Numerical Optimization of the Position in Femoral Head of Proximal Locking Screws of Proximal Femoral Nail System; Biomechanical Study

Abstract

Background: Proximal femoral fracture rates are increasing due to osteoporosis and traffic accidents. Proximal femoral nails are routinely used in the treatment of these fractures in the proximal femur.

Aims: To compare various combinations and to determine the ideal proximal lag screw position in pertrochanteric fractures (Arbeitsgemeinschaft für Osteosynthesefragen classification 31-A1) of the femur by using optimized finite element analysis.

Study design: Biomechanical study.

Methods: Computed tomography images of patients' right femurs were processed with Mimics. Afterwards a solid femur model was created with SolidWorks 2015 and transferred to ANSYS Workbench 16.0 for response surface optimization analysis which was carried out according to anterior-posterior (-10°<anterior-posterior<10°), inferior-superior (-6°<inferior-superior<7°) and tip-apex distance (10 mm<tip-apex distance<30 mm) proximal lag screw positions in the fracture region. The optimum position of the proximal lag screw was determined based on the von Mises stress values occurring on the fracture line. Initial analysis of the system was realized under the surgeon's normal positioning conditions (anterior-posterior, inferior-superior=0°; tip-apex distance=12 mm).

Results: The maximum and minimum (compression) von Mises stresses were found to be 438 MPa and 0.003 MPa, respectively, and risky stresses for the system occurred in the regions where the proximal lag screw passes through the proximal femoral nail hole, the small diameter portion of stem joints with a large diameter and lag screw mounts to the stem. The most suitable position of the proximal lag screw was found at the middle position of the tip-apex distance (20 mm) and femoral neck (anterior-posterior, inferior-superior=0°), according to von Mises compression stress values occurring on the fracture line.

Conclusion: In our study, we couldn't find any correlation between proximal lag screw movement and tip-apex distance on stresses of the fracture surfaces, but the proximal lag screw position in the inferior (inferior-superior<0)-superior (inferior-superior>0) and posterior-anterior directions of the femur neck significantly increased these stresses. The most suitable position of the proximal lag screw was confirmed as the middle of the femoral neck by using optimized finite element analysis.

Keywords: Femoral neck fractures; biomechanics finite element analysis..

FIG. 1.. Simulation of AO 31-A1 fracture and description of tip-apex distance.

FIG. 2.. Loading condition of the femoral head.

FIG. 3.. Modular nail prosthesis: (a) PFN, (b) hip prosthesis.

FIG. 4.. (a, b) Equivalent von Mises stress and total deformation of the cephalomedullary nail and femur, (c) stresses on the cephalomedullary nail, (d) stresses on the locking screw, (e) stresses on the fracture line of the pertrochanteric fracture.

FIG. 5.. Von Mises stress distribution on the fracture line according to screw position in three planes, tip-apex distance (mm), AP (°) and IS (°).