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Frog-Leg Lateral Pelvic Radiographs Are Reliable for the Measurement of Acetabular Indices In Evaluation of Developmental Dysplasia of the Hip

Sunday, October 21, 2012: 8:00 AM
Melrose (Hilton Riverside)
Lauren E. Karbach, BA1, J. Herman Kan, MD2, William A. Phillips, MD3, Wei Zhang, PhD2 and Scott Rosenfeld, MD1, (1)Department of Orthopedic Surgery, Baylor College of Medicine, Houston, TX, (2)Department of Radiology, Baylor College of Medicine, Houston, TX, (3)Pediatric Orthopaedics & Scoliosis Surgery, Texas Children's Hospital, Houston, TX

Purpose: Current recommendations from the American Academy of Pediatrics (AAP) and the American College of Radiology (ACR) for developmental dysplasia of the hip (DDH) screening in infants 4 months or older is pelvic radiographs. For DDH, acetabular indices are routinely performed on AP pelvic radiographs. There is concern, however, that variability in these measurements makes them unreliable. We have observed that frog-leg lateral (FL) positioning allows more consistent visualization of the acetabular sourcil compared with AP. To our knowledge, there are no studies comparing FL to AP radiographs in measuring acetabular index. The purpose of this study is two-fold.  First to determine if there is any difference in acetabular index measurements made on AP vs. FL radiographs.  Second, to determine if acetabular index measurements are less variable when made on FL than AP radiographs.

Methods: An IRB approved, retrospective, case-control study was performed in children who underwent screening AP and FL radiography for suspected DDH.  274 hips were evaluated in 137 patients (mean age 9 months, range 1 month – 2 years and 9 months).  233(85%) were normal, and 41(15%) had DDH.  Radiographs were blindly and independently reviewed by a pediatric orthopedist and CAQ pediatric radiologist using a standard measurement technique. For each hip, AP and FL acetabular indices were measured at separate viewings with no knowledge of AP measurements when measuring FL. Interobserver measurement differences were calculated.

Results: The interobserver difference for acetabular index on AP was 0.48 (P=0.001) and for FL was 0.18 (P=0.114). The average acetabular index for AP was 24.13 and for FL was 23.66 (P=0.147).

Conclusion: There was no significant difference between acetabular index measurements made on the AP and FL radiographs. Acetabular indices are measured with less variability on the FL view compared with the AP view. These improvements in reliability on FL positioning are likely due to a more orthogonal projection of the sourcil creating a single sclerotic line to perform measurements. The AP position may have increased pelvic tilt leading to a non-orthogonal projection of the sourcil and no distinct line to measure.  We recommend using the FL radiograph for measuring acetabular indices in the assessment of DDH.

Table 1. Interobserver Reliability for AP and FL.

Average Difference








Table 2. Average acetabular index measurements AP versus FL.

Acetabular index



Frog-leg lateral


P value