A biomechanical study on five unilateral external fracture fixation devices

Objective. This study examines plastic, slip and fatigue failures in external fixator frames. Design. Failure was examined in five commonly used unilateral external fixators using experimental models that simulated both stable and unstable diaphyseal fractures. Background. Plastic failure arises from permanent deformation of a frame initiated by bending of frame components, while slip failure arises from slip at clamping interfaces. Such failure may lead to refracture or to malalignment of the bone (transverse or angular). Fatigue failure arises from the loosening or wear of components under long-term cyclic loading; this can lead to variable interfragmentary displacement and impairment of fracture stability. Methods. Axial loads were measured at which plastic or slip failure occurred, as were changes in interfragmentary motion due to 10000 load cycles simulating walking activity. Results. In four fixators, plastic or slip failure initiated bone malalignment at only 50% of average adult weight bearing (650 N). Additionally, in three fixators a third screw at the clamp centre was found to reduce retention of the two adjacent screws, causing premature slip failure. Evidence of fatigue was found in all the fixators after only 10000 load cycles. In one fixator, the amplitude of elastic interfragmentary motion increased progressively by 30% transversely, 15% angularly and 100% torsionally. In another fixator it increased by 10% in all directions while the column was locked (non-dynamizing), and it increased a further 10% while it was unlocked and able to slide telescopically (dynamizing). Clamp slip occurred at the column of a third fixator almost immediately after commencing the cyclic loading. Conclusion. Plastic or slip failure of frames may occur prematurely during routine weight-bearing on unstable fractures, and frame fatigue may affect long-term interfragmentary stability.