Contact fatigue response of porcelain-veneered alumina model systems Christian F. J. Stappert,1,2,3 Marta Baldassarri,1,2 Yu Zhang,2 Dina Stappert,2,4 Van P. Thompson2 1
Department Department 3 Department 4 Department 2
of of of of
Periodontology and Implant Dentistry, New York University College of Dentistry, New York, New York Biomaterials and Biomimetics, New York University College of Dentistry, New York, New York Prosthodontics, Albert-Ludwigs-University, Freiburg, Germany Orthodontics, New York University College of Dentistry, New York, New York
Received 13 March 2011; revised 19 July 2011; accepted 28 August 2011 Published online 24 November 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.31977 Abstract: Fatigue damage modes and reliability of handveneered (HV) and over-pressed (OP) aluminum-oxide layer structures were compared. Influence of luting cement thickness on mechanical performance was investigated. Sixty-four aluminum-oxide plates (10 10 0.5 mm) were veneered with hand built-up or pressed porcelain (0.7 mm) and adhesively luted (50- or 150-lm cement thickness) to water-aged composite resin blocks (12 12 4 mm). Single-load-tofailure and fatigue tests were performed with a spherical tungsten carbide indenter (d ¼ 6.25 mm) applied in the center of the veneer layer. Specimens were inspected with polarized-reflected-light and scanning electron microscopy. Use-level probability Weibull curves were plotted with twosided 90% confidence bounds, and reliability at 75,000 cycles and 250 N load was calculated. For all specimens but two OP with 50-lm cement thickness, failure was characterized by flexural radial cracks initiating at the bottom surface of the alumina core and propagating into the veneering porcelain
before cone cracks could extend to the porcelain/alumina interface. HV specimens showed higher reliability compared to OP. Those with 50-lm cement thickness were more reliable relative to their 150-lm counterparts (HV_50 lm: 95% (0.99/0.67); HV_150 lm: 55% (0.92/0.01); OP_50 lm: 69% (0.84/0.48); OP_150 lm: 15% (0.53/0.004)). Similar failure modes were observed in HV and OP specimens. Radial cracks developing in the core and spreading into the veneer are suggested to cause bulk fracture, which is the characteristic failure mode for alumina core crowns. However, the highest resistance to fatigue loading was found for the HV specimens with thin cement thickness, while the lowest C 2011 Wiley occurred for the OP with thick cement layer. V Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 100B: 508–515, 2012. Key Words: ceramic, fatigue, aluminum-oxide, veneering porcelain, reliability, cement layer thickness, veneering method
How to cite this article: Stappert CFJ, Baldassarri M, Zhang Y, Stappert D, Thompson VP. 2012. Contact fatigue response of porcelain-veneered alumina model systems. J Biomed Mater Res Part B 2012:100B:508–515.
INTRODUCTION
As an alternative to metal-ceramic crowns, all-ceramic restorations offer better esthetics and biocompatibility.1 In 1991, Nobel Biocare (Gothenburg, Sweden) introduced Procera AllCeram crowns with a densely sintered alumina core.2 Flexural strength and fracture toughness of densely sintered aluminum-oxide are 680 MPa and 3–5 MPa m1/2, respectively.3–6 Clinical survival rates of aluminum-oxide crowns supported by natural teeth vary from 100% (4 years) to 93% (10 years).7–10 All-ceramic crowns are layer systems consisting of a veneer and a ceramic core.11 To understand the failure mechanics of these systems, flat layers like-crown structures have been mechanically tested in the past years.12 Under repeated contact loading in moisturized environment,
porcelain/alumina crown-like layers are still not as durable as porcelain/metal and porcelain/zirconia structures.11,13,14 Clinically, the most critical failure mode associated with alumina crowns is bulk fracture, for which radial cracks propagate through the core and reinitiate in the porcelain veneer.7,15,16 To understand the failure behavior of all-ceramic crown systems, four-layer structures with the outer layer (porcelain), the intermediate core (ceramic), adhesive interlayer (cement), and a substrate (tooth dentin) were under investigation. Characteristic failure modes of four-layers are cone cracks (outer and inner) in the porcelain veneer and radial cracks in the core. Some studies have used transparent materials, such as glass for the outer layer, to observe the evolution of cracks as a function of load and number of
Correspondence to: C. F. J. Stappert; e-mail:
[email protected] Contract grant sponsor: Nobel Biocare Contract grant sponsor: United States National Institute of Dental and Craniofacial Research; contract grant numbers: P01 DE016755, R01 DE017925 Contract grant sponsor: National Science Foundation; contract grant number: CMMI-0758530 Contract grant sponsor: NIH/NIDCR
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cycles; although inner cone cracks formed after the outer cones, they were able to reach the veneer/core interface, resulting in veneer chipping or delamination. On the other hand, flexural radial cracks forming at relatively high loads were responsible for catastrophic bulk fracture.17–23 Other studies have analyzed four-layers with the same materials of those used for all-ceramic crowns. However, most of those studies have focused on the veneer/zirconia/cement/ substrate systems, in which bulk fracture is not likely to occur due to the superior mechanical properties of zirconium-oxide.13,14 Previous investigations showed that for brittle glass layers, the critical loads for a radial crack to form are affected by the thickness of the adhesive interlayer.24 A study by Scherrer et al.25 showed a gradual decrease of the fracture strength of monolithic glass-ceramic tri-layers (glass-ceramic/cement/substrate) when the resin cement thickness was increased from thin (26 6 11 lm) to thick (297 6 48 lm). Yet, a significant difference was found only when cement thicknesses of at least 300 lm were taken into account. De Jager et al.26 showed that under static compression, thick layers (6.0 mm) of adhesive cement (Rely X ARC) were characterized by a significantly lower stiffness than thin cement layers (0.5 mm), probably providing less support for crown ceramics. The objective of this study was to investigate on the fracture mechanism of crown-like porcelain/alumina/ cement/substrate layers under simulated chewing. Failure modes were observed by monitoring cracks’ growth during fatigue testing.12 Both over-pressed (OP) and hand-veneered (HV) structures were investigated and their mechanical reliability was determined.27,28 The influence of thick (150 lm) versus thin (50 lm) cement layers on the mechanical behavior of the specimens was also investigated. This cement thickness range is the most relevant to clinical applications. MATERIALS AND METHODS
Specimens preparation Sixty-four four-layer structures were fabricated (Figure 1). Ceramic plates 10 10 0.7 mm in size were cut from alumina blocks (Procera AllCeram, Nobel Biocare), which were sintered at a temperature of 1600 C, according to manufacturer’s instructions. The alumina blocks were mounted in a cutting apparatus (Isomet 1000, Buehler, Germany) and ceramic plates of 0.7-mm thickness were produced with a water-cooled diamond wheel blade (11-4225, Buehler, Germany). To assure plates were flat, they were ground down to 10 10 0.5 mm, using 280– 600 grit diamond disks (Ecomet 4, Buehler, Lake Bluff, IL). A veneer layer of 1.5 mm was applied to the alumina plates either by over-pressing (OP: n ¼ 32) or hand-veneering (HV: n ¼ 32) (Ernst Hegenbarth, Zen Line Dental, Bruchkoebel, Germany; Nobel Rondo, Procera, Nobel Biocare), according to manufacturers’ recommendations. To obtain a flat porcelain surface, it was ground (600, 800, and 1200 grit SiC abrasive papers) down to a total thickness of 0.7 mm and polished with diamond suspensions (6- to 0.5-lm
FIGURE 1. Schematic of tested four-layer aluminum-oxide structures (porcelain veneer/alumina core/luting cement/substrate).
particle size; Ecomet 4, Buehler, Lake Bluff). The veneered aluminum-oxide plates were adhesively bonded with cement (Rely X ARC, 3M ESPE, St. Paul, MN) to 12 12 4 mm resin-based composite substrates (Z100, 3M ESPE), which were previously aged in water for at least 30 days to allow for hydroscopic expansion.21 Plastic sheet spacers were placed at the four corners of the squared specimens, between the substrate and the aluminum-oxide core, to assure for a cement thickness of 50 lm (HV: n ¼ 16; OP: n ¼ 16) and 150 lm (HV: n ¼ 16; OP: n ¼ 16), respectively. In this way, four groups of 16 specimens each were obtained and analyzed. Specimens were stored in distilled water at 37 C in a humidity chamber for 2 weeks prior to testing. Mechanical testing and failure modes analysis Mechanical failure was defined as porcelain cone cracks extending to the veneer/core interface and/or bottom radial cracks in the alumina core reaching the veneer/core interface.20 For each of the four groups, specimens were tested up to failure with a constantly increased load (1 mm/min) using an Instron machine (Canton, MA). Load was applied at the center of the specimens through a tungsten carbide spherical indenter with a 6.25-mm diameter.29 The mean failure load was then used to develop three step-stress accelerated life testing (SSALT) profiles with increasing loads and number of cycles.27,28,30 The applied load ranged from 50 to 600 N and the number of cycles varied from 20,000 to 155,000. For each of the four groups, specimens were distributed amongst the three profiles (mild:medium:aggressive)27 with a ratio of 8:4:2. A fatigue testing machine (ELF 3300, EnduraTEC Division of Bose, Minnetonka, MN) with the same tungsten carbide indenter used in the Instron machine was employed. Specimens were mounted into a metallic holder, so that they could not move during testing, and vertically loaded at the center of the veneer layer until failure occurred. At the end of each loading step, polarized-reflected-light microscopy (Model H-160, Edge Scientific Instrument Corporation, Marina del Ray, CA) was used to detect cracks initiation/ propagation. SSALT tests were performed at room temperature and specimens were kept in water throughout the entire testing period. For each layer structure, failure load
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Statistical analysis The reliability of each of the four groups was computed using a power law cumulative damage model and two parameters Weibull analysis at 250 N load for a mission of 75,000 cycles. An accelerated life testing software (Alta Pro 7, Reliasoft, Tuscon, AZ) using data (failure and suspension) from all three profiles was employed. Specimens failing by bottom radial cracks were accounted as failure, while specimens in which veneer cone cracks extended to the porcelain/core interface before a radial crack could occur were considered as suspensions. Load to cycle ratio of specimens failing during accelerated life testing due to machine error was accounted for in reliability calculations up to when the machine error occurred (suspension). RESULTS
FIGURE 2. Load to failure specimen demonstrated occlusal surface cone cracks (top) as well as cementation surface radial cracks (448 N; bottom).
and number of cycles were recorded. Specimens were embedded (epoxy resin, Epofix, Struers, Copenhagen, Denmark), cross-sectioned (Isomet 1000, Buehler, Germany) and polished (1-lm finish, Ecomet 4, Buehler, Lake Bluff), to observe failure modes and crack propagation patterns (fractographic features) with polarized-reflected-light microscopy (Leica MZ APO; Leica, Bensheim, Germany) and scanning electron microscopy (S-3500 N, Hitachi Instruments, San Jose, CA).
When cement thickness was measured, mean (6SD) values were 52 6 12 and 154 6 14 lm for the OP and 52 6 3 and 155 6 3 lm for the HV specimens. The mean single load to failure value was 459 6 46 N for specimens with 50-lm cement thickness and 413 6 21 N for those with 150-lm thickness. Single load to failure testing resulted in outer cone cracks extending to the porcelain/core interface as well as bottom radial cracks reaching the interface and propagating to the peripheral region of the loading area (n ¼ 8; Figure 2). In all specimens but two OP, SSALT failure was characterized by radial cracks initiating at the core-cement interface and propagating into the veneering layer [Figure 3(a,b)]. Cone cracks generated at lower loads/cycles (HV/profile I: 50–225 N/20,000–90,000 cycles; OP/profile I: 50–150 N/20,000–60,000 cycles) relative to radial cracks
FIGURE 3. Radial cracks determined failure by developing up to the porcelain before cone cracks could reach the alumina core. Cross-sections of two aluminum-oxide four-layer structures showed bottom radial cracks and inner cone cracks after SSALT (a) HV_115 K_375 N_40, (b) HV_120 K_400 N_40.
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FIGURE 4. Evolution of cracks: radial cracks generated at higher loads/cycles than cone cracks. (a) HV#9_130 K_425 N_40, (b) HV#9_140 K_475 N_80, (c) OP#15_90 K_250 N_80, and (d) OP#15_105 K_325 N_40.
[Figure 4(a–d)] and propagated steadily toward the veneer/ core interface, as evidenced by a series of expanding ring cracks that emanated from the indented area and extended into the porcelain. However, radial cracks, once initiating, quickly propagated up to the veneer before cone cracks could reach to the porcelain/alumina interface [Figures 3(a,b) and 4(b,d)]. Only in two OP specimens with cement thickness of 50 lm, cone cracks reached the veneer/core interface before radial cracks could develop into the porcelain [Figure 5(a,b)].
An example of the plotted Weibull distribution for HV and OP aluminum-oxide layer structures with 150-lm cement thickness is shown in Figure 6(a). As it can be seen, three OP specimens (150 lm) were accounted as suspensions, because a machine error occurred during testing.27 The 250 N load and 75,000 cycles reliability of HV specimens with cement thickness of 50 and 150 lm was 95 and 55%, respectively. The reliability of OP specimens was 69 and 15% for cement thickness of 50 and 150 lm, respectively. HV specimens showed a higher reliability relative to
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FIGURE 5. For two over-pressed layers, cone cracks reached the porcelain-alumina interface before radial cracks developed into the veneer. Cross-sections of two aluminum-oxide four-layer structures demonstrated inner cone cracks and initiated bottom radial cracks (a) OP_85 K_275 N_30 and (b) OP_125 K_400 N_30.
the OP [Figure 6(a)], as well as specimens with 50-lm cement thickness relative to those with 150 lm [Figure 6(b) and Table I]. A significantly different reliability was found between HV specimens of 50-lm cement thickness and OP specimens of 150 lm (nonoverlap of two-sided 90% confidence bounds; Table I). DISCUSSION
The present work investigated the fatigue fracture behavior and mechanical reliability of porcelain-alumina crown-like structures under simulated chewing. The influence of different veneering techniques (i.e., hand built-up and over-pressing) and resin cement thicknesses on the mechanical behavior of the specimens was investigated. The same fracture patterns were found in all groups, with failure ascribed to radial cracks initiating at the intaglio surface of the alumina
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core. However, HV systems were more resistant to bulk fracture relative to OP, as well as specimens with thin cement layer (50 lm) were more reliable than those with thick cement (150 lm). Cone cracks in the veneer initiated at lower loads/cycles than radial cracks in the core, as shown by previous in situ fatigue studies on alumina-glass layers.17–20,22,23 As both modes were competing, for all the specimens but two, radial cracks propagated to the porcelain layer before inner cone cracks could reach the veneer/core interface. Therefore, the mechanical failure of almost all the specimens was determined by radial fracture. For aluminum-oxide crowns, radial cracks developing in the core lead to catastrophic bulk fracture before chipping of the veneer can occur, contrarily to what is observed for zirconium-oxide restorations.31–33 When we performed a similar study on porcelain-zirconia crown-like structures, their mechanical failure was characterized by cone cracks reaching the veneer-core interface before radial cracks in the zirconium-oxide layer could develop.14 These findings explain the clinical situation, for which bulk fracture have been found to be the characteristic failure mode of alumina crowns contrarily to veneer chipping of zirconia crowns.7,10,34 HV specimens were more reliable than the OP, for which flexural radial cracks in the alumina layer occurred at lower load/cycles. Porcelain veneers typically are manufactured with a slightly lower coefficient of thermal expansion (CTE) relative to their corresponding core materials. In the case of OP alumina, porcelain veneer is pressed onto the alumina core with a single firing cycle. Upon cooling, residual compressive stresses are developed in the porcelain veneer and tensile stresses in the alumina core. For HV alumina, multiple layers are applied through a number of firing cycles. Refiring may alter the residual stresses due to the following reasons: increase in veneer thickness due to application of porcelain increments; slow cooling after refiring versus rapid cooling after the initial firing; and changing crystallinity, and hence CTE, of veneering material upon repeated exposures to elevated temperature. Of all these possible reasons for changes in residual stresses, the first two are likely to occur to a great degree.35–37 Crown-like systems with thin cement layer were more reliable relative to those with thick resin. Radial cracks initiating at the cementation surface of the alumina core have been associated with flexural-tensile stresses beneath the loading area.17,38 The elastic modulus of the resin cement (Rely X ARC; Ec ¼ 3.1 GPa)39 is much lower than that one of the supporting composite (Z100; Es ¼ 16–21 GPa),40,41 which is similar to that one of human dentin (Ed ¼ 18–21 GPa).42 Thus, the effective modulus of the cement/composite support is lower for thick cement layers than for thinner ones.39 The lower effective modulus associated with thicker cement layers offers less support to the flexing alumina core, leading to a lower critical load for the onset of radial cracks, relative to thinner layers.43 The cement thicknesses (50 and 150 lm) investigated in this study are clinically relevant.44,45 A previous study by Proos et al.,46 using Finite Element Analysis on In-Ceram alumina and gold coping crowns,
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FIGURE 6. Weibull-reliability calculations of SSALT data: Weibull probability graph of unreliability versus number of cycles (time) of handveneered (HV; n ¼ 28) versus over-pressed (OP; n ¼ 28) aluminum-oxide layer structures (load level 250 N) at two-sided 90% confidence bounds. (a) Comparison of hand-veneered and over-pressed specimens (150-lm cement thickness) and (b) Comparison of specimens with 150- and 50-lm cement thickness (hand-veneered).
showed that cement thickness had minor influence on the maximum stresses at the cementation surface of the coping. However, Proos et al.46 only compared cement thicknesses 50 lm with 100 lm. The differences between the two cases were rather small. Also, they did not take into account the effect of water absorption. In a moisturized environment,
such as the mouth, resin cements allow saliva absorption and consequently have the tendency to swell.47 In a previous work, the fatigue endurance of glass-veneered ceramic layers with thick cement layer have been shown to significantly decrease when the systems were stored in water from 24–48 h to 60 days.21 Water absorption is especially
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TABLE I. The Table Depicts the Calculated Reliability at 90% Confidence Intervals (CI) Using Standard Probability Calculation for the Over-Pressed and Hand-Veneered Groups Divided by Cement Thickness of 50 and 150 lm Reliability 90% CI
Hand-Veneered 50 lm cement, 75 K cycles at 250 N
Hand-Veneered 150 lm cement, 75 K cycles at 250 N
Over-Pressed 50 lm cement, 75 K cycles at 250 N
Over-Pressed 150 lm cement, 75 K cycles at 250 N
Upper Reliability Lower
0.99 0.95a 0.67
0.92 0.55 0.01
0.84 0.69 0.48
0.53 0.15a 0.004
A mission of 75,000 cycles at 250 N was considered for groups’ comparison. a Symbol for statistical significant difference.
deleterious for ceramic crown systems with thick cement layers. The present study addressed the competing failure modes in aluminum-oxide four-layer structures under vertical loading using step-stress accelerated life time testing. Similar failure patterns were found between the present study and the work by Santana et al.30 on alumina/handbuilt-porcelain systems fatigued with 30 off-axis sliding loading, using comparable parameters. Yet, compared to the current specimens tested under vertical loading, the majority of their four-layer flat specimens tested under sliding demonstrated faster progression of porcelain cone cracks extending to the porcelain/substructure interface.30 The claim that a sliding contact might result in faster crack propagation and lower load/cycles to cone cracks failure is supported by further investigations.48–51 The current study is conducted on flat-layer geometries, and thus may not be readily generalized to the clinical case. This is because dental crowns and FDPs have complex geometries that can affect the residual stress distribution and cooling rate. Therefore, we have conducted mouthmotion fatigue on anatomically correct alumina-based crowns and FDPs,52 and our findings agree with our observations in flat layers. Therefore, our current flat layer models provide a simple but effective analogy concerning the fatigue behavior of alumina-based restorations in a clinical setting.53 CONCLUSIONS
This study provides insights on bulk fracture that is observed as the main clinical failure mode of alumina core crowns. Our flat layer findings suggest that radial cracks in the core develop undetected and lead to bulk failure before inner cone cracks in the veneer are able to cause chipping of the porcelain. Similar mechanical behavior is shown for HV and OP specimens. However, the HV specimens with thin cement thickness appear to perform better relative to the other cases. ACKNOWLEDGMENTS
The authors sincerely appreciate the help of Dr. Bongok Kim, New York University, for participating in specimen preparation, mechanical testing, and imaging analysis. Optical microscopy and SEM imaging were kindly advised by Dr. Timothy Bromage. The authors are grateful to Ernst Hegenbarth, Zen Line Dental, Bruchko¨bel, Germany, for fabricating the
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specimens. They appreciate the support of Steffen Assmann (Wieland Dental Ceramics GmbH, Rosbach, Germany).
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