Lumbar Isthmic Spondylolisthesis

What it is

Here a vertebra slips forward because of a small defect in a bridge of bone called the pars interarticularis — often a stress fracture that formed during the active years of youth. This is different from the degenerative type: the slip is allowed by a specific bony defect rather than by general wear. It usually happens at the lowest level, L5 on S1. The pars defect is common, present in roughly 4 to 6 of every 100 people, and many never have a problem from it.

What it feels like

The most common symptom is activity-related low back pain. Some people also develop leg symptoms when the nearby nerve is stretched or pinched, and in adults the slip can contribute to narrowing and the leg heaviness of stenosis. Many people with a mild slip have few or no symptoms for years.

How we measure and follow it

On a side X-ray I can see the slip and grade how far it has moved on a 1-to-5 scale: Grade 1 is up to one-quarter, Grade 2 up to one-half, and Grades 3, 4, and 5 progressively further (Grade 5 means the vertebra has slipped fully off). Most slips are low-grade (1 or 2). I may add a CT to see the pars defect clearly or an MRI to check the nerves.

How we treat it

Low-grade slips (1–2) usually respond well to non-surgical care — physical therapy, activity adjustments, medication, and sometimes an injection — and often need nothing more. Surgery (a decompression with fusion, occasionally with partial repositioning in higher grades) is considered when pain stays limiting despite good non-surgical care, when there is nerve weakness, or when a higher-grade slip is progressing. High-grade slips (3–5) are less common, tend to occur in younger patients, and involve a more detailed surgical conversation, which we would have in full.

The science — for the curious

An important point about the grade: it tells us how far the vertebra has slipped, not how much it hurts. People with the same grade can feel very differently, and symptoms do not reliably track with the number. Most low-grade slips are stable over time. Progression is mainly a concern in younger patients who still have growing to do and in higher-grade slips, which is why follow-up is tailored to your age and grade. When surgery is needed, the pars defect usually means the level has to be fused to be stable — but I keep that to the single affected level whenever possible, since every fused level adds stress to its neighbors over the years.

Learn more

OrthoInfo — Adult Spondylolisthesis in the Low Back: orthoinfo.aaos.org

North American Spine Society — KnowYourBack: knowyourback.org

This handout is general education about a common condition and does not replace the personalized advice we discuss in clinic. If anything here does not match what we talked about, what we talked about wins — please ask me.