Does the Angle Between Dynamic Hip Screw and Anti-Rotation Screw Affect the Outcome of Vertically Oriented Femoral Neck Fractures? A Biomechanical Analysis and Clinical Results

Objective: To analyze the effects of the angle between dynamic hip screw (DHS) and anti-rotation screw (AS) on vertically oriented femoral neck fractures (VOFNFs) and investigate the clinical results of them. Methods: Eighteen synthetic femurs were simulated and divided into 3 groups. The angle between DHS and AS in anteroposterior-view was marked as alpha, and in lateral-view was marked as beta, thus the total angle (TA) was defined as the summation of alpha and beta. The groups were categorized as group A (TA <= 5 degrees), B (5 degrees < TA <= 10 degrees), and C (TA > 10 degrees), respectively. All samples were tested under incremental, cyclical loading, and loading to failure. In clinic, 80 consecutive VOFNFs in 78 patients were treated with DHS plus AS. The patients were divided into 2 groups, including 48 fractures in parallel group (TA <= 10 degrees) and 32 in angular group (TA >10 degrees). Results: Group A and B survived during incremental and cyclical loading and endured longer than group C. Axial stiffness and failure loads were not different between group A and B, and greater than group C. Fracture gaps compressive stress was highest in group A, followed by group B and C. Forty-one fractures in parallel group and 23 in angular group healed at final follow-up. Nonunion and osteonecrosis occurred in 3 and 4 of parallel group, and 4 and 5 of angular group. Conclusion: The construction with TA <= 10 degrees between DHS and AS showed superior biomechanical performance and clinical results than those with TA >10 degrees.