Diffusion of mepivacaine to adjacent synovial structures after intrasynovial analgesia of the digital flexor tendon sheath.
Authors: Jordana M, Martens A, Duchateau L, Haspeslagh M, Vanderperren K, Oosterlinck M, Pille F
Journal: Equine veterinary journal
Summary
# Editorial Summary: Digital Flexor Tendon Sheath Analgesia and Adjacent Structure Diffusion When performing diagnostic analgesia of the digital flexor tendon sheath (DFTS) to localise lameness, practitioners should be aware that injected local anaesthetic inevitably diffuses to neighbouring synovial structures, though this Belgian study suggests the concentrations involved are unlikely to produce clinically meaningful analgesia in adjacent tissues. Researchers injected mepivacaine into the DFTS of eight horses under general anaesthesia and measured drug concentrations in synovial fluid from the injected sheath, the neighbouring MCP/MTP, PIP and DIP joints, navicular bursa, and contralateral MCP/MTP joint at 15 and 60 minutes post-injection using ELISA analysis. Mepivacaine concentrations within the DFTS itself remained well above the therapeutic threshold (5077 mg/l at 15 minutes, declining to 3503 mg/l at 60 minutes), yet concentrations in adjacent structures peaked at just 3.2 mg/l—roughly 1–3% of levels considered necessary for synovial analgesia—with diffusion increasing significantly between the 15- and 60-minute intervals. Whilst mepivacaine did reach contralateral joints, suggesting some systemic absorption occurred, the ipsilateral MCP/MTP joint showed statistically higher concentrations than the contralateral joint, indicating local diffusion rather than purely vascular distribution. For practitioners interpreting DFTS diagnostic blocks, these findings support the reasonable specificity of this procedure, as adjacent joint involvement through passive diffusion is unlikely to confound lameness localisation, though clinicians should remain mindful that any observed improvement in gait following DFTS analgesia reflects sh
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Practical Takeaways
- •DFTS diagnostic blocks may lack specificity because mepivacaine diffuses to adjacent joints, potentially masking the true source of lameness
- •Positive response to DFTS analgesia does not definitively confirm DFTS as the pain source—adjacent joint involvement must be ruled out separately
- •Consider combining DFTS blocks with specific joint anesthesia (MCP/MTP, PIP, DIP) for more accurate diagnostic localization in cases of distal limb lameness
Key Findings
- •Mepivacaine concentrations in the DFTS at T15 (5077 mg/l) and T60 (3503 mg/l) far exceeded the 100-300 mg/l threshold needed for synovial analgesia
- •Mepivacaine diffused to all adjacent synovial structures but only reached maximum concentrations of 3.2 mg/l, well below clinically relevant levels
- •Mepivacaine concentrations in adjacent structures were significantly higher at T60 than T15 (except navicular bursa), suggesting progressive diffusion over time
- •Drug was detected in contralateral MCP/MTP joints, indicating systemic distribution, with significantly higher ipsilateral than contralateral concentrations at T60