The effect of stress distribution and displacement of open subtalar dislocation in using titanium alloy and stainless steel mitkovic external fixator – a finite element analysis

Abstract

An external fixator is normally used by medical surgeons in treating subtalar dislocation due to its biomechanical characteristics that can providing an adequate stability, preventing deformity (mal-union and non-union), reduce rate of infections, and promoting fast healing process as compared to coventional internal fixator. Apart from the configurations and fixation techniques, previous studies has mentioned that the stability of external fixator can be altered and manipulated by using different materials, e.g. stainless steel, titanium alloy and polymer. To be noted, the current available research works that have been investigated on different materials of external fixator are still lacking, therefore, the present study is aims to conduct related study. The main objective of the research work is to simulate finite element model of foot and ankle joint associated with open subtalar dislocation in which treated with Mitkovic external fixator by using two different material properties; titanium alloy (Model 1) and stainless steel (Model 2). The three-dimensional model of foot and ankle joint were reconstructed using images of CT dataset. For the soft tissues, cartilages at the ankle joint were developed by offsetting the bone surfaces with 1 mm thickness while ligaments were modelled with linear links. Homogeneous and isotropic properties were assigned to the bone, Mooney-Rivlin model for cartilage and specific stiffness value for ligaments. In order to simulate stance phase during walking condition, an axial load of 350 N was applied to the proximal tibia bone. The results of von Mises stress demonstrated that Model 2 has a low magnitude (127 MPa) at the pin-bone interface of tibia bone, compared with Model 1 (369 MPa). As for the local displacement at the bony segment of fibula, Model 2 (3.3 mm) indicated high stability of the external fixator than Model 1 (7.4 mm). In conclusion, the use of stainless steel material for Mitkovic external fixator can provide adequate stability and optimum stress distribution.