Many bones have epiphyses at both ends, others at one end only.
Long limb bones show the former, while the metacarpals, metatarsals, phalanges, clavicles and ribs have only one epiphysis, though the costal cartilages may represent epiphyses normally devoid of ossification centres.
Epiphyseal ossification is sometimes more complex; for example, the proximal end of the humerus, wholly cartilaginous at birth, develops three centres during childhood, which coalesce into a single mass before fusing with the diaphysis. Only one of these centres forms an articular surface, the others forming the greater and lesser tubercles giving muscular attachments. Similar composite epiphyses occur at the distal end of the humerus and in the femur, ribs and vertebrae.
Because some ossification centres appear in regions
1. exposed to articular pressure,
and others in regions
2. subject to muscular traction, a classification into pressure, traction and atavistic epiphyses was proposed by Parsons
The atavistic epiphyses being considered to represent skeletal elements separate at earlier evolutionary stages. Comparative morphology suggests that some mammalian bones are composites of separate reptilian or amphibian elements. The skull, clavicle, scapula and innominate bones are examples; a small centre in the human coracoid process and an epiphysis at the medial end of the clavicle may be vestiges of skeletal elements separate in earlier vertebrates, repeated during development as transient features in subsequent mammalian forms. However, the medial end of the clavicle could equally be regarded as a pressure epiphysis.