This paper presents a feasibility and evaluation study for using 2D ultrasound in conjunction with our statistical deformable bone model in the scope of computer-assisted surgery (CAS). The final aim is to provide the surgeon with an enhanced 3D visualization for surgical navigation in orthopaedic surgery without the need for preoperative CT or MRI scans. We unified our earlier work to combine several automatic methods for statistical bone shape prediction from a sparse set of surface points, and ultrasound segmentation and calibration to provide the intended rapid and accurate visualization. We compared the use of a tracked digitizing pointer to ultrasound to acquire landmarks and bone surface points for the estimation of two cast proximal femurs, where two users performed the experiments 5-6 times per scenario. The concept of CT-based error introduced in the paper is used to give an approximate quantitative value to the best hoped-for prediction error, or lower-bound error, for a given anatomy. The conclusions of this work were that the pointer-based approach produced good results, and although the ultrasound-based approach performed considerably worse on average, there were several cases where the results were comparable to the pointer-based approach. It was determined that the primary factor for poor ultrasound performance was the inaccurate localization of the three initial landmarks, which are used for the statistical shape model.
Supporting surgeons in performing minimally invasive surgeries can be considered as one of the major goals of computer assisted surgery. Excellent intraoperative visualization is a prerequisite to achieve this aim. The Siremobil Iso-C<sup>3D</sup> has become a widely used imaging device, which, in combination with a navigation system, enables the surgeon to directly navigate within the acquired 3D image volume without any extra registration steps. However, the image quality is rather low compared to a CT scan and the volume size (approx. 12 cm<sup>3</sup>) limits its application. A regularly used alternative in computer assisted orthopedic surgery is to use of a preoperatively acquired CT scan to visualize the operating field. But, the additional registration step, necessary in order to use CT stacks for navigation is quite invasive. Therefore the objective of this work is to develop a noninvasive registration technique. In this article a solution is being proposed that registers a preoperatively acquired CT scan to the intraoperatively acquired Iso-C<sup>3D</sup> image volume, thereby registering the CT to the tracked anatomy. The procedure aligns both image volumes by maximizing the mutual information, an algorithm that has already been applied to similar registration problems and demonstrated good results. Furthermore the accuracy of such a registration method was investigated in a clinical setup, integrating a navigated Iso-C<sup>3D</sup> in combination with an tracking system. Initial tests based on cadaveric animal bone resulted in an accuracy ranging from 0.63mm to 1.55mm mean error.