The hock is a complex joint of the rear limb situated between the stifle and the ankle that consists of six bones and four joints. Similar to the human ankle--but elevated and bending backward--the hock works in concert with the stifle, flexing and extending together with the stifle to achieve rear end propulsion. As the pivotal hind limb joint, the hock receives considerable forces. It's no surprise, then, that this hard-working joint is the most common site for rear limb stress injuries, performance-limiting problems, and pain.

As inflammatory changes develop, normal hock structure and function are interrupted. The joint capsule can swell or a bony enlargement can develop on the inside of the hock, signaling inflammation of the joint capsule. Eventually, degeneration of the joint cartilage and remodeling of the underlying bone can occur.

The two most common hock disorders are distal tarsal osteoarthritis (bone spavin) and tarsocrural effusion (bog spavin).

Bone Spavin

Distal tarsal osteoarthritis--also known as spavin, bone spavin, jack spavin, blind spavin, juvenile spavin, and occult spavin--is a common cause of hind limb lameness in performance horses and is probably the most common cause of hock lameness or pain in all horses, says Julie Dechant, DVM, MS, Dipl. ACVS, an equine emergency clinician at Oklahoma State University. "Spavin typically refers to problems in the narrow lower joints in the hocks," she explains. "These joints do not appear to contribute to much motion in the hock joint--the majority of motion originates from the large upper joint--but they can be a significant source of pain when inflamed or arthritic."

See diagnostic images of spavin here

Dechant, who has authored or co-authored several papers relating to hock problems and conducted a study on a surgical hock treatment, maintains a special interest in hock disorders. She says there are three main types of spavin:

  • Tarsitis (blind spavin/occult spavin) is seen in young racehorses and young performance horses, she says. "It is associated with soft tissue inflammation around or in the small joints of the hock, although there are no radiographic changes."
  • Juvenile spavin, also seen in young horses, is associated with abnormal development of the small cuboidal bones in the hock, possibly due to osteochondrosis (OCD) or incomplete ossification of these bones in premature or dysmature foals (full-term or over-term foals with signs of being underdeveloped or premature), with subsequent collapse or crushing of these bones. Notes Dechant, "Juvenile spavin is distinguished from other types of spavin in that these horses typically have not experienced sufficient work or training to cause arthritis in any joint."
  • Adult onset spavin (typically referred to as bone spavin) is probably the most common form of spavin and is associated with "wear and tear" or repetitive trauma due to performance demands in mature or older horses, Dechant states. "In addition to the factors associated with tarsitis and juvenile spavin, other risk factors include performance events that stress the hind limbs (Western performance/ rodeo events, jumping, dressage, pulling events), some shoeing practices (trailers, calks, hoof imbalance), poor conformation (post-legged, cow-hocked, sickle-hocked), and injury to the small tarsal bones from infection, fracture, luxation (complete dislocation), or crushing/collapse. Breed disposition would be associated with conformation and breed-prevalent performance demands."

Bone spavin usually presents as a chronic, intermittent lameness that worsens with increasing performance demands. "Although lameness is a frequent complaint," Dechant says, "spavin may also be associated with less specific signs such as reduced performance, reluctance to work off the hind end, behavioral/training problems, and back pain."

According to Mark D. Markel, DVM, associate dean at the University of Wisconsin's School of Veterinary Medicine, "Horses with bone spavin exhibit a gradual onset of lameness and tend to become more lame if worked hard. There is a tendency to gallop with a stiff gait because of pain on flexion of the hock. Generally, affected animals will be most lame when they are first put to work for the day, but will warm out of the lameness." (From UC Davis Book of Horses: A Complete Medical Reference Guide for Horses and Foals.)

Diagnosis is based on history, physical exam (conformation, appearance of hock), lameness exam, and flexion tests, followed by radiographs. Says Dechant, "Diagnostic local anesthesia or nuclear scintigraphy may be used to help in the assessment of atypical or complicated cases."

The two main types of bone spavin treatment are medical and surgical therapies.

Medical Therapies

"Medical treatment is the mainstay and is usually the first and only treatment required to manage most horses with spavin," Dechant states. Medical therapies often used in combination include:

  • Altered performance demands or reduced exercise used initially to manage the problem until other therapies can take effect, or used permanently for older horses or horses with breeding value which are near the end of their performance careers;
  • Intra-articular (within the joint) corticosteroids are "very commonly used as the primary treatment," says Dechant. "Intra-articular corticosteroids are not a cure, but they provide short-term, anti-inflammatory relief typically for a few months to one year, depending on the horse, severity of spavin, and performance demands."
  • Pain relievers and anti-inflammatory agents, typically phenylbutazone (Bute), can provide short-term relief for either initial management of the spavin or intermittent management of occasional-use horses. Toxic effects of Bute limit its use as a long-term treatment.
  • Shoeing changes to improve breakover and restore hoof balance can help. Avoid using toe grabs, calks, and trailers in affected horses, if possible.
  • Natural fusion (ankylosis) of the two lower joints can be allowed when cartilage degeneration is already underway. "While this should eventually eliminate the pain, for various reasons it is not successful or appropriate for many horses with spavin," Dechant warns. "It is a slow and unpredictable process that only happens successfully in a small number of horses with bone spavin."
  • Adjunctive medications including intra-articular or intravenous hyaluronic acid (HA) and intramuscular polysulfated glycosaminoglycans (PSGAG).
  • Oral nutraceutical joint therapies and complementary therapies (therapeutic ultrasound, acupuncture, chiropractic, electromagnet therapies, cold lasers, and extracorporeal shock wave therapy--ESWT).

Dechant cautions that opinions on the effectiveness of oral nutraceuticals and complementary therapies vary between veterinarians, with little good research to document benefits from complementary treatments. She does note three exceptions:

  • Research conducted by equine locomotion researcher Hilary M. Clayton, BVMS, PhD, MRCVS, Mary Anne McPhail Dressage Chair in Equine Sports Medicine, large animal clinical sciences, Michigan State University, suggested that Corta-Flx--an oral product for treatment in horses with bone spavin--resulted in a more symmetrical gait pattern (interpreting that the joint function was improved).
  • A 1997 study by Reid Hanson, DVM, Dipl. ACVS, assistant professor of surgery and co-director of large animal surgery residency training at Auburn University, evaluated Cosequin as a treatment for degenerative joint disease in seven horses (out of 25 total) that had spavin. "Overall, there was an improvement in lameness for all horses, although the results aren't subdivided for the different types of degenerative joint disease," Dechant states.
  • Studies by Scott McClure, DVM, Dipl. ACVS, assistant professor of equine surgery, now at Iowa State University, and G. David McCarroll, DVM, Dipl. ACVIM, of Interstate Equine Services in Goldsby, Okla., found extracorporeal shock wave therapy (ESWT) decreased lameness from bone spavin in 80% of treated horses. "Note that horses were improved, not necessarily sound, and the study was not controlled and the investigators were not blinded to the treatment," Dechant states. "I am impressed by the anecdotal reports of success of ESWT, but have no personal experience or observation with the procedure."

Overall, Dechant believes that oral joint nutraceuticals are harmless and might offer potential benefit; that acupuncture and chiropractic (provided by an appropriately trained and qualified equine acupuncturist or chiropractor) could be particularly beneficial in horses with significant lumbar back pain secondary to the hock pain; and that therapeutic ultrasound, magnets, or cold lasers are without research specifically supporting their use.

Surgical Treatments

When medical management doesn't work--when intra-articular medication is ineffective or the results last less than eight to 10 weeks--surgery might be the only other option, says Dechant.

One method is surgical fusion (arthrodesis) of the distal (lower) two joints (the distal intertarsal and tarsometatarsal joints). "There are multiple techniques for arthrodesis, but they can be generalized into chemical arthrodesis, drilling of the joints, and laser arthrodesis," Dechant explains.

Chemical arthrodesis is controversial and is associated with potential severe complications, such as severe pain following injection, tissue necrosis (severe damage and loss of tissue), joint infection, and progression of arthritis to other hock joints. There are variable success rates, plus the chemical used (sodium monoiodoacetate) is not yet approved by the FDA.

"Hock drilling to remove cartilage and hasten joint fusion can be performed with or without additional stabilizations (bone plates or bone pins); success rates are variable depending on the technique (0-80%) and it may take up to a year to see full effect," Dechant adds.

In her study (which has been accepted for publication in the Journal of the American Veterinary Medical Association and presented in part at a convention of the American Association of Equine Practitioners) that used the technique in 54 horses, 59% were considered successful, 11% were improved, and 30% were unimproved after hock drilling surgery.

"Laser arthrodesis is a new technique, but good success on a limited number of horses has been reported," says Dechant. "For laser arthrodesis, experience with the technique is required, and not all horses may be candidates for the technique. Further experience and research will be needed to adequately compare this technique to other methods of management."

In this technique, a laser fiber is passed into the joint through a needle while the horse is under anesthesia. The proposed goal of the procedure is to heat the cartilage and joint capsule by vaporizing the synovial (joint) fluid. Heating the cartilage should cause the cells in the cartilage to die, resulting in eventual joint fusion, and heating the joint capsule might alter the collagen in the joint capsule and stabilize the joint. This is the proposed mechanism of action, and although studies are underway to assess these results, no findings have been published as of this time.

Another surgical option for treating bone spavin is cunean tenectomy. Performed under sedation and a local anesthetic, a portion of the cunean tendon, which runs diagonally across the inside of the lower hock, is removed through a small incision. The proposed theory is that removal of this tendon might relieve pressure over the inside of the hock and alleviate pain associated with the cunean bursa (a small synovial fluid sac which lies underneath the tendon).

"This is a controversial technique that has variable benefit and frequently results in a significant cosmetic blemish, but is easy to perform by a veterinarian. In one study, it was associated with high owner satisfaction," says Dechant. "Although this technique has been used with apparent success, critical evaluation of the technique has not been as favorable, and intuitively it is difficult to understand how the surgery might work."

Because arthritic changes occurring from distal tarsal osteoarthritis are chronic and irreversible, the prognosis depends on how well the problem is managed, the severity of bony changes, and the work expectations for the horse. Markel writes that some horses will exhibit mild spavin that responds to corrective shoeing and occasional administration of anti-inflammatories, while other horses develop severe, career-limiting degenerative changes.

Horse owners might be able to minimize the risk of distal tarsal osteoarthritis, says Dechant, by promptly treating injuries, fractures, or infections of the hock, modifying performance demands in poorly conformed animals, monitoring training in young horses for the early signs of hock pain discussed previously, and promptly treating poorly developed cuboidal bones in premature or dysmature foals to prevent damage to the developing bones. Damage to poorly developed cuboidal bones might be prevented by restricted exercise, supported weight bearing (use of a sling to support the foal when he nurses or stands), and/or use of braces, splints or casts to minimize the degree of deformity or crushing in these bones during weight bearing. None of these treatments are guaranteed to prevent crushing--success might depend on the severity of bone underdevelopment, size and activity of the foal, and constraints provided by the management situation (e.g., management on the farm vs. management in an intensive care foal unit). Once the bones have been crushed, the success of subsequent management for arthritis depends on the degree of damage to the bones and joints, the joints involved, and the mature size and performance aspirations for the horse (i.e., miniature horses used as pets are more likely to tolerate damage than Warmbloods destined for advanced training or competition).

Bog Spavin

Tarsocrural effusion, also referred to as bog spavin or joint swelling, affects the large, upper hock joint. "This joint is responsible for the majority of motion in the hock," Dechant notes. "(Bog spavin) can be an unsightly, if not disabling, problem."

The most common cause of bog spavin, Markel writes, is osteochondrosis dissecans (OCD). Other causes include joint trauma, chronic inflammation, and bleeding into the joint. Horses with poor hind limb conformation tend to be predisposed (straight hocks are more subject to hock strain and injury), as are certain breeds--OCD of the hock is more prevalent in Standardbreds, Thoroughbreds, Quarter Horses, Arabians, and draft horses.

"Onset usually is acute," Dechant says, "with sudden swelling of the joint, but the swelling may become chronic if the cause is unsuccessfully or inappropriately treated." Clinical signs include variable degrees of fluid swelling in the hock joint and variable degrees of lameness; some horses will exhibit little or no lameness.

Diagnosis is made by history, physical examination, lameness examination, flexion tests, and radiographs. "If radiographs are unrewarding, the joint may be tapped to see if there is infection, abnormal joint fluid from inflammation, or bleeding into the joint," says Dechant. "In certain cases, local anesthesia may be necessary to localize the origin of lameness."

Treatment depends on the underlying cause. OCD or fracture fragments can be removed with arthroscopic surgery (see images on page 70). For problems unrelated to OCD, intra-articular corticosteroids and intra-articular or intravenous hyaluronate or intramuscular polysulfated glycosaminoglycans can be administered to reduce inflammation and swelling. "Oral nutraceuticals may be beneficial, too," adds Dechant. "Anecdotally, a number of veterinarians find that atropine added to intra-articular corticosteroids further improves the joint swelling. For show horses where cosmetic appearance is important, intermittent draining of the joint fluid (by a veterinarian), dimethyl sulfoxide (DMSO) sweats, and hock bandages may temporarily reduce the joint swelling."

When treated promptly, the prognosis for bog spavin is good to excellent. "However, if osteoarthritis develops in the joint, the prognosis is guarded because of the importance of that joint in motion of the hind limb," Dechant says. "If the joint swelling becomes chronic, it can become a permanent cosmetic blemish, but depending on the underlying cause, it may (or may not) be associated with lameness."

To minimize your horse's risk of getting bog spavin, Dechant recommends prompt and appropriate treatment of any hock injuries or hock problems in order to prevent osteoarthritis and chronic swelling. "In predisposed breeds, radiograph the hocks to assess for the presence of osteochondrosis in yearlings or long yearlings, then schedule preventive surgical removal of the osteochondrosis fragment before sale, show, or training," she says.

Bog and bone spavin don't necessarily have to end your horse's performance career, but they certainly require careful attention and care. Every horse and situation is different, so you will need to work with your veterinarian.

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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