Biomechanical study of stress in the fifth metatarsal

Objective. The stress throughout the fifth metatarsal was determined under various loading conditions, in order to better understand the causes of fractures to this bone. Design. A mathematical approach was taken, in which the stresses were analysed using the methods of beam theory. Background. Finite element analysis has frequently been used to determine the stress in bones. Beam theory provides an easier method for determining the force and moment resultant in any cross-section. The distribution of stress throughout the cross-section can then be found by solving certain partial differential equations defined on the cross-sections. Methods. Cross-sections of the bone were obtained by slicing a mould, into which the bone was placed, at numerous intervals along its length. Analytic expressions describing each cross-section were obtained by fitting a Fourier series to a sequence of points along the boundary. Results. The maximum stress found in the fifth metatarsal resulted from an oblique load, and had a magnitude less than would occur in a subject during normal walking. Conclusions. Since the magnitude of the stress is submaximal, this study lends theoretical support to the clinical observation that the diaphyseal fracture is indeed a stress fracture. Relevance Our analysis adds a biomechanical rationale to the pathomechanics of diaphyseal stress fractures of the fifth metatarsal. It suggests that inversion during repetitive activities may predispose the foot to fractures at a predictable location. (C) Elsevier Science Ltd.