BACKGROUND: Intramedullary screws frequently are used for fixation of Jones fractures of the fifth metarsal. While the ability of intramedullary screw fixation in fifth metatarsals to resist bending and tensile forces is well known, the ability to withstand torsion has not been studied. This paper compares the torsional stiffness of Jones fractures treated with 6.5-mm short- threaded intramedullary screws to those treated with 5.0-mm cannulated short-threaded intramedullary screws. METHODS: Nine fresh-frozen, matched pairs of cadaver fifth metatarsals had an acute Jones fracture simulated with an osteotomy. They were stabilized with intramedullary screws and then loaded to failure. RESULTS: The torsional stiffness of the metatarsals fixed with the 6.5-mm did not differ significantly from that using 5.0-mm screws. However, to achieve stability, the 5.0-mm screw had to be long enough to reach the metatarsal head and neck. This tended to straighten the normally curved fifth metatarsal bone and caused lateral gapping at the fracture site. CONCLUSION: Both 5.0-mm or 6.5-mm screws provide equal torsional rigidity, but 5.0-mm screws may need to be longer, which could potentially straighten the fifth metatarsal shaft in patients who have a curved fifth metatarsal.
BACKGROUND: Intramedullary screws frequently are used for fixation of Jones fractures of the fifth metarsal. While the ability of intramedullary screw fixation in fifth metatarsals to resist bending and tensile forces is well known, the ability to withstand torsion has not been studied. This paper compares the torsional stiffness of Jones fractures treated with 6.5-mm short- threaded intramedullary screws to those treated with 5.0-mm cannulated short-threaded intramedullary screws. METHODS: Nine fresh-frozen, matched pairs of cadaver fifth metatarsals had an acute Jones fracture simulated with an osteotomy. They were stabilized with intramedullary screws and then loaded to failure. RESULTS: The torsional stiffness of the metatarsals fixed with the 6.5-mm did not differ significantly from that using 5.0-mm screws. However, to achieve stability, the 5.0-mm screw had to be long enough to reach the metatarsal head and neck. This tended to straighten the normally curved fifth metatarsal bone and caused lateral gapping at the fracture site. CONCLUSION: Both 5.0-mm or 6.5-mm screws provide equal torsional rigidity, but 5.0-mm screws may need to be longer, which could potentially straighten the fifth metatarsal shaft in patients who have a curved fifth metatarsal.