The human acetabulofemoral joint is commonly modelled as a pure ball-and-socket joint, but there has been no quantitative assessment of this assumption in the literature. Our aim was to test the limits and validity of this hypothesis. We performed experiments on four adult cadavers. Cortical pins, each equipped with a marker cluster, were implanted in the pelvis and the femur. Movements were recorded using stereophotogrammetry while an operator rotated the cadaver’s acetabulofemoral joint, exploiting the widest possible range of movement. The functional consistency of the acetabulofemoral joint as a pure spherical joint was assessed by comparing the magnitude of the translations of the hip joint centre as obtained on cadavers, with the centre of rotation of two metal segments linked through a perfectly spherical hinge. The results showed that the radii of the spheres containing 95% of the positions of the estimated centres of rotation were separated by less than 1 mm for both the acetabulofemoral joint and the mechanical spherical hinge.

Therefore, the acetabulofemoral joint can be modelled as a spherical joint within the considered range of movement (flexion/extension 20° to 70°; abduction/adduction 0° to 45°; internal/external rotation 0° to 30°).

Polyethylene wear in total hip arthroplasty Is associated with generation of particulate wear debris and component failure. Wear has both mechanical and biological consequences with one of the most important of these being the stimulation of immune medicated periprosthetic osteolysis in response to polyethylene particles. It has been shown that the amouont of wear debris generated correlates with the degree of osteolysis encountered. Unfortunately, the assessment of wear of components remains difficult and we wished to apply a new digitised technique of measuring wear using engineering computer softwear on a population of uncemented total hip replacement (THR) patients.

Forty patients having primary uncemented THR (ABG 1 prosthesis) for osteoarthritis were enrolled in the study. Seventeen had a 28mm femoral head implanted and 23 had a 32mm head. There were 28 females and 12 males concerned, all having standardised (120 cm hip to x-ray tube) ;weight bearing antero-posterior plain radiographs of the hip performed in the immediate post-operative period and again at a mean of 6 years post-operatively (range 54 – 96 months). The x-rays were then scanned to computer and analysed using Autocad software. The analysis essentially involved 4 steps, namely assessing sphericity of the cup, sphericity of the head, superimposition of the post-operative and 6 year radiographs an.d obtaining computer generated analysis of both the amount and direction of wear.

The results of our analysis demonstrated that there was an overall mean wear of 0.157mm per year (range 0.08 – 0.27mm). Of the 17 patients with a 28mm head the mean wear was 0.143mm per year, whereas the 32mm heads were associated with a mean wear rate of 0.188mm per year, with the difference reaching statistical significance (p=0.004). Analysis of the direction of wear demonstrated that as expected wear typically occurred in a superolateral direction with a mean vector of 9° lateral to the vertical axis of the hip.

These results primarily demonstrate the usefulness of appropriate computer software in determining wear of components in THR. This allows for assessment of rate and degree of wear which may be important in identifying patients at particular risk of developing significant osteolysis, loosening and ultimately component failure. The results are also consistent with previous reports of increased volumetric wear with large diameter heads and direction of wear in retrieval studies. It is therefore promising as an investigative tool for the in vivo assessment of inovations in THR design in the future.