Hexapod Fixator Mechanics - The Final Link

Background: Hexapod External fixators are used for complex limb reconstruction, limb lengthening and deformity correction: recently they have also provided definitive fixation for complex fractures.An accurate knowledge of hexapod biomechanics will enable the clinician to unleash full capabilities of the device, especially its control of the biomechanical environment at the osteotomy /fracture site, creating an optimal mechanical strain with minimal biological insult.Recent studies have extensively characterized the Taylor Spatial Frame (TSF, Richards, USA) system but only some aspects of the newer Trulok Hex System (TL-HEX; Orthofix SRL Italy).Methods: Standard frame constructs of TSF and TL-HEX were mechanically tested in axial tension/compression, transverse bending and axial torsion.Frames were mounted on bone analogues (fiber glass tubes) supported by paired crossed wires and the `Fixation' constructs were tested mechanically. Findings:The maximum axial stiffness of both unmounted systems was similar: 1552 N.mm -1 and 1699 N.mm -1 .The overall mean stiffnesses were also similar: 302 vs 452 N.mm -1 .With crossed wires and bone analogues, the stiffness of fixation constructs dropped about 85% to around 26 N.mm -1 for both: the stiffness of the hexapod was overshadowed by the low stiffness of the Fixation Elements and the deformation was nonlinear.Interpretation: Hexapod fixators are commonly used in clinical practice, but their complex biomechanics are not well understood.By clarifying this to clinicians, more informed choices can be made about the fixation mechanics and hence the biomechanics of the fracture can be optimized to enhance fracture healing.