A novel radiographic finding for estimation of the cortical-cancellous boundary: A magnetic resonance imaging case series of patients with femoroacetabular impingement

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Purpose: To determine if comparing alpha angle measurements of the outer cortical margin (preoperative alpha angle) and the inner cortical margin (theoretical postoperative alpha angle) on magnetic resonance imaging (MRI) can estimate the amount of sclerotic bone that needs to be resected to restore the alpha angle to normative values (<55⁰) in patients with symptomatic femoroacetabular impingement (FAI). Methods: A retrospective review was conducted on patients with FAI undergoing primary hip arthroscopy and CAM osteoplasty from June 2013 to February 2017. Inclusion criteria were radiologist-measured alpha angles >70° on 3T oblique axial MRI, unilateral primary hip arthroscopy, and age ≥18. Exclusions included previous hip surgery, hip trauma, avascular necrosis, advanced osteoarthritis, dysplasia, poor quality images, or missing MRI images. Alpha angles were measured preoperatively using the outer cortical margin and postoperatively using the inner cortical margin of femur. Surgeries were performed by the senior surgeon. Paired t-tests compared preoperative and postoperative alpha angles, and intraclass correlation coefficients (ICC) were used to assess inter-rater and intra-rater reliability. Results: One hundred patients (100 hips) were included with an average age of 40.8 years (range, 18.7-64.9), with 75 % being male (n = 75). The average BMI was 24.7 ± 3.9 (range, 17.1-38.8). The average maximum alpha angle at the outer cortical margin was 77.0 ± 4.2° (range, 70.0-90.8), with an ICC for inter-rater reliability of 0.861, indicating excellent agreement. The average inner cortical alpha angle was 46.4 ± 3.4° (range, 36.5-55.4), significantly less than 55° (p<0.001). The mean difference between the outer and inner cortical alpha angles was -30.6 ± 5.1° (p< 0.001). Conclusions: The difference between outer and inner cortical alpha angles on MRI can accurately estimate the amount of sclerotic bone resection needed to restore the alpha angle to normative values in patients with FAI. This information may be directly applied clinically marker to assess adequacy of CAM resection.

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