Sympathetic nervous system stimulation, which releases noradrenaline, influences the nociceptor activity which develops after tissue injury. The alpha 2-adrenergic agonist, clonidine, produces analgesia through a central mechanism but also inhibits noradrenaline release at terminal nerve fibre endings. Clonidine may induce analgesia when administered at peripheral sites. This study assesses the potential analgesic effect of clonidine after intra-articular administration. Forty ASA I-III patients, scheduled for arthroscopic knee surgery under general anaesthesia were allocated randomly in 4 groups of 10 patients each, at the end of the surgical procedure. In the control group (group 1), the patients received 20 ml of intra-articular isotonic saline. In group 2, the patients received 150 micrograms of clonidine diluted in 20 ml of isotonic saline injected into the knee joint. In group 3, the patients were given 20 ml of intra-articular isotonic saline and clonidine 150 micrograms was injected subcutaneously. In group 4, morphine 1 mg, diluted in 20 ml of isotonic saline, was injected into the knee joint. Postoperative pain was assessed in a double-blind fashion using a visual analogue scale (VAS) at 1, 2, 3, 6 and 24 h after the end of surgery. VAS scores were significantly lower in groups 2 and 4, compared to groups 1 and 3, at 1 and 2 h after surgery. The delay between intra-articular injection and further postoperative analgesic administration was significantly longer (P < 0.05) in group 2 (533 +/- 488 min) compared to groups 1 and 3 (70 +/- 30 min and 132 +/- 90 min, respectively). The difference was not significant between group 4 (300 +/- 419 min) and the other groups. We conclude that a low dose of intra-articular clonidine produces analgesia unrelated to vascular uptake of the drug. This study further supports a peripheral analgesic effect of clonidine.

Both intra-articular morphine and clonidine produce analgesia. This study was designed to compare the analgesic effects of the two drugs, used separately and in combination. We studied 90 patients undergoing arthroscopy of the knee under general anaesthesia. Patients were allocated randomly to receive 20 ml of intra-articular isotonic saline solution containing morphine 2 mg, clonidine 150 micrograms or both. Pain was assessed on an visual analogue scale after operation and time for rescue medication was measured. There was no difference in VAS scores between the three groups and the time for rescue analgesic was comparable. We conclude that intra-articular morphine and clonidine have comparable analgesic effects in the doses used. The combination of both drugs did not seem to increase analgesia.

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