Subtrochanteric Fracture after Cannulated Screw Fixation of Femoral Neck Fractures: Biomechanical Analysis and Case Series

Objectives: To determine whether a condensed screw construct or a dispersed screw construct is more likely to sustain a subtrochanteric fracture after cannulated screw fixation for femoral neck fractures, and to report the characteristics of the screw constructs in our patient population that sustained a subtrochanteric fracture. Methods: We performed a biomechanical analysis of two screw constructs and a consecutive case series of patients treated with cannulated screw fixation for femoral neck fractures sustaining subtrochanteric fracture. The biomechanical study consisted of two groups of biocomposite femora: (1) Condensed screw group (CS) (n=7); (2) Dispersed screw group (DS) (n=7). Axial loading was applied to the biomechanical group until fracture, and a load deformation curve was used to quantify the mechanical behavior by measuring the load (kN) at failure, displacement (mm) at failure, and initial construct stiffness (kN/mm). The case series involved a retrospective chart review of patients treated with cannulated screw fixation who sustained a subtrochanteric fracture (n=7). Radiographs were analyzed for trends in failure during chart review. Results: There was a trend towards increased load to failure in the DS group compared with the CS group. We did not observe a statistical difference (p = 0.1023) in load to failure, but we did observe increased stiffness in the DS group (p = 0.0346). Post-hoc non-inferiority analysis demonstrated that the DS were not inferior to CS group. We found a 3.9% incidence of peri- implant fracture in our patient population who underwent cannulated screw fixation. Conclusion: The results of this study suggest that maximally dispersing screw placement within the femoral neck, may have higher load to failure than more condense screws. The authors of this study advocate maximizing spacing of the screws within the femoral neck while letting the anatomy of the femoral neck dictate the position of the distal screw relative to the lesser trochanter.