The forearm muscles are contained within three fibrous sheaths, unyielding as if they are leather. When there is a fracture of one or both of the forearm bones, the radius and/or ulna, there will be bleeding within that location. However, that bleeding is contained within the fibrous sheath, and will put pressure against the adjacent blood vessels, the muscles and nerves. If the bleeding is extensive, this creates an internal tourniquet-like effect, and unless relieved timely, this will result in irreversible gangrene of the muscles and nerves. These muscles and nerves control function of the hand.
If there is loss of motion and/or numbness in the hand, the first concern is that of a tight cast wherein the swelling from the fracture against the inside of the cast first appears to be caused by too tight a cast. The standard of care requires that both the cast and the padding beneath be severed longitudinally, and then the patient observed. If the problem was just from the cast, the hand will regain sensation and motion. Also by pinching the fingers, the capillary blood flow will be seen to have immediately resumed.
However, if that does not rapidly resolve the problem, then compartment syndrome is the correct diagnosis to be immediately treated. The the skin and the underlying fibrous sheath involved with that compartment is severed longitudinally to relieve the compression, this internal tourniquet-like effect. The failure to properly make this diagnosis and treat with urgent surgery is a departure from the accepted standards of care.