This can occur because of improper positioning of the leg in relation to the weight of the horse's body, such as when a horse clears a jump and lands wrong. Any unbalanced loading of weight on a leg, such as uneven footing, poor conformation or improper shoeing, can harm a tendon. When loaded or stretched beyond its capacity, some of the fibers that compose the tendon tear. Since a tendon is one large structure composed of many smaller fibers, a tear consists of individual fibers in one area pulling apart from others in the same area. This initial damage results in fluid buildup within the tendon itself, the result of leakage from damaged cells.
Blood is also possibly present as it leaks from damaged vessels.
These accumulations of fluid or blood appear as holes in the tendon when viewed with ultrasound and are called core lesions. Soon after the initial trauma the body begins the difficult task of healing. Fibroblasts, which are specific cells from the connective tissue in tendons, begin to produce new cells to repair the damage. These fibroblasts go through a series of changes to become tenocytes or tendon cells. Normally these tenocytes are separate bundles of cells throughout the tendon and the connective tissue between these bundles keeps the entire structure flexible and elastic.