Biomechanical comparison of a novel monocortical and two common bicortical external fixation systems regarding rigidity and dynamic stability

Abstract Aim: To biomechanically compare a monocortical single frame external fixator (Orthofix UNYCO) with two bicortical fixator systems (dual frame: Stryker Hoffmann and single frame: Synthes LEF) with respect to system rigidity and stability under cyclic loading. Methods: The fixator systems were assessed for axial rigidity under loads which would occur clinically during fixator application and dynamic stability (cyclic fatigue) under loads which would occur in the first week postoperatively. Tests were performed on porcine tibiae (n>5 per group) with characteristic frame configurations. Loads were applied with an electrodynamic material testing machine and pin and frame deformations were continuously monitored with a marker based motion capturing system. Results: The bicortical single frame fixator revealed the largest rigidity (276±55) N/mm and was 20% (p=0.116) stiffer compared to the bicortical dual frame configuration and 39% (p=0.003) stiffer compared to the monocortical system. All systems survived 4000 cycles of loading, with the smallest vertical displacement (2.44±0.54 mm) observed for the bicortical dual frame system, followed by the monocortical single frame (3±0.55 mm, p=0.85) and bicortical single frame (3.25±0.96 mm, p=0.215). Conclusion: The monocortical fixation system performed comparably to the bicortical systems for its intended use as a temporary treatment before a definitive fracture osteosynthesis by plating or nailing.