Synovial Cyst of the Spine: What Is It?

Surgeons removing synovial cysts from the spine have noticed some interesting things about these structures. Sometimes they are located quite a bit away from the joint they originate from. Sometimes they are on both sides of the joint. Sometimes they contain bits of joint cartilage, scar tissue, and even fragments of bone. These unusual findings led the researchers who wrote this article to study these cysts more closely and report on what they are made of and how they are formed.

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.

The cysts studied by these researchers were located within a spinal ligament called the ligamentum flavum. Because this ligament stretches the length of the spine and is located along the inside of the spinal canal, any thickening of the ligament can put pressure on the spinal cord inside the spinal canal. Cysts such as these synovial cysts when embedded within the ligamentum flavum contribute to a painful condition called spinal stenosis. Stenosis is a narrowing of the spinal canal.

Specimens removed from 27 patients were microscopically examined in the lab to determine structure and biologic makeup. These sections were compared with similar tissue samples taken from cadavers (bodies preserved for study after death). None of the cadavers had any evidence of spinal stenosis, so could be considered normal from that perspective and therefore function as the control or comparison group. The authors were particularly interested in finding out if the breach in joint integrity leading to fluid leaking and cyst formation could contribute to the development of joint osteoarthritis. Spinal joints are referred to as facet joints.

The cysts could be divided into four different types based on form and structure. Some did not have a lining and were very inflexible (lacking elasticity). Those that with a lining varied from having a very thin to a very thick lining made of synovial cells. Some of the cysts had walls that had calcified or hardened. Others had new formation of tissue with a good blood supply that turned clots into fibrous scar tissue.

In about three-fourths of the cysts (75 per cent), a channel formed between the cyst and the joint supplying the cyst with fluid from the joint. As a result of this direct connection, a large percentage of those cysts (89 per cent) had bone and cartilage debris from the osteoarthritic process embedded in the cyst wall. Some of the cysts were pressed up against osteophytes (bone spurs) that had formed around the joints. The cysts were filled with blood and scar tissue and surrounded by a layer of additional scar tissue. Some of the cysts did not have a connecting channel with the joint.

In addition to examination of the cysts, the joints were also X-rayed and classified using the Kellgren and Lawrence grading system for osteoarthritis. For the cadavers without cysts, the same grading system was applied to the facet joints for comparison. X-rays were also used to view alignment of the vertebral bones because it is suspected that one of the major causes of synovial cysts in the spine is degenerative spondylolisthesis.

Spondylolisthesis is the forward slippage of one vertebral body over the one below it. As the vertebra moves forward, the spinal canal narrows. Anything that narrows the spinal canal can cause pinching (impingement) or compression of soft tissue structures such as the spinal cord and spinal nerve roots. The shift in the bodies of the vertebral bones also changes the normal alignment of the spinal (facet) joints. Any change in joint alignment can contribute to uneven wear and tear and the eventual formation of bone spurs, disruption of the joint integrity, and escape of joint fluids leading to cyst formation. So you can see, one thing leads to another and another and another.

Treatment for synovial cysts causing spinal stenosis is often surgical with a procedure called decompression. The surgeon removes any structures compressing the neural tissue. If the spondylolisthesis is severe enough, it may be necessary to fuse the spine to keep it from slipping further.

You may be wondering, what about the cadaver control group? What did they find there? Well, there were no cysts and no stenosis. Some of the cadavers did have severe facet joint arthritis. There were similar communicating channels present even when no cysts were present leading the authors to suspect these channels are normally present and serve some function. Clearly, they are used to form cysts when arthritic changes in the joints results in synovial leakage and subsequent cyst formation.

It appears that the risk of synovial cysts in the ligamentum flavum 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. Arthritic changes seem to be the first step in the formation of these cysts. The new discovery of communicating channels from this study helps explain how these cysts can develop some distance away from the joints. For now, these channels are being called intraligamentous bursa-like synovial channels. Future studies may help identify their purpose in the normal anatomical spine structures. For now, they help explain the formation of the cysts under investigation here.

Reference: 

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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