How Extreme Locking of Intramedullary Nails Affects the Overall Biomechanics of Intramedullary Nail Systems

Intramedullary nails are tube-like implantable medical devices that are commonly used to treat long bone fractures. Interlocking screws, that pass from one side of the bone to the other, through holes at either end of the nail, provide additional stability. Newer designs of intramedullary nails have screws placed more extremely. The aim of this study was to use mechanics to analytically investigate how extreme locking of intramedullary nails affects the overall biomechanics of intramedullary nail systems. The nail was modelled as a tube of various sizes. The deflection of the nail from axial compression, bending and torsion was determined as the working length of the nail was varied. The screw was modelled as a simple beam, built-in at both ends. The deflection of the screw was determined as the medullary width was varied. Placing interlocking screws more extremely in intramedullary nails increases the working length of the nail and leads to the use of longer screws, since at the bone extremities, the width of the medullary cavity increases. An increased working length leads to increased rotation of the nail. The use of longer screws leads to increased deflection of the screw and an increased bending moment that has to be resisted by the bone cortex. Extreme locking of intramedullary nails changes the biomechanics of the nail system, and may well have clinical implications in terms of fracture healing.