Effects of intraarticular ropivacaine and morphine on lipopolysaccharide-induced synovitis in horses

To compare the intraarticular (IA) analgesic effects of ropivacaine and morphine in horses with experimentally induced synovitis. Randomized, blinded cross-over design. Twelve healthy mixed breed horses between 8-15 years old. Synovitis was induced in the left radio-carpal joint with an injection of lipopolysaccharide (Escherichia coli 055:B5). Six hours later, the horses were treated with an IA injection of 40 mg of ropivacaine (ROPI), 40 mg of morphine (MOR), 20 mg of ropivacaine added to 20 mg of morphine in saline (RM) or 4 mL of saline (SAL), as control. Analgesia was measured subjectively using a numerical rating scale, a simple descriptive scale, pain upon maximal flexion of the carpus and by the range of motion exhibited by the affected joint. Data are reported as mean +/- SD and were analyzed using anova. Blood and synovial data were analyzed by split plots in time with units (treatments: SAL, ROPI, MOR and RM) and subunits (times: T0-24), in a completely randomized design with six replicates. Mean comparisons were made by Tukey's test; differences were considered significant at p < 0.05. Ropivacaine had a clinical analgesic effect with a relative short duration (similar to 2.5 to 3.5 hours). Morphine had a slower onset of action than ROPI, but a stronger analgesic effect of longer duration. The RM showed an earlier onset of action than MOR and had a strong analgesic effect for the 24-hour post-injection period. All treatments caused a significant decrease in total nucleated cells compared with the control, 24 hours after administration. Morphine alone or in combination with ropivacaine produced a strong analgesic effect of prolonged duration, which may offer pain relief for acute synovitis for at least 24 hours.