The effect of clamping a tensioned wire: Implications for the Ilizarov external fixation system

Abstract

This study demonstrates that clamping a tensioned wire can cause a reduction in wire tension. Tension (about 1275 N) was applied to a wire that was subsequently clamped, using cannulated bolts, to the steel half-ring of an Ilizarov external fixator. The tension in the wire was monitored before, during and after clamping. The apparatus was disassembled and the deformations in the wire caused by the clamps were measured. This experiment was repeated 15 times. When the wire was clamped to the frame, the wire tension was reduced by 22 +/- 7 per cent (mean +/- standard deviation, SD). The drop in wire tension was linearly correlated (r = 0.96; p < 0.001) with the deformation caused by the bolts. A finite element (FE) model of the wire was also constructed. The model was pre-stressed (tensioned), and the clamping effect replicated. This analysis showed that clamping the wire could be considered to squeeze the wire outwards (like toothpaste from a tube) and so reduce its tension during fixator assembly. To assess the magnitude of this effect in the clinical situation, the FE model analysis was repeated to replicate clamping a 1.8-mm-diameter wire to a 180-mm-diameter steel Ilizarov ring component. The analysis showed that for these conditions the tension reduced by 8-29 per cent. The results of this study highlight a general engineering problem: how can a tensioned wire be secured to a structure without an appreciable loss of tension? If the performance of the structure depends on the wire tension, this performance will change when the wire is secured.

Rapid pre-tension loss in the Ilizarov external fixator: An in vitro study

“…The average pre-tension loss that occurred immediately after tightening of the bolts was almost identical for both configurations: around 277 N, and in both this represents a loss of around 24% of the initial pre-tension. These data are in accordance with the data of Watson et al (2003a), who demonstrated an average loss in wire pretension of 22% after tightening of the bolts. It seems, however, that the cause of their loss in wire pre-tension was different than ours.…”

“…It seems, however, that the cause of their loss in wire pre-tension was different than ours. Watson et al (2003a) demonstrated that the initial loss was due to plastic deformation of the wire; the wires were squeezed outwards and increased in length (like toothpaste squeezed out of a tube), thus losing pre-tension. Our set-up was similar; we tightened the bolts while the dynamometric tensioners remained attached to the ring, but we only measured a loss in pre-tension when we removed the tensioners-and not during the tightening of the bolts (Figure 3).…”

Reduction in wire tension caused by dynamic loading. An experimental Ilizarov frame study

“…no. 10-0600, Memphis, Tennessee, USA) (Aquarius et al, 2007;Aronson and Harp, 1992;Watson et al, 2003a). Fixator ''B'' was another type of cannulated bolt in combination with a custom-made slotted washer.…”

“…Unfortunately a reduction of the imposed wire tension starts already when clamping the wires to the ring and continues when the wire tensioning device is removed (Aquarius et al, 2007;Delprete and Gola, 1993;Gasser et al, 1990;Hillard et al, 1998;La Russa et al, 2011;Watson et al, 2000Watson et al, , 2003a. Further considerable tension loss is measured during the weight bearing period (Aquarius et al, 2007;Delprete and Gola, 1993;Hillard et al, 1998;Renard et al, 2005).…”

Analytical modelling of Kirschner wires in Ilizarov circular external fixator as pretensioned slender beams

“…K-wires have been subjected to numerous experimental and computational studies (Aronson & Harp 1992;Hillard et al 1998;Davidson et al 2003;Mullins et al 2003;Donga et al 2005;Renard et al 2005;Watson et al 2003aWatson et al ,b, 2005Zhang 2004a,b). They have also been investigated theoretically (Hillard et al 1998;Nikonovas & Harrison 2005;Zamani & Oyadiji 2008).…”