Improving Spanning-Knee External Fixator Stiffness: A Biomechanical Study
The purpose of this study was to test and compare external fixator construct stiffness using pin-to-bar clamps or multipin clamps across 2 external fixation systems. Constructs were tested with 8-mm and 11-mm–diameter bar systems and pin-to-bar or multipin clamps. Three construct designs were tested: construct 1 with a single crossbar and pin-to-bar clamps, construct 2 with 2 crossbars and pin-to-bar clamps, and construct 3 with 2 crossbars and multipin clamps. The stiffness of each construct (N = 24) was tested using anterior-posterior bending. Two crossbars and pin-to-bar clamps resulted in the highest mean stiffness. Constructs with a single crossbar and pin-to-bar clamps had a similar average stiffness compared with constructs with 2 crossbars and multipin clamps. Pin-to-bar clamps with 2 crossbars result in stronger spanning-knee external fixators than constructs using multipin clamps.
Another potential criticism of this study is the use of the same pin spread for constructs using pin-to-bar clamps and those using multipin clamps. We established that, to minimize confounding variables, a constant pin spread was necessary. This also mirrors our more common pin configurations for external fixators with pins placed outside the zone of injury. However, a key determinant of external fixator stability is pin spread, and this is a potential benefit to using pin-to-bar clamps over the multipin clamps that require an exact pin spread. Indeed, our results may have shown a larger difference between constructs using the pin-to-bar clamps compared with the multipin clamps had we maximized the pin spread. Future studies may be able to use a fracture model to compare the pin-to-bar clamps and multipin clamps using pin spread to maximize stability.
Conclusion
This study has shown that using pin-to-bar clamps can create strong, stable constructs for temporary external fixation. In particular, constructs made with a single bar and pin-to-bar clamps can produce easily implantable and less expensive constructs that are stiff enough to withstand deformation and allow patient transfers without excessive displacement of the fracture.
