A biomechanical comparison of closed-reduction osteosynthesis techniques for fractures of the ulnar styloid

BACKGROUND: Ulnar styloid fractures (USF) are frequently underestimated and can contribute to clinically relevant wrist instability and impaired functional outcomes. Several osteosynthesis techniques are available for the treatment of unstable USF. This study aims to compare the biomechanical stability of fixation methods applicable to closed reduction - specifically K-wire and headless compression screw fixation - to support clinical decision-making. METHODS: This biomechanical in vitro study was conducted using 30 specimens of artificial ulnae. USF were simulated through a standardized osteotomy at the base. Three groups of ten specimens each were created to compare three methods of osteosynthesis: One 1.4 mm (mm) Kirschner-wire (K-wire) (Group 1KW), two 1.0 K-wires (Group 2KW) or one 2.0 cannulated headless screw (Group CHS). Each construct underwent 1,000 load cycles up to 50 newtons (N) followed by load-to-failure testing, which was defined as displacement ≥ 2 mm. Displacement [mm], stiffness [N/mm] and load to failure [N] were recorded and compared among the groups. RESULTS: The mean fracture displacement after 1,000 cycles was 0.58 mm (SD 0.37) in the 1KW group, 0.23 mm (SD 0.09) in the 2KW group and 0.09 mm (SD 0.03) in the CHS group. Significant group differences could be shown between CHS and 1KW (p < 0.01) as well as between CHS and 2KW (p = 0.017). The mean construct stiffness was 269.95 (SD 45.72) N/mm in the 1KW group, 484.08 (SD 207.60) N/mm in the 2KW group and 730.95 (SD 241.42) N/mm in the CHS group. Significant group differences could be shown between CHS and 1KW (p < 0.01). The mean load-to-failure was 136.82 (SD 68.86) N in the 1KW group, 181.60 (SD 28.09) N in the 2KW group and 204.17 (SD 63.10) N in the CHS group. Significant group differences could be shown between CHS and 1KW (p = 0.013). CONCLUSION: Ulnar styloid refixation with a headless compression screw provides superior axial stability compared to single or double K-wire fixation. This should be considered in the treatment of unstable USF.