Conditions of the Hock
Dr. Patricia J. Provost
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.
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:
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.
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.
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.
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.