Radiographs Underestimate Lateral Center Edge Angle and Tonnis Angle Measurements Compared to CT Scan in Assessment of Borderline and Frank Acetabular Dysplasia.

Purpose: To compare lateral center-edge angle (LCEA) and Tönnis angle (TA) values measured in radiographs and CT scans with commercially available software and to determine the degree of concurrence in the classification of acetabular dysplasia as depicted in radiographs and CT scans.

Methods: Retrospectively collected data from patients undergoing preoperative CT protocol and X-rays for hip arthroscopy from June 2019 to December 2021. The preoperative anteroposterior supine view of the pelvis was utilized to measure LCEA and TA, and measurements were compared to CT scan views through commercially available software.

Results: In total, 323 patients (371 hips) were included, with 216 females (67%) and a mean patient age of 35.3±14.5. There was a statistically significant difference (P<0.001) in LCEA and TA measurements between X-ray and CT scan, with CT yielding higher mean ± SD values (LCEA: 32.9 ± 7.3, TA: 8.2 ± 5.9) compared to X-ray (LCEA: 30.3 ± 6.3, TA: 4.8 ± 4.6). The absolute mean difference between CT and X-ray was 4.1 for LCEA and 4.4 for TA. When defining dysplasia as LCEA<25°, 77 (21.8%) hips were dysplastic by X-rays (odds ratio, 2.0), and 43 (11.6%) by CT (P<0.001). Using a threshold of >10° for TA, 71 (19.1%) hips were dysplastic by X-rays (odds ratio, 0.5) and 118 (32%) by CT (P<0.001). A strong correlation was obtained between radiographs and CT on measured LCEA (r=0.78) and TA (r=0.67).

Conclusion: Measuring with X-ray underestimates both LCEA and TA values compared to CT. Hip dysplasia diagnosis was twice as likely with LCEA, and half as likely with TA when measured through X-rays. CT scans with commercially available software offer an alternative viewpoint for assessing acetabular dysplasia and can serve as a valuable complement when used in addition to X-ray measurements.

Level of evidence: Level IV, Retrospective Case Series.