An external camera tracks all surgical instruments using a passive array (reflected sphere) or active array (led). This tacking system has a reference array fixed to the patient [13–18].

Using the 3D navigation, the surgeon can simultaneously check the multilevels of spinal anatomy, making it helpful when percutaneous operations or deformity operations are performed (Fig. 2.2).

The IGS made spinal surgery safer and with more reliable results.

The spinal navigation systems can use images acquired from preoperative CT or intraoperative CT scan; in the first case, the surgeon must make a point-to-point record by matching the preoperative CT image with the surgical operating anatomy of the patient.

In the case of the computerised spinal navigation system, the images recorded from intraoperative CT in the operating room will be automatically recorded and sent to the navigator without the surgeon’s participation. In this particular case, the external camera situates the position of the CT scan in respect of the fixed reference array (Fig. 2.3).

The use of the intraoperative CT scan increases the accuracy of procedures and reduces the operative time eliminating the variability due to the action of the surgeon’s manual recording [19–22].

The intraoperative CT scan, where the patient is prone in the operating room, deletes another possible cause of error due to the patient position, which is usually supine during the preoperative CT. This variation of the position can be significant when the spine injuries are unstable, like in traumas or particular cases of degenerative deformity [23].

Another advantage of having an intraoperative CT scan allows supervising the right screws’ position before completing the procedure, removing the likelihood of going back to the operating room after a few days in order to correct the misplaced screws.

In our centre, we use the intraoperative CT spinal navigation for every surgical operation, and this has been essential to overtake the learning curve in the shortest time possible.

We use O-arm Medtronic (come beam CT) that moves 360° around the prone patient on the operative field; all the obtained images are automatically recorded by the navigator (stealth station Medtronic), which re-elaborate them to have clearer high-quality 3D images.

We have reviewed records of 1500 consecutive screws with a 98% amount of correct placement, according to the most recent literature revisions; in this percentage we consider the grade 0 and 1 of the Gertzbein and Robbins classification [24, 25]. The position of the screw was classified into five grades according to the violation of the pedicle cortex: (0) no violation, (1) up to 2 mm, (2) from 2 to 4 mm, (3) from 4 to 6 mm and (4) more than 6 mm (Fig. 2.4).

The intraoperative CT-based navigation has a significant role in the re-operation cases, where the anatomy results undermined and the surgeon cannot find the usual landmarks; the external camera replaces the surgeon’s sight so that he can see their instruments moving on the several levels on the navigator screen. This system guides him through the dense arthrodesis and tissue scars (Fig. 2.5).

Even in several deformity cases, the spinal navigator, using the multilevel vision, helps the surgeon to nullify anatomic alterations due to the deformity in order to find the right peduncle (Fig. 2.6).

It is well known that spinal surgery, using computer navigation, has a learning curve [26]; from their personal experience, the authors are convinced that we need a habitual use of it, to get to the end of the learning curve of the navigation system and the use of intraoperative CT scan. In this way our surgical team has reached a plateaued in 5 months.

After 4 years of use of CT intraoperative-based computer-assisted navigation system, according to the literature, we can state that at the end of the learning curve, the surgical timing reduces significantly.

Thanks to the possibility to run a new scan before completing the intervention, the risk of going back into surgery to correct misplaced screws is irrelevant (Fig. 2.7).

Surgeons should not underestimate the importance of having an intraoperative CT scan also for reducing the exposure to ionising radiation, to which doctors and nurses of the operating room are exposed during the spinal surgeries performed with the only fluoroscope [19].