Abstract
Background: The 45 Dunn radiograph is the preferred projection to detect cam morphology in femoroacetabular impingement (FAI). 3D CT and MRI may be obtained to further characterize cam morphology and soft tissue pathology. Axially rotated 3D MRI may be an alternative to 3D CT that allows clinicians to assess both soft tissue pathology and characterize bony deformity without exposing the patient to additional radiation. Therefore, the purpose of this study was to compare 3D imaging to standard radiographs and to compare 3D CT to 3D MRI for the assessment of cam morphology. Methods: This was a retrospective review of patients from January 2012 to February 2024 with symptomatic FAI and/or hip dysplasia that had failed non-operative management. Patients were included if they had preoperative 3D MRI, 3D CT, standard AP pelvis, and 45 Dunn radiographs. Patients with prior hip surgery were excluded. 3D MRI and CT reconstructive images were rotated along the axis of the femoral neck and utilized the clock-face localization technique for specification of the alpha angle. Alpha angle measurements were obtained using standard techniques described by Notzli. Averages and standard deviations were calculated, and independent t-tests were performed to compare the alpha angles between each modality. The frequency of alpha angles above the threshold of 55° for each imaging modality was also calculated. Results: One hundred and twenty-nine patients were included. The average alpha angles on the 45 Dunn, CT, and MRI were 56.6 ± 10.2, 62.6 ± 18.8, and 62.3 ± 17.3, respectively. There was a significant difference in the alpha angles between 45 Dunn and CT (p=0.002) and 45 Dunn and MRI (p=0.001). There was no significant difference in alpha angles between CT and MRI (p=0.913). The average maximum alpha angle positions on MRI and CT were both 2:57 (p=0.722). At an alpha angle of 55° or greater 3D CT and 3D MRI detected a similar number of patients (60%, 61%, respectively) while 45 Dunn detected 53%. Conclusion: There was no difference between 3D CT and 3D MRI, with both detecting a higher percentage of patients with cam morphology than 45 Dunn radiographs. 3D imaging is a valuable tool that should be utilized to detect and characterize 3D deformity. 3D MRI should be considered as a viable alternative to 3D CT due to its ability to characterize both bony morphology and soft tissue pathology without additional radiation exposure.