Diffusion of mepivacaine to adjacent synovial structures after intrasynovial analgesia of the digital flexor tendon sheath.

This study examines how, when used in horses, the analgesic mepivacaine diffuses from the digital flexor tendon sheath to adjacent structures. The findings suggest that while the compound does move to these areas, the concentrations are not clinically relevant.

Research Objective and Methodology

This crossover experiment aimed to investigate the extent of diffusion of the analgesic mepivacaine from the digital flexor tendon sheath (DFTS) to neighboring synovial structures in horses.

• Under general anaesthesia, the DFTS of one front limb and one hind limb of eight horses were injected with mepivacaine.
• Synovial fluid samples were then gathered from the injected DFTS and other areas 15 minutes post-injection (T15) and again 60 minutes post-injection (T60).
• Blood samples were also taken at the same intervals to assess the systemic distribution of the drug.
• After a two-week washout period, the exact process was repeated, with the timing of sample collection reversed between the front and hindlimbs.
• The concentration of mepivacaine in the samples was measured using a commercial ELISA kit.

Results and Findings

The researchers found that:

• Mepivacaine was present in every adjacent synovial structure and in the opposing metacarpo-/metatarsophalangeal (MCP/MTP) joints.
• However, the concentrations were low, the highest being just 3.2 mg/l, which is insufficient to produce effective analgesia.
• Contrary to what might be expected, the mepivacaine concentrations in the adjacent synovial structures increased at the 60 minute mark compared to the 15 minute mark (with the navicular bursa being the only exception).
• At T60, concentrations were significantly higher in the MCP/MTP joints on the same side (ipsilateral) than those on the opposite side (contralateral).
• The blood samples displayed significantly higher concentrations of mepivacaine at T15 and T60 than at the beginning of the experiment (T0).

Conclusions

The research concludes that while mepivacaine injected into the DFTS of horses diffuses towards adjacent synovial structures, it does not reach concentrations that would be considered clinically relevant for achieving analgesia.