The Influence Mechanism of Screw Internal Fixation on the Biomechanics of Lateral Malleolus Oblique Fractures

It remains inconclusive about the stability and optimal fixation scheme of screw internal fixation for lateral malleolus oblique fractures in clinical practice. In this study, the effects of different screw internal fixation methods on the biomechanics of lateral malleolus oblique fractures were investigated. These efforts are expected to lay a theoretical foundation for the selection of internal fixation methods and rehabilitation training regimens in the treatment of lateral malleolus fractures. A healthy ankle joint model and a lateral malleolus fracture internal fixation model were established based on CT data with the aid of some software. Besides, the effects of screw internal fixation modalities on the fracture displacement of fibula fractures, fibula Von Mises stress, and screw Von Mises stress under different physiological conditions and loading conditions were investigated using finite element methods (FEMs) and in vitro physical experiments. The double screw vertical fibular axis internal fixation approach had the lowest fracture displacement of fibula fractures and screw Von Mises stress values; while the double screw vertical fracture line internal fixation approach had the lowest fibula Von Mises stress values. Under different physiological conditions, the magnitude of the peak Von Mises stress of the fibula and screw was ranked as plantarflexion 20° > plantarflexion 10° > neutral position > dorsiflexion 10° > dorsiflexion 20°; and the magnitude of the peak displacement of the fibula fracture breaks was ranked as plantarflexion 20° > plantarflexion 10° > neutral position > dorsiflexion 20° > dorsiflexion 10°. The results of in vitro physical experiments and finite element analyses were in good agreement, which validated the validity of finite element analyses. The vertical fracture line screw implantation method displays a better load-sharing ability; while the vertical fibular axis screw implantation method exhibits a better ability to prevent axial shortening of the fibula and also reduces the risk of screw fatigue damage. Overall, the double screw achieves better therapeutic effects than the single screw. Given that the ankle joint has high stability in the dorsiflexion position, it is recommended to prioritize dorsiflexion rehabilitation training, rather than dorsiflexion and plantarflexion rehabilitation training with too large angles, in the treatment of lateral malleolus fractures.