The epiphysis (/ɛˈpɪfəsəs/) is the rounded end of a long bone, at its joint with adjacent bone(s). Between the epiphysis and diaphysis (the long midsection of the long bone) lies the metaphysis, including the epiphyseal plate (growth plate). At the joint, the epiphysis is covered with articular cartilage; below that covering is a zone similar to the epiphyseal plate, known as subchondral bone (see Wiktionary:subchondral).
The epiphysis is filled with red bone marrow, which produces erythrocytes (red blood cells).
There are four types of epiphysis:
- Pressure Epiphysis: The region of the long bone that forms the joint is called Pressure Epiphysis. For example the head of femur which is a part of the hip joint complex is a pressure Epiphyses. These Epiphyses assists in transmitting the weight of the human body and are the regions of the bone which is under pressure during movement, or locomotion hence they are named Pressure Epiphyses. Another example of Pressure Epiphysis is the head of humerus which is a part of the shoulder complex.
- Traction epiphyses: The regions of the long bone which are non-articular, that is, not involved in the joint formation is called as Traction Epiphysis. Unlike Pressure Epiphysis, these regions do not assist in weight transmission. However, their proximity to the Pressure Epiphysis region means that the supporting ligaments and Tendons attach to these areas of the bone. Traction epiphyses ossify later than the pressure epiphyses. Examples of these epiphyses are tubercles of humerus (greater tubercle and lesser tubercle), Trochanters of the Femur (both greater and Lesser) etc.
- Atavistic epiphyses: As Homo Sapiens evolved from being four legged to two legged, their lower limbs became stronger and hands became free from being actively involved in locomotion. This fused certain bones together due to the change in functionality over generations. These type of fused bones are called as Atavistic. Example of this is the coracoid process of scapula which has been fused in humans but separate in four legged animals.
- Aberrant epiphyses: These Epiphysis are deviations from the norm and are not always present. For example the epiphyses at the head of the first metacarpal bone.
Pathologies of the epiphysis include avascular necrosis and osteochondritis dissecans (OCD). OCD involves the subchondral bone.
Epiphyseal lesions include chondroblastoma and giant-cell tumor.
Longitudinal section of head of left humerus.