What used to be called navicular “disease” is now known to be an array of different problems which cause pain in the posterior part of the hoof capsule. “Navicular” gets its name because pathological changes to the shape of the navicular bone—actually a sesamoid—are usually prominent. Prickly looking exostosis of the front and rear aspects of the navicular sesamoid is characteristic. Exostosis impinges the tendon of the deep digital flexor muscle where it passes over the navicular bone to its insertion upon the semilunar crest on the underside of the coffin bone. The tendon rubbing on bony prickles creates intense “ischemic” pain—pain that an experienced veterinarian once defined for me as “pain out of all proportion to the size of the lesion.”
Rolf is the only horse among the four analyzed here whose skeleton shows evidence of navicular syndrome; Ethan Allen’s navicular bones are presented for comparison as he is entirely free of “navicular.”
ROLF: NAVICULAR SESAMOIDS
Prickly exostosis associated with “navicular disease” (yellow arrows) is evident on joint surface of the sesamoid (A) and the volar surface over which the tendon of the deep digital flexor muscle passes (B).
ETHAN ALLEN: NAVICULAR SESAMOIDS
Close-ups of Ethan Allen's navicular show no evidence of pathology. The rough appearance of some of the edges is due to postmortem rotting, not disease.