The covering around the spinal cord (and brain) is called the dural sac. Dural tears during spinal surgery are fairly common. Studies report a range from one to 17 percent. A recent study in Germany of over 1,200 patients placed this figure at around three percent.
Most dural tears are repaired at the time of the original surgery. They are either stitched together or patched. Sometimes the tear isn't visible and is diagnosed later on the basis of patient symptoms.