Objective: Following the advent of strong adhesive bonding, the conservative tooth preparation designs without a retentive form gained increasing popularity. This study aimed to evaluate the marginal adaptation of zirconia-reinforced lithium silicate (ZLS) overlays with different preparation designs. Materials and methods: Forty sound human maxillary first molars were collected and divided into four groups (n = 10) according to their preparation design for the fabrication of ZLS overlays: (O) anatomical occlusal reduction, (OS) anatomical occlusal reduction with rounded shoulder, (OG) anatomical occlusal reduction with a central groove, and (OSG) anatomical occlusal reduction with rounded shoulder and central groove. A video measuring machine (VMM) was used to measure the marginal gap. Statistical analysis was carried out using one-way ANOVA followed by paired t test and Tukey's test (α = .05). Results: The gap size was significantly different between the groups both before (P = .004) and after (P = .008) cementation, and it was significantly smaller in group O than group OSG (P = .002 before and P = .004 after cementation). Marginal gap increased in all groups following cementation (P < .001). Conclusions: Group O showed comparable marginal adaptation with groups OS and OG while group OSG with the most complex design showed the lowest marginal adaptation for ZLS overlays. Clinical Significance: According to the results of this in vitro study, it may be stated that for defect-free teeth requiring occlusal restoration, the occlusal surface preparation alone would suffice to achieve optimal marginal adaptation, and retentive and more invasive preparation designs are not required.
Single fi le rotary systems are frequently used for preparation root canals due to their speed, single use and decreased contamination possibility, however, fi le fracture is a main problem in these systems. This study assessed the effects of different reciprocating angles and rotary movements on the fracture resistance of OneShape instruments. 150 mesiobuccal and mesiolingual canals of mandibular molars or mesiobuccal and distobuccal canals of maxillary molars having 30-60° curvatures were selected and assigned to 3 groups; OneShape fi les were used with rotary movement, reciprocating movement with 30-150°, reciprocating movement with 30-90° until they will fracture. The time required for canal preparation, the canal's curvatures, the mean length of fractured segments and the mean number of prepared canals with one fi le were measured. In rotary movements, in 8 canals (16.0%) fi les were fractured, while in the reciprocating movement with 30-150 degree and 30-90°; in 6 (12.0%) and 5 (10.0%) canals fi les were fractured, respectively. No signifi cant differences were noted regarding the fracture incidence of 3 groups. The mean time required for canal preparation in OneShape instruments with rotary, reciprocating movements with 30-90 degrees and 30-150 degrees were 31.31±15.83, 62.89±48.87 and 46.5±35.54 seconds, respectively. The preparation time with rotary movements were signifi cantly lower than reciprocating movements with 30-90° (P=0.001). No other signifi cant differences were noted in the paired comparisons. Using OneShape instruments had higher fracture resistance by reciprocating movements with different angles, but it was not signifi cant. The preparation time with rotary movements were signifi cantly lower than reciprocating movements with 30-90°.
Aim: To assess the effect of different mechanical surface treatments on flexural strength of repaired denture base. Material and Methods: Sixty bar-shaped specimens of heat-polymerized acrylic resin were fabricated, and divided into six groups (n=10). All specimens, except the positive control group (group PC), were sectioned into halves to create a 1-mm clearance. A negative control group with no surface treatment (group NC) was also considered. Other groups underwent different surface treatments: group Laser; treated with erbium: yttrium-aluminum-garnet (Er:YAG) laser, group APA; airborne-particle abrasion (APA), group APA plus Laser; a combination of laser and APA, and group Bur; bur grinding. After measuring surface roughness (Ra) with a profilometer, all sectioned specimens were repaired by auto-polymerizing acrylic resin, and thermocycled afterward. Three-point bending test was performed by a universal testing machine. Data were statistically analyzed (α=0.05). Results: The mean surface roughness of all experimental groups were significantly higher than that of group NC (p<0.05). The mean flexural strength of all groups was significantly lower than that of group PC (p<0.05). Group B had significantly higher flexural strength than the other surface-treated groups (p<0.05). Group Laser had significantly higher flexural strength than groups APA (p=0.043) and APA plus Laser (p=0.023). No significant difference was found between groups APA and APA plus Laser (p=0.684). Conclusion: All surface treatments increased the surface roughness and flexural strength compared with the untreated group. The highest flexural strength was observed in specimens treated by bur grinding and then laser, however, it was still significantly lower than intact specimens.
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