Sticks and bones: Tree study to help orthopaedic surgeons

16 Dec 2009

A study on why tree branches buckle or split, rather than break cleanly, could help orthopaedic surgeons do a better repair job on children’s broken bones.

Children are prone to what doctors term “greenstick fractures” – their bones do not break cleanly; they break halfway across then split longitudinally. Tree branches do the same.

Dr Roland Ennos and his team at The University of Manchester are analyzing tree branches to find out why they do this.

“The cell structure in wood is like lots of drinking straws packed together along the branch,” Dr Ennos explains.

“So branches are stiffer along their length than side-to-side. When you try to snap them, you apply longitudinal and transverse pressure. This stretches one side of the branch and compresses the other.

“The transverse pressure easily crushes the straw-like cells in the lighter woods, causing them to buckle. This is why we can weave those withies into baskets.

“Denser wood has thicker-walled cells so the application of force causes the branch to break halfway across then split along its length, as the low transverse tensile strength diverts the crack longitudinally.

“The fact is living branches don’t break cleanly.”

Dr Ennos, based at the University’s Faculty of Life Sciences, adds: “I remembered children’s bones were susceptible to ‘greenstick fracture’, breaking and splitting in exactly the same way. When I read through the literature, no one knew why.

“It appears that the crystals in bone are oriented in the same way, longitudinally along the bone, like the cells in wood are. Adult bones are heavily remodeled by being dissolved and laid down again, so the crystals in their bones lie in different directions. Bones are remodeled to stop cracks from developing – the crystals dissolve and are re-laid in those places. Children’s bones have not had time to do that.

“This is something we could study further with an orthopaedic surgeon.”

The theoretical study, published in the Proceedings of the Royal Society B today (Wednesday 16th December), will be followed by a practical study on three different woods: willow, ash and hazel.

Dr Ennos came up with the idea for the study on a walk through the woods near his home in Buxton, Derbyshire: “I was walking through our local wood and breaking twigs off trees and wondering why they were breaking in these two particular ways. I remembered how difficult it was to break branches for firewood as a cub scout – you can’t break fresh branches, you need to find dead wood.

“The cells are arranged this way in order to draw water up and along branches and the minor downside is that they are more prone to buckling or splitting. But wood is a marvelous material, the best in the world, better than steel or plastic. It is stiff, strong and tough, all combined, and that’s very rare in a material. Steel is stronger but it’s heavier and both that and plastic take a lot of energy to make, which is important when we are facing climate change.

“We ought to return to an age of wood, in my opinion. We have a feel for wood that goes back to our early ancestors, when we used to cut branches off trees to make into spears and other tools. Understanding precisely how it works should help us design the tools of the future.”

Notes for editors

The paper ‘Transverse stresses and modes of failure in tree branches and other beams’, Proceedings of the Royal Society B, is available.

Images are also available.

For more information, a copy of the paper, images or an interview with Dr Roland Ennos contact Media Relations Officer Mikaela Sitford on 0161 275 2111 or Mikaela.Sitford@manchester.ac.uk.