The operation Baltimore Orioles third baseman Cal Ripken underwent yesterday appears to have been one in which a small portion of bone is delicately removed from a canal through which a nerve exits the spine on its way to the leg.

The operation "decompresses" the nerve, relieving most of the pain that is the condition's predominant symptom. About 90 percent of patients report significant improvement, experts said yesterday.

Although the bone removed is part of the lumbar spine, which bears most of the body's weight, the amount is small. Its loss rarely causes instability or limits activity.

Details of Ripken's back ailment, and of the surgery, were not released yesterday at his request. Press reports earlier in the season, however, said his recurring pain wasn't caused by a disk problem he had two years ago, but by an overgrowth of bone in a nerve canal.

If that is the case, the condition is probably one known as "foraminal stenosis." The vertebrae of the spine are stacked on each other like soup cans. Each one forms two openings, or "foramina," with the vertebra above and two with the vertebra below. In each pair, one opening carries a nerve root exiting from the left side of the spinal cord, and the other a nerve root exiting the right.

A spinal nerve root contains thousands of individual fibers, some carrying sensation, others directing the action of muscles. Symptoms occur in a relatively large area of the body, such as the lower leg or foot. Age and wear-and-tear can cause arthritis in the bones forming the canal, which over time thicken and crowd the nerve. Although pain is the predominant symptom of foraminal stenosis, muscle weakness can occur if the problem is severe and left uncorrected.

The most common site for foraminal stenosis is in the canal formed by the bottom two vertebrae of the lumbar spine (designated L4 and L5), which tends to cause pain in the shin.

"It is not uncommon to have multiple levels involved, although that happens less than half the time," said Harry N. Herkowitz, chairman of orthopedic surgery at William Beaumont Hospital, in Royal Oak, Mich., and head of the spine committee for the American Academy of Orthopedic Surgeons. In a minority of cases, canals on both the right and left side of a pair of vertebrae must be widened, he said.

The procedure is done with an open incision, not through a fiberoptic scope. A small, pliers-like device called a rongeur is used to nip away tiny amounts of the encroaching bone. Virtually all patients are on their feet in one day, and can resume moderately strenuous activity in about six weeks.

"Ninety percent plus do very well if you make the correct diagnosis," said Fraser C. Henderson, a neurosurgeon and chief of spine surgery at Georgetown University. "But there's always some residual pain. We're not fixing the back, we're simply trying to remove the bony compression of a nerve. The back still has the degenerative changes."

Henderson said that athletes are good candidates for optimal recovery, even if they continue to put unusual strain on their backs, because "they have good quality bone and supportive ligaments."