Subchondral Bone Cyst

(Courtesy of Lauren Lamb)

By Lauren Lamb

Last month we talked about osteochondritis dissecans (OCDs). This month we are going to talk about subchondral cyst, which is another type of bone disease commonly seen in horses. Unlike like OCDs, which are strictly a bone development disease, subchondral cyst can be developmental or acquired. Acquired means that the horse is not born with the cyst in their bone, but instead it develops secondary to an injury to the cartilage. Subchondral cysts are characterized as an area in the bone, usually adjacent to the joint surface, which is less dense and lined with an inflammatory lining. On radiographs the cyst will appear as a dark hole in the bone.

The cause of subchondral cyst is not completely understood. Two predominant theories have been supported by research. The first theory is called the hydraulic theory. With the hydraulic theory, the articular cartilage has full thickness damage, like a small split or tear.

Synovial fluid is forced into the cartilage defect while the horse is weight bearing. The fluid will place pressure on the subchondral bone, which will result in damage and necrosis of the bone. The subchondral bone is the bone directly beneath the cartilage. Ultimately, this damage to the subchondral bone will lead to a subchondral bone cyst.

The second theory is the inflammatory theory, which causes a cyst to form and enlarge secondary to cellular and molecular mechanisms of inflammation within the subchondral bone. Subchondral bone cysts are seen predominately, about 50 percent of the time, in the stifle.

The second most common location is the long or short pastern bone, around 25 percent. About 62 percent of subchondral bone cyst occur in males, both stallions and geldings. Thoroughbreds and quarter horses are the breeds most commonly affected.

The most common clinical sign is lameness in the affected leg. Joint effusion (excess fluid within the joint) may or may not be present. Joint effusion is more frequent when the subchondral bone cyst communicates with the joint. Only 30 percent of subchondral bone cysts communicate with the joint. The pain causing the lameness is secondary to increased pressure within the cyst or increased inflammation in the bone around the cyst. The tissue within the cyst is termed myxomatous tissue. This tissue secretes inflammatory mediators that will dissolve the bone around the cyst.

Diagnosis of a bone cyst usually requires a combination of a thorough lameness exam and diagnostic radiographs. The lameness in the leg with a bone cyst will become worse with flexion of the joint adjacent to the subchondral bone cyst. Intra-articular anesthesia will improve the lameness in most cases. Radiographs are used to definitively diagnosis subchondral bone cyst.

Size does matter when you are talking about subchondral bone cyst and the prognosis for a horse to make a full recovery. Specifically, it is not the overall size of the cyst that matters, but the size of the articular surface that is damaged by the cyst. Cysts that involve a large articular surface area carry a poor prognosis for a full recovery; on the contrary, a cyst can be large but not involve a large area of the articular surface. A cyst with a small articular component will have minimal detriment on the horse’s prognosis.

Read the September issue to learn more!