Diffusion of radiodense contrast medium after a mid‐pastern ring block

Diffusion of local anaesthetic solution after a mid‐pastern ring block has not previously been investigated. The aim of this study was to demonstrate potential distribution of local anaesthetic solution following injection of radiodense contrast medium as performed for a mid‐pastern ring block. Twelve mature horses were used and 1.5 ml radiodense contrast medium injected over the medial or lateral palmar digital nerve at the level of the proximal aspect of the ungular cartilages. A dorsal ring block was performed on the ipsilateral side, 1.5 cm proximal to the palpable palmar aspect of the proximal eminence of the middle phalanx, using 2 or 5 ml contrast medium. Both forelimbs were injected on 2 days (48 injections). Four standard radiographic views of the pastern were obtained immediately, 10 and 20 min after injections. Images were analysed subjectively and objectively. After dorsal injections, the contrast medium was distributed in a diffuse patch over the ipsilateral half of the proximal phalanx (P1), extending proximally over the half of the length of P1 in all limbs (greatest proximal extension: 89.0% of the length of P1 [from distal] after 2 ml, 94.2% after 5 ml). There was significant proximal diffusion in the first 10 min after injection and significant dorsal diffusion between all time points (P<0.01). There was significant positive association between injected volume and the proximal extension of the dorsal contrast patch (P = 0.01). The median dorsal diffusion was to the dorsal midline of P1; 5 ml contrast medium resulted in significantly greater dorsal diffusion than 2 ml (P<0.01). The dorsal and palmar contrast patches did not merge. In conclusion, diffusion to the proximal aspect of P1 following a mid‐pastern ring block may occur even if only 2 ml of local anaesthetic solution is used.