STATISTICAL SHAPE MODELLING TO COMPARE ANATOMICAL DIFFERENCES IN INDIVIDUALS WITH BILATERAL SYMPTOMATIC FEMOROACETABULAR IMPINGEMENT

Femoroacetabular impingement (FAI) is one of the leading causes of early hip osteoarthritis and can be characterized by a bony overgrowth of the femoral head-neck junction (cam-type) and/or an overcoverage of the acetabulum (pincer-type). Although many individuals with FAI indicate these characteristic features, there are often several anatomical factors in addition to the cam deformity that may lead to earlier injury risks in either one or in potentially both of their hips. The purpose was to examine patients with bilateral symptomatic cam FAI and compare the additional anatomical variations between their larger and smaller cam deformity using multimodal statistical shape modelling. Twenty-four patients with bilateral symptomatic cam FAI (n = 24, m:f = 13:11, age = 22 ± 6 years), and fifty-four control participants (n = 54, m:f = 42:12, age = 33 ± 7 years) were included in this study. Each participant was CT scanned to confirm any bilateral cam morphology and additional anatomical measurements of the femoral neck (femoral neck-shaft angle, medial proximal femoral angle, femoral version), acetabular coverage (acetabular version, lateral centre-edge angle) and spinopelvic parameters (pelvic incidence) were assessed. Paired sample (larger-cam vs. smaller-cam) and independent sample t-tests (control vs. each FAI group) were performed to compare the anatomical CT measurements (CI = 95%). Each participant's hip models were segmented and imported into a statistical shape modelling program (ShapeWorks, USA) to compare the anatomical differences in mean shape models. From the imaging data, the bilateral FAI patients’ larger-cam side showed larger alpha angles compared to the contralateral smaller-cam side. The most important finding was that bilateral symptomatic FAI patients had either a large cam deformity or a combination of a smaller femoral neck-shaft angle with a small cam deformity. Injury risks and earlier symptoms could be associated with a combination of these features where a varus neck can further bring even a milder cam deformity closer to the chondrolabral junction, resulting in earlier mechanical impingement and symptoms. An anterior sacral tilt also increases anterosuperior engagement of the cam deformity and decreases posterior stability of the hip joint. These structural asymmetries and variances may lead to compensatory pelvic instability, highlighting the need for personalized surgical plans to optimize hip stability and help alleviate symptoms for potentially either or both hips.