Turn of the Screw
Extensive testing in the Spine Biomechanics Lab gives surgeons new certainty—and improves outcomes.
Date: May 20, 2011
Ali Bydon and his colleagues at Johns Hopkins perform about 1,000 spine surgeries a year. Around two-thirds of these surgeries require instrumentation—screws, rods, and the like—to fuse or stabilize one or more of the spine’s 33 bones. But regardless of how many procedures these doctors do, Bydon says that experience doesn’t always hold all the answers. “I’ve encountered lots of things in the operating room that we don’t always have a solution for,” he says. “Should I use a certain rod or another to stabilize someone’s spine? Is one length of screw better than another?”
Such instrumentation isn’t foolproof, Bydon points out. If it fails, the cost for patients can be high, with extreme pain and additional corrective surgeries. If he and his colleagues could know whether the standard way they put in instrumentation is best—or whether there might be an even better way—they may be able to improve patients’ outcomes and prevent needless complications.
In 2007, Bydon and Risheng Xu, a first-year medical student Bydon had recruited to work with him, came up with a novel idea: Why not test instrumentation in new ways in real spines—but before it goes into patients? That’s the concept behind the team’s Spine Biomechanics Lab, housed on Hopkins’ Bayview Medical Center campus. Taking cadaver spines obtained from the Maryland Anatomy Board, Bydon, Xu, and their colleagues immobilize both ends of the backbones in a machine that can move them repeatedly in lifelike ways. They then test out various instrumentation, using screws, rods, and other hardware in new ways, or review the soundness of traditional installation methods.
While patients might take weeks or months to settle into their new hardware, the machine holding the cadaver spines twists and bends the backbones over and over, Bydon says, simulating long-term wear and tear over days—much like the machines that simulate thousands of flop-downs on IKEA couches.
“Different surgeons do things in different ways,” Xu says. “We’re trying to find the best, safest way to improve the way we practice spine surgery.”
The researchers recently reported the results of one of their tests, this one comparing two different kinds of screws commonly used in a surgery to fuse vertebrae in the neck. Most surgeons consider one of the screws, called a pedicle screw, to be longer and stronger than the other screw, called a C7 lateral mass screw. However, the pedicle screw can be tougher to place and goes in a slightly different location than the C7 lateral mass screw, right by a nerve that controls the hand—bringing a small, but significant, risk of paralyzing that hand if things go wrong.
Bydon, Xu, and their colleagues showed that the C7 lateral mass screws did the job just as well as the trickier pedicle screws.
“All of the sudden,” says Bydon, “you don’t need to go the extra mile to place tough screws. You can get away with placing the easier screws and have the same result.”
Their lab has another arm, which studies differences in surgical outcomes between procedures meant to accomplish the same thing.
He, Xu, and other researchers recently published a paper comparing various ways to treat synovial cysts, a common back problem in which fluid-filled sacs develop between vertebrae, painfully pressing on spinal nerves. Surgeons can remove the cyst and small pieces of the vertebra, giving more room for the spinal nerves, known as a “decompression,” or they can do the same surgery while also fusing the affected joints, known as “decompression with stabilization.” After looking at data from 167 patients who had one of these surgeries, the researchers found that stabilizing the joint led to significantly less pain down the road, with no additional risk for patients.
“Before this paper came out,” Bydon explains, “many surgeons thought it might be too risky to do fusion—they wanted to just do decompression and see if that helped. For the first time, we’re showing there’s a difference in outcomes and fusion is actually better. It’s evidence, instead of anecdotal medicine.” Christen Brownlee