A mechanical and three-dimensional finite element study of the optimum tooth sectioning depth during the extraction of low-level horizontally impacted mandibular third molar

The present study aims to determine the optimum sectioning depth for the extraction of low-level horizontally impacted mandibular third molar (LHIM3M) using mechanical and finite element analysis. One hundred and fifty extracted mandibular third molars were randomly divided into three groups: 1, 2 or 3 mm of tooth tissue was retained at the bottom of the crown. The breaking force of teeth was tested in a universal strength testing machine. The fracture surface was observed and the type of tooth breakage was recorded. According to the three groups, corresponding 3D finite element models were created. The breaking force obtained in the mechanical study was, respectively, applied and the stress and strain of the teeth and surrounding tissues were analysed. Breaking force decreased as sectioning depth increased. The 2 mm group produced the lowest rate of incomplete breakage (10%). In the 2 mm model, the stresses were evenly distributed in the tooth tissue at the bottom of the fissure, and the maximal stress was located in the tissue close to the root segment. The maximum values of stresses in the bone and of strains in the periodontal ligament of the second molar and bone were lower in the 1 mm model than in other models. Their distribution was similar in the three models. A sectioning depth of 1 mm group saves labour during the extraction of LHIM3M, compared to 2 and 3 mm; 2 mm might be the appropriate sectioning depth in terms of breakage shapes.