My 14-year old son has been diagnosed with proximal humeral epiphysiolysis. Can you explain what this is?

In layperson's terms, you're talking about Little League shoulder. Some call this a stress fracture of the growth plate at the top of the upper arm. It occurs when a large rotational force or load is placed on the bone during the act of throwing.

Damage of this type to the growth plate occurs with repeated trauma. A local inflammatory reaction is set up in the area of the growth plate. Eventually a fatigue fracture occurs.

Doctors prefer to use a term that describes the lesion. In this case proximal refers to the top of the humerus. The humerus is the upper arm. The growth plate is called the epiphysis. Separation of the epiphysis from the bone is an epiphysiolytic fracture or epiphysiolysis.