Hole Structure Strengthens Bone

close up of a brown horse's hoof while it stands in soil during the day

Biological Strategy

Horse

AskNature Team

Image: Devanath / Pixabay / Public Domain - No restrictions

Distribute Energy

Many forms of energy are naturally available, including kinetic, potential, thermal, elastic, radiant, chemical, and more. All living systems require energy to carry out their many activities. Energy isn’t a physical object that can be held, so it also can’t be pushed, pulled, or carried. However, it can be transmitted or transformed. For example, the golden-scale snail has a tri-layered shell that dissipates, or transfers, forces from a predator biting down on it, thereby minimizing damage to its soft body inside the shell.

Search this Function

Manage Compression

When a living system is under compression, there is a force pushing on it, like a chair with a person sitting on it. When evenly applied to all sides of a living system, compression results in decreased volume. When applied on two sides, it results in deformation, such as when pushing on two sides of a balloon. This deformation can be temporary or permanent. Because living systems must retain their most efficient form, they must ensure that any deformation is temporary. Managing compression also provides an opportunity to lessen the effects of other forces. Living systems have strategies to help prevent compression or recover from it, while maintaining function. For example, African elephant adults weigh from 4,700 to 6,048 kilograms. Because they must hold all of that weight on their four feet, the tissues of their feet have features that enable compression to absorb and distribute forces.

Search this Function

Mammals

Class Mammalia (“breast”): Bats, cats, whales, horses, humans

Mammals make up less than 1% of all animals on earth, but they include some of the most well-known species. We know first-hand some of the characteristics that make mammals unique, like having hair, being able to sweat, and producing milk through mammary glands. Another critical shared feature is a set of highly-specialized teeth. Unlike sharks or alligators, for example, whose teeth are generally all the same size and shape, mammals have differently shaped teeth in different areas of the jaws to target specific foods or foraging strategies.

Search this Living System

A metacarpal bone of a horse avoids structural weakness caused by a hole via stress-dispersing microstructure.

Zebras, horses, and other equine species put substantial stress upon their central forefoot bones, particularly the third metacarpal, with remarkable strength despite having holes in them for blood vessels to pass through. The presence of a hole (or foramen) in a structural element offers the potential for it to act as a site of stress concentration and initiation of cracks, yet these foramina do not weaken the bone or act as fracture initiation sites. Hence the foramen in the third metacarpal of equine species has been of interest to engineers to learn how to design openings in structures in a way that avoids cracking.

The key features investigators have found that minimize cracking at these sites are:

their location in regions predominantly experiencing compression
their elliptical––rather than round––shape, oriented parallel to the long axis of the bone and the lines of force
the “softening” of the material discontinuity by increased compliance of the tissue surrounding the opening that shifts peak stresses away from the foramen edge
a ring of increased stiffness reinforcing the foramen at some distance from it to absorb those stresses shifted inward from the compliant foramen edge.

Many human-made structures, such as airplane wings, need to have holes in them to accommodate wires, fuel lines, or hydraulic system elements. Hence inspiration from the design of foramina in bones could have wide application.