The effect of varus foot deformities on muscle moment arms in children with cerebral palsy

Children with cerebral palsy (CP) commonly develop deformities of the foot [1], which lead to pain and gait problems. One of the causes of such deformities is likely an imbalance in muscle forces around the foot [2]. In turn, these deformities can also alter muscle function due to altered muscle moment arms. Moment arms can be estimated using musculoskeletal models, but models based on generic bone geometry are unlikely to represent the deformity accurately. Weight-bearing computed tomography (WBCT) enables assessment of abnormal bone alignment under loaded conditions [3]. What are the changes in moment arm lengths of the main invertors and evertors (i.e. the tibialis and peroneal muscles) around the subtalar joint in children with cavovarus and equinovarus foot deformity due to CP? Six children with a severe hindfoot varus deformity due to CP (one female, aged 13.8 ± 2.3 years) and four typically developed (TD) adults (one female, aged 35.8 ± 4.8 years) were included. Personalized musculoskeletal foot models were created in OpenSim Creator [4] using WBCT scans. This foot was attached to the full-body OpenSim gait2392 model, which was scaled using gait analysis data. Muscle moment arms were calculated using OpenSim [5,6] and normalized to tibia length. A non-parametric Mann-Whitney U test was used to compare between groups. Normalized inversion-eversion moment arm lengths are shown in Fig. 1. The tibialis anterior had an inversion moment arm in the deformed CP feet, in contrast to an eversion moment arm in the TD group. No differences were found for the tibialis posterior. Although there was no overall group effect, the eversion moment arm of the peroneal muscles was smaller in most CP children compared to the TD group. Fig. 1 - Moment arms around the subtalar joint, normalized to tibia length. Note the different scales on the vertical axes. * p <.01.Download : Download high-res image (93KB)Download : Download full-size image We present the first study to evaluate altered moment arms in feet of children with CP using personalized musculoskeletal foot models based on WBCT scans. Our results indicate that the tibialis anterior becomes a more effective invertor with a varus deformity of the foot. Despite the fact that the tibialis posterior is often seen as an important cause of the varus deformity, its function as expressed by the moment arm does not change. On the other hand, the eversion moment arms of the peroneal muscles tend to become smaller, meaning they would be less effective in counterbalancing the deformity. Together, these altered moment arms would lead to an even larger varus moment in the subtalar joint with similar muscle forces, thereby pulling the foot even more towards varus. Our results could explain the progression of the deformity once the deformity is present.