SEATTLE – Smoking is related to prevalent and incident joint pain in patients who have knee osteoarthritis or are at high risk for it, but the mechanism of this association is unclear, in a study of 3,026 patients.
“Previous studies have shown that compared to nonsmokers, smokers have more musculoskeletal pain and report more severe pain,” Jingbo Niu, D.Sc. of the Clinical Epidemiology Research & Training Unit at Boston University reported at the World Congress on Osteoarthritis.
Jingbo Niu, D.Sc.
Possible mechanisms include down-regulation of the hypothalamic-pituitary-adrenal axis or triggering of nicotine-sensitive acetylcholine receptors in the dorsal root ganglion. “These factors may translate to a change in pain perception among smokers,” she said.
Dr. Niu and her colleagues analyzed data from the Multicenter Osteoarthritis Study (MOST), a prospective cohort study of risk factors for the development and progression of knee osteoarthritis and knee pain in patients initially aged 50-79 years. They performed cross-sectional analyses at baseline and at the 7-year follow-up visit, and longitudinal analyses for the interim period.
At baseline, 56% of the cohort were never-smokers, 38% were former smokers, and 6% were current smokers, reported Dr. Niu, who disclosed that she had no relevant conflicts of interest. Smoking intensity averaged 22 pack-years.
Study results showed that at baseline, current smokers had significantly more severe knee pain than did never-smokers, with an adjusted difference of 0.83 points on the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) knee pain scale.
Also at baseline, former smokers were more likely than were never-smokers to have widespread joint pain, defined as pain in all five regions of a joint pain homunculus (adjusted odds ratio, 1.3). And there was a dose-response relationship, whereby the greater the number of pack-years at baseline, the higher the risk of developing widespread joint pain during follow-up (P = .02).
However, neither smoking status nor smoking intensity was significantly associated with pain sensitization, assessed from the pressure pain threshold at the right wrist, according to the data reported at the meeting, which was sponsored by the Osteoarthritis Research Society International.
“We found smoking was related to knee pain severity and widespread pain, but not with pain sensitization,” Dr. Niu concluded.
“The number of current smokers in our analysis was limited,” she said. In addition, “subjects with knee pain were more likely to be recruited into the MOST study because they have a high risk of developing knee osteoarthritis. This inclusion criterion may lead to selection bias when we study knee pain and other pain-related outcomes.”
Dr. Timothy McAlindon
One session attendee noted that the confounders in the analyses did not include socioeconomic factors and recommended they be added. “Of course, smoking status could be confounded by socioeconomic factors, and couldn’t it just be that smoking is a surrogate measure?” he asked.
“It’s a good suggestion,” Dr. Niu agreed, noting that the data set has some socioeconomic measures, such as education, that could be incorporated into analyses.
In an interview, Dr. Timothy McAlindon, one of the session’s comoderators and a professor of medicine at Tufts University, Boston, said, “I think this is a very nicely done study looking at the association of smoking with pain in osteoarthritis.
“What they found is there is clearly a significant association of smoking with pain and incident widespread pain. What was less clear was the mechanistic relationship. Maybe they didn’t test the right mechanistic measures,” he proposed.
“If there is a causal relationship, it could perhaps suggest intervention. But I think that is very conjectural based on what they have done so far,” Dr. McAlindon concluded.