Longitudinal evaluation of fentanyl concentrations in equine plasma and synovial fluid following application of transdermal fentanyl patches over one carpal joint.
Authors: Ortega McCormack John J, Reed Rachel A, Epstein Kira L, Camus Melinda S, Knych Heather K
Journal: Veterinary surgery : VS
Summary
# Fentanyl Patches and Carpal Joints: Limited Local Delivery Transdermal fentanyl patches are used clinically in equine practice, but whether positioning them directly over an affected joint delivers higher drug concentrations to the synovial fluid—potentially improving local analgesia—remained unclear. McCormack and colleagues applied dual 100 µg/h fentanyl patches over one carpometacarpal joint in six healthy horses for 48 hours, measuring fentanyl concentrations in plasma and intercarpal synovial fluid via liquid chromatography-mass spectrometry at seven timepoints. Whilst all horses achieved detectable fentanyl in both compartments with peak concentrations at 12 hours, the critical finding was that synovial fentanyl concentrations did not differ between the treated and untreated carpus at any timepoint (p > 0.608), and synovial levels remained substantially lower than plasma concentrations throughout. For practitioners considering topical fentanyl for localised joint pain, these results suggest that direct patch placement over the affected joint offers no pharmacokinetic advantage; fentanyl uptake into synovial fluid appears governed by systemic absorption and passive diffusion rather than local penetration. This has implications for patch positioning strategies and reinforces the need for alternative approaches—such as intra-articular medication or optimised systemic dosing—to achieve therapeutically meaningful local concentrations in inflamed joints.
Read the full abstract on PubMed
Practical Takeaways
- •Applying fentanyl patches directly over a carpal joint does not increase local analgesic drug concentration in that joint's synovial fluid compared to systemic delivery
- •Local patch placement over the affected joint offers no analgesic advantage over systemic transdermal administration for carpal pain management
- •Consider alternative pain management routes (systemic, intra-articular injection, or other modalities) if local joint analgesia is the clinical goal
Key Findings
- •All six horses achieved detectable fentanyl concentrations in both plasma and synovial fluid with peak levels at 12 hours
- •Synovial fluid fentanyl concentrations were significantly lower than plasma concentrations at 6 and 12 hours post-application
- •No significant difference in synovial fentanyl concentrations between treated and untreated joints at any time point (p > 0.608)