13 Computer Navigational Guidance in Spine Surgery Abstract Computer navigation allows the surgeon to use minimally invasive techniques by pinpointing the position of the pathology and the surgeon-guided instrumentation. This chapter reviews the various applications of computer navigational guidance in spinal surgery including preoperative and intraoperative image acquisition, computer-aided navigation, and various operative procedures. Keywords: three-dimensional imaging, computer navigation, navigational space, radiographic imaging, fluoroscopic imaging, CT imaging, dynamic reference array, implantable device Computer navigational guidance has been used for multiple surgical procedures in neurological surgery, predominantly for cranial surgery. Computer navigation allows the surgeon to use minimally invasive surgical techniques by clearly pinpointing the position of the surgical pathology to be addressed and by guiding the surgeon to the precise location where the surgical procedure is to take place. In cranial procedures, the technique is limited by the relative instability of the cerebral structures that may render the precision less than desired. In spine surgery, the rigidity of the bony structures and the greater stability of the spine allow the surgeon to have greater predictability in the position of the vertebra in a three-dimensional environment. This chapter will review the technique and applications of computer navigational guidance in spine surgery: the benefits and limitations of the technique as well as the potential application to multiple spine procedures. The success of any surgical procedure rests on the ability of the surgeon to know and understand surgical anatomy, and apply it to the purpose intended by the intervention. Surgical technique requires the execution of the operative procedure in a safe and expedient manner with adequate exposure and visualization of the area, minimal damage to the surrounding structures, and limited or minimal blood loss. Open surgical procedures allow the surgeon to expose and visualize the tissues and structures to be operated on, but require large exposures, damage to collateral structures, and occasionally significant blood loss. Computer navigation in surgical procedures is premised on the anatomical understanding of the structures operated on, translated into the computer software, and used to guide the various instruments into safe positions where they will allow the surgeon to complete the procedure intended. Computer navigation allows the surgeon to access the various structures through small or limited approaches, limiting visualization of the surrounding tissues, and compromising the transmission of light into the area. Specialized instrumentation is required to have sensors that allow the computer system to capture the position of the instruments in a three-dimensional space, and integrate that position with the computerized anatomical rendering of the area being operated on. Various magnification and lighting systems are also required to allow the surgeon to visualize the surgical field adequately. Computer navigation was introduced in cranial surgery to allow access to difficult areas of the brain or the base of the skull, through limited access exposures. The surgeon prepares the operative approach using the computerized trajectory determined by the system, and follows the path through small and frequently narrow spaces to reach the target where the pathology is located. The trajectory developed on the computerized map facilitates the planning and execution of a surgical corridor with limited exposure and minimal damage to surrounding structures. Deep-seated brain tumors, arteriovenous malformations, intraventricular lesions, or placement of ablative lesions or stimulating electrodes can all be detected using three-dimensional cranial navigational techniques. The success of cranial navigation is based on the small space contained within the cranial cavity, the relative proximity of all the intracranial structures, the rigidity of the skull, and the ability to affix the skull to a rigid supporting frame. Reference points of localization for navigational purposes require a fixed reference frame or fixed anatomical structures that can be targeted and remembered by the computer software. Limitations of the cranial navigational techniques are that the intracranial structures are soft, not fixed, and frequently change position when the intracranial dynamic balance is altered by the craniotomy procedure itself. Removal of cerebral tissue or cerebrospinal fluid will result in shifts of the intracranial structures, and will create errors in the fine identification of intracranial lesions. Spinal navigation requires reference points as well which are frequently fixed to the spinous process of one of the vertebrae in the area being operated on, the skull, or the pelvis, and which can be correlated to fixed anatomical bony structures for confirmation or realignment. The mobility of the individual spinal segments and the changes that take place following distraction of the intravertebral spaces, the placement of instrumentation, or the resection of vertebral elements result in distortions of the original navigational images, and will require skill and understanding to correct for them during the operative procedure. Spine surgery for degenerative disease, spine reconstruction, and spine trauma require intensive radiographic monitoring and imaging. Initial approaches to surgical reconstruction were all done with multiple simple radiographs in the anteroposterior or lateral position, requiring considerable delays and radiation to the patient and the surgical team. Progress in radiographic imaging allowed for the introduction of fluoroscopic imaging that allowed the surgeon to see in real time the manipulation of the operative field and the placement of instrumentation. Spine fluoroscopy allows for the immediate visualization of the area in question on a fluoroscopic screen, but requires the surgeon and the team to wear protective gear, including aprons, thyroid shields, gloves, and goggles, making the procedures considerably uncomfortable. The radiation dose required in complex spinal reconstruction can increase rapidly to significant levels. Surgeons and assistants alike can be subject to significant radiation doses in cumulative bases, resulting from repeated use over time. Computer navigational guidance provides the opportunity for the surgeon and the team to acquire all the images needed at the beginning of the surgical procedure, and to navigate the instruments in a virtual computer environment with the same precision obtained with real-time fluoroscopy. The navigational space can be defined as the surgical area that will be recognized by the computer based on image acquisition and image reconstruction in a three-dimensional environment. Image acquisition then becomes the essential element to allow the surgeon to establish the areas where the surgical procedure will occur. Images can be obtained with fluoroscopic systems that incorporate a reference array directly on fluoroscopic unit and correlate the position of the reference frame located on the spine with the radiographic images reconstructed by the computer in a three-dimensional space.1 Similar images can be obtained directly with a surgical CT scanner that allows the recognition of fixed anatomical bony landmarks in relation to a rigid frame affixed to the spine during the acquisition time of the CT scan. Images acquired through either system are reconstructed by the computer software to create a three-dimensional environment in axial, coronal, and sagittal planes, as well as a true three-dimensional model, through which the surgeon will be able to navigate the different instruments during the execution of the surgical procedure. A rigid frame is currently required as the primary source of reference for the computer to merge the anatomical images acquired by the radiographic technique with the actual instrumentation that will be used to complete the surgical procedure.2 The reference frame is equipped with reflective spheres located in four or five different positions (passive arrays) within the reference frame ( Fig. 13.1 Minimally Invasive Neurosurgical Society (MINS) (Royal Oak, MI) cadaveric training lab images showing surgeons training on (a,b) O-arm image guidance navigation system for placement of (c) percutaneous pedicle screws. Newer software computer programs and devices allow the surgeon to obtain three-dimensional reconstruction of rigid structures using surface overlays that adapt to the surgical field, and are fixed in place by attaching them directly to the patient’s skin. Other systems use normal anatomical bony landmarks as the reference points for the computer, since they are permanently fixed to the patient, are not going to move in location unless the surgeon removes them, and can be reused if the system loses its tracking arrangements. Small implantable electronic radiofrequency-emitting devices have been used in pediatric surgery to serve as reference points, since the bony anatomy in children may be too weak to sustain a large dynamic reference array that may damage the normal pediatric bone. These radiofrequency-emitting devices provide an orthogonal electromagnetic field in the surgical navigational space, allowing the surgeon to track the instruments as they move through the field. Their size and dimensions limit their use at the moment to small operative fields such as the pediatric skull. Computerized navigation allows the surgeon to visualize the surgical space in a real-time environment without the need to use additional radiation. The images give a three-dimensional representation of the surgical space in which the various surgical instruments move. At any point during the surgery when an instrument equipped with a passive reflective array is used, the surgeon can view the precise localization of the instrument in the axial, coronal, and sagittal direction, and anticipate the trajectory the instrument will follow. The navigational instrument equipped with a passive reflective array can be used to identify anatomical points on the patient, precisely place points of access for penetrating instruments, plot the trajectory of implantable devices such as pedicle screws and cages, and determine the ultimate resting position of any device the surgeon wants to leave in place. The safety and convenience derived from the knowledge of the precise location of the instrument at any time during the surgery when navigational guidance is used give the surgeon predictability in the position of the surgical procedure, the extent of resections, and the placement of implantable devices, all through limited access, minimally invasive approaches.4 Radiographic images for spine surgery consist of the review of plain film radiographs, scoliosis full-length radiographs, and MRI and CT scans of the areas in question. Not all imaging modalities are used for every patient, and the surgeon will select some or all of the modalities in the evaluation. Occasionally, radioisotope studies of bones will be required to document the presence of neoplastic or infectious lesions. The correlation of the preoperative studies with intraoperative imaging allows the accurate localization of the pathology and the correction of the deformities in question. Unlike computer navigation in cranial surgery, spine surgery is predominantly done with CT images acquired either preoperatively or most commonly intraoperatively. Image acquisition for spine surgery with computer-guided navigation requires the placement of a dynamic reference array on a fixed point in the spine, skull, or pelvis. Most commonly, the spinous process of one of the immediately adjacent vertebrae is selected because of its proximity to the operative field and the ease of exposure. When the spinous process is selected, it is necessary to insure that the direction and position of the dynamic array is out of the field of view of the electro-optical camera to prevent interference in the recognition of the passive reflective arrays located in the navigational instruments. The spinous process fixation device is a simple clamp that must sit fully on its entire length on the long axis of the spinous process, and must be rigidly fixed to prevent its movement during the surgical procedure that may disrupt the accuracy of the navigational system. The second most common site is to place a rigid bar in the posterior superior iliac spine or high on the iliac crest, making sure that when the bar is placed in the iliac crest region the placement is bicortical to prevent displacement during surgery. The dynamic array is mounted on either device with a ratcheted device that allows for orientation of the dynamic array in relation to the position of the patient, the imaging, and navigational system used. For cervical procedures, the dynamic reference frame may be placed on the C7 spinous process or on the skull. The selection of the site is surgeon dependent. Having placed the dynamic array in a fixed and stable manner, the images are then acquired for the region being operated on. Fluoroscopic devices that replicate the three-dimensional reconstruction abilities of a true CT scan, such as the O-Arm or the Brainlab systems, provide imaging of an area of the spine generally limited to four or five immediately adjacent spinal levels. If surgery is required at more than four levels, a second run is required to complete the imaging acquisition before surgery. An intraoperative motorized three-dimensional C-arm acquires 190 two-dimensional projection images during a single rotation. A multiplanar reconstruction algorithm creates autoregistered high-resolution three-dimensional reconstructions from the set of projection images. The resultant volume can be reformatted, using multiplanar reformatting (MPR) computer programs to display CT-like intraoperative images. True intraoperative CT scanners are equipped with the ability to travel along the length of the entire spine and can get an entire spine reconstruction on a single run, decreasing the radiation dose to the patient and the time required to complete the study. The spine three-dimensional reconstruction obtained by either methodology provides an immediate real-time view of the position of the instruments in the operative field in the coronal, axial, and sagittal planes, allowing the surgeon to have great precision on the localization of the operative site and guiding the placement of the instruments. Furthermore, the navigational plans provide measuring tools that allow for the exact dimensions required for the specific anatomical location in all three dimensions, providing the width, height, and length specific to the anatomical location and the instrument to be used ( Preoperative preparation in spine surgery is fundamental to the successful completion of any surgical procedure. A thorough understanding of the patient’s clinical presentation and physical findings, coupled with a complete assessment of appropriate imaging and diagnostic studies, provides the surgeon with the ideal environment to perform a navigated surgical procedure. Computer navigation in spine surgery provides the surgeon with greater accuracy and safety when performing any spine reconstruction procedure. The surgeon is still required to be totally proficient in the understanding of the following: • Surgical anatomy of the operative area. • Pathology to encounter. • Surgical technique to be used. • Equipment needed. • Potential complications to face and how to solve those complications when they present themselves. Computer navigation facilitates the surgeon’s ability to visualize with greater precision and accuracy areas of the spine that are not readily visualized in the operative field, but is not a substitute for an appropriate selection of the patient or a complete understanding of the anatomy and the techniques to be used, or how to solve potential complications that arise during the operative procedure.
13.1 Introduction
13.2 Navigational Concepts
13.2.1 Navigational Space
13.2.2 Navigational Reference Points
Fig. 13.1a). The passive arrays will reflect the laser light emitted by the light-emitting diodes (LEDs; active arrays) located in the electro-optical camera tracker, and will allow the computer to establish the position of the fixation point of the reference frame, or dynamic reference array, in the specific anatomical location in the spine. The anatomical registration of the spine structures is completed in that process when the input from the imaging system is then correlated with the dynamic frame reference input registered by the optical camera tracker and integrated by the computer software merging program (
Fig. 13.1b,c). This process will then allow the system to recognize the position of the surgical instruments in the surgical navigational space. Each surgical instrument is equipped with a passive array frame that reflects the light from the LED camera, allowing the system tracker to register each instrument with its specific physical characteristics, which have been preprogrammed in the computer software, and then permitting the precise recognition of each instrument in the surgical space. The ability of the software to then reconstruct the anatomical structures being highlighted by the tip of the instrument in the surgeon’s hands allows the surgeon to view the instrument in a virtual three-dimensional space on the computer screen.3
13.3 Image Acquisition
Fig. 13.2).
13.4 Computer-Navigated Surgical Procedure
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