Sonography aided computer assisted surgery (SACAS) in orbital surgery

Background. The application of computer assisted procedures in orbital surgery is made more difficult by the intraoperative tissue shift in intraorbital structures, since this intraoperative dislocation cannot be imaged in preoperative CT/MR clatasets. Methods. After preoperative recording of CT and/or MR clatasets in five patients with orbita affected by frontobasal tumors, we used intraoperative sonography by coupling the ultrasound unit to the navigation system. Results. Registration, referencing and calibration of the ultrasound system proceeded without any difficulties. Intraoperatively, the structures of the anterior and middle thirds of the orbita and their tissue shift could be particularly well evaluated sonographically. Conclusion. The use of navigated sonography enables repeated intraoperative re-evaluation of preoperative CT/MR datasets. The fusion of intraoperative sonography with preoperative imaging visualizes the tissue shift and facilitates the identification of anatomical structures and the spatial orientation of the surgeon. This appears to allow both increased operative radicality and greater tissue protection. In our opinion, the intraoperative parallel application of a non-calibrated ultrasound system and an only CT/MRT based navigation system cannot fulfill these requirements because of anatomical complexity.