Mechanostat - Unit: Strain E

Unit: Strain E

The elastic deformation of bone is measured in μStrain. 1000μStrain = 0.1% change of length of the bone.

• Strain E at length l and change of length Δl:

It has to be considered that bone strength is highly dependent on geometry and direction of the acting forces in relation to this geometry. The fracture load for axial forces of the tibia for example is about 50 to 60 times the body weight. The fracture load for forces perpendicular to the axial direction is about 10 times lower.

Different type of bones can have different modelling and remodelling thresholds. The modeling threshold of the tibia is about 1500 μStrain (0.15% change of length), the modelling threshold for bone of the skull is lowered by the factor of 6 to 8. Since the physical material properties of bone (as a material) are not altered in the different bone types of the body, this difference in modelling threshold results in an increased bone mass and bone strength and hence in an increased safety factor (relation between fracture load and typical loads) for the skull compared to the tibia. A lower modeling threshold means that the same typical daily forces result in a ‘thicker’ and hence stronger bone at the skull.