To determine the prevalence of tibiofemoral radiographic knee osteoarthritis (OA) in people aged 35-54 years associated with chronic (> 3 months) knee pain using two different radiographic grading systems. Population based postal survey in a random sample of inhabitants in a district in southern Sweden followed by clinical examination and plain posteroanterior, weight bearing radiographical examination. The Ahlbäck criteria (focusing on joint space narrowing) and the Kellgren & Lawrence classification for knee OA were used for diagnosing tibiofemoral OA. A questionnaire was sent to 2000 randomly selected people aged 35-54 years. The response rate was 92.6%. Fifteen per cent of these people reported chronic knee pain. This group (n = 279) was offered a clinical and radiographic examination of the knee joint and 204 persons agreed to participate. According to the Kellgren & Lawrence classification 28 subjects had OA of the knee grade 2 or more and 16 grade 3 or more. Radiographically detected OA of the knee according to Ahlbäck was found in 20 cases. The minimum prevalence of radiological tibiofemoral knee OA with knee pain was thus 1.5% for Kellgren & Lawrence grade 2 or more, 0.9% for grade 3 or more, and 1.1% according to the Ahlbäck classification. The agreement between the Kellgren & Lawrence grades 2-3 versus Ahlbäck grade I as well as grade 3-4 versus Ahlbäck grade I-II was good (kappa 0.76 and 0.78 respectively). The prevalence of radiographic tibiofemoral OA combined with chronic knee pain in people aged 35-54 years was around 1% as estimated by either the Kellgren & Lawrence or the Ahlbäck classifications systems. Prospective follow up of this cohort should elucidate the significance of knee pain as a sign of developing OA.
