Nathaniel C. Lawson

The wear of polished and glazed zirconia against enamel

STATEMENT OF PROBLEM The wear of tooth structure opposing anatomically contoured zirconia crowns requires further investigation. PURPOSE The purpose of this in vitro study was to measure the roughness and wear of polished, glazed, and polished then reglazed zirconia against human enamel antagonists and compare the measurements to those of veneering porcelain and natural enamel. MATERIAL AND METHODS Zirconia specimens were divided into polished, glazed, and polished then reglazed groups (n=8). A veneering porcelain (Ceramco3) and enamel were used as controls. The surface roughness of all pretest specimens was measured. Wear testing was performed in the newly designed Alabama wear testing device. The mesiobuccal cusps of extracted molars were standardized and used as antagonists. Three-dimensional (3D) scans of the specimens and antagonists were obtained at baseline and after 200 000 and 400 000 cycles with a profilometer. The baseline scans were superimposed on the posttesting scans to determine volumetric wear. Data were analyzed with a 1-way ANOVA and Tukey Honestly Significant Difference (HSD) post hoc tests (α=.05) RESULTS Surface roughness ranked in order of least rough to roughest was: polished zirconia, glazed zirconia, polished then reglazed zirconia, veneering porcelain, and enamel. For ceramic, there was no measureable loss on polished zirconia, moderate loss on the surface of enamel, and significant loss on glazed and polished then reglazed zirconia. The highest ceramic wear was exhibited by the veneering ceramic. For enamel antagonists, polished zirconia caused the least wear, and enamel caused moderate wear. Glazed and polished then reglazed zirconia showed significant opposing enamel wear, and veneering porcelain demonstrated the most. CONCLUSIONS Within the limitations of the study, polished zirconia is wear-friendly to the opposing tooth. Glazed zirconia causes more material and antagonist wear than polished zirconia. The surface roughness of the zirconia aided in predicting the wear of the opposing dentition.

Wear of enamel opposing zirconia and lithium disilicate after adjustment, polishing and glazing.

OBJECTIVES To compare the wear and opposing enamel wear of adjusted (A); adjusted and polished (AP); and adjusted and glazed (AG) zirconia and lithium disilicate. METHODS Specimens (n=8) were prepared of lithium disilicate (A, AP, and AG), zirconia (A, AP, and AG), veneering porcelain, and enamel (control). Surface roughness was measured for each ceramic. In vitro wear was conducted in the UAB-chewing simulator (10 N vertical load/2mm slide/20 cycles/min) with lubricant (33% glycerin) for 400,000 cycles. Isolated cusps of extracted molars were used as antagonists. Scans of the cusps and ceramics were taken at baseline and 400,000 cycles with a non-contact profilometer and super-imposed to determine wear. Data were analyzed with ANOVA and Tukey-Kramer post hoc tests (alpha=0.05). RESULTS A and AP zirconia showed no detectable signs of wear, and the veneering porcelain demonstrated the most wear. All other ceramics showed significantly less volumetric loss than the veneering porcelain, comparable to enamel-enamel wear. Veneering porcelain produced the most opposing enamel wear (2.15 ± 0.58 mm(3)). AP lithium disilicate and zirconia showed the least amount of enamel wear (0.36 ± 0.09 mm(3) and 0.33 ± 0.11 mm(3) respectively). AG lithium disilicate had statistically similar enamel wear as AP lithium disilicate, but A lithium disilicate had more enamel wear. A and AG zirconia had more enamel wear than AP zirconia. No statistically significant difference was seen between the enamel-enamel group and any other group except the veneering porcelain. CONCLUSIONS Zirconia has less wear than lithium disilicate. Wear of enamel opposing adjusted lithium disilicate and zirconia decreased following polishing. CLINICAL SIGNIFICANCE Zirconia experiences less and lithium disilicate experiences equivalent occlusal wear as natural enamel. It is preferable to polish zirconia and lithium disilicate after adjustment to make them wear compatible with enamel. Veneering of zirconia and lithium disilicate should be avoided in areas of occlusal contact to prevent enamel wear.

Wear, strength, modulus and hardness of CAD/CAM restorative materials

OBJECTIVE To measure the mechanical properties of several CAD/CAM materials, including lithium disilicate (e.max CAD), lithium silicate/zirconia (Celtra Duo), 3 resin composites (Cerasmart, Lava Ultimate, Paradigm MZ100), and a polymer infiltrated ceramic (Enamic). METHODS CAD/CAM blocks were sectioned into 2.5mm×2.5mm×16mm bars for flexural strength and elastic modulus testing and 4mm thick blocks for hardness and wear testing. E.max CAD and half the Celtra Duo specimens were treated in a furnace. Flexural strength specimens (n=10) were tested in a three-point bending fixture. Vickers microhardness (n=2, 5 readings per specimen) was measured with a 1kg load and 15s dwell time. The CAD/CAM materials as well as labial surfaces of human incisors were mounted in the UAB wear device. Cusps of human premolars were mounted as antagonists. Specimens were tested for 400,000 cycles at 20N force, 2mm sliding distance, 1Hz frequency, 24°C, and 33% glycerin lubrication. Volumetric wear and opposing enamel wear were measured with non-contact profilometry. Data were analyzed with 1-way ANOVA and Tukey post-hoc analysis (alpha=0.05). Specimens were observed with SEM. RESULTS Properties were different for each material (p<0.01). E.max CAD and Celtra Duo were generally stronger, stiffer, and harder than the other materials. E.max CAD, Celtra Duo, Enamic, and enamel demonstrated signs of abrasive wear, whereas Cerasmart, Lava Ultimate, Paradigm MZ100 demonstrated signs of fatigue. SIGNIFICANCE Resin composite and resin infiltrated ceramic materials have demonstrated adequate wear resistance for load bearing restorations, however, they will require at least similar material thickness as lithium disilicate restorations due to their strength.

Influence of Etching Protocol and Silane Treatment with a Universal Adhesive on Lithium Disilicate Bond Strength

OBJECTIVES To measure the effects of hydrofluoric acid (HF) etching and silane prior to the application of a universal adhesive on the bond strength between lithium disilicate and a resin. METHODS AND MATERIALS Sixty blocks of lithium disilicate (e.max CAD, Ivoclar Vivadent) were sectioned into coupons and polished. Specimens were divided into six groups (n=10) based on surface pretreatments, as follows: 1) no treatment (control); 2) 5% HF etch for 20 seconds (5HF); 3) 9.5% HF etch for 60 seconds (9.5HF); 4) silane with no HF (S); 5) 5% HF for 20 seconds + silane (5HFS); and 6) 9.5% HF for 60 seconds + silane (9.5HFS). All etching was followed by rinsing, and all silane was applied in one coat for 20 seconds and then dried. The universal adhesive (Scotchbond Universal, 3M ESPE) was applied onto the pretreated ceramic surface, air thinned, and light cured for 10 seconds. A 1.5-mm-diameter plastic tube filled with Z100 composite (3M ESPE) was applied over the bonded ceramic surface and light cured for 20 seconds on all four sides. The specimens were thermocycled for 10,000 cycles (5°C-50°C/15 s dwell time). Specimens were loaded until failure using a universal testing machine at a crosshead speed of 1 mm/min. The peak failure load was used to calculate the shear bond strength. Scanning electron microscopy images were taken of representative e.max specimens from each group. RESULTS A two-way analysis of variance (ANOVA) determined that there were significant differences between HF etching, silane treatment, and the interaction between HF and silane treatment (p<0.01). Silane treatment provided higher shear bond strength regardless of the use or concentration of the HF etchant. Individual one-way ANOVA and Tukey post hoc analyses were performed for each silane group. Shear bond strength values for each etch time were significantly different (p<0.01) and could be divided into significantly different groups based on silane treatment: no silane treatment: 0 HF < 5% HF < 9.5% HF; and RelyX silane treatment: 0 HF < 5% HF and 9.5% HF. CONCLUSIONS Both HF and silane treatment significantly improved the bond strength between resin and lithium disilicate when used with a universal adhesive.

Effect of grain size on the monoclinic transformation, hardness, roughness, and modulus of aged partially stabilized zirconia.

OBJECTIVE Low-temperature-degradation (LTD) has been reported to cause property changes in yttria-tetragonal zirconia polycrystals (Y-TZP). The current study measured monoclinic phase transformation of Y-TZP with different grain sizes and corresponding property changes due to artificial aging. NULL HYPOTHESIS the grain size of aged Y-TZP will not influence its transformation, roughness, hardness or modulus of elasticity. METHODS Four groups of Y-TZP were examined with differing grain sizes (n=5). The line intercept technique was used to determine grain sizes on SEM images (100,000×). Artificial aging was accomplished by autoclaving at 2 bar pressure for 5 h. X-ray diffraction (30 mA, 40 kV) was used to measure tetragonal to monoclinic transformation (t→m). Surface roughness analysis was performed using a non-contact surface-profilometer. Nano-hardness and modulus of elasticity were measured using nano-indentation. RESULTS SEM analyses showed different grain sizes for each sample group (0.350 μm, 0.372 μm, 0.428 μm, and 0.574 μm). The fraction of t→m transformation increased as grain size increased; furthermore, aging of zirconia caused increased roughness. Modulus and hardness after aging displayed no significant correlation or interaction with grain size. SIGNIFICANCE Smaller grains caused less transformation, and aging caused increased roughness, but grain size did not influence the amount of increased surface roughness. Future studies are needed to determine the effects of grain size on the wear and fracture properties of dental zirconia.

Enamel Wear Opposing Polished and Aged Zirconia

Aging of dental zirconia roughens its surface through low temperature degradation. We hypothesized that age-related roughening of zirconia crowns may cause detrimental wear to the enamel of an opposing tooth. To test our hypothesis, we subjected artificially aged zirconia and reference specimens to simulated mastication in a wear device and measured the wear of an opposing enamel cusp. Additionally, the roughness of the pretest surfaces was measured. The zirconia specimens, artificially aged by autoclave, showed no significant increase in roughness compared to the nonaged specimens. Furthermore, no significant difference in material or opposing enamel wear between the aged and nonaged zirconia was seen. All zirconia specimens showed less material and opposing enamel wear than the enamel to enamel control or veneering porcelain specimens. Scanning electron micrographs showed relatively smooth surfaces of aged and nonaged zirconia following wear testing. The micrographs of the veneering ceramic showed sharp fractured edges and fragments of wear debris. Zirconia may be considered a wear-friendly material for restorations opposing enamel, even after simulated aging.

Influence of particle abrasion or hydrofluoric acid etching on lithium disilicate flexural strength

STATEMENT OF PROBLEM Lithium disilicate is a translucent, glass-containing material used for ceramic restorations. Clinicians frequently use alumina abrading or hydrofluoric acid etching to create micromechanical retention in the intaglio surface before bonding a lithium disilicate restoration to the tooth. Few studies have investigated how the etching or abrasion processes affect the flexural strength of lithium disilicate ceramics. PURPOSE The purpose of this study was to measure and compare the flexural strength of e.max CAD after alumina abrasion at differing pressures and acid etching at differing concentrations and times. MATERIAL AND METHODS Bars of e.max CAD (9 groups of 10; 22×2.5×2.5 mm) were prepared, polished sequentially with 180, 320, and 600 abrasive paper, and sintered according to the manufacturers instructions. Four groups were particle abraded (30-μm alumina particles from 10 mm at 55, 100, 200, or 300 kPa for 10 seconds). Four groups were etched with either 5% hydrofluoric acid (20 seconds or 120 seconds) or 9.5% hydrofluoric acid (20 seconds or 120 seconds). The control was polished and fired only (no treatment). Specimens were placed onto an Instron (1 mm/min crosshead speed) and loaded to failure in a 3-point flexural test. One-way ANOVA and the Dunnett t test determined intergroup differences (α=.05). RESULTS Compared with the control, the 100, 200, and 300 kPa alumina abraded groups produced significantly lower flexural strengths (P<.001); however, the flexural strength of the 55 kPa abraded group was not statistically different from the control (P=.080). The flexural strength of the 5% and the 9.5% hydrofluoric acid-etched groups also were not significantly different from the control (P>.050); however, the 9.5% hydrofluoric acid at 20 seconds group was nearly statistically significant (P=.051). CONCLUSION Alumina particle abrasion at pressures of 100 kPa and higher significantly reduced flexural strength by creating stress risers in e.max CAD and should not be used. Hydrofluoric acid etching should be used to increase micromechanical retention and clean the intaglio surface of the restoration before bonding.

Fracture load of ceramic restorations after fatigue loading.

STATEMENT OF PROBLEM A clinician must decide what ceramic coping and veneer material to prescribe based on the amount of tooth reduction possible and the desired esthetic outcome of the restoration. PURPOSE The purpose of this in vitro study was to compare the fracture strength of monolithic and bilayered lithium disilicate (IPS e.max) and zirconia (LAVA) crowns at clinically relevant thicknesses after load cycling. MATERIAL AND METHODS Crowns (n=8) were fabricated from 6 groups: 1.2-mm monolithic lithium disilicate, 1.5-mm monolithic lithium disilicate, 1.5-mm bilayered lithium disilicate with hand-layered veneer, 0.6 mm monolithic zirconia, 1.2-mm bilayered zirconia with hand-layered veneer, and 1.2-mm bilayered zirconia with milled veneer (dimension represents thickness at the occlusal pit). Crowns were cemented to identical milled resin dies with resin-modified glass ionomer cement. Cemented crowns were stored at 37°C for 24 hours and load cycled for 200,000 cycles at 25 N at a rate of 40 cycles/minute. The ultimate fracture load for each specimen was measured in a universal testing machine. Data were analyzed with a 1-way ANOVA and Tukey honest significant difference post hoc analysis (α=.05). RESULTS Mean ±SD fracture load values were 1465 ±330 N for monolithic lithium disilicate (1.2-mm thickness) and 2027 ±365 N (1.5-mm thickness) and 1732 ±315 N for bilayered hand-veneered lithium disilicate (1.5-mm thickness). Fracture loads were 1669 ±311 N for monolithic zirconia crowns (0.6mm thickness), 2625 ±300 N for zirconia milled-veneered (1.2-mm thickness), and 2655 ±590N for zirconia hand-veneered crowns (1.2mm thickness). One-way ANOVA showed a statistically significant difference among the groups (P<.01). Veneered zirconia crowns showed the highest fracture strength, 1.2-mm hand veneered zirconia was similar to that of 1.5-mm monolithic zirconia, and all other groups were not statistically different. CONCLUSIONS Crowns of 1.2-mm bilayered zirconia had higher fracture loads than 0.6-mm zirconia or 1.2-mm lithium disilicate monolithic crowns.

Two-year clinical trial of a universal adhesive in total-etch and self-etch mode in non-carious cervical lesions

OBJECTIVES To compare the clinical performance of Scotchbond™ Universal Adhesive used in self- and total-etch modes and two-bottle Scotchbond™ Multi-purpose Adhesive in total-etch mode for Class 5 non-carious cervical lesions (NCCLs). METHODS 37 adults were recruited with 3 or 6 NCCLs (>1.5mm deep). Teeth were isolated, and a short cervical bevel was prepared. Teeth were restored randomly with Scotchbond Universal total-etch, Scotchbond Universal self-etch or Scotchbond Multi-purpose followed with a composite resin. Restorations were evaluated at baseline, 6, 12 and 24 months for marginal adaptation, marginal discoloration, secondary caries, and sensitivity to cold using modified USPHS Criteria. Patients and evaluators were blinded. Logistic and linear regression models using a generalized estimating equation were applied to evaluate the effects of time and adhesive material on clinical assessment outcomes over the 24 month follow-up period. Kaplan-Meier method was used to compare the retention between adhesive materials. RESULTS Clinical performance of all adhesive materials deteriorated over time for marginal adaptation, and discoloration (p<0.0001). Both Scotchbond Universal self-etch and Scotchbond Multi-purpose materials were more than three times as likely to contribute to less satisfying performance in marginal discoloration over time than Scotchbond Universal total-etch. The retention rates up to 24 months were 87.6%, 94.9% and 100% for Scotchbond Multi-purpose and Scotchbond Universal self-etch and total-etch, respectively. CONCLUSIONS Scotchbond Universal in self- and total- etch modes performed similar to or better than Scotchbond Multipurpose, respectively. CLINICAL SIGNIFICANCE 24 month evaluation of a universal adhesive indicates acceptable clinical performance, particularly in a total-etch mode.

Crown retention and flexural strength of eight provisional cements

STATEMENT OF PROBLEM Few studies have compared the retention of resin-based and zinc-oxide noneugenol provisional cements. Crown retention testing is difficult and variable; therefore, a simpler method of testing by using flexural strength is indicated. PURPOSE The purpose of this study was to measure the retention of base metal alloy castings to dentin provided by 8 provisional cements (3 resin-based and 5 zinc oxide) and correlate the retention to their flexural strength. MATERIAL AND METHODS Flexural strength specimens (2.5 x 2.5 x 22 mm) were made of each cement. The specimens were placed in a 3-point bending testing mode and loaded at 1 mm/min until failure. Ten extracted teeth were milled to a standardized complete crown preparation, fitted with Rexillium III castings, and cemented with each provisional cement. The specimens were subjected to a tensile load (1 mm/min) until failure in a universal testing machine. The flexural strength for each cement was calculated in MPa and correlated to the retention. A 1-way ANOVA and Tukey HSD post hoc analysis were used to determine significant intergroup differences (alpha=.05). Linear regression was used to correlate flexural strength and crown retention (alpha=.05). RESULTS Significant differences were found in the flexural strengths and retention provided by the various cements. Flexure strength was correlated with cement retention for resin-based cements (r=0.998) but not zinc-oxide noneugenol cements (r=0.058). CONCLUSIONS Based on a 20-degree preparation, stronger cements provide increased retention. Therefore, the desired amount of retention should be based on both the cement and a clinical evaluation of the preparation.