The sole surface should be concave, calloused, and about as wide as it is long, measuring 12 to 15 millimeters thick under the distal rim of the coffin bone, she said. A sole with this thickness can effectively protect the coffin bone from trauma. A simple way to predict sole thickness is by placing a ruler (calibrated in millimeters) in the collateral sulci at the apex of the frog, measuring the distance between the deepest part of the sulci and the bottom of the hoof, Taylor explained.
She explained that thin-soled horses have very little or no depth of the collateral sulci at the frog apex. Horses with zero collateral groove depth at the apex of the frog generally have a sole depth of less than 7 millimeters, she said, which can predispose them to sole brusing, subsolar infection, and coffin bone remodeling or rim fractures.