We investigated the effect of the location and the number of distal screws in the efficiency of an intramedullary nail implementing the finite element method (FEM).
The left proximal femur of a 93-year old man was scanned and two series of full 3D models were developed. The first series, consisting of five models, concerned the use of a single distal screw inserted in five different distal locations. The second series, consisting of four models, concerned the use of four different pairs of distal screws. Each model was analyzed with the (FEM) twice, first considering that the femur is fractured and then considering that the femur is healed.
For nails with a single distal screw, stresses around the nail hole were reduced with proximal placement of the distal screw but the area around the nail hole where the lag screw is inserted is stressed more. Furthermore, for nails with a pair of distal screws, placing the pair of distal screws at a specific location is most beneficial for the mechanical behavior of the femur/nail assembly.
The distal area of the nail generally gets less stressed when a pair of distal screws is introduced, while the presence of two distal screws far away from each other results in lower proximal femoral head displacements. The stress field at the area of fracture is not influenced significantly by the presence of a single distal screw or a pair of distal screws.
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