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Researchers found no protective effect of smoking against knee osteoarthritis (OA), according to a new longitudinal cohort study of 620 individuals with meniscal tears treated at five sites in Denmark. An inverse association between OA and smoking had been suggested by several previous studies and a recent meta-analysis.
In the new analysis, the researchers examined patients who were undergoing knee arthroscopy for a meniscal tear, and found knee OA present in 37.7% of current smokers and 45% of nonsmokers, but no difference after adjusting for several covariates, reported Marianne Bakke Johnsen, MD, of the University of Oslo, and colleagues who reported their findings online in BMC Musculoskeletal Disorders.
Johnsen's team found that smokers had more symptoms of OA, but there was no statistically significant relationship between current smoking and knee OA in any of the models, and the results did not change substantially after excluding adjustment for body mass index (RR 1.05, 95% CI 0.88-1.27). Current smokers did, however, report having more knee pain and worse function.
The researchers found no statistically significant association between knee OA and current smoking after adjusting for multiple other variables including age, sex, education, occupation, and physical activity, with a relative risk of 1.09 (95% CI 0.91-1.30), the team reported online in BMC Musculoskeletal Disorders.
The team explained their choice of patients for the study by noting that "individuals with degenerative meniscal tears often exhibit early signs of knee OA and are considered to have a higher risk of developing the disease and may therefore be a useful population to study the relationship between early degenerative knee changes and smoking."
The study population included patients enrolled in the Knee Arthroscopy Cohort Southern Denmark study from 2013 to 2015. Patients who were scheduled for arthroscopic surgery responded to a questionnaire asking whether they smoked and if so, how much. They also completed the Knee injury and Osteoarthritis Outcome Score (KOOS), which includes five subscales measuring pain, other symptoms, activities of daily living, sports and recreation, and knee-related quality of life. Scores on these subscales ranged from 0 to 100, with 0 representing the worst knee problems.
Covariates in the analysis include age, sex, education, occupation, BMI, and physical activity. Among the 620 patients, 22.3% were current smokers, while 77.7% were nonsmokers, which included those who had stopped within the previous 6 months. Compared with nonsmokers, current smokers were younger (ages 45 vs 50.4), had lower BMI (26.7 vs 27.4), and were less likely to have advanced degrees (1.5% vs 8.9%).
Current smokers also had worse knee symptoms on the KOOS subscales:
- Pain, 52.3 vs 55.6 (P=0.06)
- Symptoms, 56.6 vs 60.9 (P=0.02)
- Activities of daily living, 60 vs 64.8 (P=0.01)
- Sports and recreation, 22.5 vs 27.3 (P=0.02)
- Quality of life, 37.9 vs 42.7 (P=0.001)
However, the differences between the two groups regarding self-reported knee pain were not statistically significant. The team measured cartilage defects according to the International Cartilage Repair Society (grades 0 to 4, with 0 representing normal cartilage) during surgery. In 37.7% of patients, these were grade 1 in two or more knee compartments or grade 2 in one compartment, and the remainder had grade 3 or 4 defects in at least one knee compartment.
When BMI was excluded from the analysis, there again was no increased risk for knee OA and current smoking (RR 1.05, 95% CI 0.88-1.27). This sensitivity analysis was done because other researchers had suggested that smoking might influence OA through effects on BMI. There also was no interaction between sex and smoking.
Cartilage defects were measured according to the International Cartilage Repair Society (grades 0 to 4, with 0 representing normal cartilage) during surgery. In 37.7%, these were grades 1 in two or more knee compartments or grade 2 in one compartment, and the remainder had grades 3 or 4 defects in at least one knee compartment.
When BMI was excluded from the analysis, there again was no increased or decreased risk for knee OA and current smoking (RR 1.05, 95% CI 0.88-1.27). This sensitivity analysis was done because other research had suggested that smoking might influence OA through effects on BMI. There also was no interaction between sex and smoking.
"Smoking has been suggested to have an analgesic effect through the constituent of nicotine," and in vitro studies have observed beneficial effects of nicotine on chondrocyte metabolism, Johnsen and co-authors explained. In one previous Australian study that included almost 300 participants, smoking was associated with higher levels of tibial cartilage, with a stronger relationship in individuals with a greater number of pack years.
Earlier studies have relied on Kellgren-Lawrence scores of 2 or higher, or the need for total knee replacement when considering the effect of smoking on OA, Johnsen, et al. noted.
In the new study, the presence of meniscal tears and knee pain may "represent a different phenotype of knee OA than has previously been studied in relation to smoking," the authors hypothesized.
One study strength, they said, is that surgeons who treated patients were not aware of how the patients replied in the email questionnaire regarding smoking status.
Among the study limitations, the researchers said, were that never and former smokers were both categorized as non-smokers although the number of former smokers was low.
Source Reference: BMC Musculoskeletal Disorders, 2019; 20:141