[Second-generation metal bearings in cementless primary total hip arthroplasty: rationale, French homologation and preliminary results].

PURPOSE OF THE STUDY: Long-term outcome of Charnley low-friction arthroplasty in young active patients is impaired worldwide due to wear of the polyethylene (PE) component and osteolysis. In the late eighties, reports of possible low wear with some former metal on metal total hip arthroplasties led to the reintroduction of metallic bearings. The aims of this work were to examine the rationale for using metal on metal bearings in primary total hip arthroplasty (THA) and report preliminary results obtained with cementless Metasul -Alloclassic hips.
MATERIALS AND METHODS: From January 1994 to March 1997, 64 cementless primary Alloclassic-THA (grit-blasted titanium SL stems and CSF treaded cups) with 28 mm Metasul bearings were performed. Mean age at surgery was 60 years (range, 36-73). Diagnoses were usual, mainly primary osteoarthrosis in 70 p. 100 of the hips. Two bearing surfaces were exchanged for late dislocation at 2.6 and 2.9 years. Thus, 62 hips in 58 active patients (4 bilateral) were reviewed after a minimum 2-year follow-up (mean 3.2 years, range 24-66 months).
RESULTS: Clinical results according to the Merle d'Aubigne and Charnley rating system were graded excellent or good in all 62 hips. Radiologically, calcar, atrophy and spot welds were noted in 93 p. 100 and 82 p. 100 of hips respectively. Proximal reactive and lucent lines and mild proximal stress shielding were observed in 8 p. 100 and 4.8 p. 100 of hips respectively. No osteolysis granuloma has thus far been observed in the vicinity of any component. Cobalt blood level remained normal, except in 6 cases due to occupational exposure (n=1), possible impingement (n=1) or an unknown cause (n=4). All elevated cobalt levels (range 7 to 25 microg/l) were nevertheless far below the toxic limit. DISCUSSION: Dislocation may be due either to the posterolateral surgical approach and/or early impingement with the first Metasul bearing design (head sleeve). Metasul acetabular component fixation is not restricted to only cementless metal-backing, unlike alumina-ceramic cups. The concern about the toxicity of metallic wear debris dissemination and the hematocarcinogenic risk must be taken into consideration as for any metallic THA. Follow-up is too short for the new polys for significant comparisons.
CONCLUSION: Metal on metal tribology is well known in vitro and Metasul bearings have functioned in vivo for 12 years (120 000 Metasul hips worldwide) as was expected from laboratory tests. Obviously, this friction couple is not the unique answer to PE-wear and THA longevity, but, in light of current data, appears as a trustworthy solution available today.