© James Rooney, D.V.M.

Toe and quarter cracks starting at the coronary band – the more common and clinically important form of cracks - are a direct result of disruption/damage of the coronary band . With such damage, of course, the process of tubular horn formation is interrupted , and a defect appears in the hoof wall as the adjacent, normal coronary band continues to produce new horn tubules. Usually such interruptions of growth are temporary with the crack eventually disappearing as the coronary band is repaired and resumes growing with the hoof growing out.

Direct trauma to the coronary band is a clear example. Such trauma occurs for a variety of reasons: interference by another foot, wire cuts, treading by the same or another horse, etc.

The purpose here is to consider the role of the horseshoe in the pathogenesis of quarter and toe cracks not attributable to such direct trauma.

As the bare foot is loaded the quarter moves outward in a smooth curve as indicated in the drawing. When the horse is shod, the quarter is constrained by the nails and will bend at or near the last nail. This bending can cause tearing and bruising of the laminae along the bend line and at the coronary band. “Crack,” then, is a misnomer. There is, in fact, failure of production of new horn tubules in an area of the coronary band (the area of the asterisk in the drawing), and this appears as a defect in the wall.

A similar mechanism holds for toe cracks. Because of the restraint imposed by the nails and shoe the toe bulges out and forward as shown by the asterisk in the drawing. In this case , the bending is in the opposite sense but still could cause damage as discussed.

An interesting feature of many toe cracks is that, if not too large, they tend to close when the foot is loaded and open when unloaded. This clinical observation has been confirmed by Thomason et al 1992. They showed with strain gauges that there is biaxial compression (two compressive loadings at right angles to each other) of the toe portion of the hoof wall. The major compression is vertical while that at right angles is caused by the circumferential bending of the hoof wall. It is this circumferential bending which tends to close the gap when the foot is loaded. The old literature also indicates that toe cracks were more common in the hind feet of draft horses. One may speculate that the damage could be done when the foot if lifting from the surface with the compressive force concentrated on a decreasing surface area. Even though the total force would be decreasing as the foot lifted off the surface, the force per unit area (the stress) at the toe could be increasing.

Given that the above hypothesis is correct or nearly so, one is faced with an age-old dilemma: why don’t all shod horses crack their hoof walls? It is not possible to answer this question here in a general way. To find an answer one must consider a variety of factors for the individual horse. A few of those factors: genetic (or otherwise) quality of hoof horn, frequency of trimming/shoeing, surfaces upon which horse habitually moves, body weight (more weight=more load=more movement of hoof wall), misplacement of nails, state of hydration of the hoof wall, contracted hoof, etc. The important point is that one can provide a reasonable general pathogenesis applicable in all cases but cannot provide general statements about other, contributing factors. Those can only come with evaluation of the individual horse.

Thomason JJ, Biewener, AA, and Bertram JEA (1992) Surface Strain on the Equine Hoof Wall in vivo: etc. Journal of Experimental Biology 166: 145-168.

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