Joint Lubrication and Injury Response (AAEP 2011)
A horse's athletic success depends on the health of his joints, and veterinarians are continually studying up on how best to maintain athletic joints and manage injury. During a presentation at the 2011 American Association of Equine Practitioners convention, held Nov. 18-22 in San Antonio, Texas, Larry Bramlage DVM, MS, Dipl. ACVS, an equine surgeon at Rood & Riddle Equine Hospital, in Lexington, Ky., gave veterinarians an "introduction to joint therapy." He described the lubrication mechanisms within the joint and the natural responses of the joint to injury.
Bramlage pointed out that when a joint is damaged, veterinarians see typical and consistent signs: a lame horse and a distended joint with increased amount of watery joint fluid. While the human impulse is to "fix" these types of signs, Bramlage pointed out that these very clinical signs are "a superficial part of the joint's response to insult."
The interior of the joint consists of the articular cartilage, which covers the bone ends, and the synovium that lines the inside of the joint capsule. Both cartilage and synovium are bathed in joint fluid, but they are lubricated by different means and have different responses to insult.
Bramlage referenced a study evaluating pain within the joint in which examiners found that while pulling on the synovium itself was excruciating, cartilage has no nerve endings and cartilage stimulation is not painful.
Hyaluronic acid (HA) lubricates the synovium, while water lubricates the cartilage through a sort of "sponge" mechanism. Weight-bearing compresses cartilage and fluid is forced out, preventing friction. Proteoglycan in the cartilage matrix reinflates it, and more proteoglycan means better inflation. Insults to the joint can exhaust proteoglycan supplies over time. When proteoglycan can no longer restore the water in the cartilage matrix, the lack of lubrication produces wear and tear. "Cartilage degrades silently and with very little pain," says Bramlage.
If damage can occur within the joint in the absence of pain, can joint pain ever be good? The answer is yes because it protects the joint. Since synovial lining lubrication plays such a large role in pain sensation, Bramlage went on to point out that under-lubricated synovium and its accompanying "drag" causes pain with joint motion and is the reason flexion tests work.
Loss of lubricating HA allows an influx of protein and fibrin into the joint, he noted. Fibrin collects debris and sticks it to the synovial villi (carpetlike projections) to prevent it from circulating in the joint. Then the synovium must eliminate it.
However, the debris removal process and its associated inflammation causes the villi to stick together and lose their capability to collect additional debris. If this situation goes on too long, the joint will eventually exhaust the villi, and lose its ability to produce HA. The result will be chronic effusion, or joint fluid buildup.
Bramlage points out the problem with a damaged joint: It is "difficult to improve upon the joint's normal response (to insult), but that normal response might not allow performance. Treatment then must aid in performance without trying to eliminate the joint's natural response to injury."
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.
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