BONE MINERAL DENSITY CHANGES AFTER SURFACE REPLACEMENT OF THE HIP: A FIVE YEAR DEXA STUDY

Abstract

The long-term performance of surface replacement devices remains unknown. One area of concern is the viability of the bone in the proximal portion of the femur. Previous studies, using a single time point, have shown reduced loss in bone stock compared to total hip replacement and minimal difference with the contra-lateral, unaffected hip. The aim of this study is use DEXA scanning to report the changes in bone mineral density (BMD) which take place at different time points up to 5 years post-op in the same patients following implantation. The effect of component placement will also be considered

Thirty patients were recruited (24 male, 6 female; 16 left hip 14 right, mean age at surgery 53 years, range 28–65). DEXA measurements were taken post- operatively and at 3, 12, 24 and 60 months. During the study 2 patients were revised and 8 were lost to follow-up. Thus the number of patients available at each follow-up were 28, 29, 23 and 20 respectively. The regions of interest were R1 (neck region), R2 (Gruen zones 2,6) and R3 (Gruen zones 3,5).

BMD in zones R1, R2 and R3 post-operatively were significantly different, 0.955, 1.114 and 1.457g/cm2 respectively (p< 0.0001). In the R1, BMD reduced at 3m to 95.0% (p=0.005) and then recovered to higher than the post-op level 102.2% (p=0.241) by 12m, and further increases to 103.5% (p=0.019) at 24m and 103.9% (p=0.057) at 60m.

In zone R2, BMD reduced at 3m (97.4%, p=0.02) but recovered to post-op levels after 12m and is maintained thereafter. In zone R3 there were no significant differences from post-op. In zone R1 at 3m, 20/28 cases (71%) had a BMD that was less than the immediate post-operative value. At 12m only 12/29 cases (41%) had reduced BMD, the balance (59%) undergoing an increase. The comparable values at 60m follow-up were 43% and 57%. There was a trend for patients with higher post-op BMD to undergo a greater reduction at 3m whilst showing a greater level of recovery after 60m. However patients with higher post-op BMD had the highest 3m and 60m values. There was a trend for older patients to have a lower post-op BMD although this was not translated into greater reductions in BMD. There was no obvious correlation between femoral component angle and BMD. However there was a trend for components with a higher cup angle to undergo a greater reduction in BMD at both 3m and 60m. The current cohort was dominated by male patients and therefore comparison by gender was not possible.

Changes in BMD were confined to the neck region (R1) and Gruen zones 2, 6 (R2). The finding that BMD reduces in R1 at 3m but by 12m has recovered to postop levels in R2 and in R1 has exceeded post-op levels, strongly suggests that whatever inter-operative trauma takes places is quickly repaired and changes beyond 12m are minimal out to 60m.