TECHNICAL EVALUATION OF THE POSITIONAL ACCURACY OF COMPUTER ASSISTED SURGICAL SYSTEMS

Abstract

The role of CAOS systems is now well established in several areas of orthopaedic surgery. The increasing use of these systems, particularly in knee arthroplasty, has been supported by clinical trials that demonstrate a more accurate final position of implanted devices compared with conventional instrumentation. CAOS technology is constantly evolving along with its expanding list of potential indications. This requires the adaptation of both software and hardware components. It is therefore essential that potential users have confidence in the accuracy of these systems. The aim of this project was to design and manufacture a standardised measurement object (phantom) to independently evaluate CAOS system performance.

The American Society for Testing and Materials (ASTM) International along with CAOS International recently drafted a standard for measuring technical accuracy of navigation systems. This proposed standard was obtained and its recommendations used to design a phantom model. This consisted of a 150×150×20mm base plate and two additional levels including a single 30° slope. This created a 3D surface on which points could be placed. Co-ordinates for 21 points were given to establish the x, y and z axes of a Cartesian system and then to have points at a variety of known locations in this 3D space. The final model was machined from a billet of marine grade aluminium alloy 6082-T6 (chosen for its dimensional stability) using a vertical computer numerical controlled (CNC) milling machine with the co-ordinate points drilled with a Ø0.8mm 60° BSO centre drill to a depth of 1.2mm. The drill holes, with chamfers of Ø1.0mm, were designed to accommodate a ball-nosed pointer tip of a known diameter. A Perspex base unit with three different sites of rigid tracker attachment was made to hold the phantom and provide its reference frame. This avoided the need to directly modify the phantom itself.

The final design has been used to measure the positional accuracy of a novel portable navigation system and demonstrate that it is not yet suitable for clinical evaluation due to errors of 1 – 6 mm in point location. It has also allowed independent technical validation of current pre-existing navigation systems.