An osselet is a term for traumatic arthritis of the fetlock joint (metacarpophalangeal joint) on the front leg of a horse. This condition should not be confused with sesamoiditis, which involves the sesamoid bones that are found at the back of the fetlock.
Osselets will begin with the swelling on the front of the fetlock joint, with the possible addition of synovial distentions on the sides of the joint (windpuffs). They can be very painful when the horse flexes the joint and can cause lameness. Because osselets often seem to occur in both front ankles at once, the lameness might manifest itself as a short and choppy gait, with no one leg visibly more off than the other. If only one fetlock is involved, the horse will point the affected leg to avoid bearing weight on it.
As the condition worsens, so does the horse’s lameness, and the interior structures of the joints become increasingly irritated. The fibrous joint capsule will start to thicken (synovitis and capsulitis). New bone growth, which is a common sign of trauma, is triggered when the periosteum (the sheath that covers the bony surfaces) is pulled and stressed by the attached joint capsule and the end of the digital extensor tendon. Extra bone will begin to form on the lower end of the large metacarpal bone (cannon bone) and the high end of the first phalanx (long pastern bone), which meet at the fetlock joint. This bony growth technically is called an exostosis and it can easily be palpated through the skin. As this progresses, it can limit the amount of flexion in the fetlock joint.
If this is left unchecked, the formation of osselets will damage the periosteum and the articular cartilage at the ends of the bones. In some cases, the point of attachment of the digital extensor tendon may become degraded, leading to chronic lameness. The joint capsule will become noticeably thickened, the ulcerated cartilage will lead to recurring inflammation and the cycle of irritation will eventually compromise the horse’s athletic potential along with his ability to move soundly and fluidly.
In many cases, if they are treated promptly and in their early stages of development, osselets will resolve to the point that the horse can resume normal work. However – the prognosis will be less optimistic once bony changes begin to accumulate in the fetlock joints and encroach on the articular surfaces. Treatments include s of polysulfated glycosaminoglycans (Pentosan and Pentosan Gold) or sodium hyaluronate (Joint-50 Hyaluronate Sodium).