Indentation damage and mechanical properties of human enamel and dentin

Use of contact testing in the characterization and design of all-ceramic crownlike layer structures: a review

Ceramic-based crowns, particularly molar crowns, can fail prematurely from accumulation of fracture and other damage in continual occlusal contact. Damage modes depend on ceramic types (especially microstructures), flaw states, loading conditions, and geometric factors. These damage modes can be simulated and characterized in the laboratory with the use of Hertzian contact testing on monolayer, bilayer, and trilayer structures to represent important aspects of crown response in oral function. This article reviews the current dental materials knowledge base of clinically relevant contact-induced damage in ceramic-based layer structures in the context of all-ceramic crown lifetimes. It is proposed that simple contact testing protocols that make use of sphere indenters on model flat, ceramic-based layer structures-ceramic/polymer bilayers (simulating monolithic ceramic crowns on dentin) and ceramic/ceramic/polymer trilayers (simulating veneer/core all-ceramic crowns on dentin)-can provide useful relations for predicting critical occlusal loads to induce lifetime-threatening fracture. It is demonstrated that radial cracking from the lower core layer surface is the dominant failure mode for ceramic layer thicknesses much below 1 mm. Such an approach may be used to establish a scientific, materials-based foundation for designing next-generation crown layer structures.

Regulated gene expression dictates enamel structure and tooth function

Enamel is a complex bioceramic tissue. In its final form, enamel is a reflection of the unique molecular and cellular activities occurring during organogenesis. From the ectodermal origins of ameloblasts, their gene activity and protein expression profiles exist for the sole purpose of producing a mineralized shell, almost entirely devoid of protein, deposited over the 'bone-like' dentine. The interface between enamel and dentine is referred to as the dentine enamel junction and it is also unique in its biology. This review article is narrow in its scope. We restrict our review to selected advances in our understanding of the genetic, molecular and structural aspects of enamel biology. We present a model of enamel formation that relates gene expression to the assembly of an extracellular protein matrix that in turn controls the structural hierarchy and mechanical aspects of enamel and the tooth organ.

A predictive formula of the contraction stress in restorative and luting materials attending to free and adhered surfaces, volume and deformation

OBJECTIVES

To find a predictive formula of stress, considering the surfaces (free, adhered) involved, the volume and characteristics of material and the deformation of the measuring system.

MATERIALS AND METHODS

231 samples of five chemically cured restoratives (Silar (SIL, 23), Clearfil F2 (CLE, 39), P10 (P10, 33), Concise (CON, 30), Isopast (ISO, 28)) and four luting (3M Experimental 241 (EXM, 20), Variolink II (VAR, 13), Vitremer LC (VTM, 20) and Dyract Cem (DYR, 25)) materials were allowed to polymerize until they reached a maximum tension (T(max), 25 min) between six pairs (null 5.81, 8.5, 11.26, 12.42, 17.02, 23.14 mm) of polished metallic discs (range of distances: 0.02-5.9 mm) mounted in a tension machine. The deformation of the measuring system was measured for the recorded forces.

RESULTS

A descriptive non-linear formula T(max)=KVol(-3.267)FS(3.283)AS(0.642)Def(0.561) was found that individualizes the material's characteristics (K) that considers volume (Vol), free (FS) and adhered (AS) surfaces and deformation (Def) of the system for each force. This formula renders good correlation (material K (r(2) coefficient)): SIL 0.9998 (0.995), CLE 1.0062 (0.989), P10 1.0224 (0.990), CON 0.9908 (0.992), ISO 0.9648 (0.974), EXM 1.0083 (0.991), VAR 0.9777 (0.996), VTM 0.9925 (0.993), DYR 0.9971 (0.997) between actual T(max) and calculated Tension. There are statistically significant differences (p=0.002) between K values of both (restorative and luting) groups.

SIGNIFICANCE

Predictive parameters have influence in a different way to what is actually considered, if the system is allowed to have deformation, as occurs naturally and volume and material's characteristics are considered.

Mechanical properties of human dental enamel on the nanometre scale

Atomic force microscopy (AFM) combined with a nano-indentation technique was used to reveal the structure and to perform site-specific mechanical testing of the enamel of third molars. Nano-indentations (size<500 nm) were made in the cusp area to measure the mechanical properties of single enamel rods at different orientations. The influence of etching on the physical properties was studied and etching conditions that did not significantly alter the plastic-elastic response of enamel were defined. Elasticity and hardness were found to be a function of the microstructural texture. Mean Young's moduli of 87.5 (+/-2.2) and 72.2 (+/-4.5) GPa and mean hardness of 3.9+/-0.3 and 3.3+/-0.3 GPa were measured in directions parallel and perpendicular to the enamel rods, respectively. Analysis of variance showed that the differences were significant. The observed anisotropy of enamel is related to the alignment of fibre-like apatite crystals and the composite nature of enamel rods. Mechanical properties were also studied at different locations on single enamel rods. Compared to those in the head area of the rods, Young's moduli and hardness were lower in the tail area and in the inter-rod enamel, which can be attributed to changes in crystal orientation and the higher content of soft organic tissue in these areas.

Mechanical properties of the dentinoenamel junction: AFM studies of nanohardness, elastic modulus, and fracture

The dentinoenamel junction (DEJ) is a complex and poorly defined structure that unites the brittle overlying enamel with the dentin that forms the bulk of the tooth. In addition, this structure appears to confer excellent toughness and crack deflecting properties to the tooth, and has drawn considerable interest as a biomimetic model of a structure uniting dissimilar materials. This work sought to characterize the nanomechanical properties in the region of the DEJ using modified AFM based nanoindentation to determine nanohardness and elastic modulus. Lines of indentations traversing the DEJ were made at 1-2 microm intervals from the dentin to enamel along three directions on polished sagittal sections from three third molars. Nanohardness and elastic modulus rose steadily across the DEJ from bulk dentin to enamel. DEJ width was estimated by local polynomial regression fits for each sample and location of the mechanical property curves for the data gradient from enamel to dentin, and gave a mean value of 11.8 microm, which did not vary significantly with intratooth location or among teeth. Nanoindentation was also used to initiate cracks in the DEJ region. In agreement with prior work, it was difficult to initiate cracks that traversed the DEJ, or to produce cracks in the dentin. The fracture toughness values for enamel of 0.6-0.9 MPa . m(1/2) were in good agreement with recent microindentation fracture results. Our results suggest that the DEJ displays a gradient in structure and that nanoindenation methods show promise for further understanding its structure and function.

The fracture behaviour of human and pig molar cusps

Masticatory efficiency depends upon the ability of the molar cusps to apply concentrated bite forces to food particles and simultaneously to withstand the dental stresses that may cause enamel fracture. This study investigated how low-crowned molar cusps in omnivorous mammals, specifically humans, Homo sapiens, and pigs, Sus scrofa, resist fracture under compressive load. A uniaxial compressive load was applied to individual molar cusps with a materials testing machine. The progressive loading and deformation of the cusps were recorded for interrupted and continuous tests. In interrupted tests, the appearance of progressive cusp fracture was recorded. Stiffness and fracture stresses were calculated from continuous test results. Pig cusps responded to both interrupted and continuous loads with greater deformation; progressive crumbling of the cusp tip resulted in new occlusal contacts on enamel lophs. Conversely, human cusps showed minimal breakage before failure. Continuous compressive tests demonstrated the greater stiffness of human cusps, as well as the capacity to sustain higher cusp tip stresses. The greater stiffness and high fracture resistance of human cusps may be attributed to the thickness of enamel. Test results reflected fundamentally different means of crown stress management that correspond with phylogenetic differences in masticatory function.

Biological organization of hydroxyapatite crystallites into a fibrous continuum toughens and controls anisotropy in human enamel

Enamel forms the outer surface of teeth, which are of complex shape and are loaded in a multitude of ways during function. Enamel has previously been assumed to be formed from discrete rods and to be markedly aniostropic, but marked anisotropy might be expected to lead to frequent fracture. Since frequent fracture is not observed, we measured enamel organization using histology, imaging, and fracture mechanics modalities, and compared enamel with crystalline hydroxyapatite (Hap), its major component. Enamel was approximately three times tougher than geologic Hap, demonstrating the critical importance of biological manufacturing. Only modest levels of enamel anisotropy were discerned; rather, our measurements suggest that enamel is a composite ceramic with the crystallites oriented in a complex three-dimensional continuum. Geologic apatite crystals are much harder than enamel, suggesting that inclusion of biological contaminants, such as protein, influences the properties of enamel. Based on our findings, we propose a new structural model.

Whisker-reinforced dental core buildup composites: effect of filler level on mechanical properties

The strength and toughness of dental core buildup composites in large stress-bearing restorations need to be improved to reduce the incidence of fracture due to stresses from chewing and clenching. The aims of the present study were to develop novel core buildup composites reinforced with ceramic whiskers, to examine the effect of filler level, and to investigate the reinforcement mechanisms. Silica particles were fused onto the whiskers to facilitate silanization and to roughen the whisker surface for improved retention in the matrix. Filler level was varied from 0 to 70%. Flexural strength, compressive strength, and fracture toughness of the composites were measured. A nano-indentation system was used to measure elastic modulus and hardness. Scanning electron microscopy (SEM) was used to examine the fracture surfaces of specimens. Whisker filler level had significant effects on composite properties. The flexural strength in MPa (mean +/- SD; n = 6) increased from (95+/-15) for the unfilled resin to (193+/- 8) for the composite with 50% filler level, then slightly decreased to (176+/-12) at 70% filler level. The compressive strength increased from (149+/-33) for the unfilled resin to (282+/-48) at 10% filler level, and remained equivalent from 10 to 70% filler level. Both the modulus and hardness increased monotonically with filler level. In conclusion, silica particle-fused ceramic single-crystalline whiskers significantly reinforced dental core buildup composites. The reinforcement mechanisms appeared to be crack deflection and bridging by the whiskers. Whisker filler level had significant effects on the flexural strength, compressive strength, elastic modulus, and hardness of composites.

The hardness and modulus of elasticity of primary molar teeth: an ultra-micro-indentation study

OBJECTIVE

Baseline information on the mechanical properties of and the effect of load upon dental hard tissue is important in the development of successful dental materials. Existing methods of measuring such properties of tissue are subject to significant experimental error. This study reports on the use of an Ultra-Micro-Indentation System (UMIS) to measure the hardness and elastic modulus of primary enamel and dentine.

METHODS

Primary molar teeth were sectioned, set in resin and polished. Thirty indentations were made in enamel and dentine using a Berkovitch indentor, 15 of which were subject to a load of 50mN and 15 to a load of 150mN. An automated computerised system converted the force/penetration graph for each indentation in to a hardness vs depth graph from which values for the mean hardness and elastic modulus were calculated.

RESULTS

Primary enamel had a mean hardness of 4.88+/-0.35GPa whilst the hardness of dentine was 0.92+/-0.11GPa The elastic modulus for enamel was 80.35+/-7.71GPa and that of dentine 19.89+/-1.92GPa. Using linear regression analysis a significant relationship could be shown between the hardness and the elastic modulus for both enamel and dentine when loaded to 150mN but only for dentine at 50mN (P<0.05). In general the elasticity of tooth structure increased as the hardness increased.

CONCLUSION

The UMIS offers a simple and reproducible method of measuring basic mechanical properties of small samples of enamel and dentine.

Effects of whisker-to-silica ratio on the reinforcement of dental resin composites with silica-fused whiskers

Resin composites need to be strengthened to improve their performance in large stress-bearing restorations. This study aimed to reinforce composites with whiskers and to investigate the effects of the whisker:silica ratio. It was hypothesized that changing the whisker-silica ratio would affect the whisker-matrix bonding and the filler's distribution, and hence alter the composite properties. Silica particles and whiskers were mixed at various whisker:silica mass ratios, thermally fused, and combined with a dental resin at filler mass fractions of 0-65%. Whisker:silica ratio and filler level had significant effects on composite properties. At 60% filler level, the silica composite (whisker:silica = 0:1) had a flexural strength (mean +/- SD; n = 6) of 104 +/- 21 MPa; that at a whisker:silica ratio of 1:0 was 74 +/- 36 MPa. However, that of the silica-fused whisker composite (whisker:silica = 5:1) was 210 +/- 14 MPa, compared with 109 +/- 23 MPa and 114 +/- 18 MPa of two prosthetic controls. Mixing silica with whiskers minimized whisker entanglement, improved filler distribution in the matrix, and facilitated whisker silanization and bonding to the matrix, thus resulting in substantially stronger composites.

Dental resin composites containing ceramic whiskers and precured glass ionomer particles

OBJECTIVES

Glass ionomer, resin-modified glass ionomer, and compomer materials are susceptible to brittle fracture and are inadequate for use in large stress-bearing posterior restorations. The aim of this study was to use ceramic single crystal whiskers to reinforce composites formulated with precured glass ionomer, and to examine the effects of whisker-to-precured glass ionomer mass ratio on mechanical properties, fluoride release, and polishability of the composites.

METHODS

Silica particles were fused onto silicon nitride whiskers to facilitate silanization and to improve whisker retention in the matrix. Hardened glass ionomer was ground into a fine powder, mixed with whiskers, and used as fillers for a dental resin. Four control materials were also tested: a glass ionomer, a resin-modified glass ionomer, a compomer, and a hybrid composite. A three-point flexural test was used to measure flexural strength, modulus, and work-of-fracture. A fluoride ion-selective electrode was used to measure fluoride release. Composite surfaces polished simulating clinical procedures were examined by SEM and profilometry.

RESULTS

At whisker/(whisker + precured glass ionomer) mass fractions of 1.0 and 0.91, the whisker composite had a flexural strength in MPa (mean (SD); n = 6) of (196 (10)) and (150 (16)), respectively, compared to (15 (7)) for glass ionomer, (39 (8)) for resin-modified glass ionomer, (89 (18)) for compomer, and (120 (16)) for hybrid composite. The whisker composite had a cumulative fluoride release of nearly 20% of that of the glass ionomer after 90 days. The whisker composites had surface roughness comparable to the hybrid resin composite.

SIGNIFICANCE

Composites filled with precured glass ionomer particles and whiskers exhibit moderate fluoride release with improved mechanical properties; the whisker-to-glass ionomer ratio is a key microstructural parameter that controls fluoride release and mechanical properties.

Indentation modulus and hardness of whisker-reinforced heat-cured dental resin composites

OBJECTIVES

Recent studies showed that ceramic whisker reinforcement imparted a two-fold increase in the strength of dental composites. The aim of this study was to investigate the indentation response and measure the elastic modulus, hardness, and brittleness of whisker-reinforced heat-cured resin composites as a function of filler level, heat-cure temperature, and heat-cure duration.

METHODS

Silica particles were fused onto silicon nitride whiskers to facilitate silanization and to roughen the whiskers for improved retention in matrix. Whisker filler mass fractions of 0, 20, 40, 60, 70, 74 and 79% were tested. Heat-cure temperature ranged from 100 to 180 degrees C, and duration from 10 min to 24 h. A nano-indentation system enabled the measurement of elastic modulus. Fracture toughness was measured and composite brittleness index was calculated. An inlay/onlay composite and a prosthetic composite were tested as controls.

RESULTS

Whisker filler level and heat-cure duration had significant effects on composite properties, while heat-cure temperature had non-significant effects. The whisker composite with 79% filler level had a modulus in GPa (mean (SD); n = 6) of 26.9 (1.0), significantly higher than 15.1 (0.2) of an inlay/onlay control, and 16.1 (0.3) of a prosthetic control (Tukey's multiple comparison test; family confidence coefficient = 0.95). The fracture toughness in MPa.m1/2 was 2.22 (0.26) for the whisker composite, higher than 0.95 (0.11) for inlay/onlay control, and (1.13 +/- 0.19) for prosthetic control. The brittleness index was (0.49 +/- 0.07) for whisker composite, lower than (1.02 +/- 0.12) for inlay/onlay control and (0.63 +/- 0.13) for prosthetic control.

SIGNIFICANCE

Whisker filler level had a profound influence, heat-cure duration had significant effects, while temperature did not have significant effects, on the properties of whisker composite. The whisker composite had significantly higher elastic modulus and fracture toughness, and lower brittleness than the inlay/onlay and prosthetic controls.

Whisker-reinforced heat-cured dental resin composites: effects of filler level and heat-cure temperature and time

Currently available dental resin composites are inadequate for use in large stress-bearing crown and multiple-unit restorations. The aim of this study was to reinforce heat-cured composites with ceramic whiskers. It was hypothesized that whiskers substantially strengthen heat-cured composites. It was further hypothesized that whisker filler level and heat-cure temperature and time significantly influence composite properties. Silica particles were fused onto the whiskers to facilitate silanization and to roughen the whiskers for improved retention in the matrix. The whisker filler mass fraction was varied from 0% to 79%, the heat-cure temperature from 80 degrees C to 180 degrees C, and cure time from 10 min to 24 hrs. Flexural strength, work-of-fracture, and fracture toughness of the composites were measured, and specimen fracture surfaces were examined with scanning electron microscopy. Filler level had a significant effect on composite properties. The whisker composite with 70% filler level had a flexural strength in MPa (mean +/- SD; n = 6) of 248 +/- 23, significantly higher than 120 +/- 16 of an inlay/onlay composite control and 123 +/- 21 of a prosthetic composite control (Tukey's multiple comparison test; family confidence coefficient = 0.95). Heat-cure time also played a significant role. At 120 degrees C, the strength of composite cured for 10 min was 178 +/- 17, lower than 236 +/- 14 of composite cured for 3 hrs. The strength of whisker composite did not degrade after water-aging for 100 d. In conclusion, heat-cured composites were substantially reinforced with whiskers. The reinforcement mechanisms appeared to be whiskers bridging and resisting cracks. The strength and fracture toughness of whisker composite were nearly twice those of currently available inlay/onlay and prosthetic composites.

Lifetime-limiting strength degradation from contact fatigue in dental ceramics

The hypothesis under examination in this paper is that the lifetimes of dental restorations are limited by the accumulation of contact damage during oral function; and, moreover, that strengths of dental ceramics are significantly lower after multi-cycle loading than after single-cycle loading. Accordingly, indentation damage and associated strength degradation from multi-cycle contacts with spherical indenters in water are evaluated in four dental ceramics: "aesthetic" ceramics-porcelain and micaceous glass-ceramic (MGC), and "structural" ceramics-glass-infiltrated alumina and yttria-stabilized tetragonal zirconia polycrystal (Y-TZP). At large numbers of contact cycles, all materials show an abrupt transition in damage mode, consisting of strongly enhanced damage inside the contact area and attendant initiation of radial cracks outside. This transition in damage mode is not observed in comparative static loading tests, attesting to a strong mechanical component in the fatigue mechanism. Radial cracks, once formed, lead to rapid degradation in strength properties, signaling the end of the useful lifetime of the material. Strength degradation from multi-cycle contacts is examined in the test materials, after indentation at loads from 200 to 3000 N up to 10(6) cycles. Degradation occurs in the porcelain and MGC after approximately 10(4) cycles at loads as low as 200 N; comparable degradation in the alumina and Y-TZP requires loads higher than 500 N, well above the clinically significant range.

A micromechanics model of the elastic properties of human dentine

A generalized, self-consistent model of cylindrical inclusions in a homogeneous and isotropic matrix phase was used to study the effects of tubule orientation on the elastic properties of dentine. Closed-form expressions for the five independent elastic constants of dentine were derived in terms of tubule concentration, and the Young's moduli and Poisson ratios of peri- and intertubular dentine. An atomic-force microscope indentation technique determined the Young's moduli of the peri- and intertubular dentine as approx. 30 and 15 GPa, respectively. Over the natural variation in tubule density found in dentine, there was only a slight variation in the axial and transverse shear moduli with position in the tooth, and there was no measurable effect of tubule orientation. It was concluded that tubule orientation has no appreciable effect on the elastic behaviour of normal dentine, and that the elastic properties of healthy dentine can be modelled as an isotropic continuum with a Young's modulus of approx. 16 GPa and a shear modulus of 6.2 GPa.

In vitro dentine hardness following gamma-irradiation and freezing

OBJECTIVES

To investigate the short term effects of gamma-irradiation and conventional freezing on the hardness of human dentine.

METHODS

Twenty-one dentine disks were produced by transverse sectioning 21 sound extracted human permanent molar teeth. The 2.1 mm thick disks were wet polished using 4000 grit polishing paper. Three lines were drawn dividing each disk into six symmetrical areas. Seven disks were randomly assigned for freezing in water at -18 degrees C for 12 days; seven were exposed to a single dose of 25.2 kGy gamma radiation in water; seven were kept in water at 20 degrees C for 12 days (control). Before treatment, three Vicker's indentations at a load of 30 kg and a dwelling time of 20 s were made in one half of each disk, at equal distances from its edge and centre. Using light microscopy and image analysis software, the indentation diagonals were measured 4 h after preparation. Vicker's hardness values (VHN = kg/mm2) were calculated for each indentation. Following the treatment, VHNs were calculated again, for three symmetrical indentations in the second half of each disk. A statistical analysis was performed using the Wilcoxon rank sum test.

RESULTS

The frozen group showed no changes following the treatment (VHN = 58 +/- 6 before vs. 57 +/- 6 after). The irradiation and control groups showed some hardening (VHN = 53 +/- 7 vs. 59 +/- 8; 57 +/- 3 vs. 63 +/- 6, respectively), however all values were within the normal variation.

CONCLUSION

Both conventional freezing at -18 degrees C for 12 days in water, and a single gamma-irradiation dose of 25.2 kGy in water, appeared to have no short term effects on the hardness of human coronal dentine.

Three-body wear of a hand-consolidated silver alternative to amalgam

Recent studies have investigated a mercury-free silver alternative to amalgam, but the silver powders required a relatively high compaction pressure to consolidate. The aim of the present study was to consolidate a precipitated silver powder into a cohesive solid using an air-driven pneumatic condenser fitted with an amalgam plugger at a clinically realistic load, and to study the mechanisms and rates of three-body wear of the consolidated silver in comparison with that of an amalgam. The silver powder was annealed, rinsed with a dilute acid, and consolidated either in a prepared tooth cavity or in a specimen mold at a load of 15 N. A four-station wear machine was used where each specimen was immersed in a slurry containing polymethyl methacrylate beads, then a steel pin was loaded and rotated against the specimen at a maximum load of 76 N. The flexural strength in MPa (mean +/- SD; n = 10) was 86 +/- 20 for amalgam, 181 +/- 45 for silver with a polished surface, and 202 +/- 21 for silver with a burnished surface. After 4 x 10(5) wear cycles, the wear scar depth in microm was 134 +/- 54 for amalgam, 143 +/- 8 for polished silver, and 131 +/- 9 for burnished silver, which were not significantly different (Tukey's multiple comparison test; family confidence coefficient = 0.95). SEM examination revealed cracks and fracture pits in the worn surface of amalgam, in contrast to a smooth surface in silver. Wear and material removal in amalgam occurred by microfracture and dislodgement of cracked segments, while wear in the silver occurred by ductile deformation and flow of materials. To conclude, the consolidated silver possesses a three-body wear resistance similar to that of amalgam, and a higher resistance to wear-induced damage and cracking than amalgam. The mechanism of wear in amalgam is microfracture and material dislodgement, while that in consolidated silver is ductile deformation and flow of material.

Dental composite resins containing silica-fused ceramic single-crystalline whiskers with various filler levels

Currently available direct-filling composite resins are susceptible to fracture and hence are not recommended for use in large stress-bearing posterior restorations involving cusps. The glass fillers in composites provide only limited reinforcement because of the brittleness and low strength of glass. The aim of the present study was to use ceramic single-crystalline whiskers as fillers to reinforce composites, and to investigate the effect of whisker filler level on composite properties. Silica particles were fused onto the whiskers to facilitate silanization and to roughen the whiskers, thereby improving retention in the matrix. The composite flexural strength, elastic modulus, hardness, and degree of polymerization conversion were measured as a function of whisker filler mass fraction, which ranged from 0% to 70%. Selected composites were polished simulating clinical procedures, and the surface roughness was measured with profilometry. The whisker composite with a filler mass fraction of 55% had a flexural strength (mean +/- SD; n = 6) of 196+/-10 MPa, significantly higher than 83+/-14 MPa of a microfill and 120+/-16 MPa of a hybrid composite control (family confidence coefficient = 0.95; Tukey's multiple comparison). The composite modulus and hardness increased monotonically with filler level. The flexural strength first increased, then plateaued with increasing filler level. The degree of conversion decreased with increasing filler level. The whisker composite had a polished surface roughness similar to that of a conventional hybrid composite (p>0.1; Student's t). To conclude, ceramic whisker reinforcement can significantly improve the mechanical properties of composite resins; the whisker filler level plays a key role in determining composite properties; and the reinforcement mechanisms appear to be crack pinning by whiskers and friction from whisker pullout resisting crack propagation.

Damage modes in dental layer structures

Natural teeth (enamel/dentin) and most restorations are essentially layered structures. This study examines the hypothesis that coating thickness and coating/substrate mismatch are key factors in the determination of contact-induced damage in clinically relevant bilayer composites. Accordingly, we study crack patterns in two model "coating/substrate" bilayer systems conceived to simulate crown and tooth structures, at opposite extremes of elastic/plastic mismatch: porcelain on glass-infiltrated alumina ("soft/hard"); and glass-ceramic on resin composite ("hard/soft"). Hertzian contacts are used to investigate the evolution of fracture damage in the coating layers, as functions of contact load and coating thickness. The crack patterns differ radically in the two bilayer systems: In the porcelain coatings, cone cracks initiate at the coating top surface; in the glass-ceramic coatings, cone cracks again initiate at the top surface, but additional, upward-extending transverse cracks initiate at the internal coating/substrate interface, with the latter dominant. The substrate is thereby shown to have a profound influence on the damage evolution to ultimate failure in the bilayer systems. However, the cracks are highly stabilized in both systems, with wide ranges between the loads to initiate first cracking and to cause final failure, implying damage-tolerant structures. Finite element modeling is used to evaluate the tensile stresses responsible for the different crack types. The clinical relevance of these observations is considered.

Indentation creep behavior of a direct-filling silver alternative to amalgam

Amalgam creep has been identified as a key parameter associated with marginal breakdown and corrosion. The aim of this study was to evaluate the time-dependent deformation (creep) of a novel silver filling material as an alternative to amalgam. We made the silver specimens by pressing a precipitated powder at room temperature to a density that can be achieved in clinical hand consolidation. The surface of the silver was either polished or burnished. To examine local contact creep and the effect of surface finishing, we used an indentation creep method in which a Vickers indenter was loaded on the specimen surface at a load of 10 N with dwell times of 5 sec to 6x10(4) sec. We used a bonded-interface technique to examine subsurface creep mechanisms. The flexural strength (mean+/-SD; n = 10) was 86+/-20 MPa for amalgam, 180+/-21 MPa for polished silver, and 209+/-19 MPa for burnished silver-values which are significantly different from each other (family confidence coefficient = 0.95; Tukey's multiple-comparison test). Indentation creep manifested as hardness number decreasing with increased dwell time. With dwell time increasing from 5 sec to 6x10(4) sec, the hardness number of amalgam was reduced by approximately 80%; that of the polished silver and the burnished silver was reduced by only 40%. Subsurface creep in amalgam consisted of the shape change of the alloy particles from spherical to elongated shapes, and the separation of matrix grains from each other, possibly due to grain-boundary sliding. Creep of the polished silver occurred by densification reducing porosity and increasing hardness; that of the burnished silver occurred by the displacement of the burnished layer. These results suggest that, due to creep-induced subsurface work-hardening and densification, the consolidated silver exhibits a higher resistance to indentation creep than does amalgam. The hardness number of silver approaches that of amalgam after prolonged indentation loading.