What is the appropriate axial position in cannulated screw fixation for femoral neck fractures? A finite element analysis.

BACKGROUND

Cannulated screw fixation is a common surgical treatment for femoral neck fractures; however, there is limited information on the optimal axial position of the screws. Herein, we aimed to investigate the impact of axial screw position on surgical stability in femoral neck fracture models fixed with three cannulated screws.

METHODS

Eighteen finite element models (FEMs) replicating Pauwels type II femoral neck fractures were constructed and tested using nine normal and nine osteoporotic bone models. Each FEM simulated combinations of three different screw positions (anterior, central, and posterior) in the axial view and three models (type 1: 8° angles, 10 mm inter-screw interval; type 2: 6° angles, 10 mm inter-screw interval; type 3: 8° angles, 6 mm inter-screw interval), assuming anatomical reduction. Stress concentrations on the screws and bone were investigated, with measurements of peak von Mises stress (PVMS) and mean stress.

RESULTS

Stress concentration on the cannulated screws was consistently observed at the inferior screw near the fracture site in all FEMs. Stress concentrations on the bone around the screws were noted around the head and tip of the inferior screw in each FEM. All PVMS on the screw and surrounding bone decreased as the screw position moved from posterior to anterior in the axial view. Additionally, these stresses decreased as the screw tilt angle increased and the inter-screw interval was maximized. The mean stresses over the region of interest in all FEMs showed similar patterns to those of the PVMSs.

CONCLUSION

To enhance fixation stability and reduce stress concentrations at the fracture site and lateral cortex in femoral neck fractures fixed with three canulated screws, positioning the screws anterior to the center in the axial view and maximizing the inter-screw interval, tailored to the patient's femur geometry, are recommended.