Biomechanical analysis of an FNS® fixation construct for femur neck fractures and clinical implications: a finite element method

Background Despite widely use of femoral neck system system (FNS®), there is little evidence for mechanical property according to type of femoral neck fracture. This study is to assess the structural/mechanical stability of fixation constructs with a femur neck system by using the finite-element (FE) analysis after simulating the femur neck fractures and to introduce the clinical implications. Methods We simulated the fracture models of subcapital, transcervical, basicervical, and vertical types by using the right femur model (SAWBONES®) and imported the implant model of FNS® to ANSYS® to place the implant in the optimal position. The distal end of the femur model was completely fixed and was abducted 7°. The force vector was set laterally at an angle of 3° and posteriorly 15° in the vertical ground. We did the analysis using Ansys® software with the von Mises stress (VMS) in megapascal (MPa). Results The max VMS of the fracture site was 67.01 MPa for a subcapital fracture, 68.56 MPa for a transcervical fracture, 344.54 MPa for a basicervical fracture, and 130.59 MPa for a vertical fracture. The max VMS of FNS® was 840.34 MPa for a subcapital fracture, 637.37 MPa for a transcervical fracture, 464.07 MPa for a basicervical fracture, and 421.01 MPa for a vertical fracture. The max VMS of the implant corresponded to the value of the entire fixation construct and thus, FNS® mainly functions as a load-bearing implant. When we compared the basicervical and vertical fractures, the stress distribution between the implant and the fracture sites differed significantly, and the basicervical fracture had higher VMS in the bone, implant, and fracture sites. Conclusion Considering the stress distribution of the assembly model, FNS® fixation should consider the osseous anchorage between the proximal bolt and cancellous bone of femoral head and might be appropriate for vertical fractures. Regarding the VMS of fracture site, FNS® might be applied cautiously just in the basicervical fracture of anatomical reduction without gap and comminution.