Location

Osselets usually occurs in the front legs of the horse, because there is more strain and concussion on the fetlock there than in the hind legs. The arthritis will occur at the joint between the cannon bone and large pastern bone

Pastern

The pastern is a part of the leg of a horse between the fetlock and the top of the hoof. It incorporates the long pastern bone and the short pastern bone , which are held together by two sets of paired ligaments to form the pastern joint...

, at the front of the fetlock.

Causes and Progression

Osselets is caused by stress on the fetlock, which results in a stretching of the joint capsule. The early stage is called green osselets, and is characterized by a hot, soft swelling at the fetlock. If inflammation damages the cartilage

Cartilage

Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

 of the joint, the swelling may become chronic and permanent. The joint capsule may also thicken.

Eventually, the bones of the joint will become involved, causing arthritis, pain, stiffness

Lameness (equine)

Lameness in horses and other equidae is a term used to refer to any number of conditions where the animal fails to travel in a regular and sound manner on all four feet...

, and periostitis

Periostitis

Periostitis, also known as periostalgia, is a medical condition caused by inflammation of the periosteum, a layer of connective tissue that surrounds bone...

 (new bone growth). The fibers of the joint capsule will also increase in size. The long pastern bone may also eventually chip at its front edge, which will leave bone fragments in the joint.

Conformation that encourages osselets to occur include short, upright pasterns

Pastern

The pastern is a part of the leg of a horse between the fetlock and the top of the hoof. It incorporates the long pastern bone and the short pastern bone , which are held together by two sets of paired ligaments to form the pastern joint...

, because this conformation promotes concussion.