Relative forefoot abduction and its relationship to foot length in vitro

Background. The human foot is often modelled as a rigid body in gait analysis. A more realistic model separates this segment into a forefoot and rearfoot. However, no three-dimensional data has been published on dynamic relative ab-adduction between these segments, and how this impacts changes in foot shape. Objective. The purpose was to quantify three-dimensionally forefoot ab-adduction relative to the rearfoot in vitro, and to determine how forefoot ab-adduction affects foot length. Methods. Video data were collected from reflective marker triads affixed to the ends of Steinmann pins drilled into the tibia, calcaneus, cuboid, and the first and fifth metatarsal bones. Medial and lateral foot length and forefoot ab-adduction relative to the rearfoot were calculated under two axial tibial loads (200 N, 600 N) and two input motions (dorsi-plantarflexion, internal external tibial rotation). Results. It was found that patterns of change for each variable were dependent on the degree of rigidity of the foot. Relative forefoot ab-adduction values ranged from 4.4 ° of adduction to 1.7 ° of abduction. Medial foot length values changed ± 0.8 mm (± 0.5%) and lateral foot length values changed ± 0.5 mm (± 0.3%). Medial foot length was correlated positively with relative forefoot abduction, and external tibial rotation was correlated positively with relative forefoot adduction.