Block vertebrae are a congenital vertebral anomaly where two or more adjacent vertebrae fail to separate.
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Clinical presentation
Block vertebrae are often considered an incidental anatomic variant. However, adjacent segment degeneration is thought to occur secondary to altered biomechanics leading to accelerated disc degeneration, and patients may present with symptoms related to spinal canal and/or foraminal stenoses 1,4,5.
Pathology
Congenital block vertebrae are most common in the cervical spine 4.
Etiology
Block vertebrae are a congenital condition caused by failed somite segmentation, which results in the abnormal fusion of adjacent vertebral bodies and/or vertebral arches 6. Vertebral body fusion can be partial or complete, depending on the amount of rudimentary fibrous tissue after failed segmentation 4.
Associations
Klippel-Feil syndrome: strong association 4
adjacent segment degeneration of the most immediate caudal segment 1,4,5
Radiographic features
block vertebrae are usually the same height as two vertebral bodies and one intervertebral disc although the AP diameter is smaller 4
intervertebral disc space is rudimentary (disc remant or scar present) or absent with an undisrupted trabecular pattern between the two vertebrae 4
wasp-waist sign: biconcave shape at the site of fusion 3,4
Differential diagnosis
acquired vertebral fusion: can usually be easily differentiated from congenital vertebral fusion based on imaging findings 4