Synovial fluid bupivacaine concentrations following single intra-articular injection in normal and osteoarthritic canine stifles

Intra-articular bupivacaine helps alleviate pain in animals receiving joint surgery, but its use has become controversial as ex vivo studies have illuminated the potential for chondrotoxicity. Such studies typically involve cell cultures incubated in solutions containing high bupivacaine concentrations for long durations. The aim of this study was to measure the actual synovial fluid bupivacaine concentrations after intra-articular injection. Eight healthy beagles with normal stifles and 22 large and giant-breed dogs with stifle osteoarthritis (OA) were treated with a single intra-articular injection of bupivacaine (1 mg/kg) into a stifle. Joint fluid samples were taken from the treated stifle immediately after injection and 30 min after injection and analyzed for bupivacaine concentrations. Immediately after injection, the median bupivacaine concentrations in normal and OA stifles were 3.6 and 2.5 mg/mL, respectively. Thirty minutes after injection, bupivacaine concentrations in normal and OA stifles were 0.4 and 0.6 mg/mL, respectively. These results provide insight into the pharmacokinetics of bupivacaine after injection into a joint. Given its immediate dilution and rapid drop in synovial fluid concentration, bupivacaine is unlikely to damage chondrocytes when administered as a single intra-articular injection.