fisis

Fisis, or physis, is the cartilaginous growth region at the end of a long bone where longitudinal growth occurs. It sits between the epiphysis (the bone end) and the metaphysis (the neck of the bone) and serves as the primary site of bone elongation during childhood and adolescence. In some languages the term is rendered as fisis.

The physis is organized into zones of reserve cartilage, proliferative cartilage, hypertrophic cartilage, and calcified cartilage, which are progressively replaced by bone through endochondral ossification. Chondrocytes divide and align in columns, mature, and are converted to bone by osteoblasts, allowing the bone to lengthen.

Growth is controlled by systemic hormones such as growth hormone and thyroid hormone, by sex steroids during puberty, and by local factors including insulin-like growth factors. Mechanical loading and nutrition also influence physis activity. Physes are present in most long bones, including the femur, tibia, and humerus.

Injury to the physis is a common concern in pediatric orthopedics. Physeal fractures can disturb normal growth and are classified by the Salter-Harris system (types I–V), ranging from simple physeal separations to injuries involving the metaphysis or epiphysis and potentially causing growth disturbance.

As growth ceases in late adolescence, the physis closes and is replaced by a bony epiphyseal line. On radiographs, the epiphyseal line marks completed fusion; before closure, the physis appears as a semi-radiolucent growth plate.