Block Vertebra¶
Definition¶
A block vertebra (congenital vertebral fusion) results from failure of segmentation of two or more adjacent vertebrae during embryological development. The fused vertebrae function as a single unit. Block vertebrae can occur at any level but are most common in the cervical spine, where they are a feature of Klippel-Feil syndrome.
Imaging Findings¶
Radiography/CT¶
- Fusion of two or more adjacent vertebral bodies, with absence or rudimentary remnant of the intervening disc
- Wasp-waist sign — Narrowing of the AP diameter at the level of the vestigial disc space, giving the fused block a waist-like appearance
- The fused segment has a smooth anterior contour without osteophytes (unlike acquired/degenerative fusion)
- Posterior elements (spinous processes, laminae) may also be fused
- The total height of the fused block is approximately equal to the expected combined height of the individual vertebrae
Distinguishing Congenital from Acquired Fusion¶
| Feature | Congenital Block Vertebra | Acquired Fusion (Degenerative/Surgical) |
|---|---|---|
| Disc remnant | Rudimentary or absent, smooth | Irregular, calcified disc material |
| Wasp-waist | Present | Absent |
| Osteophytes | Absent | Present |
| Anterior contour | Smooth, continuous | Irregular |
| Posterior elements | May be fused | Usually not fused (unless surgical) |
Clinical Pearl
The wasp-waist sign — smooth narrowing at the vestigial disc level — is the key feature distinguishing a congenital block vertebra from acquired fusion. A congenital block has smooth contours without osteophytes, while a degenerative fusion shows irregular disc calcification and marginal osteophytes.
Key Points¶
- Congenital fusion from failure of segmentation — most common in the cervical spine
- Wasp-waist sign distinguishes congenital from acquired fusion
- Multiple block vertebrae in the cervical spine = Klippel-Feil syndrome
- Adjacent unfused segments are at risk for premature degeneration
- Usually incidental and asymptomatic
References¶
- Block vertebra. Radiopaedia.org. https://radiopaedia.org/articles/block-vertebra
- Menger RP, Rayi A, Notarianni C. Klippel Feil Syndrome. In: StatPearls. Treasure Island (FL): StatPearls Publishing; updated 2024 May 11. https://www.ncbi.nlm.nih.gov/books/NBK493157/
- Aboughalia H, Noda S, Chapman T, Revzin MV, Deutsch GH, Browd SR, Katz DS, Moshiri M. Multimodality Imaging Evaluation of Fetal Spine Anomalies with Postnatal Correlation. RadioGraphics. 2021;41(7):2176-2192. https://pubmed.ncbi.nlm.nih.gov/34723699/
- Chaturvedi A, Klionsky NB, Nadarajah U, Chaturvedi A, Meyers SP. Malformed vertebrae: a clinical and imaging review. Insights Imaging. 2018;9(3):343-355. https://pmc.ncbi.nlm.nih.gov/articles/PMC5991006/
- Rufener SL, Ibrahim M, Raybaud CA, Parmar HA. Congenital spine and spinal cord malformations—pictorial review. AJR Am J Roentgenol. 2010;194(3 Suppl):S26-S37. https://pubmed.ncbi.nlm.nih.gov/20173174/
- Frikha R. Klippel-Feil syndrome: a review of the literature. Clin Dysmorphol. 2020;29(1):35-37. https://pubmed.ncbi.nlm.nih.gov/31577545/