Radiology glossary

Spondylolisthesis — What It Means on Your Report

Part of: Spine findings, explained

Full explainer: Spondylolisthesis — What It Means on Your Report

One vertebra has slipped forward over the vertebra below it — a shift that is measured and graded by how far it has moved. Picture a stack of blocks where one block has slid partway off the one beneath; that is essentially what the imaging shows. Doctors grade the slip from I to IV based on the percentage of the vertebral body's width that has shifted: grade I is up to 25 percent, grade II up to 50, and so on, with higher grades being less common. The two most common causes are degenerative — worn discs and arthritic facet joints gradually letting a vertebra drift, typically in the lower back of older adults — and isthmic, where a small stress fracture in the bony bridge behind the vertebra (spondylolysis) lets it slide, often in younger active people. A mild slip often causes no symptoms and is simply monitored; a larger slip can narrow the canal or crowd nerves, causing back pain or leg symptoms. The grade describes the anatomy — your symptoms and examination determine what, if anything, needs to be done.

What it means on MRI

MRI shows both the slipped vertebra and its soft-tissue consequences in one study: the grade of the slip, the condition of the disc at that level, and whether the shift is narrowing the canal or pinching nerve roots. It can reveal the pars defect behind an isthmic slip and show any nerve compression caused by the misalignment. Because treatment decisions hinge on whether nerves are affected — not just on the slip's size — MRI's view of the nerves makes it the key test when symptoms are present. It also helps distinguish a stable long-standing slip from one with new changes.

What radiologists look at next

For spondylolisthesis, the radiologist identifies which vertebra has slipped and in which direction — almost always forward — then measures the displacement as a percentage to assign the grade from I to IV. They determine the type: degenerative, with worn discs and arthritic facets, or isthmic, with a visible defect in the pars interarticularis. Next they assess consequences: canal narrowing, foraminal crowding, and any nerve root compression, plus disc condition at the involved level. Dynamic X-rays are checked for movement between positions, since a slip that shifts is clinically different from one that is fixed. Prior studies are compared to judge stability over time.

Where it commonly shows up

• L5-S1
• L4-L5
• lumbar spine
• L5
• L4

Questions to ask your doctor

• What grade is my slip, and what does that grade mean?
• Is the slip stable, or does it move when I bend?
• What caused it — wear and tear or a pars defect?
• Is the slip pressing on any nerves?
• What symptoms should make me call my doctor?

Common questions

What is the difference between spondylolisthesis and spondylolysis?

Spondylolysis is a crack or defect in the small bony bridge (the pars) behind the vertebra — it is the cause in many younger patients. Spondylolisthesis is the actual forward slip of the vertebra, which can result from that defect or from degenerative wear. You can have the defect without the slip, but the slip often follows it.

Is a grade I slip serious?

Usually not — grade I slips are the mildest, the most common, and very often cause no symptoms at all. Many are found incidentally and simply monitored with periodic imaging to confirm they are not progressing. Whether yours matters depends on your symptoms and whether nerves are affected, so discuss it with your doctor.

Can a slipped vertebra slip further?

It can, but many low-grade slips remain stable for years or decades. Progression is more likely in younger patients with isthmic slips and in higher-grade slips. That is why doctors recheck with follow-up X-rays — to catch change early. Ask your doctor what follow-up schedule fits your grade and situation.

Related conditions

• Spondylolysis — What It Means on Your Report
• Spinal Stenosis — What It Means on Your Report
• Nerve Root Compression — What It Means on Your Report