194. SHORT-TERM CONTRIBUTIONS OF THE LARGE DIAMETER METAL-ON-METAL BEARING FOR TOTAL HIP ARTHROPLASTY

Abstract

Purpose of the study: Use of the metal-on-metal bearing has been validated in total hip arthroplasty (THA) for conventional diameters and for resurfacing but not with large-diameter implants. The purpose of our study was to establish the short-term clinical validity of large diameter implants.

Material and methods: This was a retrospective study of a consecutive series of 106 press-fit cups (Durom^®) in 102 patients with minimum two years follow-up. Mean age at surgery was 66 years. Aetiologies were primary degenerative diseases (n=83), aseptic osteonecrosis (n=11), secondary degeneration (n=7), degeneration on acetabular dysplasia (n=2), rapid destructive osteoarthritis (n=1), fracture of the femoral neck (n=1) rheumatoid disease (n=1). Patients were reviewed with mean 30 months follow-up with the PMA and Harris scores. Radiographic measurements were made on semi-automatically (Imagicka^®) and included the ratio of prosthesis head to native head diameter, acetabular offset, and any implant migration, gap or lucency.

Results: There were two traumatic dislocations due to falls with regressive tendonitis of the gluteus medius with no later consequence. The mean Harris score was 91.6 and the mean PMA score 17. Outcome was excellent (n=70), good (n=31), fair (n=3) and poor (n=2). The ratio showed that head diameter had been restored for 65 hips (0.95 to 1.05). There were no cases of cup migration. Acetabular offset showed a mean lateralization of 1.1 mm. For 67 immediate posterior gaps, including ten measuring > 2mm, only two had not filled at review. None of the radiographic measurements had an effect on the clinical outcome.

Discussion: These results are comparable with those published for metal-on-metal bearings with 28mm heads and press fit cups. We did not find any mechanical or medical cause explaining the persistent pain observed in our patients with fair or poor outcome. The low level of dislocation (1.8%) confirms the improved prosthetic stability compared with 28 mm heads.

Conclusion: In our opinion, these early results demonstrate a clear improvement in stability, but at the cost of possible unexplained pain. Long term clinical and radiographic surveillance is needed to validate this option in terms of implant wear and survival.