Effect of Axial Dynamization in Circular External Fixation on Bone Segment Vertical and Lateral Displacements

Context: The field of deformity correction with external fixation has continued to advance since with the addition of half-pins, even though they can act to stiffen the construct and undermine healing. Dynamization increases axial motion at the fracture site and improves fracture and osteotomy healing in the experimental and clinical studies. No study compares the lateral and vertical displacements of bone segment in dynamized versus nondynamized frames. Aims: The purpose of this study was to compare the segmentary motion in the axial, coronal, and sagittal planes using nondynamized and dynamized circular external fixation frames. Subjects and Methods: Seven frame models were tested including classic Ilizarov three-wire construct and two frame configurations representing the most common modern half-pin and wire combinations. These models were either nondynamized or dynamized with two types of dynamization modules. Each model underwent axial loading up to 50 kg for 11 cycles. Statistical Analysis Used: One-way analysis of variance testing followed by post hoc Tukey's test. Results: The addition of each half pin sequentially decreased axial motion while increased sagittal motion. Dynamization had a limited effect on the sagittal motion but significantly improved axial motion. The sagittal to axial motion ratio increased with half pins in nondynamized frames but decreased equal to or beyond the Ilizarov three-wire fixation frame with dynamization. At the limit of the dynamizers' motion, there was change in the rate of displacement, suggesting that subsequent motion was strictly from the wires and half-pins. Overall, there was minimal coronal motion. Conclusions: While half pins decrease axial micromotion and increase detrimental sagittal motion, dynamization restores.