Horse Leg Bones: A Hole in the Design Argument by Don DeYoung

The sight of a galloping horse is a thing of beauty. Its bones and muscles awe equine lovers of all ages. But on closer inspection, there seems to be a terrible flaw at the point of greatest stress—a hole in its shin.

Horses have a small bone just below their front knees called the third metacarpal, or shin bone, which supports their whole weight even when galloping. Only about the thickness of a human wrist, the bone must endure a lot of stress without fracturing. To make matters worse, a pea-sized hole, called a foramen, cuts through the small bone to make room for blood vessels. In manmade structures, similar holes are a frequent source of weakness and failure. So at first glance, horse legs appear to be poorly designed.

Taking a closer look at the equine leg bone, however, scientists have discovered unexpected features that give it amazing strength and may inspire new engineering ideas. They found that the bone tissue surrounding the hole is arranged in a way that directs stresses away from the hole toward stronger regions of the leg bone. The hole is also elliptical, with its long axis pointed along the length of the bone. This geometry provides extra strength under compression, as the horse walks or runs. Occasionally a horse may experience a broken metacarpus (a special hazard for racehorses), but the fracture seldom takes place at the opening, as you might expect.

Engineers at the University of Florida have modeled the horse metacarpal bone to understand how it remains so strong under pressure. First they drilled small holes in plates of various materials. Then they surrounded these openings with polyurethane foam and tested the structures by applying forces. When the plates were stressed to the point of cracking, the fractures were found to be separate from the openings, as is the case for horse fractures. Instead of being a weak link in bone design, the foramen is a source of strength.

This discovery may point to solutions to some age-old engineering challenges. Traditionally, openings or holes in manufactured structures must be reinforced with extra thick material. Think, for example, of portholes on a ship, surrounded by heavy ring-shaped plates and bolts. The horse design suggests that by varying the type of material around such openings we may accomplish the same purpose. Such lightweight solutions are especially desirable in aircraft design, where openings are needed for wires and hydraulic lines. Engineers would love to increase the plane’s strength while reducing weight (and fuel costs).

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It is assumed that fossils show evolutionary progress and improvements over time. However, horse fossils typically have third metacarpal bones very similar to present-day horses, complete with foramen openings. Many other mammals, including people, have similar bone openings for blood vessels. This skeletal structure shows exquisite design.

It is ironic that mankind attributes to chance—and natural selection—things that we intelligent beings have not even thought of yet. But it makes more sense for us to study what the Lord has designed, looking for structural innovations that we might be able to imitate.

The Lord long ago asked Job to remember mankind’s limitations: “Do you give the horse his strength or clothe his neck with a flowing mane?” (Job 39:19, NIV).

NOTES

“From the Bone of a Horse: A New Idea for Aircraft Structures,” http://www.sciencedaily.com/releases/2002/12/021206075907.htm, December 6, 2002.

N. Gotzen, A. Cross, P. Ifju, and A. Rapoff, “Understanding Stress Concentration about a Nutrient Foramen,” Journal of Biomechanics 36, no.10 (2003): 1511–1521.