The firm and rigid manner in which the bones of the pedal articulation are held together renders dislocation of this joint an exceedingly rare occurrence, and then it is only liable to happen under the operation of great force. In the literature to our hand we have only been successful in discovering one reported instance, and, strange to say, in this, a well-marked case, the cause was altogether obscure. We quote the case at the end of this section.
Fig. 165. - Diagram Showing The Course Taken By The Needle When Setoning The Frog.
This is shown by the dotted curved line a, b. 1, The navicular bone; 2, the plantar cushion; 3, the os pedis; 4, the perforans tendon.
A partial dislocation of this articulation is the condition met with in 'Buttress Foot.' In this case the fracture of the pyramidal process, and the consequent lengthening of the tendon of the extensor pedis, allows the os coronae to occupy upon the articulatory surface of the os pedis a more backward position than normally it should.
It is quite probable, too, that slight lesions of the other restraining ligaments and tendons of the articulation may bring about a similar though less marked condition. We may be quite sure of this - that whenever such lesions (as, for example, sprain and partial rupture of the lateral ligaments) do occur, and the normal position of the opposing bones is changed, if only slightly, that great pain and excessive lameness must be the result, and this with but little to show in the foot. Many of our cases of obscure foot lameness might, if capable of demonstration, turn out to be cases of sprain and partial dislocation of the pedal articulation.
Recorded Case. - 'The animal, a trooper of the 8th Hussars, was found on the morning of April 17 unable to bear any weight on the limb (the near hind). Cause not known - the heel-rope I thought at first; but on investigation I found the heel-rope had been on the other leg.
Diagnosis. - Dislocation of the left os coronae from the articulating surface of the os pedis in a backward direction.
Every devisable means were unsuccessful in reducing the limb to its natural position. The horse was thrown, and a strong rope, with four men pulling at it, was fastened round the hoof, whilst I put my knee to the back of the pastern, using all possible force, with one hand to the foot and the other to the fetlock, but all to no purpose. Next day other means were tried. First by throwing the horse and placing him on his belly, with the fore-legs stretched out forwards, and the hind-legs backwards. This I did so as to get the injured limb placed as nearly flat on the ground as possible, with its anterior aspect downwards. Then a very heavy man, with his boots off, was made to jump on the back of the pastern, where the prominence showed most; and afterwards, when these means failed, a strong piece of wood, well covered with leather, was placed (where the hollow of the heel ought to have been) on the most prominent part, and hit several times with a heavy hammer; but all efforts were futile.
Prognosis. - Unfavourable. During the latter operations I had a very strong pressure applied to the hoof, and the horse firmly fastened in every way, and it appeared as though no amount of force would ever reduce the dislocation.
Tautological. - The case was destroyed on April 30, being of no further use to the service.
'Post-mortem. - The os coronae was found to have slipped out of the articulating cavity of the os pedis, backwards and past the lateral ligaments. These last-named structures prevented the bone being forced forward into its proper position, being firmly locked over the lateral prominences. The capsular ligament was considerably lacerated and inflamed, causing slight effusion and swelling about the region of the coronet.'[A]
[Footnote A: T. Flintoff, A.V.D., Veterinary Journal, vol. xix., p. 74.]
Treatment. - After the forcible means of reduction related by Mr. Flintoff, we may add that when they are successful, they should be followed by suitable bandaging of the parts, and rest. The first is effected by applying plaster of Paris and linen, and the second by having the animal put in slings.