This is a disease of the foot, and, excepting perhaps splints, one of the most common causes of lameness from which horses suffer. From a rough estimate, it may be said that not less than 60 to 70 per cent of our light horses, and more of the heavy ones than is generally believed, sooner or later become affected by it. Just as a splint is the bane of young horses, so is navicular disease the common affliction of old ones. While the former, however, is amenable to treatment, and seldom becomes permanently hurtful, the latter is practically incurable and progressively destructive, so that once established it goes on from bad to worse, and ultimately cripples its victim and renders him physi-callv useless.
It was not until the early part of the nineteenth century that anything was known of this most destructive ailment. Until then the lameness arising out of it had been for a long time attributed to various imaginary conditions, as well as some real ones, hence it received and became known by a variety of terms, according with the various views entertained as to its seat and origin. From its supposed existence in the muscles of the shoulder and chest it was by some designated "chest founder"; while others, regarding it as the result of contraction of the feet, spoke of it as "contraction", which was alleged by the leading veterinary authorities to arise out of a diseased condition of the sensitive laminae intervening between the pedal-bone and the hoof. About the year 1804, Moorcroft, a distinguished member of the veterinary profession, first indicated the actual seat and nature of the disease, which he spoke of as "coffin-joint lameness". A few years later Mr. James Turner very considerably added to our knowledge of the malady, since which time it has been known by the term navicular disease, and more recently as navicularthritis.
Fig. 402 - Navicular Disease.
a, Healthy bone; B, Primary disease of bone, giving rise to softening and erosion of the cartilage at numerous points; C, Bone showing loss of cartilage and underlying excavations; D, Large carious surface resulting from confluence of small excavations.
As to the precise nature of the disorder, it is now pretty generally regarded by veterinary surgeons as an ulceration or caries of the navicular bone. Commencing at first in a subacute inflammation of the ossific structure, the bony tissue gradually undergoes solution and removal at one or several centres, as a result of which minute holes occur near to the surface and gradually extend themselves inward and outward, until two or more joining together form large ulcers and excavations, during which the articular cartilage covering the bone undergoes a process of softening and removal. The bone having lost its smoothness, now presents a rough and eroding surface to the flexor tendon on which it rests, with the result that by constant rubbing of the latter against the former, the tendon becomes irritated and inflamed, and later swollen and softened, when its weakened fibres gradually break away and impart to its articular surface a rough and ragged condition. Associated with these changes in the bone and tendon, chronic changes in the synovial membrane are also observed, so that one after another all the structures of the joint sooner or later become implicated in the disease. If during its progress the patient is allowed a prolonged rest, the tendon frequently unites with the bone, when all movement between the two ceases to exist. Rupture of the tendon and fracture of the bone are ulterior consequences of the disease.
From the great difference in susceptibility to this affection presented by different animals, and the evidence afforded by the family history of some, heredity would seem to exercise an important predisposing influence in its causation.
That bad shoeing, by inducing weakness and contraction of the foot, operates also as a predisposing factor there is no room to doubt, and especially is this the case with regard to horses which are confined to the stable in a standing position for long periods, while receiving large quantities of highly stimulating food.
Fig-. 403. - Sections of Navicular Bone, healthy and diseased.
A, Section of healthy bone; B, Section of diseased bone permeated by carious tracts.
The exciting cause is almost invariably due to concussion acting upon feet deprived of their elasticity by long-continued abuse. Animals with high hammering action suffer most, but long-continued wear on hard roads affects all alike. The most sudden and acute cases are found among hunters as the result of encountering deep drops in jumping into hard roads under heavy weights.
As we have seen, under ordinary circumstances navicular disease is slowly progressive, while in others it is sudden in attack, and runs from the first a more or less acute course. In the former case the onset of the disease is hardly perceptible. If in a riding horse, the first indication of the oncoming affection is mostly experienced by the rider, who recognizes a sense of discomfort in the animal's movements arising out of a loss of his ordinary elasticity and firmness of tread; but even this for a time is subject to variation, so much so, that the owner may be led to doubt the truth of his own impressions. As time goes on, however, evidence of another kind soon presents itself. The animal is noticed to move feelingly, at first on one limb only, then after a while on the other, and then on both; but it is rarely that the defect of action is uniform on the two sides. To-day it is worst on the near one, when the animal is induced to relieve it by throwing the weight of the body mainly on the off. This, however, soon results in aggravation of mischief in the latter, when the lameness becomes worse than in the former one, and so the relative severity of the disease alternates from time to time between one limb and the other.
As to the character of the lameness, it is peculiar in the increasing shortening of the stride, the gradual loss of knee action, and tendency to move the entire limb in a stiff and stilty fashion. The action altogether becomes short and wanting in liberty, and especially so on hard ground. On a soft surface when relieved of concussion it is less marked, but in advanced cases still evident even to the inexperienced eye.
The lameness in navicular disease is seriously aggravated by work, and is very pronounced on the morning following an active day. Moreover, as the disease becomes established, there is a disposition in the animal to relieve the heel of the foot from pressure and to throw it towards the toe. This has the effect of occasioning tripping and stumbling, and rendering the horse disagreeable if not actually unsafe both to ride and to drive.
Still further evidence of the existence of navicularthritis may be found in changes taking place in the form and character of the feet. Here it is observed that the heels become gradually narrow, the sole concave or arched, the quarters deep, the frog small and wasted, and the feet generally upright and blocky in appearance. Horses suffering from navicular disease are very liable to thrush, and the feet are warmer than normal. It cannot be said, however, that cold feet are inconsistent with the existence of the disease. It should be noticed that the contraction of the feet is seldom uniform, so that one will mostly be observed to be smaller than the other.
Some assistance in diagnosis may be obtained by careful observation of the posture of the patient when in the stable. In this connection it is observed that the feet are placed alternately in positions of rest, either by raising the heel from the ground, flexing the pastern, or advancing the foot straight out in front of the body - "pointing".
There is little hope of effecting a cure in this disease when once it is established, but a good deal may be done by judicious management to restrain its progress and palliate its effects. In the very earliest stages of the malady it is quite conceivable that a long rest on a soft damp surface and a repetition of blisters to the coronet might effect a cure; but it is comparatively seldom that any serious attention is given to the case at this early period of its existence, and it is not until some considerable advance has been made that a reliable diagnosis can be effected. All cases are benefited by a run at grass, provided the ground is fairly wet and soft; but in dry seasons, when these conditions are not only reversed, but flies are also troublesome, a considerable aggravation of the disease is likely to result. When rest and blistering are resorted to, the condition of the horse should be kept up; and when he returns to the stable the feet should be irrigated with cold water morning and evening, and covered with swabs in the intervals.
Setons inserted through the frogs were once very much in vogue, but the benefit resulting from them was not found to go beyond that derived from blistering the coronets. Where these measures fail, it only remains to relieve the animal from pain by dividing the plantar nerves and removing a portion to prevent their reunion.
It must be understood that the operation referred to does not check the progress of the disease. On the contrary, it rather tends to aggravate it; for all feeling having been removed from the foot, the animal ceases to favour it, and by indiscriminate use adds to existing trouble. Where horses are worked to the end after the operation of neurotomy, either the diseased bone fractures or the flexor tendon beneath it ruptures.
The first is evidenced by a return of the pain and lameness, attended with swelling around the coronet and a separation of the hoof from the sensitive structures of the foot. The second is manifested by a sudden swelling appearing in the hollow of the heel, by which it is rendered convex instead of concave. At the same time the toe shows a disposition to turn up in consequence of the tendon having broken away from the pedal-bone.