A Biomechanical Comparison of Three Hybrid Linear-Circular External Fixator Constructs

Objective To evaluate the stiffness, displacement, ring deformation and bone model motion of 3 configuations of linear‐circular hybrid fixator constructs loaded in axial compression, craniocaudal and mediolateral bending, and torsion. Study Design Biomechanical evaluation. Sample Population Three hybrid construct configurations with 8 replicates/configuration. Methods Construct Ia used a single, 84 mm, incomplete ring and 2 tensioned olive wires to stabilize 1 bone segment and a primary hybrid rod with 3 fixation pins to stabilize the other bone segment. Constructs Iad and Ib were similar to Ia with the addition of a secondary diagonal hybrid rod. Construct Ib had a fixation pin inserted orthogonally from the diagonal rod. Constructs were loaded for 10 cycles in each mode of loading using a materials testing machine. Ring deformation was assessed by obtaining serial ring measurements. Bone model motion at the fracture gap as a result of loading was also calculated. Results Axial compression: constructs Iad and Ib were significantly stiffer than construct Ia. Craniocaudal bending: Construct Ib was significantly stiffer than construct Ia. Mediolateral bending: there were no significant differences between constructs. Torsion: Construct Ib was significantly stiffer than constructs Ia and Iad. Permanent ring deformation did not occur. Bone model translational motion decreased in constructs Iad and Ib compared to construct Ia. Conclusions Addition of a secondary hybrid rod as well as biplanar fixation pin placement improved construct stiffness in several loading modes.