It’s amazing the amount of punishment the horse’s knee — or “carpus” — can take, especially when you actually look at how it’s put together. The radius, the major bone of the upper arm, is balanced on two rows of small bones called the carpal bones, which are in turn perched on top of the horse’s cannon bone and the head of the medial (inside) splint. This arrangement allows the knee to flex freely, but it doesn’t look too stable.
Stability is achieved by multiple ligaments that connect the carpal bones to each other, as well as others that run from the radius to the carpal bones, the carpal bones to the cannon bone and bridging the entire joint running from the radius to the cannon bone. This assembly is further supported by the joint capsule of the carpus.
Speed, Twist, Repeat
Joint problems can occur that involve any of the tissues in the knee, including the joint capsule, synovial membrane, cartilage and any of the supporting ligaments.
Speed is the major enemy of the carpus. The knee is locked in an extended (straight) position when the horse’s weight is traveling over the top of it. As the body catapults forward at high speed, it can cause overextension that brings the cartilage and bone at the front of the knee in close contact, pinching the synovium, causing inflammation and eventually wear or even causing fractures.
Another high-risk time for overextension injuries occurs when landing after a jump and when going downhill.
Horses that make sharp turns at high speed (e.g. roping, barrel racing) or do upper-level lateral dressage work subject their knees to twisting forces that can cause inflammation of the supporting and connecting ligaments, which could result in arthritic changes.
Repetitive movements may also lead to strain and inflammation in the knee, just as typing all day or working on an assembly line can cause carpal-tunnel syndrome in people. Prolonged periods of trotting, especially on hard surfaces, can inflame the carpus.
A perfectly conformed knee takes enough of a beating as it is. If the horse’s conformation is not good, this only makes things worse.
• Offset Knees. The most common conformation fault involving the knee is offset knees, sometimes called bench knees. This means that the cannon bone isn’t sitting directly under the carpus when viewed from the front but is shifted to the outside.
Consequences: The head/top of the inside splint bone will be forced to do more weightbearing than normal. This predisposes the horse to inside splints and the inflammation may also involve the lower carpal bones in that area.
What to do: You can’t fix offset knees. Young horses with this conformation should be brought along slowly or you’re virtually guaranteed a splint will develop. If the horse does begin to develop a splint, do not overstress the leg with formal exercise until it’s well set up/calcified and showing no signs of inflammation. Once the splint has quieted, the area will be stabilized. Since many of these horses also toe out, it is important to trim the horse so that his foot is correctly aligned with the bones above.
• Over At The Knee. A horse with “buck knee” has knees that appears to bulge or buckle to the front when viewed from the side, rather than being flat and flush with the cannon bone. It can also be described as the cannon bone appearing to be set too far back under the knee.
Consequences: Although unsightly, this conformation usually doesn’t cause any knee problems per se. May indicate pain lower in the leg (horse voluntarily buckling forward) or that the horse had a painful condition as a foal. Horses severely over at the knee may tend to collapse on the forehand, e.g. at speed or landing over jumps.
What to do: No specific action needed.
• Back At The Knee. Also called “calf-kneed,” the horse’s leg appears to bow backwards at the knee, knee set back behind the front edge of the cannon bone.
Consequences: This is a serious conformation fault that exaggerates overextension and increases the risk of overextension injuries, including fracture. Horse may also be more prone to bowed tendons and suspensory injuries.
What to do: Keep shoeing simple, no grab effect, and foot carefully balanced. It’s important not to let the toe get too long as this will interfere with breakover. Shorter toe and rounded/beveled edge at the toe makes for easiest breakover.
• Flexion Tests: The horse’s knee should be held in the most flexed position you can obtain without the horse obviously objecting, i.e. leg up, with the cannon bone as close to the forearm as you can get it comfortably. Support the leg by a hand under the cannon bone. Don’t flex the ankle. Do not lift up on the leg so that the knee is held higher than the knee of the leg on the ground. Hold for 60 seconds, release and immediately trot off.
Advantages: Inexpensive, no trauma and will accentuate most lamenesses.
Disadvantages: False positives possible with older horses or stiff joints and if the test isn’t done correctly.
• Local Anesthesia: Injection of anesthetic around the knee’s nerves or directly into the joint.
Advantages: Can be done at home, minimal discomfort to the horse, helps confirm flexion test findings.
Disadvantages: Doesn’t tell you what’s wrong, just where the problem is. High suspensory or high-splint problems may also block out. Some risk of infection any time a joint is entered (minimal with correct technique).
• Radiographs (X-rays): Still a mainstay of lameness workups.
Advantages: Can be done at your barn, no trauma, good for picking up chips and arthritis that has advanced to the point of bone changes, some fractures.
Disadvantages: Doesn’t give any information on cartilage disease, inflammation or ligamentous strain, and fractures that are not wide or displaced may be missed
• Bone Scan: Injection of a radioactive tracer intravenously, which will then “light up” any areas of abnormal bone.
Advantages: Most useful when problem has not been unequivocally localized to the knee, or if exam/blocks point to knee but radiographs aren’t helpful. Helpful with fractures that are not displaced and not easily seen on X-ray.
Disadvantages: Primarily picks up areas of active bone breakdown/remodeling, although inflamed soft tissue may “light up” if scans are done within the first few minutes of injecting the dye. Must be done at a full-service clinic/hospital, and horse will have to stay overnight until level of radioactivity has dropped.
• Arthroscopy: Direct examination of the interior of the joint with a small fiberoptic scope.
Advantages: Only current widely available (hospitals) technique that allows for a 3-D examination of the joint, including cartilage, synovial lining and interconnecting ligaments. Also allows for any necessary treatment at the same time (e.g. removal of chips, smoothing of cartilage, removal of inflamed synovial membrane). Short stay. Horse may go home the same day or after just one overnight stay. Horses usually have a rapid recovery time.
Disadvantages: Most expensive. Requires general anesthesia. Some risk of infection any time a joint is entered/injected.
The horse’s knee is both a mechanical marvel and a disaster waiting to happen. Because of the great degree of movement required, it is stabilized to a great extent by ligaments.
Understanding the structure of the knee, how anatomy and work type influences the forces put on it, and how to recognize signs of a problem will help you keep your horse’s knees problem-free.