Fiducial-free Endoscopic Vertebra Referencing

Over the past decade computer aided, navigated surgery has evolved from early laboratory experiments to an indispensable tool for many interventions in spine surgery. Accurate patient registration with the pre-operative imaging modalities or referencing of the target anatomical structure is an essential component of these procedures. Many of today’s approaches are invasive and use either mechanical fixation devices, fiducial markers, ionizing radiation or require tedious manual registration with a pointing device. While the potential of ultrasound imaging for non-invasive registration and referencing is currently explored [1] its usability is constrained by the presence of air-filled cavities in the body. In this paper we present a novel approach that uses a tracked monocular endoscope to register and reference vertebrae during spine surgery. By tracking natural landmarks over multiple views, a 3D reconstruction of the surgical scene can be computed using photogrammetric methods. The proposed algorithm consecutively refines the 3D model with each additional view. This reconstruction provides quantitative metric data about the target anatomy which can be used for 3D-3D registration of the anatomy to the pre-operative data and for further referencing. Our aim is to create an intra-operative support environment, which relies as much as possible on the tools and instrumentation used anyway during surgery and does not increase the invasiveness of the intervention just for navigation purposes. The presented method is general and can be used for any rigid anatomical structure with sufficiently textured surfaces. In this paper, the results of a series of cadaver experiments are presented.