Mechanical properties of ankle joint and gastrocnemius muscle in spastic children with unilateral cerebral palsy measured with shear wave elastography.

The aim of this study was to describe passive mechanical and morphological properties of the ankle joint and gastrocnemius medialis (GM) muscle in paretic and contralateral legs in highly functional children with unilateral cerebral palsy (UCP) using shear wave elastography (SWE). SWE measurements on the GM muscle were performed in both paretic and contralateral legs during passive ankle dorsiflexion using a dynamometer in 11 children (mean age: 10 years 6 months) with UCP. Torque-angle and shear modulus-angle relationships were fitted using an exponential model to determine passive ankle joint and GM muscle stiffness respectively. Based on shear-modulus-angle relationship, slack angle and shear modulus of GM muscle were compared between legs. GM and Achilles tendon length were determined at rest using ultrasonography. No significant difference was found between legs for passive ankle joint (p = 0.26; 11.2%; 95 %CI: 31.9, -9.4) and GM muscle passive stiffness (p = 0.62; -4.4%; 95 %CI: 14.7, -23.4). GM shear modulus at a common angle was significantly higher on the paretic leg (p = 0.02; +56.5%; 95 %CI: 100.5, 12.6). GM slack angle on the paretic leg was significantly shifted to a more plantarflexed position (p = 0.04; +25.5%; 95 %CI: 49.7, 1.3) and this was associated with a non-significant lower muscle length compared to the contralateral leg (p = 0.05; -4.5%; 95 %CI: -0.4, -8.7). Increased passive tension on the paretic leg when compared to the contralateral one may be explained in large part by muscle shortening. The role of altered mechanical properties remains unknown.