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Towards a solution of the wires’ slippage problem of the Ilizarov external fixator

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Abstract

Clinical experience has indicated that many complications during treatment with the Ilizarov method, and mainly tract infection, are related to decreased wire tension. The aim of this work was to evaluate biomechanically a novel wire tensioning and clamping system that will minimise or even diminish the reduction of the wire pretension during treatment. The proposed approach is based on threading of the wire end in a sufficient length. The wire pretension is applied by twisting a nut on the threaded part of the wires against the ring and is recorded by an incorporated force sensor. For biomechanical evaluation, the frame, consisting of a polyethylene bar, simulating the bone fragment, suspended on two rings, was subjected to a dynamic load of 0–800 N at a frequency of 0.5 Hz. After dynamic loading for 20 min, loss of the initial wire pretension for the novel clamping system ranged between 12 and 16 %. The average loss for conventionally clamped wires was 75 %. The advantages of the novel clamping system were the much greater ability to sustain the transverse load and the easy and effectual wire re-tensioning. Although wire slippage has been avoided with the novel system, wire material yield is still responsible for a pretension loss.

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Acknowledgments

This work was financially supported by 7th Framework Programme, LEAD ERA Project, Call 2011: E-Iliza-Development of an e-health system in orthopedics.

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Correspondence to D. Deligianni.

Appendix

Appendix

The used wire thread was M2.0 and the corresponding dimensions (Fig. 9) were taken from the thread data chart (metric thread–coarse pitch): P = 0.40 mm; d 2 = 1.740 mm; d 3 = 1.509 mm; D 1 = 1.567 mm; h 3 = 0.245 mm; h 1 = 0.217 mm; r = 0.058.

Fig. 9
figure 9

Configuration and dimensions of the metric thread

The thread was pretensioned at a tension P PT = 880 N. After the application of the transverse dynamic load, each wire was loaded with an added load of ΔP = 100 N (value determined experimentally). Thus, the total maximum load for each wire was:

$$P_{\text{F}} = P_{\text{PT}} +\Delta P = \, 880\,{\text{N}} + \, 100\,{\text{N}} = \, 980\,{\text{N}} .$$

The mean load applied on the thread was:

$$P_{\text{m}} = \frac{{P_{ \hbox{min} } + P_{ \hbox{max} } }}{2} = \frac{{P_{\text{PT}} }}{2} + \frac{{P_{\text{F}} }}{2} = 920 {\text{N}}$$

and the alternating load was:

$$P_{r} = \frac{{P_{\hbox{max} } - P_{\hbox{min} } }}{2} = \frac{{\Delta P}}{2} = 50 {\text{N}} .$$

The corresponding applied stresses were:

$$\sigma_{\text{m}} = \frac{{P_{\text{m}} }}{{A_{3} }} = \frac{{P_{\text{m}} }}{{{{\pi d_{2}^{2} } \mathord{\left/ {\vphantom {{\pi d_{2}^{2} } 4}} \right. \kern-0pt} 4}}} = 391.50\,{\text{MPa}},$$
$$\sigma_{r} = \frac{{P_{r} }}{{A_{3} }} = \frac{{P_{r} }}{{\pi d_{2}^{2} /4}} = 21.30\,{\text{MPa,}}$$

where d 2 was the mean wire diameter d 2 = 1.740 mm

Using the Sodeberg criterion, the equivalent stress is given by the equation

$$\sigma_{\text{eq}} = \sigma_{\text{m}} + \sigma_{r} \frac{{s_{\text{y}} }}{{s_{\text{e}} }} \le \frac{{s_{\text{y}} }}{N}$$
(1)

where s y, is the yield limit in shear strength and s e, the yield limit in alternating shear strength. The wires used in our experiments were made of stainless steel 316L with s y = 520 MPa and s e = 90 MPa. With the above values, Eq. (1) gives σ eq = 514 MPa. Assuming a coefficient of safety N = 1, σ eq = 514 MPa < s y /N = 520 MPa and the Sodeberg criterion was satisfied.

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Bairaktari, C., Athanassiou, G., Panagiotopoulos, E. et al. Towards a solution of the wires’ slippage problem of the Ilizarov external fixator. Eur J Orthop Surg Traumatol 25, 435–442 (2015). https://doi.org/10.1007/s00590-014-1518-9

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