The influence of mandibular third molar classification on fixation stability of mandibular angle fractures: a biomechanical finite element analysis.

backgroundThe mandibular third molar (MTM) is frequently present in mandibular angle fractures treated with the Champy technique. Clinical research on MTM management has focused predominantly on postoperative complications, and several studies have suggested that retention may confer a stabilizing effect on the fracture - the so-called "splinting effect" proposed by Wolujewicz. However, this stabilizing role has never been quantitatively verified at the biomechanical level, and the geometric feature of the retained tooth responsible for this effect has not been identified. The aim of this theoretical biomechanical study was to test, by means of finite element analysis (FEA), whether a retained MTM improves the mechanical stability of Champy fixation across a range of impaction types, and to identify the key geometric determinant of any such effect.

methodsA mandibular finite element model was constructed from the cone-beam computed tomography data of an adult volunteer. Ten impaction configurations were established by combining three Pell and Gregory depth positions (A, B, C) with four Winter angulations (mesioangular, distoangular, vertical, horizontal). For each configuration, an MTM-retained and an MTM-extracted model were created, yielding 20 models in total. All models shared identical boundary conditions, material properties, and loading (bilateral molar clenching, 230 N). The primary outcome was interfragmentary displacement; secondary outcomes included plate and screw von Mises stress and proximal cortical bone maximum principal strain. Six candidate geometric variables of the retained tooth were measured and related to the stabilizing contribution (the percentage increase in displacement upon extraction) using descriptive correlation analysis.

resultsIn all ten impaction configurations, the MTM-retained model exhibited a smaller interfragmentary displacement than its extracted counterpart. Extraction increased displacement by 68%-172%, and this pattern was corroborated by consistent increases in plate stress (26%-61%), screw stress (23%-55%), and cortical bone strain (10%-47%). Among the geometric variables examined, the trans-fracture cross-sectional area of the tooth - the area of its cross-section at the fracture plane - showed the strongest association with the stabilizing contribution (r = 0.86, ρ = 0.79), whereas volume-based measures, including mesial segment volume (r = 0.14), were substantially weaker. Impaction depth and angulation did not act as independent determinants; configurations spanning all three depth positions followed a common cross-sectional-area trend.

conclusionsUnder the conditions of this finite element model, retention of the MTM was associated with improved biomechanical stability of Champy fixation in every impaction configuration studied. The magnitude of this stabilizing effect was governed primarily by the cross-sectional area of the tooth at the fracture plane rather than by the conventional angulation- or depth-based classifications, which appeared to act only indirectly through the cross-section they produce. These findings are theoretical and derived exclusively from computational simulation; they require corroboration by in vitro testing and prospective clinical studies before informing clinical decision-making.