The reciprocal effects of pain intensity and activity limitations: implications for outcomes assessment in clinical trials.

Jensen, Mark P; Molton, Ivan R

OBJECTIVES To examine the reciprocal effects of pain intensity and limitations in physical functioning over time. METHODS This study presents findings from a reanalysis of a 7-center trial conducted in Ontario, Canada, included 209 adults with chronic knee pain secondary to osteoarthritis. Patients were randomized to receive 28 days of therapy with an active solution (1.5% w/w diclofenac sodium in dimethyl sulfoxide) or 1 of 2 control solutions containing no diclofenac. The key outcome measures used in the current analyses were administered throughout the study period and assessed pain intensity, perceived activity limitations, and a composite score measuring both domains. A structural cross-lagged regression approach was used to determine the reciprocal effects of pain and activity limitations over time. RESULTS In both study groups, participants (N=209) experienced significant reductions in mean pain intensity and activity limitations from baseline to weeks 1, 2, 3, and 4 (P<0.001 for both variables). Similarly, there were significant reductions in the activity limitations outcome at weeks 1 and 4 for the active versus control group (P<0.05 for both). Higher levels of perceived activity limitations predicted more future pain at all time points. Cross-lagged associations in which pain predicted subsequent perceived activity limitations were not significant at any time point. All 3 outcome measures evidenced similar responsiveness to the treatment. CONCLUSION These analyses showed that a decrease in activity limitations results in a decrease in pain intensity. However, changes in pain intensity had no effect on subsequent activity limitations in the study sample. None of the 3 outcome variables emerged as being more responsive to treatment than the others.