Bone Cyst: The Case of the Missing Bone

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.

On radiographs, subchondral bone cysts show up as dark areas. This X-ray shows cysts in the area of a horse

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.