The CHECK study is the first prospective 10-year follow-up study of osteoarthritis in an early phase of the disease that combines biological, psychological and social aspects of osteoarthritis. Radiographic knee osteoarthritis was present only in a small number of the CHECK participants when compared with the OAI. In contrast, the participants in CHECK had more pain, more stiffness, more limitations in activities and a worse health status. The worse clinical health status is supported by the use of pain medication: At baseline, only 9% of the participants in OAI had taken any pain medication, whereas this was 46% for CHECK (data not shown). Other characteristics such as body mass index or gender appeared not to be explanatory for this observed characteristic difference between both cohorts (data not shown). It could be that the difference in radiographic joint damage between both cohorts is due to differences in implementation of the K&L grading method. However, grading of a random sample of the OAI knee radiographs by those who performed the grading for CHECK excluded this possibility (data not shown). Although not expected to be explanatory, it should be kept in mind that radiographs were taken, although according to standard protocols, in 10 different centres, whereas in the OAI only four centres are involved. It can not be ruled out that the social, cultural and healthcare system differences between the USA and Europe, in particular The Netherlands, account for (part of) the difference in reported health status. Also differences in inclusion between both cohorts can not be ruled out.
The OAI incidence cohort is recognised as an early osteoarthritis cohort. Taking the radiological findings into account, we conclude that CHECK was started in an even earlier phase of the disease compared with the OAI. Although this is apparently in discordance with the more severe clinical symptoms, the relation between radiographic damage and clinical symptoms has never been clear27
and is the subject of study in both cohorts. Therefore, it is hypothesised that in the early phase of osteoarthritis pain, stiffness and disability (of still unknown origin) are prominent and not yet accompanied by radiographic findings of osteoarthritis (CHECK). In the subsequent phase (OAI) patients are coping with the pain and physical disability, leading to a decrease in the report of these characteristics, while independently (or maybe as a consequence) structural changes, visible on radiographs, develop. In other words, earlier recruitment of patients may carry more perceived symptoms of osteoarthritis (as also seen in rheumatoid arthritis), while in a later stage coping with a new disease may ameliorate the symptoms (). In the final course of the disease the structural (radiographic) changes progress and lead to further pain and disability. It should be taken into account that, in addition, several other factors, as described in the ICF model, may add to the apparent discrepancy observed between pain and structural joint damage over time.28
Our hypothesis can be tested in the future follow-up of patients in both cohorts, in particular those with the more severe complaints (still) without radiographic joint damage. If it appears that the CHECK population with respect to pain and joint damage, independent of factors such as social background, healthcare system differences, cultural difference, variance in methodology etc, follows the OAI population, then our hypothesis may hold true. Of course other factors, independent of symptoms and joint damage, need to be evaluated regarding observed differences between both cohorts, as such giving both cohorts their surplus value.
Schematic presentation of the hypothesis as put forward in the discussion explaining the (apparent) discrepancy between both cohorts with respect to pain and joint damage. CHECK, Cohort Hip and Cohort Knee; OAI, Osteoarthritis Initiative.