M, Handel G, Rosentritt M. Two-body wear of artificial acrylic and composite resin teeth in relation to antagonist material. J Prosthet Dent 2009;101:269-78. 30. Yilmaz B, Alp G, Seidt J, Johnston WM, Vitter R, McGlumphy EA. Fracture analysis of CAD-CAM high-density polymers used for interim implantsupported fixed, cantilevered prostheses. J Prosthet Dent 2018;120:79-84. 31. Alt V, Hannig M, Wöstmann B, Balkenhol M. Fracture strength of temporary fixed partial dentures: CAD/CAM versus directly fabricated restorations. Dent Mater 2011;27:339-47. 32. Lawson NJS, Cakir D, Burgess JO. An analysis of the physiologic parameters of intraoral wear: a review. J Phys D 2013;46:1-7. Corresponding author: Dr Jonathan Esquivel Department of Prosthodontics LSU School of Dentistry Box 2221100 Florida Avenue New Orleans, LA 70119 Email: [email protected] Copyright © 2019 by the Editorial Council for The Journal of Prosthetic Dentistry. https://doi.org/10.1016/j.prosdent.2019.11.004
Esquivel et al
Wear of resin teeth opposing zirconia Jonathan Esquivel, DDS,a Nathaniel C. Lawson, DMD, PhD,b Edwin Kee, MCDT,c Karen Bruggers, DDS, MS,d and Markus B. Blatz, DMD, PhDe The placement of 4 or more ABSTRACT implants has become a comStatement of problem. The use of dissimilar materials for opposing complete-mouth mon procedure for restoring implant-supported prosthesis has become popular, especially when one arch is made from an edentulous arch with fixed anatomical contour zirconia. However, the amount of wear zirconia causes on resin and other 1-3 dental prostheses. A titadenture tooth materials is largely unknown. nium bar supporting acrylic Purpose. The purpose of this in vitro study was to determine the volumetric wear of 4 commercially resin and denture teeth is available resin materials used for denture teeth in complete-arch implant-supported prostheses the most prevalent prosthetic opposed by zirconia. design for complete-arch Material and methods. A total of 32 maxillary central incisor denture teeth were evaluated (n=8): implant-supported prosthedouble crosslinked polymethyl methacrylate (PMMA) (DCL), nanohybrid composite resin (PHO), and 3 4 ses. Box et al reported that computer-aided design and computer-aided manufacturing (CAD-CAM)-fabricated teeth made the most common prosthetic from crosslinked PMMA (TEL) and acrylate polymer (ZCAD). Antagonist cone-shaped specimens complication with metalwere milled from zirconia. Specimens were mounted in acrylic resin, polished to a flat surface by using 1200-grit SiC paper, and stored in water (37 C for 24 hours) before exposure in a custom acrylic resin complete-arch dual-axis wear simulator for 200 000 cycles with a vertical load of 20 N, a horizontal slide of 2 implant-supported prostheses mm, and a frequency of 1 Hz. Volumetric wear was measured by using a noncontact was the wear of posterior profilometer and a superimposition software program. Data were analyzed with a 1-way analysis denture teeth. Monolithic zirof variance (ANOVA) and the Tukey Honestly Significant Difference (HSD) post hoc test (a=.05). conia complete-arch implantResults. A statistically significant difference in volumetric wear was found between groups (P<.001), supported prostheses are with PHO (4.3 ±1.0 mm3)
a
Assistant Professor, Department of Prosthodontics, School of Dentistry, Louisiana State University, New Orleans, La. Assistant Professor, Department of Clinical and Community Sciences, School of Dentistry, University of Alabama at Birmingham, Birmingham, Ala. Associate Professor of Clinical Prosthodontics, School of Dentistry, Louisiana State University, New Orleans, La. d Assistant Professor and Department Chair, Department of Prosthodontics, School of Dentistry, Louisiana State University, New Orleans, La. e Professor of Restorative Dentistry, Chairman, Department of Preventive and Restorative Sciences and Assistant Dean for Digital Innovation and Professional Development, University of Pennsylvania School of Dental Medicine, Philadelphia, Pa. b c
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Clinical Implications Proper selection of resin denture teeth to oppose zirconia complete-arch implant-supported prostheses is essential to limit abrasion wear.
Different materials may be used for complete-arch implant-supported prostheses in opposing arches. Even though the survival of implants is not affected by the number of arches restored,8 a patient restored with implant-supported prostheses in both arches may experience more mechanical complications than a patient restored with an implant-supported prosthesis in a single arch. Gonzales and Triplett9 reported complications with layered zirconia complete-arch implant-supported prostheses that opposed similar layered zirconia completearch implant-supported prostheses or a natural dentition. When the same layered zirconia complete-arch implant-supported prostheses were opposing metalacrylic resin complete-arch implant-supported prostheses, all the complications occurred with the denture teeth in the opposing arch.9 In a 1-year clinical report of 2 patients with monolithic zirconia complete-arch implantsupported prostheses opposing natural dentition or nanohybrid-composite resin teeth, Cardelli et al10 reported that the wear of the nanohybrid-composite resin teeth was comparable with the wear of enamel opposing a zirconia complete-arch implant-supported prostheses. In addition, the nanohybrid composite resin teeth had more wear than the opposing zirconia prostheses.10 As wear of denture teeth is a concern for metal-acrylic resin complete-arch implant-supported prostheses and wear of the opposing prostheses is a concern for monolithic zirconia implant-supported prostheses, the effect of zirconia on the abrasion wear of opposing denture teeth needs further evaluation. The wear of zirconia has been studied extensively.7,11-13 When zirconia opposes natural tooth structures, it causes less wear on enamel than feldspathic porcelain because of its small-particle grain size and a surface that remains smooth throughout the wear process, while feldspathic porcelain fractures and becomes rougher.7 When zirconia opposes polymeric materials, it causes significantly more wear to composite resins than it does to enamel.14,15 The abrasion wear resistance of different resin-based materials has been reported,16-22 with different testing protocols used to evaluate their resistance to wear and on how these test conditions relate to intraoral use.23-25 While wear behavior of different antagonist and denture tooth materials has been tested, data on the wear of denture teeth caused by zirconia are limited.26-29 Milled polymethyl methacrylate (PMMA) has become a preferred material for interim THE JOURNAL OF PROSTHETIC DENTISTRY
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Issue
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implant-supported prostheses because of improved physical properties when compared with conventional PMMA.30,31 Although its abrasion wear resistance to opposing zirconia has not been extensively studied, it is an essential factor in the long-term clinical behavior and survival of these restorations. The purpose of this in vitro study was to measure and compare the volumetric wear of different artificial resin tooth materials opposed by zirconia. The null hypothesis was that no difference in volumetric wear would be found among the materials tested. MATERIAL AND METHODS Thirty-two maxillary central incisor denture teeth were evaluated (n=8): DCL (Blueline DCL; Ivoclar Vivadent AG), PHO (SR Phonares II; Ivoclar Vivadent AG), TEL (Telio CAD; Ivoclar Vivadent AG), and ZCAD (ZCAD Temp Esthetic; Harvest Dental Products LLC.). Groups DCL and PHO were prefabricated denture teeth, while TEL and ZCAD were designed by using a computeraided design and computer-aided manufacturing (CADCAM) software program (exocad Valletta 2.2; exocad GmbH) and milled in a 5-axis milling unit (Tizian cut 5 smart; Schutz Dental GmbH). The denture teeth specimens were mounted in the center of brass holders with autopolymerizing acrylic resin (Coldpac; Yates Motloid) with the labial surfaces exposed. The specimens were polished by using a series of SiC abrasive paper (320, 600, and 1200 grit) (CarbiMet; Buehler) under water spray for 1 minute per paper and a rotational polishing device (No: 233-0-1997; Buehler). The specimens were stored in distilled water at 37 C in an incubator for 24 hours. Antagonists were designed in a cone shape (Cerec InLab 18.1; Dentsply Sirona) and milled from zirconia (Katana LT; Kuraray Noritake Dental Inc) in a 5-axis mill (inLab MC X5; Dentsply Sirona). The specimens were sintered in a furnace (inFire HTC Speed; Dentsply Sirona) following the manufacturer’s recommended sintering schedule. The zirconia cusps were mounted in a custom dual-axis mastication simulator (Fig. 1). The test device and parameters have been described in detail in a previous publication.32 A vertical load of 20 N and a horizontal slide of 2 mm were applied for 200 000 cycles at a frequency of 1 Hz. A 33% glycerin solution (Glycerine; Sigma Aldrich) was used as a lubricant. The specimens were scanned before and after wear testing by using a noncontact 3D surface measurement instrument (PROSCAN 2000; Scantron Industrial Products Ltd). A 4×4-mm area of the specimen was scanned with a 20-mm resolution. The before and after scans were superimposed to determine volumetric material loss (ProForm Software; Scantron Industrial Products Ltd). Representative specimens were examined by digital light microscopy (VHX600; Keyence Corp). One-way analysis Esquivel et al
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Figure 1. Zirconia cusp mounted on screw for testing. Table 1. Mean ±standard deviation volumetric wear values of tested specimens Composition
Mean Volumetric Wear (mm3)
n
Group
8
DCL
Cross-linked PMMA
8
ZCAD
Cross-linked acrylate polymer (CAD CAM)
14.3 ±0.8
8
TEL
Cross-linked PMMA (CAD CAM)
11.9 ±2.0
8
PHO
Nano-hybrid composite resin
17.3 ±1.0
4.3 ±1.0
Statistically significant differences found in volumetric wear among all groups (P<.05).
of variance (ANOVA) and the Tukey Honestly Significant Difference (HSD) post hoc test were applied to analyze the data (a=.05). RESULTS The volumetric wear of the 4 materials tested are presented in Table 1. The 1-way ANOVA indicates significant differences among materials (P<.001). The Tukey post hoc analysis ordered materials into statistically distinct groups (average volumetric wear ±standard deviation): PHO (4.3 ±1.0 mm3)
materials was rejected. PHO, a nanohybrid composite resin material, demonstrated higher wear resistance to the opposing zirconia cusps than the other groups. The milled PMMA groups TEL and ZCAD had higher wear resistance than group DCL, which had the least resistance to wear. Some of the differences in the performance of the teeth may be because of their microstructure. The teeth in group PHO are described as a nanohybrid composite resin. More specifically, this material is composed of a urethane dimethacrylate matrix reinforced with inorganic fillers. The fillers in this nanohybrid composite resin affect the wear resistance of the denture teeth. The harder filler particles protect the softer resin matrix during the wear process. Inspection of the worn surfaces of the PHO denture teeth revealed fine scratches traversing in the same direction as the path of the zirconia antagonists (Fig. 3A). These scratches suggest that the denture teeth underwent abrasive wear. In this mechanism of wear, irregularities on the surface of the zirconia antagonist, called asperities, plow through the softer denture tooth surface. The filler particles in the nanohybrid composite resin were harder than the surrounding resin matrix and, therefore, were not as easily abraded by the asperities on the zirconia antagonist. Previous studies of composite resins confirm the role of filler particles on wear resistance. The denture teeth with unfilled polymer (TEL, ZCAD, and DCL) had a wear scar that is slightly differently shaped than the one in the PHO group (Fig. 3B-D). The head of the wear scar (the area that was first impacted by the antagonist) was wider than the tail of the scar. This wider area of first antagonist impact likely experienced fatigue wear. Fatigue wear occurs when microcracks form below the surface of a material and eventually coalesce to free a small fragment of the material. Materials with a lower elastic modulus, such as unfilled resins, are susceptible to fatigue wear. The tail end of these wear scars demonstrates the scratches associated with abrasive wear occurring when the antagonist performed its horizontal motion.21 Despite the differences in volumetric wear observed for the 3 unfilled polymers, no observable differences were found in the wear scar that would imply different mechanisms of wear were occurring. Additionally, the manufacturers of these materials do not provide compositional information regarding their products that would explain the differences in measured volumetric wear. The wear of cross-linked PMMA and nanohybrid composite resin denture teeth has been evaluated.16,20,28 The outcome of the present study in which the resin composite material was more wear resistant than the unfilled polymer materials is consistent with the outcomes of earlier investigations. Ghazal and Kern19 reported that composite resin denture teeth have better THE JOURNAL OF PROSTHETIC DENTISTRY
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Figure 2. Scan of representative specimens (20-mm resolution; ProForm software; Scantron Industrial Products Ltd). A, Group PHO. B, Group TEL. C, Group ZCAD. D, Group DCL. DCL, double crosslinked polymethyl methacrylate; PHO, nanohybrid composite resin; TEL, computer-aided design and computer-aided manufactured from crosslinked PMMA; ZCAD, computer-aided design and computer-aided manufactured acrylate polymer.
wear resistance than acrylic resin teeth when opposing a steatite ceramic ball. Similar results were found in a study comparing conventional acrylic teeth, cross-linked acrylic teeth, and composite resin teeth opposing a silicon nitride ceramic ball.27 In contrast with the present study, Munshi et al16 reported that nanohybrid composite resin teeth exhibited more wear than various types of PMMA denture teeth.16 In their study, the different types of denture tooth materials were abraded against the same materials. This variation in testing methodology explains the different results. A previous study29 reported that the type of antagonist material had a significant effect on the wear properties of different denture teeth. A ceramic antagonist discriminated better among different types of denture teeth than metal or polymer antagonists. However, studies with zirconia as an antagonist material in wear tests of denture teeth are lacking. Intraoral wear is complex and related to both the composition of denture teeth and the patient’s oral habits. A patient’s sex and intraoral clinical factors such as antagonist material, mechanical factors, neuromuscular force, intraoral pH, as well erosion and corrosion, have also been described as influencing parameters.18,23 Replicating these THE JOURNAL OF PROSTHETIC DENTISTRY
clinical factors in vitro is challenging; however, the results of these studies can only serve as a guide because the applied in vitro test arrangement and parameters cannot fully replicate the intraoral conditions. One advantage of the current wear device is that the load was applied with a nondampened load unlike the previous iteration of this wear device in which the load was dampened with a spring. As there are no periodontal ligaments associated with opposing implant restorations, a nondampened load should better replicate the clinical situation. In the present study, a 2-body wear test was used to evaluate the wear behavior of different resin materials for complete-arch prostheses opposing zirconia, simulating dissimilar materials for complete-arch implant-supported prostheses. A 33% glycerin solution served as a lubricant because of its similarity to saliva and to help wash away worn material particles.7 The concentration of glycerin was chosen to replicate the viscosity of stimulated saliva, acting as a boundary lubricant.7 The ability to control intraoral factors that vary among patients makes in vitro 2-body wear testing a valid model to measure antagonist wear without involving an intermediate medium that may influence the results.16,23,24 Esquivel et al
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Figure 3. Digital light microscopy of representative specimen (VHX600; Keyence). Original magnification ×15. A, Group PHO. B, Group TEL. C, Group ZCAD. D, Group DCL. DCL, double crosslinked polymethyl methacrylate; PHO, nanohybrid composite resin; TEL, computer-aided design and computeraided manufactured from cross-linked PMMA; ZCAD, computer-aided design and computer-aided manufactured acrylate polymer.
The 200 000 cycles applied in this study represent approximately 9.6 months of clinical use.25 These types of correlations of wear cycles and clinical service are based on correlating the volumetric loss with a reference restorative material in a laboratory device with clinical wear measurements. Therefore, this estimation is likely flawed. A more useful observation is that the PHO teeth had 3 to 4 times less wear than all other materials. Assuming a linear wear rate, these teeth would last 3 to 4 times longer than the others. An increased wear rate of denture teeth represents a clinical and financial disadvantage, leading to an earlier remake. As denture teeth wear more progressively, the patient will lose vertical dimension of occlusion (VDO). This loss of VDO may lead to mechanical failures such as the fracture of anterior denture teeth, the prosthetic substructures, or functional and esthetic alterations. Limitations of the present study include that only 4 different resin materials were tested from the great range of available materials. Furthermore, the creation of a flat surface on the anterior teeth tested removed part of the outside enamel layer of the denture teeth in groups PHO and DCL and may have altered their Esquivel et al
abrasion wear resistance. This procedure could not be avoided, however, because of the configuration of the wear testing device. In general, in vitro studies can only assess limited parameters and may not be able to fully represent clinical reality and conditions. Some of these parameters may include the effects of temperature and pH cycling on the wear behavior of the material. However, limiting influencing parameters allows for a focused assessment of the materials and research question at hand. Further studies are suggested to better understand the behavior of materials exposed to function and abrasion wear. Such future studies should incorporate simulated aging parameters such as temperature and pH cycling. CONCLUSIONS Based on the findings of this in vitro study, the following conclusions were drawn: 1. Denture teeth made from different materials demonstrated significantly different volumetric substance loss when subjected to occlusal wear by zirconia antagonists. THE JOURNAL OF PROSTHETIC DENTISTRY
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2. Nanohybrid composite resin denture teeth had the least amount of volumetric substance loss. REFERENCES 1. Brånemark PI, Adell R, Breine U, Hansson BO, Lindström J, Ohlsson A. Intra-osseous anchorage of dental prostheses. I. Experimental studies. Scand J Plast Reconstr Surg 1969;3:81-100. 2. Zarb GA, Zarb FL. Tissue integrated dental prostheses. Quintessence Int 1985;16:39-42. 3. Sadowsky SJ. Treatment considerations for maxillary implant overdentures: a systematic review. J Prosthet Dent 2007;97:340-8. 4. Box VH, Sukotjo C, Knoernschild KL, Campbell SD, Afshari FS. Patientreported and clinical outcomes of implant-supported fixed complete dental prostheses: a comparison of metal-acrylic, milled zirconia, and retrievable crown prostheses. J Oral Implantol 2018;44:51-61. 5. Bidra AS, Tischler M, Patch C. Survival of 2039 complete arch fixed implantsupported zirconia prostheses: a retrospective study. J Prosthet Dent 2018;119:220-4. 6. Park JH, Park S, Lee K, Yun KD, Lim HP. Antagonist wear of three CAD/ CAM anatomic contour zirconia ceramics. J Prosthet Dent 2014;111:20-9. 7. Janyavula S, Lawson N, Cakir D, Beck P, Ramp LC, Burgess JO. The wear of polished and glazed zirconia against enamel. J Prosthet Dent 2013;109:22-9. 8. Maló P, Araújo Nobre MD, Lopes A, Rodrigues R. Double full-arch versus single full-arch, four implant-supported rehabilitations: a retrospective, 5-year cohort study. J Prosthodont 2015;24:263-70. 9. Gonzalez J, Triplett RG. Complications and clinical considerations of the implant-retained zirconia complete-arch prosthesis with various opposing dentitions. Int J Oral Maxillofac Implants 2017;32:864-9. 10. Cardelli P, Manobianco FP, Serafini N, Murmura G, Beuer F. Full-arch, implant-supported monolithic zirconia rehabilitations: pilot clinical evaluation of wear against natural or composite teeth. J Prosthodont 2016;25: 629-33. 11. Preis V, Behr M, Handel G, Schneider-Feyrer S, Hahnel S, Rosentritt M. Wear performance of dental ceramics after grinding and polishing treatments. J Mech Behav Biomed Mater 2012;10:13-22. 12. Passos SP, Torrealba Y, Major P, Linke B, Flores-Mir C, Nychka JA. In vitro wear behavior of zirconia opposing enamel: a systematic review. J Prosthodont 2014;23:593-601. 13. Gou M, Chen H, Kang J, Wang H. Antagonist enamel wear of toothsupported monolithic zirconia posterior crowns in vivo: a systematic review. J Prosthet Dent 2019;121:598-603. 14. Pereira GKR, Dutra DM, Werner A, Prochnow C, Valandro LF, Kleverlaan CJ. Effect of zirconia polycrystal and stainless steel on the wear of resin composites, dentin and enamel. J Mech Behav Biomed Mater 2019;91:287-93. 15. Ludovichetti FS, Trindade FZ, Werner A, Kleverlaan CJ, Fonseca RG. Wear resistance and abrasiveness of CAD-CAM monolithic materials. J Prosthet Dent 2018;120:318.e1-8. 16. Munshi N, Rosenblum M, Jiang S, Flinton R. In vitro wear resistance of nanohybrid composite denture teeth. J Prosthodont 2017;26:224-9.
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