Children with cerebral palsy (CP) commonly have bony deformities of the foot, which lead to pain and gait problems. One of the causes of such a deformity is an imbalance in muscle forces around the foot. In turn, the bony deformity can also alter muscle function, due to, for example, altered muscle moment arm lengths. In this study, the altered muscle function due to hindfoot varus deformities in children with CP was investigated using musculoskeletal models. The first aim of this study was to create personalized musculoskeletal models of the foot based on weight-bearing computed tomography (WBCT) data. Secondly, we applied the model to get insight in how joint axis orientations and muscle moment arms are altered in varus feet, and how this leads to differences in muscle function during gait.
Models were created in OpenSim for seven children with a hindfoot varus deformity due to CP, and four adults with neutral feet. Each model was based on WBCT scans and included five degrees of freedom in the foot (talocrural, subtalar, Chopart, Lisfranc and metatarsophalangeal joints). The orientations of the foot joint axes and moment arms of the extrinsic foot muscles were calculated. Subsequently, the models were combined with motion capture and ground reaction force data to calculate muscle activations during gait.
The joint axis orientations showed greater variability in the group of children with CP compared to the control group; in most subjects, changes in axis orientation were observed that may lead to a more rigid foot. Furthermore, the dorsiflexion moment arm of the tibialis anterior decreased, while the inversion moment arm increased; thus, the tibialis anterior became an even more effective invertor when a varus deformity of the foot was present. On the other hand, the eversion moment arms of the peroneal muscles tended to become smaller, meaning they would be less effective in resisting the varus deformity. Static Optimization results showed decreased activity in the tibialis muscles, and increased activity in the peroneal muscles. This increased activity might be necessary due to the smaller moment arms, and/or to stabilize the ankle.
This is the first study in which a musculoskeletal foot model was developed based on personalized bone data of feet with bony deformities in weight-bearing conditions. Distinct changes were shown in muscle function when a varus deformity is present, which might lead to progression of the deformity. This contributes to a better understanding of the altered muscle function due to foot deformities, which may eventually contribute to improvement of treatment to prevent the progression of foot deformities.