Thomas J. Hill

Microstructural evolution and physical behavior of a lithium disilicate glass–ceramic

BACKGROUND Elucidating the microstructural responses of the lithium disilicate system like the popular IPS e.max® CAD (LS2), made specifically for computer-aided design and computer-aided manufacturing (CAD-CAM), as a temperature-dependent system unravels new ways to enhance material properties and performance. OBJECTIVE To study the effect of various thermal processing on the crystallization kinetics, crystallite microstructure, and strength of LS2. METHODS The control group of the LS2 samples was heated using the standard manufacturer heating-schedule. Two experimental groups were tested: (1) an extended temperature range (750-840°C vs. 820-840°C) at the segment of 30°C/min heating rate, and (2) a protracted holding time (14min vs. 7min) at the isothermal temperature of 840°C. Five other groups of different heating schedules with lower-targeted temperatures were evaluated to investigate the microstructural changes. For each group, the crystalline phases and morphologies were measured by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. Differential scanning calorimeter (DSC) was used to determine the activation energy of LS2 under non-isothermal conditions. A universal testing machine was used to measure 3-point flexural strength and fracture toughness, and elastic modulus and hardness were measured by a nanoindenter. A one-way ANOVA/Tukey was performed per property (alpha=0.05). RESULTS DSC, XRD, and SEM revealed three distinct microstructures during LS2 crystallization. Significant differences were found between the control group, the two aforementioned experimental groups, and the five lower-targeted-temperature groups per property (p<0.05). The activation energy for lithium disilicate growth was 667 (±29.0)kJ/mol. CONCLUSIONS Groups with the extended temperature range (750-840°C) and protracted holding time (820-840°C H14) produced significantly higher elastic-modulus and hardness properties than the control group but showed similar flexural-strength and fracture-toughness properties with the control group. In general, rapid growth of lithium disilicates occurred only when maximum formation of lithium metasilicates had ended.

Influence of the resin cement thickness on the fatigue failure loads of CAD/CAM feldspathic crowns

OBJECTIVES to evaluate the influence of the occlusal resin cement thickness on the cyclic loads-to-failure of feldspathic crowns and to compare the results to data from monotonic tests. A large range of cement thickness (50μm and 500μm) was tested, in order to better measure the influence of this variable. METHODS Feldspathic ceramic crowns (Vita Mark II blocks, Vita Zahnfabrik) were bonded to dentin analog dies (G10 (NEMA grade G10, International Paper), with occlusal resin cement thicknesses of 50μm and 500μm (Multilink Automix, Ivoclar). The dies were prepared with microchannels for water transport to the cement layer. After 96-h water storage, the specimens (n=20) were submitted to cyclic loads (500,000 cycles at 20Hz; initial maximum load=40% of monotonic load, from previous data) following a staircase sensitivity design (step size=25N). Failure loads at 500,000 cycles were compared to monotonic failure loads (from a previous study with specimens produced by the same author, using the same materials, specimen configuration and cementation protocol). RESULTS Crowns with an occlusal cement layer of 50μm were more resistant than those cemented with 500μm (246.4±22.9N vs. 158.9±22.9N), under wet cyclic testing conditions (p<0.001). The fatigue failure loads were reduced compared to monotonic loads: to 37% of monotonic for 50μm; to 53% of monotonic for 500μm. SIGNIFICANCE An occlusal cement thickness of 50μm was more favorable for the structural performance of feldspathic crowns than was 500μm. Cyclic fatigue reduced failure loads well below those found under monotonic loading.

Contributions of stress corrosion and cyclic fatigue to subcritical crack growth in a dental glass-ceramic

OBJECTIVE The objective of this study was to test the following hypotheses: (1) both cyclic degradation and stress-corrosion mechanisms result in subcritical crack growth (SCG) in a fluorapatite glass-ceramic (IPS e.max ZirPress, Ivoclar-Vivadent) and (2) there is an interactive effect of stress corrosion and cyclic fatigue to accelerate subcritical crack growth. METHODS Rectangular beam specimens were fabricated using the lost-wax process. Two groups of specimens (N=30/group) with polished (15μm) or air-abraded surface were tested under rapid monotonic loading. Additional polished specimens were subjected to cyclic loading at two frequencies, 2Hz (N=44) and 10Hz (N=36), and at various stress amplitudes. All tests were performed using a fully articulated four-point flexure fixture in deionized water at 37°C. The SCG parameters were determined using the ratio of inert strength Weibull modulus to lifetime Weibull modulus. A general log-linear model was fit to the fatigue lifetime data including time to failure, frequency, peak stress, and the product of frequency and logarithm of stress in ALTA PRO software. RESULTS SCG parameters determined were n=21.7 and A=4.99×10(-5) for 2Hz, and n=19.1 and A=7.39×10(-6) for 10Hz. After fitting the general log-linear model to cyclic fatigue data, the coefficients of the frequency term (α1), the stress term (α2), and the interaction term (α3) had estimates and 95% confidence intervals of α1=-3.16 (-15.1, 6.30), α2=-21.2 (-34.9, -9.73), and α3=0.820 (-1.59, 4.02). Only α2 was significantly different from zero. SIGNIFICANCE (1) Cyclic fatigue does not have a significant effect on SCG in the fluorapatite glass-ceramic evaluated and (2) there was no interactive effect between cyclic degradation and stress corrosion for this material.

The effect of endodontic access preparation on the failure load of lithium disilicate glass-ceramic restorations

STATEMENT OF PROBLEM Endodontic access preparation through lithium disilicate ceramic restorations may damage the restoration and compromise its load-bearing capability. PURPOSE The purpose of this in vitro research was to investigate the effect of simulated endodontic access preparation through lithium disilicate glass-ceramic restorations on their load to failure. MATERIAL AND METHODS Sixty lithium disilicate glass-ceramic (IPS e.max CAD) complete-coverage restorations were milled and crystallized. Five coats of die relief were applied internally in the crown to provide a cement space approximately 60 μm in thickness. Composite resin dies were fabricated by backfilling each crown. The specimens were then stored at 37°C and 100% humidity for 30 days. The crowns with their respective dies were divided into 6 groups: Groups M-C, M-ZR, M-SC, and M-CRF were adhesively bonded with a resin cement (Multilink Implant), and Groups F-C and F-ZR were conventionally cemented with zinc phosphate cement (Flecks). After storing all groups for 1 week, Groups M-C and F-C served as the intact controls for the 2 cementation techniques, while Groups M-ZR and F-ZR had an access prepared with a 126 μm grit-size diamond rotary instrument. For Groups M-SC and M-CRF, the endodontic access was prepared with 150 μm and 180 μm grit-size diamond rotary instruments, respectively. Access preparations were restored with composite resin. All specimens were stored at 37°C and 100% humidity for 1 week before they were loaded to failure with a universal loading apparatus (crosshead speed=1mm/min). The results were analyzed with a 1-way ANOVA followed by Tukeys HSD test (α=.05). RESULTS The highest failure loads were achieved with Groups M-C (3316 N ±483) and M-ZR (3464 N ±645) Larger grit rotary instruments resulted in lower failure-loads in Groups M-SC (2915 N ±569) and M-CRF (2354 N ±476). Groups F-C (2242 N ±369) and F-ZR(1998 N ±448) had significantly lower failure loads than their adhesively bonded counterparts (P<.05). The use of 126 μm grit size did not significantly alter the failure loads of the restorations in either cementation technique. CONCLUSIONS Adhesively bonded restorations sustained significantly higher loads to failure than those conventionally cemented. The use of a high efficiency, smaller-grit diamond rotary instrument for endodontic access preparation did not alter the load to failure of lithium disilicate restorations, regardless of the cement used. The use of a larger-grit rotary instrument did not improve the cutting efficiency and reduced the failure load of bonded restorations.

Fatigue loading and R-curve behavior of a dental glass-ceramic with multiple flaw distributions

OBJECTIVES To determine the effects of surface finish and mechanical loading on the rising toughness curve (R-curve) behavior of a fluorapatite glass-ceramic (IPS e.max ZirPress) and to determine a statistical model for fitting fatigue lifetime data with multiple flaw distributions. MATERIALS AND METHODS Rectangular beam specimens were fabricated by pressing. Two groups of specimens (n=30) with polished (15 μm) or air abraded surface were tested under rapid monotonic loading in oil. Additional polished specimens were subjected to cyclic loading at 2 Hz (n=44) and 10 Hz (n=36). All fatigue tests were performed using a fully articulated four-point flexure fixture in 37°C water. Fractography was used to determine the critical flaw size and estimate fracture toughness. To prove the presence of R-curve behavior, non-linear regression was used. Forward stepwise regression was performed to determine the effects on fracture toughness of different variables, such as initial flaw type, critical flaw size, critical flaw eccentricity, cycling frequency, peak load, and number of cycles. Fatigue lifetime data were fit to an exclusive flaw model. RESULTS There was an increase in fracture toughness values with increasing critical flaw size for both loading methods (rapid monotonic loading and fatigue). The values for the fracture toughness ranged from 0.75 to 1.1 MPam(1/2) reaching a plateau at different critical flaw sizes based on loading method. SIGNIFICANCE Cyclic loading had a significant effect on the R-curve behavior. The fatigue lifetime distribution was dependent on the flaw distribution, and it fit well to an exclusive flaw model.

Low-Friction Total Hip Arthroplasties

Total hip arthroplasty (THA) is one of the most frequently performed operations in orthopaedics. Three decades of clinical studies have resulted in a large body of data regarding the survivorship, performance, and revision of THAs. The benefits of compiling these data into a single reference lie in aiding clinicians in decision making and patient education. It is important to know whether the life expectancy of current prosthetic devices may result in device failure at the time when revision surgery may produce a high risk to the patient.