Effects of 3 different residual root treatments after post-and-core restoration: An in vitro fracture resistance experiment and finite element analysis

Statement of problem Crown-lengthening procedures (CLPs) and orthodontic eruption procedures (OEPs) improve the biomechanical properties of residual root restoration. However, their use is limited by clinic time, cost, and crown-root ratio. An inner shoulder retention form (ISRF) overcomes these limits; however, whether ISRF meets the biomechanical requirements is unclear. Purpose The purpose of this in vitro and finite element analysis (FEA) study was to evaluate the effects of 3 residual root treatments (ISRF, CLP, and OEP) on premolar fracture resistance and stress distribution after post-and-core restoration. Material and methods Thirty-two extracted mandibular second premolars were screened and randomly divided into 4 groups (n=8): a 2-mm ferrule restoration group (NPR2; control group) and 3 experimental groups (0.5×0.5-mm ISRF [ISRF0.5], 2-mm CLP [CLP2], and 2-mm OEP [OEP2]). After removal of the crown, endodontic treatment, and embedding and restoration, the specimens were loaded on a universal tester (crosshead speed of 0.1 mm per minute) at 30 degrees to the long axis of the tooth until fracture. Fracture resistance and pattern were analyzed. Four groups were added in FEA: 1-mm ferrule restoration group (NPR1), 1.0×1.0-mm ISRF (ISRF1), 1-mm CLP (CLP1), and 1-mm OEP (OEP1). The maximum principal stress peak (σmax), maximum displacement, and stress distribution were evaluated. Results Mean fracture load ±standard deviation was 796.23 ±155.61 N (NPR2), 650.32 ±150.43 N (ISRF0.5), 385.38 ±149.92 N (CLP2), and 542.93 ±79.34 N (OEP2); these differences were statistically significant (F=12.724; P<.001). The main fracture patterns were decementation from the lingual crown margin and oblique root fracture in ferrule restoration groups and post-and-core decementation and oblique fracture starting from the middle proximal surface in the ISRF groups without ferrules. In FEA, the results of NPR2, ISRF0.5, CLP2, and OEP2 were consistent with the in vitro experiments and provided stress distribution and displacement information. Conclusions Teeth with crown dentin ferrules showed higher fracture resistance and lower stress concentration. For residual roots, ISRF and OEP showed no differences, but CLP had lower fracture resistance and higher stress concentration.