Effects Of Three Different Residual Root Treatments On Fracture Resistance Of Single Rooted Teeth After Post-and-core Restoration

Objective: To evaluate the effects of 3 different residual root treatments(inner shoulder retention form,ISRF),crown lengthening procedure(CLP)and orthodontic eruption procedure(OEP)on the fracture resistance of the mandibular second premolar after post and core restoration in the laboratory experiment.Methods: Thirty-two mandibular second premolars extracted for orthodontic reasons were screened and then randomly divided into 4 groups(n = 8).The 2-mm ferrule restoration group(NPR)served as a control group,and the remaining 3 groups(group1,preparation of 0.5-mm × 0.5-mm [depth × width] ISRF [ISRF];group 2,2-mm CLP [CLP];and group 3,2-mm OEP [OEP])served as experimental groups.In each group,after the coronectomy,root canal treatment(RCT),and embedding and restoration of teeth,the specimens were forced on a universal tester by a crosshead speed of 0.1 mm per minute at 30° to the long axis of the tooth until fracture.The fracture resistance and pattern were statistically analyzed.Results: The mean fracture resistance in Newtons ± SD was 796.23 ± 155.61 in group NPR,650.32 ± 150.43 in group ISRF,385.38 ± 149.92 in group CLP,and542.93 ± 79.34 in group OEP,and its differences among 4 groups were statistically significant(F = 12.724;P<.001).Fracture resistance was statistically significant highest in NPR2 and decreased in the other groups in residual root restoration,fracture resistance was statistically significant higher in ISRF than in CLP2 and greater in OEP2 than in CLP2 but influenced by the crown-root ratio when the residual roots had the same length,no significant difference was found between ISRF and OEP2.The main fracture pattern was decementation starting from the lingual crown margin and oblique fracture of the root in the restoration groups with ferrules,as well as post and core decementation and oblique fracture starting from the middle part of proximal surface in the inner shoulder groups without ferrules.Conclusions: The tooth with crown dentine ferrule presence showed statistically significant higher fracture resistance.For residual roots,inner shoulder retention form and orthodontic eruption are 2 feasible treatments and meet the biomechanicaldemand of post and core prostheses,but crown lengthening procedure showed statistically significant lower fracture resistance.