When bones collide

The old gray mare, she ain't what she used to be. She doesn't move as easily as she once did, doesn't jump fences or wheel around the paddock with the same grace and fluidity she had in her youth. On cold and rainy days, she comes out of her stall feeling positively creaky, and she doesn't care for having her feet held up for long periods of time by the farrier, or for working on hard ground.The old gray mare, she ain't what she used to be. She doesn't move as easily as she once did, doesn't jump fences or wheel around the paddock with the same grace and fluidity she had in her youth. On cold and rainy days, she comes out of her stall feeling positively creaky, and she doesn't care for having her feet held up for long periods of time by the farrier, or for working on hard ground.

Story originally posted by: Karen Briggs

The old gray mare, she ain’t what she used to be. She doesn’t move as easily as she once did, doesn’t jump fences or wheel around the paddock with the same grace and fluidity she had in her youth. On cold and rainy days, she comes out of her stall feeling positively creaky, and she doesn’t care for having her feet held up for long periods of time by the farrier, or for working on hard ground.

What’s up with the old gray mare? In a word, arthritis. Whether it was years of wear and tear that started the process, or a single traumatic injury, she’s now experiencing the painful after-effects – when synovial fluid in her joints thins and fails to lubricate, the cartilage that provides a smooth articulating surface breaks down, and bone ends up grinding excruciatingly against bone. No wonder she doesn’t want to flex her joints the way she once did!

Osteoarthritis accounts for the end of more equine performance careers than any other single cause. And until recently, it was considered an irreversible process. New research, however, offers some hope. There are now several treatments that can significantly improve joint lubrication, slow or halt the deterioration of the joint surfaces, and reduce pain and inflammation – giving horsemen some options for coping with this destructive condition.

INSIDE THE JOINT
To understand how arthritis works, we first have to take a look inside your horse’s skeletal structure and see how it operates.

A horse’s joints are marvels of biological engineering, especially considering the extreme pressures brought to bear on the equine skeleton in motion. That the bones involved can glide frictionlessly over, under, around, and against each other is due to the ingenious interior structure of each joint.

In a normal, healthy joint, the ends of the bones are coated with a thin layer of friction-reducing articular cartilage, and are surrounded by a joint capsule with a tough outer layer (to connect the bones and protect the joint) and a permeable inner layer, or synovial membrane, which secretes synovial fluid and allows the passage of nutrients and other elements from the bloodstream. Synovial fluid, a slippery, viscous liquid the consistency of egg whites, fills the joint capsule, nourishes the articular cartilage, and provides essential lubrication when the cartilage-covered bone surfaces move against each other.

Synovial fluid is a nutrient-rich brew which contains proteins and enzymes, water, leukocytes, and the key ingredient sodium hyaluronate. Formerly known as hyaluronic acid, sodium hyaluronate is a negatively-charged sugar chain, or glycosaminoglycan (GAG), which arranges itself in complicated coils, adapting to the pressure changes in the joint capsule as the horse moves. It assures the unhindered passage of metabolites to and from tissues throughout the joint, and also serves as a stabilizer and shock-absorber.

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 along the matrix. Chondrocytes produce giant proteoglycan molecules that resemble bottle brushes and bind GAG’s; the GAG’s in turn, extract and loosely hold large amounts of positively charged water molecules.

Cartilage has many talents. It conforms to the bone surfaces for a tight fit between weight-bearing bones; it spreads pressure out 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 joint is ‘unweighted’ – which transports nutrients throughout the cartilage. Essentially, it acts like a sponge, conforming to loading demands by changing its shape and size, and regaining its original shape when the pressure is off.

But here’s the down-side: cartilage is one of the body’s most primitive structures. It has no blood or nerve supply of its own, so cartilage has little or no ability to heal or repair itself. Although cartilage performs admirably under normal conditions, it only takes a five to 10 per cent overload of work stresses to begin the process of cartilage breakdown.

A VICIOUS CYCLE
The effects of the wear and tear of hard work, uneven or unyielding footing, and poor conformation may trigger the deterioration of cartilage, which is where arthritis begins. Though the horse notices no pain at this stage, and continues to work with no observable lameness, inside his joints, havoc is ensuing.

When cartilage cells begin to erode, the synovial membrane responds by cleaning up the debris – a classic inflammatory response. Though the debris-eating cells are simply doing their job, the action of breaking down and removing the foreign bodies from the area changes the chemical makeup of the fluid in the joint, introducing excess fluids and a greater concentration of destructive enzymes and prostaglandins into the closed space, and destroying the lubricating sodium hyaluronate. The synovial fluid begins to lose viscosity, becoming less like egg whites and more like water.

Deprived of its protective cushioning and lubricating, the cartilage breaks down further. More enzymes are released to clean up the second round of debris, and create even more inflammation. Now swollen and sore, the joint experiences increased friction as the synovial fluid gets thinner, the synovial capsule thickens, and the vicious cycle accelerates.

Eventually, chondrocytes, suffering from a compromised nutrient supply, can’t keep up with repairs, and the cartilage develops pits, holes, and fissures, opening the bone ends to direct trauma. The bone responds with a defence that only causes further destruction – it lays down new bone to strengthen the surface (a process called sclerosis) and extend its margins (bone spurs and osteophytes). In its final stages, if left unchecked, arthritis causes the fusing of equine joints.

WHEN IS IT ARTHRITIS?
Once cartilage damage is noticeable, researchers categorize the condition of the joints as arthritic. But in the early stages, cartilage damage may be very subtle (and because cartilage has no nerve endings and does not register pain, the horse may appear sound). The two earliest stages of degenerative joint disease – a catch-all term that encompasses arthritis, developmental orthopaedic disease, and infectious arthritis caused by bacteria invading the joint – are synovitis (inflammation of the synovial membrane) and capsulitis (inflammation of the joint capsule). Both are common signs of overworked joints and possible conformational weaknesses.

Other precursors of arthritis include injuries of the ligaments within a joint; pre-existing articular cartilage damage (sometimes originating with DOD episodes in the horse’s youth, causing abnormal cell growth and thickened, brittle cartilage); and osteochondral fractures (commonly called ‘bone chips’, and especially common in the fetlock and knee joints). Any of these can start the cycle of inflammation in a joint, and instigate the breakdown of cartilage.

It’s a rare horse who doesn’t suffer some symptoms of arthritis as he ages. But there are several ways you can help your horse cope.