Parallel trajectory of cannulated screws is biomechanically advantageous to diverging trajectory in moderate Pauwels angle femoral neck fractures.

Abstract

Femoral neck fractures are significant injuries, often seen in the elderly and less commonly in patients younger than 65 years old. If internal fixation is preferable and feasible, many options for cannulated screw orientation exist. One of the standard approaches remains three screws in a parallel trajectory and inverted triangle formation. It is hypothesized that the diverging constructs are non-superior biomechanically to the parallel configuration when treating moderate Pauwels angle, transcervical femoral neck fractures. This study used 32 identical femur models (16 parallel trajectory, 16 diverging) representing healthy bone. A fracture with Pauwel's angle of 40 degrees was created for all femurs, and a 3D printed guide was utilized for screw placement for both parallel and divergent screw trajectories. Torsional stiffness, axial stiffness, and axial load to failure were tested for both screw trajectories. The models were examined immediately after testing for anatomical points of failure. Statistical analysis included two-tailed t-tests and Fisher exact testing. Thirty femurs were included for final analysis. One model from each group exceeded the testing machine's limits and was excluded. Parallel screws demonstrated significantly greater axial stiffness compared to the diverging screws (431 N/mm ± 45 N/mm vs 367 N/mm ± 104 N/mm, respectively, p = 0.032). No significant difference was found between torsional stiffness and axial load to failure between the parallel and diverging screws (6.36 Nmm/deg ± 2.46 Nmm/deg vs 6.69 Nmm/deg ± 2.14 Nmm/deg, respectively, p = 0.691) (4461 N ± 402 N vs 4095 N ± 601 N, respectively, p = 0.063). No one failure mode occurred more often when comparing the two groups (p > 0.05). Parallel trajectory of cannulated screws in moderate Pauwels angle femoral neck fractures is biomechanically superior to diverging trajectory of cannulated screws. With widely available guides for consistent placement and reproducibility of parallel screws, this clinically feasible and efficient screw trajectory can most likely be relied upon for robust femoral neck fracture fixation. Controlled laboratory study.