Finite Element Study of Implant Subsidence and Medial Tilt in Agility Ankle Replacement.

BACKGROUND Clinical studies indicate that in total ankle arthroplasty, postoperative implant subsidence and medial tilt become two significant concerns of the ankle replacement system, and which are associated with the contact between the bones and the talar component. Up to now, little attention has focused on the contact between the bones and the talar component. MATERIAL AND METHODS In order to address implant subsidence and medial tilt, one three-dimensional finite element model of contact between the bone and the talar components was built with the material properties of the cancellous bone interpolated from the experimental data, which represents variation of material properties through the cancellous bones. The finite element model was used to study the following: variation of the Young's modulus of the bones, stiffness of the talar component, loading direction, and loading magnitude with the implant subsidence. RESULTS The computational results reveal that a variety of Young's modulus of the cancellous bones causes the medial tilting of the talar component and that big plastic strains are associated with tilting. The implant subsidence increases from 0.169 mm to 0.269 mm when the loading changes from 272 kg to 408 kg. However, to the contrary, the implant subsidence decreases from 0.2676 mm to 0.1783 mm when Young's modulus of the bones increases 50%. However, the implant subsidence shows little change with a different Young's modulus of the talar component from 88 GPa to 132 GPa. CONCLUSIONS Our study indicates that a variety of different Young's modulus of the cancellous bones cause the medial tilting of the talar component. To solve subsidence and tilting, both the contact area and the variation of material properties should be taken into account.