Image-Guided Lumbar Instrumentation

NARESH P. PATEL, JAE Y. LIM, KEE D. KIM, AND J. PATRICK JOHNSON

Pedicle screw fixation is a widely used method to achieve rigid fixation of the lumbar and sacral spine. It is effective to augment fusion for the treatment of fractures, spondylolisthesis, scoliosis, tumors, and appropriately selected cases of degenerative disk disease with segmental instability.¹^–⁵ Although many surgeons have become adept with placement of lumbar and sacral pedicle screws, considerable knowledge and technical skill are required for accurate screw placement. Screw placement errors may result in either failed fusion or neurovascular injury. The current literature reports pedicle screw placement error rates as high as 30%.⁶^–^8a Recent advances in stereotactic image-guided surgical techniques may provide spinal surgeons the ability to decrease pedicle screw error rates and maximize safety.⁹^–¹³ This chapter reviews the traditional methods and imaging of pedicle screw placement, then focuses on the current state-of-the-art techniques and applications of image-guided lumbar and sacral pedicle screw placement.

Traditional Localization Methods

Accurate pedicle screw placement relies on the surgeon’s experience and three-dimensional (3-D) conceptualization and understanding of spinal anatomy. Initially spinal surgeons used plain anteroposterior and lateral view standard intraoperative radiographs. However, plain radiographs required significant additional time in obtaining and processing x-ray films, which made plain radiographs an impractical and unpopular method for intraoperative imaging. Also each plain radiograph is acquired independently and cannot be immediately updated as with fluoroscopy. Subsequently, intraoperative C-arm fluoroscopy has been used to assist in accurate pedicle screw placement and still remains a primary method of intraoperative imaging where image guidance is not available. Fluoroscopy can be used to obtain multiple images in rapid sequence, and it allows precise positioning for imaging oblique or other unusual views, particularly if there is any anatomic spinal deformity. This flexibility of fluoroscopy allows intraoperative real-time imaging for accurate “fine-tuning” of pedicle screw trajectories. The disadvantages of fluoroscopy include a potentially higher than acceptable radiation exposure as well as the cumbersome size of the C-arm, which hinders the surgeon’s access to the operative field. Regardless of the type of intraoperative radiographic imaging used, successful pedicle screw placement depends upon high-quality imaging that demonstrates pedicle anatomy with each vertebra oriented and aligned anatomically to ensure ideal screw placement.

Despite appropriate intraoperative radiographic techniques or modern image guidance, accurate screw placement cannot be guaranteed, nor is it always feasible. Creating a pilot hole through the pedicle by manual probing requires locating a proper entry point and trajectory with a medial-directed angulation that the surgeon estimates from the preoperative computed tomography (CT) or magnetic resonance imaging (MRI). Screw diameter and length can also be estimated by measuring the preoperative axial CT or MRI. Pedicle screw placement can be extremely difficult when using conventional radiographic imaging technology in patients with altered anatomy due to previous surgery, severe degenerative changes, or deformity. The limitations of plain radiographic and fluoroscopic guidance provided some of the stimuli and indications for computerized image-guided spine surgery to potentially maximize the accuracy of lumbar and sacral screw placement.⁹^–¹¹ Current image-guided systems are based either on optical imaging [i.e., light emitting diodes (LEDs) or light reflectors], magnetic fields, ultrasound, virtual fluoroscopy, or an articulating arm. This chapter focuses on the optical imaging and virtual fluoroscopy technologies that have applications to the other guidance systems.

CT-Guided Frameless Stereotaxy: Anatomy and Preoperative Planning

Image-guided spinal applications were adapted from frameless cranial stereotactic technology that was well established. The lumbar and sacral spinal column is well suited for stereotactic surgical applications because the individual bony vertebral segments are large in size and have distinct and identifiable anatomic prominences on the dorsal surface. Registration requires open surgical exposure of the selected vertebral segment because closed registration with skin fiducials is neither feasible nor accurate for spinal surgery due to mobility of the skin.¹⁴ The spinous processes, facets, and transverse processes are the most frequently used anatomic fiducials that can be identified at surgery and on the 3-D surface rendering images on the workstation, but any other distinct bony prominences are useful. Intersegmental motion between two vertebral segments remains a potential problem and may require registration of each segment intraoperatively to avoid errors. Because preoperative CT scans are obtained in a supine position, the relative position of each vertebra may be significantly different when with the patient is in the prone position for surgery.

A preoperative CT scan of the surgical region must be obtained with a specific protocol that is similar for most image-guided systems using 1 mm contiguous slices over a 10 to 14 cm field of view. Three-millimeter CT slices can also produce acceptable-quality detail for accurate image-guided procedures in the lumbar and sacral spine, but 5 mm scan slices may produce poor-quality imaging for surgery. The scan protocol for the CT technicians can be obtained by the image-guided vendor to assure correct details of the protocol. The CT scan data of the patient are then transferred to the image-guided computer workstation where they are reformatted into axial, coronal, sagittal, and 3-D views.

Optical Tracking Image-Guided System Components

Several different optical tracking image-guided surgical systems are now commercially available. Although computer hardware and software profiles may differ somewhat between manufacturers, each system has similar basic components and clinical applications. The main hardware components include a computer workstation, a digitizer, and a camera (Fig. 23–1A). The peripheral components are a dynamic reference frame (DRF) (Fig. 23–1B), a standard pointer probe (Fig. 23–1C), and various instrument arrays that provide universal instrument registration (UIR) (Fig. 23–2). The computer workstation consists of a monitor, optical disc drive or CD-ROM or digital audiotape (DAT) drive, as well as an internal hard drive. The CT scan data is received by an optical disc, CD-ROM, or DAT drive or through a hospital network (ethernet) data transfer system. The computer workstation operating system runs the software needed to produce 3-D reconstructions from the scan data and perform surgical tracking with the images. In addition, the computer will run the software that takes the surgeon through the steps of registration, planning, and navigation (Fig. 23–3).

The optical tracking camera system detects LEDs (or passive, reflective ball-markers) on the instruments in the field and digitally detects their positions in space. The spatial location of the instruments (as defined by Cartesian coordinates) is determined by the computer to provide real-time navigation. The use of several distinctly different LED/passive marker configurations now allows simultaneous tracking of multiple instruments during the image-guided procedure.

The DRF is attached to the spine segment of interest (or the adjacent segment being operated) and has a unique pattern of LED/markers that the computer recognizes (see Fig. 23–1B). The DRF is securely attached to the spinous process with a clamp or a modified screw that is attached to the clamp, which allows the digitizer to track any patient movement and immediately update the computer-generated scan image on the monitor screen. During the registration and image-guided surgical procedure, it is essential that the DRF remain undisturbed and within view of the optical tracking camera.

Only gold members can continue reading. Log In or Register to continue