Statistical Shape Modeling of an Experimental, Induced Cam–Femoroacetabular Impingement Deformity in a Rabbit Model: A Platform to Study Mechanism of Hip Disease

Background: Femoroacetabular impingement (FAI) is a common determinant of hip pain in young adults and an established risk factor in the subsequent development of osteoarthritis (OA). The mechanism of hip OA secondary to FAI is unknown. Small-animal models are critical translational tools to understand mechanisms of disease and develop interventional therapies. Kamenaga and colleagues proposed a novel animal model to mimic cam-FAI; however, 3D morphology of the induced deformity has not been objectively investigated. Purpose: To use statistical shape modeling to quantitatively describe the induced proximal femoral head-neck deformity in order to take the necessary step in validating this animal model as a translational model for human cam-FAI. Study Design: Controlled laboratory study. Methods: Six-week-old immature New Zealand White rabbits (n = 13) were subject to right femur physis injury, with left femurs serving as controls. Micro–computed tomography images of femurs were taken at minimum 4 weeks after injury. 3D reconstructions were aligned and underwent statistical shape modeling with 2048 particles placed on each femur. Differences between mean shapes were calculated and analyzed using the Hotelling T 2 test. Principal component analysis was used to describe shape variation, and parallel analysis was used to determine the statistically significant modes. Results: Hotelling T 2 test demonstrated significant differences between cam-FAI and control mean shapes ( P < .01). The cam-FAI mean shape protruded above the control mean by a maximum of 0.8 mm in the anterolateral head-neck junction with sustained protrusions of ~0.6 to 0.8 mm over the anterosuperior aspect and anteroposterior midline of the femoral head-neck junction. Maximum deviations between individual cam-FAI femurs and the mean control femur ranged between 0.1 and 1.6 mm in the same region. The first 6 modes explained 92.1% of the cumulative variation, and the first 13 modes were statistically significant, confirming the deformity. Conclusion: The proposed model resulted in a head-neck cam deformity similar to human cam-FAI. Clinical Relevance: This proposed animal model creates a cam-type deformity similar to that observed in human FAI, helping validate the model as a platform to study mechanisms of hip FAI OA and develop future interventional therapies for this disease.