There was a substantial lack of consensus relating to marginal adaptation of various crown systems due to differences in testing methods and experimental protocols employed. Direct view technique was the most commonly used method of reproducible results. Also, conducting an experimental set-up of testing a minimum of 30 specimens at 50 measurements per specimen should produce reliable results. Additionally, using a combination of two measurement methods can be useful in verification of results.

The studies reviewed showed a level of heterogeneity, as testing parameters were considered through different setups. The current study demonstrated that various setting of the testing parameters and having a lack of testing standardization has likely led to inconsistency in the reported results. The obvious heterogeneity in the setting of testing variables-especially the magnitude of load and number of cycles applied-made it impractical to run direct comparisons between the reviewed studies. Therefore, specific international standardization of fatigue testing of dental restorations is urgently needed to ensure the delivery of consistent, indicative, and comparable data.

PURPOSEThe aim of this in vitro study was to investigate the fracture resistance under chewing simulation of implant-supported posterior restorations (crowns cemented to hybrid-abutments) made of different all-ceramic materials.MATERIALS AND METHODSMonolithic zirconia (MZr) and monolithic lithium disilicate (MLD) crowns for mandibular first molar were fabricated using computer-aided design/computer-aided manufacturing technology and then cemented to zirconia hybrid-abutments (Ti-based). Each group was divided into two subgroups (n=10): (A) control group, crowns were subjected to single load to fracture; (B) test group, crowns underwent chewing simulation using multiple loads for 1.2 million cycles at 1.2 Hz with simultaneous thermocycling between 5℃ and 55℃. Data was statistically analyzed with one-way ANOVA and a Post-Hoc test.RESULTSAll tested crowns survived chewing simulation resulting in 100% survival rate. However, wear facets were observed on all the crowns at the occlusal contact point. Fracture load of monolithic lithium disilicate crowns was statistically significantly lower than that of monolithic zirconia crowns. Also, fracture load was significantly reduced in both of the all-ceramic materials after exposure to chewing simulation and thermocycling. Crowns of all test groups exhibited cohesive fracture within the monolithic crown structure only, and no abutment fractures or screw loosening were observed.CONCLUSIONWhen supported by implants, monolithic zirconia restorations cemented to hybrid abutments withstand masticatory forces. Also, fatigue loading accompanied by simultaneous thermocycling significantly reduces the strength of both of the all-ceramic materials. Moreover, further research is needed to define potentials, limits, and long-term serviceability of the materials and hybrid abutments.

Purpose: To investigate the effect of core/veneer thickness ratio on the fracture strength of lithium disilicate crowns subjected to cyclic loading in a simulated oral environment. Materials and Methods: A typodont molar tooth received a standard complete crown preparation according to the manufacturer's instructions. Sixty lithium disilicate crowns were prepared and assigned to three groups with the following core/veneer thickness ratios A: 0.7 (0.6/0.9), B: 1.1 (0.8/0.7), and C: 2 (1.0/0.5). The cores were milled from lithium disilicate e.max CAD blocks and hand-layered using e.max Ceram. Ten specimens from each group acted as control, while the remaining ten specimens were subjected to thermal mechanical loading in a chewing simulator. All specimens were then subjected to the single load to fracture test at 1 mm/min crosshead speed. Data were analyzed using two-way ANOVA, Tukey multiple comparison test, Pearson correlation test, and quadratic regression (p < 0.05).