CT-Based Navigation Systems

Abstract

Medical imaging modalities in diagnostic and clinical use today offer a wide range of possibilities for design and implementation of computer assisted surgical systems. Among those modalities, Computed Tomography (CT) scan has a prominent role, since it provides three dimensional (3D) images of relatively high accuracy. It is especially well suited for orthopaedics, since the bone separates from the rest of the tissue by its intensity, which makes the bone segmentation a relatively simple task. CT scanners are large machines, typically housed in the hospital's radiology departments or at separate imaging facilities. Attempts to integrate CT scanners in the operating room, or to develop navigation systems integrated with CT scanners did not have much success so far, especially in orthopaedics. However, because bones don't change shape between the time of the scan and the time of the surgery, it was possible to design procedures in which the imaging, planning and image guided intervention could be done at different times, as three separate consecutive events. Most of the early surgical navigation procedures and all of the robotic procedures in orthopaedics have been designed this way - sometimes referred to as «canned reality» procedures. This overview identifies the most important technical components of CT-based CAS systems, using the example of the HipNav system, developed by our team. HipNav (CASurgica, Inc.) is the surgical navigation system developed initially to address the problem of cup alignment in total hip replacement [3]. It has introduced several new concepts in the computer-assisted surgery, most notably the concept of patient-specific pre-operative simulation for joint replacement procedures. Proper alignment of implant components is one of the most important factors that contribute to the joint stability; Misalignment of components reduces the safe range of motion and can lead to dislocation of the joint, or repeated impingement of components that can result in excessive wear and generation of wear debris. Especially sensitive is the alignment of the cup part, since the orientation of the cup is not restricted by the shape of the bone cavity. Currently used mechanical tools for positioning of the cup aligns the cup with respect to global body landmarks, often resulting in inadequate cup alignment. HipNav relies on the CT-scan image for planning, and both the cup and the femoral stem of the desired type and size are selected from the 3D model database and placed in the respective bones.