Diffusion of mepivacaine between adjacent synovial structures in the horse. Part 1: forelimb foot and carpus

This paper tests the hypothesis that the local analgesic agent mepivacaine diffuses between adjacent equine synovial structures in the forelimb and with greater frequency than latex, gelatine dye or contrast media. We report the incidence of diffusion of mepivacaine between the distal interphalangeal joint (DIPJ) and navicular bursa (NB) of the forelimbs and between the intercarpal (IC) and radiocarpal (RC) joints of 31 fresh equine cadavers. The DIPJ of one forelimb and the NB of the contra lateral forelimb and the RC joint of one forelimb and the IC joint of the contra lateral forelimb were injected with mepivacaine. After flexion and extension of the joints, synovial fluid was obtained from the synovial structures adjacent to the injected synovial structures. The concentration of mepivacaine in these samples was determined using an enzyme linked immunosorbent assay. For samples obtained by dilution of synovial fluid, the concentration of mepivacaine was determined by comparing the concentrations of urea in the diluted synovial fluid and the concentration of serum urea. Mepivacaine diffused from the DIPJ to the NB or from the NB to the DIPJ in 25/25 (100%) limbs. Mepivacaine diffused from the IC to RC joints in 24/25 (96%) limbs and from the RC to IC joints in 21/25 (84%) limbs. It was detected at concentrations >0.3 mg/l in 9/25 (36%) of IC joints after RC joint injection and in 25/25 (100%) of the NB after DIPJ injection; at concentrations >100 mg/l in 2/25 (8%) of IC and RC joints and 12/25 (48%) of NB following DIPJ injection; and at concentrations >300 mg/l in 1/25 (4%) in the IC joints following RC joint injection and in 11/25 (44%) of DIPJ following NB injection. The results show greater diffusion of mepivacaine between adjacent synovial structures than assumed from previous anatomical, latex injection and contrast arthrographic studies. This study showed that commonly performed intrasynovial analgesic techniques in the forelimb of the horse are not as specific as previously reported.