FACTORS AFFECTING BONE MINERAL DENSITY CHANGES AFTER SURFACE REPLACEMENT OF THE HIP: A DEXA STUDY

Abstract

Surface replacement is widely used as a treatment for younger patients requiring hip replacement. However the long-term performance of such devices remains unknown. One area of concern is the viability of the bone in the proximal portion of the femur. These concerns are related to the trauma which the proximal femur is exposed to during the operative procedure and the level of vascularity which is subsequently attained. Previous studies have used a single time point and 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 to report the changes in bone mineral density (BMD) which take place at different time points in the same patients following implantation using DEXA scanning. The effect of component placement and metal ion levels will also be considered

A total of twenty-six patients were recruited (18 male, 5 female; 15 left hip 11 right, mean age at surgery 56 years, range 31–69) who had DEXA measurements at all three time points post-op, 120 and 420 days (4 and 14 months). Measurements were taken in the neck region and Gruen zones 6 and 7. Metal ion levels were measured in whole blood using the high resolution ICP-MS technique.

The BMD in the neck region, zone 7 and 6 post-operatively were 0.945, 1.092 and 1.451g/cm2 respectively. In the neck region BMD reduces at 120 days (96.5%) and then increases to higher than the post-op level at 420 days (103.6%, differences between all three groups: p< 0.008).

In zone 7, BMD drops at 120 days (98.2%, p=0.03) but recovers to higher than post-op levels after 420 days (101.65%)but not significantly so (p=0.13). In zone 6 there are no significant differences at any ime points.

Despite the wide variation in the immediate postoperative bone density (0.70–1.25g/cm2), there is no obvious relationship between this value and the capacity of the bone to recover from the operative trauma

Twenty-three of these patients were also part of a metal ion study previously reported. There was no relationship between the combined metal ion levels (chromium+cobalt) at 12 months and the changes in BMD in the neck region at 4 or 14 months.

There was no significant difference in the response of the male and female patients. There was no significant relationship between changes in BMD and cup abduction angles, femoral component inclination nor acetabular component diameter.

This is a short-term study, however it is reassuring that whilst BMD reduces in the neck and Gruen zone 7, by 420 days (14 months) it has recovered to postop levels and in zone 7 has exceeded the post-op levels. Furthermore this response appears to be unrelated to patient factors and component position.