The disposition and local effects of intra-articular morphine in normal ponies

Morphine (15 mg in 5 ml saline) was injected into the left, and 5 ml saline into the right, tarsocrural joint of 8 ponies. Venous blood samples were collected before and at 0.5, 1, 2, 6 and 24 h after the intra-articular morphine injection and analysed for morphine and its metabolites. Synovial fluid was sampled from both tarsocrural joints before and 24 h after injection. Synovial white blood cell and red blood cell counts, protein and hyaluronate concentrations were measured in all the samples; and the synovial fluid morphine concentration from the left tarsocrural joint was measured 24 h after the injection. The peak mean plasma morphine concentration (7.1 μg/l) was detected in samples taken 0.5 h after the intra-articular morphine injection, but neither morphine nor its metabolites were found in plasma 6 h or more post injection. Morphine was detected in the synovial fluid of each pony 24 h after the injection. The plasma morphine or morphine-6-glucuronide concentrations were lower than those likely to have any systemic effect. The synovial fluid white blood cell count and protein concentration were increased and hyaluronate concentration decreased in samples taken 24 h after the intra-articular morphine injection, compared to the pre-injection samples. No differences were found between morphine and saline injected joints. It was concluded that morphine did not irritate the joint more than saline.