Subtalar joint biomechanics in children with cerebral palsy with midfoot break: A pilot study

BACKGROUND: Midfoot break deformity is common in children with cerebral palsy, presenting as extreme planovalgus, with midfoot dorsiflexion. Muscle imbalance, particularly between tibialis posterior (tib-post) and tibialis anterior (tib-ant), is frequently blamed. Recent work has shown that midfoot break is associated with an altered subtalar joint axis orientation but the impact on dynamic moments and moment arms remains unknown. This pilot study aims to determine whether a novel methodology for determining subtalar joint biomechanics reveals clinically important biomechanical differences in a sample of children with midfoot break compared with typically developing children, informing future trial design. METHODS: Weight-bearing cone-beam CT scans of the foot and ankle were obtained, and gait analysis conducted on nine typically developing children and six with midfoot break. Personalized musculoskeletal models were generated. Subtalar joint moments were calculated and muscle moment arms of tib-post and tib-ant estimated throughout the gait cycle. FINDINGS: The novel methodology successfully identified differences between children with midfoot break and typically developing children. Five out of six children with midfoot break exhibited significantly larger everting subtalar joint moments for at least part of the gait cycle, with ground reaction force moment arms 60% greater. Tib-ant acted as a stronger evertor for those with midfoot break and tib-post had significantly lower inverting moment arms for four midfoot break children. INTERPRETATION: This novel methodology has demonstrated a relationship between subtalar joint alignment and midfoot break, revealing biomechanical pathways which may be contributing to the development of the deformity in children with cerebral palsy.