The Relation between the Dynamization of Hexapod Circular External Fixator and Tibial Mechanical Properties

Dynamization of the external fixator, defined as gradually decreasing construct-stability of the fixator, is widely accepted as a method for treatment during the late phase of the bone healing process. However, the dynamization is mostly based on the subjective experience of orthopaedists at present, without unified standards and a clear theoretical basis. The objective of the study is to investigate the influence of the dynamization operations on the tibial mechanical properties with a hexapod circular external fixator and standardize the dynamization process.

A 3D-printed tibial defects model with Young's modulus of 10.5 GPa and Poisson's ratio of 0.32 simulated the clinically fractured bone. A 10 × ∅ 45 mm silicone sample with Young's modulus of 2.7 MPa and Poisson's ratio of 0.32 simulated the callus in the fracture site. Furthermore, a hexapod circular external fixator whose struts were coded from #1 to #6 was fixed on the model with six half-pins (5 mm diameter). Corresponding to the action of removing and loosening the struts, 17 dynamization operations are designed. For each construct after different dynamization operations, the mechanical environment changes in the fracture site were recorded by a triaxle forces sensor under gradually increasing external load from 0 to 500 N.

The results show that the bone axial load-sharing ratio of each construct in the removal group was generally higher than that in the loosening group. The ratio increased from 92.51 ± 0.74% to 102.68 ± 0.27% with the number of operated struts rising from 2 to 6. Besides, the constructions with the same number of operated struts but with different strut codes such as constructions 3-5, had similar bone axial load-sharing ratios. In addition, the proposed dynamization method of the hexapod circular external fixator can gradually increase the bone axial load-sharing ratio from 90.73 ± 0.19% to 102.68 ± 0.27% and maintain the bone radial load-sharing ratio below 8%.

The laboratory study verified the effects of the type of operations and the number of operated struts on the bone axial load-sharing ratio, as well as the slight influence of the choice of the strut code. Besides, a dynamization method of the hexapod circular external fixator was proposed to increase the bone axial load-sharing ratio gradually.