Intramedullary nails with two lag screws.

Objective: To investigate the structural integrity of intramedullary nails with two lag screws, and to give guidance to orthopaedic surgeons in the choice of appropriate devices.

Design: Alternative designs of the construct are considered, and the use of a slotted upper lag screw insertion hole is analysed.

Background: Intramedullary fixation devices with a single lag screw have been known to fail at the lag screw insertion hole. Using two lag screws is considered. It has also been proposed to use a slot in the nail for the upper lag screw to prevent the upper lag screw from sticking.

Methods: Bending and torsion load cases are analysed using finite element method. Consideration of both load conditions is essential.

Results: The results present the overall stiffness of the assembly, the load sharing between lag screws, and the possibility for cut-out to occur.

Conclusions: While the slot for the upper lag screw might be advantageous with regard to the stresses in the lag screws, it could be detrimental for cut-out occurring adjacent to the lag screws.

Relevance: Comparative analyses demonstrate that two lag screws may be advantageous in patients whose cancellous bone quality is good and who impose large loads on the lag screw/nail interface. However, the use of two screws might pre-dispose to failure by cut-out of the lag screws. The addition of a slotted hole for the upper lag screw appears to do nothing significant to reduce the risk of such a failure.