Correct positioning for hip radiographs allows reliable measurement of hip displacement in cerebral palsy

Aim The pelvic radiograph in children with cerebral palsy (CP) can inform the degree of hip displacement by calculation of the migration percentage. However, concerns have arisen about the reliability of this measurement. The present study examined the reliability of radiographic assessment of displacement and the importance of positioning and reporting experience. Method Two pelvic radiographs, taken at least an hour apart, were performed in 20 children (total 40 hips) in the standard position by a trained paediatric radiographer. Children (13 males, seven females) were aged 30 months to 10 years with severe bilateral spastic CP in Gross Motor Function Classification System levels IV (n=10) and V (n=10). The migration percentage of each hip was measured on two occasions 3 months apart by two experienced radiologists independently. Comparisons of migration percentage were made in three ways by (1) the same observer at the same time, (2) the same observer 3 months apart, and (3) different observers 3 months apart. Results Migration percentage (mean [SD]) was (1) 3.2% (3.5), (2) 3.3% (3.2), and (3) 3.7% (3.8) respectively. Interpretation Reliable measures of migration percentage can be obtained with correct positioning and if reported by suitably experienced radiologists, making this a valid surveillance method. Clinical decisions can be made taking into account an expected error in hip displacement measurements.