An in vitro study of the effect of different restorative materials on the reliability of a veneering porcelain Matthew R. Anderson, DMD, MSD,a Kwok-Hung Chung, DDS, MS, PhD,b Brian D. Flinn, MS, PhD,c and Ariel J. Raigrodski, DMD, MSd University of Washington, Seattle, Wash Statement of problem. Implant-supported, porcelain veneered restorations experience a greater rate of porcelain fracture than tooth-supported restorations. For completely edentulous patients, one approach to minimizing porcelain fracture is to use acrylic resin in the mandible, although its efficacy is unknown. Purpose. The purpose of this study was to evaluate the reliability of a veneering porcelain fatigued with different restorative materials in vitro. Material and methods. Fifty-nine veneering porcelain disk specimens were fabricated by layering veneering porcelain on nickel-chromium base metal alloy disks. Four groups of different indenter materials fatigued the porcelain specimens: group WC, tungsten carbide served as a control; group FC, pressed leucite glass ceramic; group NHC, nanohybrid composite resin denture tooth; and group AR, unfilled acrylic resin denture tooth. Porcelain specimens were randomly divided into 4 groups (n¼14). A step-stress accelerated life-testing model was used. Use-level probability Weibull plots were generated, and the reliability of each group was estimated for a theoretical completion of 50 000 cycles at 150 N. Results. Nanohybrid composite resin and unfilled acrylic resin denture tooth groups had higher reliability than tungsten carbide and leucite glass ceramic groups. No significant differences existed between the reliability of the tungsten carbide and leucite glass ceramic groups and the nanohybrid composite resin and acrylic resin denture tooth groups. Conclusions. Veneering porcelain disk specimens fatigued with the unfilled acrylic resin and nanohybrid composite resin denture tooth indenters exhibited higher reliability than the specimens fatigued with either the tungsten carbide or leucite glass ceramic indenters. All of the veneering porcelain disk specimens failed with the same mode of fracture, although the surface posttest exhibited different fracture characteristics among specimens fatigued with the 4 different materials. (J Prosthet Dent 2013;110:521-528)
Clinical Implications In completely edentulous patients with dual-arch implant-supported fixed dental prostheses, a different restorative material, such as acrylic resin denture teeth, in one arch should improve the fatigue resistance of a veneering porcelain in the opposing arch. Implant-supported fixed dental prostheses (FDP) are a popular treatment alternative for completely edentulous patients.1,2 These fixed restorations are
frequently fabricated with either a metal or ceramic infrastructure. Similar to complete mouth rehabilitations with tooth-supported FDPs, one approach to
treating these patients is to veneer the framework of both arches with porcelain,1,3 even though esthetic dental porcelain is susceptible to premature failure
Supported by a Stanley D. Tylman Research Grant from the American Academy of Fixed Prosthodontics. First place, 2012 Tylman Award Program, American Academy of Fixed Prosthodontics. a
Graduate student, Graduate Prosthodontics, Department of Restorative Dentistry, School of Dentistry, University of Washington, Seattle, Wash. b Professor, Department of Restorative Dentistry, University of Washington School of Dentistry, Seattle, Wash. c Research Associate Professor, Materials Science and Engineering, University of Washington School of Engineering, Seattle, Wash. d Professor, Graduate Prosthodontics, Department of Restorative Dentistry, University of Washington School of Dentistry, Seattle, Wash.
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Volume 110 Issue 6 with repeated loading in moist conditions.4 Moreover, recent studies have demonstrated a significant increase in the incidence of porcelain fracture in implant-supported, porcelain veneered FDPs compared with porcelain veneered tooth-supported FDPs.5-8 The increased incidence of veneering porcelain fracture can be attributed to several factors, including the occlusal scheme used in the restorations, occlusal interferences, and the parafunctional habits of a patient.5 In addition, the lack of a periodontal ligament may be a significant contributing factor in the higher incidence of veneering porcelain fracture.9,10 The periodontal ligament provides proprioception and helps transmit sensory feedback about jaw position during function; its absence has been demonstrated to result in increased mandibular muscle discoordination and decreased tactile sensitivity in edentulous patients.11-13 It also permits a degree of tooth movement, from 25 mm to 150 mm in periodontally healthy teeth and dampens forces that act on teeth and tooth-supported restorations compared with implants that lack mobility.14,15 By comparison, osseointegrated implants typically only move up to 10 mm.16,17 Owing to mandibular discoordination, off-axis loading and excursive contacts are to be expected at a higher rate, and, with the absence of a force-absorbing function with osseointegrated implants, these forces combined may influence the rate of veneering porcelain fracture for implant-supported crowns versus similar tooth-supported restorations.9 Kinsel and Lin18 conducted a retrospective analysis of implant-supported metal ceramic (ISMC) crowns opposing ISMC, tooth-supported metal ceramic, and natural teeth.18 Both patient-specific factors and implant-specific factors influenced porcelain fractures in this study. When patient-specific factors were controlled, the researchers concluded that an ISMC restoration opposing another ISMC restoration increased the odds of porcelain fracture by 7 compared with an ISMC restoration opposing either
a tooth-supported metal ceramic restoration or natural teeth and that 79% of major porcelain fractures within the study occurred in the ISMC-ISMC group.18 The rate and types of complications associated with veneered porcelain may be amplified in complete mouth implantsupported rehabilitations. Intraoral repair of porcelain fractures have only limited success, and problems of color matching and the bonding of repair material may persist.19-21 An ongoing need for porcelain repair requires frequent maintenance and additional cost. Different materials or a combination of them, such as veneering porcelain opposing a more resilient material, may address this clinical challenge with ISMC/ISMC rehabilitations.18 The use of acrylic resin denture teeth with implant-supported restorations was originally advocated by Brånemark22 to mitigate occlusal force transmission to implant parts and the bone-implant interface.23-25 Although acrylic resins have been shown to reduce force transmission in vitro when compared with dental porcelain and gold,26 these studies did not examine the role of a material in the longevity of a prosthesis. The type of restorative material has not been shown to influence implant survival.27 To minimize porcelain fractures, one treatment option is to veneer the maxillary arch infrastructure with porcelain and the mandibular arch with acrylic resin denture teeth.28 The complications associated with using acrylic resin in fixed prosthodontics, for example wear, are well documented.29-32 However, if its use in conjunction with porcelain leads to a substantial minimization of porcelain fracture, then maintaining one arch as an implant-supported metal acrylic resin prosthesis may be advantageous. Porcelain fatigue studies commonly use indenters such as stainless steel or tungsten carbide (WC) with a high elastic modulus.33,34 Applying results from such studies to clinical situations in which porcelain may be interacting with antagonists of different materials is difficult. In addition to the size of the
The Journal of Prosthetic Dentistry
contact area, the indenter material can affect the apparent strength of brittle materials.35,36 Steel-ball indenters as opposed to glass-ball indenters have been shown to lead to a statistically significant increase in fracture strength in glass slabs. This increase is attributed to the differences in interfacial friction, stress distribution, and elastic constants of the 2 materials.37 The purpose of this study was to evaluate the reliability of a veneering porcelain on metal with different restorative materials in an in vitro environment that simulated the rigidity of osseointegrated implants. The first null hypothesis was that the reliability of the veneering porcelain would not be affected when fatigued by different restorative materials. The second null hypothesis was that no differences would be observed in the mode of failure or in the surface and fracture characteristics of the veneering porcelain after testing.
MATERIAL AND METHODS Veneering porcelain disk specimen preparation Fifty-nine metal disk-shaped specimens (101.5 mm) were fabricated with autopolymerized acrylic resin (Pattern Resin LS; GC America Inc) by placing the acrylic resin into a custom, milled aluminum mold. After 30 minutes, the disks were removed from the mold and invested with a carbon-free phosphate investment (CeraFina; Whip Mix Corp). After the investment had set for a minimum of 90 minutes, it was placed in a burnout furnace at 850 C. A nickel-chromium base metal alloy (Rexillium V; Jelenko) was used to cast the metal disks with a nonvacuum centrifugal casting machine (Centrifico; Kerr Dental Laboratory Products). After devesting and sprue removal, the disks were airborneparticle abraded with 50-mm aluminum oxide particles (Kavas) for 10 seconds at 0.2 MPa pressure and at an approximate distance of 10 mm. The disks were cleaned ultrasonically
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Indenter material preparation Four different materials served as indenters: (1) group WC, 6.35-mmdiameter WC indenters served as a control; this diameter provided a uniform 0.2 mm2 contact area (r¼0.25 mm) as verified with articulating paper; (2) group FC, pressed leucite glass ceramic (FC) (IPS InLine PoM; IvoclarVivadent AG); (3) group NHC, nanohybrid composite resin denture teeth (NHC) (SR Phonares Lingual NHC; Ivoclar-Vivadent AG), which contained inorganic filler; and (4) group AR: unfilled acrylic resin denture teeth (AR) (33 Classic; Dentsply Intl). Each group contained 14 antagonist specimens, except for group WC, which contained only 4 because the material properties permitted multiple uses. A light-body polyvinyl siloxane (Imprint 3; 3M ESPE) impression was made of a WC indenter. Autopolymerizing acrylic resin (Pattern Resin LS; GC America Inc.) was applied into the polyvinyl siloxane mold to a depth of 1.5 mm. With the
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400
300
Newtons
for 30 minutes in distilled water, steam cleaned, dried, and degassed at 980 C in a porcelain furnace (Austromat 3001; DEKEMA Service GmbH). A leucite glass veneering porcelain (EX3; Noritake) was hand layered onto the metal disks with a custom-milled aluminum mold. Gauze and light finger pressure was used to condense the powder. The porcelain was applied in 2 steps. After the first layer was condensed and fired in a porcelain furnace at 920 C under vacuum (Austromat 3001; DEKEMA Service GmbH), the specimens were allowed to cool to room temperature. A second layer of porcelain was applied by following the same procedure. Each specimen was ground by using sintered diamond rotary instruments (Fine grit laboratory diamond rotary cutting instruments; Brasseler USA) to a porcelain thickness of 1.5 mm, followed by hand polishing with successive polishing instruments (Dialite; Brasseler USA) and a single glaze firing cycle at 920 C, although no glaze material was applied.
523
Light Moderate Aggressive
200
100
0
20 000
40 000
60 000
80 000 ~120 000
Cycles 1 Step-stress profiles. same protocol and materials for metal disk fabrication, metal posts that measured 35 mm were made. Once polymerized, the acrylic resin pattern was removed and attached with wax to the top of a metal post. Wax sprues were attached to the sides of the resin patterns. Care was taken not to alter the tip of the resin patterns. Once the sprues were attached, the patterns were invested with phosphate-bonded investment (CeraFina; Whip Mix Corp) and allowed to set for 90 minutes. Once set, the investments were placed in an oven for 40 minutes, and the acrylic resin was eliminated at 850 C. FC (IPS InLine PoM; Ivoclar-Vivadent AG) was pressed into the mold according to manufacturer’s instructions. Once cooled, the FC specimens were devested and polished with the same protocol as the veneering porcelain disk specimens. For the denture teeth groups, the palatal cusps of the maxillary second premolars of the NHC and AR denture teeth were sectioned and placed into specimen holders. The bases contacted metal, and autopolymerizing acrylic resin (Pattern Resin LS; GC America) was added between the holder and the sectioned cusps. The FC indenters were assembled in the same manner. Each specimen’s contact area was adjusted by using a diamond rotary cutting instrument (Brasseler USA) to an area of 0.2 mm2 and was measured with digital calipers (Digital precision calipers
CP8806-T; Carrera Precision). Only the borders of the contact area were adjusted to preserve the as-received surface from the manufacturers. A step-stress accelerated life-testing model was used for this study, which permitted a product’s reliability to be estimated from a relatively small sample size with reduced testing time.38,39 Data were gathered across different fatigue profiles, light, moderate, and aggressive, which were used in a ratio of 4:2:1 (Fig. 1).38 This ratio has been shown to result in data that correspond well to a material’s actual service life.40 The minimum number of specimens per group (n¼14) was determined by doubling the ratio.41,42 Specimens were numbered by using software (Research Randomizer v4.0; Geoffrey C Urbaniak and Scott Plous)43 and were randomly divided into 4 groups that corresponded to the 4 categories of antagonist materials. With the exception of the WC group, 1 indenter was used against 1 veneering porcelain disk specimen. To determine the baseline load for testing, 3 porcelain specimens were subjected to a single load to failure test with the same WC indenters used in the study. The 3 values were averaged, and 30% of the mean load to failure was used as the baseline to begin testing.38 The maximum load applied was 400 N, which was the limit of the testing apparatus. Each veneering porcelain disk specimen was subjected to sinusoidal, cyclic
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RESULTS The single load to failure mean (standard deviation) was 222 32 N. These data were used to develop stepstress profiles with a load range from 75 N to 400 N and up to 120 000 cycles. All the specimens were cyclic loaded and experienced fracture. The elastic modulus (E) values are listed in Table I. The use-level probability Weibull plot was generated at a stress of 150 N with 90% CB (Fig. 2) and revealed no significant differences among the groups because the CB overlapped at
Table I.
Elastic modulus value (E) for materials tested
Pressed Nanohybrid Unfilled Acrylic Tungsten Leucite Glass Composite Resin Resin Denture Material Carbide Ceramic Denture Tooth Tooth Elastic modulus value
530 GPa
67 GPa
2.5 GPa
1 GPa
99
Unreliability
loading at 1.5 Hz in a 37 C water bath in a coil cycler electromechanical fatigue machine (Fatigue Tester; ProtoTech) until failure. The loading cycle consisted of the indenter making contact with the specimen, lifting 0.25 mm off its surface, and then contacting the surface again at the desired load. The maximum load for all profiles was 400 N. The specimens were visually inspected every 5000 cycles with a light stereomicroscope (SMZ 1500; Nikon Corp) with 112.5 magnification. Failure was defined as the presence of radial cracks with either inner or outer cone cracks. Photographs representative of the posttest were made of the surfaces of porcelain specimens and the contacting areas of the indenters. The load and the number of cycles at which failures occurred were recorded, and the data were used to generate uselevel probability Weibull plots with 2sided 90% confidence bounds (CB).39 The reliability of each group was estimated for a theoretical completion of 50 000 cycles at 150 N (Alta Pro 8; Reliasoft). The modulus of elasticity for the 2 denture tooth materials was measured in compression on rectangular specimens (224.5 mm) machined from the denture teeth with a universal testing machine (Instron 5500R; Instron) at a crosshead speed of 0.01 mm/min. The values of the elastic modulus of the WC and FC materials were determined from previous studies.44,45
WC FC NHC AR
10
1 100
1000
10 000
100 000
1000 000 1E+07
Cycles 2 Use-level probability Weibull plots at 150 N with 2-sided 90% confidence bounds. Veneering porcelain disk specimens fatigued with denture tooth indenters, nanohybrid composite resin denture teeth (NHC), and unfilled acrylic resin denture teeth (AR) exhibited significantly higher reliability than those fatigued with either tungsten carbide (WC) or pressed leucite glass ceramic (FC) indenters. No statistical significance existed between WC and FC groups and NHC and AR groups. lower stresses and cycles. However, at higher stress loads and cycles, the CB of groups WC and FC did not overlap with groups NHC and AR, which indicated statistically significant differences among the groups. No significant differences existed in reliability between the paired groups WC and FC and the groups NHC and AR. The slope of the Weibull plot, called the shape parameter, is denoted by a value, b. This value describes the failure rate of the specimens over time. Values 1 indicate failure rates that are the result of cumulative stress and damage.46 All 4 groups had b values >1, which indicated that the damage to the porcelain disks was cumulative (Table II). The calculated reliability for a theoretical mission of 50 000 cycles at 150 N with 2-sided 90% CB of the porcelain specimens fatigued with the different indenters is listed in Table II. All of the WC and nearly all of the FC fatigued specimens showed failures, whereas approximately 92% of the NHC group and nearly 81% of the AR group were free of complications or failure at the end of a theoretical load of 150 N for 50 000 cycles.
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Results of calculated 90% confidence bounded intervals for 50 000 cycles at 150 N for tested groups
Table II.
Material
Nanohybrid Composite Unfilled Acrylic Pressed Tungsten Leucite Glass Resin Denture Resin Denture Tooth Tooth Carbide Ceramic
Reliability upper confidence bound
0.253
Reliability lower 1.065 E-10 confidence bound
0.007
0.989
0.951
7.004 E-26
0.626
0.626
Reliability
0.001
0.004
0.813
0.813
b value
2.58
3.98
2.63
1.66
Representative veneering porcelain disk specimens were examined after fatigue testing and revealed failure by inner and outer cone cracks for all groups (Fig. 3). Differences were noted in the
fracture characteristics among specimens fatigued with the 4 different materials. Pronounced circular zones of hackle were noted in the porcelain specimens fatigued with FC indenters
(Fig. 3B). Those specimens fatigued with NHC indenters had a greater amount of surface spar, or flakes, than the other groups and exhibited surface radial cracks that formed perpendicular to the contact area (Fig. 3C). The indenter posttest contacting surfaces were also inspected, and differences were observed for each of the 4 groups (Fig. 4). The WC indenters (Fig. 4A) did not change discernibly, whereas the FC indenters (Fig. 4B) exhibited slight scratches along their surfaces. The NHC indenters exhibited small chips and minor wear with cracks that radiate from the contacting surface through the body of the material (Fig. 4C). Within this study, the AR indenters exhibited the greatest amount of wear, and their contacting surfaces were characterized as being smooth and covered with multiple small indentations with no chipping (Fig. 4D).
3 Posttest photographs of representative porcelain disk specimens at 112.5 magnification. A, Tungsten carbide fatigued specimen, exhibiting inner and outer cone cracks. B, Pressed leucite glass ceramic fatigued specimen; arrow points to pronounced circular hackle zone. C, Nanohybrid composite resin denture teeth fatigued specimen; upper arrow points to spar, or flaking, whereas lower arrow points to radial crack. D, Unfilled acrylic resin denture teeth fatigued specimen.
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4 Posttest photographs of representative indenters at 112.5 magnification. A, Tungsten carbide. B, Pressed leucite glass ceramic. C, Nanohybrid composite resin denture teeth; chip is evident in upper left, and cracks are radiating from contact surface. D, Unfilled acrylic resin denture teeth; small indentations are evident across smooth surface.
DISCUSSION This study evaluated the effect of different indenter materials on the axial loading fatigue of layered veneering porcelain on metal disks in a wet environment. The different indenter materials led to significant differences in the reliability of the veneering porcelain disk specimens between those fatigued with the WC and FC groups and with the NHC and AR groups. Therefore, the first null hypothesis was rejected. All specimens failed because of inner and outer cone cracks, although different fracture characteristics and surface appearance also were noted between these 2 sets of groups. Therefore, the second null hypothesis was partially rejected. Any changes to the indenters themselves after cyclic loading were not quantified in this study. However, posttest visual inspection of the 4
types of indenters did reveal physical changes. No material loss, cracks, or fractures were seen in the WC and FC indenters. The NHC indenters exhibited mild wear on the contacting surface and cracks that originated from the contact point and radiated out through the body of the material were observed after the test. None of the NHC indenters experienced a chip or gross loss of material during the test. The AR indenters did not exhibit any cracks, although this group exhibited noticeable loss of material. The AR indenters that exhibited the largest volume loss of material after testing had contact areas approximately 3 times their original size. The process of fretting, which is a special wear process in which 2 bodies in repeated load are subjected to minute movements and vibrations and which leads to material loss, may account for the increase in contact areas in both
The Journal of Prosthetic Dentistry
the NHC and AR indenters. The axial loading of the specimens did not have a horizontal, or sliding, component, although it is reasonable to assume that a degree of slight movement of the piston occurred during loading. The indenter material loss witnessed was consistent with previous studies that documented the wear of NHC and AR against porcelain surfaces.31,32 Within this study, the veneering porcelain disk specimens fatigued with the 2 denture tooth indenters exhibited the highest reliability. Noticeable differences existed between both the fracture characteristics of the porcelain disk specimens fatigued with the 2 kinds of indenters and the indenters themselves. The amount of flaking seen on the surface of the NHC-fatigued specimens and the presence of cracks running through the body of the NHC indenters may be of concern. If these characteristic findings are an indication of how these 2
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December 2013 materials affect one another when veneered on implant-supported FDPs in an in vivo scenario, then the use of NHC materials may not decrease the incidence of porcelain-associated complications. The inorganic fillers typically used in NHC materials may decrease the fracture resistance of porcelain. For these reasons, using an unfilled acrylic resin may be more advantageous despite the absence of statistical significance in reliability between the 2 denture tooth groups and despite both having similar elastic moduli. The limitations of this study include the method and design of the protocol for fatiguing the veneering porcelain disk specimens. Vertical loading perpendicular to a flat test surface in which contact, load, and lift-off are cyclically performed approximates intraoral contacts in vitro, although it does not simulate the range of forces that can be generated by a patient during function and parafunction. In addition, flat specimens do not replicate the complex geometry of anatomically contoured restorations. Another limitation is the contact area. Although it is likely that contact area surfaces would be adjusted in vivo, the NHC and AR denture teeth with the as-purchased surface texture were tested. Only the periphery of the contact area was modified. Therefore, uniform contact geometry was not standardized across all groups. Although this approach may have influenced some findings, it was done to avoid the inclusion of additional design variables that may result from altering the contacting surface texture and consistency. Presently, few studies have examined the complication rates of implantsupported FDPs in edentulous patients.8,29,30 The authors are not aware of a study that investigates the prosthodontic complications of metal ceramic opposing metal acrylic resin implantsupported FDPs in vivo. As more edentulous patients seek implants to facilitate their oral rehabilitation, clinical studies that examine the influence of various restorative materials on complications and outcomes are indicated. In the interim, research on this topic should encompass the influence of specific
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527 adjustment and the polishing protocols of denture teeth on the fracture resistance of porcelain. Combinations of different ceramics and other common restorative materials such as pressable glass ceramics and highly cross-linked acrylic resins and other popular restorative materials such as those found in ceramic restorations are also indicated. Currently, this study indicates that using a different restorative material in one arch to improve the fatigue resistance of the veneered porcelain in another may have merit. If such an approach is taken, then a material with greater wear properties is more desirable than materials that are either brittle or have greater wear resistance.
CONCLUSIONS Within the limits of this in vitro study, the following conclusions may be drawn: 1. Veneering porcelain disk specimens fatigued with the AR and NHC denture tooth indenters exhibited higher reliability than the specimens fatigued with either the WC or FC indenters. 2. All of the veneering porcelain disk specimens failed with the same mode of fracture, although the posttest surfaces exhibited different fracture characteristics among specimens fatigued with the 4 different materials.
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6. Pjetursson BE, Tan K, Lang NP, Brägger U, Egger M, Zwahlen M. A systematic review of the survival and complication rates of fixed partial dentures (FPDs) after an observation period of at least 5 years. Clin Oral Implants Res 2004;15:625-42. 7. Pjetursson BE, Brägger U, Lang N, Zwahlen M. Comparison of survival and complication rates of tooth-supported fixed dental prostheses (FDPs) and implantsupported FDPs and single crowns (SCs). Clin Oral Implants Res 2007;18:97-113. 8. Bozini T, Petridis H, Tzanas K, Garefis P. A meta-analysis of prosthodontics complication rates of implant-supported fixed dental prostheses in edentulous patients after an observation period of at least 5 years. Int J Oral Maxillofac Implants 2011;26:304-18. 9. Brägger U, Aeschlimann S, Bürgin W, Hämmerle CH, Lang NP. Biological and technical complications and failures with fixed partial dentures (FPD) on implants and teeth after four to five years of function. Clin Oral Implants Res 2001;12:26-34. 10. El-Sheikh AM, Hobkirk JA, Howell PG, Gilthorpe MS. Passive sensibility in edentulous subjects treated with dental implants: a pilot study. J Prosthet Dent 2004;91:26-32. 11. Guichet NF. Biological laws governing functions of muscles that move the mandible: part I. Occlusal programming. J Prosthet Dent 1977;37:648-56. 12. Hämmerle CH, Brägger U, Lussi A, Karayiannis A, Joss A, Lang NP. Threshold of tactile sensitivity perceived with dental endosseous implants and natural teeth. Clin Oral Implant Res 1995;6:83-90. 13. Tartaglia GM, Testori T, Pallavera A, Marelli B, Sforza C. Electromyographic analysis of masticatory and neck muscles in subjects with natural dentition, teeth-supported and implant-supported prostheses. Clin Oral Implants Res 2008;19:1081-8. 14. Daly CH, Nicholls JI, Kydd WL, Nansen PD. The response of the human periodontal ligament to torsional loading e I. Experimental methods. J Biomech 1974;5:517-22. 15. Picton DC, Wills DJ. Viscoelastic properties of the periodontal ligament and mucous membrane. J Prosthet Dent 1978;40:263-72. 16. Sekine H, Komiyama Y, Hutta H, Yoshida K. Mobility characteristics and tactile sensitivity of osseointegrated mobility characteristics and tactile sensitivity of osseointegrated fixturesupported systems. In: van Steenberghe D, editor. Tissue-integration in oral and maxillofacial reconstruction. Amsterdam: Exerpta Medica; 1986. p. 326-32. 17. Nyman SR, Lang NP. Tooth mobility and the biological rationale for splinting teeth. Periodont 2000;1994;4:15-22. 18. Kinsel RP, Lin D. Retrospective analysis of porcelain failures of metal ceramic crowns and fixed partial dentures supported by 729 implants in 152 patients: patient-specific and implant specific predictors of ceramic failure. J Prosthet Dent 2009;101:388-94. 19. Walton JN, MacEntee MI. A retrospective study on the maintenance and repair of implant-supported prostheses. Int J Prosthodont 1993;6:451-5.
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Volume 110 Issue 6 20. Yanikoglu N. The repair methods for fractured metal-porcelain restorations: a review of the literature. Eur J Prosthodont Restor Dent 2004;12:161-5. 21. Kupiec KA, Wuertz KM, Barkmeier WW, Wilwerding TM. Evaluation of porcelain surface treatments and agents for composite-toporcelain repair. J Prosthet Dent 1996;76: 119-24. 22. Brånemark PI. Osseointegration and its experimental background. J Prosthet Dent 1983;50:399-410. 23. Davis DM, Rimrott R, Zarb GA. Studies on frameworks for osseointegrated prostheses: part 2. The effect of adding acrylic resin or porcelain to form the occlusal superstructure. Int J Oral Maxillofac Implants 1988;3: 275-80. 24. Cibirka RM, Razzoog ME, Lang BR, Stohler CS. Determining the force absorption quotient for restorative materials used in implant occlusal surfaces. J Prosthet Dent 1992;67:361-4. 25. Hobkirk JA, Psarros KJ. The influence of occlusal surface material on peak masticatory forces using osseointegrated implantsupported prostheses. Int J Oral Maxillofac Implants 1998;13:781-90. 26. Gracis SE, Nicholls JI, Chalupnik JD, Yuodelis RA. Shock-absorbing behavior of five restorative materials used on implants. Int J Prosthodont 1991;4: 282-91. 27. Brunski JB, Puleo DA, Nanci A. Biomaterials and biomechanics of oral and maxillofacial implants: current status and future developments. Int J Oral Maxillofac Implants 2000;15:15-46. 28. Balshi TJ, Wolfinger GJ. Teeth in a day for the maxilla and mandible: case report. Clin Implant Relat Dent Res 2003;5:11-6. 29. Purcell BA, McGlumphy EA, Holloway JA, Beck FM. Prosthetic complications in mandibular metal resin implant-fixed complete dental prosthesis: a 5 to 9 year analysis. Int J Oral Maxillofac Implants 2008;23:847-57.
30. Hemmings KW, Schmitt A, Zarb GA. Complications and maintenance requirements for fixed prostheses and overdentures in the edentulous mandible: a 5-year report. Int J Oral Maxillofac Implants 1994;9:191-6. 31. Ekfeldt A, Oilo G. Occlusal contact wear of prosthetic materials. An in vivo study. Acta Odontol Scand 1988;46:159-69. 32. Ghazal M, Yang B, Ludwig K, Kern M. Twobody wear of resin and ceramic denture teeth in comparison to human enamel. Dent Mater 2008;24:502-7. 33. Lawn BR, Deng Y, Thompson VP. Use of contact testing in the characterization and design of all-ceramic crownlike layer structures: a review. J Prosthet Dent 2001;86:495-510. 34. Kim B, Zhang Y, Pines M, Thompson VP. Fracture of porcelain-veneered structures in fatigue. J Dent Res 2007;86:142-6. 35. Baran GR, McCool JI, Boberick KG, Zhang HQ. Size effect in resin / glass composite flexural strengths. J Oral Rehabil 1999;26:775-80. 36. Green DJ. An introduction to the mechanical properties of ceramics. Cambridge: Cambridge University Press; 1998. p. 276-9. 37. Johnson KL, O’Conner JJ, Woodward AC. The effect of the indenter elasticity on the Hertzian fracture of brittle materials. Proc R Soc Lond A 1973;334:95-117. 38. Nelson W. Accelerated testing: statistical models, test plans, and data analysis. New York: Wiley; 1990. p. 493-520. 39. Zhao W, Elsayed EA. A general accelerated life model for step-stress testing. IEEE Trans Reliabil 2005;37:1059-69. 40. Meeker WQ, Hahn GJ. Asymptotically optimum over-stress tests to estimate the survival probability at a condition with a low expected failure probability. Technometrics 1977;19: 381-99. 41. Silva NR, de Souza GM, Coelho PG, Stappert CF, Clark EA, Rekow ED, et al. Effect of water storage time and composite cement thickness on fatigue of a glassceramic trilayer system. J Biomed Mater Res B Appl Biomater 2008;84:117-23.
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42. Coelho PG, Silva NR, Bonfante EA, Guess PC, Rekow ED, Thompson VP. Fatigue testing of two porcelain-zirconia allceramic crown systems. Dent Mater 2009;25:1122-7. 43. Urbaniak GC, Plous, S. Research Randomizer (v4.0). Available at: http://www.randomizer. org/. Accessed March 8, 2012. 44. Liu K, Li XP, Rahman M, Liu XD. CBN tool wear in ductile cutting of tungsten carbide. Wear 2003;225:1344-51. 45. Gonzaga C, Cesar P, Okada CY, Fredericci C, Neto FB, Yoshimura HN. Mechanical properties and porosity of dental glass-ceramic hot-pressed at different temperatures. Mater Res 2008;11:301-6. 46. Abernethy R. The new Weibull handbook. 5th ed. North Palm Beach: Robert B. Abernethy; 2006. p. 2-10.
Corresponding author: Dr Kwok-Hung Chung Department of Restorative Dentistry University of Washington 1959 NE Pacific Street, D-770 HSC Box 357456 Seattle, WA 98195-7456. E-mail: [email protected] Acknowledgments The authors thank Karen Verina for assistance in obtaining photographs of representative specimens and for measuring the modulus of elasticity of the test materials and Lloyd Mancl, PhD, for statistical consultation. Copyright ª 2013 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Clinical Implications In completely edentulous patients with dual-arch implant-supported fixed dental prostheses, a different restorative material, such as acrylic resin denture teeth, in one arch should improve the fatigue resistance of a veneering porcelain in the opposing arch. Implant-supported fixed dental prostheses (FDP) are a popular treatment alternative for completely edentulous patients.1,2 These fixed restorations are
frequently fabricated with either a metal or ceramic infrastructure. Similar to complete mouth rehabilitations with tooth-supported FDPs, one approach to
treating these patients is to veneer the framework of both arches with porcelain,1,3 even though esthetic dental porcelain is susceptible to premature failure
Supported by a Stanley D. Tylman Research Grant from the American Academy of Fixed Prosthodontics. First place, 2012 Tylman Award Program, American Academy of Fixed Prosthodontics. a
Graduate student, Graduate Prosthodontics, Department of Restorative Dentistry, School of Dentistry, University of Washington, Seattle, Wash. b Professor, Department of Restorative Dentistry, University of Washington School of Dentistry, Seattle, Wash. c Research Associate Professor, Materials Science and Engineering, University of Washington School of Engineering, Seattle, Wash. d Professor, Graduate Prosthodontics, Department of Restorative Dentistry, University of Washington School of Dentistry, Seattle, Wash.
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Volume 110 Issue 6 with repeated loading in moist conditions.4 Moreover, recent studies have demonstrated a significant increase in the incidence of porcelain fracture in implant-supported, porcelain veneered FDPs compared with porcelain veneered tooth-supported FDPs.5-8 The increased incidence of veneering porcelain fracture can be attributed to several factors, including the occlusal scheme used in the restorations, occlusal interferences, and the parafunctional habits of a patient.5 In addition, the lack of a periodontal ligament may be a significant contributing factor in the higher incidence of veneering porcelain fracture.9,10 The periodontal ligament provides proprioception and helps transmit sensory feedback about jaw position during function; its absence has been demonstrated to result in increased mandibular muscle discoordination and decreased tactile sensitivity in edentulous patients.11-13 It also permits a degree of tooth movement, from 25 mm to 150 mm in periodontally healthy teeth and dampens forces that act on teeth and tooth-supported restorations compared with implants that lack mobility.14,15 By comparison, osseointegrated implants typically only move up to 10 mm.16,17 Owing to mandibular discoordination, off-axis loading and excursive contacts are to be expected at a higher rate, and, with the absence of a force-absorbing function with osseointegrated implants, these forces combined may influence the rate of veneering porcelain fracture for implant-supported crowns versus similar tooth-supported restorations.9 Kinsel and Lin18 conducted a retrospective analysis of implant-supported metal ceramic (ISMC) crowns opposing ISMC, tooth-supported metal ceramic, and natural teeth.18 Both patient-specific factors and implant-specific factors influenced porcelain fractures in this study. When patient-specific factors were controlled, the researchers concluded that an ISMC restoration opposing another ISMC restoration increased the odds of porcelain fracture by 7 compared with an ISMC restoration opposing either
a tooth-supported metal ceramic restoration or natural teeth and that 79% of major porcelain fractures within the study occurred in the ISMC-ISMC group.18 The rate and types of complications associated with veneered porcelain may be amplified in complete mouth implantsupported rehabilitations. Intraoral repair of porcelain fractures have only limited success, and problems of color matching and the bonding of repair material may persist.19-21 An ongoing need for porcelain repair requires frequent maintenance and additional cost. Different materials or a combination of them, such as veneering porcelain opposing a more resilient material, may address this clinical challenge with ISMC/ISMC rehabilitations.18 The use of acrylic resin denture teeth with implant-supported restorations was originally advocated by Brånemark22 to mitigate occlusal force transmission to implant parts and the bone-implant interface.23-25 Although acrylic resins have been shown to reduce force transmission in vitro when compared with dental porcelain and gold,26 these studies did not examine the role of a material in the longevity of a prosthesis. The type of restorative material has not been shown to influence implant survival.27 To minimize porcelain fractures, one treatment option is to veneer the maxillary arch infrastructure with porcelain and the mandibular arch with acrylic resin denture teeth.28 The complications associated with using acrylic resin in fixed prosthodontics, for example wear, are well documented.29-32 However, if its use in conjunction with porcelain leads to a substantial minimization of porcelain fracture, then maintaining one arch as an implant-supported metal acrylic resin prosthesis may be advantageous. Porcelain fatigue studies commonly use indenters such as stainless steel or tungsten carbide (WC) with a high elastic modulus.33,34 Applying results from such studies to clinical situations in which porcelain may be interacting with antagonists of different materials is difficult. In addition to the size of the
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contact area, the indenter material can affect the apparent strength of brittle materials.35,36 Steel-ball indenters as opposed to glass-ball indenters have been shown to lead to a statistically significant increase in fracture strength in glass slabs. This increase is attributed to the differences in interfacial friction, stress distribution, and elastic constants of the 2 materials.37 The purpose of this study was to evaluate the reliability of a veneering porcelain on metal with different restorative materials in an in vitro environment that simulated the rigidity of osseointegrated implants. The first null hypothesis was that the reliability of the veneering porcelain would not be affected when fatigued by different restorative materials. The second null hypothesis was that no differences would be observed in the mode of failure or in the surface and fracture characteristics of the veneering porcelain after testing.
MATERIAL AND METHODS Veneering porcelain disk specimen preparation Fifty-nine metal disk-shaped specimens (101.5 mm) were fabricated with autopolymerized acrylic resin (Pattern Resin LS; GC America Inc) by placing the acrylic resin into a custom, milled aluminum mold. After 30 minutes, the disks were removed from the mold and invested with a carbon-free phosphate investment (CeraFina; Whip Mix Corp). After the investment had set for a minimum of 90 minutes, it was placed in a burnout furnace at 850 C. A nickel-chromium base metal alloy (Rexillium V; Jelenko) was used to cast the metal disks with a nonvacuum centrifugal casting machine (Centrifico; Kerr Dental Laboratory Products). After devesting and sprue removal, the disks were airborneparticle abraded with 50-mm aluminum oxide particles (Kavas) for 10 seconds at 0.2 MPa pressure and at an approximate distance of 10 mm. The disks were cleaned ultrasonically
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Indenter material preparation Four different materials served as indenters: (1) group WC, 6.35-mmdiameter WC indenters served as a control; this diameter provided a uniform 0.2 mm2 contact area (r¼0.25 mm) as verified with articulating paper; (2) group FC, pressed leucite glass ceramic (FC) (IPS InLine PoM; IvoclarVivadent AG); (3) group NHC, nanohybrid composite resin denture teeth (NHC) (SR Phonares Lingual NHC; Ivoclar-Vivadent AG), which contained inorganic filler; and (4) group AR: unfilled acrylic resin denture teeth (AR) (33 Classic; Dentsply Intl). Each group contained 14 antagonist specimens, except for group WC, which contained only 4 because the material properties permitted multiple uses. A light-body polyvinyl siloxane (Imprint 3; 3M ESPE) impression was made of a WC indenter. Autopolymerizing acrylic resin (Pattern Resin LS; GC America Inc.) was applied into the polyvinyl siloxane mold to a depth of 1.5 mm. With the
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400
300
Newtons
for 30 minutes in distilled water, steam cleaned, dried, and degassed at 980 C in a porcelain furnace (Austromat 3001; DEKEMA Service GmbH). A leucite glass veneering porcelain (EX3; Noritake) was hand layered onto the metal disks with a custom-milled aluminum mold. Gauze and light finger pressure was used to condense the powder. The porcelain was applied in 2 steps. After the first layer was condensed and fired in a porcelain furnace at 920 C under vacuum (Austromat 3001; DEKEMA Service GmbH), the specimens were allowed to cool to room temperature. A second layer of porcelain was applied by following the same procedure. Each specimen was ground by using sintered diamond rotary instruments (Fine grit laboratory diamond rotary cutting instruments; Brasseler USA) to a porcelain thickness of 1.5 mm, followed by hand polishing with successive polishing instruments (Dialite; Brasseler USA) and a single glaze firing cycle at 920 C, although no glaze material was applied.
523
Light Moderate Aggressive
200
100
0
20 000
40 000
60 000
80 000 ~120 000
Cycles 1 Step-stress profiles. same protocol and materials for metal disk fabrication, metal posts that measured 35 mm were made. Once polymerized, the acrylic resin pattern was removed and attached with wax to the top of a metal post. Wax sprues were attached to the sides of the resin patterns. Care was taken not to alter the tip of the resin patterns. Once the sprues were attached, the patterns were invested with phosphate-bonded investment (CeraFina; Whip Mix Corp) and allowed to set for 90 minutes. Once set, the investments were placed in an oven for 40 minutes, and the acrylic resin was eliminated at 850 C. FC (IPS InLine PoM; Ivoclar-Vivadent AG) was pressed into the mold according to manufacturer’s instructions. Once cooled, the FC specimens were devested and polished with the same protocol as the veneering porcelain disk specimens. For the denture teeth groups, the palatal cusps of the maxillary second premolars of the NHC and AR denture teeth were sectioned and placed into specimen holders. The bases contacted metal, and autopolymerizing acrylic resin (Pattern Resin LS; GC America) was added between the holder and the sectioned cusps. The FC indenters were assembled in the same manner. Each specimen’s contact area was adjusted by using a diamond rotary cutting instrument (Brasseler USA) to an area of 0.2 mm2 and was measured with digital calipers (Digital precision calipers
CP8806-T; Carrera Precision). Only the borders of the contact area were adjusted to preserve the as-received surface from the manufacturers. A step-stress accelerated life-testing model was used for this study, which permitted a product’s reliability to be estimated from a relatively small sample size with reduced testing time.38,39 Data were gathered across different fatigue profiles, light, moderate, and aggressive, which were used in a ratio of 4:2:1 (Fig. 1).38 This ratio has been shown to result in data that correspond well to a material’s actual service life.40 The minimum number of specimens per group (n¼14) was determined by doubling the ratio.41,42 Specimens were numbered by using software (Research Randomizer v4.0; Geoffrey C Urbaniak and Scott Plous)43 and were randomly divided into 4 groups that corresponded to the 4 categories of antagonist materials. With the exception of the WC group, 1 indenter was used against 1 veneering porcelain disk specimen. To determine the baseline load for testing, 3 porcelain specimens were subjected to a single load to failure test with the same WC indenters used in the study. The 3 values were averaged, and 30% of the mean load to failure was used as the baseline to begin testing.38 The maximum load applied was 400 N, which was the limit of the testing apparatus. Each veneering porcelain disk specimen was subjected to sinusoidal, cyclic
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RESULTS The single load to failure mean (standard deviation) was 222 32 N. These data were used to develop stepstress profiles with a load range from 75 N to 400 N and up to 120 000 cycles. All the specimens were cyclic loaded and experienced fracture. The elastic modulus (E) values are listed in Table I. The use-level probability Weibull plot was generated at a stress of 150 N with 90% CB (Fig. 2) and revealed no significant differences among the groups because the CB overlapped at
Table I.
Elastic modulus value (E) for materials tested
Pressed Nanohybrid Unfilled Acrylic Tungsten Leucite Glass Composite Resin Resin Denture Material Carbide Ceramic Denture Tooth Tooth Elastic modulus value
530 GPa
67 GPa
2.5 GPa
1 GPa
99
Unreliability
loading at 1.5 Hz in a 37 C water bath in a coil cycler electromechanical fatigue machine (Fatigue Tester; ProtoTech) until failure. The loading cycle consisted of the indenter making contact with the specimen, lifting 0.25 mm off its surface, and then contacting the surface again at the desired load. The maximum load for all profiles was 400 N. The specimens were visually inspected every 5000 cycles with a light stereomicroscope (SMZ 1500; Nikon Corp) with 112.5 magnification. Failure was defined as the presence of radial cracks with either inner or outer cone cracks. Photographs representative of the posttest were made of the surfaces of porcelain specimens and the contacting areas of the indenters. The load and the number of cycles at which failures occurred were recorded, and the data were used to generate uselevel probability Weibull plots with 2sided 90% confidence bounds (CB).39 The reliability of each group was estimated for a theoretical completion of 50 000 cycles at 150 N (Alta Pro 8; Reliasoft). The modulus of elasticity for the 2 denture tooth materials was measured in compression on rectangular specimens (224.5 mm) machined from the denture teeth with a universal testing machine (Instron 5500R; Instron) at a crosshead speed of 0.01 mm/min. The values of the elastic modulus of the WC and FC materials were determined from previous studies.44,45
WC FC NHC AR
10
1 100
1000
10 000
100 000
1000 000 1E+07
Cycles 2 Use-level probability Weibull plots at 150 N with 2-sided 90% confidence bounds. Veneering porcelain disk specimens fatigued with denture tooth indenters, nanohybrid composite resin denture teeth (NHC), and unfilled acrylic resin denture teeth (AR) exhibited significantly higher reliability than those fatigued with either tungsten carbide (WC) or pressed leucite glass ceramic (FC) indenters. No statistical significance existed between WC and FC groups and NHC and AR groups. lower stresses and cycles. However, at higher stress loads and cycles, the CB of groups WC and FC did not overlap with groups NHC and AR, which indicated statistically significant differences among the groups. No significant differences existed in reliability between the paired groups WC and FC and the groups NHC and AR. The slope of the Weibull plot, called the shape parameter, is denoted by a value, b. This value describes the failure rate of the specimens over time. Values 1 indicate failure rates that are the result of cumulative stress and damage.46 All 4 groups had b values >1, which indicated that the damage to the porcelain disks was cumulative (Table II). The calculated reliability for a theoretical mission of 50 000 cycles at 150 N with 2-sided 90% CB of the porcelain specimens fatigued with the different indenters is listed in Table II. All of the WC and nearly all of the FC fatigued specimens showed failures, whereas approximately 92% of the NHC group and nearly 81% of the AR group were free of complications or failure at the end of a theoretical load of 150 N for 50 000 cycles.
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Results of calculated 90% confidence bounded intervals for 50 000 cycles at 150 N for tested groups
Table II.
Material
Nanohybrid Composite Unfilled Acrylic Pressed Tungsten Leucite Glass Resin Denture Resin Denture Tooth Tooth Carbide Ceramic
Reliability upper confidence bound
0.253
Reliability lower 1.065 E-10 confidence bound
0.007
0.989
0.951
7.004 E-26
0.626
0.626
Reliability
0.001
0.004
0.813
0.813
b value
2.58
3.98
2.63
1.66
Representative veneering porcelain disk specimens were examined after fatigue testing and revealed failure by inner and outer cone cracks for all groups (Fig. 3). Differences were noted in the
fracture characteristics among specimens fatigued with the 4 different materials. Pronounced circular zones of hackle were noted in the porcelain specimens fatigued with FC indenters
(Fig. 3B). Those specimens fatigued with NHC indenters had a greater amount of surface spar, or flakes, than the other groups and exhibited surface radial cracks that formed perpendicular to the contact area (Fig. 3C). The indenter posttest contacting surfaces were also inspected, and differences were observed for each of the 4 groups (Fig. 4). The WC indenters (Fig. 4A) did not change discernibly, whereas the FC indenters (Fig. 4B) exhibited slight scratches along their surfaces. The NHC indenters exhibited small chips and minor wear with cracks that radiate from the contacting surface through the body of the material (Fig. 4C). Within this study, the AR indenters exhibited the greatest amount of wear, and their contacting surfaces were characterized as being smooth and covered with multiple small indentations with no chipping (Fig. 4D).
3 Posttest photographs of representative porcelain disk specimens at 112.5 magnification. A, Tungsten carbide fatigued specimen, exhibiting inner and outer cone cracks. B, Pressed leucite glass ceramic fatigued specimen; arrow points to pronounced circular hackle zone. C, Nanohybrid composite resin denture teeth fatigued specimen; upper arrow points to spar, or flaking, whereas lower arrow points to radial crack. D, Unfilled acrylic resin denture teeth fatigued specimen.
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4 Posttest photographs of representative indenters at 112.5 magnification. A, Tungsten carbide. B, Pressed leucite glass ceramic. C, Nanohybrid composite resin denture teeth; chip is evident in upper left, and cracks are radiating from contact surface. D, Unfilled acrylic resin denture teeth; small indentations are evident across smooth surface.
DISCUSSION This study evaluated the effect of different indenter materials on the axial loading fatigue of layered veneering porcelain on metal disks in a wet environment. The different indenter materials led to significant differences in the reliability of the veneering porcelain disk specimens between those fatigued with the WC and FC groups and with the NHC and AR groups. Therefore, the first null hypothesis was rejected. All specimens failed because of inner and outer cone cracks, although different fracture characteristics and surface appearance also were noted between these 2 sets of groups. Therefore, the second null hypothesis was partially rejected. Any changes to the indenters themselves after cyclic loading were not quantified in this study. However, posttest visual inspection of the 4
types of indenters did reveal physical changes. No material loss, cracks, or fractures were seen in the WC and FC indenters. The NHC indenters exhibited mild wear on the contacting surface and cracks that originated from the contact point and radiated out through the body of the material were observed after the test. None of the NHC indenters experienced a chip or gross loss of material during the test. The AR indenters did not exhibit any cracks, although this group exhibited noticeable loss of material. The AR indenters that exhibited the largest volume loss of material after testing had contact areas approximately 3 times their original size. The process of fretting, which is a special wear process in which 2 bodies in repeated load are subjected to minute movements and vibrations and which leads to material loss, may account for the increase in contact areas in both
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the NHC and AR indenters. The axial loading of the specimens did not have a horizontal, or sliding, component, although it is reasonable to assume that a degree of slight movement of the piston occurred during loading. The indenter material loss witnessed was consistent with previous studies that documented the wear of NHC and AR against porcelain surfaces.31,32 Within this study, the veneering porcelain disk specimens fatigued with the 2 denture tooth indenters exhibited the highest reliability. Noticeable differences existed between both the fracture characteristics of the porcelain disk specimens fatigued with the 2 kinds of indenters and the indenters themselves. The amount of flaking seen on the surface of the NHC-fatigued specimens and the presence of cracks running through the body of the NHC indenters may be of concern. If these characteristic findings are an indication of how these 2
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December 2013 materials affect one another when veneered on implant-supported FDPs in an in vivo scenario, then the use of NHC materials may not decrease the incidence of porcelain-associated complications. The inorganic fillers typically used in NHC materials may decrease the fracture resistance of porcelain. For these reasons, using an unfilled acrylic resin may be more advantageous despite the absence of statistical significance in reliability between the 2 denture tooth groups and despite both having similar elastic moduli. The limitations of this study include the method and design of the protocol for fatiguing the veneering porcelain disk specimens. Vertical loading perpendicular to a flat test surface in which contact, load, and lift-off are cyclically performed approximates intraoral contacts in vitro, although it does not simulate the range of forces that can be generated by a patient during function and parafunction. In addition, flat specimens do not replicate the complex geometry of anatomically contoured restorations. Another limitation is the contact area. Although it is likely that contact area surfaces would be adjusted in vivo, the NHC and AR denture teeth with the as-purchased surface texture were tested. Only the periphery of the contact area was modified. Therefore, uniform contact geometry was not standardized across all groups. Although this approach may have influenced some findings, it was done to avoid the inclusion of additional design variables that may result from altering the contacting surface texture and consistency. Presently, few studies have examined the complication rates of implantsupported FDPs in edentulous patients.8,29,30 The authors are not aware of a study that investigates the prosthodontic complications of metal ceramic opposing metal acrylic resin implantsupported FDPs in vivo. As more edentulous patients seek implants to facilitate their oral rehabilitation, clinical studies that examine the influence of various restorative materials on complications and outcomes are indicated. In the interim, research on this topic should encompass the influence of specific
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527 adjustment and the polishing protocols of denture teeth on the fracture resistance of porcelain. Combinations of different ceramics and other common restorative materials such as pressable glass ceramics and highly cross-linked acrylic resins and other popular restorative materials such as those found in ceramic restorations are also indicated. Currently, this study indicates that using a different restorative material in one arch to improve the fatigue resistance of the veneered porcelain in another may have merit. If such an approach is taken, then a material with greater wear properties is more desirable than materials that are either brittle or have greater wear resistance.
CONCLUSIONS Within the limits of this in vitro study, the following conclusions may be drawn: 1. Veneering porcelain disk specimens fatigued with the AR and NHC denture tooth indenters exhibited higher reliability than the specimens fatigued with either the WC or FC indenters. 2. All of the veneering porcelain disk specimens failed with the same mode of fracture, although the posttest surfaces exhibited different fracture characteristics among specimens fatigued with the 4 different materials.
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6. Pjetursson BE, Tan K, Lang NP, Brägger U, Egger M, Zwahlen M. A systematic review of the survival and complication rates of fixed partial dentures (FPDs) after an observation period of at least 5 years. Clin Oral Implants Res 2004;15:625-42. 7. Pjetursson BE, Brägger U, Lang N, Zwahlen M. Comparison of survival and complication rates of tooth-supported fixed dental prostheses (FDPs) and implantsupported FDPs and single crowns (SCs). Clin Oral Implants Res 2007;18:97-113. 8. Bozini T, Petridis H, Tzanas K, Garefis P. A meta-analysis of prosthodontics complication rates of implant-supported fixed dental prostheses in edentulous patients after an observation period of at least 5 years. Int J Oral Maxillofac Implants 2011;26:304-18. 9. Brägger U, Aeschlimann S, Bürgin W, Hämmerle CH, Lang NP. Biological and technical complications and failures with fixed partial dentures (FPD) on implants and teeth after four to five years of function. Clin Oral Implants Res 2001;12:26-34. 10. El-Sheikh AM, Hobkirk JA, Howell PG, Gilthorpe MS. Passive sensibility in edentulous subjects treated with dental implants: a pilot study. J Prosthet Dent 2004;91:26-32. 11. Guichet NF. Biological laws governing functions of muscles that move the mandible: part I. Occlusal programming. J Prosthet Dent 1977;37:648-56. 12. Hämmerle CH, Brägger U, Lussi A, Karayiannis A, Joss A, Lang NP. Threshold of tactile sensitivity perceived with dental endosseous implants and natural teeth. Clin Oral Implant Res 1995;6:83-90. 13. Tartaglia GM, Testori T, Pallavera A, Marelli B, Sforza C. Electromyographic analysis of masticatory and neck muscles in subjects with natural dentition, teeth-supported and implant-supported prostheses. Clin Oral Implants Res 2008;19:1081-8. 14. Daly CH, Nicholls JI, Kydd WL, Nansen PD. The response of the human periodontal ligament to torsional loading e I. Experimental methods. J Biomech 1974;5:517-22. 15. Picton DC, Wills DJ. Viscoelastic properties of the periodontal ligament and mucous membrane. J Prosthet Dent 1978;40:263-72. 16. Sekine H, Komiyama Y, Hutta H, Yoshida K. Mobility characteristics and tactile sensitivity of osseointegrated mobility characteristics and tactile sensitivity of osseointegrated fixturesupported systems. In: van Steenberghe D, editor. Tissue-integration in oral and maxillofacial reconstruction. Amsterdam: Exerpta Medica; 1986. p. 326-32. 17. Nyman SR, Lang NP. Tooth mobility and the biological rationale for splinting teeth. Periodont 2000;1994;4:15-22. 18. Kinsel RP, Lin D. Retrospective analysis of porcelain failures of metal ceramic crowns and fixed partial dentures supported by 729 implants in 152 patients: patient-specific and implant specific predictors of ceramic failure. J Prosthet Dent 2009;101:388-94. 19. Walton JN, MacEntee MI. A retrospective study on the maintenance and repair of implant-supported prostheses. Int J Prosthodont 1993;6:451-5.
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Corresponding author: Dr Kwok-Hung Chung Department of Restorative Dentistry University of Washington 1959 NE Pacific Street, D-770 HSC Box 357456 Seattle, WA 98195-7456. E-mail: [email protected] Acknowledgments The authors thank Karen Verina for assistance in obtaining photographs of representative specimens and for measuring the modulus of elasticity of the test materials and Lloyd Mancl, PhD, for statistical consultation. Copyright ª 2013 by the Editorial Council for The Journal of Prosthetic Dentistry.
Anderson et al
