Effect of tip-apex distance and calcar referenced tip-apex distance on the stability of intertrochanteric fractures treated with cephalomedullary nailing assessed by the finite element method

doi:10.3969/j.issn.2095-4344.2917

Abstract

Abstract:

BACKGROUND: Tip-apex distance is widely used to predict lag screw cut-out. However, some clinical studies found that lag screw is more stable when it is positioned in the infero-central zone of femoral head in the frontal plane, so in order to find more appropriate position of lag screw, calcar referenced tip-apex distance was proposed. However, the application value of calcar referenced tip-apex distance needs more verification.

OBJECTIVE: According to the standard formula of tip-apex distance and calcar referenced tip-apex distance, lag screw is put into different quadrants of femoral head. The finite element method was used to assess and compare the stability of the models.

METHODS: A left femur finite element model was established using the data of computed tomography scan. The finite element model of Zimmer Natural Nail was established via 3D scanner and modeling. The lag screw was inserted into five different areas of the femoral head (posterior-superior, anterior-superior, anterior-inferior, posterior-inferior and central) according to tip-apex distance (15, 20, 25, 30, and 35 mm), so 24 internal fixation models were established. The calcar referenced tip-apex distance was calculated when lag screw was put into central area. Sixteen models of calcar referenced tip-apex distance were established when the lag screw was put into posterior-superior, anterior-superior, anterior-inferior and posterior-inferior quadrants of the femoral head. Thereafter, each finite element model was given a vertical downward force to compare the stability of the femoral head of the model established by the standards of tip-apex distance and calcar referenced tip-apex distance.

RESULTS AND CONCLUSION: (1) Putting the lag screw into the femoral head according to the standard formula of tip-apex distance, the maximum axial displacement of the femoral head relative to the model without lag screw was 0.008 205 5 mm when the lag screw was located in the anterior-superior position of femoral head and the tip-apex distance was 35 mm. When the lag screw was located in the posterior-inferior position of the femoral head and the tip-apex distance was 20 mm. The difference value of maximum axial displacement relative to the model without lag screw was maximum 0.023 524 0 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 was minimum 0.008 794 1 mm when the lag screw was located in the anterior-superior position of femoral head and the calcar referenced tip-apex distance was 37.886 mm. When the lag screw was located in the posterior-inferior position of the femoral head and the calcar referenced tip-apex distance was 25.256 mm, the difference value of maximum axial displacement relative to the model without lag screw was maximum 0.023 183 2 mm. (2) Higher maximum principal stress was found in the posterior-inferior quadrant of femoral head. The average value was 82.339 4 MPa when the lag screws were put into the femoral head according to the standard formula of tip-apex distance and 79.118 8 MPa when the lag screws were put into the femoral head according to the standard formula of calcar referenced tip-apex 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 49.535 9 MPa when the lag screws were put into the femoral head according to the standard formula of tip-apex distance and 49.642 8 MPa when the lag screws were put into the femoral head according to the standard formula of calcar referenced tip-apex distance. It is concluded that 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. 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 posterior-inferior area quadrant of the femoral head.

Key words: bone, internal fixation, tip-apex distance, calcar referenced tip-apex distance, finite element analysis, intertrochanteric fracture, stability, tension

CLC Number:

Qiao Wen, Lü Xin, Liu Jinyuan, Zhang Jing, Li Yuanyuan, Kong Lingbao. Effect of tip-apex distance and calcar referenced tip-apex distance on the stability of intertrochanteric fractures treated with cephalomedullary nailing assessed by the finite element method[J]. Chinese Journal of Tissue Engineering Research, 2020, 24(36): 5755-5763.

Figures/Tables 18

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