Torsional strains in the proximal fifth metatarsal: implications for Jones and stress fracture management.
BACKGROUND: Reports of nonunion of proximal fifth metatarsal fractures treated by internal fixation indicate that current fixation methods do not always adequately address the stresses to which the bone is subjected during ambulation. In particular, the insertion sites of the peroneus brevis and peroneus tertius tendons on the fifth metatarsal suggest that their actions can impose torsional stresses on the areas of the bone in which Jones fractures and stress fractures occur. Intramedullary screw fixation, however, offers little resistance to rotation of the proximal and distal fragments relative to one another. METHODS: To determine the potential for the existence of torsional stresses in the fifth metatarsal during post-operative ambulation, a simplified cadaver model of single-limb stance was used in which cadaver feet were subjected to concurrent axial and tendon forces while monitoring the outputs of stacked rosette strain gauges placed at the typical sites of Jones and stress fractures. Principal strain and shear strain magnitudes and directions were measured. RESULTS: The shear strain magnitudes and strain axis directions indicated the presence of torsional stresses in the underlying bone potentially capable of causing internal rotation of the proximal fragment relative to the distal end of the bone. CONCLUSIONS: This finding has implications for the treatment of both Jones fractures and stress fractures of the proximal fifth metatarsal. An internal fixation device that has the capability to resist torsion as well as tension and bending would appear optimal to treat these fractures.