DESTIN, FLA. – Facet joint osteoarthritis likely accounts for much of what is classified as "nonspecific" low back pain, according to Dr. Alfred C. Gellhorn.
"Amazingly – and I think sadly for us – the lumbar facet joints really have received very little attention in the literature," Dr. Gellhorn, who works in the department of rehabilitation medicine at the University of Washington, Seattle, said at the annual Congress of Clinical Rheumatology.
Eight of every ten American will experience low back pain at some point during their lifetime; low back pain is second only to the common cold in frequency, is the most common reason for time off work, and has a total social cost of more than $100 billion annually. Up to 85% of patients never receive a definitive diagnosis and are classified has having nonspecific pain, he said.
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Back pain is an often-overlooked symptom of facet joint osteoarthritis, said Dr. Alfred Gellhorn.
The facet joints may be responsible for a significant proportion of that pain.
Facet joint cartilage is aneural, but a number of nociceptors exist in the subchondral bone, the synovial folds, and the capsule. Once activated – by synovial inflammation or mechanical factors such as trabecular microfractures, capsular distention, pressure on the subchondral bone as joint load increases, or intramedullary hypertension, for example – the nociceptors may cause secondary reflex contraction of paraspinal muscles.
Patients will report this as spasms, and the contractions can be palpated, Dr. Gellhorn said, noting that prolonged inflammation in and around the facet joints can lead to central sensitization, neuronal plasticity, and development of chronic low back pain.
Facet joint osteoarthritis (OA) is distinct from disc degeneration, but the two conditions are interdependent. Radiographic hallmarks of disc degeneration include disc height loss, dehydration, and endplate sclerosis, whereas radiographic hallmarks of facet joint OA include narrowing of the facet joint space, osteophytosis of articular processes, hypertrophy of articular processes, sclerosis, subchondral erosion, and subchondral cysts.
Older studies that looked at facet joint OA by comparing findings on imaging and symptoms found either no association or only minimal association between facet joint OA and low back pain, but the threshold used in those studies was mild to moderate OA in young and middle-aged subjects.
"That’s the wrong criterion to use," Dr. Gellhorn said, noting that mild facet joint OA is "essentially ubiquitous" by middle age. Moderate to severe facet OA, however, is more symptomatic, and predominantly affects older adults – and should be the criterion used in studies of the condition.
In a recent study of 252 patients with a mean age of 67 years who were participants in the Framingham heart study, severe OA affecting the facet joint was significantly associated with frequent low back pain (odds ratio, 2.2). Disc height narrowing was not associated with low back pain in these patients (Osteoarthritis Cartilage 2013;21:1199-206).
The findings contrast with those from prior studies, likely because the cohort was older (mean age of 67 years vs. 30s to 50s), he said.
It may be that with age, back pain classified as "nonspecific" shifts from discogenic pain in younger adults to facetogenic pain in older adults, he suggested.
Findings with respect to disc pathology and low back pain in young and middle-aged adults seem to support this hypothesis, he noted.
For example, in a study of patients with a mean age of 49 years, low back pain was associated with a twofold increased likelihood of disc height loss and annular tears, and in a study of patients aged 18-50 years, moderate disc height loss was also associated with a twofold increased likelihood of low back pain. In another study of patients with a mean age of 50 years, advanced disc height loss was associated with a threefold increased likelihood of prevalent low back pain.
In another study, severe disc height narrowing was associated with a threefold increase in the odds of low back pain in those younger than age 60 years but not in those over age 60 years.
There are markers for symptomatic facet joint OA. Despite the known association between severe facet joint OA and low back pain, "the truth is there is still limited positive predictive value for that," he said.
"Many older adults with severe facet joint OA on imaging are relatively asymptomatic," he added.
There are some additional imaging makers, however. Symptomatic facet join OA is apparent on single-photon emission computed tomography/computed tomography (SPECT/CT) or fluid-sensitive, fat-suppressed MRI. Also, 64% of patients with suspected facet joint pain in one study had bone marrow lesions on short T1 inversion recovery (STIR) MRI, which were well correlated to the side of pain, he said.