Exploring subtalar joint axis alignment in pathological feet of children with cerebral palsy using weight bearing CT

The subtalar joint (STJ) is essential for proper foot functioning and fundamental to the biomechanics of this joint is its axis orientation. It is currently unknown how this orientation is altered in cerebral palsy (CP) across the range of foot deformities including varus, planovalgus and midfoot break (MFB). The aim of this study was to quantify STJ axis orientations in children with CP and typically developing (TD) children. In this multicentre study, weight-bearing CT (WBCT) data of participants (21 CP and 9 TD) were collected and foot bone surfaces were segmented. STJ axes were derived from the talar surfaces using shape fitting techniques. STJ axis medial deviation and inclination angles were measured with respect to two anterior/posterior axes, one derived from the tibia and the second from the foot. Significant differences were identified between groups in both medial deviation and inclination angles. Planovalgus feet without MFB exhibited greater medial deviation (26.2±3.2°) and lower inclination angles (18.2 ± 7.3°) than TD feet (medial deviation: 19.4±3.6° and inclination: 25.7 ± 3.8°). Children with MFB had even greater medial deviations (37.7 ± 7.2°) and lower inclinations (-1.7 ± 10.8°) with the mean STJ axis orientation switching to pointing towards the ground. Varus feet had the greatest STJ axis inclination (32.3 ± 7.7°) and the widest range of medial deviations (0.8 ± 21.5°). This study presents STJ axis alignment in CP for the first time. The use of WBCT imaging has identified changes in alignment of key foot bones, with potential for more detailed biomechanical analysis (e.g. muscle moment arms) and individualised management in the future. This study presents STJ axis alignment in CP for the first time. The use of WBCT imaging has identified changes in alignment of key foot bones, with potential for more detailed biomechanical analysis (e.g. muscle moment arms) and individualised management in the future.