Background: EOS imaging offers a low-radiation alternative to conventional radiography (CR) and has little to no magnification effects. However, it is unclear how radiographic measures may be affected using EOS. The present study aims to determine the reproducibility of measures of acetabular morphology on EOS images as compared with CR, and to directly compare the 2 imaging modalities. Methods: A total of 21 consecutive patients (66.7% female; 14.4±4.7 y) indicated for an open hip preservation procedure with both an anterior-posterior pelvis radiograph and EOS image performed preoperatively were included. Three orthopaedic surgeons measured Tönnis angle, lateral center edge angle (LCEA), acetabular depth-width ratio (ADR), and extrusion index (EI). Measurements were performed twice, 2 weeks apart. Reliability between observers and time points was measured using intraclass correlation coefficients, and agreement between time points and modalities was measured using Bland-Altman analysis. Results: On EOS images, inter-rater reliability was 0.86 for Tönnis angle, 0.86 for LCEA, 0.74 for ADR, and 0.93 for EI. On CR, inter-rater reliability was 0.86 for Tönnis anlge, 0.90 for LCEA, 0.82 for ADR, and 0.84 for EI. In the agreement analysis, biases between imaging modalities were observed. On average, raters measured Tönnis angle and EI higher on EOS images than CR (2.22 degrees, 1.09%, respectively); and LCEA and ADR lower on EOS images than CR (1.54 degrees, 1.14% respectively). Limits of agreement (LOA) between modalities were similar to that of LOA observed in intra-rater analysis. Conclusions: Measures of acetabular morphology performed on EOS images have similarly high intra-rater and inter-rater reliability compared with CR. Measures performed on EOS also have similar intra-rater agreement as compared with CR. Intermodality agreement had similar LOA as intra-rater agreement on either individual imaging modality. Small biases between imaging modalities were detected. Levels of Evidence: Level I—diagnostic study: investigating a diagnostic test.

INTRODUCTION: Young patients followed for hip disorders often require serial imaging and incur significant radiation to the abdomen and pelvis. The EOS imaging system offers a low-radiation alternative. However, consistency with radiographic parameters from conventional radiographs (CR) have not been studied. We evaluated the reliability and validity of EOS imaging in comparison to CR in evaluating acetabular morphology. METHODS: 48 hips (27 patients, 66.67% female, 14.4 ± 3.6 years old), indicated for open hip preservation procedure between 2015-2017, which had both EOS imaging and AP pelvis CR taken pre-operatively were examined. Both symptomatic and contralateral hips were measured, excluding hips with prior osteotomies. Three orthopaedic surgeons measured Tonnis angle, lateral center edge angle (LCEA), acetabular depth ratio (ADR), and extrusion index (EI) in both imaging modalities. Images were measured twice, two weeks apart. Reliability was assessed using intraclass correlation coefficients (ICC). Bland-Altman analysis was used to evaluate test-retest agreement and inter-modality agreement. RESULTS: Test-retest reliability was excellent for all raters on all tests (ICC range 0.8047 to 0.9714). Inter-rater reliability was also excellent for all measures on both modalities (ICC range 0.8593 to 0.9547). ICC values were consistent across EOS and CR. By Bland-Altman analysis, test-retest agreement was high, with no statistically significant mean differences in any test or modality (p > 0.6) except LCEA measured on EOS images (p = 0.02), which was 1.16° lower. Tonnis angle (p < 0.005) and ADR (p < 0.001) were statistically different from EOS to CR. Bias was also not uni-directional across tests: LCEA and ADR were, on average, higher on CR while Tonnis angle and EI were, on average, higher on EOS images. Reliability between the EOS and CR images was excellent for all raters on all tests (ICC range 0.8402 to 0.9458). DISCUSSION AND CONCLUSION: LCEA, ADR, EI, and Tonnis angle have consistently high test-retest reliability, test-retest agreement, and inter-observer reliability in both EOS imaging and CR to evaluate acetabular morphology. Inter-modality disagreement was observed between EOS and CR, but the difference was small and likely not clinically significant. When establishing acetabular morphology, these results suggest that EOS may provide an opportunity to minimize radiation exposure for patients followed long-term for hip pathology while maintaining the accuracy of diagnostic measures. [Table: see text][Table: see text]