To analyze the effect of contralateral knee pain on sensitivity of patient-reported outcomes and objectively measured functional performance tests in subjects with knee osteoarthritis (OA).
Subjects with discordant knee pain status (i.e., 1 knee being painful [≥4 on a numeric pain rating scale (NPRS)], with the contralateral knee being pain free [NPRS 0]) were selected from the Osteoarthritis Initiative and matched to subjects with bilaterally pain-free and painful knees by age, sex, body mass index, and radiographic knee OA. The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) physical function score, the global Physical Activity Scale for the Elderly (PASE), and objective functional performance tests were cross-sectionally compared in a matched case-control design.
A total of 378 subjects with discordant knee pain status were matched to 359 controls with bilaterally pain-free knees and to 323 controls with bilaterally painful knees. WOMAC scores in pain-free knees of discordant knee pain cases significantly differed compared to scores of bilaterally pain-free knees (P = 0.003). Likewise, scores in painful knees of discordant knee pain cases significantly differed compared to scores of bilaterally painful knees (P < 0.001). PASE levels between these groups were not significantly different (P > 0.68). Functional performance tests differed in subjects with discordant knee pain compared to subjects with bilaterally pain-free knees and when compared to subjects with bilaterally painful knees, with the chair stand test showing the strongest effect size (standardized response mean 0.28 and 0.33, respectively).
The WOMAC physical function score, although knee specific, is impacted by the contralateral knee pain status. The repeated chair stand test appears to be the most sensitive assessment in differentiation between groups with different status of knee pain.