Condition monitoring of Ilizarov frames using vibration sensors

Condition monitoring of Ilizarov frames using vibration sensors

William Bolton
University of Leeds, Leeds, United Kingdom

Introduction: Maintaining adequate tension within the wires of an Ilizarov frame is key to clinical efficacy. There is currently no method for in-situ measurement of wire tension after the initial pretension has been applied during fitment. Being able to measure this may be valuable in helping to advise frame adjustment, fracture healing and timing of frame removal.

Methodology: One method of assessing the tension of a wire is through analysis of the vibrating frequency after an impulse has been applied; like a guitar string, increased tension results in higher frequency vibration (emitted as a sound) in output. We have developed a piezoelectric vibration sensor which can be placed on the fixation bolt at the end of the wire and used to directly measure the transmitted wire vibration and thus minimise the influence of external environmental (auditory) noise. The vibration signal is measured and recorded by a microcontroller system, post-hoc analysis then uses a Fast Fourier Transform to determine the dominant vibrational frequency in the response. The sensor was fixed to a standard Ilizarov frame, modelled using two acrylic tubes to simulate a fractured bone. Static axial loads of 200-800 N where then applied to vary the tension in the wire. At each condition, a metal key was used to stimulate the wire and 5 repeat measures were recorded and analysed. This was then repeated with silicon pads added around the tubes to simulate soft tissues of the leg.

Results: Results show that a dominant vibrational frequency can be identified between 1500-2500 Hz. The frequency was sensitive to changes in the axial load, showing a repeatable and significant linear trend of increasing frequency with wire tension. This response was also evident with the addition of the silicon soft-tissue simulant, demonstrating that the technique has potential clinical relevance.

Conclusions: The sensing system presented here offers a viable method to quickly and easily monitor any tension loss that occurs either during the initial installation of the frame, or over time due to cyclic loading during use. This has wider implications in areas where X-ray access is limited, there are limited orthopaedic specialists, or where travelling from remote regions may make follow-up after frame surgery challenging and costly.

Keywords: Ilizarov, sensing, trauma, rural surgery, innovation