The effect of functional orthotic rearfoot posting on in-vivo tibial strain

Abstract

Medial tibial stress fractures occur secondary to failure in shear. It has been proposed that the use of functional foot orthotics (FFOs) may decrease their incidence by positioning the subtalar joint in the neutral position at heel strike. The purpose of this study was to evaluate the scientific basis of this concept by in-vivo measurement of tibial principal strain, the principal strain angle and shear strains during treadmill walking from rosette strain gauges bonded to the tibia of a male subject at two sites prone to stress fracture. Recordings were made while wearing Rockport walking shoes without orthotics, with non-posted graphite orthotics, with neutral rearfoot posted polypropylene orthotics, with 4° varus rearfoot posted polypropylene orthotics, and with neutral rearfoot posted kinetic wedge polypropylene orthotics designed to treat hallux limitus. None of the various modifications of FFOs tested in this study had a statistically significant effect on the compression strains during treadmill walking compared to the walking shoe alone, indicating that they were not affective in attenuating the ground reaction force. Their use was associated with a 22–51% increase in principal (p < .001) and a 9–35% increase in tibial shear strains (p = .003). The compression and shear strains and the angle of the principal strains (p < .001) were higher at the distal than at the proximal recording site, (p < .001 for all). The findings do not support the use of FFOs in any of the posting configurations tested as a means for lowering tibial shear strains or the concept of the biomechanical importance of the neutral position of the subtalar joint at heel strike.