Investigation into the effect of varus-valgus orientation on load transfer in the resurfaced femoral head: a multi-femur finite element analysis.

BACKGROUND: Femoral head resurfacing is a popular procedure for younger active hip replacement patients. Whilst the current generation of metal-on-metal resurfacing arthroplasties appear to have cured the osteolysis problems that plagued earlier resurfacing implants, fracture of the femoral head and aseptic loosening are still factors of concern in its survivorship. Several studies have shown a tendency towards failure in resurfaced femurs where the implant has been set at a varus angle. This work aims to investigate the influence of varus-valgus orientation on load transfer within the resurfaced proximal femur. METHODS: This study uses Computer Tomography based finite element analysis to determine the effect of implant orientation on load transfer in the proximal femur with respect to the intact femur. A group of 16 femurs were studied to take into account inter-patient variation; four models were produced for each femur, one of the intact femur, one resurfaced with the implant set inline with the femoral neck and one each representing varus and valgus implant alignment. FINDINGS: Results showed the valgus aligned resurfaced femur to produce strain patterns more akin to the intact femur. As the implant's angle to the femoral shaft increases from varus to valgus the strains in the superior femoral neck are reduced while those in the inferior neck are increased. INTERPRETATION: The study concluded that valgus alignment of the resurfacing arthroplasty is preferential to varus alignment; as it induces a more physiological strain pattern and reduces the risk of femoral neck fracture. These findings are in line with clinical experience, which has shown an increase of failure with varus implanted prostheses.

Investigation into the affect of cementing techniques on load transfer in the resurfaced femoral head: a multi-femur finite element analysis.

BACKGROUND: Femoral head resurfacing is a popular procedure for younger active hip replacement patients. Two major cementing techniques are used in femoral component fixation; producing varying cement mantle thicknesses. Each technique has benefits, but it is not known if cement mantle thickness is detrimental to load transfer within the proximal femur. METHODS: This study uses computer tomography based finite element analysis to determine the effect of cement mantle thickness on strain in the resurfaced femoral head. A group of 16 femurs were investigated to take into account inter-patient variation; a factor often over-looked in such studies. FINDINGS: Results showed strain in the proximal femoral head to decrease with increasing cement thickness, with a slight increase in strain in the posterior-superior neck. Cementing of the implant guide-pin produced marked reductions in strain in the head and neck except for the posterior-inferior neck; where it increased slightly. INTERPRETATION: The study concluded that a thicker cement mantle increases strain shielding within the superior resurfaced femoral head; although short to medium term clinical trials do not suggest these factors to be of much concern. The analysis also found strain to increase around the inferior head-neck junction with a thinner mantle. Cementing the guide pin resulted in increased strain shielding whilst simultaneously shifting peak loads distally along the femoral neck. This work found that modelling a group of femurs was a useful tool for better interpreting the loading in the resurfaced femur.

Determination of suitable sample sizes for multi-patient based finite element studies.

Finite element analysis is used extensively to assess joint replacements, but the majority of these are single sample studies. Recent investigations have suggested that such studies are unable to account for natural inter-patient variation in bone geometry and material property distribution. Recent developments in computer tomography based analyses make multiple sample studies possible; the question remains how many femurs are required to perform a study which accounts for such variations. This work investigates the factors that should be considered in answering this question. It explores sample sizing techniques when comparing strain distribution in the intact and implanted femur and when comparing two or more implant designs in a group of femurs. An example analysis of the effect of femoral head resurfacing was undertaken. Two sample sizing calculations were utilised, one based on achieving the desired precision in results, the other based on determining if a significant difference exists between two designs. The analysis shows that reasonable statistical precision can be achieved with a group of femurs. The study was also able to determine a suitable sample size for the analysis of a statistically significant difference between two groups of femurs with varying design parameters. The study concluded that while sample sizing is recommended for an accurate analysis, consideration must be made for the practicality of such a task.