Evaluate The Effect Of Tip-apex Distance And Calcar Referenced Tip-apex Distance On The Stability Of Intertrochanteric Fractures Treated With Cephalomedullary Nailing By Finite Element Analysis

Objective:According to the standard formula of tip-apex distance and calcar referenced tip-apex distance,put lag screw into different quadrant of femoral head to make finite element models,then compared the stability of the models.Methods:Obtain complete femurs image through computed tomography scan,and then used these images to build a femur finite element model.The finite element model of Zimmer Natural Nail was established via 3D scanner and modeling.The lag screw was inserted into five different area of the femoral head(posterior-superior,anterior-superior,anterior-inferior,posterior-inferior and central)according to tip-apex distance(15mm,20 mm,25mm,30 mm,35mm),so 24 internal fixation models were established.According to calcar referenced tipapex distance calculated when lag screw was put into central area,put the lag screw into posterior-superior,anterior-superior,anterior-inferior and posterior-inferior quadrants of femoral head.Thereafter,each finite element model was given a vertical downward force to compare the stability of the femoral head.Results:(1)Putting the lag screw into the femoral head according to the standard formula of tipapex distance,the maximum axial displacement of the femoral head relative to the model without lag screw is 0.0082055 mm when the lag screw is located in the anterior-superior position of femoral head and the tip-apex distance is 35mm;When the lag screw is located in the posterior-inferior position of the femoral head and the tip-apex distance is 20 mm,The difference value of maximum axial displacement relative to the model without lag screw is 0.0235240 mm.Putting the lag screw into the femoral head according to the standard formula of calcar referenced tip-apex distance,the maximum axial displacement of the femoral head relative to the model without lag screw is 0.0087941 mm when the lag screw is located in the anterior-superior position of femoral head and the calcar referenced tip-apex distance is 37.886mm;When the lag screw is located in the posterior-inferior position of the femoral head and the calcar referenced tip-apex distance is 25.256 mm,The difference value of maximum axial displacement relative to the model without lag screw is 0.0231832 mm.(2)Higher maximum principal stress was found in the posterior-inferior quadrant of femoral head.The average value is 82.3394 MPa when the lag screws were put into the femoral head according to the standard formula of tip-apex distance and 79.1188 MPa when the lag screws were put into the femoral head according to the standard formula of calcar referenced tipapex distance.When lag screws were located in the posterior-inferior quadrant of the femoral head,the maximum principal stress at the proximal femur decreased to 49.5359 MPa when the lag screws were put into the femoral head according to the standard formula of tip-apex distance and 49.6428 MPa when the lag screws were put into the femoral head according to the standard formula of calcar referenced tip-apex distance.Conclusion:(1)The calcar referenced tip-apex distance did not show a significant advantage over the tip-apex distance as an evaluation factor of stability in the treatment of intertrochanteric fractures with the intramedullary nail.(2)The stability of the proximal femur of fracture is more affected by the position of the lag screw,which is best when it is in the posteriorinferior area quadrant of the femoral head.