The same movement of the joint is given by causing the animal to put full weight upon the diseased limb, a small wedge of wood being first placed under the toe. In this manner the pressure of the perforans tendon upon the bursa is greatly increased, and the animal is caused to show symptoms of distress.
The lameness may also be increased, and diagnosis helped, by paring the heels, so as to leave the frog prominent and take the whole of the body-weight. The same end is also obtained by applying a bar shoe. This was originally pointed out by Brauell, and is quoted by Zundel and by Möller.
The changes in the form of the hoof may now be noticed. These are largely dependent on the fact that more or less constantly the patient saves the heel. The horn of the walls in this region, and the horn of the frog, is thereby put out of action and induced to atrophy. The hoof gradually assumes a more upright shape, and the heels contract. We thus get a hoof which is visibly narrowed from side to side, with a frog that is atrophied and often thrushy, and with a sole that is abnormally concave, hard, and affected with corns.
When occurring in the hind-feet—a condition that is rare, but which has been noticed by Loiset, and quoted by Zundel—the animal is stiff behind, walks on his toes, and gives one the impression that he is suffering from some affection in the region of the loins.
One such case is reported by an English veterinary surgeon, and we quote it here:
'A gray gelding, and a capital hunter, the property of a gentleman in this neighbourhood, became lame in the near fore-foot after the hunting season of 1859. The lameness was believed to be due to navicular disease. The operation of neurectomy was ultimately had recourse to. The horse subsequently did his work as well as ever, and was ridden to hounds regularly till the end of the year 1861, when he went lame of the off fore-foot. From this date he also showed very peculiar action behind, and was at times lame of both hind-limbs without any apparent cause.
'In the year 1862, from the groom's indiscreet use of physic, super-purgation was brought on which caused the animal's death. On a post-mortem examination being made, the horse was found to have navicular disease of all four feet. It is worthy of note that this horse had always "extravagant" action behind, but was a remarkably quick and good jumper.'[A]
[Footnote A: F. Blakeway, M.R.C.V.S., Veterinarian, vol. ii., p. 21.]
Differential Diagnosis.—Navicular disease may be mistaken for ordinary contracted foot. It will be remembered, however, that in the early stages of navicular disease contraction is absent, and that it is only when the disease in the bursa is of long standing that contraction comes on. With ordinary contracted foot, too, careful paring and suitable shoeing soon sees a diminution in the degree of lameness, and a return to the normal in shape (see Treatment of Contracted Foot, p. 125). With navicular disease, however, such shoeing as is beneficial in the treatment of contracted foot (notably the various methods of giving to the frog counter-pressure with ground) soon brings on an aggravation of the lameness.
It is, perhaps, even more likely to be confounded with contraction when we have with the contraction a state of atrophy and thrush of the frog. With a frog in this condition pressure will give rise to pain, and navicular disease be erroneously judged to be present. In such a case we must rely wholly upon either extreme flexion or extreme extension of the joint to guide us, when, if contraction only is the offending condition, no symptom of pain will be shown.