The Association of Femoral Neck Stress Fractures with Femoral Acetabular Impingement

OBJECTIVES: To determine if there is an increased incidence of femoral acetabular impingement (FAI) in patients presenting with stress fractures of the femoral neck. METHODS: After IRB approval, the imaging studies of 25 athletes (22 females, 3 males, mean age 26, range 19 - 39 years) with femoral neck stress injuries were assessed for the presence of features suggesting FAI, including acetabular retroversion, coxa profunda, abnormal femoral head-neck junction, fibrocystic change, os acetabulae, labral tear and chondral injury. All subjects had to have an adequate AP Pelvis radiograph, a lateral radiograph of the affected hip, and an MRI of the affected hip. The alpha angle, anterior offset ratio, and center to edge (CE) angle were measured on radiographs. The grade of stress injury was determined on MR images. All images and measurements were made by a musculoskeletal fellowship trained radiologist, a fellowship trained orthopaedic surgeon, an orthopaedic sports medicine fellow and a physical medicine and rehabilitation resident. Charts were reviewed to determine treatment of the stress fracture, outcome and final follow up, as well as to determine if the patient had any further treatment for their hip. RESULTS: Of the 25 hips (18 right, 7 left) with femoral neck stress reactions, 9 were grade 2 (bone marrow edema), 5 were grade 3 (high T2 and low T1 marrow signal), and 11 were grade 4 (stress fracture). Twenty patients (80%) had coxa profunda - where the floor of the cotyloid fossa touches or extends beyond the ilioischial line (incidence in general population is 15.2% of males, and 19.4% of females). Coxa profunda, defined by the floor of the cotyloid fossa touching or extending beyond the ilioischial line and a center edge angle of more than 35(o), was present in 28% of subjects. Acetabular retroversion as assessed by the crossover sign was present in 42% (normal incidence is 5% of population). Center edge angle was greater than 35(o )in 20% and greater than 40 (o) in 12% of subjects. Any radiographic sign indicating pincer impingement was present in 66% of subjects. An abnormal alpha angle > 50(o) was seen in 4, and an abnormal anterior offset ratio < 0.18 was seen in 7. There were no cases of combined FAI. Labral tears were seen in 14, an abnormal femoral head/neck junction in 5, fibrocystic change in 2, os acetabuli in 2 and cartilage damage in 3. Four of the patients had surgery for their femoral neck stress fracture, while 5 patients (20%) had surgery for FAI within the 2 years after their hip stress fracture for persistent intra-articular hip pain. CONCLUSION: Our results suggest that patients with femoral neck stress injuries have a higher incidence of bony abnormalities associated with pincer impingement, such as coxa profunda and acetabular retroversion. Over coverage of the femoral head may place additional stresses on the femoral neck, predisposing an athlete to stress injury as they take their hips through the extremes of motion while running. This may explain why most femoral neck stress fractures occur on the medial, compressive, side of the femoral neck. We hypothesize that leverage of the lateral femoral neck against the over-covered acetabular rim may result in tensile forces on the medial femoral neck resulting in the stress injury. Some of our patients also had features of cam impingement, as evidenced by an increased alpha angle, decreased anterior offset ratio or fibrocystic change, although the incidence is similar to the values expected in the general population.