Although the primary lesion lies with disc degeneration, facet joint damage follows closely. Degenerative changes of the facet joints show synovitis as their earliest feature. This is seen on MRI as high signal intensity in the intraarticular space consistent with fluid on T2-weighting (25). This progresses to joint space narrowing and articular cartilage damage in a variety of forms, ranging from total loss to chondromalacia in the form of fibrillation, fragmentation, and loose bodies. Joint capsule laxity ensues, owing to the cartilaginous changes and chronic joint effusions that distend the capsule (20). Ligamentum flavum hypertrophy, articular process overgrowth, and osteophytosis are found in later stages (25). The net result is instability at the facet joints with possible anterior or posterior subluxation.
Osteophytosis of the articular processes may encroach on different areas of the spinal canal. Because of the anterolateral location of the superior articular process of the inferior vertebra, osteophytes may grow anteromedially to narrow the lateral recess of the spinal canal. The postero-medially located inferior articular processes may form osteophytes that directly impinge upon the central canal (20).
With facet joint degeneration, synovial cysts may develop. Oftentimes, they are found at the medial aspect of the facet joint and protrude into the spinal canal. They may also encroach on intervertebral foramina and affect the exiting spinal nerves (20) (Fig. 5). These cysts are believed to originate from degenerative changes that cause chronic joint effusions with proliferation and expansion of the joint capsule. They are most common in the lumbar spine and are rarely bilateral (26). Because they are fluid filled, they are seen as high signal masses on T2-weighted images. This is in distinction to fibrous and chondroma-tous masses that can be found in the same area, which possess low signal intensity (20). Enhancement may be seen in the cyst wall with post-contrast imaging.
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