Reliability and Accuracy of Ultrasound Measurement of Hip Displacement in Children with Cerebral Palsy

Objective: Hip migration percentage (MP) measured on anteroposterior pelvis radiographs is the gold standard to assess the severity of hip displacement in children with cerebral palsy (CP). Repeated exposure of these children to ionizing radiation under a hip surveillance program is undesirable. Recently, a semi-automatic approach to measure MPUS on ultrasound (US) images was validated in a phantom study. This pilot in vivo study applied the previous phantom method and aimed to determine the reliability and accuracy of the MPUS. Methods: Thirty-four children (23 boys and 11 girls) aged 8.9 +/- 3.1 y old and diagnosed with CP were recruited. A total of 59 hips were scanned once, while 43 of these were scanned twice to evaluate the test-retest reliability. Two raters (R1 and R2) manually measured MPUS; procedures included selecting images of interest, cropping a region of interest and removing soft tissues on hip US images. Custom software was developed to measure MP automatically after the manual pre-image processing. Results: The intra-class correlation coefficients (ICC2,1) for the test-retest (R1), intra-rater (R1) and inter-rater (R1 vs R2) reliabilities were 0.90, 0.94 and 0.82, respectively. The standard error of measurement of MPUS for all three evaluations was <= 3.0%. The mean absolute difference between MPUS and MPX-ray and the percentage of MPUS within clinical acceptance error of 10% for R1 and R2 were (R1: 6.2% +/- 4.9%, 84.7%) and (R2: 7.6% +/- 6.1%, 73.7%), respectively. Conclusion: This study demonstrated that US scans were repeatable and MPUS could be measured reliably and accurately.