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No statistical differences found in functional scores between THA head sizes

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Joann E. Higgins

DUBLIN – No statistical differences in functional scores were found between large and small heads in total hip arthroplasty at 5 years, according a presenter here at the British Orthopaedic Association and the Irish Orthopaedic Association Combined Meeting 2011.

“Large heads theoretically produce stability, however, these have been in vitro studies. We therefore believe ours to be the first in vivo study to compare functional outcome to a large-small head. We hypothesized that a large head would confer greater stability, increased range of movement and therefore better functionality to the patient,” Joann E. Higgins, BA(Hons), MA, said.

The cohort for the multi-surgeon, prospective, matched study consisted of 458 patients with a new 38 mm uncemented metal-on-metal articulation total hip replacement (THR) (M²A/Bi-metric, Biomet UK). The control group consisted of 459 patients with a 28 mm metal-on-polyethylene articulation THR (Exeter/Exeter or Exeter/Duraloc, Stryker UK) from the local joint registry database. The demographics for the 38 mm and 28 mm groups had mean ages of 65.9 years and 66.3 years and were 40.4% and 39.3% male, respectively. Preoperative Oxford Hip and WOMAC scores were comparable.

All patients attended a standard preoperative assessment Joint Clinic Review regardless of prosthesis used. Patients were seen at 6 weeks, 12 weeks, 1 year and 5 years in Joint Review Clinic with annual postal questionnaires, and the functional scores were scored preoperatively and at yearly intervals. Range of movement was measured preoperatively and at 1 year.

At each review point, there was no statistical difference in either Oxford or WOMAC scores. These results were sustained at 5 years. Dislocation rates in the 38-mm group were lower (3.5% vs. 5%) although Higgins said this was not statistically significant. The number of revision was statistically significant, however, with 21 revisions (4.6%) in the 38-mm group and 9 (2%) in the 28-mm group, she said. In the large-head group, Higgins said that 2% of the revisions were due to due adverse metal reactions and 1.3% to loosening.

“There was no statistically significant functional advantage in using a large head prosthesis. It did not confer increased stability. Given the difference in revision rates seem to be due to an adverse reaction to metal debris, we did conclude that a large head is a property of a metal-on-metal that did not confer any advantage,” she said.

Reference:

• Higgins JE, Pearce AR, Price M, et al. Large diameter metal-on-metal articulation total hip replacement versus 28-mm metal-on-polyethylene - is there a functional difference at 5 years?. Paper #2. Presented at the British Orthopaedic Association and the Irish Orthopaedic Association Combined Meeting 2011. Sept. 13-16. Dublin.

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