The importance of mediolateral forces on foot rehabilitation after injury

Abstract

Displaced fractures of the foot and ankle take considerable time to heal. Throughout this rehabilitation period, the goal is to provide sufficient stimulus to the bone to prevent bone loss while limiting the chance of re‐injury. Both of these goals require understanding the ground reaction forces (GRFs) applied to the foot while walking. Most understanding of the forces to which bones of the foot are subjected is premised on data developed from force plates in laboratories that evaluate movement. The coordinate system of force plates is typically aligned with the physical surroundings as well as the coordinate system of motion capture equipment to create a world coordinate system (WCS). This WCS also aligns with most skeletal elements and joints. The anatomical directions of the foot, particularly during walking, do not necessarily match up to these primary axes. The degree to which foot placement impacts the internal forces and moments of the foot and ankle remains unclear; however, the variation of foot placement has the potential to substantially change internal forces. Consequently, the purpose of this work is to determine if forces aligned in a foot coordinate system (FCS) differ from those in the WCS in order to inform orthopedic treatment and rehabilitation.Twenty participants were recruited to walk unshod for this study. Kinematic and kinetic data were collected using an eight‐camera Qualisys system with four Kistler force plates. The FCS, which tracks the orientation of foot as it moves through space, was calculated from the position of the heel and first and fifth metatarsal heads. GRFs were translated and rotated from WCS to FCS using custom‐developed MatLab code. Proximodistal, mediolateral, and superioinferior forces were compared between WCS and FCS. In FCS, mediolateral forces developed during the propulsive phase are 150–200N larger than in WCS (p<0.001), and the maximum mediolateral force is directed toward the centerline of the body in WCS, while in FCS it is directed laterally.Rotating the GRF into FCS produces a different picture of internal forces than is usually understood from WCS. Typically, mediolateral forces are considered less consequential when considering treatment for foot and ankle injuries. These data show that mediolateral forces (relative to the foot) are larger than previously understood. Injuries that are sensitive to these mediolateral forces include metatarsal fractures, Lisfranc injuries, talar neck fractures, and distal fibular fractures, among others. Implants and interventions used to stabilize these injuries should take into consideration these forces including potential modifications to the rehabilitation protocol.