A new external ring fixator for limb reconstruction: a mechanical study

Background and purpose: The mechanical behavior of an external ring fixator has a direct influence on the bone healing process. In fact, the axial stiffness of the frame and the stability of its elements under repeated loading are essential to promote the bone healing process. In this study we aimed to compare the mechanical behavior of the Stryker Hoffmann LRF (Limb Reconstruction Frame) to two different configurations of the Smith and Nephew Ilizarov system. Material and method: The axial stiffness was measured with quasi static loading on complete frames. The dynamic stability was assessed applying a cyclically increasing force on multiplanar connection elements, the ball joints and the conical washers. Finally, complete frames were tested dynamically under an angle of 15°to simulate the angularly scenario during walking. Results: The Stryker Hoffmann LRF frame showed significantly higher stiffness than both Ilizarov frames under static loading. As multiplanar connecting elements, both ball joint and conical washer showed no slippage or rotation prior mechanical failure. Complete Hoffmann LRF frames showed higher number of cycles at failure compared to the Ilizarov frames. Conclusion: The Hoffmann LRF frame showed higher axial stiffness with fewer components and a higher stability against permanent deformation under a repeated load than the Ilizarov system.