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The role of image guidance to enhance the safety and efficacy of complex spine surgery is well documented.1–8 This technology can improve the accuracy and efficiency of the techniques used for decompression of the neural elements and placement of instrumentation, therefore ensuring effective stabilization while protecting the neighboring neurovascular elements. In addition, this method dramatically reduces or eliminates the surgeon’s exposure to radiation.⁹ Despite the aforementioned advantages, this technology has been, unfortunately, poorly and slowly adopted by all spine surgeons.

The Evolution of Image-Guided Spine Surgery

Image-guided spine surgery has evolved to its current form through a step-by-step improvement in three-dimensional (3-D) imaging technology, which can be applied in the operating room environment to localize spinal bony elements accurately and reliably. Initially, in the 1990s, the use of two-dimensional (2-D) spine surgery imaging to guide surgery was onerous and mastered by few surgeons. The process had several limitations: first, a preoperative computed tomographic (CT) scan was required and then imported to a computer workstation in the operating room. After a reference array was affixed to the patient, the surgeon chose a series of anatomical landmarks on the workstation and co-registered the corresponding landmarks on the patient to the ones preselected on the workstation.1 Unlike the relative ease of touching preplaced fiducials on the head for cranial neuronavigation, arriving at the preselected point in the spine was difficult. If an acceptable error range was not met, then a surface merge was employed. This process entailed co-registering 50 to 100 points on the spine. Finally, the surgeon was ready to navigate the one level attached to the reference array; navigating away from the frame decreased the accuracy. Re-registering multiple levels was usually a time-prohibitive exercise. Whereas navigating “open” posterior procedures was cumbersome, navigating minimal access or anterior procedures was impossible. Once the hardware was placed, conventional means, including postoperative imaging, were employed to verify proper hardware placement.

Although most neurosurgeons easily adopted the technology for their cranial cases, few employed it for the spine. The barriers to success for image guidance in spine surgery can be grouped into the following:

1. A need for additional preoperative imaging

2. A relatively extended intraoperative time commitment requiring

a. Point to point registration and providing

b. A limited range of navigation

3. Availability of only 2-D versus 3-D images

4. Reliance on old technology for verification of placement of instrumentation, including

a. Conventional radiographs and fluoroscopy

b. Pedicle screw stimulation

c. Postoperative imaging requiring return of the patient to the operating room if hardware position is not satisfactory

5. Cost constraints

The next foray was virtual fluoroscopy.¹⁰ In this circumstance, a standard C-arm was fitted with a calibration target, and multiple 2-D images were obtained and transferred to a computer workstation intraoperatively. There was no need to register anatomical points or employ surface merging because the images were only in 2-D. Although the system allowed multiple images to be displayed, real-time biplanar views were available with out a need for a C-arm. Nevertheless, the accuracy and effectiveness of this technology were questioned in some studies.¹¹ Given that the patient was imaged in the operative position using this imaging technique, navigating around multiple segments may have been associated with less error. Virtual fluoroscopy overcame two of the barriers to the adoption of image guidance in spine surgery: the need for preoperative imaging, and extended intraoperative time commitment. However, this technique also suffered from shortcomings: providing only 2-D images and requiring postoperative imaging for confirmation of instrumentation position.¹⁰ Some surgeons believed this technique did not add enough value to warrant its routine use in everyday practice. Although this method eliminated radiation exposure to the surgeon, it eventually failed to gain widespread popularity.

In 2002, Siemens introduced the Iso-C 3-D image guidance system.¹² This technology was revolutionary and served as a foundation that led to the development of today’s image guidance systems. Iso-C is a motorized C-arm that acquires and reformats multiple 2-D images into a 3-D dataset and provides multiplanar images intraoperatively.13 This development obviated the need for a preoperative scan and the imaging of the patient in a position different from the operative position.¹⁴ Most importantly, intraoperative 3-D imaging became a reality.

For the first time, intraoperative 3-D confirmation of instrumentation position was available before leaving the operating room.¹⁴ Unfortunately, some shortcomings remained evident: the image quality was not always acceptable, especially for obese patients,^13,14 and for images at the cervicothoracic junction, the initial scans had a limited field of view (typically three lumbar vertebrae), and scanning took ~2 minutes.

Moreover, the presence of prior instrumentation significantly degraded the image quality. Although this was a quantum leap in advancing image guidance for spine surgery, it also faced a limited acceptance. The advocates of the old-style 3-D systems touted its better image quality, whereas others failed to see a clear benefit for immediate feedback and the opportunity for intraoperative 3-D confirmation of instrumentation position.

In 2006, Breakaway Imaging (Littleton, MA), funded in part by spine surgeons, released the O-arm. This modality has the same functionality as Iso-C but provides a vastly improved image quality and field of view. The O-arm can image up to four or five lumbar segments, six thoracic segments, or the entire cervical spine, and it provides good-quality images despite the presence of prior instrumentation and obesity. The O-arm is able to adequately image the previously considered “difficult to image areas” such as the cervicothoracic junction. In fact, scans may be done with the retractors in place. Additionally, a scan takes less than 25 seconds to complete. This device lifted most of the barriers to the widespread adoption of image guidance during spine surgery. One limitation of this modality is its inability to guide K-wires; the presence of a K-wire demands real-time imaging with fluoroscopy. The only other remaining barrier has been the cost of the device. In the current health care economy, it may be impossible for all spine surgeons, especially in small medical centers, to have access to cutting-edge navigation technology. The O-arm itself costs in excess of $600,000. A more cost-effective option that can provide navigation to a larger subset of surgeons is greatly needed.

As the advantages of more minimal-access procedures are recognized and used by surgeons, the normal anatomical landmarks become less visible intraoperatively, and the importance of image guidance is more evident. Intraoperative image guidance can increase the accuracy and efficiency of instrumentation placement and ensure adequacy of neural decompression or diskectomy. Furthermore, this method can significantly decrease the extent of radiation exposure to the surgeon. These superior qualities allow for the introduction of further innovative applications for image guidance in the future.

Application of Image Guidance to Minimally Invasive Spine Surgery

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Promising Advances in Minimally Invasive Spine Surgery Minimally Invasive Lumbar Diskectomy Minimally Invasive Transforaminal Lumbar Interbody Fusion Minimally Invasive Lumbar Interbody Fusion: Choosing Between Approaches Minimally Invasive Lumbar Laminectomy for Stenosis Alternative Approaches for Lumbar Fusion: Axial Lumbar Interbody Fusion (AxiaLIF)

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