Spondylolisthesis is a Latin term for "slipped vertebral body". For approximately 5% of the population, a stress fracture occurs (usually between the ages of five and seven) in the lowest lumbar vertebral segment (L5). As a result, the L5 vertebral body slips forward on the S1 (sacral 1) vertebral body. This process is almost never due to trauma (see Figure 1).
The occurrence of spondylolisthesis often results in no noticeable symptoms, and probably 80% of people never even know they have the problem. In the 20% who do develop low back pain, their pain is usually due to disc degeneration in young adulthood.
Without the stabilizing effect of the bony arch, the disc is forced to work harder to resist shearforces (forward subluxation). The disc is designed to function very effectively in compression, but tends to break down when shear forces are applied to it.
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