Dye Injection Volume and Allocation

The initial objectives of perineural injections with methylene blue solution were to observe tissue dye staining and test the injection selectivity for the ramus ventralis considering possible diffusion to the adjacent cranial and/or caudal ispilateral ramus in addition to diffusion to the epidural space across midline. Three equine cadavers stored for <36 h at 4°C were used after being at room temperature for 3 h. They received unilateral perineural ramus ventralis injections of C7 and C8 with the methylene blue solution (6 injections in toto). The use of a single dye color limited the number of injection possibilities and testing conditions. As a result and to facilitate cadaver handling and dissection, it was decided to test 2 volume conditions on the same side of the neck. While considering the ramus ventralis size difference (width of C8 1.5 to 2 times larger than C7), the two conditions tested were C7–7 ml/C8–14 ml on two horses (horse 1 and 3) and C7–7 ml/C8–7 ml on horse 2. Perineural injection volumes selected (7 and 14 ml) were extrapolated from similar studies using an allometric scale (12, 16, 26). Left and right neck injections and condition assignment for the 3 cadavers were randomly selected using a closed envelope technique.

Following the results on the 3 methylene blue cadavers and to confirm the findings, an additional cadaver received, four injections with a solution of permanent marking tissue dye within 30 min from euthanasia. The use of 4 different colors allowed to perform bilateral injections without compromising the evaluation of individual injection selectivity for each ramus ventralis. Opposite randomized conditions were tested: C7–7 ml/C8–14 ml on the left and C7–14 ml/C8–7 ml on the right.