Diffusion of mepivacaine between adjacent synovial structures in the horse. Part 1: forelimb foot and carpus.
Authors: Gough M R, Mayhew G, Munroe G A
Journal: Equine veterinary journal
Summary
Mepivacaine diffuses between adjacent synovial structures in the equine forelimb far more readily than previously recognised, with penetration occurring in 84–100% of cadaver specimens depending on the joint pair examined. Gough, Mayhew and Munroe injected mepivacaine into either the distal interphalangeal joint or navicular bursa in one forelimb of 31 fresh cadavers, then alternated sides for the radiocarpal and intercarpal joints, before harvesting synovial fluid from adjacent structures and quantifying mepivacaine concentration via enzyme-linked immunosorbent assay. Complete bidirectional diffusion occurred between the DIPJ and navicular bursa (100%), with notably high concentrations (>100 mg/l) detected in 48% of navicular bursa samples after DIPJ injection; diffusion between radiocarpal and intercarpal joints was similarly extensive (96% in one direction, 84% in the other), though concentrations remained lower and less consistent. These findings challenge the assumption that intrasynovial regional anaesthesia techniques are anatomically specific and have important implications for diagnosis of subtle lameness: analgesia obtained following injection into one structure cannot definitively localise pain to that structure alone, potentially masking multi-site pathology or leading to incomplete pain relief if an adjacent affected structure is overlooked.
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Practical Takeaways
- •Joint injections in the foot (DIPJ/navicular) cannot reliably target one structure alone—expect substantial diffusion between these compartments, which may explain variable clinical responses to regional analgesia
- •Carpal joint injections (IC/RC) show less dramatic diffusion than foot structures but still cross-communicate in most cases—interpret diagnostic anesthesia results cautiously and consider both joints may be affected
- •When performing intrasynovial anesthesia for lameness localization in the forelimb, acknowledge that multiple adjacent structures will be anesthetized simultaneously; this limits diagnostic specificity but may improve analgesia for multi-compartment problems
Key Findings
- •Mepivacaine diffused between DIPJ and navicular bursa in 100% (25/25) of forelimbs, with 44% achieving concentrations >300 mg/l in DIPJ following NB injection
- •Mepivacaine diffused from IC to RC joints in 96% of limbs and from RC to IC joints in 84% of limbs, with only 4% achieving concentrations >300 mg/l in IC joints
- •Diffusion of mepivacaine between adjacent synovial structures occurs more frequently and at higher concentrations than previously demonstrated with latex, gelatine dye, or contrast media
- •Intrasynovial analgesic techniques targeting individual forelimb joints are not as anatomically specific as previously assumed due to substantial diffusion between adjacent structures