10 Navigated Subaxial Cervical Spine Pedicle Screw Instrumentation

10.1055/b-0039-172721

10 Navigated Subaxial Cervical Spine Pedicle Screw Instrumentation

John A. Buza III and Peter G. Passias

Abstract:

Computer navigation increases the accuracy of pedicle screw placement in the cervical spine, but further studies must elucidate clear proof of benefit to the patient before more widespread adoption of this technology. Cervical pedicle screw placement is technically difficult, and computer navigation has been employed as a means of increasing the accuracy of this procedure. Computer navigation is generally classified by the associated imaging modality, and may utilize either CT scan or fluoroscopy. CT scans may now be obtained either preoperatively or intraoperatively. Traditional fluoroscopy was two-dimensional, but newer technology has allowed for three-dimensional fluoroscopy to be obtained intraoperatively. The process of intraoperative registration is vitally important, and should be performed at each spinal segment individually. The major advantage of computer-assisted navigation is increased accuracy and reliability, and a decreased rate of major pedicle cortical violations. This would in theory decrease the rate of vascular or neurological injuries, although this is yet to be shown. The major disadvantages of navigation are increased operating room time and cost. No studies have demonstrated a clear clinical benefit to the patient from the adoption of this technology. Future high-level studies are required demonstrating a clear clinical benefit. The available series has demonstrated that while navigation-assisted cervical pedicle screw placement may increase accuracy and reliability, there is still a relatively high rate of major cortical violations. Therefore, it is critically important to take great care when placing cervical pedicle screws, even with the use of this technology.

10.1 Introduction

Pedicle screw placement in the cervical spine is challenging, and the rate of cervical pedicle screw misplacement varies from 2.5 to 29.1% in the literature. ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ ^, ⁶ There are a number of possible reasons for the relatively high rate of pedicle screw misplacement in the cervical spine. The anatomy is smaller in size compared to the lumbar spine. There are frequently three-dimensional (3D) deformities, and a number of vital anatomic structures lie in close proximity to the pedicle, including the spinal cord, cervical nerve roots, and vertebral artery. There is extremely limited space for deviation of a pedicle screw from the ideal trajectory between the spinal canal medially and the vertebral artery laterally between C3 and C7, and therefore a significant risk of neurovascular injury by inaccurate screw position. The standard posterior approach to the cervical spine allows a two-dimensional (2D) visualization of a complex 3D structure. Conventional orientation to the spine is achieved by identifying this surface anatomy, in combination with 2D fluoroscopy. However, several situations can make identification of surface anatomy more challenging. Minimally invasive approaches to the spine limit the exposure of surface anatomy, decreasing the number of known landmarks. The aging population typically presents with advanced degenerative changes in the cervical spine, which may also mask the surface anatomy. Instrumentation of the cervical spine is therefore very complex and requires significant spatial awareness and experience.

Due to the inherent challenges and higher precision required in the cervical spine, intraoperative navigation has been developed in an effort to provide the surgeon with spatial information to enhance the accuracy of screw placement. This has the potential to significantly increase screw accuracy and limit the amount of intraoperative radiation exposure.

10.2 Intraoperative Techniques

Several techniques are available for navigation in cervical pedicle screw placement, which differ in both the imaging modality and registration method. The imaging modality may be broadly classified as CT imaging, 2D fluoroscopy, or intraoperative 3D fluoroscopy. While CT-based methods were frequently used in the development of spinal navigation, intraoperative imaging and, in particular, 3D fluoroscopy are being used with increasing frequency. The registration method has largely changed from surface matching methods to automatic registration of the intraoperative fluoroscopy imaging dataset.

There are advantages and disadvantages to these techniques, which will be further reviewed below.

10.2.1 CT-Based Navigation

CT-based navigation was the first imaging modality for image-guided spinal navigation. CT-based navigation traditionally requires that the patient undergo a preoperative CT scan with a special protocol acquiring consecutive axial images of 1.5 mm thickness. Screw entry point and screw direction are then determined using surgical software planning to minimize potential risk to surrounding neurovascular structures. Both screw length and diameter can be determined using this software.

At the beginning of the surgical case, intraoperative landmarks are mapped and then correlated with the virtual data obtained by CT scan (Fig. 10‑1 , Fig. 10‑2). This process is known as “registration.” There are three different methods by which registration may occur, including paired-point matching, region matching, and CT-fluoro matching. In paired-point matching, anatomically distinct points are identified in the CT scan and these points are “presented” to the navigation system by surgeon with the use of a pointer after attachment of a reference array to the spine. Region matching is a similar process, but the navigation system constructs a 3D model of the spine, and a random cloud of surface anatomy points is correlated by the surgeon with that 3D model. The third method of registration is CT-fluoro matching, in which the preoperative CT scan is matched with intraoperative 2D fluoro images taken from multiple angles.

**Fig. 10.1** Intraoperative CT scan is performed using the Brainlab Airo Mobile Intraoperative CT.

**Fig. 10.2** Intraoperative registration is performed by verifying the accuracy of the pointer on three known anatomical landmarks (Brainlab Airo Mobile Intraoperative CT).

This process of registration is the key influencing factor for the accuracy of any navigation-based technology. The patient’s anatomy must be carefully and accurately registered to the imaging data set. When using CT-based navigation, the spatial relation between vertebral segments may change greatly as the patient goes from a supine position during CT scan to the prone position during a posterior surgery. Therefore, a careful and repeated control of registration accuracy using distinct anatomic points is necessary during surgery, particularly for instrumentation at more than one level. Tauchi et al demonstrated this in a retrospective study of 46 patients undergoing CT-based navigated pedicle screw placement. ⁷ The authors found that the pedicle screw misplacement rate for single-time multilevel registration was 23.4%, compared to 6.2% for separate registration times. ⁷ The inaccuracy of single-time registration was most pronounced for patients with increased preoperative cervical range of motion, and the authors concluded that it is particularly important to perform separate registrations in this patient population. ⁷

Regardless of the registration method used, each method allows for identification of the surgical instruments in relation to the surgical field. The surgeon can then use the virtual field to achieve precise positioning of all instruments, which allows for entry point identification and screw trajectory (Fig. 10‑3). A reference frame attached to the patient can also track any movement of the patient, such as excursion of the thorax caused by breathing, which may alter the relation of the virtual field to the surgical field.

**Fig. 10.3** Pedicle screw placement is planned using intraoperative navigation into the left C6 pedicle (Brainlab Airo Mobile Intraoperative CT).

Given some of the limitations surrounding the use of preoperative CT scans for navigation-based pedicle screw placement, the use of intraoperative CT scan has been advocated. In a study of 129 pedicle screws placed in the cervicothoracic junction (C5–T3), Barsa et al found that the use of intraoperative CT scan resulted in a relatively low rate of pedicle screw misplacement (5 of 129, 3.9%). ⁸ The authors used a 32-slice portable CT-scanner (BodyTom; NeuroLogical, Danvers). While the imaging quality is less than a traditional helical CT-scan, the authors concluded that the quality of intraoperative CT imaging sufficient for navigation was obtained at all spinal segments regardless of patient’s habitus or positioning. The authors did report a higher radiation exposure for the patient, and an average increase of 27 minutes of operative time for this technique. ⁸

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