Soft Tissue Injuries in the Equine Foot: Dancing in the Dark
- Feb 1, 1998
You know he's off. You can feel it. Your trainer says it's easy to see. Your vet says, "Hmmmm..." as she looks at the expensive radiographs hanging on the viewer. "I didn't think we'd see much," she says sympathetically. "It's probably just soft tissue. We'll just have to wait and see."
Wait and see? Your horse means the world to you, and he's lame. And you want to know when you can get back in training. What can you do?
"Soft tissue injury" sounds like a vague diagnosis, designed to frustrate the owner, but it is a legitimate set of sports injuries that can disable your horse. With new diagnostic tools, such as nuclear scintigraphy, you might be able to obtain a more accurate (and expensive) idea of the exact location of your horse's injury...but you won't necessarily be able to speed recovery.
What Are Soft Tissues?
Technically speaking, a soft tissue would be anything other than bone. In the horse's foot, there are three bones (the distal--or lower end--of P2, the navicular bone, and P3 or the coffin bone). Traditionally, we have been taught that the foot bones, and therefore the horse's weight, "hang" inside the hard hoof capsule, which is designed to protect the bones from being damaged and to support the weight. Tiny threads called "laminae" knit the bone to the hoof wall.
The deep digital flexor tendon runs down the back of the leg and attaches the bottom of P3 to the main muscle of the upper leg; the extensor tendon runs down the front of the pastern and attaches to the front of P3. The suspensory ligament (which is technically a tendon) zigs and zags, splits, and unites down the back of the leg, then wraps around the pastern to join the extensor tendon in its attachment to P3, creating an area on the bone called "the extensor process." Of course, we all see and hear about enough P3 fractures and enough occasional navicular fractures to know that the "protection" of the hoof capsule varies greatly from horse to horse, and that even the toughest hoof wall can't protect a weakened, demineralized bony structure or the mammoth "whap!" of a trailing hind hoof on a solid timber fence.
But, if you think about the hoof capsule as a coffee cup--and you know that the navicular bone, P2, and P3 combined are a cookie sitting in the coffee--you know that there is still a lot of coffee in the cup. That coffee sloshing around your cookie would represent the "soft tissue" in your horse's foot.
Horses' feet vary greatly among individuals and among breeds, and also according to how a horse has been trained, shod, and used (or overused) in its lifetime. New research is showing the horse industry that it might be a good idea to take a closer look at the soft tissue of the foot and see what role it plays in the overall soundness of the horse.
"Soft tissue" in the foot could be any of four categories: The first is the laminae and inner hoof wall; the second is the inner (sensitive) sole and frog on the bottom of the foot; the third is the digital cushion; the fourth is the coronary band. Think of the laminae as soft tissue around the bone, the sole and frog as soft tissue under the bone, the digital cushion as soft tissue behind the bone, and the coronary band as the soft tissue above the bone.
An additional category, the ungual cartilage, might be "soft" or "hard" depending on whether it has become ossified (bonelike, called sidebones) or not. The ungual cartilages are extensions of the wings of P3, but they create a framework for the digital cushion to sit in, and they are increasingly referred to in the same breath as the digital cushion.
Injuries To Soft Tissue
Hoof problems can be conditions, diseases, or injuries. Within the category of injuries, there are "emergency" situations, such as lacerations and puncture wounds, and there are "sports" injuries, related to performance and the knocks that an athlete has to endure.
The Inner Hoof Wall
The most common soft tissue injury in the foot is hemorrhaging of soft tissue. This is caused by a sharp rap of a hoof (such as on a fence rail), repeated kicking, or hemorrhage of non-specific origin. Wall bruises generally are invisible on dark feet, but they can be horrifically visible on a white foot, particularly if it has been sanded for showing. Lift the feathers on a hard-working, flat-footed Clydesdale or Shire and you will probably be able to see bruising somewhere through the wall.
Bruises are common in the hoof because the blood supply is literally "between a rock (the bones) and a hard place (the hoof wall)." In addition, the network of laminae that attaches P3 to the hoof wall contains an important part of the foot's blood supply. Break off your own fingernail "below the quick" and you will see your own laminae's blood supply; crush your fingernail with a hammer and you will have a bruise. Now imagine supporting your weight on that bruised fingernail--ouch!
More often than not, the hoof wall itself did not allow the laminae to become bruised. Probably, the original injury was trauma to the coronary band, and the hemorrhage becomes visible as it grows down with the new hoof wall. Visible bruises probably are not causing lameness, but they do indicate that the foot is susceptible to injury.
Farriers sometimes find a red stain in the "white line" on the bottom of the hoof. All or part of the white line might be red. Again, this is an indication of a past injury that is growing out. Sometimes a horse with feet that are misshapen, or a horse which is landing irregularly to compensate for a lameness in another foot, will show a stain in the part of the foot that has been under the most stress.
Other problems often lumped under soft tissue injury in the hoof wall could be a "hot" horseshoe nail (the tissue around the nail becomes inflamed), or an abscess (infection) under the wall, but these are technically conditions rather than injuries.
The Sole And Frog
The most common injuries to the sole and frog are puncture wounds to the outer tissue, but sports injuries such as "stone bruises" and "stinging feet" are very common. Another injury is incorrectly applied pressure to the sole or frog, causing necrosis (death) of tissue as pressure is put on areas where the weight of the horse impedes the normal functioning of the blood supply under the foot.
Often, a semi-circular mark surrounds the point of the frog in overworked horses, or horses working on hard surfaces. This classic "half moon" bruise indicates hemorrhaging inside the foot that is mimicking the shape of P3, perhaps caused by the pressure of the bone on a weak sole. It also is a classic sign in the soles of foundered horses. Horses can wear down and damage their soles with excessive pawing, being worked on hard ground or concrete, or by improper shoeing or trimming.
Some horses have fleshy, protruding soles that seem overly sensitive. These often are flat-footed horses, or horses with underrun heels. A good farrier can help keep the horse comfortable, after making sure that there is no other cause for the sensitivity.
The coronet is the "crown" of the hoof. It is a semi-circle of tissue that provides a blood supply and growth to the hoof wall. It also is painfully exposed and unprotected, and it often can suffer from the wear-and-tear of hard going, or from direct impact of jumping or trail riding over rocky ground.
The most common injury to coronets is bruising or scraping of the outer tissue by horses wearing studs on their shoes. Make sure that your horse is protected by bell boots if you work your horse with studs.
Lacerations to the coronary band are serious injuries that require expert first aid and quality care so that the growth and shape of the hoof will not be compromised. Injuries to the coronet often will be visible on the horse for months, as the insult leaves a scar in the growth pattern of the hoof wall.
New Foot Findings
The digital cushion and ungual cartilages are not subject to direct injury in the foot, but they can be affected by fatigue, deformation, and poor circulation in the foot. This part of the foot is suspected of having a much more significant role than previously thought in the diagnosis of heel pain and the treatment of horses with navicular-like pain.
At the 1997 American Association of Equine Practitioners Convention, Robert Bowker, VMD, PhD, of Michigan State University, reported on his comparative dissection analysis of Standardbred and Arabian horse feet. He found that in most Arabians, the cartilage was more "robust," that it extended down and created a "floor" of cartilage under the digital cushion, and that the forelimb's cartilages always were thicker than the hind limb's.
Bowker also noted that the location of veins passing through the cartilage varied in horses, with some passing to the inside sooner than others.
The composition of the digital cushion seems to be related directly to the thickness of the ungual cartilage that overlies the bars of the sole. Different horses had different fatty content and cartilage content in their digital cushions.
If you look at dissected horse feet, you will see an amazing variety in the size, shape, and location of the digital cushion within the foot. While the digital cushion might not be subject to direct trauma, it can be that if it is weak, it is deformed easily. Many horses have a section of the digital cushion underlying the navicular bone; in others, the digital cushion is "squished" out the back of the foot, with no padding under the bone at all.
A good prognostic indicator of recovery to soundness might be whether a deformation to the digital cushion, such as a change in hoof capsule shape due to long-term underrun heels, is reversible or not. The hoof capsule cannot be expected to return to a more upright posture if the inner structures have collapsed.
The digital cushion always has been believed to be part of the energy dissipation system in the foot, subject to the upward pressure of the frog. New ideas are forthcoming on how the foot bears the horse's weight, and how energy is dissipated. An inferior or weakened digital cushion might not be discernible by any diagnostic tools currently available--with the horse alive--but it is easy to see how it could affect a horse's soundness and athletic ability.
Therapy For Foot Injuries
The best treatment is protection. Get your horse the best shoeing available to provide the most complete protection and support while the foot is healing. Your farrier might suggest bar shoes or support shoes to take stress off part of the foot.
Work with your veterinarian to develop a schedule for turnout. You might want to keep the horse in a small paddock, or not turn the horse out with others until healing is well underway.
If you think your horse has weak feet, start him on a supplement designed to increase hoof quality and growth. Consider using a hoof hardener if you think the horse has "mushy" feet.
Give your horse good rub-downs, with special attention to rubbing the lower legs and coronet, particularly if the horse is stallbound. Practice stretching and suppling exercises with the horse.
If your horse has a coronet or wall injury, do not turn the horse out without boots. If you are considering Easy Boots, make sure that the sharp pull needed to remove them, and the clamps, will not hurt the horse.
- "Macroscopic and Microscopic Anatomy of the Ungual Cartilage," by Robert Bowker, VMD, PhD, Proceedings of 43rd AAEP Convention (1997).
- "Intimate Interior Anatomy: The Digital Cushion," by Alan Bailey, Bruce Chase, Simon Curtis, and Fran Jurga in Hoofcare & Lameness: The Journal of Equine Foot Science, Spring 1992.
- The Lame Horse, by James R. Rooney, DVM.
- Color Atlas of the Horse's Foot, by Christopher Pollitt.
About the Author
Fran Jurga is the publisher of Hoofcare & Lameness, The Journal of Equine Foot Science, based in Gloucester, Mass., and Hoofcare Online, an electronic newsletter accessible at www.hoofcare.com. Her work also includes promoting lameness-related research and information for practical use by farriers, veterinarians, and horse owners. Jurga authored Understanding The Equine Foot, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.
POLL: University Equine Hospitals