SpineThoracolumbar Spine/Thoracolumbar Spine

Adult Isthmic Spondylolisthesis

Assoc. Prof. Ozcan Kaya· Medical Park TEM Hospital
Apr 21, 2026

Definition

  • Forward translation of a vertebral body due to a defect in the pars interarticularis (spondylolysis)

  • Most commonly occurs at L5–S1

  • Typically results from a chronic pars defect acquired in adolescence, becoming symptomatic in adulthood

Epidemiology

  • More common in males

  • Often asymptomatic in youth → symptoms develop in adulthood

  • L5–S1 involved in the vast majority of cases

Etiology / Pathophysiology

  • Repetitive stress → pars fracture (spondylolysis)

  • Bilateral pars defect → loss of posterior stability → vertebral slippage

  • Degeneration of the disc over time → progression of slip

Classification

Wiltse Classification

  • Type II (Isthmic):

    • IIa: stress fracture

    • IIb: elongated pars

    • IIc: acute fracture

Meyerding Grading (Slip Severity)

  • Grade I: <25%

  • Grade II: 25–50%

  • Grade III: 50–75%

  • Grade IV: 75–100%

  • Grade V: >100% (spondyloptosis)

Clinical Presentation

  • Low back pain (most common)

  • Pain with:

    • Extension

    • Activity

  • Radiculopathy (L5 nerve root)

    • Leg pain

    • Paresthesia

  • Tight hamstrings (occasionally)

Imaging

X-ray

  • Lateral view → slip percentage

  • Oblique view → “Scotty dog” sign (pars defect)

MRI

  • Nerve root compression

  • Disc degeneration

  • Foraminal stenosis

CT

  • Best for visualising pars defect

Diagnosis

  • Clinical + imaging

  • Evaluate:

    • Slip grade

    • Neurological status

    • Foraminal stenosis

Treatment

Nonoperative (First-line)

Indications:

  • Low-grade slip (Grade I–II)

  • No significant neurological deficit

Management:

  • Activity modification

  • NSAIDs

  • Physical therapy (core strengthening)

  • Epidural steroid injections (selected cases)

Operative Treatment

Indications:

  • Persistent pain despite conservative treatment

  • Radiculopathy

  • Progressive slip

  • High-grade spondylolisthesis

Surgical Options

  • Decompression + fusion (standard)

    • Most common approach

  • Posterolateral fusion

  • Interbody fusion (PLIF / TLIF / ALIF)

Fusion preferred over decompression alone (instability risk)

Complications

  • Persistent pain

  • Nonunion (pseudoarthrosis)

  • Adjacent segment disease

  • Neurological injury

Prognosis

  • Good outcomes with appropriate treatment

  • Surgical patients → high rates of pain relief

  • Progression usually slow in adults

Pits & Pearls

  • Isthmic = pars defect, not degeneration

  • Most cases are L5–S1

  • Low-grade → conservative first

  • Fusion > decompression alone

  • Radiculopathy often due to foraminal stenosis

Pitfalls

  • Confusing isthmic with degenerative spondylolisthesis

  • Over-treating asymptomatic patients

  • Decompression alone → postoperative instability

  • Ignoring sagittal balance in surgical planning

Mini Decision Algorithm

Low-grade + no neuro deficit → Conservative
Persistent pain → Consider surgery
Radiculopathy → Decompression + fusion
High-grade → Surgical stabilisation

Feature

Isthmic Spondylolisthesis

Degenerative Spondylolisthesis

Primary Cause

Pars interarticularis defect (spondylolysis)

Facet joint + disc degeneration

Pathophysiology

Bilateral pars defect → loss of posterior stability → slip

Disc degeneration + facet arthropathy → segmental instability

Typical Age

Adolescence onset → symptomatic in young adults

Older adults (>50 years)

Sex Predilection

More common in males

More common in females

Common Level

L5–S1 (most common)

L4–L5 (most common)

Slip Direction

Anterior (anterolisthesis)

Anterior (usually low-grade)

Slip Severity

Can be high-grade

Usually low-grade (I–II)

Pars Defect

Present

Absent

Facet Joints

Normal or mildly affected

Degenerative, sagittal orientation

Disc Degeneration

Secondary (later stage)

Primary driver

Neurological Symptoms

Radiculopathy (foraminal stenosis)

Central canal stenosis + neurogenic claudication

Imaging Key

“Scotty dog” defect on oblique X-ray

Facet hypertrophy, disc collapse

Progression Risk

Higher in younger patients

Usually slow progression

Treatment Approach

Conservative → fusion if needed

Often decompression + fusion (if unstable)

Decompression Alone

Not preferred (instability risk)

Possible in selected stable cases

Clinical Pearls

  • Level difference is key:
    L5–S1 → think isthmic
    L4–L5 → think degenerative

  • Symptoms differ:
    Isthmic → radicular pain
    Degenerative → neurogenic claudication

  • Pars defect = diagnostic clue for isthmic

Pitfalls

  • Mixing up the two in exams (very common)

  • Assuming all spondylolisthesis behaves the same

  • Missing central stenosis in degenerative cases

  • Performing decompression alone in isthmic cases

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