Several theories have been suggested on the origin of synovial cysts. Most are contiguous with the facet joint capsule itself. Other theories suggest displaced cell rests of synovium, degeneration of the collagenous tissue of the joint capsule, or neoformation from articular tissue resulting from chronic irritation and inflammation1,7)
. Symptomatic synovial cysts of the facet joints in the lumbar spine have been the focus of study recently5,11)
. The most common cause reported is degenerative facet joint disease with hypertrophy and herniation of synovium through a tear of the capsule. Rarely, cysts have been observed to develop secondary to rheumatoid arthritis2,6,8,9)
. However, there have been no cases reported in association with AS; although Vergne et al.10)
considered the possibility of this association. As the synovial fluid not only exists in the spinal canal, but also appears in ligament or tendons, there is possibility for it to be occured without attaching with facet joint. We, however, could not find any case which developed foraminal location at L2-L3 level. In this case, as the actual section of the facet joint was undefinable due to ankylosis of AS, we could not conclude whether the connection with facet joint exists or not, but considering the levels and location, in the case of synovial cyst that was not detected in the initial MRI taken 5 years ago, the possibility of formation of intraspinal synovial caused by chronic irritation by posterior segment fracture as the natural course of AS is worth taking into consideration. Although a casual relationship cannot be definitely established, spondylolisthesis was absent and there was no history of trauma. The occurrence of this finding in a patient with AS might be explained predominantly by an enthesitis rather than arthritis, which might make it less likely to cause a synovitis initially. A synovial cyst that develops from arthritis in a facet joint later in the course of the disease might not protrude through the ossified periarticular ligaments. Osteoporosis is a well-known complication of AS, and bone loss is the result of changes in the material and structural properties of bone; these changes lead to an increased risk of stress fractures. During the early course of AS, a simple osteoporotic compression fracture occurred after a minor injury in the patient reported here. Over time, with the chronic spinal inflammation the vertebrae likely fused together at the posterior segment fracture through the facet joints and appeared as chronic findings identified as sclerosis at the fracture margins along with pseudoarthrosis. This case showed sagittal imbalance as a consequence of the old compression fracture in the first lumbar vertebral body. Kyphosis secondary to a compression fracture indicates greater axial distraction stress or force loaded onto the posterior segment and the facet joint adjacent to the fracture level compared to normal spinal alignment. Finally, it is possible that the long-standing effects of repetitive stress caused by sagittal imbalance and the natural course of the increased vertebral stiffness were concentrated at the adjacent posterior element, through the facet joints and might have been contributing factors causing the stress fracture of the posterior element and the synovial cyst.