Correlations and Reproducibility Between Radiographic and Radial Alpha Angles in the Evaluation of Cam Morphology

Naomi Kobayashi; Kosuke Sumi; Shota Higashihira; Hyonmin Choe; Taro Tezuka; Takayuki Oishi; Yohei Yukizawa; Akira Morita; Yutaka Inaba

doi:10.1177/2325967120932922

Correlations and Reproducibility Between Radiographic and Radial Alpha Angles in the Evaluation of Cam Morphology

Orthop J Sports Med. 2020 Jul 9;8(7):2325967120932922. doi: 10.1177/2325967120932922. eCollection 2020 Jul.

Authors

Naomi Kobayashi¹, Kosuke Sumi², Shota Higashihira², Hyonmin Choe², Taro Tezuka², Takayuki Oishi¹, Yohei Yukizawa¹, Akira Morita², Yutaka Inaba²

Affiliations

¹ Department of Orthopaedic Surgery, Yokohama City University Medical Center, Yokohama, Japan.
² Department of Orthopaedic Surgery, Yokohama City University, Yokohama, Japan.

Abstract

Background: The alpha angle used to evaluate cam morphology can be determined on different imaging views; however, 2-dimensional (2D) imaging can present limitations in terms of the reproducibility of the radial alpha angle. Recent developments in 3-dimensional (3D) high-resolution magnetic resonance imaging (MRI) have allowed detailed evaluations of the radial alpha angle.

Purpose: To determine whether there are any correlations or discrepancies between the 2D alpha angle on plain radiography and the maximum radial alpha angle on 3D MRI.

Study design: Cohort study (diagnosis); Level of evidence, 2.

Methods: A total of 42 hips from 39 patients (19 males, 20 females) were analyzed, including 22 hips with femoroacetabular impingement (FAI; mean age, 41 years) and 20 hips with borderline developmental dysplasia of the hip (BDDH; mean age, 43 years). Radial images were reconstructed from 3D multiple echo recombined gradient echo (MERGE) MRI. Differences in the maximum radial alpha angle on MRI between hips with FAI and BDDH were evaluated. Correlations and discrepancies between the maximum radial alpha angle on MRI and alpha angles on the anteroposterior, cross-table lateral, and 45° Dunn views of radiography were also evaluated.

Results: The maximum radial alpha angle was significantly higher for hips with FAI than for hips with BDDH. On average, the greatest alpha angle on radial MRI was higher than the alpha angle on each of the 3 radiographic views for both FAI and BDDH. The 45° Dunn view revealed the smallest discrepancy for both FAI (P = .005) and BDDH (P = .002). The cross-table lateral view had the highest correlation with the maximum radial alpha angle for BDDH (P < .001).

Conclusion: We reconfirmed the utility of the 45° Dunn view, with it presenting the best reproducibility for the maximum radial alpha angle in the evaluation of cam morphology, while the cross-table lateral view revealed the best correlation with the maximum radial alpha angle, particularly for hips with BDDH.

Keywords: alpha angle; borderline developmental dysplasia of the hip; cam morphology; femoroacetabular impingement; radial MRI.