Biomechanical Efficacy of Three Methods for the Fixation of Posterior Malleolar Fractures: A Three-Dimensional Finite Element Study.

Introduction: We investigated the biomechanical behaviour of different fixations of the tibial posterior malleolus (TPM), simulating distinct situations of involvement of the tibiotalar articular surface (TTAS) through a finite element model (FEM). Material and methods: A 3D computer-aided design model of the left ankle was obtained. The materials used were divided according to their characteristics into ductile and non-ductile, and all materials were assumed to be linear elastic, isotropic, and homogenous. Three different fracture lines of the TPM were defined, with sagittal angles of 10°, 25°, and 45°. For biomechanical comparison, different constructions using a trans-syndesmotic screw (TSS) only (Group T), a one-third tubular plate only with (Group PT) and without (Group PS) a TSS, and a locked compression plate with (Group LCPT) and without (Group LCPS) a TSS were tested. FEM was used to simulate the boundary conditions of vertical loading. Load application regions were selected in the direction of the 700 N Z-axis, 90% on the tibia and 10% on the fibula. Data on the displacement and stress in the FEM were collected, including the total principal maximum (MaxT) and total principal minimum (MinT) for non-ductile materials, total displacement (desT), localized displacement at the fragment (desL), localized displacement at syndesmosis (desS), and Von Mises equivalent stress for ductile materials. The data were analysed using ANOVA and multiple comparison LSD tests were used. Results: For TPM fractures with sagittal angles 10° and 25°, desL in the PT and LCP groups was significantly lower, as well as Von Mises stress in Group LCPT in 10°, and PT and LCPT groups in 25°. For TPM fractures with a sagittal angle of 45°, desL in the LCP group and Von Mises stress in Group LCPS and LCPT were significantly lower. We found that any TPM fracture may indicate instability of the distal tibiofibular syndesmosis, even when the fragment is small. Conclusion: Our study showed that in fragments involving 10% of the TTAS, the use of a TSS is sufficient, but when the involvement is greater than 25% of the TTAS, either a non-locked or locked plate must be used to buttress the TPM. In posterior fragments affecting 45% or more of the TTAS, the use of a locking plate is recommended.