[Metal-on-metal hip replacement using Metasul cups cemented into Muller reinforcement rings after a mean 5-year (3-8) follow-up: improvement of acetabular fixation by comparing with direct cementation to bone].

PURPOSE OF THE STUDY

Early loosening, before a three-year follow-up, has been observed with cemented cups having a metal-on-metal insert in a polyethylene cup. The metal-on-metal bearing has been incriminated as the source of the problem because of its rigidity (particularly for small cups measuring less than 50 mm) and the creation of stress conditions unfavorable for a cemented fixation. The purpose of this retrospective study was to determine whether this phenomenon is observed when the cement is fixed not directly into the bone, but via a Muller reinforcement ring.

MATERIAL AND METHODS

From 1998 to 2004, 23 arthroplasties using a cemented Metasul cup in a reinforcement ring were implanted in 22 patients (16 women and six men) aged on average 44 years (range 24-56 years). The series included six primary total hip arthroplasties (three for dysplasia, two for protrusions, one for rheumatoid arthritis and one for arthritic degradation) and seventeen revisions (two septic). The Metasul cup (Zimmer-Centerpulse) combined a 28 mm modular head anchored in a femoral implant (two cemented, 21 pressfit) and a polyethylene cup with a Metasul insert (13 of 23 measuring<50mm). In all cases, the cup was fixed with low-viscosity cement in a Myller metal reinforcement ring fixed with screws (Zimmer-Centerpulse). All patients were reviewed clinically and radiographically at a mean 5-year follow-up (range 3-8 years). Acetabular and femoral fixation were analysed (search for lucency and implant migration).

RESULTS

Revision was not necessary in any patient for failure of the acetabular fixation. The mean Postel-Merle-d'Aubigné score improved from 12.9 points (range 7-17) to 17.5 points (range 16-18). The radiographic analysis did not reveal any sign of lucency between the cup and ring, nor any migration of the ring. There was no evidence of femoral osteolysis but one femoral revision was needed due to fracture of the lateral cortical identified six weeks after implantation.

DISCUSSION AND CONCLUSION

Cementing the metal-on-metal cup into a reinforcement ring can avoid the risk of loosening observed after direct cementing into bone. In our study, the large number of small cups (13/23) would have been expected to produce a high rate of acetabular lucent lines and/or a high rate of early revision, as reported by others, as early as 24 months. Our series was also different from others by the use of pressfit femoral implants in most patients, which should reduce the risk of cement debris in the bearing. Longer follow-up will be necessary to confirm the good results observed to date which suggest that direct cementing of the cup into the bone should be incriminated rather than the metal-on-metal bearing to explain the reported failure of cemented Metasul cups.