Spondylolisthesis occurs when one vertebra slips forward on the vertebra immediately below. About 5% of the population develop a spondylolisthesis at some point in their life.

There are four grades of severity, determined by the amount the upper vertebra has slipped in relation to the lower vertebra.

1. Grade 1 is a slip of less than 25%
2. Grade 2 is 25-50%
3. Grade 3 is 50-75%
4. Grade 4 is 75-100%

Most slips are grade 1 and are generally stable.

There are two major types of spondylolisthesis:

Degenerative

Degenerative spondylolisthesis usually occurs in older women, most often at the L4/5 level. It causes lower back pain and symptoms due to compression of the lumbar nerve roots, weakness, numbness, and tingling down one or both legs.

This type of slip is degeneration of the disc, which causes hypertrophy at the facet joints and is generally stable. Decompression can often be performed without the need for fusion.

Lytic or spondylotic listhesis

Isthmic spondylolisthesis usually occurs at L5/S1 and normally presents in the teenage years or 20s. It often appears in cricketers, when they sustain a stress fracture of the pars region of the L5 vertebra. It happens due to repetitive stress in the lumbar spine. The fracture occurs through a thin portion of the vertebra between the two articular processes or joints. This part of the vertebra is called the pars or the pars inticularis.

Initial treatment is exercise and physiotherapy. Most patients will eventually require a posterior fusion with pedicle screws when the symptoms become significant. Surgery generally produces a cure.

Because fusion surgery can have long term ramifications, symptoms generally need to be significant before considering surgery.