The Role of Neuronavigation in Lumbar Spine Surgery

Fig. 1.1

The basic methods of spinal navigation

Fig. 1.2

The main differences between 2D fluoroscopy-based method and 3D CT-based method to navigate the spine. With fluoroscopy, the surgeon must deduce the third dimension and is directly exposed to radiation. A 3D navigation system allows to see directly the three planes of the anatomy

The essential components of a 3D system are:

An image-processing computer workstation
An infrared camera
A reference array made up of reflective spheres, attached to the spinous process of the interested level
An arrangement of passive reflective spheres mounted on the surgical tools

The infrared light, projected by the camera toward the surgical field, is reflected by the spheres and then sent to the computer workstation. This reckons the data received. Hence, the position of the surgical tool is visible on the screen of the workstation, superimposed on axial, coronal, and sagittal radiological planes.

The decisive step in navigated procedures is the “registration,” the process through which the images and the surgical anatomy are “matched.”

The technique of the registration depends on the navigation system adopted and on the kind of surgery: whether open or percutaneous.

With a preoperative CT-based system, the matching process is performed directly by the surgeon who may use a “paired-point” or a “surface matching” technique that are detailed in Figs. 1.3 and 1.4 [13, 26].

Fig. 1.3

Our usual operative setting for spinal navigation

Fig. 1.4

The workflow in preoperative CT-based system starts with the acquisition, before surgery, of a CT in prone position. The intraoperative images acquired with a C-arm are then matched with the preoperative ones

If an intraoperative CBCT or MDCT is used, the registration process is an automatic one, without any significant surgeon input.

In Sect. 1.5, we will see how many sources of possible mistakes are inherent in this crucial moment of the navigation.

Once the registration is completed, the workflow starts with the choice of the entry point and the planning of the trajectory of the screw. The camera tracks the surgical tools, and, following the images displayed on the screen, the surgeon selects the adequate caliper, length, and orientation of the pedicular screw (Figs. 1.5 and 1.6).

Fig. 1.5

The planning of trajectory of pedicular screw in percutaneous technique

Fig. 1.6

The guided piercing of the pedicle on the left and the insertion of screw along the planned way on the right

During the navigated surgical procedure, any fluoroscopic control is not necessary. At the end of the surgery, the correctness of the screws’ placement is checked through a CT acquisition before the closure or, if an intraoperative CT is not available, an AP and LL fluoroscopy.

1.1.1 How to Assess the Screws’ Placement

Before analyzing the differences of the results between free-hand technique and navigated technique as well as among the different available techniques of navigation, it is useful an attempting to define a method to evaluate the accuracy of the screws’ placement. Kosmopoulos and Schizas, in their meta-analysis, found 35 different methods to assess screws placement (but some are only slight modifications of others) [17, 30]. There are, in essence, two critical points:

The first one is: we have, on one side, an “in/out” way to evaluate the position of the screws; all those violating the cortical bone are “out,” whatever is the degree of the cortical breach. On the other side, we have extremely graduated or incremental methods, like that proposed by Gerbzstein and Robbins [11]. These authors distinguish five different groups: (1) when the screw is entirely within the pedicle; (2) if the encroachment is <2 mm; (3) if it is between 2 and 4 mm; (4) when it is between 4 and 6 mm; and (5) if the screw is more than 6 mm (i.e., about the screw size). If the first way may be too much rough, the other one may result impractical in clinical routine. At this regard, we have also to consider the concept of “safe zone,” a space interposed between the pedicles and the neural elements. Lien relieved, according with other authors, that this space is greater superiorly and laterally than medially and inferiorly. The distance of the roots from the lateral surface of the pedicles is in the range of 2.4–9.4 mm, and it is smaller in the low lumbar levels [8, 9, 19].

The second critical point regards the radiological planes on which the screws’ location is evaluated: if coronal and sagittal planes are also considered, the incidence of cortical violations is higher than when only axial plane is analyzed.

For these reasons, there are so relevant differences among papers regarding screws’ placement accuracy. And then every comparison among different methods of navigation in terms of accuracy and misplacement must be cautiously considered, not forgetting the differences among different descriptive statistics [17].

1.1.2 Comparison Between Free-Hand Technique and Navigated Technique

With the limitations reported in the above paragraph, we may compare the results of surgeries supported only by 2D fluoroscopy and the navigated procedures, globally considered.

In a meta-analysis on papers regarding large series of patients, operated on with and without the help of navigation systems, Verma et al. relieved there were no reported cases of neurological complications in the navigated procedures, whereas there was an incidence of 2.3% of these complications without navigation aid. The accuracy of screws placement (evaluated with different methods) was of 93.3% with navigation and of 84.7% without it [31].

If the difference in terms of complications in favor of navigational procedures was not statistically significant, the advantage of navigation in terms of accuracy (93.3% vs. 84.7%) was statistically relevant [31].

Referring to fusion rate and functional outcome, there are no literary sources to compare the results of navigated and conventional techniques [31].

The lack of visualization of bony landmarks makes the guidance of a 3D navigation system particularly advantageous in percutaneous procedures.

Hence, it is of remarkable interest the comparison between 2D fluoroscopic navigation and 3D stereotactic navigation in percutaneous transpedicular screws’ insertion. With this aim, Bourgeois et al. compared their series of 599 patients, operated percutaneously with the guide of intraoperative CBCT, with ten papers, reporting series of percutaneous procedures, assisted by 2D fluoroscopy [3]. In the 3D-navigated series, there was a rate of pedicle breach of 1.15% on per-patient basis and 0.33% on per-screw basis. In the 2D series, the lowest rate of incorrect screw placement was 9% per patient and 1.7% per screw.

Compared with conventional percutaneous screws’ placement, the 3D technique presented an absolute risk reduction of 17% [3].

Other studies confirm that 3D navigation systems lessen the rate of screws misplacement in percutaneous procedures [3, 17, 25].

1.1.3 Comparison Among the Different 3D-Navigated Techniques

3D navigation systems based on intraoperative CT (CBCT or MDCT) present the following main advantages over the systems based on preoperative CT:

Automatic registration process instead of a manual one and, thus, less probability to make mistakes in this crucial moment of navigation.
The CT images are acquired with the patient in prone position on the surgical table, with no risks of incongruence due to the possible changes of vertebrae position after the induction of anesthesia and muscle relaxation.
The possibility to perform a CT control at the completion of surgery and, thus, to revise a misplaced screw before the closure [4, 16].

In an accurate comparison between a series of patient operated with a preoperative CT-based system and a series in which an intraoperative CT (O-Arm) was used, Costa et al. observed the following advantages with O-arm: higher accuracy in screws placement (95.2% vs. 91.8%), shorter mean operative time (92 min vs. 128 min), merging procedures less time consuming (1.15 min vs. 6.5 min), and shorter mean insertion time per screw (2.9 min vs. 3.8 min) [4, 5].

1.1.4 Spinal Navigation in Pediatric Patients

In children with deformity, the risk of screws’ misplacement is higher than in adult patients due to the smaller pedicle size and of a relevant coronal deformity [18].

In pediatric population, the 3D navigation systems improve the accuracy of screws’ placement. Larson reports a rate of accuracy of 96.4%, compared with an 84.3% of studies where the assessment was based on postoperative CT and the procedures were without navigation [18].

Luo reports a series of young children (range 1–10 years), in whom the screws were implanted from C1 to L5 for different types of congenital deformities. In his navigated series, the accuracy rate was 97.8%, significantly higher than the 90.9% reported by Baghadi in a non-navigated series of the same age group [2, 20].

In addition to a safer screw placing, 3D spinal navigation offers another remarkable advantage in scoliosis surgery. A CT performed after the screws’ insertion allows to revise or remove the malpositioned screws before the correction maneuver, reducing the risk of loss of fixation and screws’ migration during this act [2, 18, 20].

1.2 Uses of Spinal Navigation Other than Screws’ Placement

If the improvement of correctness and safety in pedicular screw placement is the main reason to use navigation systems, we also have to consider there are other procedures, in instrumented spinal surgery, that may be usefully supported by a stereotactic guidance system.

In our personal experience, we found navigation useful to perform a transpedicular vertebral biopsy. In cases in which we proceed to stabilize a spine, jumping the pathological vertebra and putting the screws above and below it, we complete our surgery taking different samples within the pathological body, through a transpedicular route. The availability of a navigation system allows us to be sure to have wholly centered the critical areas of the lesion (Fig. 1.7).