Abstract
Total disc replacement is an alternative to spinal fusion in treating degenerative disc disease, whilst preserving motion and reducing the risk of subsequent DDD at adjacent levels. Current designs have evolved from technology used in total hip replacements with metal-metal or metal-PE bearing surfaces. These articulating systems may be prone to wear and it is essential the medical engineering community assess their performance using appropriate simulators
Utilising previous Leeds simulation design experience, current knowledge on spinal kinetics and prevailing Standards for spinal testing, a comprehensive set of requirements was generated from which a simulator design was produced. The Leeds Spine wear simulator, developed in conjunction with Simulation Solutions Ltd, incorporates five active degrees of freedom: axial compression, axial rotation, flexion-extension, lateral bending and anterior-posterior displacement. The fifth DOF, unique to the Leeds simulator, is anticipated to be particularly important for the study of mobile bearing devices such as the Charité. Loads and motions are applied by electro-mechanical actuators, providing accurate and precise control without the low band width suffered from pneumatics or contamination from hydraulic systems. This validation study determines the accuracy and precision of the simulator with regards to the degrees of freedom required by the newly published standard ISO 18192-1. Here, loads and motions have to be within ±5% of the maximum value and ±0.5degrees, respectively. The simulator’s response to demand input signals was determined for load and motion using independent measuring devices; a digital inclinometer for motions and load cell for force.
The load calibration was found to be within ±1% of the maximum load within the specified load range of 600–2000N. Flexion-extension, lateral bending and axial rotation were found to be within ±0.5, ±0.3 and ±0.5 degrees respectively, within and beyond the operating ranges specified by ISO.
The Leeds spine wear simulator is the first orthopaedic wear simulator to include electro-mechanical actuators for all active DOF, and the first spinal wear simulator to include a minimum of 5 active DOF. This novel simulator meets the demanding tolerances required by ISO for testing of total disc replacements. Validation of the simulator is currently being undertaken to determine its suitability against explanted devices and debris located within tissues.
Correspondence should be addressed to Dr Roger Bayston, Division of Orthopaedic and Accident Surgery, Queen’s Medical Centre, Nottingham, NG7 2UH, England.
