Pain mechanisms in osteoarthritis of the knee: Effect of intraarticular anesthetic

Objective. The causes of pain in osteoarthritis (OA) remain unclear. We examined the effect of intraarticular (IA) local anesthetic (LA) on pain perception in OA knee to test the hypothesis that pain arises from structures in contact with the intraarticular surface. The effect of intervention on one knee on perception of pain in the other was also studied. Methods. Using a single blind, blind observer design, 20 subjects with bilateral symptomatic OA knee were randomized to receive either IA LA (bupivacaine 0.25%, 5 mi) or placebo into the most painful knee. Pain from both knees was assessed at 1 h, 24 h, and 7 days using a 100 mm visual analog scale (VAS). Pain descriptors using a standardized questionnaire (McGill) were recorded. Results. At 1 h there was a significant fall in pain in LA knees (median VAS 61.5 to 0.0; p = 0.007). The change at VAS was significantly different between knees receiving LA and placebo (median change -45.5 vs + 3.5; p = 0.03). 6/10 LA knees scored 0 on VAS at 1 h. Noninjected knees also showed a fall in pain scores. For knees contralateral to those receiving LA, the fall was from median 28.0 to 1.0 (p = 0.08). The reduction in pain was much less in knees contralateral to those receiving placebo (median 43.5 to 38.0), though the difference between the changes did not reach statistical significance. VAS scores remained below baseline for 7 days, though this difference was not significant. McGill pain scores fell in the LA group (for all categories of pain), but did not change in the placebo group. Conclusion. Pain in some cases of OA knee can be abolished by IA LA, suggesting that the structures responsible for pain are in contact with the intraarticular environment. As well, interventions in one knee have significant effects on pain perception in the contralateral knee.