Cases of intertrochanteric hip fractures as a result of osteoporosis have been increasing in recent years. Treatment of these types of fractures is often performed with intramedullary (IM) nails or compression hip screws (CHS) ^[1]. IM nails are composed of a stem, which is inserted into the medullary canal of the femur, and a lag screw that is placed inside the head of the femur. One problem with this type of device is that both the left and right femurs are fixed with IM nails that have right-hand threaded lag screws. Therefore, on left femurs, the right-handed threads may not provide satisfactory fixation in the bone. This insufficient fixation could cause rotary motion and slippage in the femoral head, which would inhibit fracture healing. This study used three-dimensional finite element analysis (FEA) to examine the fixation and rotational characteristics in reference to the thread direction of the lag screw and the relative angle between the stem and lag screw.

In this study, a 3D CAD model of a left femur and four proximal femoral IM nail designs were analyzed in FEA for stress and displacement. An intertrochanteric femoral fracture was created so that the femoral head and diaphysis were separated. The four IM nails were designed to with either left or right-handed lag screw threads (figure .1) and with relative stem-lag screw angles of 125 or 135 degrees. (Traditional IM nails use a right-handed screw and a relative angle of 125 degrees.)

The results showed the femoral head displacement was smaller when using the left-handed lag screw. It is thought that this difference between the left and right-handed screws is caused by the direction of rotation, which would cause the left-handed screw to tighten and the right-handed screw to loosen within the femoral head. The femoral head displacement also decreased with a screw-stem angle of 135 degrees in comparison to the standard 125 degree angle. The standard right-handed screw with 125 degree relative angle was shown to have the largest displacement of all four types of tested IM nails, whereas the left-handed, 135 relative degree design produced the smallest displacement of all four implants.

These results show how using a left-handed lag screw with proper relative angles in the left femur, effectively reduces femoral head displacement when compared to traditional right-handed lag screw IM implants. This is important for the promotion of intertrochanteric fracture healing.