Background/Purpose: To provide current estimates of the prevalence of knee pain, radiographic, symptomatic, and clinical knee osteoarthritis (OA) in middle-aged and elderly in Sweden.

Methods: In 2007 a random sample of 10 000 56 to 84 year old Region Skåne residents from the Malmö Diet and Cancer Study (Manjer et al 2001) were sent a mailed questionnaire about knee pain in the last 12 months; this being the first part of the Malmö Osteoarthritis Study (MOA). We classified subjects reporting knee pain with duration of at least 4 weeks as having frequent knee pain. A random sample of 1300 subjects with frequent knee pain and a random sample of 650 subjects without (out of the 7737 questionnaire responders) were invited for a clinical and radiographic examination including assessment of clinical knee OA according to the American College of Rheumatology (ACR) clinical criteria. Participants underwent radiography of both knees in weight-bearing and semi-flexion. An independent radiologist who was blinded to clinical data assessed all frontal and patellofemoral radiographs. We considered subjects who fulfilled criteria approximating Kellgren and Lawrence (KL) grade 2 or worse to have radiographic knee OA. Those with frequent knee pain and having radiographic knee OA were classified as having symptomatic knee OA. We used weighting to adjust for different sampling probabilities depending on the knee pain status as well as for the nonresponse and volunteer bias in MOA study using data on age, sex, body mass index and highest education level collected within the Malmö Diet and Cancer Study and the knee pain status from the MOA survey.

Results: The 10 000 random sample had mean (SD) age of 70.3 (7.6) years, mean (SD) body mass index was 27.1 (5.0) and 62% were women. The response rate in mailed questionnaire was 77.4% and 1527 invited subjects (78.3%) attended the clinical visit. The prevalence of frequent knee pain in one or both knees during the last 12 months was 25.1% (95%CI: 24.1 to 26.1), 20.8% in men and 27.7% in women and was similar in all age groups (Figure 1). The prevalence of radiographic knee OA (KL grade ≥2) was 25.4% (95%CI: 22.6 to 28.5), the prevalence of symptomatic knee OA was 10.5% and both increased with increasing age. The prevalence of clinical knee OA according to the ACR criteria was 9.0%. The prevalence of frequent knee pain in the subjects with radiographic knee osteoarthritis was 42.1%. Among those with symptomatic knee OA 40% fulfilled the ACR clinical knee OA criteria. In the study sample 11.8% of subjects reported frequent knee pain but did not fulfill OA criteria, neither for clinical ACR nor radiographic knee OA (Figure 2).

Conclusion: The prevalence of knee OA in southern Sweden in the age group 56 to 84 years varied from 9.5% to 25.4% depending on the definition used. The estimates from Sweden are lower than those from the United States. A lower body mass index in the Swedish population may be one explanation.