BORDERLINE ACETABULAR DYSPLASIA: INDEPENDENT PREDICTORS OF HIP INSTABILITY VERSUS IMPINGEMENT
Authors: Nepple Jeffrey J. , Graesser Elizabeth A. , Wells Joel , Clohisy John 
Published: March 2019

Background: Hips with borderline acetabular dysplasia (lateral center-edge angle, LCEA, between 20° and 25°) are challenging in terms of diagnostic and treatment decision-making. It is accepted that a portion of this population has primarily hip instability-based symptoms consistent with symptomatic acetabular dysplasia, while others have primarily hip impingement-based symptoms consistent with femoroacetabular impingement (FAI). Nevertheless, the diagnostic characteristics that differentiate hip instability and FAI have not been identified. The purpose of this study was to examine a cohort of patients with minor acetabular dysplasia features in order to identify the preoperative clinical characteristics and imaging findings that differentiate patients with hip instability from patients with FAI. Methods: A retrospective cohort study of patients with borderline acetabular dysplasia was performed. Utilizing our institution’s hip preservation database, we identified 143 consecutive hips in 134 patients undergoing hip preservation surgery in the setting of borderline acetabular dysplasia. All patients were identified by prospective radiographic evaluation with an LCEA between 20° and 25°. Inclusion criteria included age 14-40 years and primary surgical treatment. Medical records were reviewed to determine patient demographics, details of clinical presentation, baseline patient-reported outcome scores, physical exam findings, plain radiographic findings, and the operative procedures performed. Statistical analyses were used to compare the clinical features and imaging parameters of the symptomatic acetabular dysplasia and FAI subgroups. Results: Of the 143 hips in the cohort, 39.2% (n = 56) had the diagnosis of symptomatic instability, while 60.8% (n = 87) had the diagnosis of FAI. The cohort included 109 females (76.2%) and 34 males (23.8%). Hips with instability (compared to FAI) had a significantly lower LCEA (21.8° vs. 22.8°; p < 0.001), lower ACEA (23.3° vs. 26.6°; p = 0.002), a higher AI (11.8° vs. 8.5°; p < 0.001), and a lower maximum alpha angle (54.4° vs. 61.1°; p = 0.001). The odds of instability increased 1.7 times for each one-degree decrease in LCEA, 1.4 times for each one-degree decrease in ACEA, and 1.1 times for each one-degree increase in acetabular inclination (all p < 0.003). Sex was strongly associated with the clinical diagnosis, with instability present in 48.6% of females compared to only 8.8% of males (p < 0.001). Patients with instability presented with significantly greater disability, as indicated by the modified Harris hip score, UCLA activity, SF-12 physical function, and HOOS (pain, activities of daily living, sports and recreation, and quality of life) scores (all p = 0.05). The symptomatic acetabular dysplasia subgroup had significantly greater range of motion in terms of internal rotation in flexion (IRF, 22.7° vs. 12.4°, p < 0.001) and total arc of rotational motion (IRF+ERF, 61.2° vs. 47.4°, p < 0.001). Lateral hip pain was present in 42.9% (24/56) of hips in the instability group compared to 25.3% (22/87) of hips in the impingement group (p = 0.03). Conclusions: We found significant differences in the clinical characteristics and radiographic features of the symptomatic acetabular dysplasia and FAI subgroups within the borderline dysplasia cohort. Patients with symptomatic instability tend to have lateral hip pain, greater functional limitations, higher range of motion, and a greater AI, while patients with impingement symptoms tend to have more limited range of motion (especially IRF), a greater ACEA, and a greater alpha angle.