Analysis of the Concentric Distraction and Distraction Loss of the Bridged Elbow by a Dynamic Fixator

The mechanisms of concentric distraction and distraction loss within the elbow-fixator-pin construct remains unclear. Furthermore, the literature reports are inconsistent regarding the correct distraction distance between the articulating surfaces of a bridged elbow. This study investigated the mechanism of the distraction loss in terms of the relevant fixator design and elbow angle. Four elbow joints flexed at 90°, 120°, 150°, and 180° were developed in this study. The contribution of each humeral and ulnar distractor to the concentric distraction at the elbow center was evaluated by the finite-element method. The distraction loss within the elbow-fixator-pin construct was studied along the specific load-transferring paths on both elbow sides. Among four elbow models, both concentric and eccentric distractions simultaneously occur at the elbow center. The distraction loss always exists in the joint distraction of the bridged elbow. Comparatively, the 120° elbow model showed the more effectively concentric distraction. For the 180° elbow model, the distraction loss was the highest. This distraction loss was mainly attributed to the lateral deflection of the fixing pins and the vectorial transformation of the distracted length of the distractors. The lateral deflection of the fixing pins is a function of the stiffness of both pins and periarticular tissues. The spatial relationship of the elbow anatomy, elbow angle, and fixator frame plays a significant role in the vectorial transformation of the effective distraction between the elbow articulating surfaces.