Closed intramedullary nailing is an accepted method of treatment for femoral shaft fractures. Technical complications of the procedure include fracture instability, which may result in proximal nail migration, malrotation, delayed union, and occasionally femoral bursting during insertion of the nail, sometimes leading to fracture instability as well as shortening. This study defines the effect of starting hole position, fracture component length, reamed diameter, and nail type on the potential for femoral bursting and fracture instability. The most significant factor in the proximal femoral component was found to be the position of the starting hole. Anterior displacement by greater than 6 mm from the neutral axis of the medullary canal consistently caused high hoop stresses at the level of the fracture, which resulted in bursting of the proximal femoral component by lifting off the anterior cortex. Hoop stresses at the level of the fracture were less sensitive to lateral or medial placement of the starting hole. Distally, fracture stability was governed by femoral component length and reamed diameter. In the proximal and distal components, fracture stability and the potential for bursting were influenced by the particular nail used. This was due to significant differences in mechanical geometric properties between nails of different manufacturers. Case reports are presented to illustrate these biomechanical principles as they apply to clinical situations.

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