Hip abduction force measurements accurately predict success for total hip arthroplasty patients

BACKGROUND: Whether hip abduction strength recovery influences patient-reported function is unknown. Consequently, in patients undergoing anterior-based muscle-sparing (ABMS) Total Hip Arthroplasty (THA), the present study determined: (1) the abduction torque ratio of the operative hip relative to the unaffected contralateral hip, and (2) whether an abduction torque ratio threshold is present, which is predictive of improved hip function at six weeks (≥ 11-point Oxford Hip Score improvement), and a successful outcome one year after THA (Forgotten Joint Score ≥ 74).

METHODS: This study examined 118 consecutive patients undergoing ABMS THA with an unaffected contralateral hip. Bilateral peak isometric hip abduction torque was measured and compared at preoperative, 6-week, and 1-year time points to compute the abduction torque ratio. A non-inferiority test determined the significance of the comparison with a torque ratio ≥ 94% defining non-inferiority. A receiver operating characteristic curve analysis computed the threshold of abduction torque ratio associated with hip function improvement at six weeks and a successful outcome one year after THA.

RESULTS: Mean (± standard deviation) abduction torque ratios and associated significance of the non-inferiority test before, 6 weeks, and 1 year after THA were 73 ± 14% (p = 1), 93 ± 5% (p = 0.99), and 98 ± 5% (p < 0.01). A 90% and 91% abduction torque ratio at 6 weeks and 1 year had a 0.8 and 1.0 positive predictive value, respectively, of improving hip function and a successful outcome.

CONCLUSIONS: A ≥ 91% abduction torque ratio is associated with a successful outcome after ABMS THA. Hence, a ≥ 91% abduction torque ratio provides a meaningful target when measuring abduction force and, if not achieved one year after THA, may prompt further investigation for causes of hip abduction weakness.

LEVEL OF EVIDENCE: IV.