Ossification of posterior longitudinal ligament

Overview

• Definition: Pathological calcification/ossification of the posterior longitudinal ligament (PLL) leading to spinal canal stenosis.
• Epidemiology:
  • Prevalence: High in East Asian populations (Japan/Thailand ~2–4%) vs Caucasian (<0.5%).
  • Gender: M > F (2:1).
  • Age: > 50 years.
• Associations:
  • DISH: Strongest association (~50% overlap).
  • Metabolic syndrome: DM, obesity (same as DISH).
  • Ankylosing spondylitis: Rare, but can co-exist.

Anatomical distribution

• Cervical spine: Most common site (C4–C6).
• Thoracic spine: Upper thoracic segments (often missed).
• Lumbar spine: Rare.

Clinical presentation

• Cervical myelopathy:
  • Slow, progressive compression of the cord.
  • Signs: Hyperreflexia, Hoffmann's sign, gait disturbance, numbness/dexterity loss in hands.
• Trauma risk:
  • Patients usually present acutely after minor trauma (e.g., a fall).
  • Mechanism: Stiff spine (DISH) + Narrow canal (OPLL) = Central cord syndrome or catastrophic cord injury.

Imaging features

Plain radiograph

• Lateral view:
  • Dense, linear, ossified strip along the posterior margins of vertebral bodies.
  • Often overlooked due to overlying shoulder artifact or prominent anterior osteophytes (DISH).
  • Pearl: A "double cortical line" at the posterior vertebral body margin.

CT (Gold standard)

• Essential for diagnosis, measuring canal diameter, and surgical planning.
• Classification (Tsuyama Classification):
  1. Continuous: Long flowing lesion over multiple vertebrae.
  2. Segmental: Patches behind each vertebral body (discontinuous).
  3. Mixed: Combination of continuous and segmental.
  4. Localised: Confined to the intervertebral disc level (circumscribed).

MRI

• Signal:
  • T1/T2: Hypointense (cortical bone).
  • Marrow signal: Large OPLL may have central high signal (fatty marrow).
• Cord Signal: Look for T2 high signal in the cord (myelomalacia/oedema) indicating compression.

The "Double layer sign" (surgical alert)

• Definition: An imaging sign on CT indicating dural ossification.
• Appearance:
  • Anterior rim: Ossified OPLL.
  • Lucent line: A thin layer of connective tissue between the OPLL and the dura.
  • Posterior rim: Ossified dura.
• Significance:
  • If the double layer is PRESENT:
    • It confirms dural ossification. The dura has become brittle bone.
    • Surgical risk: Extremely high risk of dural tear (CSF leak).
    • Why? The surgeon cannot "peel" the OPLL off the dura because the dura is the bone. Attempting to remove it anteriorly will rip the dura open.
    • Management: Surgeon usually switches to a posterior approach (laminoplasty) to avoid touching the anterior mass.
  • If the double layer is ABSENT:
    • Usually implies Classic OPLL without dural ossification.
    • Surgical risk: Standard risk. The dura is likely soft and pliable, allowing the surgeon to dissect the bony OPLL away from it via an anterior approach.

Management principles

• Conservative: If asymptomatic or mild (no myelopathy signs).
• Surgical: Indications = Myelopathy or severe stenosis.
  • Anterior approach (ACDF/Corpectomy): Direct removal of OPLL. Higher risk of dural tear.
  • Posterior approach (Laminoplasty/Laminectomy): Decompression by widening the canal (shifting the cord backwards away from the OPLL). Preferred for multi-level continuous OPLL (common in Asia).

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