A more rapid, though less efficient method of immobilizing a toe-or a quarter-crack is by the use of the Vachette hook. A special apparatus is required ([see Fig. 219]). The burning iron ([Fig. 219, A]) is brought to a yellow heat, its end applied to the wall so that the two ears are on opposite sides and equidistant from the crack, when it is pressed firmly till the shoulder ([Fig. 219, b]) touches the surface of the wall. A Vachette hook, the distance between the points of which equals the distance between the ears of the firing iron, is seized by the special pincers (C), pressed into the slots burned to receive it, and is then driven into the horn by compressing the pincers. At the toe these hooks are frequently stripped off by the heels of the opposite shoe (in hind feet). Free application of hoof ointment, and maceration of the horn by melting snow or mud tends to loosen them so that they often drop out.
An efficient method of fastening either a toe-or a quarter-crack is by using a metal plate one-sixteenth (¹/₁₆″) of an inch thick, provided with four to eight holes for the reception of screws four- to five-sixteenths of an inch long. The plate is heated, bent to conform to the curvature of the wall and pressed against the horn till it burns a bed for itself, when it is screwed fast. It will not loosen (see [Fig. 220, b]). In every complete crack of the wall the growing down of coherent horn is favored by thinning the horn for an inch on both sides of the crack directly over the coronary band ([see Fig. 221, a]), so that any gliding movement between the sides of the crack below can not be transmitted through the thinned area to the crack in the velvety tissue of the coronary band. Cutting a “V” at the coronet acts similarly, but is less efficient.
Fig. 220.
Hoof with coronary quarter-crack, shod with a bar-shoe. The part of the quarter relieved of pressure a, is indicated by the dotted lines; b, iron plate secured by small wood screws ⁴/₁₆-⁵/₁₆ of an inch in length.
Fig. 221.
Hoof with complete quarter-crack, shod with a bar-shoe: a, area thinned almost to the pododerm; b, ¹/₁₆ inch metal plate secured by screw ⁵/₁₆ of an inch long; c, quarter relieved of pressure from bottom of crack to a perpendicular dropped from top of crack.
Quarter-crack is usually associated with contraction of the heels. It occurs on the inner quarter of base-wide (toe-wide) hoofs, and rarely in the outer quarter of base-narrow hoofs. For quarter-cracks we use a bar-shoe and determine the extent of the wall to be laid free in the following manner: We imagine the crack to be prolonged in the direction of the horn-tubes to the plantar border, and drop a perpendicular line from the upper end of the crack to the plantar border. That part of the plantar border lying between these two points is then to be lowered sufficiently to prevent pressure from the shoe until the next shoeing ([Figs. 220, a], and [221, c]).
This method should be followed even when the perpendicular line falls behind the buttress.