Erlenmeyer flask deformity | Radiology Signs

Erlenmeyer flask deformity results from defective metaphyseal modeling, specifically undertubulation of long bones, causing flaring and lack of normal diaphyseal constriction. This abnormality is seen most commonly in the distal femora but may also involve proximal humeri, tibiae, distal ulnae, and radii. It is associated with several clinical conditions including osteopetrosis, Gaucher disease, thalassemia, sickle cell disease, craniometaphyseal dysplasia, fibrous dysplasia, Niemann-Pick disease, hereditary multiple exostoses, lead poisoning, and various skeletal dysplasias.

Why is it called so?

The deformity is named for its resemblance to the Erlenmeyer flask, a conical laboratory flask with a wide, flared base and narrow neck, designed by the German chemist Emil Erlenmeyer. The radiographic appearance shows a broad metaphyseal region flaring outward from a relatively narrow diaphysis, mimicking the flask’s shape.

Pathophysiology

The sign develops due to impaired or defective bone modeling at the meta-diaphyseal junction during bone growth, leading to failure of normal tubular constriction (undertubulation). Instead of a tapered diaphysis with a defined metaphyseal flare, the metaphysis remains broadened and flared with straight or convex lateral borders and thinning of the cortical bone. This results from altered osteoclastic resorption or abnormal remodeling depending on the underlying disease process.

Alternative names: Metaphyseal flaring

Other associated named signs: None specific to this sign