Taiseer A. Sulaiman

Optical properties and light irradiance of monolithic zirconia at variable thicknesses

OBJECTIVES The aims of this study were to: (1) estimate the effect of polishing on the surface gloss of monolithic zirconia, (2) measure and compare the translucency of monolithic zirconia at variable thicknesses, and (3) determine the effect of zirconia thickness on irradiance and total irradiant energy. METHODS Four monolithic partially stabilized zirconia (PSZ) brands; Prettau® (PRT, Zirkonzahn), Bruxzir® (BRX, Glidewell), Zenostar® (ZEN, Wieland), Katana® (KAT, Noritake), and one fully stabilized zirconia (FSZ); Prettau Anterior® (PRTA, Zirkonzahn) were used to fabricate specimens (n=5/subgroup) with different thicknesses (0.5, 0.7, 1.0, 1.2, 1.5, and 2.0mm). Zirconia core material ICE® Zircon (ICE, Zirkonzahn) was used as a control. Surface gloss and translucency were evaluated using a reflection spectrophotometer. Irradiance and total irradiant energy transmitted through each specimen was quantified using MARC® Resin Calibrator. All specimens were then subjected to a standardized polishing method and the surface gloss, translucency, irradiance, and total irradiant energy measurements were repeated. Statistical analysis was performed using two-way ANOVA and post-hoc Tukeys tests (p<0.05). RESULTS Surface gloss was significantly affected by polishing (p<0.05), regardless of brand and thickness. Translucency values ranged from 5.65 to 20.40 before polishing and 5.10 to 19.95 after polishing. The ranking from least to highest translucent (after polish) was: BRX=ICE=PRT<ZEN<KAT<PRTA (p<0.05). The ranking from least to highest total irradiant energy was: BRX<PRT<ICE=ZEN<KAT=PRTA (p<0.05). There was an inverse relationship between translucency, irradiant energy, and thickness of zirconia and the amount was brand dependent (p<0.05). SIGNIFICANCE Brand selection, thickness, and polishing of monolithic zirconia can affect the ultimate clinical outcome of the optical properties of zirconia restorations. FSZ is relatively more polishable and translucent than PSZ.

Survival rate of lithium disilicate restorations at 4 years: A retrospective study

STATEMENT OF PROBLEM Ceramic restorations are frequently being placed due to the esthetic demand and the cost of noble metals that has increased considerably. One major disadvantage of ceramic restoration is failure of the material due to fracture by crack propagation. In vitro studies are of little clinical significance and in vivo studies are too short to support clinical success. PURPOSE The purpose of this retrospective study was to evaluate the failure rate of lithium disilicate restorations (monolithic and layered) at 4 years. MATERIAL AND METHODS Data were collected over 45 months from 2 commercial laboratories. Restorations were categorized into monolithic restorations and layered restorations. Each category was further classified into complete coverage single crowns, fixed dental prostheses, e.max veneers, and inlay/onlay restorations. Failure rates were compared and analyzed using Chi-square (α=.05). RESULTS A total of 21,340 restorations were evaluated in this study and included 15,802 monolithic restorations and 5538 layered restorations. The failure rate for single crown monolithic restorations was 0.91% and was 1.83% for single crown layered restorations. For fixed dental prostheses, 4.55% of monolithic restorations failed. For e.max veneers, 1.3% of monolithic veneers fractured and 1.53% of layered veneers fractured. Of the inlay/onlay restorations group, 1.01% of monolithic restorations fractured. CONCLUSION In the short term (45 months), restorations fabricated with the lithium disilicate material (IPS e.max) had relatively low fracture rates. Layered single crowns fractured at approximately 2 times the rate of monolithic crowns.

The effect of staining and vacuum sintering on optical and mechanical properties of partially and fully stabilized monolithic zirconia

The effect of staining and vacuum sintering on optical properties and the bi-axial flexural strength of partially and fully stabilized monolithic zirconia (PSZ, FSZ) were evaluated. Disc-shaped specimens divided into three subgroups (n=15): non-stained, stained and non-stained with vacuum sintering. After staining and sintering, optical properties were evaluated using a reflection spectrophotometer and bi-axial flexural strength was tested using the piston-on-three balls technique. Statistical analysis was performed using multivariate analysis of variance (MANOVA) followed by post-hoc Tukeys tests (p<0.05). Staining decreased translucency parameter (TP) values of FSZ (p<0.05). Sintering under vacuum enhanced TP values for PSZ (p<0.05). Staining enhanced surface gloss for both types of zirconia (p<0.05). Staining increased bi-axial flexural strength of FSZ (p<0.05), while it decreased the strength of PSZ (p<0.05). Sintering under vacuum provided minimal benefits with either type of zirconia.

Fracture of layered zirconia restorations at 5 years: A dental laboratory survey

Statement of problem. The use of ceramic restorations has significantly increased in recent years because of their esthetic properties and the high cost of noble metals. However, given the lack of long‐term clinical trials and the limitations of in vitro studies, the durability of ceramic restorations is still questionable. Purpose. The purpose of this retrospective study was to determine the fracture rate of layered zirconia restorations at up to 5 years of clinical performance by using a dental laboratory survey model. Material and methods. Data up to 5 years were collected from 2 commercial dental laboratories. Layered zirconia restorations returned to the laboratory for remaking because of failure (fracture) were identified and included. Restorations were categorized as anterior or posterior. Each category was further divided into complete coverage single crowns (SCs) and multiple unit fixed dental prostheses (FDPs). Fracture rates were compared and analyzed using the chi‐square test (&agr;=.05). Results. A total of 31 594 restoration records were reviewed and included 13 419 anterior restorations (10 134 SCs and 3285 FDPs) and 18 175 posterior restorations (12 810 SCs and 5365 FDPs). The overall fracture rate up to 5 years for all restorations (anterior and posterior) was 3.31%. The fracture rates for anterior restorations were less than for posterior restorations. The fracture rates for anterior SCs were approximately half those of the posterior SC fracture rate, and the combined fracture rate (anterior and posterior) was 3.25%. For FDPs, anterior FDPs fractured at a rate similar to that of posterior FDPs. The combined fracture rate (anterior and posterior FDPs) was 3.47%. Conclusions. Layered zirconia restorations displayed relatively low fracture rates in the relatively short term of 5 years.

Effect of different treatments on the flexural strength of fully versus partially stabilized monolithic zirconia

Statement of problem. Recent monolithic zirconia materials used for indirect restorations are predominantly fully stabilized zirconia with claims of enhanced optical properties. These restorations may behave differently from the conventional partially stabilized zirconia restorations, which may negatively affect some of the core properties required for restoration success. Purpose. The purpose of this in vitro study was to evaluate and compare the effects of staining, airborne‐particle abrasion, and artificial aging on the flexural strength of fully and partially stabilized zirconia material. Material and methods. Each partially stabilized monolithic zirconia (PSZ) and fully stabilized zirconia (FSZ) material and a zirconia core material (control) were prepared as bar‐shaped specimens (2×2×25 mm) and divided into 6 groups (n=8/subgroup): regular sintering, vacuum sintering, stained, airborne‐particle abrasion, artificially aged regular sintering, and artificially aged vacuum sintering. Critical load to fracture was determined for all groups by using monotonic uniaxial loading in accordance with International Organization for Standardization standard 6872. Data were analyzed using univariate analysis of variance, followed by the Tukey honest significant difference post hoc test (&agr;=.05). Results. The control and PSZ (1034 and 1008 MPa) displayed a significantly higher (P<.05) flexural strength than FSZ (582 MPa). Airborne‐particle abrasion significantly (P<.05) enhanced the strength of the control and PSZ (1413 and 1227 MPa) but significantly (P<.05) reduced the flexural strength of the FSZ (442 MPa). Staining, artificial aging, and vacuum sintering had no significant effects on any of the groups. Conclusions. Fully stabilized zirconia may behave differently from conventional PSZ, especially with regard to airborne‐particle abrasion, which may weaken the FSZ. The strength of PSZ is approximately double the strength of FSZ. Both of the zirconia materials showed resistance to artificial aging.

Impact of gastric acidic challenge on surface topography and optical properties of monolithic zirconia

OBJECTIVE To evaluate the surface topography and optical properties of monolithic zirconia after immersion in simulated gastric acid. MATERIALS AND METHODS Four partially stabilized (PSZ) and one fully stabilized (FSZ) zirconia materials were selected for the study: Prettau (PRT, Zirkonzahn), Zenostar (ZEN, Ivoclar), Bruxzir (BRX, Glidewell), Katana (KAT, Noritake) and FSZ Prettau Anterior (PRTA, Zirkonzahn). IPS e.max (Ivoclar) was used as a control. The specimens (10×10×1.2mm, n=5 per material) were cut, sintered, polished and cleaned before immersed in 5ml of simulated gastric acid solution (Hydrochloric acid (HCl) 0.06M, 0.113% solution in deionized distal water, pH 1.2) for 96h in a 37°C incubator. Specimens were weighed and examined for morphological changes under scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDX). Surface roughness was evaluated by a confocal microscope. Surface gloss and translucency parameter (TP) values were determined by a reflection spectrophotometer before and after acid immersion. The data was analyzed by one-way ANOVA followed by Tukeys HSD post hoc test (p<0.05). RESULTS PRTA displayed the most weight loss (1.40%) among the zirconia specimens. IPS e.max showed about three times more weight loss (3.05%) than zirconia specimens as an average. SEM examination indicated areas of degradation, bead-like shapes and smoothening of the polishing scratches after acid immersion. EDX displayed ion interactions and possible ion leaching from all specimens. Sa and Sq values for PRTA, ZEN and IPS e.max were significantly lower (p<0.05) after acid immersion. TP values increased significantly for PRT, ZEN and IPS e.max (p<0.05), while the surface gloss of ZEN, PRTA and IPS e.max increased (p<0.05). SIGNIFICANCE Monolithic zirconia materials show some surface alterations in an acidic environment with minimum effect on their optical properties. Whether a smoother surface is in fact a sign of true corrosion resistance or is purely the result of an evenly progressive corrosive process is yet to be confirmed by further research.

Mechanical properties of resin-based cements with different dispensing and mixing methods

Statement of problem. Resin‐based cements are frequently used in clinical practice. To reduce time and technique sensitivity, manufacturers have introduced the same brand of cement with different dispensing methods. The effect of this change on properties of the cement is unknown. Purpose. The purpose of this in vitro study was to evaluate the mechanical properties of resin‐based cements with different dispensing systems. Material and methods. Specimens of resin‐based cements (n=14) PANAVIA SA Cement Plus Handmix, PANAVIA SA Cement Plus Automix, RelyX Unicem Handmix, RelyX Unicem 2 Automix, G‐CEM Capsule Automix, G‐CEM LinkAce Automix, Variolink II Handmix, and Variolink Esthetic Automix were prepared for each mechanical test. They were examined after thermocycling (n=7/subgroup) for 20 000 cycles as to fracture toughness (FT) (ISO standard 6872; single‐edge V‐notched beam method), compressive strength (CS) (ISO 9917‐1), and diametral tensile strength (DTS). The specimens were mounted and loaded at a crosshead rate of 1 mm/min (0.5 mm/min for FT) with a universal testing machine until failure occurred. The 2‐and 1‐way ANOVA followed by the Tukey HSD post hoc test were used to analyze data for statistical significance (&agr;=05). Results. Thermocycling had a significant effect in reducing the FT property of all resin‐based cements except RelyX Unicem 2 and G‐CEM LinkAce (P<.05). Variolink II and G‐CEM LinkAce showed better FT properties than their automixed counterparts (P<.05). The overall CS of all automixed resin‐based cements was better than that of their hand‐mixed counterpart, except for Variolink II. PANAVIA SA Automixed and G‐CEM LinkAce had higher DTS than their hand‐mixed counterparts (P<.05). Conclusions. Changing the dispensing method alters the mechanical properties of resin‐based cements. The clinical significance of these results is yet to be determined.