EC Orthopaedics

Background: Acetabular index (AI) is the most commonly utilized radiographic measure for developmental dysplasia of the hip (DDH). However, there is variability in measurement of AI, thus impairing reproducibility. Frog-leg lateral (FL) positioning allows more consistent visualization of the acetabular sourcil compared with anteroposterior (AP). The purpose of this study is to determine if there is any difference in AI measurements on AP versus FL radiographs and if there is less measurement variability on FL than AP radiographs.

Methods: An IRB approved, retrospective study was performed on 274 hips that underwent screening AP and FL radiography for DDH. Radiographs were reviewed using a standard measurement technique. Interobserver measurement differences and the differences between measurements made on AP and FL radiographs were calculated.

Results: 233 (85%) were normal, and 41 (15%) had DDH. The average AI on AP vs FL was not significantly different (24.13 and 23.66 degrees, respectively; P = 0.147). There was a significant difference in measurement in variability for the interobserver difference for AI on AP (0.48 degrees; P = 0.001), whereas there was no significant difference for AI on the FL (0.18 degrees; P = 0.114).

Conclusion: AI had less variability on the FL view than the AP view, likely due to a more orthogonal projection of the sourcil creating a single sclerotic line with which to perform measurements. We recommend using the FL radiograph as a supplement to the AP radiograph for measuring AI for following up patients diagnosed with DDH.

Level of Evidence: Level III.

Keywords: Developmental Dysplasia of the Hip (DDH); Acetabular Index; Frog Lateral

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