Large diameter head metal-on-metal bearings total hip arthroplasty: preliminary results.

INTRODUCTION: Although the use of the metal-on-metal bearings has been validated over the long term in total hip arthroplasty (THA) for standard 28 and 32 mm diameters, and over the medium term in resurfacing procedures, the use of larger metal head size in conventional THA has not yet been extensively reported.

HYPOTHESIS: The large-diameter metal-on-metal head is beneficial in terms of implant stability without altering the result in terms of function and bone fixation compared to the standard 28 and 32 mm diameters.

OBJECTIVE: The objective was to test this hypothesis by assessing the short-term clinical and radio graphic results of a metal-on-metal large-diameter heads THA system, using cups from the resurfacing hip concept.

MATERIAL AND METHODS: We conducted a retrospective study on a continuous series of 106 uncemented acetabular cups (Durom) implanted in 102 patients (mean age, 66 years): 93 cases of primary or secondary coxarthrosis, 11 cases of aseptic osteonecrosis, one fracture of the femoral neck, and one case of rheumatoid arthritis of the hip. At 30 months of follow-up,the Harris Hip Score and the Merle d'Aubigné (PMA) score were calculated. The radiological investigation included comparison of the implant head with native head diameters, variations of acetabular center of rotation, inspection for implant migration, and search for a gap or radiolucent line.

RESULTS: The series included two post-traumatic dislocations as well as spontaneously receding tendinitis of the gluteus medius with no further recurrence. The mean Harris Hip Score improved from 49.3 preoperatively to 91.6 at the latest follow-up and the mean PMA score ranged from 12 to 17. The results were excellent for 70 cases, good for 31 cases, fair for three cases, and poor for two cases. In the last five cases, the overall results were undermined by low pain subscore,with no identifiable explanation. Restoration of the original head diameter was verified for 65 hips. No cup migration was observed. Measurement of the acetabular centre of rotation showed a mean lateralization of 1.1mm. Of the 67 immediate postoperative gaps, only two did no disappear at follow-up. Implant head diameter, cup position, and the existence of a gap were not correlated with the clinical results.

DISCUSSION: These results are comparable to 28 mm-diameter metal-on-metal heads in uncemented cups but with improved stability but without demonstrable alteration of the quality of the bone fixation. We found no mechanical or medical cause that could explain the five cases of persistent pain leading to fair or poor results. Long-term follow-up will validate these theoretical advantages in terms of wear and implant survival.

LEVEL OF EVIDENCE: IV. Retrospective series.