INTRODUCTION Veneer chipping is a common failure of zirconia based restorations, especially in the presence of high occlusal loads. Modifying zirconia design may influence veneer chipping. OBJECTIVES: Purpose of this study was to evaluate fracture resistance of complete and partial contour zirconia crowns. MATERIALS AND METHODS: Twenty standardized epoxy resin dies, prepared to receive zirconia based crowns, were divided into 4 groups, 5 specimens each. Zirconia framework designs were: (a) Full contour zirconia crown (FC), (b) Partial contour zirconia coping with buccal veneering (PC), (c) Anatomically reduced zirconia coping with palatal and proximal collar (ARD) and (d) half mm zirconia coping without collar (CON) (control group). Dual scanning technique was done for the test groups to fabricate zirconia copings with the use of standardized resin patterns, while normal scanning was made for the control group. After milling and sintering of zirconia specimens, veneering was done for all desired surfaces. All crowns were adhesively luted to their corresponding dies, and subjected to thermal cycling and mechanical loading corresponding to one year clinical service. Single load to fracture was applied to all the specimens. Type of failure was recorded for each specimen, investigated visually then by using Stereomicroscope and SEM. RESULTS: Complete and partial contour zirconia restorations showed high resistance to fracture. The presence of collar was insignificant for increasing fracture resistance, but had an influence on the fracture extension toward the proximal surface of the veneered restorations.

Background. The purpose of this clinical trial was to assess soft tissue reaction around custom dental implant abutments by evaluation of Matrix Metalloproteinase-8 (MMP-8) level in peri-implant sulcular fluid (PISF) and clinical parameters. Methods. Thirty participants with missing mandibular first molar were enrolled in this randomized controlled trial. Each participant underwent implant placement following the standard 2-stage surgical protocol. All participants were randomly allocated to one of the two groups; Control Group (n=15): Pre-machined stock titanium abutment, and Study Group (n=15): Custom titanium CAD-CAM abutment. Combined screw- and cement-retained zirconia crowns were used to restore implants. The PISF samples were collected at 1, 3, 6 and 12 months after placement of restorations. MMP-8 level was assessed by Enzyme-Linked ImmunoSorbent Assay. Peri-implant probing depth (PPD), simplified mucosal index (SMI) and modified plaque index (mPI) scores were assessed at all time-points. Crestal bone loss (CBL) was assessed by using bitewing radiographs made at baseline and 12-month time points. Two-way repeated-measures ANOVA and Friedman test were used to analyze data. Results. There was a significant effect of the interaction of time and abutment type where the change in MMP-8 and PPD in CA was higher than in SA. SMI, mPI and CBL recorded no statistical differences between the two groups after one year (P > 0.05). Conclusions. The change in the level of MMP-8 and in PPD after one year is significantly higher in custom abutments. However, this change was of low clinical impact. Trial Registery: the trial was registered in the Pan African Clinical Trial Registry under identification number PACTR201809736563842 (25/ 09/ 2018)

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Background Mechanical surface treatments can deteriorate the mechanical properties of zirconia. This study evaluated and compared the biaxial flexural strength, fracture toughness, and fatigue resistance of high translucency (HT) to low translucency (LT) zirconia after various mechanical surface treatments. Methods Four hundred eighty zirconia discs were prepared by milling and sintering two HT (Katana and BruxZir) and LT (Cercon and Lava) zirconia blocks at targeted dimensions of 12 mm diameter × 1.2 mm thickness. Sintered zirconia discs received one of the following surface treatments: low-pressure airborne particle abrasion (APA) using 50 µm alumina particles, grinding using 400 grit silicon carbide paper, while as-sintered specimens served as control. Internal structure and surface roughness were evaluated by scanning electron microscope (SEM) and a non-contact laser profilometer, respectively. Half of the discs were tested for initial biaxial flexural strength, while the rest was subjected to 106 cyclic fatigue loadings, followed by measuring the residual biaxial flexural strength. Fractured surfaces were examined for critical size defects (c) using SEM to calculate the fracture toughness (KIC). The effect of surface treatments, zirconia type, and cyclic fatigue on the biaxial flexural strength was statistically analyzed using three-way analysis of variance (ANOVA) and Tukey HSD post hoc tests (α = 0.05). Weibull analysis was done to evaluate the reliability of the flexural strength for different materials. Results The initial biaxial flexural strength of LT zirconia was significantly higher (p < 0.001) than that of HT zirconia in all groups. While low APA significantly increased the biaxial flexural strength of LT zirconia, no significant change was observed for HT zirconia except for Katana. Surface grinding and cyclic fatigue significantly reduced the flexural strength of all groups. High translucency zirconia reported higher fracture toughness, yet with lower Weibull moduli, compared to LT zirconia. Conclusion LT zirconia has higher biaxial flexural strength, yet with lower fracture toughness and fatigue resistance, compared to HT zirconia. Low-pressure APA has significantly increased the biaxial flexural strength in all zirconia groups except BruxZir. Grinding was deteriorating to biaxial flexural strength and fracture toughness in all zirconia types. Cyclic fatigue has significantly decreased the biaxial flexural strength and reliability of HT and LT zirconia.