Background/Purpose: Knee pain is the primary reason that patients with OA seek medical care. The goal of this study is to describe pain trajectory over three years in a cohort of patients with radiographic, symptomatic knee OA.

Methods: We used data from the Osteoarthritis Initiative (OAI), a multi-center, longitudinal, prospective observational study of knee OA. Pain assessments were done at baseline and at yearly visits for 4 years. We defined patients with symptomatic knee OA as those with central reader Kellgren/Lawrence (KL) score >2 and a WOMAC pain score >0. We used group-based trajectory modeling to identify distinct patterns of pain progression. To minimize the impact of baseline flare effect we restricted our analysis to visits occurring in months 12 through 48. We also built multivariable generalized linear models to determine factors affecting change in pain severity over time. Factors examined included sex, race, education, baseline comorbidities, 12-month age, BMI, alignment, KL grade, and depression.

Results: We used the data from 1,447 OAI study participants with radiographic, symptomatic knee OA at month 12. The average WOMAC pain at month 12 was 24 (0-100 scale with 100 = worst) with standard deviation 17.6. Individual pain reports varied markedly over time. For example, at 48 months, 51% reported a lower pain score than at 12 months and 36% reported a higher score. Also, 32% of all patients reported having no pain at one or more subsequent follow-up visits. Group based trajectory modeling identified 4 distinct pain trajectories [Figure]. Two-thirds of patients (groups 2 and 3 in Figure) started with moderate pain and showed little change on average over three years. The 12.6% in the highest pain trajectory tended to increase in pain by an average of 6 points over 3 years of follow-up, while patients in the first category (lowest pain at 12 months) decreased by an average of 9 points. Higher BMI, depression, and KL grade at the beginning of observation were associated with pain worsening over time in multivariable models.

Conclusion: We found that knee pain is highly variable over time and that pain neither worsens nor improves, on average, over three years in a majority of patients. Just 12.6% of patients demonstrate progressive worsening. These observations of highly variable pain reports with relatively little change on average over time contrast with the frequent clinical teaching that osteoarthritis symptoms progressively worsen over time.