Computational Biomechanics of the Hip Joint

This chapter introduces the hip joint and hip disorders that may lead to total replacement of the joint. The internal morphology of the proximal femur, which can be categorised into one of three types, is of primary importance as it will affect the type of implant most suitable for the treatment. Osteoarthritis is one of the most common reasons for hip replacement where in certain cases can severely limit patient mobility and reduce quality of life. Implants for total hip arthroplasty (THA) can be categorised into one of two types, cemented and cementless. Whilst cemented has been regarded as the gold standard, the cementless coutnerpart is gaining popularity. However, the issue of primary stability has to be addressed and tackled if the cementless approach was to be widely accepted. Finite element method will be used to analyse the stability of the cementless implants for hip arthroplasty.

This chapter explains the methodology to simulate total hip arthroplasty. Three-dimensional (3D) model of a femur was created from the Visible Human Project computed tomography dataset. Four different models of cementless hip stem were constructed from various file formats. Triangular surface mesh was manually repaired to ensure good finite element model for analyses. The implant surface mesh was then aligned in the femoral canal and the complete arthroplasty model was then converted into solid tetrahedrals. The implant was assigned with linear isotropic properties and the bone was assigned based on their greyscale values. Loads simulating the gait cycle and stair-climbing were used for the simulation. An algorithm to calculate implant-bone relative motion was developed to analyse the interface micromotion. A convergence study was performed and the micromotion algorithm verified.

This chapter analysed the effect of implant design on primary stability. Three different categories were formed from a collection of cementless hip stem designs—straight cylindrical, taper and anatomical. A representative of each category was analysed where similar stress distributions and magnitudes were obtained under the simulated physiological loadings. The three most common materials used for implant—cobalt chromium alloys, titanium alloys and isoelastic composite—were also analysed. The isoelastic stem, though touted as mechano compatible due to its similarity with bone properties, produced a ten-fold increase in relative micromotion. In anticipation of the use of short stem in conservative approach, the finite element analysis showed that a very short stem that covers the proximal region compromised the primary stability. A comparison was also made between the proximal and distal fixation stem where the proximal design was found to be unstable under the gait and stair-climbing activities.

This chapter analysed two of the most important parameters affecting micromotion and therefore stability of hip implant. It has been accepted that interfacial gaps exist when broaching is used to prepare the bone bed for implantation. An experiment was carried out to quantify the gaps and analysis was performed to check on primary stability. No significant differences were observed between those with gaps and those without gaps. Other analyses were performed interfacial gaps set at potential locations of a particular implant. Similar results were observed where no significant differences were found in terms of primary stability. Undersizing and malalignment is another important error during surgery and is more prominent in young surgeons. The results show that undersizing and malalignment is more problematic for straight cylindrical stem compared to tapered stem. The effect of bone properties were also analysed where an extra care should be given to osteoporotic bone as the thicker implant diameter may not only caused potential fracture but also produced higher micromotion.