An osselet is a traumatic arthritis of the metacarpophalangeal joint (fetlock) of the equine front leg, writes Karen Briggs in the September edition of The Horse. The condition should not be confused with sesamoiditis, which involves the sesamoid bones found at the back of the fetlock.

Osselets begin with swelling on the front of the fetlock joint, with the possible addition of synovial distentions on the sides of the joint (commonly known as windpuffs). They are painful when the horse flexes the joint, and can cause lameness. Because osselets often brew in both front ankles at once, the lameness might manifest itself as a short, choppy gait, with no one leg visibly more sore than the other. If only one fetlock is involved, the horse will "point" the affected leg and try to avoid bearing weight on it. At that stage, the condition is sometimes called "green osselets."

As the condition worsens, so does the horse's lameness, and the interior structures of the joints become more and more irritated. The fibrous joint capsule starts to thicken (synovitis and capsulitis). New bone growth, a common sign of trauma, is triggered when the periosteum (the sheath covering the bony surfaces) is pulled and stressed by the attached joint capsule and the end of the digital extensor tendon. Extra bone begins to form on the lower end of the third 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 it progresses, it can limit the amount of flexion in the fetlock joint.

Left unchecked, the formation of osselets damages the periosteum and the articular cartilage at the ends of bones. In some cases, the point of attachment of the digital extensor tendon can become degraded, leading to chronic lameness. The joint capsule becomes noticeably thickened, the ulcerated cartilage leads to recurring inflammation, and the cycle of irritation eventually compromises the horse's athletic potential and his ability to move soundly and fluidly.

In many cases, if treated promptly in their early stages of development, osselets resolve to the point that the horse can resume normal work. However, the prognosis is considerably poorer once bony changes begin to accumulate in the fetlock joints and encroach on the articular surfaces. Treatments include injections of polysulfated glycosaminoglycans (Adequan) or sodium hyaluronate.