The accidental overstretching and rupture of the fibres composing the back sinews is of common occurrence, and is usually designated breakdown.
The structure most frequently implicated in this accident is a short ligament (fig. 3G6), which proceeds from the upper and back part of the canon-bone, and joins the flexor pedis or main tendon of the leg about three inches below the knee. The seat of injury is invariably at the point of union of the two parts.
Horses light of bone, and whose sinews lack size and strength, are more predisposed to this disease than others of stouter build, as are also those with big, heavy frames and small limbs. The exciting causes are heavy draught, slipping and sliding on smooth pavement, and severe efforts in galloping and jumping, particularly under circumstances of fatigue, as at the end of a quickly-run race or steeplechase, when the muscles are tired and the weight of the body is forcibly thrown on the passive structures.
As a result of this accident an enlargement mostly makes its appearance in the course of the tendons, about three inches below the knee. It is sometimes small and hardly perceptible, while at others it may reach the size of a walnut, or even larger, and extend a considerable distance downwards. When pressed, the horse winces and lifts the leg sharply from the ground. The part, moreover, is hot and inflamed. In standing, the heel of the foot is slightly raised, or the leg may rest on the toe. The amount of lameness will depend upon the severity of the strain. In progression the horse moves short, and imposes the weight mainly on the front part of the foot to relieve the injured part from traction.
In this connection it is important that the injured structure should, as far as possible, be placed in a state of rest. For this purpose the ordinary shoe will require to be removed at once and replaced by one having a thick or wedge heel. Hot fomentations should then be applied for an hour or two, and followed by the continuous use of hot flannel bandages to the leg. A dose of physic and absolute quiet must also be enjoined.
When the inflammation and pain have been in a large measure reduced, hot fomentations may be changed for cold water irrigation and cold linen bandages. It may be that in slight cases these, with a short period of rest, will suffice; but where the sprain is severe and the enlargement considerable, a blister will afterwards require to be applied to the leg between the knee and the fetlock, and repeated once or oftener according to the progress of the case. It often happens that the shortest way to a cure is to fire the part as soon as the inflammation has dispersed, and blister over the fired surface.
In bad sprains there is a tendency to contraction and shortening of the injured tendon, resulting in "knuckling over" of the fetlock-joint. To guard against this the wedge-heeled shoe should not be worn too long, but gradually reduced in thickness until the heel is brought to a level bearing with the parts in front. If in spite of this the tendon should contract unduly, a shoe having a short lever projecting from the toe (fig. 380), and a low, thin heel, must be adjusted to the foot and worn for a few weeks. If this fail, then the shortened tendon may require to be cut through in order that the parts displaced may resume their normal position.
Fig. 380. - Shoe to prevent Contraction of the Back Sinews of the Leg after treatment for Sprain.