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Spine surgeons seek improved accuracy and safety with navigated procedures
Surgeons can place arrays up to two disc spaces from surgical site and still maintain accuracy.
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Surgical navigation is gaining popularity in knee surgery, and now spine surgeons are exploring the technology to help them perform meticulous procedures.
“I think there are many different reasons to learn this type of technology,” said D. Greg Anderson, MD, an orthopedic spine surgeon with the Rothman Institute in Philadelphia. “One of the best is the increased accuracy and safety that you get from it. It also allows you to do less invasive procedures, which have become popular in recent years [and] it’s very helpful when you’re doing highly technical procedures.
“There are some procedures in spine surgery where the margin of error is very small,” he said.
Anderson cited a study by Holly and Foley that demonstrated 100% accuracy of 30 pedicle screws placed into cadaver spines using image guidance.
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“In the thoracic spine, accuracy wasn’t quite as high, but the breeches that they found were minimal breeches, probably clinically insignificant,” Anderson said.
Less radiation
In another study comparing fluoroscopy alone to navigated fluoroscopy, Sagi and colleagues found that electromagnetic field-based image guidance was at least as accurate as fluoroscopy with less radiation exposure.
In addition to potentially increasing accuracy, surgical navigation can produce concurrent intraoperative views of surgical instruments and limits surgeons’ exposure to radiation.
“I think that’s becoming an increasing concern as we’re using more and more fluoro for our less-invasive procedures,” Anderson said.
Although surgeons typically use navigation to guide implant placement, he said the technology is beneficial during decompression surgeries and revisions, where the original anatomic landmarks are absent.
Surgical navigation can also help spine surgeons obtain the desired correction for osteotomies by providing detailed preoperative planning.
Types of systems
According to Anderson, all navigation systems consist of the following three components: a computer station and monitor, tracking system and reference arrays.
The reference arrays are fixed to the patient’s anatomy and surgical tools. The tracking system details the position of these objects in space and relays the data to the computer station. Finally, the monitor displays the information to the surgeon. Navigated systems can be CT or fluoroscopic-based. CT-based systems require the surgeon to match preoperative scans with the actual location of the spine and input this data into the computer.
During fluoro-based navigation, surgeons fix a reference array to the fluoroscope and use the fluoroscopic images to guide the procedure.
Axial images obtained from CT-based systems give surgeons a clear view of the canal — a location not accurately depicted in fluoro-based systems.
“The nice thing with the fluoro systems, however, is that they’re very quick and simple to use,” he said.
New types of navigation systems include isocentric CTs and O-arms. Isocentric CTs include a motorized C-arm that travels around the patient, taking up to 100 scans.
“Then it produces something like a ‘poor man’s CT scan,’ where it gives you images that can be axial or reformatted in different planes,” Anderson said during an instructional course presentation at the American Academy of Orthopaedic Surgeons 73rd Annual Meeting.
The O-arm works like an intraoperative CT scanner.
“Once you bring it onto the field and you drape it out, you can obtain either fluoro images or CT images [in] real-time. And although this is not yet linked to navigation, that [will be] the next step,” he said.
Navigation tips
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Apart from the system used, the keys to a successfully navigated spine surgery are careful equipment setup, adequate images and frequent equipment checks, Anderson said.
“I can’t overemphasize how important it is to obtain good images for navigation, especially if you’re using fluoroscopic images,” he said. “You’re really only as good as your images are.”
Image quality plays an especially crucial role when using fluoroscopic systems with patients who are osteopenic and obese. “If you can’t see their bone well enough, then you need to get an alternative technique,” Anderson said.
To obtain adequate images, surgeons should also know the distance of the arrays to the surgical site. “If you attach a reference array to the iliac crest, it usually has a reasonable clinical accuracy to work on a couple of the lower vertebral segments,” Anderson said.
Surgical views
He cited incorrect C-arm positioning and inadequate orthogonal views as a common mistake during surgically navigated spine procedures.
“[When viewed correctly], you’re going to see the endplate as though it’s a single parallel line,” Anderson said. “Your pedicles are going to look like they’re coming off of the upper portion of the vertebrae [and] your spinous process is going to be equidistant between the pedicles.”
For pedicular fixation, surgeons should get an oblique view that focuses on the axis of the pedicle.
“The clue here is … you want to see that the medial aspect of the superior articular facet lines up with the medial portion of the pedicle,” Anderson said. “That tells you that you’re looking right down the pedicle, and so this is sort of a bull’s-eye target view for you to use while you’re doing your imaging.”
Although surgeons typically place reference arrays at the surgical site, Anderson said that specific situations, such as a laminectomy case, might require a different position.
“You can put things into the vertebral body,” he said. “A lot of times if you’re doing a revision case, where you maybe have to do an osteotomy for a flat back, you can put a pin right through the fusion mass and then use that.”
He also noted that old implants or the posterior/superior iliac spine could also offer surgical alternatives.
While surgical navigation for spinal procedures may be beneficial, Anderson said surgeons should not underestimate the learning curve for the technique.
“I would recommend that if you are going to adopt this, you spend some time really learning the system well before you do your first case, and that will really add to your comfort as you move through,” he said.
For more information:
- Anderson DG. Surgical navigation for trauma and spine surgery. Instructional course #224. Presented at the American Academy of Orthopaedic Surgeons 73rd Annual Meeting. March 22-26, 2006. Chicago.
- Orthopedics Today was unable to determine whether Dr. Anderson has a direct financial interest in the products discussed in this article or if he is a paid consultant for any companies mentioned.