Finite element modelling of the mechano-biological interface changes around transfemoral osseointegrated implants and the simulated dynamic properties

This study investigated the ability of non-invasive vibration analysis to monitor the healing progression of implants in transfemoral amputees. This is an advancement of its application in monitoring dental implant stability. Studies of physical femur-implant models have used vibration to detect interface alterations that only represent gross changes at the interface in vivo. This study describes the development of a finite element (FE) model of the femur-implant system to simulate incremental interface changes during osseointegration. The model was used to validate experimental results and further investigate changes in the dynamic properties.
A geometrical model of an amputated femur with an implant in the medullary canal was developed. A cylindrical interface region was created around the implant to simulate the volume of damaged bone due to implant insertion, which subsequently remodels. Appropriate material properties, boundary conditions, and finite element mesh parameters were applied. The dynamic properties of resonant frequency and mode shape were calculated computationally. These parameters were iteratively calculated as changes were made to the material properties of the interface region, simulating bone remodelling.
The femur-implant resonant frequencies calculated when the interface properties were set to minimum and maximum values corresponded well with resonances found in experimental models of similar conditions and validated the use of the model. The FE model identified axial and torsional resonances that were not detected experimentally. The torsional resonance was the most sensitive to interfacial changes.
The axial and torsional resonances were not found experimentally due to the selected experimental methodology. Therefore, the FE model provides additional information about the dynamic behaviour of the femur-implant system. As torsional resonance was the most sensitive to interfacial changes, amending the experimental methodology to enable its detection and expand the feasibility of vibration analysis for this application is a priority.