Tuesday, February 2, 2010

Tuesday, February 7: The Fetlock and Metacarpus

Vet tip of the Day:

First a quick review: we've looked at the anatomy of the bones in the pastern and foot and the attachment of the deep flexor tendon onto the back of the second pastern bone and coffin bone. We've discussed how irregularities in the navicular bone can contribute to heel pain by irritating the flexor tendon and navicular bursa as the tendon slides up and down against the back of the navicular bone. And we've reviewed ring bone, an arthritic condition that occurs at the P1-P2 and P2-P3 joints.

Today we will move on to the bones and major soft tissue structures between the knee (carpus) and the pastern. The cannon bone, with a much smaller splint bone on either side, spans this distance. The bottom of the cannon bone rests on the top of P1, the long pastern bone, and forms the fetlock joint. The cannon bone and the splint bones are all metacarpal bones, which correspond to the bones in your hand between your wrist and your knuckles. At the level of the fetlock joint, just behind the cannon bone, lie the sesamoid bones. The suspensory ligament, which arises from the back of the knee, lies along the back of the cannon bone and splits just above the sesamoids, with one branch attaching to the top of each sesamoid bone. The deep and superficial flexor tendons originate behind the knee and attach at the level of the pastern. The deep flexor tendon attaches to the back of P1 and P2 (the long and short pastern bones), and the superficial tendon to P2 and P3 (the short pastern and coffin bones).

The distal, or lower cannon bone, suspensory branches and fetlock joint are like an amazing suspension bridge which is critical in supporting the weight of the horse during exercise. They are very common sites of injury in athletic horses. You may have seen pictures of the x-rays of Barbaro, whose catastrophic injury involved not only a fracture of the end of the cannon bone, but also his sesamoid bones, suspensory ligament and pastern. In our less extreme athletes, small strains in the suspensory branches, chip fractures in the fetlock, and inflammation of the sesamoid bones are all relatively common causes of lameness.

Pick up your horse's right front leg and hold the foot in your left hand. Place your right thumb on the inside, and your index finger on the outside, of the cannon bone mid-way between the knee and the fetlock. Squeeze firmly and let your fingers slide over the structures moving toward the back of the leg. The suspensory ligament is difficult to feel because it is very firm and lies closely attached to the back of the cannon bone. However, it is easy to feel the deep and superficial digital extensor tendons which lie just under the skin. Learn to distinguish between them, and then move your hand up and down the leg, feeling the length of each tendon from knee to fetlock. Just above the fetlock, on the inside and outside, it is possible to clearly palpate the branches of the suspensory ligament where they attach to the top of the sesamoid bones. Your hands are very sensitive indicators of changes in the soft tissue structures of the leg that can alert you to the presence of injuries.


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