It is thought that the plantar flexion musculature of the toes help to dissipate stress on the metatarsals. It has been demonstrated that dorsal strains are significantly reduced by simulated contraction of the plantar flexion musculature. It is therefore possible that fatigue of these muscles during strenuous or prolonged running may result in decreased dissipation of forces by the musculature and increased exposure of the stress to the metatarsals [29,30].
A metatarsal biomechanical model has been proposed as a link between the increased incidence of second and third metatarsal stress fracture, and the relative bending strain forces and shear forces as measured beneath these lesser metatarsal heads during distance running. The bending strain in the second metatarsal has been reported as 6.9 times greater than the bending strain in the adjacent first metatarsal bone. Shear forces are also reported as greatest in the second metatarsal in comparison with other metatarsal bones. Axial forces are greatest in the first metatarsal . The second through fourth meta-tarsals have been reported as the weakest metatarsals in terms of their cross-sectional geometric properties; however, the second and third metatarsals experience relative increased stress during walking and running . The relative lengths of the first and second metatarsal do not seem to have an increased incidence of associated stress fracture risk to the second metatarsal .
MRI is useful for imaging stress injuries of the metatarsals. Plain films of metatarsal stress injuries are often negative. Nuclear scintigraphy often is less specific as to the exact location of injury in the small bones of the foot. MRI of stress response typically shows intramedullary low T1 signal and corresponding increased T2, fat-saturated, or STIR signal intensity, and may show enhancement of the corresponding marrow as well as surrounding soft tissues after contrast administration. It is critical to correlate these MRI findings with the clinical presentation because neoplasm and infection may show similar findings (Fig. 2).
An actual stress fracture will show the above findings associated with stress response, with the additional finding of a low T1, low T2 signal intensity line extending to the cortex representing the fracture plane (Fig. 3) [24,34-36].
Fractures of the proximal fourth metatarsal bone are less common than distal fourth metatarsal fractures, and have a longer healing time. This is similar to proximal fifth metatarsal injuries and stress fractures. Patients may continue to be symptomatic even after 3 months of rest and immobilization. Ideal treatment appears to involve prolonged combination of non-weight-bearing casting followed by weight-bearing casting . The fifth metatarsal stress fracture may occur in the metatarsal shaft in runners in contradistinction to the Jones fracture, which is a fracture through the base of the fifth metatarsal (Fig. 4).
Delayed union and nonunion may occur in a significant number of these injuries. Delayed unions of Jones fractures may occur in up to 67% of cases treated conservatively. Immediate intramedullary screw fixation of Jones fractures and proximal shaft fifth metatarsal fractures has been reported to have nearly 100% union rates, with an average time to union being approximately 6 to 8 weeks. Intramedullary fixation has been recommended as the treatment of choice for these fractures to achieve improved union rates [38,39]. More recently, however, it has been suggested that intramedullary screw fixation alone does not always adequately address the torsional stress created by the insertion of the peroneus brevus on the proximal fragment of the fifth metatarsal in fifth metatarsal fractures. It has been suggested that optimal internal fixation appears to require internal devices or fixation that also addresses the torsional stresses
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