The lateral hip instability test: Diagnostic accuracy for lateral or posterolateral femoral head undercoverage.

Purpose: To (i) describe the lateral hip instability test, developed to discriminate between stable versus unstable hips with lateral or posterolateral femoral head undercoverage, (ii) evaluate differences between painful hips that tested positive versus negative, and (iii) evaluate the accuracy of this test as defined by radiographic references for acetabular dysplasia and/or retroversion. Methods: A consecutive series of patients were evaluated for hip pain from 1 January 2019 to 31 January 2021. Routine assessment included the new lateral hip instability test, which is positive when inducing deep lateral hip pain and consists of maximum passive adduction of the painful hip, and application of a force in the long axis of the femur. Descriptive statistics were used to compare patients with positive versus negative lateral hip instability tests. The accuracy of the test was assessed using a lateral centre-edge angle (LCEA) ≤ 20° for frank dysplasia, LCEA ≤ 25° for borderline dysplasia and a combination of positive cross-over sign, posterior wall sign and ischial spine sign for acetabular retroversion. Results: Sixty-nine of the 159 (43%) painful hips had a positive lateral hip instability test. Positive hips had higher Tönnis angle (9 ± 8° vs. 3 ± 5°, Conclusions: The lateral hip instability test can discriminate between stable versus unstable painful hips with lateral and/or posterolateral femoral head undercoverage. When the test is positive, clinicians can be quite certain that a patient has hip instability resulting from lateral and/or posterolateral undercoverage. Level of Evidence: Level II.