What causes H-shaped vertebrae on radiography and MRI of the spine?
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Answer:
H-shaped vertebrae, also called Lincoln log vertebrae, show a sharply delimited, squared-off central depression of both endplates with the anterior and posterior thirds of the vertebral body preserved at normal height, so the body takes the outline of a capital H. The step-off is abrupt rather than gradual, and this is the feature that separates it from the smooth, corner-to-corner biconcavity of the codfish vertebra of osteoporosis or osteomalacia. It is a hallmark of sickle cell disease, present in roughly 10% of patients, and is reported less often in Gaucher disease and thalassaemia. MRI shows the central endplate depression together with heterogeneous marrow signal from chronic red marrow hyperplasia and, in the acute setting, marrow oedema at the affected endplates. Its presence in an unexplained anaemia is a strong pointer to a haemoglobinopathy, and it should prompt a search for the other skeletal manifestations of sickle cell disease.
Why is it called so?
The preserved anterior and posterior segments with a depressed centre reproduce the shape of the letter H, or of the notched logs of a toy Lincoln Log set.
Pathophysiology
The central portion of each vertebral endplate is supplied by a discrete set of small nutrient arterioles with little collateral. In sickle cell disease, vaso-occlusion by rigid sickled erythrocytes infarcts precisely this central subchondral zone, while the peripheral endplate, supplied by the richer perianular circulation, survives. Growth then continues normally at the preserved periphery and ceases centrally, so the healed vertebra ends up with a squared central depression bounded by abrupt shoulders. The sharply angular geometry reflects the discrete vascular territory involved, in contrast to osteoporosis, where uniform structural weakness lets the whole endplate bow smoothly under axial load.
Alternative names: Lincoln log vertebrae, step-off vertebrae, fish vertebrae (imprecise, see codfish vertebra)
Other associated named signs: Codfish vertebra, bone-within-a-bone, hair-on-end sign, and the dactylitis of hand-foot syndrome
References
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