Intra-articular morphine for pain relief after knee arthroscopy

BACKGROUND: Peripheral opioid analgesia is well documented. But the clinical usefulness of intra-articular morphine after surgery is uncertain. The aim of the present study was to evaluate the analgesic effects of intra-articular morphine after knee arthroscopy. METHODS: In this parallel-group, double-blind study, 90 patients were randomised to receive either morphine 1 mg, morphine 2 mg or placebo in 5 ml saline intra-articularly at the end of arthroscopic knee surgery. Anaesthetic technique was local infiltration and intra-articular injection of lidocaine. Analgesic efficacy was evaluated by a global pain score, pain intensity (visual analogue scale), and analgesic requirements (paracetamol) during the first 48 h postoperatively. RESULTS: No significant differences between the groups were found for any of the efficacy variables. A majority of the patients had mild pain throughout the study, thus possibly compromising study sensitivity. In a subgroup with more intense pain early after arthroscopy, intra-articular morphine 2 mg reduced pain intensity (P < 0.05) and analgesic requirements (P < 0.05) compared with placebo. CONCLUSION: Postoperative analgesic effect of intra-articular morphine was found only in a subgroup of patients with higher pain intensity in the immediate postanaesthetic period. Possible reasons for our overall negative findings include low study sensitivity due to weak pain stimulus, lack of inflammation that may be a prerequisite for peripheral opioid analgesia, and the local anaesthetic, which impedes local inflammatory reaction and expression of peripheral opioid receptors. These factors may also explain the conflicting results in other studies.