Finite element modelling of plantar pressure beneath the second ray with flexor muscle loading

INTRODUCTION:: Little is understood about the effects of flexor loading on plantar pressure distribution. The goal of the current work is to model flexor muscle loading applied to the distal phalanges in order to study the effect of these loads on plantar normal stress (pressure) beneath the metatarsal head. METHODS:: The finite element model is a two-dimensional, plane strain sagittal section incorporating the second metatarsal, proximal phalanx, and plantar and dorsal soft tissue (Figure 1). The metatarsophalangeal joint is simulated by a nodal hinge that transfers loads and produces reasonable kinematic motion between the articular surfaces of the proximal[Figure: see text] phalanx and metatarsal head. Soft tissues are simulated by a uniform continuum. A single flexor tendon passes over the condyle of the metatarsal heads with sliding contact against intervening soft tissue, and is attached to the distal end of the proximal phalanx. A rigid element at the proximal end is fixed by boundary conditions to simulate reactions at the distal cuneiform joint. Material properties of bone are from published values, one tenth the stiffness of bone is used for the flexor tendon, and the soft tissue continuum is hyperelastic using coefficients obtained from compression of the heel plantar fat pad. A 188 N vertical ground reaction force and a flexor tendon load at a 10 degree angle from the X (horizontal) axis are applied to the model. RESULTS:: Figure 2 shows Y direction normal stress distribution along the plantar surface for two load cases: no load and a 250 N load to the flexor tendon. DISCUSSION:: Bending moments at the proximal metatarsal correspond to values obtained by Sharkey et al. Tension in the flexor tendon served to counter the moment in the metatarsal created by the vertical load, and at the same time, to apply an additional axial load. Under flexor loading, focal plantar pressure shifts toward the proximal phalanx and yields a 60% reduction in peak pressure, indicative of the load sharing between the sub-metatarsal head and subphalangeal regions. [Figure: see text] CONCLUSIONS:: The model yields verifiable and reasonable reactions and a significant relationship between flexor muscle loading and peak plantar pressure. Refinement of the model, such as adding the middle and distal phalanges, should reveal further insight into the mechanics of plantar loading.