MRI findings of gluteus minimus muscle damage in primary total hip arthroplasty and the influence on clinical outcome.

Introduction: The influence of surgical trauma on gluteus minimus muscle in total hip arthroplasty (THA) and the impact on functional outcome has been hardly investigated up to now. Potential risks of minimus damage during the approach or femoral preparation is due to its attachment to the anterior facet of the greater trochanter. Possible trauma-associated functional deficits of minimus muscle may result in reduced abduction force or in an unstable hip joint. The aim of the present study was to assess the pre- and post-operative gluteus minimus muscle (tendon defects and fatty atrophy) in patient with anterolateral minimally invasive and modified lateral approach by means of magnetic resonance imaging (MRI) and to investigate the associated impact on functional outcome.

Materials And Methods: Thirty-eight patients [average age, 64 years (35-80); BMI, 28 kg/m(2) (19-35)] with primary coxarthrosis were prospectively enrolled in the study. A cementless hip prosthesis was implanted either via a minimally invasive anterolateral or a modified direct lateral approach. Patients were clinically and radiologically (MRI) examined preoperatively, 3, and 12 months postoperatively. Additionally, the Harris hip score, a pain score (NRS 0-10) and a satisfaction score (1-6) were recorded. To test the function of the abductor muscles the Trendelenburg's sign and the abductor muscle strength were evaluated. MRI evaluation includes the assessment of tendon defects and fatty atrophy of the minimus muscle.

Results: Tendon defects and fatty atrophy were seen in nearly 50% of the patients after THA. Harris hip-, pain-, and satisfaction scores did not correlate with the MR findings. There was also no impact on the abduction strength or the Trendelenburg's sign. Furthermore, the frequency of minimus damage was neither influenced by age, gender, BMI nor by the applied approach.

Conclusion: Muscle atrophy and tendon defects of the minimus muscle appear frequently after THA without any favored relation to the lateral or anterolateral approach. The extent of injured minimus muscle has a minor impact on the clinical outcome particularly not on the abduction strength within the first postoperative year. The main function of the gluteus minimus is rather the centralization of the femoral head in the joint during the gait cycle than hip abduction and stabilization of the pelvis. The use of a straight stem with the associated need for lateral femoral preparation may be a risk factor for minimus tendon damage. Therefore, the surgeon should pay special attention to the prevention of surgical trauma to the gluteus minimus muscle during femoral preparation.