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  • Title: Mechanical stress on tensioned wires at direct and indirect loading: a biomechanical study on the Ilizarov external fixator.
    Author: Gessmann J, Jettkant B, Schildhauer TA, Seybold D.
    Journal: Injury; 2011 Oct; 42(10):1107-11. PubMed ID: 21392761.
    Abstract:
    BACKGROUND: The biomechanical effect of indirect weight loading with the Ilizarov ring fixator using a weight-bearing platform has not yet been investigated. The problem of wire loosening and breakage occurs more frequently when patients are mobilised with a weight-bearing platform. Therefore, the aim of this research was to compare the influence of direct and indirect weight loading on the tensioned wires. METHOD: A universal testing machine (UTS, Germany) was used in this study. A composite tibia model with a standard four-ring Ilizarov fixator and 1.8-mm wires in anatomical position was used to simulate a clinical situation. Wire strain was measured with two strain gauges positioned at the ring-wire interface of each wire. After a standardised 2-mm mid-diaphyseal osteotomy, an axial load of up to 1000 N was applied to the bone; the different methods of weight loading were evaluated in two experimental set-ups. RESULTS: A higher axial load was necessary to achieve an osteotomy gap closure at indirect loading. Mechanical stress on the tensioned wires was 400% higher on the proximal wires and 250% higher on the distal wires at a maximum axial loading of 1000 N. Mechanical stress remained on the wires in indirect loading, even after bone end contact, and led to excessive stress under higher weight-bearing amounts. CONCLUSION: There is a substantial change in the biomechanical characteristics of the Ilizarov ring fixator when mobilising a patient with a weight-bearing platform. The considerable higher mechanical stress on the wires needs to be considered when patients are mobilised with a weight-bearing platform.
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