Mechanical and morphological aspects of experimental overlaod and fatigue in bone
- “Slip lines” is a term used to explain deformation of bone along the direction of high shear stress.
- when compressing bone, there’s a period of elastic deformation (reversible) followed by plastic deformation (only partially reversible) (Dempster & Liddicoat 1952)
- Apply compressive forces to dog ulnas and see when slip lines/fractures develop.
- Make a qualitative force-deformation curve for dog ulnas.
- 30 pairs of dog ulnas, no muscles, stored in freezer. “fresh and moist” bones.
- applied compressive force using an old-school materials testing machine.
- Tested ulnas in pairs: one compressed to failure, other underwent a cyclical fatigue test
- Observations were made with electron microscope
Mechanically Loading Bone
Multiple “zones” of bone loading:
- Zone of Load Where deformation is mostly linear, elastic, and follows Hooke’s Law. No amount of cyclical loading will cause a fracture or permanent deformation (plastic), a closed hysteresis loop. But if I applied a small force fast enough and long enough, would a stress fracture occur?
- Zone of Fatigue Where the curve increases (less deformation per unit force). slightly open loop, enough cycles will cause a right shift towards plastic deformation.
- Zone of Overload deformation drastically increases per unit force until fracture.
In this instance, microfractures develop in the compressed cortex, progressively weakening the bone. P was around 1-1.5BW, Zone of fatigue was 0.8-1.2BW.
I guess there is probably a point where bone repair does outpace the loads you impose on a bone. I guess I just thought that the Zone of Fatigue is a bit bigger. Also, the authors suggest that a dog running is in the zone of overload and that the bone “does not have time to resume its final form before a new force is applied to it”. Could muscles help return a bone to it’s original shape between steps? Like dorsiflexion during swing?
- Dempster Liddicoat 1952 Compact bone as a nonisotropic material Am J Anat
- “Nonisotropy” meaning here that a material does not have uniformity in all directions.