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Biomechanical Analysis of the Risk of an Unused Guide Pin Hole on the Postoperative Subtrochanteric Fractures After Femoral Neck Fracture Pinning

Subtrochanteric fractures after pinning for femoral neck fractures, though occurred rarely but serious complication, are supposed to relate with unused guide pin holes around the distal pin. The purpose of this study is to evaluate and analyse the effect of stress distribution around the distal pin hole using finite element analysis. Five 3D finite element models of fractures treated with hook pins were created based on the location of an unused guide pin hole: Model I (no unused pin hole), Model II (unused pin hole cranial to the distal pin), Model III (unused pin hole anteriorly), Model IV (unused pin hole distally), and Model V (unused pin hole posteriorly). The stress distribution were examined in each model. The region surrounding the distal pin was divided into four quadrants: cranial, anterior, caudal, and posterior, with the highest stress value in each quadrant representing the peak stress. In Model I, the peak stresses in the cranial, anterior, caudal, and posterior quadrants, were 67.8, 79.1, 28.1, and 70.6 MPa, respectively, as is almost similar in Model II and IV. In Model III, peak stresses were 78, 188, 19.2, and 71 MPa, respectively. In Model V, peak stresses were 60.7, 92.3, 22.7, and 110.6 MPa, respectively. An unused guide pin hole, in particular anterior or posterior to the distal pin poses a high risk of postoperative complications of subtrochanteric fractures. Therefore, patients should be instructed to avoid early weight bearing, postoperatively. We believe the result of this study can be applicable to three screws fixation in an inverted triangle.