J. Robert Kelly

Clinically relevant approach to failure testing of all-ceramic restorations ☆ ☆☆

STATEMENT OF PROBLEM One common test of single-unit restorations involves applying loads to clinically realistic specimens through spherical indenters, or equivalently, loading curved incisal edges against flat compression platens. As knowledge has become available regarding clinical failure mechanisms and the behavior of in vitro tests, it is possible to constructively question the clinical validity of such failure testing and to move toward developing more relevant test methods. PURPOSE This article reviewed characteristics of the traditional load-to-failure test, contrasted these with characteristics of clinical failure for all-ceramic restorations, and sought to explain the discrepancies. Literature regarding intraoral conditions was reviewed to develop an understanding of how laboratory testing could be revised. Variables considered to be important in simulating clinical conditions were described, along with their recent laboratory evaluation. CONCLUSIONS Traditional fracture tests of single unit all-ceramic prostheses are inappropriate, because they do not create failure mechanisms seen in retrieved clinical specimens. Validated tests are needed to elucidate the role(s) that cement systems, bonding, occlusion, and even metal copings play in the success of fixed prostheses and to make meaningful comparisons possible among novel ceramic and metal substructures. Research over the past 6 years has shown that crack systems mimicking clinical failure can be produced in all-ceramic restorations under appropriate conditions.

Ceramics in dentistry: Historical roots and current perspectives

This article presents a brief history of dental ceramics and offers perspectives on recent research aimed at the further development of ceramics for clinical use, at their evaluation and selection, and very importantly, their clinical performance. Innovative ceramic materials and ceramics processing strategies that were introduced to restorative dentistry since the early 1980s are discussed. Notable research is highlighted regarding (1) wear of ceramics and opposing enamel, (2) polishability of porcelains, (3) influence of firing history on the thermal expansion of porcelains for metal ceramics, (4) machining and CAD/CAM as fabrication methods for clinical restorations, (5) fit of ceramic restorations, (6) clinical failure mechanisms of all-ceramic prostheses, (7) chemical and thermal strengthening of dental ceramics, (8) intraoral porcelain repair, and (9) criteria for selection of the various ceramics available. It is found that strong scientific and collaborative foundations exist for the continued understanding and improvement of dental ceramic systems.

Fracture-surface analysis of dental ceramics

This study demonstrated that quantitative fractography can be used to study failed aluminous and glass-ceramic central porcelains. Fracture surfaces of DICOR and Vitadur-N core porcelain modulous-of-rupture bars were studied to identify fracture mirror features useful in (1) locating the source of fracture and (2) calculating the stress at fracture in clinically failed restorations. The morphology of fracture surfaces results from events related to the initiation and propagation of the crack front during failure. Modulus-of-rupture testing was performed in four-point bending. Fracture surfaces were studied by scanning electron microscopy. The mean fracture stress for the Vitadur-N porcelain was 94.7 +/- 12.4 MPa (13,730 psi); for DICOR the fracture stress was 55.4 +/- 10.6 MPa (8,030 psi). The standard quantitative fractography relationship between in mirror radius and ln fracture stress was followed for both materials. This quantitative fractography relationship was used to calculate the in vivo stress at failure in a clinically fractured DICOR molar crown. Five clinically failed DICOR crowns were seen to fail from the internal surface.

Development of a clinically validated bulk failure test for ceramic crowns

STATEMENT OF PROBLEM Traditional testing of ceramic crowns creates a stress state and damage modes that differ greatly from those seen clinically. There is a need to develop and communicate an in vitro testing protocol that is clinically valid. PURPOSE The purpose of this study was to develop an in vitro failure test for ceramic single-unit prostheses that duplicates the failure mechanism and stress state observed in clinically failed prostheses. This article first compares characteristics of traditional load-to-failure tests of ceramic crowns with the growing body of evidence regarding failure origins and stress states at failure from the examination of clinically failed crowns, finite element analysis (FEA), and data from clinical studies. Based on this analysis, an experimental technique was systematically developed and test materials were identified to recreate key aspects of clinical failure in vitro. MATERIAL AND METHODS One potential dentin analog material (an epoxy filled with woven glass fibers; NEMA grade G10) was evaluated for elastic modulus in blunt contact and for bond strength to resin cement as compared to hydrated dentin. Two bases with different elastic moduli (nickel chrome and resin-based composite) were tested for influence on failure loads. The influence of water during storage and loading (both monotonic and cyclic) was examined. Loading piston materials (G10, aluminum, stainless steel) and piston designs were varied to eliminate Hertzian cracking and to improve performance. Testing was extended from a monolayer ceramic (leucite-filled glass) to a bilayer ceramic system (glass-infiltrated alumina). The influence of cyclic rate on mean failure loads was examined (2 Hz, 10 Hz, 20 Hz) with the extremes compared statistically (t test; α=.05). RESULTS Failure loads were highly influenced by base elastic modulus (t test; P<.001). Cyclic loading while in water significantly decreased mean failure loads (1-way ANOVA; P=.003) versus wet storage/dry cycling (350 N vs. 1270 N). G10 was not significantly different from hydrated dentin in terms of blunt contact elastic behavior or resin cement bond strength. Testing was successful with the bilayered ceramic, and the cycling rate altered mean failure loads only slightly (approximately 5%). CONCLUSIONS Test methods and materials were developed to validly simulate many aspects of clinical failure.

Fracture toughness (KIc) of a dental porcelain determined by fractographic analysis

OBJECTIVES Fractographic analysis of indentation cracks is performed following flexure testing as part of the ASTM (1999) standard for fracture toughness, KIc, determination in advanced ceramics. This method depends on the conduciveness of the material towards fractographic interpretation. The purpose of this study was to evaluate the use of fractography in fracture toughness methods with a feldspathic dental porcelain, in which KIc was measured fractographically as well as numerically using two controlled-flaw beam bending techniques. METHODS The following methods for KIc determination were applied to a dental porcelain containing a leucite volume fraction of 15-20%: (1) surface crack in flexure (SCF) (dependent upon fractographic analysis); and (2) indentation strength (IS) at indentation loads of 9.8 and 19.6 N (applying both standard numeric calculations and fractographic analysis). The testing environments were (1) ambient air (IS and SCF) and (2) flowing dry nitrogen (SCF). RESULTS No significant differences were found between numeric and fractographic KIc values for the IS technique at both indentation loads (9.8 and 19.6 N) in ambient air, although KIc values were sensitive to indentation load. Due to the presence of residual stresses, stable crack extension was observed fractographically in all IS specimens, as evidenced by differences between initial (ainitial) and critical (acritical) crack dimensions. For the SCF method, there was a significant difference in toughness between specimens tested in air versus dry nitrogen, however no fractographic evidence for chemically assisted slow crack growth (SCG) was observed. SIGNIFICANCE The SCF method as described by the ASTM standard was applicable to the feldspathic porcelain and produced very comparable results with the numeric toughness calculations of the IS procedure. However, fractographic analysis of the surface crack was somewhat difficult for this glassy ceramic compared with polycrystalline ceramics. Knowledge about stable crack extension or slow crack growth and its fractographic appearance is essential when estimating the toughness from examination of flaw dimensions on fractured surfaces since large calculation errors may occur if these effects are not taken into account.

Nonprecious alloys for use in fixed prosthodontics: A literature review

The physical properties of nonprecious alloys can differ significantly from those of alloys containing a high percent of gold. Relationships among constituents, physical properties, and handling characteristics of base metal alloys were surveyed. Toxicity of nickel, beryllium, and their compounds was discussed with attention given to the dental environment. Allergic contact dermatitis appears to be a health risk to certain patients from nickel-containing prostheses. Beryllium dust is apparently not a hazard in properly ventilated and exhausted grinding and polishing areas. Lack of data on nickel-related health problems in dental laboratory workers should be interpreted with caution. This article also reviewed research on porcelain bonding and corrosion of nonprecious alloys. Although this research cannot yet predict an alloys porcelain bonding behavior in mouths, little or no porcelain bond problems have been reported. A few controlled clinical studies report little corrosion in up to 4 years.

Evaluation of thermal compatibility between core and veneer dental ceramics using shear bond strength test and contact angle measurement

OBJECTIVES To test the hypotheses that shear bond strength (sigma(s)) and contact angle (theta) are influenced by the thermal expansion/contraction behavior of bilayered all-ceramic systems. METHODS Glass-infiltrated ceramics (A - In-Ceram ALUMINA) and zirconia-reinforced alumina (Z - In- Ceram ZIRCONIA) were veneered with feldspathic ceramics (7 - VM7; 9 - VM9; 13 - VM13), yielding 6 experimental groups. Surface roughness (Ra) of A and Z core ceramic disks (12 mm diameter x 1.2 mm high) was measured to assure that values were similar. A cylinder of veneer (2 mm diameter x 2 mm high) was fired onto the center of all core disks, cooled under identical conditions and tested for sigma(s) to failure (n=20). For the theta evaluation (n=10) similar specimens (veneer 1 mm high) were overfired to develop an equilibrium theta that was then measured from digital images (AutoCAD 2006). Published thermal expansion data were used. RESULTS The mean values of sigma(s) (MPa) and theta (degrees) for the core-veneer ceramic groups were, respectively: A7 (19.4+/-4.7; 49+/-4.7); Z7 (23.4+/-6.2; 56+/-4.9); A9 (0.9+/-1.6; 55+/-5.8); Z9 (9.8+/-5.7; 59.8+/-2.9); A13 (0; 70+/-6.0); Z13 (0; 67.2+/-6.3). As the difference in coefficient of thermal expansion (coreveneer) increases, the theta value increases (r=0.95) and the sigma(s) value decreases (r=-0.92), p<or=0.01. CONCLUSION The contact angle measurement and the shear bond strength test of core-veneer ceramics are influenced by the thermal expansion behavior of these all-ceramic systems.

Influence of loading frequency on implant failure under cyclic fatigue conditions

OBJECTIVES Preliminary studies on implant fatigue testing suggested that fractures were more likely to occur at 2 Hz than at 30 Hz (chi(2), p<0.05). This investigation explores frequency and base elastic modulus effects on strain, strain rate and failure. METHODS A total of 66 implants were mounted in different base materials (acrylic, glass-filled epoxy, aluminum) and loaded up to 10(6) cycles per ISO 14801 (20 N to 420-500 N) at frequencies of 2 Hz and 30 Hz (chosen to accelerate frequency as the stressor). Absolute strain magnitudes and strain rates under varying loading conditions and with different base materials were measured using one strain-gauged implant. Failure probability distributions were analyzed by both Weibull and life data analysis. Measured strain was used to validate an FEA model. Fracture surfaces were examined by SEM. RESULTS Number of failures and failure-rates-per-cycle differed significantly between implants tested at 2 Hz versus 30 Hz (p<0.05). Strain magnitude was independent of frequency. Strain rates were highly correlated with frequency (linear r(2)>0.99) and differed significantly under failure conditions (420 N): 2 Hz=8292 microstrain/s; (500 N): 30 Hz=80,840 microstrain/s. Measured and FEA-calculated strains were similar. Fracture surfaces were indistinguishable (2 Hz versus 30 Hz). SIGNIFICANCE Fatigue failure was significantly more likely at 2 Hz than 30 Hz whereas base material and loading magnitude seemed to have only minor influence. Absolute strain was identical at these frequencies suggesting strain rate sensitivity for this commercially pure titanium implant. Both the Weibull and SEM analyses support an identical failure mechanism with damage accumulation more severe at lower frequencies, an interpretation consistent with strain rate sensitivity.

Influence of thermal gradients on stress state of veneered restorations

OBJECTIVES To assess transient and residual stresses within the porcelain of veneered restorations (zirconia and metal) as a result of cooling rate and porcelain thickness. METHODS Porcelain-on-zirconia (PZ) and porcelain-fused-to-metal (PFM) crowns were fabricated with 1 or 2mm of porcelain. Thermocouples were attached both internally and externally to the crowns to record transient temperatures. For fast cooling, the furnace was opened after the holding time and switched off. Slow cooling was accomplished by opening the furnace at 50°C below the glass transition temperature (Tg) of the material. An axially symmetric FEA model simulated thermal stresses. Time-dependent temperature equations from thermocouple readings were set as boundary conditions. Framework materials and the porcelain below Tg were considered to behave elastically. Visco-elastic behavior was assumed for porcelain above the Tg modeling properties as dependent on cooling rate. RESULTS Differences in residual stress were found for fast and slow cooled PZ and PFM crowns. Significant transient stress waves were observed within the porcelain when fast cooling through Tg. They are believed to be related to non-uniform volumetric changes originated from thermal gradients. Results were confirmed by modeling and physical testing of crowns containing a defect. SIGNIFICANCE Residual stresses do not distinguish PZ from PFM. High magnitude transient stresses observed within the porcelain during fast cooling may explain clinical fractures involving internal defects. Stress waves may also originate internal micro-cracking which could grow under function. Therefore, slow cooling, especially for all-ceramic crowns with thick porcelain, is important to prevent thermal gradients and high-magnitude transient stresses.

Comparison of an absolute and surrogate measure of relative translucency in dental ceramics

PURPOSE The objective was to compare absolute translucency with a frequently used surrogate measure of relative translucency (contrast ratio) from 14 all-ceramic materials having a wide range of translucencies. MATERIALS AND METHODS Standardized disks were fabricated from fourteen ceramics, varying both thickness (0. 3mm, 0.5 mm, 1.0 mm, and 1.5 mm) and chroma (high and low). Absolute translucency (percent transmission) was measured using a spectroradiometer with an integrating sphere. Relative translucency was measured using a spectrophotometer as contrast ratio (contrast ratio; ratio of L* values recorded on black and white backgrounds). Non-linear regression was used to compare measurements of absolute versus relative values for each of the spectrophotometers. RESULTS Contrast ratio was not able to characterize translucency across the range of materials studied and became less sensitive with increasing opacity. A non-linear correlation was found between percent transmission and contrast ratio down to 50% transmission (r(2)=0.97) and contrast ratio was insensitive to transmission differences below 50% transmission. CONCLUSION Contrast ratio is not a direct measure of translucency and cannot be used below 50% transmission.