A thoroughpin is recognized as a fluctuating enlargement situated above and behind the hock-joint, between the tendo-Achilles or "ham-strings" and the lower part of the leg-bone (fig. 382). The swelling is more or less rounded or ovoid in form, and goes through from one side to the other, hence the term "thoroughpin". The enlargement consists of a distension of the synovial sheath of the flexor pedis tendon with fluid, and is of the same nature as those smaller swellings which appear on and about the fetlock-joints, termed "Wind-galls". The liability to this disease is especially marked in heavy draught-horses, and particularly when in early life they are forced by high living and idleness to rapid accumulation of flesh for show purposes.
As in bog-spavin, so in thoroughpin, the predisposition to it is unquestionably hereditary, so much so in some instances that the writer has known it to appear in every animal bred by certain mares, and in many of the produce of certain affected sires. Horses with straight, weak hocks, short at the point, are its most common victims.
Fig. 382. - Thoroughpin.
The inducing causes are excessive feeding, and the too early and severe working of ill-conditioned colts. It is often contracted by yearlings at play, and by older animals from severe sprains otherwise induced. Hard work on slippery ground is a common cause of thoroughpin in its more chronic and progressive form. It is frequently associated with bog-spavin, of the nature of which it very much partakes.
The enlargement which constitutes the special feature of thoroughpin may develop in a few hours to a considerable size. This is especially the case in young animals, and in older ones also where it arises out of severe sprain. Other cases are less sudden in their appearance, and gradually increase in size through a long period of time. In the former case pain and lameness is usually sudden and severe, the swelling hot and tense, like a forcibly distended bladder, and there may be more or less general filling of the joint. The enlargement is sometimes much more considerable on one side than the other, at others it is uniform on the two aspects. In action the limb is moved stiffly with an inclination outward. At rest it is maintained in a flexed condition, the weight being imposed on the sound leg. Where the disease comes on gradually lameness may for a time be altogether absent, owing to the accommodation which by growth and expansion the tendon-sheath is enabled to afford the slowly-increasing fluid. The time, however, comes sooner or later when the sheath is unduly and suddenly stretched, and pain and lameness result in consequence. In old chronic cases of this kind the tendon-sheath becomes very much thickened, and sometimes by undergoing calcareous degeneration is converted into a hard bone-like substance. It sometimes occurs that an enlargement similar to the one described presents itself on both sides of the hock as a result of bog-spavin. Where this is the case a similar fullness will mostly be observed in front over the seat of the last-named disease.
Fig. 383. - Compresses for Thoroughpin.
The line of treatment to be adopted will depend entirely on the way in which the disease presents itself. If the enlargement is sudden in its appearance, and attended with inflammation of the structures involved, the patient must be put to rest and subjected to the same treatment as that prescribed under similar circumstances for bog-spavin.
In chronic cases firing and blistering will be resorted to at once, after which pressure applied by means of a suitable compress (fig. 383) may further reduce the enlargement. In some instances it may be desirable to open the sac, and after letting out its contents inject it with tincture of iodine or some other irritant. This, however, is a procedure that can only be safely decided upon and undertaken by the experienced practitioner.