Usually, this vertebra has slid forward over the vertebra directly below it. Spondylolisthesis is most common in the lower back, and occurs more often in adolescents and adults.
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Types and grades

Spondylolisthesis commonly affects the lower back.
Due to the varying degrees and causes of the misalignment, spondylolisthesis is broken down into types and grades. The six main types are based on the cause of the spondylolisthesis.
Type I - In some cases, people are born with spondylolisthesis.