420
EQUINE VETERINARY EDUCATION / AE / AUGUST 2020
a)
b)
Fig 9: a) A rasp positioned vertically dorsal to the coronet to mark the position of breakover. b) Breakover being created in the wooden shoe with a rasp at the designated point.
Using a 2 mm diameter drill bit, multiple guide holes are drilled from the solar surface proximally through the lateral and medial side of the hoof wall on the abaxial side of the sole wall junction (white line) beginning at the widest part of the foot and continuing towards the heel. Screws placed palmar to the widest part of the foot will maintain the two planes created by the trim and help unload the toe. These holes are predrilled in the solar surface of the foot to ensure accurate screw placement in the wall. The author prefers drywall screws, although they tend to be brittle, they are thinner in diameter and have a coarser thread. A 3.9 9 38 mm course thread tapered drywall screw or 4.2 9 38 mm tapered wood screw is placed in each hole on the outer hoof wall directed distally towards the ground surface and screwed in until just visible on the ground surface. Any exfoliating horn is removed from the frog and the frog sulci and the adjacent area is cleaned with a wire brush. Copper sulfate crystals (powder) are often applied to the cleaned area for its antiseptic properties before applying the impression material. To recruit the sole, bars, frog and frog sulci for weight bearing, deformable impression material (IM)4 is applied to the palmar section of the foot being careful that the impression material just covers the intended
structures and does not extend beyond the bearing border of the foot surface to create pressure. With the foot off the ground, the shoe is now set in place, two or three screws are screwed into the shoe and the foot is placed on the ground to bear weight. This allows the impression material to conform between the palmar section of the foot and the shoe, thus creating two flat surfaces. After the remaining screws are inserted, additional screws can be placed in the wooden shoe against the outer surface of the hoof wall at the heels to act as struts to provide stability and act as an anchor for the casting tape. With the foot on the ground and using a rasp as a straight edge, a vertical line is viewed from the dorsal aspect of the coronary band to the ground, marked on the side of the wooden shoe and then a line is drawn across the ground surface of the shoe (Fig 9a). Setting the breakover to this point in the shoe is easily accomplished by extending the bevel to this line using a hoof rasp with the foot being held between the farrier’s knees (so called farrier position) (Fig 9b). The line across the shoe is where the breakover point of the shoe should be positioned which will consistently be just dorsal to the solar margin of the distal phalanx when viewed on a radiograph (Fig 10).
Fig 10: Wooden shoe applied to the foot with corresponding lateral radiograph. Black line is the COR that correlates with the widest part of the foot and red line is the position of breakover. Note the screw placed against the out side of the hoof wall at the heel termed a strut to anchor the casting tape.
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