For weeks you've been puzzled by your horse's on-again/off-again hind-leg lameness. It first showed up after he took an extended romp in the pasture; you gave him some time off, and it seemed to improve. But not long after you put him back in work, the hitch in his gait returned.
Now your veterinarian is on the phone with the results of X-rays she took of your horse's stifle. The pictures show the cause of his problem: a subchondral bone cyst, or SBC. That's not good news, she says.
An SBC is a pocket of missing bone: a fluid- and scar-tissue-filled cavity just under the cartilage that covers the bone surface at a joint. ("Subchondral" means "below cartilage.") These lesions can turn up in many locations, but they're most common in the stifle and fetlock. They're a significant cause of lameness at those sites, and they're difficult to treat. Even surgery may not return the horse to full soundness.
Searching for better results, veterinarians are working on some promising new approaches to treatment using bone grafts and stem cells. In this article we'll look at some of those treatments and describe what's known about the ways in which SBCs form. Twenty years ago, most veterinarians considered SBCs to be a form of developmental bone disease, or osteochondrosis--a problem of young, growing horses. Now, however, there's evidence that development is just one of several possible causes.
What and Why
In joints such as the fetlock and stifle, smooth cartilage provides protective covering on the ends of bones. This covering lets the bone ends glide past each other for pain-free joint movement. The whole joint is wrapped in a fibrous capsule and lubricated by thick fluid. Although SBCs develop in bone, some of the other joint tissues are also involved.
A cyst begins when a gap or hole in the cartilage allows joint fluid to seep into the bone, where it erodes tissue. Many SBCs grow larger over time and cause painful irritation in the bone. If fluid and debris from the cyst leak out through the cartilage hole, an SBC can also create inflammation within the joint lining.
Bone and joint inflammation are both involved in causing pain, lameness, and loss of performance in the horse. Over time, the chronic inflammation degrades the joint, producing osteoarthritis.
SBCs are most common in horses age three or younger, but they've been reported in older horses--even some in their late teens. Quarter Horses have the highest incidence of stifle SBCs, followed by Thoroughbreds; but no breed is immune.
Why the cysts form is not always clear, but it seems that several factors can be involved.
Development. Bones develop from cartilage that gradually mineralizes, becoming hard. At joints, a layer of cartilage remains over the bone ends to provide a slick working surface for the joint. This part of bone development is complete in most horses at about eight months of age (although bone growth continues for some time). But a glitch in the cartilage-to-bone process just under this layer can leave a retained core of cartilage in the underlying bone. In some cases lesions form and a fragment of cartilage or bone may break away, a condition called osteochondritis dissecans (OCD). Some bone cysts may be linked to OCD--like OCD, they usually appear in young horses--and sometimes a horse has both conditions.
Injury.There's strong evidence that direct injury to joints can cause SBCs to form. For one thing, the cysts form in adult horses as well as in youngsters whose bones are still maturing. The trigger is probably a one-time impact that's strong enough to cut into the cartilage, rather than gradual wear and tear over a long period of time. In many cases SBCs are discovered after injury to a joint, suggesting that trauma caused the lesion. In studies, researchers have created these lesions by cutting cartilage.
Weight bearing. The location of the cysts points to a connection with use; they appear at weight-bearing joint surfaces. In the fetlock, lesions are most common in the front legs: at the lower end of the cannon (metacarpal) bone, on the inside--the spot that bears the most weight. In the stifle, they nearly always turn up in the inside lower end of the femur, which also bears more weight than other parts.
Conformation. SBCs are common in horses with straight, upright conformation--that is, those with upright pasterns and straight shoulders and hind legs. The straight angles mean that the joints flex less to absorb shock, increasing impact and raising the risk of injury.
A combination of upright conformation and fast growth, which is linked to developmental bone disease, seems to set up young horses for this problem. In many cases, though, the cysts don't cause lameness until the youngster begins training. It's not clear whether they developed before or after increased stress on the joints.
When older horses develop these cysts, it's usually in conjunction with arthritis; many of them have bone spurs and other problems in the affected joints. Humans get SBCs, too, typically in connection with arthritis in their knees.
If your horse has a bone cyst, he may be severely lame with or without obvious heat or swelling at the site. The lameness is typically intermittent--it improves with rest but then, when the horse goes back to work, worsens again. This happens because the rest period allows the inflammation in the joint to ease but the cyst itself remains. When the horse begins to exercise and put more stress on the joint, the gap in the cartilage acts like a ball valve: It opens to let the cyst contents leak into the joint, and the inflammation flares up again.
Your veterinarian can pinpoint the site of the problem through a lameness exam and local nerve blocks. With this condition, blocking the affected joint typically doesn't bring full relief. That's because SBCs create inflammation both in the bone and in the adjacent joint, and the anesthetic numbs the joint but not the bone.
If the examination leads your vet to suspect a cyst, she'll take x-rays to confirm the diagnosis. On radiographs, SBCs show up as dark areas, in some cases shallow and saucer-shaped, in others round or cone-shaped and maybe half an inch deep. The defect in the cartilage over the cyst doesn't appear.
A subchondral bone cyst is bad news because once these lesions begin to cause soreness, they generally continue to do so. Rest (up to six months), a course of nonsteroidal anti-inflammatory drugs (such as phenylbutazone or flunixin meglumine--"bute" or Banamine?) and injections of sodium hyaluronate (such as Legend?) or polysulfated glycosaminoglycan (such as Adequan?) may bring some relief. But the cyst doesn't resolve. It's always there, ready to flare up when the horse starts to work again.
The outlook is especially poor for an older horse that has extensive arthritis as well as a cyst. He may have to step down to light work, or he may need to retire. For younger horses and those without multiple problems, surgery may offer the best hope.
There are a number of surgical treatments for SBCs, all done with arthroscopy. In this technique, the surgeon inserts an arthroscope--a tubelike instrument with optical fibers and lenses--into the joint through a tiny incision. The arthroscope is connected to a video screen so the surgeon can see inside the joint. Additional small incisions are made to insert other instruments used in surgery.
Arthroscopic surgery causes less trauma and pain and promotes a faster recovery than traditional "open" surgery. But almost all arthroscopic procedures still call for general anesthesia, as well as for the special equipment and expertise available at a good equine clinic.
Debridement. The traditional and most common surgery is debridement. The surgeon basically digs out the cyst, scraping away the fibrous scar-tissue lining that forms inside it and leaving as much cartilage intact as possible. The goal is to leave a healthy bone surface that will heal for a pain-free, functioning joint. Post-op care varies; usually the horse will need two or more weeks of stall rest after surgery, to allow inflammation to subside and healing to begin. That's followed by weeks of hand-walking and then a gradual return to work over several months.
The cost varies but is typically around $2,000. Success rates are mixed: In two- to four-year-old Thoroughbreds, 50 to 60 percent return to racing after the surgery; in Quarter Horses, only about 40 percent go back to full performance.
Bone grafts. One reason for the spotty success of traditional surgery is that the old cyst doesn't fill (or fills only partly) with new bone, leaving a less-than-perfect surface at the joint. To get better outcomes, veterinary surgeons have long been searching for bone- and cartilage-grafting techniques. One approach is to surgically remove some bone tissue from the horse's sternum (breastbone) and pack it into the lesion after debridement. Because the tissue is the horse's own, his body will not reject it.
The graft isn't just a plug; it encourages new bone to form at the lesion site. Bone tissue has an amazing ability to remodel, and eventually a graft can morph into a seamless part of the bone. However, getting the tissue for the graft does involve additional surgery, increasing pain for the horse and cost for the owner. And to succeed, the graft must be sealed in place so that it won't fall out into the joint. At Cornell, where I work, we've developed a technique in which the bone graft is sealed with injectable cartilage grafts. Preparation of these grafts is currently available only at Cornell (which has the laboratory facilities required for the process), but we are working on developing similar techniques that can be applied by any equine surgeon.
Bone substitutes. With an eye to putting better results within reach for more horses, we're also working on bone substitutes, which would eliminate the need for taking a bone graft from the horse. Primarily, we're investigating tricalcium phosphate, which stimulates bone growth. After this substance is placed in a lesion, the body gradually replaces it with new bone tissue.
Stem cells. Instead of using actual cartilage for grafts, we are working with stem cells. These cells are found in various parts of the body and have the ability to transform into different types of tissue. Our procedure involves concentrating stem cells derived from bone marrow (by spinning them in a centrifuge) and injecting them into the site.
We're studying this now to see if these stem cells will reliably form cartilage. If they do, the procedure could be made widely available. Using stem cells and bone substitutes would probably add only $500 to the cost of traditional surgery.
Lisa A. Fortier, DVM, PhD, is an Assistant Professor of Large Animal Surgery at the College of Veterinary Medicine, Cornell University, in Ithaca, N.Y. Her laboratory there is investigating the underlying cellular and molecular mechanisms involved in the development of arthritis, with the goal of identifying new and better ways to treat and prevent that disease. The lab's website is www.vet.cornell.edu/labs/fortier.
This article first appeared in the August 2006 issue of Practical Horseman magazine.