Pain trajectory groups in persons with, or at high risk of, knee osteoarthritis: findings from the Knee Clinical Assessment Study and the Osteoarthritis Initiative

Objective: The authors aimed to characterize distinct trajectories of knee pain in adults who had, or were at high risk of, knee osteoarthritis using data from two population-based cohorts. Method: Latent class growth analysis was applied to measures of knee pain severity on activity obtained at 18-month intervals for up to 6 years between 2002 and 2009 from symptomatic participants aged over 50 years in the Knee Clinical Assessment Study (CAS-K) in the United Kingdom. The optimum latent class growth model from CAS-K was then tested for reproducibility in a matched sample of participants from the Osteoarthritis Initiative (OAI) in the United States. Results: A 5-class linear model produced interpretable trajectories in CAS-K with reasonable goodness of fit and which were labelled "Mild, non-progressive" (N = 201, 35%), "Progressive" (N = 162, 28%), "Moderate" (N = 124, 22%) "Improving" (N = 68, 12%), and "Severe, non-improving" (N = 15, 3%). We were able to reproduce "Mild, non-progressive", "Moderate", and "Severe, non-improving" classes in the matched sample of participants from the OAI, however, absence of a "Progressive" class and instability of the "Improving" classes in the OAI was observed. Conclusions: Our findings strengthen the grounds for moving beyond a simple stereotype of osteoarthritis as "slowly progressive". Mild, non-progressive or improving symptom trajectories, although difficult to reproduce, can nevertheless represent a genuinely favourable prognosis for a sizeable minority. (C) 2014 The Authors. Published by Elsevier Ltd and Osteoarthritis Research Society International. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).