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A block vertebra is a congenital or acquired fusion of two or more adjacent vertebral bodies. It results from failure of normal segmentation of the somites during embryological development (typically around the 3rd to 8th week of gestation).

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Commonly involve the cervical spine (C2–C3, C5–C6)
Often asymptomatic but can sometimes cause stiffness, adjacent degeneration

Etiology

1. Congenital Block Vertebra

Due to failure of vertebral segmentation
May be:
- Isolated
- Part of syndromes (e.g., Klippel–Feil syndrome (KFS), Alagille syndrome, VACTERL association)

2. Acquired Block Vertebra

Due to:
- Infections (e.g., tuberculosis – “bony ankylosis”)
- Surgery (post-fusion)
- Inflammatory arthritis (e.g., ankylosing spondylitis)
- Trauma or degenerative fusion

🧬 Associated conditions:

Klippel–Feil syndrome: Triad of short neck, low posterior hairline and limited neck motion
Congenital scoliosis
Spinal canal stenosis
Tethered cord
Neurological symptoms in adulthood due to altered biomechanics

Radiologic Features

Imaging features:

Imaging Modality	Findings
X-ray	• Fused vertebral bodies with absent or narrowed intervertebral disc
• Reduced anteroposterior (AP) diameter at fusion site (hourglass or "wasp waist" deformity)
• Single, merged spinous process
• May show reduced vertebral height at involved levels
CT	• Confirms vertebral fusion with absence of normal cortical separation
• Evaluates canal diameter and any bony stenosis
• Useful to assess complex segmentation anomalies
• Detects associated vertebral malformations (e.g., hemivertebrae)
MRI	- Evaluates spinal cord

Detects disc signal, helps differentiate congenital vs acquired: - Congenital fusion = absent IV disc; acquired fusion = fused IV disc |