The Erlenmeyer flask bone deformity in the skeletal dysplasias.

Erlenmeyer flask bone deformity (EFD) is a long-standing term used to describe a specific abnormality of the distal femora. The deformity consists of lack of modeling of the di-metaphysis with abnormal cortical thinning and lack of the concave di-metaphyseal curve resulting in an Erlenmeyer flask-like appearance. Utilizing a literature review and cohort study of 12 disorders we found 20 distinct disorders were associated with EFD. We interrogated the International Skeletal Dysplasia Registry (ISDR) radiographic database (1988-2007) to determine which skeletal dysplasias or syndromes were highly associated with EFD, whether it was a uniform finding in these disorders, and if forms of EFD could be differentiated. EFD was classified into three groups. The first catogory was the typical EFD shaped bone (EFD-T) resultant from absent normal di-metaphyseal modeling with relatively normal appearing radiographic trabecular bone. EFD-T was identified in: frontometaphyseal dysplasia, craniometaphyseal dysplasia, craniodiaphyseal dysplasia, diaphyseal dysplasia-Engelmann type, metaphyseal dysplasia-Pyle type, Melnick-Needles osteodysplasty, and otopalatodigital syndrome type I. The second group was the atypical type (EFD-A) due to absence of normal di-metaphyseal modeling with abnormal radiographic appearance of trabecular bone and was seen in dysosteosclerosis and osteopetrosis. The third group was EFD-marrow expansion type (EFD-ME) in which bone marrow hyperplasia or infiltration leads to abnormal modeling (e.g., Gaucher disease). Further, radiographic review determined that it was not always a consistent finding and that there was variability in both appearance and location within the skeleton. This analysis and classification aided in differentiating disorders with the finding of EFD.