The hock is a complicated structure made up of 11 bones, numerous ligaments, several bursa, and 4 distinct joints:
In addition, a number of tendons insert around or pass over the hock. With so many structures the hock is prone to both cosmetic blemishes and various acute and chronic conditions of lameness. The hock is a common source of chronic lameness due to arthritis, most often of the distal joints.
Arthritis of the hock can occur in any aged horse, including horses less than 1 year of age (juvenile arthritis) and affects all breeds. Arthritis can result secondary to trauma, poor conformation or as in most cases, simply as a wear and tear phenomenon of everyday work. Horses with heavy workloads and poor conformation would be expected to develop arthritis at an earlier age than those horses with perfect conformation participating in limited work.
More common to the tarsocrural (tibiotarasal) joint is the condition osteochondrosis dissecans or OCD. OCD of the tarsocrural joint is a developmental condition, which means that the horse has had the problem since it was a neonate. The development of OCD is still not completely understood but it is thought to be influenced by genetics and nutrition. Horses' with OCD of the tarsocrural joint may or may not be lame and may or may not have joint swelling.
What to Watch For
Arthritis usually involves both hocks and is insidious in onset. Initial gait changes include stiffness at the onset of exercise with gradual improvement as the horse warms up.
Other signs include:
Your veterinarian will give your horse a physical examination, which will include the following:
Caring for a horse with degenerative joint disease of the hock is not difficult but does require common sense.
There is no known method of care or treatment to eliminate or prevent arthritis of the hock. Attention should be paid to having the horse's feet properly shod or trimmed to minimize abnormal distribution of forces across the joint surfaces. Riders and trainers should also have reasonable expectations of what each horse is capable of and tailor their exercise programs accordingly.
Arthritis of The Hock
Arthritis can occur in any one of the multiple joints that horses have but the lower joints of the hock, the tarsometatarsal and the distal intertarsal joints, are common sites. Arthritis develops in horses usually secondary to "repeated, cyclic trauma," or the wear and tear of everyday riding, racing, and training exercise, that inflames the joint capsule and synovial membrane lining. Arthritis also develops secondary to any single major external or internal trauma, such as a fracture of the joint, a sprain of the soft tissue surrounding the joint, a joint infection or osteochondrosis.
Inflammation of the joint is thought to be the first step in the development of arthritis in most horses. If not treated appropriately, the inflammation leads to early degeneration of the joint through degradation of joint cartilage.
The synovial membrane releases enzymes that are mediators of joint disease, causing breakdown of joint cartilage components:
The end result is a vicious circle of joint inflammation and cartilage degradation. Changes in the cartilage ultimately affect the ability of the cartilage to withstand compressive and tensile forces placed upon it during exercise. Cartilage develops fibrillation (fine cracks in the smooth surface), then partial and full thickness areas of cartilage loss.
The tarsometatarsal and the distal intertarsal joints are considered high load, low motion joints, meaning that they transfer and absorb concussive forces of locomotion but do so with very little movement. As cartilage is lost, bony changes occur and over a period of time the joint surfaces fuse (ankylose) together. Until they do, not only is the "shock absorbing" function of the joint less effective, but the horse experiences pain as bone impinges on bone.
In addition to arthritis of the hock, other conditions that can cause a hind limb lameness include but are not limited to arthritides of other joints, such as the hip, stifle, fetlock, pastern, and coffin joints; impingement of the dorsal spinous processes of the thoracolumbar spine; and polysaccharide storage disease.
Arthritis of the lower hock joints should be suspected in horses that have a history of a stiff gait that improves during exercise. The following may be true:
A lower limb flexion test should also be performed (flexion of the ankle and foot) and the gait reassessed. The degree of lameness should be less than that seen with the upper limb flexion.
The overall goal of treating horses with arthritis of the distal joints of the hock is to eliminate the pain and allow them to continue to participate in their occupation. Retarding the progression of the disease within the distal intertarsal and tarsometatarsal joints is less of a concern than it is in other highly mobile joints. There are several types of drugs that are being used today to manage equine arthritis. Different drugs and combinations of drugs may work better in one horse than another.
The most commonly used drug of this class is phenylbutazone. Others include flunixin meglumine, ketoprofen, meclofenamic acid, naproxen, and carprofen. Many forms of arthritis often can be successfully managed with rest and short term or intermittent use of an oral or systemically administered nonsteroidal anti-inflammatory drug. Unwanted side effects can occur with the use of these drugs and they should always be used under the direction of a veterinarian.
Optimal treatment for a horse with arthritis of the hock requires a combination of owner and professional veterinary care. Follow-up can be critical, especially if improvement is not seen or if the lameness becomes more severe despite therapy.
Any horse that becomes more lame 1 - 10 days following intra-articular therapy should be seen immediately by a veterinarian to determine if the horse has developed a joint infection or a reaction to the medication.
Arthritis at this point is not curable. Successful management will depend on a correct diagnosis and diligent long-term care. Changes in management, including increased pasture turnout and decreased intensity of work, along with changes in medication may be necessary to allow the horse to continue to have a pain free productive life.