Synovial Cysts

Juxtaarticular or synovial cysts are synovium-lined cystic masses that arise adjacent to degenerated facet joints and usually communicate with the joint space. Synovial cysts are most common in the middle and lower lumbar regions, with cervical lesions being relatively uncommon. The result is an extradural mass that lies posterior or posterolateral to the thecal sac within the spinal canal. Cyst contents are variable and include serous fluid, more viscous gelatinous material, hemorrhage, or gas.

This accounts for the varied appearance of synovial cysts on imaging studies. On CT, cysts range from hypodense to hyperdense (the latter indicating hemorrhage or protein-rich fluid), and the cyst wall may be calcified (Fig. 4). The lesions may be difficult to detect on CT or, when of soft tissue attenuation, may be misinterpreted as a free disc fragment. The MR appearance on T2-weighted images is usually characteristic, with a low signal intensity rim and high signal centrally. A fluid level may be present within the cyst cavity. The capsule may enhance following contrast administration. On injection of the adjacent facet joint, the cyst will often fill with contrast, indicating communication with the joint space. For that reason, one of the management strategies for symptomatic synovial cysts is corticosteroid injection into the affected joint. Others have advocated percutaneous drainage under image guidance, with or without the instillation of steroids. Surgical removal via laminectomy is generally reserved for large lesions with significant mass effect.

PARS INTERARTICULARIS DEFECTS (SPONDYLOLYSIS)

Spondylolysis represents a unilateral or bilateral defect in the pars interarticularis of the vertebra. Alignment may be normal or there may be accompanying spondylolisthesis. The etiology of spondylolysis has long been debated. Current consensus would favor that the lesion is an acquired fatigue fracture secondary to repetitive stress rather than congenital. The recognized spike in incidence in school-age children would support that perception. It is thought to be present in as many as 3-7% of the population, with a male predominance. The lower lumbar region is most commonly affected with involvement at the L5 level in two thirds of cases. The incidence decreases at each ascending level in the lumbar spine. Involvement of the cervical spine is uncommon.

When plain radiography is used, pars defects are most clearly visible on oblique projections. A radiolucent cleft is identified through the neck of the "Scottie dog," which describes the appearance of the pedicle, facet joint, and lamina in that imaging plane. Plain films may be nondiagnostic owing to poor technique, improper positioning, or if there is superimposed facet osteophyte and sclerosis. CT is more definitive and will clearly demonstrate a break through the region of the pars interarticularis (Fig. 5). In the axial plane, contiguous images will fail to demonstrate a complete ring at the affected level. The appearance on sagittal reformations mimics that of the oblique radiograph. In the unusual situation of a unilateral pars defect, hypertrophy and sclerosis of the contralateral pedicle and lamina will be seen, related to asymmetric loading stress. The findings may lead to an erroneous diagnosis of osteoid osteoma. Similar changes can result from congenital absence of a pedicle, lamina, or articular facet. MRI is frequently diagnostic in the setting of pars defects, with CT being confirmatory in equivocal cases. All of the standard imaging modalities will depict spondylolisthesis associated with spondylolysis, although CT and MRI have the added advantage of quantifying the degree of secondary central canal and neural foraminal stenosis. Increased activity on bone scan in the region of the spondylolysis has been correlated with clinical activity.

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