Introduction: The acetabular cup comprises a 3mm thick bearing surface of UHMPE, a 1.5mm backing of 30% carbon fibre reinforced polybutyleneterephthalate (CFRPBT). Young’s modulus, of CFRPBT layer is similar to natural subchondral bony plate. The cup deforms, when loaded, with the surrounding acetabular bone so that micro-motion at the bone-prosthesis interface is reduced. We measured BMD in the periacetabulum

Method: BMD was analysed for 2 years (n=11 females) with Mean Barthel Index at 2 years 19. Regions of interest were defined according to De-Lee and Charnley (ROI I-III) for the acetabulum. BMD during follow-up was compared with immediate post-operative values. Mean precision error (CV%) was 1.8±0.87%.

Results: By 2 years the mean BMD in HA cup was 0.73gms/cm2 representing a decrease of 7% and 0.78gms/cm2, which representing a 4% decrease with the non-HA cup. With the HA cup at 2 years we measured an increase in ROI I of 3% and a reduction in ROI II and III of 3% and 20% respectively. With the non-HA cup at 2 years we measured an increase in ROI II of 7% and a reduction in ROI I and III of 3% and 16%. There was a significant difference (p< 0.05) in the BMD changes measured in non-weight bearing zone III and one of the weight bearing zones (zone II).

Conclusion: Changes in BMD measured reflect a pattern of maximally reduced stress in zone III followed by zone II and least in the load bearing axis of the acetabulum zone I.