The posterior approach in THR: assuring capsular stability.

Repair of the posterior capsule is advocated to reduce dislocation after total hip replacement (THR). To date, no predictable physical findings are reported in the literature to assess the capsular integrity and risk of hip dislocation after the posterior approach. The internal rotation test is performed at 4 to 6 weeks postoperatively with the hip and knee flexed at 90° while the patient is supine. The test is positive if a firm endpoint is felt with internal rotation of ≤15°. Between January 2007 and January 2008, twenty-three patients who had magnetic resonance imaging (MRI) at a mean 1.8±0.7 years after posterior-approach THR were included in this study. Magnetic resonance images were reviewed by a blinded radiologist for the integrity of the posterior capsule and quadratus, the distance between the piriformis and conjoined tendon to bone, and the amount of piriformis obturator internus atrophy. Magnetic resonance imaging results in patients with positive internal rotation tests showed an intact posterior capsule and scarring of the tendons with no severe muscle atrophy. In patients with a negative internal rotation test, MRI showed incomplete healing of the tendons and severe muscle atrophy. Sensitivity and specificity of the internal rotation test for tendon/scar healing were 100% and 93%, respectively. The internal rotation test is a simple, reproducible test that, if positive, has a high correlation with capsule/tendon healing.