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The bones of the spine are arranged to give the spinal column stability. Damage or defects within the supporting structures of the lumbar spine can be a source of back pain. A crack in the bony ring of the spinal column is called spondylolysis (spon-dil-low-lie-sis). If a crack occurs on both sides of the bony ring, the spine is free to slip forward. This condition is called spondylolisthesis (spon-dil-low-liss-the-sis). About 5% to 6% of people are affected by these two conditions.
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Spondylolysis mainly affects the lowest lumbar vertebra. The bony ring, formed by the pedicle and lamina bones, protects the spinal cord and spinal nerves. The bone is weakest between the pedicle and lamina, an area called the pars interarticularis - "pars" for short. A pars defect is believed to be a stress fracture. A stress fracture happens from repeated strain on a bone. At first your body is able to heal the damage. If strains repeat and happen faster than your body can respond, the bone eventually fractures. People are not born with spondylolysis. It commonly first appears in childhood. Football linemen and gymnasts are affected the most.
Spondylolisthesis occurs when a spondylosis occurs on both sides of the bony ring. A crack on both sides of the bony ring separates the facet joints from the back of the spinal column. The facet joints can no longer steady the vertebra, and the vertebra on top starts to slide forward, slowly stretching the disc below. In adults, there is usually no danger that the top vertebra will slide completely off the vertebra below. But teenagers sometimes have a unique type of spondylolisthesis in which one vertebra slips forward and slides completely off the vertebra below.
Spondylolysis and spondylolisthesis can be a source of low back pain. Having one of these conditions does not mean you are certain to have back problems, but it does put you at higher risk. These conditions can cause mechanical pain - the kind that comes from within the moving parts of the spine. They can also cause compressive pain, which is from pressure on the nerves of the low back.
Pinched or irritated nerves produce compressive symptoms. This occurs in spondylolysis when a lump of tissue forms around the crack where the body has tried to heal itself. The lump can cause pressure on the spinal nerves where they leave the spinal canal. A pinched nerve can also happen in spondylolisthesis when the vertebra slides forward and squeezes the nerve. The forward slip of the vertebra also makes the spinal canal smaller, leaving less room for the nerve roots. Pressure on the nerve can produce pain that radiates down to the foot. It can also cause numbness in the foot and weakness in the muscles supplied by the nerve.