Systemic Equine Joint Therapy: Choosing Wisely

Jennifer L. Wright, DVM

Adequan, Legend, MAP-5, Acetyl-D-Glucosamine, Pentosan, Polyglycan…as a horse owner, you have probably heard these names a million times.  You also probably recognized them as products that are used in the prevention and treatment of equine articular, or joint, diseases.  Numerous articles and advertisements extol the virtue of dozens of different joint therapy products, but what do they really do?  What product is the most appropriate for your horse?  Joint medications and supplements are available in 4 main forms of delivery, namely intravenous (IV), intramuscular (IM), intraarticular (IA, i.e. in the joint) and oral.  This article will discuss a few of the most commonly used injectable products.  Keep posted for a discussion about oral products in a future newsletter!  But first, a little background information about equine joint disease…

Normal Joint Anatomy

Normal Joint Anatomy

Your horse’s joints are designed to flex, compress and extend hundreds of times per day while carrying his weight for years on end.  Add a rider’s weight and increase athletic demand, and some level of joint inflammation will result due to repeated trauma or stress to the joint.  The definition of a joint is “the junction between two or more bones.”  Joints have several components, including:  collateral ligaments (which prevent the lateral or side to side movements of bones), synovial fluid (the fluid which fills the space between the bones and provides lubrication and nourishment to the cartilage), joint capsule (which stabilizes the joint and contains all the structures of the joint), synovial membrane (the inner lining of the joint capsule which produces and regulates the synovial fluid), and articular cartilage (the soft structural tissue that covers and cushions the ends of the bones where they meet to make up the joint).  The cartilage is a framework of tissue composed of collagen fibers, which give the structure its strength.  The collagen is interlaced with proteoglycans, which are long, protein-based molecules with multiple attached glycosaminoglycan (GAG) chains that trap water within its matrix.  As a joint flexes, the cartilage compresses and expands, forcing water in and out of its collagen matrix to provide a shock absorbing effect.

Cartilage Proteoglycan Structure

Cartilage Proteoglycan Structure

The term “arthritis,” or inflammation of a joint, can be used to describe any spectrum of joint disease ranging from early, mild, barely detectable soft tissue inflammation within the joint to chronic, severe, crippling cartilage degeneration and bone spurring.   Traumatic joint disease in horses includes synovitis (inflammation of the fluid producing synovial membrane), capsulitis (inflammation of the fibrous joint capsule) and osteoarthritis (degeneration of the articular cartilage and underlying subchondral bone).  In many cases, every day repetitive microtrauma produces minute damage within the joint structures that triggers mild inflammatory responses to make repairs.  Normally, the body’s own defenses control inflammation and the joint remains healthy and sound.

Wear and Repair Cycle

Wear and Repair Cycle

Sometimes, however, the rate of degeneration exceeds the rate of regeneration and arthritis begins to develop.  If the inflammatory process overwhelms the body’s ability to contain it, either from a single acute injury or from many years of use, a cascade of events ensues:  inflammatory enzymes break down the thick, slippery lubricating synovial fluid, which becomes thin and watery.  Proteoglycans are lost and collagen fibers lose structure, diminishing the cartilage’s ability to retain lubricating water.  The cartilage becomes eroded or cracked, and this damage stimulates even more inflammation.  The joint capsule fills with more watery fluid, leading to pressure, pain and stiffness.  The cascade of inflammation leads to a buildup of more inflammatory enzymes that further degrade the synovial fluid and cartilage.  Left un-checked, this inflammatory cycle can continue until eventually the cartilage tears or erodes away entirely, leaving the exposed ends of the bones to rub against each other.

Osteoarthritis Inflammation Cycle

Osteoarthritis Inflammation Cycle

It is not possible to cure arthritis at this time, but appropriate treatment can halt or slow the inflammatory cycle that results in permanent or progressive joint damage.  The key is early identification of joint inflammation, ideally before any cartilage damage has occurred.  Truthfully, most joint supplements do far more to prevent inflammation and joint damage than they do to treat it once it has started.  Once marked inflammation has taken hold within a joint (resulting in radiographic changes and the clinical signs of lameness, stiffness, joint swelling, poor performance, etc.) direct treatment of the joint with intra-articular injections (“joint injections”) is often indicated.  Your veterinarian will decide if it is most appropriate to begin therapy with either intramuscular and/or intravenous joint therapy, intra-articular joint therapy, or all of the above.

So what joint therapy product would be the most appropriate for YOUR horse?  First, let’s talk a little more about articular cartilage.  Think of cartilage as a tree…the roots, trunk and branches can all be represented by GAG’s (i.e. hyaluronan/hyaluronic acid, glucosamine, chondroitin).  It would seem intuitive to treat o