The physis (growth plate) is the location toward the end of the long bones of the limbs in which lengthening takes place. At the growth plate, there is laying down of a cartilage model that lengthens the bone. This template is turned to bone (the process is called endochondral ossification).

In normal growing horses, there is mild flaring of this area associated with this lengthening process. Depending upon the bone in question, the growth plate stops lengthening and turns to solid bone (closure of the growth plate) at a variable age. The shorter, lower bones tend to "close" earlier in life, the cannon bone lower growth plate above the fetlock closes at 4-6 months of age, whereas the growth plate at the end of the radius and tibia close around 2 years of age.

Abnormal growth of the physis is termed physitis or epiphysitis. In horses, it usually results from nutritional imbalances and excesses, notably protein and overall energy of the diet. Another factors can be either too much exercise or too little.

Overload of the physis also causes disruption of growth and epiphysitis. An example of this is physitis in the supporting limb in a young horse with a severe lameness condition on the opposite limb. Angular limbs cause overload of one side of the physis, causing further retardation of growth of that side. Direct trauma can interrupt physeal growth too.

The net appearance of these factors is swelling of the physis near the joint and often angular limb or flexural deformity. In some cases, there is lameness.

Change in management and feeding (reduced total calories and protein), and supplementation of minerals, and a variety of surgical procedures are used in foals with this combination of conditions.