October 6, 1997
Volume 50, No. 7
A three-dimensional computer model of the spinal column developed earlier this year by William Hutton, a professor in the Department of Orthopaedics, and his colleagues dispels several commonly held views about the cause of prolapsed, or "slipped," spinal disks.
The model, which was compiled from scans of people's spines, took about a year and a half to construct. It takes about three-and-a-half hours to run the program on a supercomputer. The computerized model allows researchers to tinker with various parameters. "We could see what happens, for example, when you bend forward and lift a heavy weight," said Hutton.
The model helped him and his colleagues discover that-contrary to popular belief-a tear in the disk begins at the center and works its way out. Hutton compared the disk to a car tire. The hub is the fluid-filled nucleus, he said, and the thick outer ring is the anulus. It was once believed that when people bend forward the outer edge of the anulus is stretched and becomes prone to tears. "What often happens is the nucleus bursts through like a puncture in a car tire and blasts away through the anulus to leak onto the outside disk," Hutton explained.
Stress that exceeds the strength of the anulus fibers results in a slipped disk. This can come from bending, twisting, heavy lifting and fluid saturation of the disk. Hutton and his team found that a slipped disk is more likely to occur in the morning after disks have absorbed water and nutrients overnight. People are taller in the morning, Hutton said, because their disks have swelled during the night while they lie in sleep.
Disks are already under stress because they are swollen when we awake in the morning, and heavy lifting increases that stress. "If you're going to lift a piano," Hutton quipped, "wait until the afternoon."
Bending was found to cause disk injury more often than twisting. Bending creates the majority of the stress required to produce a fracture, and twisting is the "straw that breaks the camel's back," Hutton said. "It's commonly held that twisting is bad for the spine. This research showed that twisting on its own isn't terribly important."
Slipped disks seem to be a gradual process. The computer simulation revealed that failure happens when anulus fibers tear away from the bone. In the event the stress is not severe enough to cause immediate prolapse, individual fibers can be broken, and the load is redistributed among remaining fibers. If the fibers don't heal properly, the disk is susceptible to future damage, according to Hutton.
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