Ring block
A ring block, performed at the level of a PDNB is unlikely to result in a positive
response after a negative response to a PDNB because the dorsal branches of the
palmar digital nerves contribute little to sensation within the foot.1,4,5 The PDNB will
already have anaesthetised the entire foot. The use of a pastern ring block
performed at mid pastern level, however, should be considered as a better
alternative to the basilar sesamoid (basisesamoid) nerve block (BSNB) or abaxial
sesamoid nerve block (ASNB) because these blocks may inadvertently, partially or
entirely desensitise the metacarpo/metatarso phalangeal (MC/MT-P) (fetlock) joint in
addition to the entire foot an d pastern, which erroneously localise pain to the foot or
pastern in horses with fetlock pain. A positive response to a ring block performed at
mid pastern level, after a negative response to a PDNB, localizes lameness to the
pastern region.
Basilar or Abaxial sesamoid nerve block (BSNB or ASNB)
Bassage and Ross6 distinguish between anaesthesia of the palmar or plantar digital
nerves where local anesthetic is deposited at the base of the proximal sesamoid
bones (i.e., a BSNB) and anaesthesia of the palmar or plantar digital nerves where
local anaesthetic solution is deposited more proximally alongside the proximal
sesamoid bones (i.e., an ASNB). Anaesthesia of the digital nerves and their dorsal
branches, at either level, alongside or at the base of the proximal sesamoid bones,
desensitises the foot, the pastern joint, the middle phalanx and associated soft
tissues, and the distal and palmar aspects of the proximal phalanx.
Clinicians should be aware that an ASNB or BSNB may ameliorate or abolish pain
within the MC/MT-P joint.2,6 Performing the nerve block at the base of the proximal
sesamoid bones (i.e., a BSNB) decreases the likelihood of partially desensitizing the
MC/MT-P joint.2 Using a small volume of local anaesthetic solution (i.e., <2 mL) may
also decrease the likelihood of partially anaesthetising the MC/MT-P joint.
Analgesia of the distal interphalangeal joint (DIP joint)
Local anaesthetic solution is most easily and accurately administered into the DIP
joint using a dorsal parallel or dorsal inclined approach, where a needle is placed into
the dorsal pouch of the DIP joint on or near midline, through or just proximal to the
coronary band.
The effect of local anaesthetic solution in the DIP joint has been reviewed.8 Local
anaesthetic administered into the DIP joint desensitises the DIP joint, the navicular
bursa, the navicular bone, the toe region of the sole and the digital portion of the
deep digital flexor tendon (DDFT) of most horses. When a large volume of
mepivacaine HCl (e.g.,10 mL) is administered, the heel region of the sole may also
be desensitised.
Possible explanations for analgesia of the navicular apparatus (i.e., the navicular
bone and its associated ligaments) after local anaesthetic solution is administered
into the DIP joint is the desensitization of the palmar digital nerves where they lie in
close proximity to the palmar pouch of the DIP joint.9 Occasionally, the palmar
aspect of the coronary band is desensitised after anaesthesia of the DIP joint;10 this
finding, as well as desensitisation of the solar region of the toe support the theory
that analgesia of the DIP joint causes desensitisation of the palmar digital nerves. A
negative response to intra-articular analgesia of the DIP joint may not eliminate the
Proceedings
of
the
South
African
Equine
Veterinary
Association
Congress
2016
44