How are the osteochondrodysplasias classified?
By international consensus, these disorders are formally classified based on etiopathogenetic information concerning specific gene and/or protein deficits. It is useful and practical, however, to group these disorders according to where the most prominent abnormalities in growth occur. The mnemonic EMPD (“empty”) can help to broadly group these syndromes.
E — E piphyseal dysplasias: the epiphysis is at the end of tubular bone and is formed as a secondary site of ossification. Normal development of the epiphysis is required if the joint surface is to be normal.
M — M etaphyseal dysplasias: the metaphysis is the wider part of a tubular bone between the diaphysis and physis.
P — P hyseal dysplasias: the physis, or epiphyseal cartilage plate, separates the metaphysis from the epiphysis during growth. It is the primary site responsible for elongation of tubular bones.
D — D iaphyseal dysplasia: the diaphysis is the shaft of a long or tubular bone. It is composed of the spongiosa and cortex and is covered with periosteum.
These disorders can be further differentiated depending on whether the spine is involved: spondyloepiphyseal dysplasia (SED), spondylometaphyseal dysplasia, or spondyloepimetaphyseal dysplasia