Computational simulation of axial dynamization on long bone fractures

Background. Axial dynamization has been shown in previous studies to promote callus formation, improve bone healing at fracture sites, and enhance bone remodeling. However, the possibility of non-axial movements or uniform fracture site compression during dynamization, and the appropriate relaxation of fixator joints to achieve such function, have not been investigated. Methods. This study used previously developed computational models based on two commercially available unilateral external fixators (Dynafix and Orthofix) to analyze the fixator joint adjustments used and the fracture site movements generated during dynamization. Findings. When none of the fixator's sliding joints were parallel to the long bone axis, significant non-axial movements occurred during dynamization. The dual sliding joint design of the Dynafix fixator was beneficial in reducing these non-axial movements. When all of the fixator joints were allowed to adjust simultaneously during dynamization, exact axial movement or uniform compression at a complicated fracture site was achievable. Interpretation. This study revealed that significant non-axial movements may occur during dynamization, and that such a deficiency can be corrected by relaxing certain fixator joints in addition to the sliding mechanism. The same modeling technique can also be applied in bone lengthening application to assure desirable limb alignment during the distraction process. These analysis results can aid the performance assessment of an external fixator and facilitate appropriate application of such a device to achieve either active or controlled axial movement.