A Novel Model of Hip Femoroacetabular Impingement in Immature Rabbits Reproduces the Distinctive Head-Neck Cam Deformity

Abstract

Background: Femoroacetabular impingement (FAI) is a leading cause of hip pain in young adults and often leads to degenerative osteoarthritis (OA). A small animal model of hip deformities is crucial for unraveling the pathophysiology of hip OA secondary to FAI. Purposes: To (1) characterize a new minimally invasive surgical technique to create a proximal femoral head–neck deformity in a skeletally immature rabbit model and (2) document the effect of an injury to the medial proximal femoral epiphysis on head–neck morphology at 28 days after the injury. Study Design: Controlled laboratory study. Methods: Six-week-old New Zealand White rabbits (n = 10) were subjected to right hip surgery, with the left hip used as a control. An epiphyseal injury in the medial femoral head was created using a 1.6-mm drill. Hips were harvested bilaterally at 28 days after surgery. Alpha and epiphyseal shaft angles were measured on radiographs. Alpha angles at the 1- and 3-o’clock positions were measured on the oblique axial plane of micro–computed tomography images. Bone bar formation secondary to growth plate injuries was confirmed using alcian blue hematoxylin staining. Results: All hips in the study group showed a varus–type head-neck deformity, with lower epiphyseal shaft angles on anteroposterior radiographs versus those in the control group (133°± 8° vs 142°± 5°, respectively; P = .022) and higher epiphyseal shaft angles on lateral radiographs (27°± 12° vs 10°± 7°, respectively; P < .001). The mean alpha angles in the study group were higher at both the 1- (103°± 14° vs 46°± 7°, respectively; P < .002) and 3-o’clock (99°± 18° vs 35°± 11°, respectively; P < .002) positions than those in the control group. Alcian blue hematoxylin staining of all hips in the study group indicated that the injured physis developed a bony bar, leading to growth plate arrest on the medial femoral head. Conclusion: The proposed model led to growth arrest at the proximal femoral physis, resulting in a femoral head–neck deformity similar to human FAI. Clinical Relevance: Our novel small animal model of a femoral head–neck deformity is a potential platform for research into the basic mechanisms of FAI disease progression and the development of disease–modifying therapies.