Statistical Shape Modeling of Cam-type Femoroacetabular Impingement

Cam femoroacetabular impingement (FAI) is characterized by a malformed femoral head that may cause shearing between the femur and acetabulum, leading to intraarticular damage and early hip osteoarthritis. Radiographic measurements are used to diagnose cam FAI, but provide only a planar view of the femoral head and often assume the ideal femur shape to be spherical. Statistical shape modeling (SSM) can be used to objectively compare complex 3D morphology without the need to assume ideal geometry. The objective of this study was to generate accurate 3D reconstructions of femoral heads and apply statistical shape modeling to quantify 3D variation and morphologic differences between control and cam femurs. Femurs from 33 controls and 15 cam FAI patients were CT scanned and 3D surfaces were generated by image segmentation. Correspondence particles were optimally positioned upon each surface using a gradient descent energy function. Resulting particle configurations were used to generate mean shapes for each group. Morphological differences were calculated as the distance between mean control and patient geometries. Differences were consistent with the location and approximate shape of cam lesions found intra-operatively. Deviations in mean shape between groups were pronounced at the anterolateral headneck junction, where the mean cam femur protruded from the mean control femur by a maximum of 2.7mm. Sustained protrusions of ~1.0-2.5mm were distributed from the anterior-posterior midline of the femoral neck along the entire anterolateral head-neck junction and distally along the anterior section of the neck. Future work will refine our statistical shape modeling software to quantify, on a patient-specific basis, the severity of cam lesions for pre-operative planning.