Osselets (Traumatic Arthritis of the Fetlock)

Osselets begin with swelling on the front of the fetlock joint, with the possible addition of synovial distensions on the sides of the joint (commonly called windpuffs). It’s painful when the horse flexes the joint, and can cause lameness.
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Osselets–it’s one of those obscure and shadowy terms out of the bowels of a Victorian veterinary manual, something you could lump in with mysterious and vaguely distasteful-sounding conditions called lampas, sweeny, or fistulous withers. We don’t hear these terms all that often today; many were problems suffered mostly by 19th-century workhorses with punishing schedules and ill-fitting tack, and some have since been redefined (or, in the case of lampas, are now understood to be perfectly normal, and not a pathology at all!). But osselets seem to have endured –even if many horse owners aren’t exactly sure what the term describes.

As set forth by today’s veterinary medicine, an osselet is a traumatic arthritis of the metacarpophalangeal joint (a.k.a. the fetlock) of the front leg–not to 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 distensions on the sides of the joint (commonly called windpuffs). It’s 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 joint become more and more irritated. The fibrous joint capsule starts to thicken (synovitis and capsulitis), and the periosteum–the sheath covering the bony surfaces–when pulled and stressed by the attached joint capsule and the end of the digital extensor tendon (which runs down the front of the cannon and helps extend the front legs) begins to trigger new bone growth, a common response to trauma. 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 (a bony protuberance), 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 (periostitis) and the articular cartilage at the ends of the 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

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Written by:

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She’s written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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