Abstract
Background
Currently existing optical navigation systems have ergonomic disadvantages such as size, the “line of sight” problem and extended registration procedures. The operation room becomes crowded by additional installations and competitive supporting devices around the patient. These points reduce and limit the acceptance of navigation systems for further applications. But especially for surgical quality management, navigation systems have a high potential as objective measurement systems.
Method
A miniaturised measuring and navigation system, which is directly fixed at the surgical tool, could overcome these limitations and fulfil the requirements demanded by current and future operation rooms. Minimising the distance between situ and camera promises an increased accuracy, a reduced “line of sight problem,” intuitive handling and one coordinate transformation (Tool2DRB) less. However, such a setting reduces the navigation working space available, needs a sterile system, a new marker design and special requirements for the cameras. The developed prototypes were tested in vitro using Synbones™ and ex vivo at anatomical specimen. Following surgical pilot applications were defined and considered during the studies: maxillofacial restoration osteotomy, hip replacement and unicondylar knee replacements (UKR). Special emphasis was placed on measured and recorded accuracy and miniaturised hardware.
Results
Several miniaturised measuring system prototypes with high resolution cameras mounted directly onto a surgical instrument have been developed and tested. One prototype includes a laser device which is used in combination with the cameras to register 3D surfaces like the rotational centre of an acetabular cup from a prosthetic hip joint. Other prototypes demonstrate the miniaturising aspect of this development and their ergonomic advantages. Corresponding algorithm and software developments include calibration, marker identification, network components and surgical planning modules.
Hard and software components have been tested for UKR application in an ex vivo study. Clinical trials for maxillofacial restoration osteotomy are prepared at the University Hospital Basel.
The accuracy of the presented systems was evaluated in vitro with two setups. After intrinsic and extrinsic camera calibration with a 3D calibration specimen, the accuracy (RMS) of a single point of the 3D point coordinates of the calibration specimen could be determined with 0,020 mm in z-axis and 0,010 mm in x/y-axis. In another setup the accuracy was measured in 3D with a fixed camera system and two markers rigidly fixed together. The marker system was moved around working space. The repeat accuracy of the distances between the two markers was 0,025 mm (RMS).
Discussion
The total development of the miniaturised measuring system, consisting of a video system, an optional laser scanner, calibration, image processing algorithms and planning modules was successful. The current prototype has proved to be accurate and usable. Users (surgeons) and suppliers of surgical implants, who have been exposed to the system, have expressed their keen interest, as it opens up new applications and fulfils their needs for improved ergonomics and smarter, cost reducing work flows.
But of course there is still potential for improvements. In the next iteration of the development process, the usability and accuracy of the system could still be improved. The currently used optics limit the possible accuracy because the aperture of F = 2.8 is too large for photogrammetric applications and its optics distortions are too large. Therefore it will be exchanged by an optimised solution. Another optimisation target is the camera electronics. The currently used ones cannot be synchronised, which limits the usability, respectively reduces the accuracy, if it is moved during a measurement shot.
