When a saddle is crooked, or has uneven stuffing, one of the thoracic vertebrae can develop a subluxation. (See post of February 1st for a definition.) This in turn can cause a rib to rotate. You will see the rib sticking out on one side, and the corresponding one on the other side will look sunken. Other events that cause this is a rider landing on the horse’s back a bit too hard, or if the horse is bent on the landing of a jump. A kick to the ribcage in pasture can also cause subluxation.
As a result of a rib dislocation, the horse will canter with haunches in or out, depending on which lead you are on. The horse will also have difficulty trotting on a circle.
There is an excellent myofascial release technique to correct this condition which usually takes about 5 minutes.