STATISTICAL SHAPE MODELLING TO DIFFERENTIATE THE SYMPTOMATIC AND ASYMPTOMATIC HIP IN BILATERAL CAM FEMOROACETABULAR IMPINGEMENT

Femoroacetabular impingement (FAI) is one of the leading causes of early hip osteoarthritis and mostly characterized by an enlarged femoral head-neck junction (cam-type). Although patients commonly present with bilateral FAI morphologies, they typically only experience unilateral symptoms. This suggests that additional anatomic factors other than the cam deformity may contribute to earlier symptoms. The purpose of this study was to directly compare the anatomic variations between the symptomatic and asymptomatic hips in bilateral FAI patients using planar imaging measurements and 3D statistical shape modelling. Twenty-seven patients awaiting surgery (n = 27, m:f = 15:12, age = 25 ± 6 years) were included if they indicated bilateral cam FAI morphologies but unilateral symptoms (IRB #114897). Patients were excluded if they indicated other hip pathologies (e.g. dysplasia, slipped capital femoral epiphysis, Legg-Calve-Perthes) or history of lower-limb trauma or surgery. Each patient's pelvic and knee regions were imaged using a computed tomography scanner (SIEMENS Somatom, Germany) to confirm cam morphology (e.g. 3:00 alpha angle > 50.5° or 1:30 alpha angle > 60°). Anatomic measurements of the femoral neck (femoral neck-shaft angle, medial proximal femoral angle, femoral torsion), acetabular coverage (lateral centre-edge angle, acetabular version at 1:00, 2:00, 3:00) and spinopelvic parameters (sacral slope, pelvic tilt, pelvic incidence) were assessed from each patient's CT data. Paired sample t-tests compared the measurements between symptomatic and asymptomatic sides using statistical software (CI = 95%; GraphPad Prism, USA). Each patient's symptomatic and asymptomatic 3D hip models were segmented (Synopsys Simpleware, USA) and statistical shape modelling compared anatomic differences between mean symptomatic and asymptomatic shape models (ShapeWorks 6.4, USA). From imaging measurements, although both symptomatic and asymptomatic sides showed similar femoral cam deformities, femoral torsions, lateral centre-edge angles, and spinopelvic parameters, the symptomatic sides had smaller femoral neck-shaft angles (p = 0.001), medial proximal femoral angles (p = 0.001), and acetabular versions at 1:00 (p = 0.004) than the contralateral asymptomatic sides. From the statistical shape models, the symptomatic group showed a more varus neck and a retroverted acetabulum with slightly increased anterosuperior and decreased posterior acetabular coverage. Additionally, the symptomatic sides’ sacroiliac joint and sacral alar were smaller and anteriorly tilted compared to their contralateral asymptomatic sides. The most important finding was that the symptomatic sides indicated a more varus neck, retroverted acetabulum, and anterior sacral tilt in addition to the cam deformities. A varus neck can bring the cam deformity closer to the chondrolabral junction and restrict hip mobility, resulting in earlier mechanical impingement and symptoms. Moreover, an anterior sacral tilt and retroverted acetabulum increases anterosuperior cam engagement and also decreases posterior hip joint stability. These anatomic variances may provide insights into symptomatic hip impingement and instability, therefore, including them would help optimize treatment planning. As the cam deformity alone may not be the only indicator for symptoms, characterizing these anatomic relationships through statistical shape modelling can help guide patient-specific diagnosis and treatment strategies for FAI to improve patient outcomes and delay the progression of early hip osteoarthritis.