The Reimers’ hip migration percentage (MP) is commonly used to examine the amount of lateral displacement of the femoral head in children with cerebral palsy (CP). Abnormal forces of the hip flexor and adductor muscles are proposed as the contributing factors of hip dislocation in children with CP. Shear wave elastography (SWE) is an ultrasound based technique which provides a quantitative measurement of the tissue stiffness. The aim of this study was to assess the correlation between the MP and the stiffness of the hip muscles.

This prospective study included 50 hips of 25 children with spastic CP. All muscles were evaluated in longitudinal plane by SWE with a linear transducer that enables scanning with a frequency ranging of 9−4MHz. The stiffness of the gluteus medius and maximus, adductor magnus and iliopsoas muscles was measured by SWE. The MP was measured on anteroposterior radiographs. The correlation between the Reimer's MP and the stiffness of hip flexors and adductors were analyzed. The correlations among the stiffness of the hip flexor-extensor and adductor-abductor muscle groups were also assessed.

There is a moderate positive correlation between MP and the stiffness of hip flexor muscles (r=0.70, P<0.05). There is a moderate positive correlation between MP and the stiffness of hip adductor muscles (r=0.71, P<0.05). The correlation between the stiffness of flexor and extensors is weak (r=0.10, P≥0.05). There is a weak correlation between the stiffness of abductor and adductor muscles (r=0.32, P≥0.05).

We suggest that increased flexor and adductor muscle stiffness would be a potential high risk factor of hip dislocation in children with CP. Measuring muscle stiffness may provide helpful information during follow-up of these patients.