horse's splint bone

His splint bone may be small and apparently without function, but without proper support it can lead to painful and frustrating injuries in your horse. This article focuses on the splint bone’s true function, and how to keep it healthy.

If your horse has ever “popped a splint”, you’re familiar with this little bone in your horse’s lower leg. We are often told the splint bone doesn’t serve a purpose, but for a structure without a purpose it certainly gets injured easily and often. By taking a look at this bone’s true function, we can help support it and prevent frustrating injuries to our horses.

Anatomy of the splint bone

The embryologic changes that modify a five-toed foot or hand into an equine limb are impressive. The equine “knee”, also called the carpus, is equivalent to the human wrist. The horse’s hock, or tarsus, is the ankle in humans. It’s below the level of the carpus/tarsus that the species-specific differences between humans and equidae become most apparent.

The thumb and pinky finger that are part of the human hand completely disappear in the equine limb. The first and third fingers recede into vestigial bones known as the splint bones – the only remaining finger being that from the center of the hand. The middle digit hypertrophies and develops into the large, weight-bearing cannon bone of the equine limb, along with the long and short pasterns and coffin bone as you move distally from the carpus.

The function of the splint bones

Because the splint bones are vestigial and not weight-bearing, it’s logical to question whether they have any function. A splint bone can certainly fracture, causing great pain. If the bone has no biological function, one wonders if it really needs to remain on the limb, thereby avoiding that potential pain. While it’s true the splint bones are vestigial, they do have a kinesiological function.

While it’s true the splint bones are vestigial, they do have a kinesiological function.

The splint bones essentially act as placeholders, maintaining the position of the carpal/tarsal bones, those of the knee/hock. It’s the interplay between the multiple rows of carpal/tarsal bones that allows for the wide freedom of movement in the carpal/tarsal joints. While the splint bones do not have a direct physiological function, they remain critical to joint mechanics.

The splint bones also have a secondary function – that of anchoring fascia and other tendon/ligamentous attachments to the limb. For this reason, a horse may be in severe pain when one of these attachments is “merely” strained. Tendons attach muscle to bone, ligaments connect bone to bone, and both the tendons and ligaments contain nerve receptors which detect processes that will present as “pain” if damaged or inflamed; therefore, a strain can be exquisitely painful.

Common injuries to the splint bone

The most common reason for splint bone pain is fracture; strained ligaments and tendons are the second most common cause. Because horse limbs are incredibly modified adaptations of the five-toed foot, balance is critical to maintaining strength and function in the equine limb. Therefore, something as “simple” as an improper hoof angle is enough to result in a fractured splint bone, resulting in great pain to the horse. The best way to prevent a fractured splint bone is to maintain excellent angulation in the hoof and limb.

The best way to prevent a fractured splint bone is to maintain excellent angulation in the hoof and limb.

Angular limb deformity and its effect on the splint bones

Sometimes additional factors can result in improper angulation of the hoof and limb. One of these is angular limb deformity (ALD), which presents as a limb, or limbs, which deviates to either the inside (pigeon-toed) or outside (splay-footed). There is some argument as to whether angular limb deformity can actually be prevented or if it is solely a birth defect; regardless, once it appears, the negative effects of ALD can be minimized through good nutrition, moderate exercise and excellent hoof care. Furthermore, not all farriers have the same level of expertise when it comes to limb deformities; therefore it’s good to ask many questions. The sooner a potential ALD is noticed to be developing, the better the foal will respond – and perhaps even grow out of it.

Once an angular limb deformity has developed, chiropractic care, acupuncture, balanced nutrition avoiding “hot” rations, and excellent hoof care are a must to prevent damage or fracture to the splint bones. “Hot” rations not only promote rapid bone growth, but are also often inflammatory, which will worsen any physical condition, including ALD. Ration balancers made from whole foods, like flax seed and mushrooms, are much less likely to cause rapid growth and/or inflammation. While less common, it is possible to feed home-prepared diets to horses. Horse owners typically feed carrots and apples, but how about cucumbers, celery, mushrooms and pears? Early introduction to novel foods is important to the development of a wide dietary palate in the horse.

 Chiropractic care for angular limb deformity

Overly “hot” rations and creep feed can lead to unequal limb growth and development – angular limb deformity (ALD) is not only a birth defect, it can also arise during development. Slow growth is ideal. Feeding quality grass/hay and a whole food ration balancer is the best way to achieve slow limb growth in foals, along with keeping a growing foal in trim body condition. However, if a foal begins to develop a bow in his limbs, there are also surgical methods that can help. Surprisingly, early intervention with chiropractic care may head off deformities and potentially avoid costly and painful surgical procedures. Animal chiropractors familiar with inter-osseous faults can provide a great service to these animals by keeping the body in balance and possibly avoid worsening of bone growth imbalances and ALD.

An inter-osseous fault is a condition known best to practitioners using applied kinesiology techniques. These practitioners understand that tissues that attach to bones can make microscopic changes in the microcrystalline structure of the bone; these microscopic changes can develop into deformities over time. Some human and animal chiropractors are trained in advanced techniques that include identification and correction of inter-osseous faults. (These faults may also be involved in abnormal jaw development in miniature horses.)

While vestigial, the splint bone is critical to proper carpal/tarsal joint function. Slow growth, balanced whole food rations, and chiropractic care, coupled with other alternative health care modalities, will keep the splint bones, and the rest of the horse, healthy and strong.