The horse’s heel is an exciting structure. Like the rest of the horse’s foot, it’s designed to absorb shock in a way that protects all the tissues above it all the way up to the horse’s back and neck. What makes the heel unique compared to the rest of the foot’s anatomy, though, is that it’s located right where much of the initial landing impact occurs. Lined up with the pastern, the heel often gets the first line of attack when striking a surface, before the foot then vertically rotates and breaks over the toe in push-off.
Just above the heel—and in many ways as part of the heel structure itself—is a robust, flexible kind of “padding” tissue, appropriately called the digital cushion. A healthy digital cushion does a good job of covering and protecting the deep digital flexor tendon (DDFT) coming down from the leg into the foot, and it has an important role in absorbing all that impact shock.
Outer heel health and digital cushion health go hand-in-hand (or should I say foot-in-foot?). While science is still working out the details of cause and effect, it looks like a bad cushion could create bad heels, and bad heels could create a bad cushion.
Keeping digital cushions and outer heels in good shape requires good genetics, to start with, but also a good working and living environment. (I’ll go into more detail about environment in a later blog.)
A real advantage of keeping a horse barefoot is preserving his heels from all the friction and wear from moving against the shoe, according to farrier and veterinarian Stephen O’Grady of Northern Virginia Equine in the U.S. While it might appear that the hoof is immobilized by the shoe, that’s actually only true for the parts where it’s nailed in. At the back of the shoe, there are no nails, so the heel’s hoof capsule gets a lot of micro-movement that ends up wearing down the heels at an uneven rate compared to the toe, which is nailed down. At the same time, the shoe creates enough grip on the foot to keep the heel from expanding the way it normally would to absorb shock. In other words, steel shoes keep the heel from doing its normal shock absorption job. This can cause damage to the outer heel and digital cushion as well as all the tissues above the heel which aren’t getting protection from shock.
Removable hoof boots differ from being barefoot in that they create an interface between the hoof and the ground. Still, when correctly fitted, the boots allow horse’s feet to move and expand within the boot as they naturally would. And since they’re used so intermittently—a few hours a day or even week—“hoof boot horses” spend most of their time enjoying the heel (and other) benefits of being barefoot.
Good heel and digital cushion management also require smart farriery, with keen observation and some essential geometry, according to Dr. O’Grady.
Knowing your horse’s feet angles is critical, he says. A lot of publications talk about aiming for about a 50% angle between the hoof and the ground at the toe, and about the same for hoof at the heel. But a better approach, he says, is aligning both those angles with the natural angle of the horse’s pastern, creating one unbroken line from the hoof up through to the pastern joint. This essentially gets all three “P” bones in a straight line.
Achieving that angle requires adjusting heel height so as to get that foot on a matched plane with the pastern. (An X-ray could make this lining-up process more accurate.) If heels are too low, this creates what’s called a “broken-back pastern axis,” meaning that nice line down from the pastern gets broken with an angle pointing down towards the ground. Most foot-knowledgeable people seem to know that this kind of “underrun” heel situation can be disastrous. Specifically, it upsets the natural shock absorption system, modifies the integrity of the digital cushion, places undue strain on the DDFT, and puts the navicular bone at risk of injury and disease.
Nobody wants that! So it might be no wonder that some people, in an effort to be over-cautious, leave the heel too long. However, a too-long heel also disrupts that nice pastern-to-foot line. By tipping the foot slightly forward, the too-long heel creates a “broken-forward pastern axis,” breaking the line with an angle pointing up away from the ground. This creates undue strain on the sensory ligament and can put the coffin joint at risk of injury and disease, Dr. O’Grady says. What’s more, heels grow forward, and that means a reduced amount of foot surface that actually touches the ground—concentrating forces in all the wrong way. Nobody wants that, either!
Better then to aim for a heel angle that, in association with the toe angle, adjusts the P3 (coffin) bone in a straight line with P1 (long pastern) and P2 (short pastern) bones. Farriers should also rasp horizontally across the heels with an aim of getting the heels on the same plane as the frog, he says.
Special note: Thrush infections can cause heels to contract, making them tighten up and appear longer. Good heel health and trimming thus requires that thrush is actively and even aggressively treated for the horse’s comfort and for trimming accurate angles.