Comprehensive biomechanical analysis of three clinically used fixation constructs for posterior malleolar fractures using cadaveric and finite element analysis

Different fixation modalities are available for fixation of posterior malleolar fractures (PMFs), but the best method is still unclear. The purpose of this study was to carry out a comparative biomechanical analysis of three commonly used fixation constructs for PMFs using experimental and finite element analysis (FEA). 15 human cadaveric ankle specimens were randomly divided into three groups. Specimens in group-A were fixed with two anteroposterior (AP) lag screws, group-B with two posteroanterior (PA) lag screws, and for group-C, a posterior plate was used. Each model was subjected to axial load. Outcomes included loads for 0.5 mm, 1 mm, 1.5 mm, and 2 mm vertical displacements of posterior fragments were noted. 3D FE models were reconstructed from computed tomography (CT) images and subjected to vertical loads. The model's stress, fracture step-off, and resultant strains in implants were also studied in 3D FE models. Significantly higher amounts of mean compressive loads were observed to cause the same amount of vertical displacements in plate group (265 +/- 60.21 N, 796 +/- 57.27 N, 901.18 +/- 8.88 N, 977.26 +/- 13.04 N) than AP (102.7 +/- 16.78 N, 169.5 +/- 19.91 N, 225.32 +/- 15.92 N, 269.32 +/- 17.29 N) and PA (199.88 +/- 31.43 N, 362.80 +/- 28.46 N, 431.3 +/- 28.12 N, 541.86 +/- 36.05 N) lag screws respectively (P<0.05). Simulated micro-motion analysis demonstrated that fracture step-off values in plate group (0.03<plus/minus>0.001 mm, 0.06 +/- 0.003 mm and 0.13 +/- 0.010 mm) were the lowest among the three groups (P<0.001). The cancellous bone showed the highest amount of stress in AP and PA lag groups respectively, whereas the lowest stress was noted in the plate-group. This biomechanical study concluded that posterior plating is biomechanically the most stable fixation construct for PMFs fixation. AP and PA lag screws with higher bone stress and fracture step-off values have a high tendency of bone cut-through and loss of fixation respectively.