A Study of Plantar Stresses Underneath Metatarsal Heads in the Human Foot

Abstract

A musculoskeletal finite element model (FEM) of the human foot was developed to investigate plantar stresses underneath metatarsal heads (MTHs) in the forefoot. Vertical and shear forces at individual MTH sites were also measured in vivo through use of a specially-designed novel gait platform system, which allows three-dimensional interfacial stresses and local shear traction ratios to be calculated. Average peak vertical pressures obtained are within the range of published data based on commercial capacitance sensors. The measurements from experiments were employed as inputs to the FEM model to define sliding contact with Coulomb friction at the foot-support interface, as well as to validate the foot model. The mechanical properties of the soft tissue under MTHs were quantified by means of an instrumented indentation device. It was found that sub-MTH pad tissue behavior is unique and dependent on metatarsophalangeal joint dorsiflexion angle. The data obtained was also employed to determine the material constants in the hyperelastic constitutive model adopted to describe such tissue. This facilitated accurate estimation of the sub-MTH stresses induced during foot-ground interactions. Simulations were undertaken to predict changes in foot mechanism, in terms of internal joint configurations and plantar loads distributions, which would occur to accommodate any reduction in muscle effectiveness of the gastrocnemius-soleus complex. The results correspond well with clinical observations in diabetic patients who underwent tendo-Achilles lengthening procedures. Stress reductions at individual MTHs were found to be site-specific and possibly dependent on foot structures, such as intrinsic alignment of the metatarsals. These highlight the clinical relevance of the model established in terms of analyzing the foot mechanism. To illustrate possible application of the model developed to therapeutic footwear intervention, the effects of modified foot-support interfaces were also examined. This showed that inclusion of a metatarsal support into a flat foam pad has the potential to relieve sub-MTH plantar stresses. Forefoot plantar stress distribution is sensitive to positioning of the metatarsal support, as well as the material used. The level of complexity of the foot model established is unprecedented and enables examination of the biomechanical interplay among muscular control, bony joint movement, and foot/support interface interactions. This is also the first attempt at such a comprehensive investigation of the foot mechanism. The instrumented tissue tester and gait platform system developed are unique experimental tools which facilitate determination of material model parameters, as well as model validation. This is also a pioneer study of the efficacy of design variables for therapeutic footwear in relieving contact stress, including plantar shear, at sub-MTH region.