Bones forming joints are held together by ligaments and muscles. When dislocations occur these are sprained or ruptured to a greater or less degree in the forcible separation of the bones.
Dislocations may be divided into congenital, and acquired or accidental, in each of which displacement may be only partial or complete. In the former it is invariably brought about by some arrest of development, or injury to parts sustained during gestation.
"At the period of birth we occasionally meet with congenitally malformed limbs, depending either on the flexors or the extensors being too short. It is by far most commonly the flexors which are implicated, thus causing the animal to go on his toes, thereby distorting the limb or limbs. This distortion may be so slight, that in the process of development, if due attention be paid to the shape and position of the feet, no operation, either mechanical or surgical, is required. But should the limb be to a greater extent malformed, it will be necessary to pay even more than usual attention to the feet, and, as soon as time will admit of shoes being placed on them, let it be done. These shoes should have-a piece of iron projecting from the toe, from 2 to 3 inches in length, and slightly curved upwards (fig. 380). At the same time the heels must be kept low.
"The kinds of deformities met with in the growing horse are: first, congenital and extreme flexure of the pedal bone upon the os corona. (Plate XLII, fig. 1), depending upon the perforans tendon being too short;, secondly, permanent flexure of the pastern bones upon the large metacarpal bone (Plate XLII, fig. 2). The perforatus tendon being too short will be found productive of this distortion; sometimes it is also connected with disease of the dense membranous sheath which invests it posteriorly. Depending upon the degree or angle at which the bones are placed, must be our remedial measures. If the distortion be but slight, mechanical means only need be resorted to; if greater, a surgical operation conjoined with some mechanical contrivance will be found to be absolutely necessary.
"In the second instance, where the extensors are too short the toe will be turned up (Plate XLII, fig. 4), the animal going on the posterior part of the foot and maybe the fetlock. This depends upon the tendon of the extensor pedis being too short. The ligament also, which extends from the outer and inferior part of the knee to the upper and anterior part of the first phalanx, takes its share in producing this effect. Here also, as in the first instance, must our remedies be regulated by the amount or degree of distortion.
PLATE XLII. MALFORMATIONS OF THE LEGS.
1. Contraction of Perforans tendon. 2. Contraction of Perforatus tendon. 3. Contraction of Perforatus and Perforans tendor 4. Contraction of Extensor Pedis tendon. 5. Overshot Fetlock Joint. 6. Contraction of Subcarpal or Check Ligament.
"Thirdly, - Occasionally we meet with instances of a bowing outwards of the fetlock-joints. In such cases the external lateral ligaments are too long, and the internal ones too short. Or in others, which are rare, the reverse of this exists; when the fetlock-joints will approach each other too near, the feet turning outwards. Of course, under such circumstances, the lateral ligaments would be the reverse of the former as to their comparative length. In either instance we should not be justified in resorting to any surgical operation. In some slight cases, if proper attention be paid to the shape of the feet during the period of development, much may be done towards improving the position of the bones we are now considering.
"Another kind of deformity often exists below the parts we have been describing, namely, at the joints formed by the pedal bone and os coronae, and to a slight extent between the os coronae and os suffraginis. This, although there may be a natural tendency to it, often develops during the growth of the animal, from a neglect of those who superintend the rearing of colts, especially as to the wearing away of the hoofs; allowing the inner and front part of the foot to be elongated, thereby throwing the weight and the wear upon the outside of the foot, and thus producing that condition commonly called 'pigeon-toed'. Or, on the other hand, the outer and anterior part of the hoof may become similarly elongated, and the weight is then thrown upon the inner side, the effect of which is obvious.
"In a horse with congenital malformed limbs, the bones, in the process of growth, become fashioned to the form of the limb, so that if the animal be neglected until he has arrived at adultism, no procedure, either surgical or mechanical, will produce the effect we are desirous of obtaining. But during growth, the parts being then pliable, by altering the position of the foot, and by some contrivance placing the limb in a state favourable to cause a proper development, our object may be gained.
"In young horses it is common to meet with a knuckling forwards of the hind fetlocks (Plate XLII, fig. 5), so much so that at times the front of the joint is on a perpendicular line with the toe. This condition is seen in most cases only at times, generally when the animal is standing; or, he may walk in this manner, except at intervals. The veterinary surgeon is often called upon to give an opinion respecting these cases. He examines the joints by careful manipulation, and he minutely traces the tendons along their length, but finds no lesion of any kind present. There may be, however, more or less fulness of the bursae above the fetlock. The question has often been asked: 'What is the pathology of this affection, and what parts are implicated which cause the animal to place himself in this position?' It seems to be a commonly received opinion that putting horses to hard work too early is the principal exciting cause, and the result of my own observations leads me to think that this is correct. It is likewise asserted by some persons, that bad constitutions, independent of early and hard work, tend to induce it. This probably may be the case; still, we see the affection in horses having the best constitutions. Colts with naturally formed upright pasterns are, if worked too early, particularly predisposed to become thus affected.
"The next question is: What can be done to restore the parts to their normal condition? At this stage of the affection no operation is indicated, because we can detect no shortening of the tendons or ligaments. I would advise that the animal be thrown altogether out of work; that mild blisters be applied to the fetlock-joints, and repeated a second or a third time; after which the horse should be allowed as long rest as may be considered necessary. But even after this, the owner must not be too sanguine as to the benefit to be derived from such treatment. If due time is allowed, the horse may return to his work and show no signs of his former affection, and in some instances he will continue sound; but it is more likely, after having been worked hard for a time, we shall find that the parts will return to the same condition as at first. This may continue for an indefinite period, depending upon the severity of the work the animal is put to, before the affection merges into permanent flexure of the fetlock-joint. If we now examine the limb from a little above the fetlock posteriorly downwards to the insertion of the perforatus tendon, we shall detect a rigidity of structure, which in most cases is attended with thickening. Nothing now will restore the limb to its former position but an operation, and the propriety of its being performed we will now consider.
"The incapability to extend the foot depends, no doubt, as in all deformities of the kind, upon the length of the tendons or ligaments, between their two fixed points, being from some cause or other diminished. We shall here find that this abnormal condition of the tissue exists at such a part as either forbids or allows of relief by operation, which is the only means by which the obstacle to either flexion or extension can be remedied. The cases referred to, in most instances, depend on the perforatus and its outer dense fibrous membrane, extending from the superior part of the sesamoid bones to where the tendon becomes inserted into the supero-posterior part of the os coronse, and which is continuous with the fibrous frog, becoming more or less thickened, and sometimes semi - cartilaginous; consequently they are shortened at this particular part. Taking a view of the anatomical relationship of these structures, forming as they do a synovial sheath for the perforans, and also their pathological condition, we should not be justified in using the scalpel, especially as the only chance of effecting our object would be by operating below the fetlock-joint, and this we should not be warranted in doing." - Varnell.