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the null hypothesis that there would be no influence of each variable or of the interaction among them on the fatigue survival of lithium disilicate and resin nanoceramic monolithic posterior crowns was rejected. for fatigue parameters, ld and rnc crowns presented statistically similar fatigue survival to a 2 million cycles challenge with constant 0-350 n load at 2 hz; presenting no debonding, cohesive or catastrophic failures. carvalho, et al.66- carvalho ao, bruzi g, giannini m, magne p. fatigue resistance of cad/cam complete crowns with a simplified cementation process. j prosthet dent. 2014;111(4):310-7. (2014) investigated the fatigue resistance of 1.5 mm ld and rnc crowns and reported statistically similar failure rates despite applying different fatigue parameters.
in this study, ld crowns had higher flexural strength and fracture toughness than rnc crowns. the flexural strength of ld was similar to or slightly higher than that of rnc. however, rnc was more resistant to crack propagation than ld, resulting in lower fracture toughness. the main factors that influence the flexural strength of ceramic restorations are the ceramic type, the fabrication procedures, and the size and shape of the restoration. therefore, the authors believe that differences in the number of constituent materials and in the filler content may have influenced the flexural strength of the rnc and ld specimens. ld specimens were composed of three different materials;  whereas rnc specimens contained a single material, consisting of 55 wt% of zro2 and 3 wt% of al2o3, with the rest of the ceramic being a glass. the flexural strength of a three-piece ld was found to be lower than that of a one-piece crown. however, the fracture toughness of a three-piece ld was higher than that of a one-piece crown, indicating better resistance to crack propagation. this finding is consistent with the findings of other studies.1010- hwang hy, park s. comparing the fracture toughness of cad/cam and conventional ceramic crowns. j prosthet dent. 2012;106(6):1262-70. (2012) d8a7b2ff72
in this study, rncs were successfully fabricated, using contemporary fabrication techniques. this study was performed in vitro, simulating a clinical loading situation. the results are promising, however, further in vitro studies, evaluating crack initiation sites, are required to further investigate the failure mechanisms.
damage mode distribution was different for rnc and ld crowns, with the former presenting more inner cone cracks, which would eventually result in bulk fracture. rnc crowns showed less resistance to fatigue and were more likely to present cohesive fractures when compared to ld crowns, which presented more outer and inner cone cracks. however, both types of crowns presented catastrophic failure, as well as cohesive fractures. it is worth noting that rnc crowns presented high subsurface damage scores, showing the need for improving the material and manufacturing methods.
scoring was performed according to subsurface damage severity. debonded crowns were considered failures and excluded from subsurface damage analysis. cohesive and catastrophic fractures were scored as failures, as well as radial cracks and inner cone cracks that reached the cementation surface, due to their potential to lead to bulk fracture.
ceramic crowns (cs) used in this study were all lithium disilicate glass ceramic (mean vickers hardness, 50 vhn; mean fracture toughness, 14 mpa·m33- sailer i, makarov na, thoma ds, zwahlen m, pjetursson be. all-ceramic or metal-ceramic tooth-supported fixed dental prostheses (fdps) a systematic review of the survival and complication rates. part i: single crowns (scs). dent mater. 2015;31(6):603-23. by number of cycles to failure) with five different dimensions (4 mm in diameter, 3.5 mm in height, 2 mm in occlusal diameter, and 1 mm in occlusal thickness, and the occlusal surface was polished to 600-grit. cs were anatomically reduced for full crown preparation; composite resin crowns (cr) (e.max cr, ivoclar vivadent; liechtenten, germany) with identical anatomic contours were designed and milled in cerec system (inlab 4.0 and mc xl, sirona dental systems; long island city, ny, usa) with minimum occlusal thickness of 2 mm. lithium disilicate crowns were crystallized and glazed and resin nanoceramic crowns were polished, according to manufacturer's instructions. the study was approved by the ethics committee of the university of lís (ce-caae: 02068912.8.0000.5248) and by the ethics committee of the school of dentistry of university of rondonópolis (ce 061/13). all participants and guardians signed informed consent form and an informed consent was obtained from each patient/guardian prior to study. four millimeters below the cemento-enamel junction (cej), an incision was made in the crown margin and a gingival incision was made with a round bur in a high-speed handpiece. the margins were then finished and polished. a thin layer of mineral trioxide aggregate (mta) was placed over the pulp chamber orifice and the pulp chamber was covered with composite resin (filtek z350, 3m espe; st. paul, mn, usa) for approximately 40 s to ensure adequate adaptation of the cement to the pulp chamber. all crowns were cemented with composite resin cement (panavia f 2.0, kuraray; osaka, japan). cementation was performed after complete setting of the mta. patients were advised to avoid chewing hard foods and to refrain from brushing the restoration for 2 months. after the initial adjustment period, patients were examined every 3 months for 12 months. at each visit, the following clinical parameters were recorded: gingival health (using the gingival index [gi] and plaque index [pi]), soft tissue health (using the modified gingival index [mgi]), loss of occlusal contact points (using a probe), and marginal discoloration. all assessments were performed by the same investigator. patient's satisfaction was also recorded.