Potential for change in US diagnosis of hip dysplasia solely caused by changes in probe orientation: patterns of alpha-angle variation revealed by using three-dimensional US.

Pediatric Imaging Purpose: To use three-dimensional (3D) ultrasonography (US) to quantify the alpha-angle variability due to changing probe orientation during two-dimensional (2D) US of the infant hip and its effect on the diagnostic classification of developmental dysplasia of the hip (DDH). Materials and Methods: In this institutional research ethics board-approved prospective study, with parental written informed consent, 13-MHz 3D US was added to initial 2D US for 56 hips in 35 infants (mean age, 41.7 days; range, 4-112 days), 26 of whom were female (mean age, 38.7 days; range, 6-112 days) and nine of whom were male (mean age, 50.2 days; range, 4-111 days). Findings in 20 hips were normal at the initial visit and were initially inconclusive but normalized spontaneously at follow-up in 23 hips; 13 hips were treated for dysplasia. With the computer algorithm, 3D US data were resectioned in planes tilted in 5 degrees increments away from a central plane, as if slowly rotating a 2D US probe, until resulting images no longer met Graf quality criteria. On each acceptable 2D image, two observers measured alpha angles, and descriptive statistics, including mean, standard deviation, and limits of agreement, were computed. Results: Acceptable 2D images were produced over a range of probe orientations averaging 24 degrees (maximum, 45 degrees) from the central plane. Over this range, alpha-angle variation was 19 degrees (upper limit of agreement), leading to alteration of the diagnostic category of hip dysplasia in 54% of hips scanned. Conclusion: Use of 3D US showed that alpha angles measured at routine 2D US of the hip can vary substantially between 2D scans solely because of changes in probe positioning. Not only could normal hips appear dysplastic, but dysplastic hips also could have normal alpha angles. Three-dimensional US can display the full acetabular shape, which might improve DDH assessment accuracy. (C) RSNA, 2014