Spondylolisthesis is one word for the translation of one vertebra on the one below it. The definition is anatomical: the cranial vertebra is translated anteriorly in reference to the vertebra below. Patients call it a slipped vertebra. But the word covers two different conditions, and the difference between them is a small bridge of bone called the pars interarticularis. In one type, the pars is intact and the listhesis comes from wear. In the other, the pars is fractured and the listhesis goes through the fracture. The surgery follows from which type it is — and for the degenerative type, the surgery is itself a question.
The shared definition, and the classification
The classification spine surgeons use is Wiltse's, from 1976: five types by cause — dysplastic, isthmic, degenerative, and two uncommon ones (Wiltse et al., Clinical Orthopaedics and Related Research, 1976). Two of the five account for nearly every case an adult sees: the isthmic type, where the pars is defective, and the degenerative type, where the pars is intact and the listhesis comes from wear. The other types — a congenital defect, a fracture elsewhere in the arch, a bone disease — are uncommon, and this piece does not cover them. The two common types share the definition and the word. They differ in the pars, the level, the age, and the operation.
Spondylolysis: the pars and the defect
The pars interarticularis is a narrow bridge of bone on each side of the vertebra, joining the facet joints to the body. It is the isthmus — the narrow part — of the posterior arch. A defect in that bridge is spondylolysis: the pars is fractured, and the two sides of the vertebra are no longer joined by bone. The defect is most often at L5, and usually on both sides — 95% involve the L5 pars, usually bilaterally (Ye et al., Journal of Orthopaedic Surgery and Research, 2024).
Spondylolysis with a listhesis is isthmic spondylolisthesis: the vertebra has translated anteriorly through the defect. The distinction matters because the defect is the diagnosis that decides the operation — and because a defect can exist for years without a listhesis, and a listhesis can exist without symptoms. The pars is the hinge of the whole comparison.
Degenerative: the pars is intact
In degenerative spondylolisthesis, the pars is intact. The listhesis comes from wear: the facet joints wear, disc degeneration sets in, the joint capsules stretch, the disc loses height, and the segment becomes unstable enough for the upper vertebra to translate. It is most often at L4–L5, and most often in women over 40. The listhesis is almost always low-grade — a small translation, not a large one — because the intact posterior elements translate with the rest of the vertebra, and they limit how far it can go.
The stenosis is the main problem. Because the back of the vertebra moves with the front, the canal narrows as the vertebra translates: the central canal, the lateral recess where the traversing root runs, and the foramen where the exiting root leaves. Lumbar stenosis brings leg pain with walking — neurogenic claudication, the legs aching and tiring after a distance, easing with rest. The degenerative listhesis is a stenosis problem first and a listhesis problem second, and the operation is planned around the nerves as much as the listhesis.
Isthmic: the pars is fractured
In isthmic spondylolisthesis, the pars is defective. The defect is a stress fracture of the pars — the bridge fails under repeated stress, and the two sides separate. The front of the vertebra — the body and the disc — translates anteriorly through the defect, while the back — the lamina and the spinous process — stays behind, held by the intact posterior structures. The listhesis is most often at L5–S1, where the pars carries the most load.
The isthmic listhesis behaves differently from the degenerative one. Because the back of the vertebra does not move with the front, the canal does not narrow the same way — the stenosis, when it occurs, is in the foramen, where the exiting root is pinched by the defect and the fibrous tissue that forms around it. The disc above the defect is at risk: the segment moves more than it should, the disc takes the load, and disc degeneration advances faster. The isthmic listhesis can progress, and the disc above it can wear out — which is why the operation for the isthmic type is different from the operation for the degenerative type.
The comparison
The pars: intact in degenerative, defective in isthmic. The level: most often L4–L5 in degenerative, most often L5–S1 in isthmic. The age: older adults in degenerative, younger in isthmic. The stenosis: central and lateral recess in degenerative, foraminal in isthmic. The progression: the degenerative listhesis is low-grade and slow; the isthmic listhesis can progress, and the disc above the defect is at risk. The disc: worn as the cause in degenerative, worn as a consequence in isthmic. Two conditions, one word — and the pars is what tells them apart.
The surgical question: degenerative
For the degenerative type, the question is decompression only vs fusion. Decompression relieves the stenosis — the nerves get room. Fusion adds stability — the segment is fixed so it cannot translate further. The answer depends on instability and patient-specific symptoms and goals of treatment. A stable listhesis with stenosis and leg pain may need only decompression. An unstable listhesis — one that moves on flexion-extension X-rays, or one with back pain that tracks the instability — may need the fusion as well. The decision is framed by symptoms, examination, imaging, spinal stability, and treatment history.
The evidence is a small randomized trial. Ghogawala and colleagues randomized 66 patients with stable grade I degenerative spondylolisthesis and symptomatic stenosis to laminectomy alone or laminectomy with instrumented fusion (Ghogawala et al., New England Journal of Medicine, 2016). The fusion group had a greater increase in physical health scores at 2 years — 15.2 vs 9.5, a difference of 5.7 points — and a lower reoperation rate: 14% with fusion vs 34% with decompression alone. Disability scores did not differ significantly between the groups. The trial was small, and it does not settle the question — it is the evidence for the frame that the decision depends on the patient in front of the surgeon, not on a rule. More blood loss and a longer hospital stay came with the fusion. The trade is real, and it is weighed case by case.
The surgical question: isthmic
For the isthmic type, the operation is fusion. The listhesis goes through a fractured pars, and the segment needs to be fixed: a spinal fusion joins the vertebra to the one below it, and the listhesis is held. The fusion is the treatment because the defect cannot heal itself — the pars, once fractured, does not reunite on its own, and the segment moves through the fracture with every step. Conservative care — activity, physical therapy, medication — manages the symptoms while the decision is made, but it does not repair the pars.
In select adolescents, the operation is different: instead of a fusion, the pars itself can be repaired. Direct repair — a Buck screw or a Morscher hook-screw, with bone graft — fixes the defect and preserves the motion of the segment, which matters in a young spine that has decades of movement ahead of it (Ye et al., Journal of Orthopaedic Surgery and Research, 2024). The repair is for the defect without a significant listhesis, in a patient whose conservative treatment has not worked. It is a different operation for a different age — and it is the one place in this piece where the adolescent spine enters the discussion.
How the decision is made
The decision starts with the type — degenerative or isthmic — and the pars is what tells them apart. Flexion-extension X-rays show whether the segment moves: the motion study for instability, read as the difference between the flexion and extension positions. The examination shows which nerve is affected and how much function is lost. The MRI shows the stenosis and the disc. And the patient shows the rest: the symptoms, the walking distance, the goals of treatment — what the patient wants the operation to do, and what the patient is willing to accept to get it. Surgery should be recommended thoughtfully — not simply because it is technically possible. That standard holds for both types: the fusion is considered for a specific structural reason, never for a listhesis alone, and never for a number on an X-ray without the symptoms to match it.
Frequently asked questions
What is the difference between degenerative and isthmic spondylolisthesis?
The pars interarticularis — a small bridge of bone on each side of the vertebra. In degenerative spondylolisthesis, the pars is intact and the listhesis comes from facet joint and disc wear, most often at L4–L5. In isthmic spondylolisthesis, the pars is defective — a stress fracture of the bridge — and the listhesis goes through the defect, most often at L5–S1. The classification is Wiltse's, from 1976 (Wiltse et al., Clinical Orthopaedics and Related Research, 1976).
What is spondylolysis?
A defect in the pars interarticularis — the pars is fractured, and the two sides of the vertebra are no longer joined by bone. The defect is most often at L5, and usually on both sides — 95% involve the L5 pars, usually bilaterally (Ye et al., Journal of Orthopaedic Surgery and Research, 2024). Spondylolysis with a listhesis is isthmic spondylolisthesis.
What is a slipped vertebra?
The patient phrase for spondylolisthesis. The definition is anatomical: the cranial vertebra is translated anteriorly in reference to the vertebra below. The clinical term is spondylolisthesis, and the type — degenerative or isthmic — is what decides the treatment. The lay word belongs in the conversation; the clinical term belongs in the diagnosis.
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Does degenerative spondylolisthesis need a fusion?
Not always. The question is decompression only vs fusion, and the answer depends on instability and patient-specific symptoms and goals of treatment. A stable listhesis with stenosis and leg pain may need only decompression. An unstable listhesis — one that moves on flexion-extension X-rays, or one with back pain that tracks the instability — may need the fusion as well. A small randomized trial found the fusion group had a greater increase in physical health scores and a lower reoperation rate — 14% with fusion vs 34% with decompression alone — but the trial was small and does not settle the question (Ghogawala et al., New England Journal of Medicine, 2016).
Does isthmic spondylolisthesis need a fusion?
Yes. The listhesis goes through a fractured pars, and the segment needs to be fixed: a spinal fusion joins the vertebra to the one below it, and the listhesis is held. The defect cannot heal itself — the pars, once fractured, does not reunite on its own, and the segment moves through the fracture with every step. In select adolescents, the pars itself can be repaired instead — a Buck screw or a Morscher hook-screw with bone graft — preserving the motion of the segment (Ye et al., Journal of Orthopaedic Surgery and Research, 2024).
Can the pars be repaired instead of fused?
In select adolescents and young adults, yes. Direct repair — a Buck screw or a Morscher hook-screw, with bone graft — fixes the defect and preserves the motion of the segment, which matters in a young spine (Ye et al., Journal of Orthopaedic Surgery and Research, 2024). The repair is for the defect without a significant listhesis, in a patient whose conservative treatment has not worked. In adults with a listhesis, the operation is fusion.
How do you tell which type it is?
By the pars. Flexion-extension X-rays show whether the segment moves. CT shows the pars itself — intact or defective — better than any other study. MRI shows the stenosis and the disc. The examination shows which nerve is affected. The type is decided by the anatomy: the pars intact or fractured, the level, and the pattern of the listhesis.
Why does the degenerative type cause leg pain?
Because the back of the vertebra moves with the front, the canal narrows as the vertebra translates: the central canal, the lateral recess, and the foramen. Lumbar radiculopathy (sciatica) follows the nerve that is pinched — leg pain with walking, the legs aching and tiring after a distance, easing with rest. The degenerative listhesis is a stenosis problem first and a listhesis problem second.
Will the listhesis keep getting worse?
The degenerative listhesis is low-grade and slow — a small translation, and it rarely progresses far, because the intact posterior elements limit how far it can go. The isthmic listhesis can progress, and the disc above the defect is at risk. Progression is followed with standing X-rays over time, and it is one of the reasons surgery is considered — alongside the pain, the walking distance, and the nerves.
What if I have back pain but no leg pain?
Back pain alone, without leg pain, is usually managed conservatively first — activity, physical therapy, medication. Surgery is considered when the pain is not controlled, when the nerves are affected, or when the deformity is progressing. The decision is framed by symptoms, examination, imaging, spinal stability, and treatment history — never by a number on an X-ray without the symptoms to match it.