Conditions of the Hock

The hock is a complicated structure made up of 11 bones, numerous ligaments, several bursa, and 4 distinct joints:

• The tarsometatarsal joint
• The distal intertarsal joint
• The proximal intertarsal joint
• The tarsocrural (tibiotarasal) joint

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.

• Tarsometatarsal joint and distal intertarsal joint. Arthritis of the two distal joints of the hock is also known as degenerative joint disease, tarsitis, or in layman's terms as "bone spavin."
• Proximal intertarsal joint and the tarsocrural joint. Arthritis can also affect these proximal joints, although less commonly.
• Tarsocrural (tibiotarasal) joint. Inflammation in the tarsocrural joint results in an increase of joint fluid within the joint producing a visible swelling of the joint and sometimes behind the joint. This joint swelling is referred to in laymen's terms as a "bog spavin."

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:

• A narrow based gait,
• Shortened anterior stride
• Stabbing the toe into the dirt
• Swinging the hindlimb beneath the body rather than flexing the hock

Diagnostic Tests

Your veterinarian will give your horse a physical examination, which will include the following:

• Observation of the horse's conformation and gait at the walk, trot, and canter in a straight line and in a circle.
• Palpation and passive range of flexion of the hock.
• Performance of hind limb flexion tests.
• Performance of intra-articular anesthesia to localize the lameness to a specific area of the leg or joint.
• Radiograph the area of interest and examine for signs of arthritis or OCD.

Treatment

• Early stages of arthritis, which are manifested by mild stiffness at the start of exercise, can be managed with the judicious use of non-steroidal anti-inflammatory drugs, such as phenylbutazone and flunixin meglumine. Many of the non-steroidal drugs are available in oral as well as injectable forms. These drugs may lead to gastric ulceration or kidney problems if used in excess.
• Horses with persistent or progressive signs of arthritis will often benefit from intra-articular therapy with either corticosteroid or hyaluronan or a combination of the two. Intra-articular injection of the joint may be necessary several to multiple times a year.
• Horses with arthritis may also be treated intravenously with hyaluronan or intramuscularly with polysulfated glycosaminoglycan.
• Horses which have severe arthritis not responsive to the above medications can be treated chemically or surgically to fuse the arthritic joints. Success rates vary from 60 to 80 percent.
• There are numerous over-the-counter oral joint supplements currently available that are being marketed for treating arthritis. Anecdotally it appears that some of these products may be beneficial, however, there is limited scientific data to support this. Before selecting a particular product, discuss the potential benefits with your veterinarian.
• Horses that have lameness secondary to the presence of OCD often benefit from surgical removal of the OCD lesions followed by 1 –3 months of rest.

Home Care

Caring for a horse with degenerative joint disease of the hock is not difficult but does require common sense.

• Horses that become stiff when they are stalled over night or for extended periods of time during the day often are more comfortable if they are housed in a larger stall or one that has a run-out paddock attached to it.
• Horses should be allowed sufficient time to "warm-up" and become less stiff before being asked to engage in exercises that require them to flex their hocks and propel from the hind limb. This warm-up time will vary with each horse but should be between 20 to 40 minutes of walking and slow trotting.
• Horses that fail to warm-up out of their stiffness should not be worked further, as doing so may injure the back and other joints, as well as result in the horse developing a very sour attitude. These horses definitely require a veterinarian's attention.
• Horses recovering from surgery either to fuse an arthritic hock or to remove an OCD lesion will require several weeks to several months of stall rest. During the initial 2 – 3 weeks the hock will be protected with a sterile bandage. The bandage will need to be changed every day to every few days.
• Following intra-articular medication with corticosteriods and/or hyaluronan, the horse is usually rested for 24 –48 hours then gradually returned to his regular exercise program. An increase in lameness following injection of these drugs should be brought to the immediate attention of your veterinarian.
• Administration of hyaluronan, polysulfated glycosaminoglycans, phenylbutazone, flunixin meglumine and ketoprofen should be done so under the care of a veterinarian.
• Consult your horse's veterinarian prior to the purchase and use of over-the-counter oral joint supplements.

Preventative Care

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:

• Metalloproteinases: MMP- 1, 2, 3, and 9 actively degrade components of joint cartilage.
• Prostaglandins increase blood flow, enhance pain perception, cause bone demineralization and deplete cartilage proteoglycans.
• Oxygen free radicals cleave proteoglycans, collagen, and hyaluronic acid.
• Cytokines [interleukin 1 (IL-1)] function to stimulate the release of metalloproteinases.

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.

Diagnosis In-depth

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:

• The horse will appear sound at the walk but short stride with the affected limb(s) when trotted. When observed from the side at the trot and at the canter, the horse will have a shortened anterior stride and will tend to drag or stab the toe into the ground. These actions occur because the horse is not flexing the limb fully. Gait abnormalities may be more apparent when the horse is lunged or ridden in a circle.
• Palpation of the hock is typically unremarkable. The hock, however, may appear boxy or enlarged in the region of the lower hock joints due to bony changes associated with arthritis.
• Horses with moderate to advanced arthritis of the hock are uncomfortable when the limb is flexed passively. These horses also resent lifting their feet to have them cleaned and are uncomfortable standing with the limb elevated in a fixed position for the farrier to work on their feet.
• As part of the lameness evaluation, a flexion test should be performed on each hind limb. This test involves flexing the hind limb with the cannon bone held parallel to the ground for 1 –2 minutes. The horse's degree of lameness will be assessed as he trots away from the observer immediately upon release of the leg. A normal response is 2 – 4 abnormal steps then a return to soundness. An abnormal response is continued lameness following the first several steps.

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.

• Despite the gait abnormalities and an increase in lameness following an upper limb flexion test, a definitive diagnosis of arthritis of the hock cannot be made without the use of diagnostic anesthesia and radiographic confirmation. Multiple other conditions of the hind limb and soreness of the back may also result in similar gait abnormalities.
• The tarsometatarsal and distal intertarsal joints will be anesthetized by intra-articular injection of a local anesthetic. Prior to placement of the needles and injection of the anesthetic, the skin surface will be prepared sterilely to minimize the risk of infecting the joint with skin surface bacteria. After injection, the horse will again be examined at the trot and flexion tests for lameness. Improvement of the lameness is expected if these joints are responsible for the lameness.
• Radiographs of the hock are taken to determine if arthritis or any other abnormalities are present that may account for the animal's lameness. Generally, four standard radiographic views are taken: a lateral to medial view, a lateral to medial oblique view, an anterior to posterior view, and a medial to lateral oblique view. Each of these views highlights a different surface of the bones that make up the joints of the hock. Special views or a different radiographic exposure technique may be required to further evaluate a specific area that appears questionable on the original standard views.
• Changes seen in radiographs that are consistent with arthritis of the distal hock joints include new bone production along the margin of the bone edges (known as bone spur formation) and loss of joint space . The latter occurs as the cartilage thins due to erosion or full thickness loss. When this occurs the distance between the two opposing bones becomes narrower. In advanced arthritis, opposing bones may start to fuse together.
• A nuclear medicine bone scan (scintigraphy) may be recommended to help determine the significance of questionable lesions or to help diagnose a horse that has multiple sites of hind limb lameness.

Therapy In-depth

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.

• Nonsteroidal anti-inflammatories. The nonsteroidal drugs inhibit the release of several enzymes that are not only responsible for the production of pain but also play a role in cartilage degradation. Numerous ongoing scientific studies are examining the effects of the various nonsteroidal drugs on joint cartilage.
• Corticosteriods. Corticosteroids are a class of potent anti-inflammatory drugs. When injected into affected joints, the corticosteroid causes a rapid reduction in inflammation by decreasing the concentration of inflammatory cells and release of enzymes. Experimentally the use of small doses of corticosteroid has been shown to protect joint cartilage. Drugs in this class include, but are not limited to, betamethasone, methylprednisolone acetate, and isoflupredone acetate. Dependent on the drug used and the degree of arthritis, intra-articular administration may be necessary every several months to once or twice a year.
• Sodium Hyaluronan. Hyaluronan is a glycosaminoglycan that is found in normal joint cartilage and joint fluid. Use of exogenously administered hyaluronan has been shown to decrease pain, increase joint mobility, and decrease cartilage degradation in experimental and clinical studies. The drug can be injected directly into the affected joint or it may be given intravenously. The drug is often given the nickname of "HA" regardless of the manufacturer's product name. The drug has been reported to have its best effects in acute arthritic stages.
• Polysulfated glycosaminoglycan. Polysulfated glycosaminoglycan (PSGAG) has anti-inflammatory effects and cartilage protective effects. The drug can be administered directly into the joint or can be given intramuscularly. The latter route reduces the risk of intra-articular infection. Intramuscular injections are recommended every 4 days for 28 days.
• Oral supplements. There are numerous oral supplements being marketed today for the treatment of equine arthritis. The two most common components in these products include chondroitin sulfate and glucosamine. The chondroitin sulfate is reported to have similar effects as those of PSGAG. There is some question as to whether the biologically effective form of the chondroitin sulfate is absorbed from the gut. The glucosamine salts have been shown to be absorbed after oral administration and may have a variety of anti-inflammatory actions. There are more anecdotal than scientific reports of their use in horses regarding the benefits of the oral supplements. It is always wise to discuss the benefits of a particular supplement with your veterinarian before purchasing and administering it.
• Horses that have severe arthritis not responsive to the above medications can be treated chemically or surgically to fuse the arthritic joints. Success rates vary from 60 to 80 percent.

Follow Up

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.