Navigation in MISS

7 Navigation in MISS

Rodrigo Navarro-Ramirez, Alisson R. Teles, Franziska Anna Schmidt, Sertac Kirnaz, and Roger Härtl

Summary

Three-dimensional navigation options offer new opportunities for more precise navigation in minimally invasive spinal surgery (MISS), reduce radiation exposure for the surgical team, and accelerate surgical workflow. Recently, the concept of “total navigation” using intraoperative computed tomography (iCT) navigation (NAV) in spinal surgery has been introduced. Total navigation makes MISS safer and more accurate, and enhances efficient and reproducible workflows. Fluoroscopy and radiation exposure for the surgical staff can be eliminated in the majority of cases.

Keywords: navigation neuronavigation spine surgery imaging intraoperative imaging localization

7.1 Introduction

Spine surgery and spine instrumentation have been influenced by technological development, allowing treatment of more complex spinal pathology, while maintaining or restoring spinal stability and alignment.1^,²^,³ Nowadays, spinal procedures have become more complex, and it is crucial to minimize complications like spinal cord injury, nerve root injury, and vascular injuries as well as dural tears and subsequent cerebrospinal fluid (CSF) leakage .²^,³ Minimizing complications requires surgeon’s expertise and familiarity with surgical anatomy that in some cases is greatly facilitated through intraoperative imaging. This allows the surgeon to be precisely oriented despite the real-time anatomical shifting during surgery or when the patient’s surgical field is reduced especially in MISS. When using MISS procedures, direct visualization of anatomical landmarks may be compromised and may make it necessary to count on the state-of-the-art imaging technology to reduce the risk of complications.²^,⁴

The technology used to acquire intraoperative imaging has evolved from the discovery of X-rays in the late 19th century to the highly sophisticated intraoperative computed tomography (iCT) based navigation tools used today. In this chapter, we will describe the two main modalities of iCT available today, namely, Airo and O-arm.²^,³^,⁵

Computer-assisted spine surgery (CAS) is a computer-based technology that links spine image, acquired by conventional techniques, such as fluoroscopy and intraoperative cone beam computed tomography (iCBCT), with an accurate representation of intraoperative anatomy.⁶^,⁷ This gives the surgeon the ability to manipulate multiplanar CT or fluoroscopic images obtained during the surgical procedure in order to gain orientation intraoperatively and, therefore, enhance the accuracy of spine surgery, which are of upmost importance during spinal instrumentations. CAS can also help to minimize the surgical team’s radiation exposure.

Accuracy of navigation systems for pedicle screw placement has been studied extensively in recent years. In summary, CAS has shown to increase screw placement accuracy and decrease the rate of malpositioned screw-related complications (>95–99%)⁸^,⁹^,¹⁰^,¹¹^,¹²^,¹³^,¹⁴^,¹⁵^,¹⁶^,¹⁷^,¹⁸ when compared with free-hand and/or fluoroscopy-based techniques.¹⁰^,¹¹ In addition, the use of three-dimensional iCT for intraoperative confirmation has the potential to eliminate reoperation for screw malposition.¹⁹ Furthermore, the navigation techniques like iCT (fan beam) may eliminate radiation exposure of the surgical staff. However, the dose received by the patient may be still high. For example, the amount of radiation received from a chest X-ray (two views) is 0.1 mSv, for an upper GI fluoroscopic study, it is 6 mSv, and the mean of an iCT (fan beam) spine procedure is 13 mSv.¹⁴^,²⁰

Here, we present the tips, pearls, and workflow that we have used to perform three-dimensional iCT navigation using the two most common iCT systems available, Airo (Brainlab) and O-arm (Medtronic).

The increasing popularity of MISS has led to dependence upon image guidance, primarily intraoperative fluoroscopy, mainly because limited muscle dissection may prevent adequate exposure of the anatomical landmarks used for traditional free-hand techniques.²¹ Although three-dimensional navigation increases instrumentation accuracy and reduces surgeon’s exposure to radiation,²² its implementation into the clinical practice has been challenged by its costs, and by surgeon’s concerns about increased operative times, ease of use, integration into the surgical flow, and safety.¹⁴

In 2013, Härtl et al²³ published a global survey on the use of computer-aided spine surgery that provided interesting data on the use of spinal navigation. In this survey, approximately 80% of the respondents were favorable to the use of spinal navigation. However, despite its widespread availability, only 11% of spine surgeons in North America and Europe used navigation. The routine users of navigation mentioned following advantages: its accuracy, the potential of making complex surgery safer, and minimizing radiation exposure. The nonusers not only indicated lack of equipment and high costs as the most important reasons not to use navigation, but also inadequate training and increasing operative time.

It has been proposed that navigation systems might prolong overall operative time due to the time used for operating room (OR) setup and navigation workflow, as well as the time required to scan and register the patient intraoperatively. In fact, it has been shown that even when the overall operative time is prolonged, the time per screw insertion is shorter due to quicker identification of bony anatomy.²⁴^,²⁵ Of note, the new generation navigation settings require less scanning and registration time.

Hence, application of step-by-step guidelines might overcome potential delays related with the setup and workflow with intraoperative image-guided navigation, making it more efficient, reproducible, and diminishing the overall operative time.²⁶

Every navigation system has its own characteristics and, consequently, the workflow, pearls, and pitfalls vary according to the navigation system being used.

We personally have used different systems over time, a journey of transition from conventional fluoroscopy to cone beam three-dimensional navigation, and lastly to three-dimensional iCT navigation in a process that has proven to be effective and perfectly applicable to any clinical practice. Also, we have introduced the concept of “Total Navigation,” implying the use of iCT navigation in the steps of surgery, as an attempt to eliminate the radiation exposure for the surgical staff, elimination of K-wires for instrumentation,²⁷ and elimination of pedicle probe.

The elimination of K-wires represents a major improvement in this setting. Usually, the systems that use three-dimensional navigation for percutaneous screw placement work via K-wires and require a separate navigation of multiple instruments, such as the drill guide, awl, tap, and, finally, the screw. The idea behind the creation of a navigated guide tube was to reduce the number of instruments that need to be navigated and the potential risks associated with the K-wires (i.e., they can break or bend during the procedure and pose risk of visceral or vascular injuries). This navigated tube is attached to a reference array and is used to determine the ideal pedicle trajectory; an appropriately sized pedicle screw is simulated using the navigation software, and the navigated tube is gently impacted so that the teeth would hold on to the bony anatomy. A drill, tap, and finally a pedicle screw without screw head can be inserted through this guide tube 27 (Fig. 7.1).

Fig. 7.1 (a) The navigated guide tube comprises a 170-mm-long tube with a 10-mm outer diameter and 8.3-mm cannulation and handle. A tissue protection sleeve and a trocar are made of polyetheretherketone (PEEK) material which facilitate percutaneous placement of the guide tube. An interface for attachment to an infrared reference array is positioned on a 270-degree rotatable collar on the proximal end to allow flexible positioning of the guide tube with respect to the navigation camera. (b–e) A drill, tap, and finally a pedicle screw without screw head can be inserted through this guide tube.

Total three-dimensional navigation makes MISS safer and more accurate, eliminates fluoroscopy, and enhances efficient and reproducible workflows.

7.2 Technology

The initial step in integrating CAS with any type of spinal procedure is the acquisition of multiple successive images of the region of interest, a process that can be accomplished with either fluoroscopy or computed tomography (CT).²⁸ The CAS or image guidance technology (IGT) is available in a variety of setups, which may be differentiated according to the manner in which these images are captured, processed, and presented to the surgeon.²⁸ Usually, the elements that compose a navigation system are: (1) a system for image acquisition that allows for the tracking of specialized instruments in relation to a single or multiple reference points attached to suitable anatomic landmarks and (2) a computer workstation that reconfigures this data set into a series of multiplanar images that are displayed on a monitor along with the relative position of any instrumentation within the operative field.²⁸

7.3 Image Acquisition

Usually, the elements that allow for image acquisition consist of a camera optical localizer that interfaces with the image-processing computer workstation through emission of infrared light to the operative field or an electromagnetic (EM) registration system. Passive reflective spheres placed in a handheld navigational tool serve as the connection between the surgeon and the computer workstation. These passive reflectors can be also attached to the traditional surgical instruments like the drill guide, a tap, or a pedicle screwdriver. In order to calculate accurately the position of the instruments in the surgical field and the anatomic points where the tip of the instruments is resting, the spacing and positioning of the passive reflectors on each navigational probe or customized trackable surgical instrument are programmed into the computer workstation. In fact, after the infrared light is transmitted toward the operative field and reflected back to the optical localizer by the passive reflectors, the information is conveyed to the computer workstation allowing calculation of the spatial location, through matching spinal image data (CT or fluoroscopic images) to its corresponding surgical anatomy.2^,³

7.4 Registration System

The accurate translation of spatial information into detailed renderings of spinal anatomy necessitates a stable frame of reference that enables the computer assisted system to calculate the relative positioning of instruments within the surgical field in all three dimensions. This process of establishing a relation between the “real” coordinate system, as defined by the patient’s array, and the “virtual” coordinate system of the imaging data is called registration. Different registration techniques can be applied.²^,⁵

7.4.1 Point-Matching Registration Technique

Several anatomical points are selected in CT and magnetic resonance imaging (MRI) data set and in the corresponding anatomy. These points have to be selected for each spinal level to be instrumented. Any anatomical landmark that can be identified, both preoperatively and intraoperatively, can be used as a reference point. Examples of some common points are: the tip of a spinous or transverse process or the apex of a facet joint. After selecting one of these points in the CT image data, the tip of the navigation tool is placed on the corresponding point in the surgical field, with the reflective spheres on the tool handle aimed toward the camera. Infrared light from the camera is reflected off the spheres, back toward the camera, and into the computer, which calculates the spatial position of the probe’s tip and the anatomic structure it is resting on. This effectively “links” the point selected in the image data with the point selected in the surgical field. If a minimum of three points are registered, when the probe is placed on any other point in the surgical field, the corresponding point in the image data set will be identified on the computer workstation. The disadvantage of this technique is that any error on the part of the surgeon in selecting the specific anatomic points in the surgical field will result in varying degrees of navigational inaccuracy. This paired point protocol can also be performed in conjunction with surface matching.²

Another method of registration is CT-fluoro matching. This technique is used sometimes when the navigation system is preoperative CT based. When using this technique, the preoperative CT is matched to intraoperative two-dimensional fluro images of the spine, taken from different angles of the patient.²⁹

7.4.2 Surface-Matching Registration Technique

Surface matching is a supplementary registration technique in which the surgeon randomly selects multiple anatomical points on the exposed posterior elements to provide supplementary topographic data. This technique does not preclude prior selection of the points in the image set, although several discrete points in both the image data set and in the surgical field are frequently needed to improve the accuracy of surface mapping. The positional information of these points is transferred to the workstation, and a topographic map of the selected anatomy is created and “matched” to the patient’s image set. The concomitant use of the point-to-point and surface-matching approaches was shown to result in a significantly lower mean registration error compared with that of paired point matching alone for the insertion of pedicle screws.²

7.4.3 Automated Registration

Automated registration is performed without any input from the surgeon and with less potential for registration error. It can be performed only when the image data is acquired intraoperatively. This technique involves attaching a reference frame with reflective spheres to some site in the exposed spinal anatomy or, in lumbar surgery, to the iliac crest. A second reference frame is built into the intraoperative CT imaging scanner or fluoroscope. As the intraoperative images are acquired, the two reference frames allow for registration to occur without the need for the surgeon’s input. The CT scanner or fluoroscope can then be removed, and real-time navigation of up to five separate spinal levels can be performed.²^,²⁸^,³⁰

7.5 Tracking System

The image guided navigation systems utilize either optical or EM tracking systems.²⁸

7.5.1 Optical Tracking

As previously mentioned with the optical systems, a source produces a series of pulsed beam infrared light that are passively reflected off the spheres on the surgical instruments and afterwards captured by a specialized camera or, alternatively, the camera may detect the infrared light that is actively emitted by an array of diodes that are fixed to the instruments and any number of reference points. The positional information obtained with these infrared signals is merged with the reference data that was previously acquired during the anatomic registration process, allowing specific location of the instrument to be identified in space and displayed on multiplanar images of the spine. The use of infrared wavelengths minimizes the distortion that may be caused by any surrounding metal or electrical fields present within the OR. The successful functioning of the optical system depends on a clear “line-of-sight” between the tracking device and the surgical field. The reflective array integral to all optical tracking systems increases the size and weight of these specialized instruments, which may make them more unwieldy for the surgeon to handle. It has also been suggested in the anesthesia literature that this infrared technology may interfere with pulse oximetry monitoring during anesthesia.

7.5.2 Electromagnetic Registration Systems

These systems have been developed as another method for tracking the location of instruments during surgical navigation to address the disadvantages of optical devices, namely, the need of a clear “line-of-sight” between the tracking device and the passive signal emitters (reflective spheres) in the surgical field.31 This may restrict the operator’s normal range of movement and thus limit the intuitive handling of the instruments. Moreover, the trackers needed for optical systems with active and passive reflectors are attached to the instruments and to the operation areas to be referenced and have anatomical and ergonomic disadvantages. For one thing, the instruments used are significantly larger and heavier, resulting in poorer ergonomics and handling for the operator.

In the EM systems, three orthogonal EM fields are generated by a transmitter attached to a fixed anatomic reference point, such as a spinous process. The positional data of these instruments are collected by a receiver and integrated to facilitate navigation. Since a line of sight is not required, the surgeon and nursing staff are able to work freely within the operative field. However, EM image guidance may be compromised by metal artifacts, including surgical implants, as well as by any EM fields originating from other equipment in the OR such as monopolar electrocautery, electrocardiogram monitoring, and cell phones. Given the limited area of these EM fields, the transmitter may also need to be repeatedly transferred to additional anatomic structures to obtain sufficient tracking information for multilevel procedures.²⁸^,³²

7.6 Types of Navigation Systems

The two primary options for intraoperative imaging of the spine are still radiography and fluoroscopy. C-arm fluoroscopy remains a low cost and widely available mode of intraoperative image acquisition, which allows for the rapid and serial visualization of two-dimensional images in real time.³³

Nonetheless, the past three decades of image-guided spine surgery have witnessed the development of multiple modalities for intraoperative imaging and navigation. The ultimate utility of these technologies depends on a critical appraisal of the unique advantages and disadvantages of each system (Fig. 7.2).²^,³⁴

Fig. 7.2 Types of navigational systems.

7.6.1 Preoperative CT-Based Navigation/Computer-Based Tomography

The first available mode of intraoperative navigation was this modality. This technique utilizes preoperative thin-slice scans and one of several registration processes to create a data set, which forms the basis for intraoperative navigation. In fact, prior to the surgery, a two-dimensional thin-cut CT through the region of interest is obtained and uploaded to a workstation where it creates a virtual three-dimensional reconstruction that can be used for planning the surgery, simulating the implants, etc. On this preoperative reconstruction, anatomical landmarks are selected for intraoperative registration.29^,³⁴

However, preoperative CT scans are acquired with the patient in a supine position, while during surgery patients are usually in the prone position. The resulting vertebral shift and realignment creates a risk for navigation errors. Therefore, to account for shifting anatomy during surgery, each level must be registered separately to accurately plan and perform the surgery.³³^,³⁵ One significant disadvantage of CT-based guidance systems is the need for surgeon-dependent registration of anatomic landmarks on preoperative CT images and the corresponding anatomy of the patient intraoperatively. In addition, extensive bony exposure is required for adequate registration and it may be difficult to identify landmarks for registration in patients with prior laminectomies.³³^,³⁴

During navigation, the surgeon is presented reformatted CT images, on virtual three-dimensional multiplanar image reconstructions, with the selected screw entry point and trajectory superimposed on the images. This information is updated in real time as adjustments are made to the selected trajectory in the surgical field.³⁶

One of the disadvantages of this technology was increased radiation exposure to the patient preoperatively.³³

7.6.2 Intraoperative Image-Based Navigation

Intraoperative image-based navigation systems eliminate the need for surgeon-dependent registration step because the system is automatically registered through the acquisition of images intraoperatively. Thus, the need for spinal exposure for point matching is obviated. Furthermore, in this setting, as images are obtained after patient positioning, they are an accurate representation of vertebral anatomy at the time of surgery.³⁰

Two-Dimensional Navigation—Fluoroscopy Based

“Virtual fluoroscopy” or two-dimensional fluoroscopy-based navigation is a strategy that combines a standard two-dimensional C-arm with a computer navigation system. It uses standard anteroposterior (AP) and lateral images of the spine acquired immediately before the start of the procedure.³⁶ Registration is performed automatically with a reference frame attached to the C-arm.³⁴ A series of fluoroscopy images in AP, lateral, and sometimes the pedicle oblique views are acquired with a reference frame attached to a stable anatomical landmark, often a spinous process in the vicinity of the vertebrae that will be operated. These images are transferred to the navigation workstation, and this data set is used to navigate implants on the virtual anatomy viewed on the screen (Fig. 7.3). An infrared camera aimed at the reference arc and navigation tools allows continuous recognition of the navigation tools in relation to the relevant anatomy. A continuous “line of sight” must be kept between the infrared camera, the reference arc, and the navigation tools. The accuracy of the system will be maintained as long as the stability of the reference arc is maintained, motion segments do not change their position compared to acquired images, and the navigation tools are kept in line with the desired trajectory. Therefore, fluoro-based navigation allows a completely automatic registration of the spine, correction of image distortion, and reduction of radiation exposure to the staff; however, it is limited to two-dimensional projection images. As it does not provide three-dimensional visualization of the spinal anatomy during navigation, the risk of navigation errors is increased and abnormal axial anatomy is more likely to remain unrecognized. Errors may also be greater in cases of: poor bone quality, excess intra-abdominal gas, morbid obesity, spinal deformity, prior surgery, and congenital anomalies. Furthermore, image resolution is typically best in the center of the field and any structures around the periphery may appear distorted secondary to parallax, so to maintain the accuracy of navigation across several spinal segments, the process of data acquisition and anatomic registration may need to be repeated several times.³⁰^,³³^,³⁴

Fig. 7.3 Workstation screen demonstrating a fluoroscopic navigational system. (a) Standard anteroposterior (AP) and lateral views are provided with superimposed simulation to determine the ideal pedicle entry point, trajectory, and the appropriate pedicle screw size. (b) Simulation to determine the ideal medial/lateral trajectory—to avoid medial pedicle violation, initially a pedicle screw is simulated that is no longer than the pedicle. If this short, simulated screw does not breach the medial border of the pedicle on the AP view and seems to be in perfect medial/lateral position, simulation is lengthened until the desired screw length is achieved. (Used with permission from Njoku et al.³⁷)

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