Diffusion of mepivacaine between adjacent synovial structures in the horse. Part 2: tarsus and stifle.
Authors: Gough M R, Munroe G A, Mayhew G
Journal: Equine veterinary journal
Summary
# Editorial Summary: Mepivacaine Diffusion in the Equine Hindlimb Intra-articular local anaesthesia is fundamental to equine lameness diagnosis, yet the specificity of these blocks may be substantially compromised by diffusion of the drug between adjacent joint compartments. Gough and colleagues injected mepivacaine into individual tarsal and stifle joints in 33 cadaver limbs, then systematically aspirated synovial fluid from non-injected neighbouring structures and quantified drug concentrations using enzyme-linked immunosorbent assay. The findings were striking: mepivacaine reached 100% of centrodistal joints when the tarsometatarsal joint was injected (and vice versa), diffused to the tarsocrural joint in 88–92% of specimens, and spread between all three stifle compartments with frequencies ranging from 85–100%. Notably, clinically relevant concentrations (>0.3 mg/l) were detected in 60–90% of non-injected joints, and in some cases concentrations exceeded 100–300 mg/l—levels capable of producing analgesia independently. This work demonstrates that the anatomical barriers between hindlimb synovial structures are far more permeable than previous contrast studies suggested, meaning that lameness localisation based on conventional regional blocks may be considerably less precise than practitioners have assumed, particularly when interpreting equivocal or multilateral responses during diagnostic anaesthesia protocols.
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Practical Takeaways
- •Intrasynovial local anesthetic injections in the equine hindlimb are less specific than traditionally assumed; expect significant diffusion to adjacent joint compartments
- •When performing tarsus or stifle joint blocks, anticipate anesthetic effects in multiple adjacent structures rather than a single isolated joint
- •Clinical interpretation of joint anesthesia response should account for multi-compartment anesthetic involvement, which may confound diagnostic anesthesia results
Key Findings
- •Mepivacaine diffused between tarsometatarsal and centrodistal joints in 100% of cases (25/25), bidirectionally
- •Mepivacaine reached the tarsocrural joint in 92% of cases from tarsometatarsal injection and 88% from centrodistal injection
- •Diffusion between femoropatellar and medial/lateral femorotibial joints occurred in 85-100% of cases
- •Mepivacaine diffusion between adjacent hindlimb synovial structures is substantially more frequent than previously documented by anatomical, latex injection, or contrast studies