Comparison of periprosthetic bone remodelling after implantation of anatomic and straight stem prostheses in total hip arthroplasty

Abstract

Total hip arthroplasty changes bone loading conditions in the proximal femur and induces adaptive remodelling of the periprosthetic bone. These remodelling processes depend on many implant-specific qualities, e.g. material and elasticity of the stem. The objective of this study was to investigate the effect of the stem design on periprosthetic bone remodelling after insertion of an anatomic stem with proximal fixation and the direct comparison to a straight stem prosthesis. In a prospective study, the changes in periprosthetic bone mineral density (BMD) after implantation of 68 CTX-S anatomic and 22 PPF straight stem prostheses were assessed in the first post-operative year by means of DEXA and zone analysis by Gruen (Clin Orthop 141:17-27, 1979) "Modes of failure" of cemented stem-type femoral components: a radiographic analysis of loosening. Furthermore all patients with CTX-S prostheses were monitored in the second post-operative year. The correlation of adaptive bone remodelling and the systemic bone density was also investigated. In the distal one-third of the straight stem prosthesis, a clearly greater, although not significant, hypertrophy of the periprosthetic bone was observed. No differences in the extent of bone loss between the two prostheses in the regions of interest (ROI) of the proximal bone were observed. The greatest decrease in BMD was registered in the medial femoral neck in both groups. Bone atrophy decreased progressively as the ROI moved distally, ending in a slight increase in BMD in the distal ROI. No significant changes in periprosthetic BMD occurred in the second post-operative year. A strong positive correlation in the regions with the greatest BMD decrease with the systemic BMD was ascertained. After implanting a CTX-S prosthesis, as opposed to PPF prostheses, a different pattern of periprosthetic bone remodelling with a slighter hypertrophy of the distal periprosthetic parts was observed. This implies that the extensive proximal, more physiological bone loading of the anatomic stem as well as the removal of less bone while implanting the stem reduces the negative effects of unphysiological strain distribution and stress shielding. The BMD loss in the medial proximal neck cannot be avoided with this stem design either. The lack of significant BMD changes in the second post-operative year suggests that a stabilisation of bone remodelling processes occurs.