Spondylolisthesis is defined as forward translation of a vertebral body with respect to the vertebra below. [1, 2, 3, 4, 5, 6] The term is derived from the Greek roots spondylo, meaning spine, and listhesis, meaning to slide down a slippery path.
Spondylolisthesis can occur at any level of the spinal column, although it is most common in the lower lumbar spine. Most cases are thought to result from minor overuse trauma, particularly repetitive hyperextension of the lumbar spine. Spondylolysis, a break in the vertebra typically in the region of the pars interarticularis, may or may not be associated with a spondylolisthesis. If the pars defect is bilateral, it may allow slippage of the vertebra, typically L5 on S1, resulting in spondylolisthesis.
Both spondylolysis and spondylolisthesis are often asymptomatic, and the degree of spondylolisthesis does not necessarily correlate with the incidence or severity of symptoms, even when a patient is experiencing back pain. However, these 2 entities have been reported to be the most common underlying causes of persistent low back pain among children and adolescents, despite the fact that most cases are asymptomatic. [3, 5, 7, 8, 9]
Spondylolisthesis can be classified into the following 6 distinct categories.
- Congenital (dysplastic)
- Caused by agenesis of the superior articular facet
- Isthmic (spondylolytic)
- Caused by pars interarticularis defects
- Secondary to articular degeneration
- Caused by fracture or dislocation of the lumbar spine, not involving the pars
- Postsurgical (iatrogenic)
A new computer-assisted classification has been recommended by the Spinal Deformity Study Group based on slip grade, pelvic incidence, and sacro-pelvic and spinal balance. Software enabled observers to identify all 6 types of spondylolisthesis and to identify 7 anatomical landmarks on each radiograph. 
A variety of methods are also used to measure the degree of spondylolisthesis. The primary focus of this article is isthmic spondylolisthesis only, because it is the most common variety and because it is relevant to sports medicine.
Isthmic (spondylolytic) spondylolisthesis usually occurs in children older than 5 years, most commonly in those aged 7-8 years, and it rarely occurs before walking begins. Slip progression is minimal after skeletal maturity.
Isthmic spondylolisthesis is further divided into the following 3 subtypes:
Type IIA, or lytic spondylolisthesis, involves a defect in the pars area and is thought to result from recurrent microfractures from the impact of the articular processes against the pars while in extension. This defect usually occurs by age 6 years and is occasionally associated with developmental anomalies such as lumbarization, sacralization, and spina bifida occulta.
Type IIB involves an intact but elongated pars, probably resulting from repetitive microfractures that heal in an elongated position, much like pulled toffee.
The prevalence rate of isthmic spondylolisthesis is approximately 5% at age 5-7 years, with an increase to 6-7% by age 18 years. This condition is twice as common in males as in females, and the prevalence is lower in blacks (2.8%, black men; 1.1%, black women) than in whites (6.4%, white men; 2.3%, white women). Despite the higher prevalence in males, progression, although still rare, has been reported to be more common in females.
Additional risk factors include having a first-degree relative with a slip, occult spina bifida at S1, and the presence of scoliosis.
A study by de Schepper et al that described MRI findings of patients presenting for a lumbar MRI examination as referred by their GP found that among 94% of the 683 participants that had abnormal MRI findings, 18% of the patients showed signs of spondylolisthesis. 75% of these were grade I. 
Mechanical stresses play an important role in this process. Erect posture produces a constant downward and forward thrust on the lumbar vertebrae. Stresses on the pars interarticularis are accentuated during repetitive hyperextension, which results in increased contact of the caudal edge of the L4 inferior articular facet with the L5 pars interarticularis. This collective trauma may eventually result in a stress fracture of the pars interarticularis. Spondylolisthesis may occur when bilateral pars defects are present, which allows forward slippage of the vertebra (typically L5 on S1). Spondylolisthesis has never been reported in quadrupeds or people who are chronically bedridden.
Sports that involve repetitive hyperextension and axial loading of the lumbar spine may result in repetitive microtrauma to the pars interarticularis, resulting in spondylolysis and sometimes spondylolisthesis. Examples of such activities include gymnastics, football (lineman), wrestling, weight lifting (particularly standing overhead presses), rowing, pole vaulting, diving, hurdling, swimming (especially the butterfly stroke), baseball (especially pitching), tennis (especially serving), sailing (particularly the hiking maneuver), and volleyball. Gymnastics and football are generally considered the highest risk sports. [4, 5, 6, 12]
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