Extended mechanical loads for the analysis of acetabular cages.

To analyse the strength and mechanical behaviour of hip implants, it is essential to employ an appropriate loading model. Generating computational models supplemented with muscle forces is a complicated task, especially in the initial phase of implant development. This research aims to expand the possibilities of the simpler acetabular cage model based on joint loads without significantly increasing the demand for computing resources. A Python script covered and grouped the loads from daily activities. The ten calculated major loads were compared with the maximum of the walking and stair climbing loads through the finite element analyses of a custom-made acetabular cage. Sensitivity analyses were performed for the surrounding bones' elastic modulus and the pelvis boundary conditions. The major loads can geometrically cover the entire load spectrum of daily activities. The effect of many high-magnitude force vectors is uncertain in the approach that uses the most common maximum loads. Using these resultant major loads, a new stress concentration area could be detected on the acetabular cage, besides the stress concentration areas induced by the loads reported in the literature. The qualitative correctness of the results is also supported by a control computed tomography scan: a fracture occurred in an extensive, high-stress zone. The results are not sensitive to changes in the elastic modulus of the surrounding bone and the boundary conditions of the model. The presented load vectors and the algorithm make more extensive static analyses possible with little computational overhead. The proposed method can be used for checking the static strength of similar implants.

Equivalent loads from the life-cycle of acetabular cages in relation to bone-graft transformation.

BACKGROUND AND OBJECTIVES: Bone grafts placed behind acetabular cages change their structure in response to mechanical stimuli. The full consideration of lifestyle loads is extremely resource-intensive, so a method using substitutive loads was proposed to reduce the calculation cost. The aim of the study is to present and prove this method. METHODS: By means of mechanical equations and using the force vectors from the literature which have the same initial point and their relative frequency, while applying a linear model, the average strain energy density distribution for all load cases can be calculated, compiling a matrix from the external loads. From the elements of this matrix, three substitutive load vectors can be calculated, which can be proven to produce the same strain energy density distribution by averaging their effects. The feasibility of using this to model the transformation of bone grafts placed behind acetabular cages is demonstrated with a finite element model, along with a reference calculation. RESULTS: The substitutive load vectors could be calculated in closed form and the simulations showed that they produced a similar density distribution to the reference model with a numerical calculation error range. Accordingly, the density distribution calculated from bone graft transformation is almost the same. CONCLUSIONS: In addition to the aforementioned linearity and the same initial point limitations, the applied method is able to produce the substitutive load vectors with which the calculation of the strain energy density distribution and the bone graft's new density distributions can be carried out faster.