DEVELOPMENT AND EXTERNAL VALIDATION OF A NOVEL CLINICAL SCORE TO QUANTIFY THE PRESENCE OF INSTABILITY CHARACTERISTICS IN PATIENTS WITH BORDERLINE ACETABULAR DYSPLASIA

BACKGROUND: Treatment of borderline acetabular dysplasia (BD) is controversial with some patients having primarily instability-based symptoms while others have impingement-based symptoms. The existing literature lacks direct comparisons of different treatment approaches, and generally fails to report other important diagnostic characteristics beyond the lateral center edge angle (LCEA). PURPOSE: The purpose of this study was (1) to identify the most important patient characteristics influencing the diagnosis of instability vs. non-instability, (2) to develop a clinical score (Borderline Hip Instability Score, BHIS) to collectively characterize these factors and (3) to externally validate BHIS in a multicenter cohort BD patients. METHODS: In Part 1, this study utilized a retrospective cohort study of 186 hips undergoing surgical treatment of BD (LCEA 20°-25°) from a single surgeon experienced in arthroscopic and open techniques. Multivariate analysis determined characteristics associated with presence of instability (treated with PAO +/- hip arthroscopy) or absence of instability (treated with isolated hip arthroscopy) based on clinical diagnosis of the single surgeon. During the study period, 39.8% of the cohort underwent PAO. Multivariate analysis with bootstrapping was performed and results were transformed into a BHIS nomogram (higher score representing more instability). In Part 2, BHIS was externally validated in 114 BD patients enrolled in a multicenter prospective cohort study across 10 surgeons (with varied treatment approaches from arthroscopy to open procedures). RESULTS: In Part 1, the most parsimonious, best fit model included 4 variables associated with the diagnosis of instability: acetabular inclination (AI), anterior center edge angle (ACEA), maximum alpha angle, and internal rotation in 90 degrees of flexion (IRF). Sex and LCEA were not significant predictors. Mean BHIS in the population was 50.0 (instability 57.7 ±7.9; non-instability 44.8±7.3, p<0.001). BHIS demonstrated excellent predictive (discriminatory) ability with c-statistic=0.89. In Part 2, BHIS maintained excellent c-statistic=0.92 in external validation. Mean BHIS in the external cohort was 53.9 (instability 66.5±11.5; non-instability 43.0±10.8, p<0.001). DISCUSSION: In patients with BD, key factors in diagnosing significant instability treated with PAO were AI, ACEA, maximum alpha angle, and IRF. The BHIS score allowed for differentiation of patients with and without instability in the development and external validation cohort.