In vivo diffusion characteristics following perineural injection of the deep branch of the lateral plantar nerve with mepivacaine or iohexol in horses

REASONS FOR PERFORMING STUDY: Hindlimb proximal suspensory desmopathy is a common injury of sport horses but diagnosis can be difficult because diagnostic analgesia of the region lacks specificity. Perineural analgesia of the deep branch of the lateral plantar nerve (DBLPN) has been proposed as a more specific method of isolating pain of the proximal aspect of the suspensory ligament but the technique has not been evaluated in vivo. OBJECTIVES: To determine the extent of diffusion of contrast medium and mepivacaine following DBLPN analgesia using a single‐needle injection technique and to determine if there is inadvertent involvement of the tarsal sheath and/or tarsometatarsal (TMT) joint using this technique. STUDY DESIGN: In vivo experimental study. METHODS: Perineural injection of the DBLPN was performed in 16 limbs with 3 ml of either mepivacaine hydrochloride or positive contrast medium. Contrast medium‐injected limbs were radiographed 5, 15, and 30 min post injection and diffusion characteristics were described. In mepivacaine‐injected limbs, synovial fluid from the TMT joint was obtained 10 and 20 min post injection and mepivacaine concentrations were analysed. RESULTS: At 5, 15 and 30 min post injection, the contrast medium extended, on average, 19.6, 20.6 and 21.0 mm proximal and 38.0, 43.5 and 51.9 mm distal to the injection site, respectively. Three of 8 (37.5%) limbs had evidence of contrast medium in the tarsal sheath. Two of 8 (25%) limbs had mepivacaine concentrations within the TMT joint sufficient to produce analgesia (>300 mg/l) at 10 min post injection. CONCLUSIONS: Contrast medium diffused further in a distal direction than in a proximal direction. Analgesia of the DBLPN can result in inadvertent involvement of the tarsal sheath and/or TMT joint.