Bone remodeling around the Cambridge cup: a DEXA study of 50 hips over 2 years

In a prospective 2-year study we have used dual-energy X-ray absorptiometry to measure periprosthetic bone mineral density (BMD) following implantation of a novel, "physiological", acetabular component designed using composite materials.

The acetabular components were implanted in hydroxyapatite (HA) and HA-removed options. They were implanted in conjunction with a cemented femoral component in 50 female patients who presented with displaced, subcapital, fractures of the neck of the femur. Regions of interest (ROI) were defined according to De Lee and Charnley. BMD during follow-up was compared with immediate postoperative values for the affected limb.

The mean precision error (CV%) was 1.01%, 2.26% and 1.12%, for ROI I, II and III respectively. The mean change in BMD, for both cups, was analyzed. There was no significant difference between the BMD changes induced with the HA- and non-HA-coated cups.

After an initial fall in BMD in all 3 ROI at 6 months, ROI I and ROI II showed return to baseline BMD by 2 years. ROI III showed no significant decrease in BMD beyond 6 months, but did not return to baseline levels. Statistical analysis revealed no significant decrease in BMD in ROI I and ROI II at 2 years, compared with immediate postoperative values. The changes in BMD reflect a pattern of maximally reduced stress in the non-weight-bearing zone (ROI III), with preservation of bone density in weight bearing zones ROI I and ROI II. These results support the design principles of the Cambridge cup.