Figure 3. Anatomical location of pars interarticularis and stages leading to spondylolisthesis.

Spondylolysis is the medical term for a spine fracture or defect that occurs at the region of the pars interarticularis. The pars interarticularis is the region between the facet joints of the spine and more specifically the junction of the superior facet and the lamina. Spondylolysis is thought to be caused by repeated strains that damage the lower spine over time. Repeated strains can eventually lead to an overuse injury in the pars interarticularis. The most common location for this to occur is in the lowest vertebra of the spine. It is common for the defect to occur on both sides. When this happens, the vertebra is no longer held firmly in place by the facet joints on the back of the ring. As a result, the vertebra is free to slip forward over the one below. This slippage which is closely related to spondylolysis is called spondylolisthesis.