Disk problems causing back pain? Think again
Jeffrey P. Johnson, M.D.
Neurosurgeon, Providence Brain and Spine Institute
Published March 2012
The facet joints are the Rodney Dangerfield of the spine – they get no respect. Because of their prominence on MRI images, intervertebral disks have assumed a disproportionate stature, leading most patients with back problems to assume that they have a bulging disk or a bone spur. It’s possible, however, that these patients have a problem not with a disk, but with a small, difficult-to-image facet joint.
The facets, also known as zygapophyseal joints, are true joints. They have articular surfaces lined with cartilage and are lubricated with synovial fluid, just like knee or hip joints. I often describe them as “knuckles.” A pair of facets exists at each level of the spine, forming the posterior portion of the motion segment.
The body’s 46 facets work in conjunction with the spinal column of vertebral bodies and disks. At each motion segment, the facets limit anterior shear forces and twisting movement.
The importance of the facet joint is most evident in the clinical condition of spondylolysis, or congenital absence of the pars interarticularis. The pars is the structure that attaches the facet joint to the rest of the spine. This developmental abnormality occurs at the last mobile segment, usually L5-S1, and results in the facets at that level being disconnected from the rest of the motion segment.
Over time this can often lead to spondylolisthesis – anterior displacement of the upper vertebral segment over the lower – and premature disk degeneration at that level. The tendency is to blame the disk for wearing out, but in reality the disk gets no assistance from the disconnected facets.
A similar etiology occurs with degenerative spondylolisthesis. Here, the culprit often is severe bilateral facet degeneration. This produces the same lack of posterior support and resultant disk degeneration and listhesis. Facet fracture and iatrogenic removal of the facet also can lead to similar instability of the motion segment.
Spinal stenosis, the most common condition related to the facet joint, usually is due to facet arthritis. It occurs most commonly at L3-4 or L4-5, although any level can be affected. The incidence of spinal stenosis increases with age, consistent with a degenerative condition.
MRI scanning reveals that the facet joints are enlarged and irregular. There is thickening of the ligamentum flavum and the joint capsule. This impinges on the underlying nerve root in the lateral recess or “subarticular recess.”
Symptoms can include radicular pain, numbness, tingling or weakness in the territory of that root. Often these symptoms follow the pattern of neurogenic claudication, or worsening on standing or walking which improves with sitting. This is believed to be a mechanical issue, as flexion of the pelvis and lower lumbar spine with sitting causes the facet capsule to stretch out and relieve the redundancy and buckling that occurs with standing. This also explains why these patients report relief when leaning on a grocery cart while shopping. Patients sometimes report being able to ride a bike all day but are unable to walk 100 feet.
A facet condition closely related to spinal stenosis is a synovial cyst. These cysts also reflect the inflammatory process of facet arthritis. They arise from the synovial membrane that lubricates the joint. They can occur dorsally, where they usually do not produce specific symptoms, or ventrally within the spinal canal. Here, they can impinge on the nerve root and produce symptoms identical to spinal stenosis.
The cysts often are filled with hemosiderin-stained material or old hemorrhage. This evidence of old bleeding also can occur within the joint capsule and ligamentum flavum. This might explain why symptoms can develop acutely in spinal stenosis, even though the condition is thought to progress slowly over time. Repeated small hemorrhages might be part of the slow progression of the condition.
The pain from nerve root impingement from an arthritic facet usually can be relieved by a simple decompressive surgery. Here, the thickened ligamentum flavum or joint capsule, or the synovial cyst, is removed while preserving the integrity of the joint itself. Although this surgery can be dramatically effective, it does not treat the underlying arthritic condition of the joint.
Facet joint pain
An abnormal facet joint can produce pain in ways other than nerve root impingement. Pain from an abnormal facet is typically localized to the back, although it may radiate in a manner that can be similar to radicular pain. It is often said to be associated with tenderness in the region, although the fact that the facets are deep under the paraspinous muscles and other layers (deeper in some of us than others) makes this finding questionable. Worsening with movement – particularly sitting, bending or twisting – also is believed to be consistent with a diagnosis of facet pain.
Facet pain can be treated with anti-inflammatory medications or corticosteroids injections. The presumptive pain fibers to the facet also can be blocked with a procedure known as a medial branch block, or disrupted with percutaneous thermoablation. These procedures can be effective, although their efficacy is hampered by a limited understanding of how to distinguish a painful joint radiographically or clinically, or how to identify the correct joint to treat.
The facets are difficult to image in detail. The largest facets in the lower lumbar spine are only about 20 mm in diameter. Also, the articular surfaces are not oriented perpendicular to the planes of the standard MRI protocols of sagittal, axial and coronal images. To complicate matters further, the facets change in orientation as you progress down the spine. This makes it difficult to get a three-dimensional sense of the shape of the joint.
Besides the findings of facet and ligamentous hypertrophy, there are other MRI findings that suggest facet joint pathology. These include fluid in the facet surface, excessive splaying of the joint and marrow edema in the bone. But these findings can be nonspecific, and it is unclear to what extent they may exist in individuals without pain. Also, the changes can frequently be found at multiple locations in the same patient, forcing targeted therapies such as injections to rely on guesswork.
Surgical repair of the facet joint currently is not possible. There have been efforts directed at facet joint replacement, but these have not gained traction. At present, the only surgical treatment for facet pain is spinal fusion. This also can be effective, provided the pain generator has been correctly identified. The widely reported failures for these surgeries almost certainly relate to the same difficulties in pinpointing the pathology to specific motion segments or facets.
Someday we will understand the subtleties of the facet joint. We will be able to perform high-resolution imaging of individual facets. We will learn about small injuries or degenerative changes that produce pain. We may understand how younger patients have different facet problems than the elderly. Back injuries currently misdiagnosed, or missed entirely, will be identifiable. We will then have elegant ways to repair these problems, like knee arthroscopy, to treat back pain.
We will look back on our current era and marvel at how far we have advanced. And we will have learned that not every spine problem is a “bulging disk.” The facet joint will get the respect it deserves.
Clinical articles by Jeffrey Johnson, M.D.
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