This term is used to denote a fissure or rent in the crust or wall of the hoof. Usually the crack extends from above downward or from below upward, in the direction of the horn fibres. It may appear in any part of the hoof, but there are certain positions specially liable to it. In the draught-horse it is most commonly seen in front of the hind-feet, while in light horses it chiefly arises in the inner quarter of the fore-feet. The reasons for this difference may possibly be found in the fact that in the former the front of the foot is subjected to considerable strain in heavy draught, while the inner and weaker quarter of the light horse is most exposed to concussion.
The predisposing conditions to sand crack are found in the two opposite states of inordinate thickness and density, and thinness and laxity of hoof-horn. Horses with strong upright feet are specially liable to this disease, as are also animals whose feet are flat and weak, with horn of coarse texture. When these conditions are associated with undue dryness, as occurs in the heat of summer, and in horses whose feet are in a chronic febrile condition, the liability to crack is much increased. The exciting causes are concussion and strain to the feet under draught or on slippery ground. It may also be induced by treads over the coronet.
Cracks usually commence at the coronet, and extend downward. When slight they are frequently covered with overhanging hair, and for a time evade detection. If the animal be continued in work, they increase in length and depth day by day, and extend through the crust to the quick. In this condition the lameness - at first only slight - becomes severe, owing to the sensitive structures being torn and pinched between the edges of the fissure. Blood may ooze from the line of the crack, and if neglected, fungous growth (proud flesh) sprouts out, and with it an offensive discharge of pus or "matter". Lameness in this disease is always present where the rent reaches the sensitive parts. It is aggravated by fast movement, and when ascending or descending a hill. In those cases where cracks are superficial, they may continue and grow out without causing inconvenience.
For the radical treatment of sand crack the horse should be thrown out of work, the edges of the crack pared away from top to bottom, and the sensitive parts thoroughly cleansed and dressed with antiseptic solution. With a view to prevent the fissure extending downward, a groove should be burnt into the hoof across the line of the crack below. This may be done with an ordinary firing iron. All that is now required is to protect the wound from dirt by cotton-wool secured by means of tape, and to dress it repeatedly with a solution of carbolic acid. The growth of horn should be stimulated by the application of a mild blister over the coronet, and this may be repeated twice or thrice at intervals of ten days if necessary.
Where it is desired to continue the horse in work, various expedients are resorted to for keeping the edges of the crack together and preventing movement. This may be effected by drilling one or more holes through the edge of the crack on both sides, and driving fine nails or pieces of wire through them, and clinching them at both ends (fig. 387). A more effective means of doing this is the Vachette clamp (fig. 388), but to insert it a special set of instruments is required, comprising a cautery iron and forceps (fig. 389). A piece of tape tightly bound round the hoof will sometimes suffice to keep the parts together where the work is not heavy. In all cases of this disorder the crust immediately under the crack should be pared away sufficiently to prevent any bearing at this point on the shoe (fig. 390). In quarter cracks a bar shoe should be substituted for the ordinary one.
Fig. 387. - Sand Crack.
Fig. 388. - Sand Crack Clamped.
Fig. 390. - Sand Crack, showing method of paring the crust.
Fig. 389. - Tools for Clamping Sand Crack.
A, Iron ; B, Forceps for turning-in the ends of the clamp C.