Dorsiflexion and Carpal Damage

Last time we talked about carpal arthrosis, pointing out that too much bowing--dorsiflexion--of the foreleg at the knee was the immediate cause of damage to the articular cartilage. Further, with a bit of mechanics, we saw that too much dorsiflexion at the knee occurred because the resisting moment (turning force) generated by the muscles and tendons on the back of the forearm and carpus was not enough to balance the turning force generated by the body weight acting on the foreleg.

While the next, obvious question is why the resisting moment is too small, let's hold that for a moment. It has long been well-known to horsemen that the horses with the conformation back-on-the knee are not a good risk because of the almost inevitable development of carpal arthrosis. The reason for that now becomes crystal clear. If too much dorsiflexion is the cause of carpal arthrosis, as it is, the horse which starts with the foreleg in this position is predisposed to the damage. The foreleg is already dorsiflexed, ready to be damaged. Looking at that in terms of the equation for equilibrium of moments, with the leg already dorsiflexed the value of 1 is greater than with the normal leg. That is, the moment tending to cause dorsiflexion, F1, is greater to start with in the horse which is back-on-the-knee.

Let's return now to the question of why the resisting moment is too small. There are two, intimately related answers: the moment, F1 becomes too large and/or the moment, Tp, becomes too small. The moment, Tp, becomes too small because the muscles and tendons on the back of the foreleg (for those who would like to know: the deep flexor, superficial flexor, ulnaris lateralis, and flexor carpi ulnaris) become fatigued. After the horse has been galloping along at a high rate of speed for some time, the muscles (and tendons) begin to tire, no different than you or me running. The tiring of the tissues means that the value of T, the muscle/tendon force, decreases. F1 does not decrease, of course, so the equilibrium of moments at the carpus is lost, and too much dorsiflexion of the knee occurs.

Why does this fatigue occur so commonly in racing horses? First, we add weight, whether as a rider or driver, thereby increasing the force, F. Obviously, that means that more T force must be generated by the muscle/tendons, and that additional work means fatigue will occur earlier. You know that even more weight is added to saddle horses, event horses, trail rides horses, etc. without carpal arthrosis being a problem in such animals. Therefore, we must add a second factor, and that is carrying weight at a high rate of speed. The faster one goes, the sooner one tires.

An interesting corollary to this is the horse which experiences a so-called slab fracture of the central carpal bone. This can occur once damage has already been done to the articular cartilage, weakening the bone. It also can occur, though less commonly, in a perfectly normal bone if the horse makes a grievous misstep such as that shown in Figure 3. Here, the foot has come to the ground far ahead of its normal impact position (the animal might do this, for example, if bumped, putting the foot down too soonwhile trying to keep from falling). Clearly, the value of 1 has increased remarkably, and the moment, F1, is too great for Tp to deal with. So, sudden, marked dorsiflexion occurs with fracture of the central carpal bone
the result.

The word fatigue is fraught. While we all think we know what we mean by it, it is a complex and difficult physiological and biochemical penomenon, and I shall not pretend to expertise. What is clear, however, is that fatigue is a major factor in many kinds of lameness (not all) and that most training of horses for high-speed work is not adequate for preventing or, at least, delaying onset of fatigue. I do not offer a solution or solutions to that problem, only say that one of the major goals of the race training regimen must be reduction of fatigue for the distances and speeds to be run.

Finally, the frequency of carpal arthrosis can be looked at a bit more closely. In clinical surveys of Thoroughbred racehorses, carpal arthrosis was the third most common cause of lameness. In an old survey of Prussian cavalry horses, on the other hand, it was the seventh most common cause, clearly reflecting the relationship of high speed to carpal arthrosis. The clinical survey is somewhat misleading; at postmortem examination, the frequency of damage to the articular cartilage of the carpus in Thoroughbreds which have raced approaches very nearly 100%. The frequency is somewhat lower in Standardbred racehores, but is still more than 75%.

About the Author

James R. Rooney, DVM

The late James R. Rooney, DVM, was Professor Emeritus of the Gluck Equine Research Center, Department of Veterinary Science, at the University of Kentucky. Rooney was a 1949 graduate of Dartmouth College with a bachelor's degree in English drama; a 1952 graduate of New York State Veterinary College at Cornell University; and a Diplomate, Emeritus, of the American College of Veterinary Pathologists. Rooney authored more than 100 articles and books on diseases and locomotion of horses, including: Biomechanics of Lameness in Horses, The Lame Horse, Clinical Neurology of the Horse, Autopsy of the Horse, and Mechanics of the Horse.

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