Background/Purpose: Knee pain is a prominent symptom among
patients with knee osteoarthritis (OA), but the impact of knee pain frequency
on physical function has been understudied. The purpose of this
study was to evaluate whether knee pain frequency at baseline and its trajectory over 72
months were associated with baseline-to-72-month physical function outcome.

Methods: This analysis was based on data from the Osteoarthritis
Initiative (OAI).  Knee pain frequency was
assessed using an item from the Knee Injury and Osteoarthritis Outcome Score
(KOOS). Physical function was assessed using self-report measures including the
WOMAC function score and the Short Form 12 physical component scale (SF-12). Longitudinal models
and mixed effects regression were used to evaluate the relationship between
baseline knee pain frequency level and the trajectory of physical function,
adjusting for potential body mass index (BMI), sex, and race. KOOS knee pain
frequency was aggregated to the following categories: Never/Monthly, Weekly, and
Daily/Always. Knee pain trajectory over 72 months was defined based on its
changes over this time period as: 1) Maintain Never/Monthly knee pain
(reference group); 2) Maintain Weekly knee pain; 3) Maintain Daily/Always knee
pain; 4) Monotonic worsening of knee pain frequency; 5) Monotonic improvement
of knee pain frequency; and 6) Fluctuating knee pain. We used a random effects model to account
for the clustering of two knees within a person.

Results : The sample included
4,796 men and women with or at high risk of developing knee osteoarthritis. At
baseline, the “Daily/Always” knee pain group had the lowest SF-12 throughout 72
months, with a mean (95% CI) ranging from 41.6 (40.6, 43.8) at baseline to 38.39 (37.4, 39.3) at 72 months; whereas, the group who maintained
“Never/Monthly” had the highest SF-12 throughout 72 months, ranging from 53.2
(52.6, 53.8) at baseline to 51.7 (50.9, 52.4) at 72 months. Interestingly, in the “monotone improvement” group, the SF-12 increased
significantly over time, while in all other groups SF-12 decreased
significantly compared to the “maintain Never/Monthly” group (P<0.001).
Comparing the knee pain trajectory groups that had worsening SF-12, the SF-12
for the “monotone worsening” group decreased three times more compared to the
“maintain Never/Monthly”, “maintain Daily/Always”, and “fluctuating KP” groups.
The results are similar for the knee pain trajectory groups and changes in
WOMAC over 72 months.

Conclusion: These findings
indicate that higher knee pain frequency at baseline is associated with
worsening function over time. In addition, worsening knee pain frequency
trajectory is associated worsening function over time, while improving knee
pain trajectory is associated with improvement in function.