Stereotactic navigation is especially useful in patients with more complex anatomy, such as significant spondylolisthesis or degenerative scoliosis. Navigation can also be used to determine the best trajectory for intervertebral cage placement and for trans-sacral fixation [12] (Fig. 11.8). In the lumbar spine, it is used to determine the length of rods and to align screws during a multilevel fusion so that the percutaneous rod placement is facilitated. In the cervical spine, CAS facilitates the minimally invasive resection of odontoid masses via a transnasal route, which is a significant improvement when compared to conventional maximally invasive transoral surgery [13, 14].

In more complex thoracolumbar deformity cases, the interbody part is frequently being accomplished via a separate anterior or lateral approach. This surgery may also include the placement of iliac crest screws [15]. In principle, navigation is performed in a similar fashion as described above (Fig. 11.9a, b).

A few differences or additional challenges, however, apply. Some authors have reported good accuracy with multilevel cases where the reference array was placed > 10 levels away from the surgery site [8]. We disagree and recommend placement of the reference array into the iliac crest for cases up to L3. If the fusion extends above L3, we will typically reposition the reference array more cranially using a spinous process clamp. In our experience this will maximize accuracy and safety. Another challenge is that current intraoperative portable cone beam CT systems have a limited field of view and therefore only allow the imaging of up to 3–5 vertebral bodies. This adds time, radiation exposure, and complexity to multilevel deformity cases. The solution here is the use of a true intraoperative CT scanner.

The current advantage of using navigation in complex anatomy cases is that screw placement is clearly more accurate and technically more straightforward than with conventional AP/lateral fluoroscopy. Screw fit is maximized and there is no need to “skip” levels due to small or complex pedicle anatomy as is frequently seen in multilevel cases with conventional MIS techniques. A recent systematic literature review confirmed that navigation provided higher screw placement accuracy compared with conventional methods especially in scoliosis cases [16]. We found that CAS was associated with improved screw placement accuracy and that it was employed in cases with a higher degree of surgical complexity such as MIS cases, deformity, and revision surgery [17]. As the technology improves, it is likely that CAS will become more important in deformity surgery.

The use of K-wires can be harmful to the patient as they can break or bend during the procedure and cause visceral or vascular injury. In addition, the surgical workflow using K-wires is complex and requires the use of multiple instruments that go back and forth between the surgeon and the scrub nurse. We introduced a navigated guide tube that allows drilling, tapping, and the placement of the final screw without the need for K-wires [10]. This instrument facilitates the workflow in the operating room by reducing the number of instruments that need to be navigated and reduces the potential risks associated with current techniques for the insertion of percutaneous or mini-open pedicle screws by eliminating the need for K-wires (Fig. 11.4). Nottmeier recently described an approach to implant pedicle screws without the use of K-wires using precalibrated instruments including an awl, pedicle probe, tap, and screwdriver [15].

When used intelligently, CAS can help make spine surgery safer for the patient as well as the surgeon and the operating room staff: The issue of radiation exposure using 2nd-generation CAS for MIS has been addressed by Nottmeier et al. [18]. In 25 MISS cases with 228 screws placed using a portable cone beam CT navigation, there was no radiation exposure to the surgeon. This requires, though, that K-wires are not used.

Navigation does not replace surgical experience, judgment, meticulous preparation, and technique. For the surgeon who uses navigation for the first time, it will neither make surgery “easier” nor will it facilitate the workflow. Navigation requires careful planning and training not only for the surgeon but also for the whole team: the scrub nurse, the assistants, the X-ray technician, and others. There is a learning curve and initially some additional time will be required to successfully incorporate navigation. Many of the initial negative reports on navigation were due to the first generation systems not being user-friendly and that surgeons did not spend the time to really master this new technique. In the author’s experience, one of the hardest tasks is to teach navigation to assistant surgeons who do not have the experience and understanding and who may believe that navigation enables “videogame” or “plug and play” surgery. The contrary is true: Accurate navigation requires very meticulous and gentle surgical technique and constant vigilant interpretation of what the computer screen shows versus the surgeon’s tactile feedback. Subtle discrepancies may indicate a mismatch between the actual anatomy and what the screen shows and this requires immediate troubleshooting. In the majority of cases, this does not mean that navigation failed. Easily correctable reasons include: