> Damage to the bars
Damage to the bars of the mouth where the bit sits, is normally caused by hard impacts from the bit, nearly always a curb bit is responsible. Leverage from the curb bit and chain sometimes put enormous pressure on this thin layer of soft tissue covering bone. Injuries to the bars cause swelling of the soft tissue, bone to die off, or boney fragments appearing through the soft tissue.
Injuries to the bars are extremely painful for most horses, they will display discomfort and will resist going on the bit. Unfortunately, time off to let the bars heal is the only course of treatment for your horse. In time your horse will become more comfortable again, though if damage has ocurred to the bone these injuries never fully recover.