Your farrier sounds like a raving moron to me. "Navicular", as in navicular
disease, is relatively rare in relation to the number of times it's diagnosed
by shoers as a dodge for longterm poor shoeing technique. If your shoer made
this diagnosis, I would replace him with someone a little more intelliget who
knows how to get the heels of the foot wider--providing circulation to the
area of the navicular.
George Maylin at Cornell once showed me a whole roundup of polo ponies the
kids were using for arena polo that his shoer had "cured" after these very
expensive thoroughbreds had been donated to the university with "incurable
navicular" diagnoses. He thought is was a great joke.
Navicular, when truly diagnosed (very seldom) is a deterioration of the
navicular bone.
ti