Sometimes all it takes to break a bone is a step in the wrong direction or, more specifically, a force applied in the same direction as the line-up of the nano-size collagen fibers that give bone its strength. New research shows that bones react to forces from certain directions like reinforced plastics and from others like brittle ceramics.

From an engineering standpoint, bones are riddled with structural flaws, from openings for blood vessels to microscopic canals for cells. As a result, bone requires a mechanism to radiate big local stresses over a large area to avoid massive breaks. Previous research revealed various sources of bone strength, such as sacrificial bonds between fibers that break and re-form to dissipate stress, but researchers had yet to discover exactly how they all worked.

So material scientist Peter Fratzl of the Max Planck Institute in Potsdam, Germany, and his colleagues subjected bone from a deceased 52-year-old woman to a three-pronged stress test. (Prior to the test, a machine cut the bone into fragments of identical shape and size.) Two prongs pushed up from the end of the fragment, while one pushed down in the middle. Without a notch to start the breaking process, these bone fragments snapped naturally.

The researchers found that it took only 375 Joules of energy to crack the bone when they applied the force within five degrees of the orientation of the collagen fibers. But the necessary force increased exponentially when they applied it at anything over 50 degrees away from that orientation, up to a whopping 9,920 Joules when they applied a nearly perpendicular force. In addition, the cracks formed by parallel force appeared relatively smooth and straight--like those that characterize broken pottery--whereas the perpendicular cracks created heavy distortion and ragged gaps in the break, as the bone sought to resist snapping.

The finding, detailed in a report published online yesterday by Nature Materials, explains how bones avoid most fractures by stopping the propagation of perpendicular forces: the collagen fibers line up against the hard edge of a table you run into and spread its blunt force out. It also suggests why sometimes simply putting the wrong foot forward can lead to a nasty break.