Paper 39: How to reliably diagnose hip microinstability: a multicenter cohort study of 802 patients with surgically confirmed diagnoses

Objectives: Hip microinstability (MI) has emerged as an important, though often underrecognized, source of pain and dysfunction in young, active individuals. Unlike an ACL Tear with a physical examination maneuver (Lachman) or imaging modality (MRI) that has a high sensitivity and specificity, along with high positive and negative predictive values, the diagnosis of hip microinstability has been difficult…more akin to shoulder instability. The aim of this study is to provide the clinical features that are highly indicative of hip microinstability to improve the ability to correctly diagnose hip MI.
Methods: Using prospectively obtained data, we retrospectively analyzed 802 patients that underwent arthroscopic hip surgery from 2023-2025 across three international high-volume hip preservation practices. Patients were categorized into two groups: those with and without intraoperative confirmation of hip microinstability. All patients underwent the same pre-operative evaluation, including patient demographics (age, sex), clinical history (symptom duration, mechanical symptoms, diagnosis of dysplasia, connective tissue disorders), and family history. Patients were queried about specific mechanical symptoms. Physical exam tests were analyzed including: objectively measured hip range of motion, Impingement test, the Scour test, the Prone External Rotation (PER) test, the Abduction-Hyperextension External Rotation test (AB-HEER), the Hyperextension-External Rotation (HEER) test, and Axial Distraction test. Radiographic features including lateral center edge angle (LCEA), anterior center edge angle (ACEA), acetabular roof angle / tonnis angle, as well as other features including femoral version, femoral neck shaft angle, cliff sign, divot sign, posterior wall sign, chondral damage (acetabulum and femoral head) and signs of FAI (alpha angle, crossover sign, femoral neck cysts) were assessed. A multivariable mixed-effects logistic regression model was used to analyze the relationships between clinical and demographic variables on the odds of microinstability diagnosis, accounting for site-level differences. Variables with insufficient sample sizes were dropped from the model to prevent quasi-complete separation. Odds ratios and 95% confidence intervals were reported. Variance inflation factors (VIFs) were assessed to rule out multicollinearity. Analyses were completed in RStudio using a two-sided level of significance of 0.05.
Results: Of the 802 hips, 240 (30%) were diagnosed with microinstability intraoperatively. There were 405 female patients (50.5%) with an average age of 34.4 (+ 11.4) years. Patients with instability were predominantly female (77.9% vs. 38.8%, OR for male sex 0.18 [0.10-0.32], p <0.001). Instability was more common in right hips (59.2% vs. 50.4%, OR 1.60 [1.01-2.52], p=0.04). No other historical features contributed significantly to help differentiate MI patients from non-instability patients. Mean hip flexion was notably higher in the instability group (117° vs. 108°, OR 1.04 [1.02-1.05], p <0.001), as was internal rotation in both seated (27.4° vs. 23.2°, OR 1.03 [1.02-1.04], p <0.001) and prone positions (32.8° vs. 26.6°, OR 1.04 [1.02-1.05], p <0.001). Seated external rotation was also increased slightly (38.2° vs. 34.5°, OR 1.01 [1.00-1.03], p=0.018). Flexion plus rotational arc is strongly associated with microinstability (182° + 30.6° vs 166° + 24.7°, OR 1.02), more than each measure alone, and especially in women (p < 0.001). Further, seated hip rotational arc (hip in 90° flexion) is more strongly associated with MI than prone rotation measures. The AB-HEER test (OR 2.43 [1.70-3.49], p <0.001), demonstrated strong predictive value, as did the HEER test (OR 1.99 [1.39-2.87], p <0.001) and the PER test (OR 1.65 [1.11 - 2.46], p = 0.013). Notably, the Axial Distraction Apprehension test, although less frequently positive (7.0% of instability vs. 1.7% of stable hips), had the highest odds ratio after multivariate analysis (OR 3.65 [1.03 - 12.96], p=0.04)Radiographic features were also helpful in the diagnosis of hip microinstability. Features of dysplasia, particularly LCEA ( <25°- OR 2.90 [1.39 - 6.23], p=0.005) and Tonnis Angle (>10°- OR 2.21 [1.34 - 3.62], p=0.002) were associated with microinstability. Further, the Cliff sign was present in one third of microinstability patients and had an OR of 1.93 [1.32-2.82], p<0.001.
Conclusions: Hip microinstability, like shoulder instability, is a dynamic problem, making static imaging less helpful in diagnosing hip microinstability. Barring the existence of a single examination maneuver with a high sensitivity and specificity (like the Lachman maneuver for the knee), a constellation of physical examination findings are critical to making the clinical diagnosis of hip microinstability with a high degree of confidence. This study of more than 800 patients, with surgically confirmed diagnoses, provides a major step forward in making the diagnosis of hip microinstability. The broad range of findings observed in this cohort supports a multifactorial model of hip instability, combining elements of osseous morphology, soft tissue laxity, and dynamic instability.This study adds valuable data on the diagnostic value of history, physical exam maneuvers and imaging in the diagnosis of hip microinstability with surgical confirmation as the gold standard. We present a simplified evaluation for the diagnosis of microinstability utilizing this multivariable analysis of 802 patients with surgically confirmed diagnoses. Our findings may serve as a foundation for developing a standardized clinical scoring system or diagnostic algorithm to guide future evaluation, referral, and treatment planning in this challenging population.