My 72-year-old father had surgery to remove some cysts from inside his spine. The pathology report just came in the mail today. It described them as synovial cysts with intraligamentous bursal communicating channels connected to the facet joint at the L3-4 level. Evidently, they were taking up space inside the spinal canal putting pressure on the nerves and causing a heck of a lot of back and leg pain. What are these cysts and what causes them?

A synovial cyst is a mass linked with a joint. It is formed when leakage of synovial fluid from inside the joint forms a gel-filled pouch lined withepithelial cells or cells that are epithelial-like. Epithelial cells are special cells that line cells the cavities and surfaces of structures throughout the body, including cysts. In the case of these cysts, there is a channel connecting joint to cyst. The channel may be long enough that the cyst isn't even next to the joint.

Studies show there are different types of spinal synovial cysts based on form and structure. Some have no lining and are very inflexible (lacking elasticity). Those that have a lining vary from having a very thin to a very thick lining made of synovial cells. Some of the cysts have walls that are calcified or hardened. Others have new formation of tissue with a good blood supply that turned clots into fibrous scar tissue.

Arthritic changes or trauma seem to be the first step in the formation of these cysts. One study showed that in about three-fourths of cysts examined, a channel formed between the cyst and the joint. Fluid from the damaged joint leaked out, traveled down the channel and filled the cyst with fluid from the joint. The channel is what is called the intraligamentous bursal communicating channel mentioned in your father's pathology report.

As a result of this direct connection, a large percentage of these cysts have bone and cartilage debris from the osteoarthritic process embedded in the cyst wall. Some of the cysts can be pressed up against osteophytes (bone spurs) that have formed around the joints. The cysts are filled with blood and scar tissue and surrounded by a layer of additional scar tissue. It appears that the risk of synovial cysts is greatest in the lower lumbar spine where the arthritic changes are the most severe. It is here that debris collects and blocks the channels.

Martin J. Wilby, MA, MB, BChir, PhD, FRCS, et al. The Prevalence and Pathogenesis of Synovial Cysts Within the Ligamentum Flavum in Patients with Spinal Stenosis and Radiculopathy. In Spine. November 1, 2009. Vol. 34. No. 23. Pp. 2518-2524.

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