Toward a clinical definition of early osteoarthritis: onset of patient-reported knee pain begins on stairs. Data from the osteoarthritis initiative.

Objective: Early detection of osteoarthritis (OA) would increase the chances of effective intervention. We aimed to investigate which patient-reported activity is first associated with knee pain. We hypothesized that pain would occur first during activities requiring weight bearing and knee bending.

Methods: Data were obtained from the Osteoarthritis Initiative (OAI), a multicenter, longitudinal prospective observational cohort of people who have or are at high risk of OA. Participants completed the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC; Likert scale) annually for up to 7 years. Rasch analysis was used to rank the WOMAC pain questions (activities) in order of affirmation as the pain score increased from 0. For each total WOMAC score category (0-20) we selected 25 individuals at random based on their maximum score across all visits. Fit to the Rasch model was assessed in this subset; stability of question ranking over successive visits was confirmed in the full OAI.

Results: WOMAC data on 4,673 people were included, with 491 selected for subset analysis. The subset data showed good fit to the Rasch model (χ(2) = 43.31, P = 0.332). In the full OAI, the "using stairs" question was the first to score points as the total pain score increased from 0 (baseline logit score ± 95% confidence interval -4.74 ± 0.07), then "walking" (-2.94 ± 0.07), "standing" (-2.65 ± 0.07), "lying/sitting" (-2.00 ± 0.08), and finally "in bed" (-1.32 ± 0.09). This ordering was consistent over successive visits.

Conclusion: Knee pain is most likely to first appear during weight-bearing activities involving bending of the knee, such as using stairs. First appearance of this symptom may identify a group suitable for early intervention strategies.