Intramedullary screw fixation of Jones fractures: a biomechanical study

The management of proximal fifth metatarsal ("Jones") fractures in athletes has become increasingly more aggressive, despite a lack of biomechanical data in the literature. A cadaver biomechanical study was conducted to evaluate the strength of intramedullary fixation of simulated Jones fractures loaded to failure via three-point bending on a Materials Testing System machine. In a series of eight intact fifth metatarsal control specimens, the force to failure (fracture) was measured for comparison with repaired specimens. Acute fractures were simulated in 10 pairs of feet via osteotomy at the typical fracture location and were fixed with either a 4.5-mm malleolar screw or a 4.5-mm partially threaded, cancellous, cannulated screw, both placed using conventional intramedullary techniques. Force at initial displacement averaged 73.9 N (SD, 64.7 N) for the malleolar screws and 72.5 N (SD, 42.3 N) for the cannulated screws. Force at complete displacement averaged 519.3 N (SD, 226.2 N) for the malleolar screws and 608.4 N (SD, 179.7 N) for the cannulated screws. The force to failure of the intact specimens was significantly greater than the initial and complete forces to failure for the fixed specimens (P < 0.05, independent measures analysis of variance). There was no statistical difference between the average forces at initial displacement or at complete displacement in the fixed metatarsal specimens for the two different types of screws, but the forces at complete displacement for each screw type were significantly greater than the forces at initial displacement (P < 0.05). On the basis of literature review and data generated from this study, it is apparent that the forces necessary to cause displacement of the stabilized Jones fracture are above what would be transmitted within the lateral midfoot during normal weightbearing. The choice of screw and intramedullary technique of fixation is a matter of surgeon preference, because the choice of screw makes no biomechanical difference.