The hoof wall of a horse’s foot is divided into three parts: the toe, the quarters, and the heel. A common ailment of the hoof that affects all three areas is a condition known as the “underrun heel”. This refers to a structural remodeling of the outer wall due to numerous causes and often disastrous consequences. Quite simply, an underrun heel occurs when the fibres of the hoof wall at the heel are not parallel to the wall fibres at the toe. For instance, if the angle of the toe is forty five degrees and the angle of the fibre at the heel is anything less than, for example, forty or thirty degrees, or even twenty degrees, then the heel is considered to be underrun.
Visualize a healthy hoof with a shape similar to a bell and a gradual widening taper from the coronary band to the ground surface. An underrun hoof has an almost inverted shape where the wall tends to curve in at the quarters toward the heel, producing a shape similar to an upsidedown funnel. This causes the volume inside the hoof to be decreased. As a result, the internal structures of the hoof can be pinched or even displaced, and this can lead to soft tissue damage as well as lameness. It is quite common to see horses with underrun heels diagnosed with posterior digital heel pain, navicular disease, side bone, pedal osteitis , and negative palmar angles. As well as setting the horse up to be a candidate for bowed tendons, suspensory ailments and all manner of connective tissue injuries, the underrun heel can affect gait, causing the horse to stumble, trip or overreach. In addition, toe cracks and hyper-expansion of the quarters (flare) can be a direct result of heels that have become underrun.
Underrun heels cause the vertical load to be behind the hoof and thus deprive the horse of its ability to absorb shock via the frog and digital cushion. Horses whose posture puts their feet in front of them are heavy on the forehand. Jumping horses, for example, will land “behind their feet” and it can seem like these horses are landing in a heap or jumping into a ditch as they struggle to rebalance.
Forging occurs due to the fact that the hoof is still on the ground too late in the stride sequence. The posture of hoof-out-front, resulting from the underrun heel, delays the hoof from leaving the ground in a correctly timed sequence as the scapular hinge passes over it. The scapular hinge is roughly in the middle of the scapula. Many times when the toe of the hoof is in a perpendicular line with the point of the shoulder rather than in a vertical alignment with the middle of the scapula, the front foot can then be struck by the hind foot as the body of the horse comes forward. These horses will be prone to stumbling and tripping as well as forging.
What constitutes the underrun heel? A normal healthy hoof has a toe to heel ratio of three to one and can, at its extreme, have a ratio as much as two to one. Because the heel is a much stronger geometric shape than the rest of the foot, it does not generally wear or break down as quickly as the rest of the hoof. Since hoof fibre is on an angle, it will grow forward on an angle. So, in essence, if a horse has a geometric disproportion wherein the heel has become long enough to grow in front of the cannon bone, then the fibres of the hoof can begin to bend downward on an even flatter angle than they were originally, and subsequent loading can cause the fibres to break out at the quarters, diminishing support and causing the hoof to rotate back and downward. This increases the load on the heels and subsequently causes the heel fibres to become crushed even further.
When underrun at the heels, the hoof inevitably becomes positioned in front of the limb. When viewed from the side it is likely that the fetlock joint is behind the bulbs of the heels. (pics 2 and 3) When viewed from underneath, the hoof can look more like an oyster shape than a disc shape. The extra width at the quarters results from an overall decrease in the distance from toe to heel .The sole surface may appear to be too small for the size of the hoof. The bulbs themselves may appear to be very close to the ground and seem to protrude rather prominently out the back of the hoof. As a result of this posture, the frog is not able to be loaded properly and it’s function as a primary weight bearing structure is lost. The horse then is hanging off the back of the hoof, much like a person standing on a step or the rung of a ladder with no support under the bones of the leg. This incurs all manner of strain as the responsibility of holding the animal up is born by the connective tissue. The resulting fatigue has been responsible for many common lameness diagnoses.
Solutions to the underrun heel
There are trimming and shoeing strategies that have been developed to correct the underrun hoof; many with success and some without. Wide web shoes, in addition to longer, wider fitted shoes, are often prescribed, yet they are rarely sufficient to address the root cause of the problem. As a result, some very expensive shoe jobs may enable a horse to ‘get by’; yet do nothing to rectify the underlying circumstances. If the horse is shod, then it is almost invariably in the horse’s best interests to have the shoes pulled to let the hoof relax. In addition, the feet must be trimmed into a shape that that allows for a uniform taper that strictly observes the toe to heel ratio of three to one without allowing the quarters to be excessively long. It is not uncommon that the quarters are left too long in the mistaken belief that in the absence of heel the responsibility of support must be born by the quarters. This only places the fulcrum further ahead on the hoof and does not unload the heels. It is important to note that pulling shoes on such horses is not something that need be a forever thing. On the contrary, the changes that need to occur can take place in as little as a few days or perhaps a week or two. It is important that the feet be filed again as they relax.
Bare feet can be just as underrun as shod feet and that can come as a result of the hoof becoming too long or being left too long between trims. Sometimes it is just a result of the toe wearing away faster than the heel, thus skewing the ratios. It is essential then that a horse’s hooves are kept filed into an appropriate shape and that the required support lines are met.
This can be a difficult proposition for many horse owners, because people have become accustomed to the practice of having farrier visits every five, six, or seven weeks. In order to rehabilitate severely malformed feet, it is sometimes necessary to have a more closely scheduled set of farrier visits even for a seemingly small amount of trimming.
The potential to remodel hooves by giving them the appropriate attention at two or three week intervals by far surpasses any changes that can be made with shoes alone. The reason for this is that the hoof will react to changes in posture as well as accumulated horn length and it just isn’t always practical to try and change this through shoeing alone. The biggest problem is that the hoof will become excessively perforated by the nails when there is too short an interval between shoeings.
Of all the strategies that are required to resolve this affliction, it is a commitment to diligent maintenance that will correct the underrun heel.