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The Equine Joint


A joint allows controlled movement of bones relative to each other, thus allowing the skeleton to move and ultimately the limb to bend. The equine joint endures incredible pressure in resisting the damaging effect that can occur when bones collide. Normal movement begins with muscle contraction, which moves the bones via  tendons and ligaments that attach muscle to bone.

 

In a normal healthy joint, the ends of the bones are coated with a thin layer of ‘friction reducing’ articular cartilage. They are surrounded by a joint capsule with a tough outer layer (to connect the bones and protect the joint) and a permeable inner layer (synovial membrane), which secretes synovial fluid and allows the passage of nutrients and other substances from the bloodstream. Synovial fluid (a slippery, viscous substance of similar consistency as egg whites), fills the joint capsule, nourishes the articular cartilage, and provides essential lubrication.

 

Synovial fluid is a nutrient-rich brew that contains proteins, enzymes, water, leukocytes and the most important ingredient Sodium Hyaluronate , which is responsible for the fluids elasticity and viscosity.

Sodium Hyaluronate (HA) is a Glycosaminoglycan (GAG) , which arranges itself in complicated coils, adapting to the pressure changes in the joint capsule as the horse moves. It assures the free passage of metabolites to and from tissues throughout the joint, and also serves as a stabilizer and shock absorber for the structures that are undergoing continual, changing mechanical stresses.

 

Articular cartilage, the other main shock-absorbing component of a joint, is an efficient but flawed structure. Its structural framework is a web of collagen fibres, with cells called chondrocytes scattered among the matrix. Chondrocytes produce giant proteoglycan molecules which bind the GAG’s. The GAG’s in turn extract and loosely hold large amounts of positively charged water molecules. When cartilage is damaged, there is a decrease in the number of GAG’s, therefore the cartilage hold less water, which in turn decreases the cushioning effect.

Cartilage conforms to the bone surfaces for a tight fit between weight-bearing bones. It spreads pressure evenly over a broad area, and it ‘manages’ the water in its matrix, squeezing it out when the joint is under pressure, and drawing it back in when the pressure is decreased. This in and out movement of the fluid transports nutrients throughout the cartilage. Cartilage also acts like a sponge, conforming to loading demands by changing its shape and size, and regaining its original shape when the pressure is off.  This cartilage lubrication acts to decrease friction.

Cartilage has no blood or nerve supply of its own, so it therefore has little or no ability to heal or repair itself. Therefore, although cartilage performs admirably under normal conditions, it only takes  a 5–10% overload of work stresses to begin the process of cartilage breakdown.

 

Maintaining joint mobility will
keep your horse performing
at his best.