Definition of Hair-on-end skull

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Hair-on-end skull: Thin fine linear extensions radiating out from the skull that look on an X-ray like hair standing "on-end" from the skull, an appearance associated with hemolytic anemias such as sickle cell disease and thalassemia.

The "hair" represents the accentuated trabeculae extending between the inner and outer skull tables through the diploe in the expanded bone marrow space (because the bone marrow has expanded due to the excessive breakdown of red blood cells). The "hair" appears to be "on end" because the trabeculae are oriented perpendicular to the inner and outer tables of the skull.