Spondylolysis is a condition in which the there is a defect in a portion of the spine called the pars interarticularis (a small segment of bone joining the facet joints in the back of the spine). With the condition of spondylolisthesis, the pars interarticularis defect can be on one side of the spine only (unilateral) or both sides (bilateral). The most common level it is found is at L5-S1, although spondylolisthesis can occur at L4-5 and rarely at a higher level.

Spondylolysis is the most common cause of isthmic spondylolisthesis, in which one vertebral body is slipped forward over another. Isthmic spondylolisthesis is the most common cause of back pain in adolescents; however, most adolescents with spondylolisthesis do not actually experience any symptoms or pain. Cases of either neurological deficits or paralysis are exceedingly rare, and for the most part it is not a dangerous condition. The most common symptom is back and/or leg pain that limits a patient’s activity level.

Since spondylolysis is the most common cause of spondylolisthesis, it may be referred to as an isthmic spondylolisthesis and sometimes these terms are used interchangeably, although this is not correct. There are at least 6 recognized causes of slippage as seen in spondylolisthesis in the literature. According to Dr. Leon Wiltse, these causes are listed as:

Dysplastic spondylolisthesis (which includes congenital)
Isthmic spondylolisthesis (which includes lytic or stress fracture, an elongated but intact pars or an acute fracture of the pars)
Degenerative spondylolisthesis (Pseudospondylolisthesis) — secondary to long-standing degenerative arthrosis (degenerative disc disease and degeneration of the facet joints)
Traumatic spondylolisthesis (secondary to a fracture of the neural arch)
Pathologic spondylolisthesis (from bone disease such as metastatic disease, tumor, osteoporosis, etc.)

Importantly, spondylolysis only refers to the separation of the pars interarticularis (a small bony arch in the back of the spine between the facet joints), whereas spondylolisthesis refers to anterior slippage of one vertebra over another (in the front of the spine). Therefore, although the terms are sometimes used interchangeably, this is incorrect and the two are technically not interchangeable.

The underlying cause of spondylolysis has not been firmly established. According to major researchers in spine medicine (including Wiltse, Yochum and Rowe) there have been no recorded cases of spondylolisthesis in a new born and therefore the condition is not believed to be genetic. Some physicians believe that repetitive trauma (such as from certain sports) may either cause or contribute to the development of spondylolysis.