Historically, veterinarians and farriers have spent far more time evaluating and adjusting front hoof balance than rear hoof balance. This is because front imbalances typically manifest as obvious foot pain. Rear limb imbalances are typically less obvious but just as critical to monitor, however, because they can cause pain higher up in the limb and body.
At the 2017 American Association of Equine Practitioners Convention, held Nov. 17-21 in San Antonio, Texas, Tracy Turner, DVM, MS, Dipl. ACVS, ACVSMR, described hind-limb biomechanics and how to address common rear hoof imbalances. He owns Turner Equine Sports Medicine and Surgery, in Stillwater, Minnesota.
“I’ve spent the majority of my career talking about front feet,” he said. “I started looking and realized that rear hoof imbalance has been largely ignored. But rear hoof imbalance might have more effect on the upper body due to its direct attachment to the axial skeleton (the vertebrae, skull, ribs, and sternum).”
He began with a brief overview of how the horse moves. The hind leg attaches directly to the pelvis, which attaches to the sacroiliac joint, which attaches to the vertebrae. “You can’t move those parts until the foot leaves the ground,” said Turner. “There’s a lot more effect on all those joints.” In comparison, the horse can move and adjust his front limbs much more easily and without affecting his upper body.
When the horse lifts his hind foot off the ground, he must contract his stifle, hamstring, hip, and gluteal muscles to propel himself forward. Any imbalance in the hooves is going to alter these biomechanics and potentially lead to lameness, said Turner.
He then described three of the most commonly documented rear hoof imbalances:
Broken Hoof-Pastern Axis
In any foot the front (dorsal) hoof wall should align with the pastern. In the rear feet this axis can change for many reasons, said Turner, but often because of age-related suspensory ligament weakness in older horses with hyperextended fetlock joints. It can also occur if the horse is placing his hind feet farther under his body to compensate for painful or underrun (also known as long toe, low heel) front feet, thus overloading his rear heels.
Most commonly, Turner said, farriers and veterinarians encounter a broken-back hoof-pastern axis, in which the hoof wall angle is lower than that of the pastern. In the rear feet, “this imbalance causes the hoof to stay on the ground longer than normal,” he said. “The strain on the deep flexor tendon will be markedly increased over normal before heel lift. This can lead to many lameness issues, the most serious of which is tenosynovitis (inflammation of the membrane surrounding the tendon sheath) of the distal (lower) tendon sheath.”
This, in turn, can lead to hock inflammation and an increased need for hock injections. It might also cause stifle and/or gluteal pain.
Correcting a negative solar angle (when the back of the coffin bone is lower than the front of the bone, rather than both ends being level) by simply raising the heels is easier said than done, Turner explained. The hind foot naturally has a more exaggerated heel-first landing than the front, and raising the heels only exaggerates it further, increasing heel pressure and potentially damaging the hoof capsule. Turner said he’s had better success applying rockered shoes to improve rollover (at the toe) and reduce tendon strain.
Abnormal heel conformation should be easy to recognize, said Turner. The horse will have a broken-back hoof-pastern axis and a “bull-nosed” dorsal hoof wall. The horse’s frog is usually large and pushed down below the hoof wall, bearing the horse’s full weight when placed on the ground.
Fortunately, said Turner, damage to the hind heels is usually easier to correct than damage to the front, “possibly due to the difference of the load encountered on the hind limbs vs. the forelimbs.”
He said he takes one of three approaches to correcting underrun hind heels:
- If possible, remove the horse’s shoes, leaving them off for four to eight weeks, trim the heel back until you reach solid hoof wall, round the toe, and allow the horse to self-correct. “Over the next few weeks, the pressure on the frog will compress and displace the frog until it assumes the same plane as the heels on either side,” said Turner. “This approach can also be used with horses that are resting due to lameness issues.”
- For horses that must stay in work and wear shoes, Turner trims the foot similar to the previous approach, then applies a frog wedge “to compress the prolapsed frog to have a flat, even plane.” Then he places that foot in a water-saturated poultice for 48 hours to soften the foot and correct the frog. The horse can then be reshod, with a wedge pad as needed for additional heel elevation.
- Lastly, Turner again trims the heel back to the hoof wall but trims the front half of the hoof in a different plane than the rear to maximize rollover. “The effect has been to reduce strain on tendon sheaths and the deep flexor tendon, but it has not been successful in restoring the hoof capsule,” he said.
When the horse moves, his hind limbs naturally have a slight rotation to them. “This is necessary to ensure that the feet are wider than the forelegs when galloping,” Turner said. “In many animals this causes … the hoof capsule to be high on the inside (medial).”
This uneven wear can lead to suspensory injuries, fetlock lameness, and hock and tibia (the long bone between the hock and stifle) pain.
The goal when addressing this imbalance, he said, is to get the horse to bear weight evenly and equally on his hind feet.
Rear hoof imbalances are not well-understood. “Instead of causing foot pain directly, rear feet seem to cause many more problems higher up the leg—to the hocks, stifles, glutes, sacroiliac joint,” said Turner. Therefore, addressing these asymmetries is just as important for treating or preventing lameness as it is in the front hooves. In closing, he emphasized the importance of radiographs when assessing rear imbalances, keeping in mind the natural differences in rear hoof shape and alignment.