Foot cracks are fairly common and are usually attributed to poor hoof wall integrity. Poor hoof wall integrity, in turn, is usually considered to be a consequence to poor nutrition and/or unsatisfactory stabling conditions. Although this may be true in some cases, most horses nowadays are exposed to good nutrition and relatively healthy environments.
The vast majority of horses with foot cracks, therefore, have excellent hoof wall integrity. In our experience, cracks are frequently a consequence of compromised laminar integrity coupled with excessive internal forces within the hoof capsule.
Two forces typically acting on the cracked foot include:
EXCESSIVE DEEP DIGITAL FLEXOR TENDON (DDFT) TENSION
In many cases the cause of excessive DDFT tension is unknown. Several factors have been implicated, such as developmental orthopedic disease (DOD), a difference in length between thoracic limbs, eating habits (i.e. standing with one limb forward and the other back), pain higher up in the affected limb(s), the horse being “right-” versus “left-handed” (or vice-versa), genetics, and rate of growth. Despite the actual cause(s), the consequences of increased DDFT tension are fairly consistent.
Excessive DDFT tension causes a downward to rotation pull on the coffin bone (P3), resulting in distraction along the laminar interface. Tension on the laminae and/or the proximity of the third phalanx (P3) to the ground surface can cause discomfort in horses experiencing mechanical P3 rotation. Navicular inflammation is another consequence of excessive DDFT tension. Click HERE to learn more about the effects of DDFT tension.
Excessive distraction of the laminar interface may result in "delamination". This condition is often called "white line disease", and represents a separation between the insensitive laminae of the hoof wall and the sensitive laminae of the underlying tissue. The underlying tissue is directly attached to the third phalanx (often referred to as the coffin bone or pedal bone). The separation of these tissues results in the creation of a space within the laminar interface (i.e. white line) of the hoof. The presence of this space invites contamination with soil, gravel, as well as infectious agents (bacteria and fungi) which can cause white line disease. Click HERE to learn more about White Line Disease.
There is a basement membrane within the perioplic corium of the hoof (located at the coronet band/ hairline). The basement membrane is responsible for producing the horny tubules that comprise the hoof wall. The basement membrane uses the third phalanx (P3) as a guide as it produces the horny tubules, which normally grow parallel to the dorsal aspect of the third phalanx. As the third phalanx becomes repositioned more vertically, the basement membrane is having to make constant adjustments in the orientation of the tubules. Consequently, we see alterations in the direction of the horny tubules that coincide with P3 repositioning. In the case of delamination, the hoof wall is most horizontally-oriented at the ground surface (where it is the oldest) and most vertically-oriented at the hairline (where it is the newest). This results in the "dished" appearance that often accompanies horses experiencing mechanical P3 rotation. Horizontal "ridging" of the hoof wall is also sometimes observed in cases where acute significant P3 repositioning has occurred.
The only way the dorsal wall can "dish" is if it becomes separated from the underlying tissue. This separation is what we refer to as delamination.
Delaminated hoof wall is weaker than normal hoof wall, not because the wall itself is weak, but because it is not adequately "secured" to the tissue beneath. Once the wall is separated from the underlying tissue, it is predisposed to cracking... especially in the case of medial-to-lateral foot imbalance.
The hoof capsule is designed to bear weight uniformly from one side to the other. In other words, the foot should experience relatively equal compressive forces all the way around the capsule. Medial-to-lateral (i.e. inside-to-outside) imbalance results in altered weightbearing around the capsule; one side experiences excessive compression and the other side experiences excessive tension relative to what is normal. A stress riser in the hoof wall is created at the point at which opposing forces meet. The foot will have an increased propensity to crack along this stress riser, which represents the junction of the two sides of the imbalanced foot.
Fetlock varus angular deformities (toeing-in), for example, generally results in excessive compression along the lateral (outside) aspect of the foot and tension along the medial (inside) aspect. The hoof wall to the lateral side of the crack is getting pushed proximad (upward) due to the force on the ground surface and the wall to the medial side of the crack is moving distad (downward) toward the ground surface. A stress riser develops between the two forces, usually somewhere along the dorsal wall (i.e. the front of the foot). Horses with fetlock varus angular deformity, therefore, are more likely to develop hoof wall cracks along the front of the feet than horses without angular deformity.
Normal hoof wall is quite expandable and usually handles the altered forces without much consequence. Delaminated hoof wall, which is inherently weaker, is more apt to crack along the stress riser created by the imbalance.
We find that a combination of a) hoof wall delamination and b) medial-to-lateral imbalance are responsible for most dorsally-oriented hoof wall cracks.
The Formation of a CRACK
Most cracks that develop along the side of the foot are a consequence of clip "pinching" and/or underrun heels. In all cases, a stress riser is created at the junction of tubules attempting to move independently of one another.
We suggest considering some of the following strategies to address hoof wall cracking:
Treating white line disease/ intralaminar infection as needed to discourage laminar and associated wall devitalization. Although this is considered to be a secondary issue, the presence of intralaminar infection can further weaken associated hoof wall. We usually recommend the use of Vet's Complete Thrush Eliminator and Sole Toughener for this purpose.
Reducing hoof wall delamination. Horses with broken-back distal limb (or pastern) axes have increased tension on the deep digital flexor tendon (DDFT). Excessive DDFT tension and associated wall delamination is usually confirmed via dishing of the feet. DDFT tension can generally be reduced by raising the heels relative to the toe. This can be performed through serial corrective trimming and/or the application of wedged pads.
Relocation of the stress riser. Even if we can't eliminate the stress riser, we can relocate it away from the current crack site. By improving medial-to-lateral foot balance, we reduce the intensity of the stress riser as well as change it's location relative to the hoof wall. In horses with fetlock varus angular deformities, this can often be accomplished via trimming slightly more off of the medial (inside) aspect of the foot relative to the lateral (outside) aspect. As we do this, the stress riser moves further towards the inside of the hoof capsule.
Decreasing the weightbearing load absorbed by the hoof wall. We can achieve this by transferring the load to other structures of the foot, such as the frog and sole. This is easily accomplished by trimming back hoof wall in the barefoot horse or by applying a sole- and/or frog-support system (e.g. pour-in pad, frog support pad or heart-bar shoe) in the shod horse. In addition to reducing challenge to the hoof wall these devices are effective at accommodating abnormal weightbearing forces by disbursing them over a greater area.
Eliminating side clips when necessary. In some cases (especially when the shoe has been "set" underneath the toe), side clips can "pinch" the hoof wall as the foot tries to slide forward on the shoe. Excessive pressure on small areas of the hoof wall can promote cracking. The easiest way to resolve this issue is to remove the clips or relocate them further forward on the shoe (e.g. use toe quarter clips or a single toe clip). Vertical grooving is often evident along tubules getting "pinched" by clips. To learn more about vertical grooving of the wall click HERE.
Floating approximately one inch of wall on either side of the crack off of the ground or shoe surface. This will disperse abnormal weightbearing forces over tissue further away from the crack site.
Notching the proximal (top) aspect of the crack just below the coronet band to discourage its further proximal (upward) propagation into sensitive tissue. The wall must be notched down to the level of sensitive laminae for this technique to be most effective.
We will be happy to supply a detailed Farrier’s Prescription (which implements these principles) at your request.