third metatarsal bone. In addition, the medial plantar metatarsal nerve may also
continue distally into the coronary dermis. Because of this additional nerve supply to
the distal portion of the pelvic limb, some clinicians recommend that the dorsal
metatarsal nerves, in addition to the plantar metatarsal nerves, be anaesthetised
when performing nerve blocks of the lower portion of the pelvic limb. The authors
and others29 believe, however, that most lameness involving the distal portion of the
pelvic limb can usually be evaluated without anesthetising the dorsal metatarsal
nerves.
Diagnostic analgesia of the collateral ligaments of the DIP joint
Lameness caused by disease of the collateral ligaments of the DIP joint often fails to
improve significantly after analgesia of the DIP joint itself.30 Only 24% of 30 horses
with MRI evidence of collateral ligament disease showed a significant improvement
in lameness after intra-articular analgesia of the DIP joint, and none of the horses
improved after intra-bursal analgesia of the navicular bursa. A PDNB, however,
improved lameness significantly in 72% of these horses, and all became sound after
an ASNB. Consequently, when lameness is abolished by a PDNB but not by
analgesia of the DIP joint or navicular bursa, and no radiographic abnormalities of
the foot can be detected, collateral desmitis of the DIP joint must be considered as a
possible cause of lameness, as long as the presence of solar pain can be eliminated
with hoof testers.
Clinical impressions suggest that the degree of improvement in lameness associated
with injury to a collateral ligament after PDNB is determined by the extent of the
injury and the level at which the palmar digital nerve is anaesthetised. The further
proximal the level of the injury within the collateral ligament, the less likely that
lameness is affected by analgesia of the DIP joint or anaesthesia of the palmar
digital nerves. This observation suggests that the branches innervating the proximal
aspect and origin of the collateral ligaments arise from the palmar digital nerves
proximal to the site where the palmar digital nerves lie alongside the palmar pouch of
the DIP joint or from the dorsal branches of the palmar digital nerve. A pastern ring
block or ASNB may be required to abolish lameness when osseous remodelling is
present at the origin of the collateral ligament at the level of the middle phalanx.
a
Caldwell, F.J. and Schumacher, J., 2015, Unpublished data, Auburn University,
Auburn AL., USA
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