A system for modelling forces on the hip joint in one-legged stance

Is stem length important in uncemented endoprostheses?

In an uncemented total hip replacement where the femoral stem is thinner than the medullary canal it may be hypothesized that the distal portion of the stem has no mechanical function. In this study an uncemented total hip replacement has been modelled mathematically using finite element analysis and mechanical tests of a similar system have been carried out loading implants in cadaveric proximal femora. Two implants have been tested mechanically; one with a full length stem and the second with the stem shortened, but three implants have been modelled with an additional intermediate length stem analysed. Additionally the finite element analysis has been done with a high neck resection and a standard level resection. The finite element model showed that the full length stem produced lower interface contact stress levels under the proximal neck, particularly when the nec was resected, and this was borne out by the mechanical testing.

Effect of interface conditions on the behaviour of a Freeman hip endoprosthesis

The femoral element of a total hip replacement is a composite structure of two, or perhaps three, components--the endoprosthesis, the bone and, where present, the cement. The interfacial conditions are such that complete structural continuity does not necessarily obtain. That this is so has often been suspected due to the observed loosening which can occur in vivo. In modelling the system, typically for finite element analysis, it has usually been considered to be monolithic, such that tensile and shear stresses could be transmitted across the interfaces as well as the normal compressive stress. Here the femoral component of a Freeman hip replacement is considered, implanted without bone cement, and analyses are carried out under monolithic, i.e. fully bonded, and non-bonded assumptions. Simultaneously the effect of retaining the neck of the femur, one of the features of using this particular prosthesis, is also examined.