In Vitro Wire Tension in Circular External Fixation Frames

Circular frames are applied to control fracture and osteotomy mechanics, which is dependent on frame design.As the clinical application of these devices is individualised, clinical performance can be difficult to compare from one case to another.In fine wire constructs, stability is dependent on wire tension, which can degrade over time.As such, wire tension is a parameter that surgeons may attempt to keep consistent between cases or indeed adapt to the clinical situation.However, there is little evidence regarding what variables in fixator application might influence this.The aim of the study was to monitor wire tension in an in vitro symmetrical circular frame construct using strain gauge instrumented wires and to use these to investigate the clinical factors in assembly and use of a frame that may be associated with tension loss.The results demonstrated that variability in wire tension was associated with the application of the tensioner itself, along with wire slippage associated primarily with the bolt torque used to tighten the ends of the wires to the frame.These two variables could easily lead to a 50% variation in the actual wire tension achieved.Surprisingly, under dynamic loading, despite an initial loss in tension due to slippage, wire tension generally remained constant and was not significantly different between wires applied with different wire pre-tensions after the initial three cycles.A calibrated tensioner with a high initial tension (110 kg), along with a controlled wire clamp bolt torque (14 Nm), is required to achieve the most consistent frame performance.