Isthmic spondylolisthesis occurs when one the vertebral bodies in the spine have slipped forward over the vertebra below it due to a small fracture in a piece of bone that connects the two joints on the back side of the spinal segment. The fracture is caused by stress to the bone and occurs most commonly when an individual is young (around 5 to 7 years old), although symptoms such as lower back pain and/or leg pain do not typically show up until adulthood.

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