• Computational model of prototype device
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External fixator for limb fractures. Motorised device lengthens bone fracture site at rate of healing to allow bone regrowth to correct absence of bone due to disease or trauma
Group Members
Tim Hartley, Matt Lisle, Charlotte Orledge, Mike Parkin, James Womald, Ian Wright
Professor Martin Browne, Dr. Alex Dickinson
Dr. Andrew Taylor
Fraction fixation devices are used for limb lengthening or to correct bone deformities. The fixator is manually adjusted by the patient 2-4 times daily, a painful process, that can result in complications. Excessive pain or patient error can result in poor compliance or inefficient treatment. By automating the lengthening process, these issues would be eliminated. Automation enables additional features such as sensors to be incorporated.

This project aimed to design an automated external fixator, and to assess its feasibility for clinical use. Requirements were established, including supporting maximum loads expected and transmitting force for bone extension among other features. This was complemented by feedback from clinicians and patients.

The new design built on existing fixator designs by using six stepper motors to provide actuation in six degrees of freedom. A design was created for the mechanical structure to provide the required mechanical properties for load bearing and enable the actuation provided by the electronic systems. Finite element analysis was performed with SolidWorks and Ansys to determine the structural response under loading. A prototype was created to assess the design and validate the computational simulations.

An electronics system was designed to automate and control movement. Sensors were incorporated into the system to check for error during operation. Power consumption of the device was reduced to extend the operational time. A printed circuit board was created to investigate size reduction of the circuitry and ease of assembly.

The designs presented demonstrate that there is potential viability for an automated external fixator within clinical use.
Cutaway of the bevel gears used to apply torque to the frame struts during extension of the device.
Image of the control code used to translate desired movement of the rings into motor speeds.
Computational model of frame strut
Electronic system circuit diagram, including multiplexing, power, sensor control and visual feedback