In a horse that is ¾ lame and a palmar nerve block relieves the lameness, which structure is not affected by this block?

• A. P3 (distal phalanx)
• B. Bulbs of the heel
• C. Navicular bone
• D. Digital cushion

Answer: (A)

Understanding how nerve blocks localize lameness in horses hinges on which tissues the palmar digital nerves actually supply. These nerves desensitize the soft tissues of the hoof region—the sole, frog, digital cushion, and the bulbs of the heel—and they also give branches to the coffin joint and the navicular apparatus. Because those structures are the ones you’re blocking, relief of lameness from a palmar block points to a problem in those tissues or in the navicular apparatus.

The distal phalanx, or P3, is not primarily targeted by this block. Its pain pathways aren’t reliably blocked by this palmar digital nerve block, so a lesion confined to the distal phalanx would be unlikely to be fully desensitized by it. Therefore, if the lameness is relieved by the block, the structure not affected by the block—and thus not likely to be the source of pain in this scenario—is the distal phalanx.