Spondylolisthesis occurs when one lumbar vertebra slips forward in relationship to the adjacent vertebra. In the Greek language, the term spondylolisthesis means slipped vertebra. The neural arch (lamina) and the paired facet joints are anatomical structures that prevent vertebrae from slipping (Fig 1).

Figure 1. Posterior elements of the spine.
The neural arch (lamina) is located in the middle,
between the facet joint complex.

A vertebra may slip following the development of a stress fracture through the neural arch. The defect in the lamina separates the back part of the vertebra from the remaining part; the vertebral body and disc. The stabilizing role of the paired facet joints is lost and the vertebral body slips forward. The laminar stress fracture (in Greek called spondylolysis) occurs in a specific region of the lamina called the pars interarticularis or isthmus. Hence the condition is called isthmic spondylolisthesis (Fig 2).

Figure 2. Isthmic Spondylolisthesis.

In some cases, the stress fracture occurs in early childhood and by age-six spondylolysis has developed. Incidence of this is found in about 4.4% of the population. In many cases the stress fracture does not cause any symptoms or discomfort and goes unrecognized.

The actual vertebral slip (spondylolisthesis), as a result of spondylolysis develops later on in life, somewhere during adolescence. In adults, the incidence of spondylolisthesis is about 6%. In rare cases, the slip may develop later in adult life (e.g. after age 20).

In the vast majority of cases, isthmic spondylolisthesis occurs at the junction of the lumbar spine and the sacrum (pelvis). In medical terms, this junction is the L5-S1 level, which is between the 5th lumbar vertebra and the 1st sacral vertebra. There seems to be a genetic predisposition to the condition. In some cases, several of the patient's family members may have acquired spondylolisthesis.