Disorders of the tarsus include the conditions known as bog spavin, bone spavin, and curb. The tarsus can also be affected by displacement of the tendon from the hock, fracture of the tarsus, hindlimb tendon and muscle ruptures, stringhalt, and thoroughpin (see Bone, Joint, and Muscle Disorders in Horses: Fractures of the Splint Bones).
Bog Spavin (Inflammation of the Hock Joint)
Bog spavin is a longterm, low-grade inflammation of the synovial membrane that surrounds the tarsal, or hock, joint. Poor conformation may lead to weakness of the hock joint and increased production of joint fluid. In such cases, both limbs are affected. Inflammation of just one hock is more likely to be a consequence of a sprain or some underlying problem within the joint (for example, osteochondritis dissecans).
The horse usually is not lame. The joint swells mainly on the middle back surface of the hock, with smaller swellings on each side of the joint near the back. Uncomplicated bog spavin rarely interferes with the usefulness of the horse but is an unsightly blemish. It should be evaluated by a veterinarian, including x-rays. The swelling may spontaneously appear and disappear in weanlings and yearlings.
Excess fluid within the joint membrane may be removed by a veterinarian using a needle and syringe. Corticosteroid injections to the joint provide variable and short-term relief. Surgery using an endoscope may be necessary when bone and cartilage involvement is suspected. Bog spavin tends to recur, especially in cases where poor conformation is involved.
Bone spavin is a degenerative condition with a gradual loss of cartilage of the joint (osteoarthritis) or inflammation of the bone or bony tissue (osteitis) of the hock joint. Portions of the joint may degenerate, particularly the upper middle part of the hock, and the formation of new bone near the joint may eventually cause the bones to fuse. Although bone spavin usually causes lameness, this may not always be obvious. The cause is not always clear, but may be related to improper conformation of the hock, excessive jarring injury, or mineral imbalance. All breeds can be affected, but it is most common in Standardbreds and Quarter Horses.
In cases of lameness, the horse tends to drag the toe. The forward movement of the hoof is shortened, and hock action is decreased. If the surfaces of the joint have been affected, lameness can be continuous. The heel may become elongated. Standardbreds develop soreness in the gluteal muscles—known as trochanteric bursitis (see Bone, Joint, and Muscle Disorders in Horses: Trochanteric Bursitis (“Whirlbone” Lameness))—as a result of the spavin. When standing, the horse may rest the toe on the ground with the heel slightly raised. The lameness often disappears with exercise and returns after rest. The bones of the affected joints may fuse spontaneously, leading to a return to soundness.
Diagnosis of bone spavin is based on the horse's history, a physical examination, and x-rays to look for joint degeneration and bony growth. Local anesthesia of the individual tarsal joints may be necessary to isolate the exact site of the pain responsible for the lameness.
In the early stages, injection of corticosteroids to the joint may help. Nonsteroidal anti-inflammatory drugs (for example, phenylbutazone) may minimize or eliminate signs. After treatment the horse may be worked to increase the fusion rate of the bones. Fusion may also be done surgically. Corrective shoeing by raising the heels and rolling the toe may help in milder cases but is unlikely to get rid of lameness on its own.
Curb is a thickening or bowing of the plantar tarsal ligament that runs down the back of the hock. The cause is typically strain. This ligament may become inflamed and thickened after falling, slipping, jumping, or pulling. The condition is most common in Standardbreds, in which poor conformation of the hock makes the horse more inclined to develop the condition. An enlargement over the bone about 4 inches (10 centimeters) below the point of the hock may be seen when observing the horse from the side. A recently formed curb typically produces noticeable inflammation and lameness. The horse stands and favors the limb with the heel elevated. In cases of chronic curb, there is rarely any lameness or pain.
Cold packs and rest are recommended for acute cases. Little can be done to overcome curb that results from poor conformation of the hock. Fortunately, the problem seems to be self-limiting, without lasting effects on performance.
Displacement of the Superficial Flexor Tendon from the Point of the Hock
The superficial flexor tendon can be dislocated by damage to its attachment to the point of the hock. A sudden bending of the hock typically causes the injury, after which the tendon may slip to the inside of the hock. The limb may initially become lame with heat and swelling at the point of injury. Treatment involves rest for up to 3 months, possibly with a cast. The horse may be left with a permanently displaced flexor tendon and a rather jerky hock action. This usually causes no difficulty during fast exercise or jumping, but dressage movements may be affected. Surgery has been reported in a limited number of cases, but the results have not been very successful, particularly in larger horses.
Fracture of the Tarsus
Fractures of the tarsus or hock are usually caused by trauma or the complications of degenerative joint disease. Because the hock is a complex joint made up of 8 bones, a wide range of fractures can occur. Specific diagnosis depends on careful x-ray examination.
Chip fractures are among the more common fractures of the tarsus. Slab fractures are also seen, particularly in Standardbreds. Because these often are quite small and may not cause lameness, your veterinarian may need to use local anesthesia to positively identify the site of lameness. In many instances, a rest period of 3 to 6 months is all that is required for full recovery, although with large chip fragments surgical removal may be better. The condition responds well to surgery with an endoscope. Slab fractures may require the placement of bone screws.
Hindlimb Tendon and Muscle Ruptures
Horses rarely tear an entire Achilles tendon (involving both the calf muscle and superficial flexor), but in such cases the outlook for recovery is grave. The hock drops towards the ground and is unable to bear weight.
Gastrocnemius (calf muscle) rupture is more common and can result from excess stress applied to the hock, such as when the horse has to stop suddenly. It can occur in both hindlimbs and weight can be borne, but the excess bending of the hock makes walking difficult. No satisfactory treatment exists. Splinting the limb and slinging the horse have been attempted but are usually unsuccessful.
Hindlimb tears often injure the extensor tendons and digital extensors as well. When only one tendon is involved, the outlook for recovery is usually good. If both extensor tendons are severed, the horse's performance gait may be lost, although the horse may remain useful for slower work or for breeding. Conservative treatment will heal the wound, but surgical repair and casting should be considered if both tendons are completely severed or if a return to performance status is desired.
Racing injuries sometimes rupture the superficial and deep flexor tendons. The tendons may also rupture when other tendons are torn. These are serious injuries with obvious lameness and varying degrees of overextension of the fetlock and pastern. Treatment involves surgical repair with splinting and casting the limb, but the outlook for recovery is poor for future performance.
Injury to the peroneus tertius muscle affects the hindlimb and disrupts the action of the stifle and hock joints. The most characteristic identifying feature of this condition is the ability to extend the hock and flex the stifle at the same time. The horse is lame but usually is able to bear weight on the limb. The affected hindlimb exhibits a jerking motion as it is brought forward. Treatment consisting of prolonged rest (usually 4 months) is recommended, and the outlook for recovery is favorable.
Stringhalt is the involuntary contraction (spasm) of the muscles of one or both hindlimbs and is seen as overflexing of the joints. Its cause is unknown, but it is possible that an abnormality in the nervous system may be involved in some cases. Severe forms have been attributed to sweet pea poisoning in the United States and possibly to flat weed or dandelion intoxication in Australia. Horses of any breed may be affected, although it is rare in foals.
Any degree of overflexion in the joint may be seen; in mild cases the horse may spasmodically lift and lower the foot, while in extreme cases the foot is drawn sharply up until it touches the belly and then is struck violently on the ground. In severe cases, the side thigh muscles waste away. In Australian stringhalt and sweet pea poisoning, the condition may be progressive, and the gait abnormality may become so severe that euthanasia must be considered.
Mild stringhalt may come and go. Signs may diminish or even disappear during warmer weather. The signs are most obvious when the horse is sharply turned or backed. In some cases, the condition is seen only during the first few steps the horse takes out of its stall. Stringhalt may not materially hinder the horse's ability to work, except in severe cases when the constant jarring injuries give rise to secondary complications. The condition may also make the horse unsuitable for equestrian sports (for example, dressage).
Diagnosis is based on a veterinarian's examination but can be confirmed by electromyography, which measures the electrical activity of the muscles and associated nerves. False stringhalt sometimes appears as a result of some temporary irritation to the lower pastern area or even a painful sore in the foot. A stringhalt-like gait may occasionally be seen in a horse with momentary “locking” of the patella (kneecap) in an extended position.
When sweet pea (US) or flat weed (Australia) ingestion is suspected, moving the horse to another paddock may correct the problem. Many such cases recover spontaneously. In longterm cases, surgical cutting of a digital extensor, including removal of a portion of the muscle, may yield good results. Improvement may not be evident until 2 to 3 weeks after surgery, however. The outlook for recovery after surgery is considered guarded; not all cases respond.
Thoroughpin is a swelling of the covering of the deep digital flexor tendon just above the hock. It is characterized by fluid-filled swellings on both sides above the tarsal joint (distinguishing it from bog spavin). It usually affects only one limb and varies in size. It may produce no detectable inflammation, pain, or lameness. Essentially a blemish, thoroughpin is chiefly important in show horses. It is treated by withdrawing the fluid and injecting hyaluronic acid or a long-acting corticosteroid. This procedure may need to be repeated until the swelling does not come back. Radiation therapy also helps reduce the fluid buildup from the tendon sheath.
Last full review/revision July 2011 by Russell R. Hanson, DVM, DACVS, DACVECC; Joerg A. Auer, DrMedVet, Dr h c, MS, DACVS, DECVS; Andrew P. Bathe, MA, VetMB, DACVS, DEO, MRCVS; Leo B. Jeffcott, MA, BVM, PhD, FRCVS, DVSc, VD; Svend E. Kold, DMV, MRCVS, RCVS Specialist in Equine Surgery (Orthopedics); C. Wayne McIlwraith, BVSc, PhD, DSc, FRCVS, DACVS; Dale A. Moore, MS, DVM, MPVM, PhD; Sheldon Padgett, DVM, MS, DACVS; Tracy A. Turner, DVM, MS, DACVS, DABT; Stephanie J. Valberg, DVM, PhD, DACVIM; John F. Van Vleet, DVM, PhD