Spondylolisthesis

What It Is

Spondylolisthesis occurs when one vertebra slips out of alignment with the one below it, usually forward (anterolisthesis) and occasionally backward (retrolisthesis). It describes a position problem with several possible causes, and doctors classify it by cause. Slips are graded by severity, from grade I (a slight shift) to grade IV (more than half the width of the vertebra). A higher grade needs active treatment more often, though the type matters as much as the grade.

Degenerative Spondylolisthesis

This is the most common type in adults. It develops when the small stabilizing joints at the back of the spine — the facet joints — and the disc between two vertebrae wear down with age until they can no longer hold the vertebra firmly in place, letting it gradually shift. It typically appears after age 50, is more common in women than men, and occurs somewhat more often in Black patients than in other groups. Because the same wear-and-tear that allows the slip also crowds the space around the nerves, spinal stenosis frequently develops alongside it.

Isthmic Spondylolisthesis and Spondylolysis

This type starts with a stress fracture in a small bridge of bone called the pars interarticularis — the segment of the vertebra that connects its upper and lower joints. That fracture on its own, before any slippage has happened, is called spondylolysis, or simply a pars fracture. The fracture usually occurs during the teenage years, often in athletes whose sport repeatedly extends and loads the lower back — gymnastics, football, and weightlifting are common examples — though it also happens with no sports involvement at all, and some people appear to be born with a naturally thinner segment of bone in this area that fractures more easily under ordinary stress. Many people carry a pars fracture for decades without knowing it; it's frequently discovered incidentally on an X-ray taken for an unrelated reason, sometimes not until well into adulthood. If the fracture doesn't heal, the front and back sections of the vertebra stay disconnected, and ordinary disc wear over the following years can gradually let the front section slide forward — that progression, from a stress fracture to an actual slip, is isthmic spondylolisthesis. Not everyone with a pars fracture develops a slip, and a slip that has developed only rarely progresses much further.

Dysplastic Spondylolisthesis

This is the least common type and the only one present from birth. It comes from a sacrum or vertebra that did not form in the shape needed to resist the spine's forward-leaning forces, with joints that are abnormally angled or bone that is underdeveloped. Because the deformity is structural, the slip tends to appear earlier, in childhood or adolescence. The malformed back portion of the vertebra tends to shift forward with the rest of it. In the isthmic type, that back portion stays in place. As a result, the dysplastic type carries a higher risk of affecting the nerves and more often needs monitoring or treatment during a child's growth.

Symptoms

• Back pain combined with leg pain or cramping that worsens with standing or walking
• Some patients have primarily back pain; others feel it mostly in the legs
• Symptoms often improve with sitting or leaning forward
• Many people with any type of spondylolisthesis — including a pars fracture that hasn't progressed to a slip — have no symptoms at all; it is often found incidentally on imaging done for an unrelated reason
• In children and adolescents with the dysplastic type, changes in posture or gait can accompany back pain