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Image Guidance in Cervical Spine Surgery

One of the most important refinements of the past 20 years in cervical spine surgery has been the development and advancement of real-time imaging technology referred to as frameless intra-operative spinal stereotaxy. This technology, also called intra- operative (neuro)navigation, is discussed in detail along with modern- day use of the StealthStation® Surgical Navigation System coupled with the O-arm® Surgical Imaging System applied to cervical spine surgery and instrumentation.

Surgery of the cervical spine has seen dramatic advances over the past two decades with regards to the marriage of technology and intra- operative imaging. One of the greatest advances involves the ongoing development of intra-operative image guidance. In the past, fluoroscopy (a form of portable continuous real time x-ray) has been used primarily to provide lateral (side-to-side) and AP (anterior- posterior) images of the patient while under general anesthesia to assist in intra-operative anatomic localization. However, this method has its limitations:

  1. it provides only for 2-dimensional imaging (with its inherent error of localization as it is unable to produce cross- sectional or axial images);
  2. it can adequately image only bony (radiodense) anatomy; and
  3. it exposes all in the room to radiation.

New techniques such as frameless intra-operative spinal stereotaxy (FISS) provide for improved 3-dimensional accuracy with minimal radiation exposure as well as providing imaging of both bony and soft- tissue structures. Stereotaxy refers to pinpoint 3-D localization in space using any of a variety of methods based on geometric calculations; it allows us to determine the exact location of any point imaged inside the body and has been applied to biopsies, tumor resections, coordinated direction of therapeutic radiation beams (referred to as “stereotactic radiosurgery” -- see the article entitled “Stereotactic Radiosurgery for the Treatment of Metastases to the cervical Spine” elsewhere on Wascherspineinstitute.com), etc.

We prefer to use the StealthStation® Surgical Navigation System coupled with the O-arm® Surgical Imaging System for cervical surgeries that would benefit from FISS. It is a high-tech surgical navigation system that allows the surgeon to precisely track the location of his or her surgical instruments in relationship to the patient’s actual anatomy throughout the operation. In essence it acts like a mini-GPS system for inside the body. It consists of a computer, a display monitor, software, a tracking tower containing an optical camera and optical emitters (light-emitting diodes or LED’s); and probes and instruments that interface with the system. The O-arm® is a portable fluoroscopic x-ray system designed for use in the operating theater. It consists of a doughnut shaped x-ray emitter-receiver that is positioned around the patient centered over the region of interest; fluoroscopic images using a low dose of radiation are taken from multiple positions as the x-ray tube rotates around the patient within the doughnut. The images are then fused by the computer to create reconstructed axial, coronal, and sagittal images as well as 3D images just like a high-end CT scan. The process of obtaining images is referred to as a “spin.”

General workflow is as follows: prior to surgery, the tracking tower with the optical camera and LED’s is positioned where the receivers have an unobstructed view of the operative field in an area

that does not threaten the sterile field. After the patient is under general anesthesia and the cervical anatomy is exposed, a sterile localizing reference clamp containing light-reflecting balls is applied to an exposed immovable portion of the patient’s anatomy close to or within the surgical field (such as a spinous process during a posterior cervical approach). An image is then “spun.” Images on the monitor can then be manipulated to obtain measurements, angles, diameters, projection of the depth of penetration of a screw tip, etc. Instruments such as a localizing probe or a drill guide (which also contain reflective balls) are registered by the surgeon. Light produced by the LED’s reflects from the multiple reflective balls; this reflected light is recognized by the optical camera and the computer immediately calculates the position of each of the balls on the instrument in relation to the reflecting balls on the localizing clamp, allowing the position of the instrument to be displayed in real time fashion on the computer screen. This allows the surgeon to visualize the location, angle, trajectory, depth, etc. of the instrument or stabilizing screw during any or all manipulations, theoretically increasing the margin of safety around critical structures like the spinal cord, nerve roots, vertebral artery, etc. It also theoretically increases the probability of optimal screw placement, allows for a more complete resection of a tumor, permits the evaluation of the extent of recreation of normal alignment during deformity surgery for kyphosis or subluxation, etc. Often, a second spin is performed after completion of the surgery to insure the final surgical result is as intended.

Advantages of this FISS system include:

  1. No need to utilize pre-operative images to navigate.
  2. Ease of use, including planning screw trajectories, measuring distances, determining the appropriate screw size, etc. in real- time.
  3. Ability to image the pertinent anatomy at any time during the procedure by obtaining a “spin,” which takes about 5 minutes.
  4. Ability to precisely track movements of instruments in real- time intra-operatively and relay those movements on the computer screen relative to the patient’s anatomy.
  5. By loading in pre-operative studies, the system allows for the ability to plan for the unique nuances of a surgery before induction of anesthesia, if required.
  6. It provides the ability to check the final result after tumor resection, placement of grafts, screw insertion, etc. to insure complete decompression and acceptable placement of hardware.
  7. Low dose radiation exposure occurs only twice during most procedures; once to obtain a registration spin and another to check the end result. The overall effect is significantly lower radiation exposure to the patient, the surgeon, and the staff.

Movement of the localizing reference clamp inadvertently during the surgery destroys the registration, requiring the entire process to be repeated to insure appropriate accuracy. Likewise, pressure from adjacent soft tissue can deform or deflect the instruments, throwing off the accuracy as well. In addition, the camera must have full view of the multiple LED’s for the system to function appropriately. The systems are costly (over $1M) and require training and technical expertise to use appropriately. Nevertheless, in spite of these limitations, the StealthStation® Surgical Navigation System coupled with the O-arm® Surgical Imaging System has consistently shown to exhibit an accuracy in the range of 1.5-2 mm for screw insertion. We have found the systems in conjunction to be sufficiently accurate, cost-saving, and time-efficient. Multiple recent studies have shown increasing use of these and similar systems with high degree of accuracy, no significant increase in operative time, and no increase in adverse events like infection. Future studies evaluating fusion of pre- operative MRI with intra-operative CT-based neuronavigational systems will show tremendous potential for accurate visualization of soft tissue and osseous boundaries to enhance spinal tumor resection. We utilize these systems in concert for virtually all posterior cervical screw insertion surgeries, especially for difficult cases such those involving C1, C2, or the upper thoracic segments. This has allowed us to successfully insert over 4810 posterior cervical (lateral mass and pedicle) screws without a single return to the operating room for a misplaced screw, as of 1/1/20.

If you or a loved one has been recommended for cervical spine surgery and could possibly benefit from frameless intra-operative spinal stereotaxy, please call us today at 1-855-854-9274 for a free MRI review. Second opinions are always available.