Hyaluronan Essential to Healthy Horse Joints
It's easy to take healthy equine joints for granted. Thousands of times each day, week after week, year after year, the mobile joints in a horse's legs bend and flex, bearing his weight and enduring punishing shocks--all without attracting undue attention. After a hard workout, the muscles may be tired but not the joints. With a bit of rest, the horse is galloping around the paddock, good as new.
All of which is especially amazing when you consider that trouble-free joint function hinges on two seemingly mundane substances: water--ordinary H2O--and hyaluronan (HA), one of the simplest, but also most essential, molecules in the animal kingdom. Together, HA and water are the primary components of synovial fluid, the slippery lubricant that fills the spaces between the bones at the joints and enables them to flex without friction.
HA also has an anti-inflammatory effect that minimizes the potentially debilitating consequences of everyday wear and tear.
Keeping the synovial HA healthy--even replenishing it when necessary--is at the heart of maintaining equine soundness. For 30 years HA injections have been used to relieve pain and inflammation in equine and human joints.
Today, HA (usually labeled as sodium hyaluronate or hyaluronic acid) is also an ingredient in a number of over-the-counter oral preparations intended to sustain joint health and function. Researchers are still uncovering many of the secrets of how supplemental HA works and how it is best administered, but much already is known about this vital substance and how it keeps our horses on the move.
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Simple but Versatile
Chemically, hyaluronan is a rather simple substance. Each molecule consists of only two sugars strung together in a long chain of alternating pairs, as many as 10,000 units long. The oxygen bonds that link these chains are powerful, giving the molecule the solidity it needs to help form the foundation of strong bodily structures, such as bone, cartilage, skin and blood vessel walls.
The versatile nature of the oxygen bonds also allows the chain to be near-infinitely flexible, giving the molecule the elasticity it needs to twist and bend without losing cohesion. With it, skin can stretch, ribs can expand and contract and blood vessels can pulse for a lifetime.
Another property that makes hyaluronan indispensable to life as we know it is that it is extremely hydrophilic--it attracts and holds onto water molecules--and, when the molecule takes on certain configurations, there are regions along the chain that are hydrophobic, repelling water.
As a result, when pure HA is placed in a solution, the hydrophobic patches tend to be attracted only to each other, so the long molecules double back on themselves and stick together to form a three-dimensional mesh. Filling the spaces within the mesh are the water molecules, trapped in place by their attraction to and repulsion from different segments of HA. The result is a thick, viscous fluid where water shifts and slides but cannot flow freely. This interaction between HA and water (and other assorted molecules) is vital to functions throughout the body.
Through Thick and Thin
The horse's key locomotor joints--including the hocks, knees and ankles--are formed by bones that do not interlock but instead stack on top of each other like building blocks, connected only by flexible, fibrous tissue. Called synovial joints for their ability to flex freely, these mobile structures consist of two bone ends with a protective covering of articular cartilage that meet face-to-face.
They are connected on the sides by collateral ligaments to prevent lateral movements of the joint, and enclosed in a fibrous covering called the joint capsule that stabilizes the structure and retains the fluids that protect the inner surfaces. At the center of all of this is the synovia, the sticky yellow fluid secreted by the synovial membrane lining the interior of the joint capsule.
The synovial fluid is as crucial to a healthy joint as motor oil is to a car's engine. Its physical value, as a lubricant that protects the bone surfaces from abrasion as they slide over each other, is crucial.
But HA also has anti-inflammatory properties: Even when there are no big injuries, the daily wear and tear an equine athlete imposes on his limbs causes minor inflammation throughout his tissues, including the joint capsule and its lining. Among other effects, that inflammation leads to the production of several enzymes that can damage local tissues. HA binds with those enzymes before damage becomes extensive.
However, if an acute injury does occur, or if a horse repeatedly is asked to work beyond his ability to recover, the inflammatory enzymes produced can overwhelm the amount of HA present.
"HA is a good free-radical scavenger, but it can't handle too much," says C. Wayne McIlwraith, BVSc, PhD, director of orthopedic research at Colorado State University. "Excessive inflammation leads to depolymerization of the molecule. That is, it breaks down."
