Comparison of Hydroxyapatite-coated stems in total hip arthroplasty after a minimum 10-years follow-up.

PURPOSE OF THE STUDY Hydroxypatite coating (HAC) was introduced into total hip arthroplasty (THA) practice to improve the fixation interface between bone and prosthesis. To test this assumption however, long-term follow-up investigations are needed. In this study, we present data for two consecutive series of THA stems with HAC and a minimum ten-year follow-up. MATERIAL Overall, 249 patients (271 hips) were included in the study, of these 122 (135 hips) had Walter hip arthroplasty (WHA group) with a two-layered TiO2/HAC at the proximal part of the stem and 127 (136 hips) had ABG I prostheses (ABG I group) with a single-layered HAC at the proximal part of the stem. Mean length of follow-up was 11.4 years (0.8-13) and 9.8 years (4-12) in WHA and ABG I groups, respectively. Mean age at the time of surgery was 62 years (23-79) and 47 years (21-65) in WHA and ABG I groups, respectively. METHODS Probabilities of implant survival were estimated using the Kaplan-Meier method. Radiographic data were included to construct the worst-case scenario. Differences in survival curves were evaluated by Gehan's Wilcoxon test. Harris hip score was used to compare preoperative status with that of final follow-up. RESULTS The overall survival of WHA was significantly better than the ABG I (0.85 versus 0.66; p < 0.05). The main reason for a high revision rate in ABG I was periprosthetic osteolysis followed by aseptic loosening. With regard to stems, the survivorship curve for the Walter stem was significantly better than for the ABG I stem even when radiographic results were included (p = 0.0002). In the WHA group, two stems (1.5%) were revised due to sepsis, in contrast to thirty-one stems (23.5%) revised in the ABG I group due to osteolysis and aseptic loosening (p < 0.05). Significant improvement was achieved in both groups under study in terms of Harris hip score. DISCUSSION Data presented here appear surprising at first glance because the differences between the stems under study are only minor. The failure in ABG I was most probably caused by poor polyethylene quality and poor locking mechanism of polyethylene liner in the metallic shell. In addition, HAC used in ABG I prosthesis was not able to prevent the development of polyethylene disease stimulated by high wear rate. CONCLUSION This study revealed excellent survivorship for WHA stems after a minimum ten-year survival and significantly poorer survivorship for the ABG I stems.This may be explained at least particularly by combined two-layered HAC used in WHA stems which provide simultaneously endurable bone interlocking and effective barrier against expansion of polyethylene disease.