Diffusion of mepivacaine between adjacent synovial structures in the horse. Part 2: tarsus and stifle

Abstract

Summary This paper tests the hypothesis that the local analgesic agent mepivacaine diffuses between adjacent equine synovial structures in the hindlimb and with greater frequency than latex, gelatine dye or contrast media. We report the incidence of diffusion of mepivacaine between the tarsometatarsal, centrodistal and tarsocrural joints, and the 3 synovial compartments of the stifle in 33 fresh equine cadavers. The tarsometatarsal joint and one synovial compartment of the stifle in the left limb and the centrodistal joint and a different synovial compartment of the stifle in the right limbs were injected with mepivacaine. Following flexion and extension of the limb, synovial fluid was aspirated from the noninjected centrodistal and tarsometatarsal joints and the tarsocrural joints of the hock and the noninjected compartments of the stifle. Concentrations of mepivacaine in these samples were assayed using an enzyme linked immunosorbent assay. For samples obtained by dilution of synovial fluid the concentration of mepivacaine was determined by comparing the concentration of urea in the diluted synovial fluid and the concentrations of the serum urea. Mepivacaine was detected in 25/25 (100%) adjacent tarsometatarsal and centrodistal joints after diffusion in both directions, in 23/25 (92%) of tarsocrural joints after diffusion from tarsometatarsal joints and in 22/25 (88%) tarsocrural joints after diffusion from centrodistal joints in the hocks. Diffusion from the femoropatellar to medial and lateral femorotibial joints and between the medial and lateral femorotibial joints in both directions were 20/20 (100%). Diffusion from the lateral femorotibial to the femoropatellar joint was 18/20 (90%) and from the medial femorotibial to femoropatellar joints 17/20 (85%). Mepivacaine was detected at concentrations >0.3 mg/l in a proportion of samples ranging from 15/25 (60%) in the tarsocrural joint following tarsometatarsal joint injection to 18/20 (90%) in the lateral femorotibial joint after femoropatellar joint injection. At mepivacaine concentrations >100 mg/l, detection ranged from 3/20 (15%) in the lateral femorotibial joint from the medial femorotibial joint to 19/25 (76%) in the centrodistal joint from the tarsometatarsal joint. At mepivacaine concentrations >300 mg/l, detection ranged from 1/25 (4%) in the tarsocrural joint from the tarsometatarsal joint to 16/25 (64%) in the from centrodistal joint the tarsometatarsal joint. The results show greater diffusion of mepivacaine between these adjacent synovial structures than assumed from previous anatomical, latex injection and contrast arthrographic studies. Therefore, commonly performed intrasynovial local analgesic techniques in the hindlimb of the horse are not as specific as first thought.

In vivodiffusion characteristics following perineural injection of the deep branch of the lateral plantar nerve with mepivacaine or iohexol in horses

“…The former is considered less likely since none of the 8 contrast medium‐injected limbs showed evidence of contrast medium within the TMT joint. Diffusion to adjacent structures has been shown in similar studies and considering the small molecular weight of mepivacaine (246 g/mol) compared with iohexol (841 g/mol), it is likely that mepivacaine could diffuse while iohexol may not. Mepivacaine could also travel via lymphatic drainage; this was demonstrated by the faint, thin lines of contrast medium seen extending away from the primary contrast region.…”

“…Mepivacaine is 1.5–2 times as potent as procaine so Gough et al . (2002) estimated that mepivacaine concentrations >100 mg/l would be sufficient to produce analgesia of the tarsocrural joint . Keegan et al .…”

Musculoskeletal Injuries in Nonracing Quarter Horses

“…Blocking the tarsometatarsal joint and the distal intertarsal joint alleviates the lameness in many cases, although horses with advanced joint degeneration and extensive subchondral bone pathologic findings may not respond. The distal intertarsal and tarsometatarsal joints communicate in approximately 30% of horses [44]; however, in some horses, it is possible for local anesthetic to diffuse into the tibiotarsal joint, which may confuse interpretation of results [45]. Radiographs of the tarsi may reveal pathologic changes ranging from mild osteophytosis to marked exostosis, areas of subchondral sclerosis and lysis, and narrowing or intermittent loss of joint spaces.…”

“…Thorough clinical examination and diagnostic analgesia are essential, because many soft tissue lesions are not apparent on radiographs. The femoropatellar joint has been shown to communicate with the medial femorotibial joint in approximately 60% of horses [56], and diffusion of mepivacaine between compartments has been demonstrated to occur in approximately 85% of horses [45]. Despite the potential for analgesia of multiple joint compartments from a single injection, sequential injection of specific compartments may be necessary.…”

Radiography of the equine stifle

“…However, communication between the lateral femorotibial compartment and the femoropatellar joint has been described at 1 to 25 per cent, with no direct communication between the medial and lateral femorotibial joint compartments (Fowlie and Stick 2011). While mepivacaine diffusion between compartments is known to occur, achieving adequate levels of anaesthesia within the joint compartments is not confirmed unless separate injections of each joint compartment is performed (Gough and others 2002). …”