Adjacent neural arches of the third cervical vertebra through the fifth lumbar vertebra articulate by pairs of facet (zygapophyseal) joints. These are diarthrodial joints between articular processes that project inferiorly and superiorly from the neural arches. The superior processes are usually anteromedial or anterolateral to the inferior processes, although the exact relationship varies from region to region and from individual to individual. The joint surfaces can lie nearly in an axial (e.g., cervical), sagittal (e.g., thoracic), or coronal (e.g., low lumbar) plane.
Figure 1. The facets are the “bony knobs” that meet between each vertebra to form the facet joints that join your vertebrae together. There are two facet joints between each pair of vertebra, one on each side. They extend and overlap each other to form a joint between the neighboring vertebra facet joints. Without the facet joints, you would not have flexibility in your spine, and you could only move in very straight and stiff motions. The facet joints are what are known as synovial joints. A synovial joint, such as the knee or elbow, is a structure that allows movement between two bones. In a synovial joint, the ends of the bones are covered with a material called articular cartilage. This material is a slick spongy material that allows the bones to glide against one another without much friction.
The facet joints of each level are usually symmetric. Depending on the orientation of the joint, gliding movement between the superior and inferior articular processes facilitates rotation or flexion and extension of the spine. The proportion of the spine’s weight born by the facet joint increases progressively toward the caudal end of the spine, and with the additional stress resulting from lumbar lordosis the weight home by the facet joints is maximal at L4-5 and L5-S 1. Therefore the facet joints and articular processes are larger in the lumbar spine than in the thoracic or cervical spine. The normal articular processes have smooth, even, uniformly thick cortical margins and approximately equal articular surfaces. Each articular surface is covered with articular cartilage.
The normal width of the articular cartilages as measured in CT images is 2 to 4 mm. A fibrous capsule with a synovial lining surrounds each joint. Small nerves that originate from the adjacent spinal nerves in the neural foramena innervate the joints. Stimulation of these nerves causes pain that is poorly localized and referred in the distribution of the adjacent spinal nerves. Therefore osteoarthritis in a lower lumbar facet joint may cause pain referred to the lower extremities simulating sciatic pain caused by a herniated disc. Hypertrophic changes in the superior articular facets that form the posterior border of the neural foramen may also cause sciatic pain by compressing a spinal nerve root in the neural foramen. Referred pain from arthritis and sciatic pain from compressive radiculopathy may be difficult to distinguish clinically.
The neural foramen immediately anterior to the facet joint contains fat, a portion of the ligamentum flavum, a root sheath containing the anterior and posterior spinal nerve roots, and small arteries and veins. The nerve roots, which are less than 1 mm in diameter, occupy only a small portion of the canal. All spinal nerves except the first and second cervical nerve roots traverse the upper part of a neural foramen. The C I spinal nerve roots have a course adjacent to the atlantooccipital articulation; the C2 spinal nerve roots have a course adjacent to the atlantoaxial articulation.
Because of their oblique orientation and curved surfaces, the lumbar and thoracic facet joints are visualized more effectively with CT than with conventional radiography or tomography. Therefore until the advent of CT, facet arthropathy could not be reliably and simply demonstrated. Facet joint disease may be demonstrated in 5% to 43% of patients with back pain, depending on the indications for CT scanning. CT can demonstrate facet joint disease and may save patients from unnecessary myelography and laminectomy. CT facilitates selection of patients for intra-articular injection of an anesthetic and antiinflammatory drugs,foramenotomy, or other surgical management. Although treatment of facet joint disease has not been perfected, articular injections temporarily and even permanently relieve pain that is secondary to facet osteoarthritis in some patients.
DEGENERATIVE DISEASES OF THE FACET JOINTS
Degeneration of the facet joints proceeds from synovitis to joint capsule laxity, articular cartilage erosion, osteophytes, and subperiosteal new bone formation. CT demonstrates findings from each of these stages of degeneration. The frequency of the findings in CT studies depends on the indications for CT scanning. Facet joint degenerative changes are most frequent at L4-5 and L5-SI and in patients with degenerative disc disease, previous spinal surgery, and advanced age.
As articular cartilage in the facet joint is eroded, the space between the articular processes narrows, and where cartilage is completely denuded, joint material herniates into the adjacent articular process. CT studies, without anatomic confirmation, suggest that a space between the lumbar superior and inferior processes measuring less than 2 mm indicates significant cartilage degeneration. Most joints narrowed to this degree have other evidence of osteoarthritis, such as subarticular erosions and sclerosis.
Figure 2. Plain CT scan showing normal facet [left] and articular facet degenerative disease [middle, right], notice reduction of the joint space, asymmetric orientation of the joint axis and osteophytosis
The first change as joint contents herniate through articular cartilage is subchondral erosions in the articular cortex, and then there is sclerosis in the adjacent medullary cavity. In 20% of patients with facet osteoarthritis and subchondral erosions, CT shows cystic changes in the osseous margins of the joint. Large erosions near the lateral joint margin may contain synovium.
As in other joints, cartilage erosion may cause pain. In the case of the lumbar facet joints, the pain may be referred in a sciatic distribution. Joint narrowing, subarticular erosions, and adjacent sclerosis, all demonstrated by CT, may explain sciatic pain in some patients. One technique used to evaluate the relationship of sciatic pain to the abnormal facet joint is a therapeutic trial of intra-articular steroids and local anesthetic, which in patients with CT changes of osteoarthritis often produces a temporary or even permanent relief of pain.
Figure 3. Plain CT scan showing articular facet degenerative disease, notice reduction of the joint space, asymmetric orientation of the joint axis and osteophytosis
Figure 4. Plain CT scan showing articular facet degenerative disease, notice reduction of the joint space, asymmetric orientation of the joint axis osteophytosis and facet hypertrophy (right)
The most common facet joint abnormality in patients with back pain is an osteophyte . The process is characterized as progressive thickening of the cortical margin of the articular facets that eventually results in dense bone encroaching on the adjacent spinal canal and neural foramen. One conventional way of measuring spinal canal narrowing caused by facet osteophytes is the interfacet distance. . In CT images, osteophytes can usually be diagnosed by a thickened cortex and the abnormal shape of the process without measuring the interfacet distance. Osteophytes commonly cause radiculopathy but not referred pain. Therefore facet joint osteophytes are not an indication for intra-articular injections, but possibly for foramenotomy.
Figure 5. Plain CT scan showing articular facet degenerative disease, notice reduction of the joint space ,asymmetric orientation of the joint axis osteophytosis and facet hypertrophy (right)
Enlargement of an articular process without thickening of the cortical margin is defined as hypertrophy. Because the normal development of the facet processes is governed by stress, hypertrophy may result from abnormal stress. CT shows hypertrophied articular processes having abnormal size but normal, uniform cortical margins . The incidence and clinical significance of hypertrophy have been insufficiently studied. Hypertrophy may possibly narrow neural foramena to cause radiculopathy.
Calcification is occasionally found in the juxta-articular soft tissues or ligamentum flavum around abnormal facet joints. The significance of the calcification is not known but may be the end result of synovitis. Clinical symptomatology and facet joint calcification have not yet been correlated. CT shows the calcification as regions of abnormally high density with CT numbers less than those of bone in the ligamentum flavum and soft tissues adjacent to the facet joint . Calcification in the ligamentum flavum adjacent to the joint should be distinguished from ossification in the insertion of the ligamentum flavum in the lamina, which is a normal finding.
Abnormal laxity of the joint capsule can be demonstrated in CT as an abnormally wide joint space or as gas in the facet joint (vacuum joint). A vacuum joint phenomenon may be demonstrated in 15% of patients with low back pain and abnormal facet joints. The pathognomonic CT finding is attenuation coefficients less than -200 HU between the articular processes . Because of partial volume averaging, CT rarely shows numbers of -I 000 HU in the joint. The joint space may be abnormally wide or narrow. Although the significance of a vacuum joint has not been sufficiently studied, it logically signifies distraction of the facet joint secondary to an abnormal joint capsule or the abnormal alignment of vertebrae . Narrowing of the neural foramen may result from abnormal distraction of the facet joint.
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