Effect of coronal fracture angle on the stability of screw fixation in medial malleolar fractures: A finite element analysis.

Malleolar screw fixation is the most widely used treatment method for medial malleolar (MM) fractures. Here, although buttress plate fixation is advocated for vertical MM fractures, the angular discrimination between oblique and vertical MM fractures is still not fully understood. The purpose of this study is to test the adequacy of screw fixation in MM fractures with different angles and determination of a 'critical fracture angle' to guide surgeons in the decision-making for screw fixation for MM fractures by utilizing an advanced engineering simulation approach. In addition to loading of the healthy tibia structure, various cases of the MM fracture double screw fixation (14 simulation scenarios in total with fracture angles between 30° and 90°, in 5° increments) were considered in this research and their static loading conditions just after fixation operation were simulated through nonlinear (geometric and contact nonlinearity) finite element analysis (FEA). Patient-specific computed tomography scan data, parametric three-dimensional solid modelling and finite element method (FEM) based engineering codes were employed in order to simulate the fixation scenarios. Visual and numerical outputs for the deformation and stress distributions, separation and sliding behaviours of the MM fracture fragments of various screw fixations were clearly exhibited through FEA results. Minimum and maximum separation distances (gap) of 3.75 and 150.34 µm between fracture fragments at fracture angles of 30° and 90° were calculated respectively against minimum and maximum sliding distances of 25.87 and 41.37 µm between fracture fragments at fracture angles of 90° and 35°, respectively. The FEA results revealed that while the separation distance was increasing, the sliding distance was decreasing and there were no distinct differences in sliding distances in the scenarios from fracture angles of 30°-90°. The limitations and errors in a FEA study are inevitable, however, it was interpreted that the FEA scenarios were setup in this study by utilizing acceptable assumptions providing logical outputs under pre-defined boundary conditions. Finally, the fracture healing threshold for separation and/or sliding distance between fracture fragments was assigned as 100 µm by referring to previous literature and it was concluded that the screws fixed perpendicular to the fracture in a MM fracture with more than 70° angle with the tibial plafond results in a significant articular separation (>100 µm) during single-leg stand. Below this critical angle of 70°, two screws provide sufficient fixation.