In Vivo Dynamic Evaluation of Ankle Joint and Muscle Mechanics in Children With Spastic Equinus Deformity Due to Cerebral Palsy: Implications for Recurrent Equinus.

Equinus is the most common deformity in children with cerebral palsy. Spastic equinus is typically defined as the inability to dorsa-flex the foot above plantigrade, with the hindfoot in neutral position and the knee in extended position. Approximately 90% of the deformities in cerebral palsy occur in the ankle and foot region alone with the incidence of equinus being around 75%. Spastic equinus exhibits poor muscle control and muscle weakness around ankle and foot, resulting in bone deformities and gait abnormalities. Non-operative conservative management of equinus is typically undertaken up until 8 years in order to prevent recurrent equinus or overcorrection by avoiding high-growth phase of child's development for surgical intervention. Despite these precautions, long term follow-up studies report up to 48% of recurrence rate post-surgery. Recurrence surgery not only increases the economic burden on the society but also has a debilitating impact on children and their families. Previous research is focused on extrinsic risk factors such as CP type, demographic parameters, and clinical gait parameters for surgical recurrence and none assessed the dynamic impact of intrinsic bone deformity on ankle joint and muscle mechanics. A primary reason for this recurrence could be a lack of understanding of bone deformity that might be forcing the child to adapt altered ankle joint and muscle mechanics (bone kinematics, cartilage contact parameters, muscle strain) during dynamic activities. In fact, the surgical treatment of fixed equinus does not consider any bone corrections and focus on muscle release or lengthening only. Being a dynamic pathology, it is critical to understand the in vivo effect of weak ankle joint musculature on joint mechanics and the resultant bone deformity. However, no such efforts have been made so far in the literature. With the advent of technology, researchers have developed and validated dynamic magnetic resonance imaging techniques to analyze in vivo muscle and joint mechanics. Processing this data enables researchers to analytically track bones without having to identify specific points or anatomical landmarks and thus provides the ability to track muscle motion as well as skeletal motion. Thus properties such as bone kinematics, cartilage contact mechanics, musculotendon moment arms, muscle strain and tendon strain are available from these analyses. These techniques can be successfully employed in equinus research to evaluate ankle joint and muscle mechanics in vivo.