Intraoperative registration for robotic-assisted orthopaedic surgery : a digital X-ray photogrammetry based technique

Changes in orthopaedic practice have led to an increased reliance upon fluoroscopic image-guidance during fracture fixation (osteosynthesis) procedures. The resulting complexity of surgical techniques, and concerns over X-ray radiation exposure levels to orthopaedic surgeons, has prompted an investigation of robotic-assisted orthopaedic surgery, with a view to improving the precision, repeatability and radiation safety of existing fluoroscopically-guided bone drilling procedures. A prerequisite for such an application is to establish the patient's location with respect to the robotic manipulator. Owing to the inherent nature of osteosynthesis procedures, this intraoperative registration process can only be performed by quantifying the existing fluoroscopic examination process. A digital X-ray photogrammetry based registration technique, which uses a radiolucent robot-mounted X-ray calibration frame, has therefore been investigated. When this calibration frame is simultaneously imaged with the patient, discrete registration markers, corresponding to radiopaque fiducials embedded in the frame, are superimposed into the standard intraoperative X-ray images. Digitising these images, using a PC-based frame grabber card, has allowed semi-automatic image analysis routines to be implemented. Applying correction-calibration software, which provides on-line compensation for image distortion effects, then allows the imaged part of the patient's skeleton to be located, with respect to the calibration frame's coordinate system, thus establishing intraoperative registration. In vitro laboratory-based trials of this registration technique indicate that reconstruction errors are in the sub-millimetre range. As such, this new approach represents a low cost non-invasive registration option, which fully adheres to "operating room compatibility" criteria, and is applicable to a wide range of osteosynthesis procedures. Significantly, by extending the technique to include quantification of the trajectory plarming process, it has also been possible to demonstrate major improvements over existing surgical techniques.