In younger patients (under twenty years old), spondylolisthesis usually involves slippage of the fifth lumbar vertebra over the top of the sacrum. There are several reasons for this. First, the connection of L5 and the sacrum forms an angle that is tilted slightly forward, mainly because the top of the sacrum slopes forward. Second, the slight inward curve of the lumbar spine creates an additional forward tilt where L5 meets the sacrum. Finally, gravity attempts to pull L5 in a forward direction.
A condition called can also cause the slippage that happens with spondylolisthesis. Spondylolysis is a defect in the bony ring of the spinal column. It affects the pars interarticularis, mentioned above. This defect is most commonly thought to be a "stress fracture" that happens from repeated strains on the bony ring. Participants in gymnastics and football commonly suffer these strains. Spondylolysis can lead to the spine slippage of spondylolisthesis when a fracture occurs on both sides of the bony ring. The back section of the bony ring separates from the main vertebral body, so the injured vertebra is no longer connected by bone to the one below it. In this situation, the facet joints can't provide their normal support. The vertebra on top is then free to slip forward over the one below.
In the cervical spine, if the posterior bony ridges are large, they can cause repeated trauma tothe spinal cord with neck motion, eventually resulting in cord deformity, atrophy, and a myelopathy.In the lumbar region, osteophytes encroaching on neural foramina are contrasted nicely by foraminal fat on T1-weighted scans.
Sasai K, Umeda M, Maruyama T, WakabayashiE and Iida H: Microsurgical bilateral decompression via aunilateral approach for lumbar spinal canal stenosis includingdegenerative spondylolisthesis. J Neurosurg Spine. 9:554–559. 2008. :