Discussion

Several techniques have been described for diagnostic analgesia of the proximal metacarpal region, but potential distribution of local anaesthetic solution following different techniques has not been assessed in live horses.

Eight mature horses were used in the live horse study. Four different injection techniques were performed: 1) medial and lateral palmar metacarpal injections performed at the lateral and medial aspects of the metacarpus; 2) lateral palmar metacarpal injection (the needle redirected from the lateral approach towards the medial palmar metacarpal nerve); 3) lateral approach for injection of the lateral palmar nerve; and 4) medial approach for injection of the lateral palmar nerve. For techniques 1 and 2, 2 ml radiodense contrast medium was used and for techniques 3 and 4, 3 ml contrast medium was used. All injections were performed with the limb in a semi-flexed position. All techniques were performed once in all horses in either the left or the right forelimb, selected randomly, resulting in 8 sets of injections for each technique.

For techniques 1 and 2, dorsopalmar (DPa) and dorsolateral- palmaromedial oblique (DL-PaMO) and dorsomedial- palmarolateral oblique (DM-PaLO) radiographic images were obtained 0, 10 and 20 min after injection. For techniques 3 and 4, DPa, DL-PaMO and DM-PaLO or lateromedial (LM) images were obtained 0, 10 and 20 min after injection.

To confirm the anatomical location of the radiodense contrast medium, injections were performed in 4 cadaver limbs, using the 4 techniques described above, one in each limb. A mixture of
0.5% methylene-blue solution and iohexol (1 ml of each for techniques 1 and 2 [i.e. a total volume of 2 ml] and 1.5 ml of each for techniques 3 and 4 [i.e. a total volume of 3 ml]) was injected. Dorsopalmar, DL-PaMO and DM-PaLO (techniques 1 and 2) or LM (techniques 3 and 4) radiographic images were obtained. Limbs were dissected and the location of the dye was recorded.
Palmar metacarpal injections performed at the medial and lateral aspects of the metacarpus
In all limbs of live horses the contrast medium was distributed in an elongated pattern just axial to the second and fourth metacarpal bones. In one limb the contrast medium injected around the medial palmar metacarpal nerve extended proximally to the level of the middle carpal joint. Proximal diffusion of contrast medium with time was noted in only one limb. In 4 limbs, in addition to the distribution axial to the second and fourth metacarpal bones, contrast medium was also identified in the middle carpal and carpometacarpal joints.

In the cadaver limb, the contrast medium was distributed in an elongated pattern axial to the second and fourth metacarpal

bones. Dissection confirmed that the dye was distributed along the medial and lateral palmar metacarpal nerves.

Palmar metacarpal injections performed from the lateral aspect of the metacarpus
In 6/8 limbs of live horses the contrast medium was distributed in an elongated pattern axial to the second and fourth metacarpal bones. In 2 limbs, the majority of the contrast medium was distributed in an elongated pattern just axial to the fourth metacarpal bone with minimal or no contrast medium axial to the second metacarpal bone. In one limb, in addition to the distribution axial to the second and fourth metacarpal bones, contrast medium was also identified in the middle carpal and carpometacarpal joints.

In the cadaver limb, the contrast medium was distributed in an elongated pattern axial to the second and fourth metacarpal bones. There was also some contrast medium between the distal aspects of the medial and lateral contrast patches. Dissection confirmed that the dye was distributed along the medial and lateral palmar metacarpal nerves; there was also some dye distributed along the deep metacarpal fascia.

Lateral palmar nerve lateral approach:
In 7/8 limbs of live horses, the contrast medium extended from the distal aspect of the accessory carpal bone to the level of, or up to approximately 1 cm distal to, the carpometacarpal joint. In one limb, the contrast medium was distributed in an elongated pattern along the entire length of the carpal sheath.

In the cadaver limb, the contrast medium was diffusely distributed around the injection site in the DPa radiograph, and the dye was identified within the accessoriometacarpal ligament and around the lateral palmar nerve.

Lateral palmar nerve medial approach:
In 5/8 limbs of live horses, the contrast medium was distributed in an elongated pattern medial to the accessory carpal bone. In 4 of these limbs, the contrast medium extended proximally to the level of the radius, and in one of the 8 limbs to the level of the proximal aspect of the accessory carpal bone. In 2 limbs, the contrast medium showed a Y-shaped distribution, with 2 branches extending proximally in the distal third of the antebrachium. In one limb the contrast medium was diffusely distributed medial to the accessory carpal bone.

In the cadaver limb, the contrast medium was distributed in a Y-shape in the DPa radiograph. Dissection confirmed distribution

of the dye along the median nerve and the palmar branch of the ulnar nerve, in addition to a small patch at the injection site.

This study highlighted potential pitfalls of different injection techniques for diagnostic analgesia of the proximal metacarpal region. Perineural injection of the medial and lateral palmar metacarpal nerves using medial and lateral approaches carried a greater risk of inadvertent penetration of the carpometacarpal joint than did perineural injection of both nerves from a lateral approach.

False negative results may be more likely to occur if injections are performed from only the lateral approach. The medial approach to the lateral palmar nerve seemed to be the safest for avoiding inadvertent penetration of synovial structures. Marked proximal diffusion was seen in some horses with this technique; similar diffusion of local anaesthetic solution may lead to desensitisation of the entire palmar carpal region and the caudodistal aspect of the antebrachium.