Diarthrodial Joint Anatomy: A diarthrodial joint is composed of bones, soft tissue and synovial fluid. The bones provide the ability to bear weight on the limb. The soft tissue structures include cartilage, ligaments, the soft tissues around the joint, the joint capsule, and the synovial membrane. The ligaments and soft tissues around the joint provide mechanical stability (side-to-side and front-to-back) to prevent injury. The joint capsule is composed of collagen and also provides mechanical stability. It has a good blood supply and is highly sensitive to painful stimuli due to the rich source of nerves which run through it. The synovial membrane contains cells (synoviocytes), which help remove any unwanted material (bacteria, etc) and produce synovial fluid (helps maintain a normal joint environment). The synovial membrane has the capacity to regenerate and can stretch and contract during joint motion. Like the joint capsule, the synovial membrane has a rich blood supply. However, it is relatively insensitive to painful stimuli. Finally, the normal cartilage within a diarthrodial joint is hyaline cartilage. It is made up of collagen and ground substance. The ground substance is composed primarily of proteoglycan, chondroitin sulfate, and hyaluronic acid. The collagen fibers and ground substance provide the cartilage with the ability to compress and expand during weight bearing, thereby acting as a shock-absorber. Cartilage, unlike the bone underneath, contains no nerve fibers. Therefore, as long as the cartilage is not damaged, the horse should not be painful. When arthritis sets in, the cartilage no longer is intact, exposing the underlying bone. The bone contains many nerve endings which causes the pain associated with arthritic conditions.

High-motion vs Low-motion Joints: A high-motion joint is one that acts like a hinge - opens and closes. Examples of high-motion joints in horses include the coffin joint, fetlock (ankle) joint, carpus (knee), elbow, shoulder, big hock joint, stifle and hip. A low-motion joint is one that does not open and close but serves to act as a shock absorber. Two examples of this type of joint are the pastern joint and small (lower) hock joints. It is important to understand the difference between high-motion and low-motion joints because the treatment for arthritis will vary.

Diagnosis of Arthritis: The diagnosis of arthritis (also known as degenerative joint disease) is based on several factors. The history is very important, since the majority of arthritic cases in performance horses are due to repeated joint trauma (unlike rheumatoid arthritis in people which is immune-mediated). A thorough physical (lameness) examination may yield findings such heat , swelling, pain on palpation and flexion of the joint, reduced range of motion of the joint, and response to nerve/joint blocks to isolate the cause of the lameness. Once the source of the problem as been localized, radiographs of the affect joint should be taken. Radiographic signs of arthritis may include a decrease in joint space and bone spurs. Additional diagnostic tests that may need to been undertaken include analysis of the synovial fluid, scinitigraphy (bone scan), thermography, or arthroscopy. Arthroscopy is the best method to assess the degree of cartilage damage present. However, it is a relatively expensive diagnostic procedure and is performed under anesthesia. Future directions for more accurately diagnosing the extent of joint damage in the horse will include MRI (magnetic resonance imaging) and CT (computed tomography).

Goals of Treatment: Once an accurate diagnosis of arthritis has been obtained from the lameness examination and diagnostic procedures, appropriate treatment recommendations can be made. The primary goals to achieve include reduction in the pain, inflammation and lameness, stabilization of the internal environment of the joint, and protection of the cartilage from further insult. There are numerous products available that can help accomplish these goals.

Medications for Joint Diseases

Nonsteroidal Anti-inflammatory Drugs (NSAIDs): NSAIDs suppress numerous components involved in the development of inflammation. The most commonly used NSAID is phenylbutazone (bute). Other NSAIDs used to treat joint disease include meclofenamic acid, flunixin [Banamine®], naproxen, carprofen [Rimadyl®] (relatively new and commonly used in small animals), and ketoprofen [Ketofen®] (shown to be not as effective as phenylbutazone). NSAIDs can have side effects, such as gastric ulceration and kidney problems. However, when used at appropriate dose rates, these problems are fairly uncommon.

Corticosteroids: These agents are very potent anti-inflammatories which act quickly and are relatively inexpensive. One problem with their use is that they suppress the local immune response within the joint. When used for the treatment of arthritis, these agents are injected directly into the joint. It is vital that proper injection technique be used to prevent introduction of bacteria into the joint. When used in high doses, corticosteroids can suppress cartilage cell metabolism and deplete glycosaminoglycan concentration, which ultimately will result in further cartilage damage. Low doses have been shown to be therapeutic, whereas high doses can be detrimental (particularly methylprednisolone acetate). Common products used include methylprednisolone acetate [Depo-Medrol®] (intermediate to long-acting), triamcinolone [Vetalog®] (short to intermediate-acting), and betamethasone (intermediate to long-acting).

Dimethylsulfoxide (DMSO): This anti-inflammatory agent can be applied topically to the affected joint or can be administered intravenous to provide generalized anti-inflammatory effects.

Methylsufonomethane (MSM): MSM is a naturally occurring dietary derivative of DMSO. It has anti-inflammatory effects and provides sulfur for connective tissue cross-linking (increases strength and flexibility of the tissue).

Polysulfated Glycosaminoglycan (Adequan®): Adequan® has anti-inflammatory effects and can counteract specific enzymes that can degrade articular cartilage. Additionally, it has been demonstrated to promote healing of damaged cartilage. Different protocols for its use are available. I recommend starting off with one injection (500 mg) every 4 days for 7 treatments, then every 30-90 days thereafter (as needed).

Hyaluronic Acid: Hyaluronic acid is an integral component of synovial fluid and articular cartilage. It is secreted by synoviocytes (synovial membrane cells) and chondrocytes (cartilage cells). Hyaluronic acid helps to normalize the joint environment. It provides lubrication of both the soft tissues in the joint, as well as the articular cartilage. During arthritic processes, hyaluronic acid becomes depleted.. There are 2 routes that this product can be administered: into the joint directly or intravenously (Legend®).

Neutriceuticals: These products as not considered drugs. They are classified as supplements. The ingredients include a combination of the following: chondroitin sulfate, glucosamine, antioxidants, and different miscellaneous ingredients (ie Ascorbic Acid [Vit C]). These products are administered in the feed and can help normalize the joint environment and assist with cartilage healing. Examples include Cosequin®, Flex-Free®, and Synoflex®

Joanne Tetens, DVM, MS, PhD, Diplomate of the American College of Veterinary Surgeons
Chief Equine Surgeon Aeolus Animal Hospital & Equine Center
145 Harmony Lane
Manchester Center, VT 05255
(802) 362-2241
e-mail: aahec@sover.net