Fracture resistance and Weibull reliability of lithium disilicate molar endocrowns with deep margin elevation and polyethylene fiber reinforcement: an in vitro study

The biomechanical effect of immediate dentin sealing and deep margin elevation on lithium disilicate endocrowns after thermomechanical aging remains uncertain. This in vitro study compared fracture resistance, failure behavior, and Weibull reliability of lithium disilicate molar endocrowns restored using conventional, deep margin elevation, and polyethylene fiber-reinforced protocols. Forty-two endodontically treated mandibular first molars received standardized mesio-occluso-distal preparations and were distributed among three groups ( n = 14). Group EC received conventional endocrowns with 4-mm pulpal chamber engagement. Group DME-EC received immediate dentin sealing and 2-mm deep margin elevation, with reduced pulpal chamber engagement. Group BIO-EC received immediate dentin sealing, deep margin elevation, and a polyethylene fiber-reinforced composite foundation with no pulpal chamber extension. Lithium disilicate endocrowns were fabricated using a digital workflow and heat pressing. Specimens underwent 10,000 thermocycles and 480,000 loading cycles at 50 N. Fracture loads were measured under compressive loading to failure by a blinded examiner. Fracture loads were compared among groups using Welch analysis of variance. Failure-mode distribution and reparability were evaluated using exact tests, and Weibull reliability was estimated. Mean fracture resistance was highest for Group EC (4009.70 ± 1024.73 N), followed by Group BIO-EC (3336.90 ± 474.18 N) and Group DME-EC (3271.26 ± 734.03 N). Welch analysis of variance did not identify a statistically significant difference among groups (F(2, 23.77) = 2.74, p = 0.085). Failure-mode distribution ( p = 0.777) and reparability ( p = 0.591) did not differ significantly. Group EC had the highest characteristic strength (4402.83 N), whereas Group BIO-EC had the highest Weibull modulus (9.81). Conventional endocrowns showed the highest mean fracture resistance, but the difference among protocols was not statistically significant. Because pulpal chamber engagement differed among protocols, the findings reflect the total effect of each restorative protocol rather than the isolated effect of deep margin elevation or fiber reinforcement. The BIO-EC protocol showed greater fracture-load consistency without higher ultimate strength.