Cementless cups do not increase osteolysis risk in metal-on-metal total hip arthroplasty. Apropos of 106 cases.

BACKGROUND: Metal-on-metal bearings in total hip arthroplasty (THA) were introduced to reduce the production of wear debris and debris-induced periprosthetic osteolysis. Analysis of various series according to the type of selected acetabular fixation highlights different evolutions: favourable results with uncemented cups contrasting with loosening and radiolucent lines (RLL) evolution for cemented cups.

HYPOTHESIS: Combining metal-on-metal bearings to uncemented cups does not increase the osteolysis risk at a minimum 5 years' follow-up.

MATERIALS AND METHODS: From January 1999 to December 2002, 106 Metasul THAs were implanted in 95 patients using a Hardinge anterolateral approach (40 women and 55 men with an average age of 59.2 years). The cups were of cementless, hydroxyapatite-coated Cedior type (Zimmer) housing a Metasul insert in a polyethylene sandwich. The femoral stem used was the cemented Acora, then the Exafit (Zimmer) type with Metasul 28-mm head mounted on a 8/10 Morse taper. Patients were evaluated clinically using the Postel-Merle-d'Aubigné (PMA) scoring system and radiologically using various markers: cup inclination angle, eventual RLL presence, appearance of osteolysis images, ectopic ossifications and finally, eventual implant migration. In the eventuality of suspected RLL evolution or osteolysis, advanced imaging was performed; joint aspiration liquid and cobalt serum level were studied.

RESULTS: We reviewed 94 prostheses (85 patients) with an average follow-up of 6.4 years (4.3 to 9.3 years, median of 6.3 years). The rate of patient loss from follow-up and death was 12.1%. The PMA score of non-revised patients increased from 11.4+/-1.5 to 17.6+/-0.2 at follow-up. We numbered ten re-operations not attributable to the type of bearings used. Three revisions were directly related to the metal-on-metal bearing: two metallosis due to impingement and one case of hypersensitivity. Cup inclination angle was 45.7+/-5.49 degrees . No implant migration was noted during the follow-up duration. Only one cup undergoing revision presented a substantial osteolysis. On the femoral side, with non-revised implants, we observed 13 cement/bone RLL images around nine femoral stems and six calcar resorptions. The survival rate at more than 6 years was 95.8% (91.8-99.8) for the cups and 94.8% (90.3-99.2) for the femoral stems (95% confidence interval).

DISCUSSION: The present study confirms our hypothesis: with Metasul bearings equipped cementless cups, the rates of aseptic loosening, RLL or acetabular osteolysis are low and remain stable over time, unlike the results observed for Metasul series with polyethylene directly cemented on bone. Specific complications, such as metallosis due to impingement and hypersensitivity, can rarely be encountered. These results encourage us to continue employing metal-on-metal bearings with non-cemented cups in active patients.

LEVEL OF EVIDENCE: Level IV: Therapeutic study.