Can Hip Passive Range of Motion Predict Hip Microinstability? A Comparative Study

BACKGROUND: Hip microinstability is an increasingly recognized cause of pain and disability in young adults. It is unknown whether differences in passive hip range of motion (ROM) exist between patients with versus without hip microinstability. HYPOTHESIS: Underlying ligamentous and capsular laxity will result in differences in clinically detectable passive ROM between patients with femoroacetabular impingement (FAI), patients with microinstability, and asymptomatic controls. STUDY DESIGN: Cross-sectional study; Level of evidence, 3. METHODS: A retrospective review of all patients undergoing hip arthroscopy between 2012 and 2018 was conducted. Patients with a diagnosis of isolated microinstability based on intraoperative findings were identified and classified as having isolated FAI, instability, or FAI + instability. Patients without a history of hip injury were included as controls. Range of motion was recorded in the supine position for flexion, internal rotation, and external rotation. Univariate and multivariate analysis was performed on each measurement in isolation as well as combinations of motion to include total rotation arc, flexion + rotation arc, and flexion + 2× rotation arc Models were then created and tested to predict instability status. RESULTS: In total, 263 hips were included: 69 with isolated instability, 50 with FAI, 50 with FAI + instability, and 94 control hips. A higher proportion of patients in the instability and FAI + instability groups were female compared with the FAI and control groups (P < .001). On univariate analysis, differences were found in all groups in all planes of motion (P < .001). Multivariable analysis demonstrated differences in all groups in flexion and flexion + rotation arc. In symptomatic patients, the best performing predictive model for hip microinstability was flexion + rotation arc ≥200° (Akaike information criterion, 132.3; P < .001) with a sensitivity of 68.9%, specificity of 80.0%, positive predictive value of 89.1%, and negative predictive value of 51.9%. CONCLUSION: Patients with hip microinstability had significantly greater ROM than symptomatic and asymptomatic cohorts without hip microinstability. Symptomatic patients with hip flexion + rotation arc ≥200° were highly likely to have positive intraoperative findings for hip microinstability, whereas instability status was difficult to predict in patients with a flexion + rotation arc of <200°.