Smoking Makes Knee Osteoarthritis Worse

ROCHESTER, Minn., Dec. 8 -- Smoking accelerates knee cartilage loss and increases pain in men with knee osteoarthritis, researchers here reported.

Current male smokers had more than twice the increased risk of cartilage loss at the tibiofemoral and patellofemoral joints compared with current nonsmokers men, according to a study published online in the Annals of the Rheumatic Diseases.

Pain scores, both at baseline and at follow-up, were also more than 30% higher in the smokers, reported Shreyasee Amin, M.D., of the Mayo Clinic, and colleagues.

The finding that cigarette smoking plays a role in the progression of knee osteoarthritis is important, Dr. Amin said, because it is a potentially modifiable risk factor. "Few studies have examined the association between smoking and symptomatic knee osteoarthritis, and findings have been conflicting," he said.

In a 30-month prospective, natural-history study of 159 male smokers with osteoarthritis, the more symptomatic knee was assessed by MRI at baseline and at 15- and 30-months follow-up.

Cartilage was scored using the WORMS semi-quantitative method at the medial and lateral tibiofemoral joint and at the patellofemoral joint. At baseline and follow-up visits, the severity of knee pain was assessed using a visual analogue scale (VAS) pain score (0-100 mm).

Among the 159 men, 19 (12%) were current smokers at baseline. Current smokers were younger (mean age SD: 62 9 vs. 69 9 years) and leaner (mean body mass index [BMI]): 28.9 3.2 vs. 31.3 4.8 kg/m²) than men who were not current smokers.

Adjusted for age, BMI, and baseline cartilage scores, men who were current smokers had 2.3-fold increased risk for cartilage loss at the medial tibiofemoral joint (odds ratio: 2.3, 95% CI: 1.0 to 5.4) and a 2.5-fold increased risk at the patellofemoral joint (OR: 2.5, CI: 1.1 to 5.7). No loss at the lateral tibiofemoral joint was seen among smokers, but there was very little overall loss.

Current smokers also had significantly higher adjusted pain scores at baseline (60.5 vs. 45.0, P<0.05) and at follow-up (59.4 vs. 44.3, P<0.05) compared with men who were not current smokers.

There is evidence that cigarette smoke has a negative effect on cartilage metabolism, the researchers said. In vitro studies have shown that components of tobacco smoke have a harmful effect on disc chondrocyte function, inhibiting cell proliferation and extracellular matrix synthesis.

These findings, they said, raise concern about the negative effects of smoking on chondrocyte function in articular cartilage. In addition, the deleterious effect of smoking may be greatest when cartilage is already damaged.

Other theories to explain the cartilage damage, according to the investigators, are increased oxidant stress, which contributes to cartilage loss. They also suggested increased carbon monoxide levels in arterial blood, adding to tissue hypoxia, which could impair cartilage repair.

The mechanism behind the increased pain in these patients is not clear, the researchers said, as cartilage does not have pain fibers. Smoking, they suggested, may affect other knee-joint structures mediating knee pain or may affect an individual's musculoskeletal pain threshold.

On the other hand, they said, socioeconomic status could have influenced pain perception. Current smokers were less likely to have more than a grade 12 education or to be currently employed.

Among the study's limitations, aside from the absence of women for analysis, was the small number of men who were current smokers, limiting the ability to address all possible confounders. However, the researchers said they did consider the effects of pain severity, physical activity level, and knee alignment. Bone marrow lesions, another possible confounder, were similar in both groups as were knee injuries.

In conclusion, Dr. Amin said that these findings are provocative and deserve further study, especially considering the negative effect of smoking on articular cartilage.

This study was supported by several grants including one from the Bayer Corporation.