The patella is a small floating bone situated at the point of the stifle and corresponds to the knee-cap of man (fig. 378). It enjoys a large range of up - and - down motion in playing over two prominent ridges on the lower extremity of the femur or thigh-bone. The outer ridge is naturally much smaller than the inner one, and this will require to be borne in mind when considering the particular direction in which the displacement usually occurs. Moreover, a large muscle, the "biceps fe-moris", is inserted in the outer part of the patella, to which there is no corresponding muscle on the inner side in antagonism with it.
Dislocation of this bone in the horse, as in man, is comparatively common, and is most frequently seen in young animals from a few weeks to two years old, although older subjects are by no means exempt from it.
a, Pelvis; b, Femur; c, Patella; D, Fibula; E, Tibia.
The early occurrence of the displacement of this bone in foals, and its persistence, in the absence of any obvious cause, led the late Professor Varnell to attribute such cases to a congenital smallness or want of development of the outer ridge referred to above, which permits the bone in certain movements to be displaced outwardly by the pull of the biceps fern oris.
Post-mortem examinations on animals so affected have convinced the writer of the accuracy of this conclusion, and further that, associated with this deficiency of development of the articulation, there is also a corresponding attenuation and weakness of its connecting ligaments, which favours the displacement of the bone.
General weakness is no doubt a cause of luxation of the patella, as evidenced by its frequent occurrence in poor, weakly, overgrown foals, and after such debilitating diseases as influenza and strangles.
The displacement usually occurs in an upward and outward direction, and although it is impossible to say precisely and fully how this comes to pass, there can be no doubt that it is in great measure determined by the smallness of the outer ridge of the trochlea of the thigh-bone, and the outward pull of the biceps femoris muscle.
These will vary somewhat with the nature of the case. When the mishap is the effect of weakness the displacement is usually sudden and transient, coming and going without any apparent reason, and being repeated at varying intervals again and again, each time passing away without assistance. The affected limb is noticed to be suddenly jerked backward and held for a moment in an extended position, and then to resume its place again. This may be confined to one leg, but it is commonly the case that both are more or less affected. Here the displacement is only partial, and although it may be repeated from time to time for some days or weeks, it ultimately passes away without assistance as the animal acquires strength.
In more complete displacement the leg is fully extended backward (fig. 379) and cannot be advanced, and the animal stands in this position helpless and immovable. The displaced bone may be felt on the upper and outer side of the joint.
Replacement of the bone may sometimes be caused to take place by a mere cut of the whip. For the most part, however, it is difficult to effect in cases of forcible displacement. Before attempting to reduce the dislocation the extended limb must be brought forward by means of a rope attached to the pastern, and then passed through a collar or a ring in the manger, or over a beam (Plate XLIII, fig. 1). This having been done, the hand is placed beneath the bone, which is pressed in an upward and forward direction. If the leg at the same time continues to be forcibly advanced, the patella will soon yield to the pressure, and with a sudden click fall into its place.
Drawing the Leg forward.
The Leg kept in Position.
PLATE XLIII. DISLOCATION OF THE PATELLA.
To keep it there is the next requirement, and for this purpose the leg must be made secure to a collar round the neck (Plate XLIII, fig. 2), and the animal tied up to the rack, or put in slings, and kept perfectly quiet.
Fig. 379. - Dislocation of the Patella.
A sharp blister must then be applied over the region of the stifle, and, in addition, it is recommended that a shoe, thicker at the toe than at the heels, and having a projecting piece of iron attached to the former, be put on. So soon as the effects of the blister have passed away, it may be desirable to repeat it before the animal is relieved of restraint.
Where the mishap results from general weakness, or slow growth and development of the parts concerned, a liberal supply of good food is of the first importance, and this should be supplemented by sulphate of iron and nux vomica, given alternately night and morning in the corn.
The patient should be confined to a small shed or loose-box, and blistered over the stifle from time to time, until the displacement ceases to occur.