• The amount of internal forces acting over a given area of an object. The units of measure for stress are thus N/m2 (Pascal [Pa]), but for most orthopaedic applications are more on the order of MPa (N/mm2). (OKU-11)
  • stress = force (in newtons) / area (in meter2) = N/m2.
  • 1N/m2 = pascal (Pa)

Strain (Normal) 

  • The amount of deformation per given length of an object. The units for strain are dimensionless, but they are often referenced as mm/mm or % strain. (OKU-11)
  • The ratio of elongation to unit lenth.
  • Stain = change in length / unit length.
  • Stain is a ration and has no units. Can be presented as a percentage or as microstrain.
  • Can be positive (tensile) or negative (compressive)

Shear Strain

  • Change in angle between tow adjacent faces of an object that formerly were perpendicular to one another.
  • Expressed in radians (360° = 2pie radians)

Modulus of Elasticity

  • Modulus of Elasticty = ratio of stress / strain of a material = E.
  • Modulus of elasticity = the slope a stress-verses-strain graph.
  • Depends on the material being tested, not on the shape of the structure.


  • The ratio of force to displacement (stiffness = force ÷ displacement). Units of measure are most commonly N/mm or N/m.
  • Stiffness = the slope of the linear postion of a force versus displacment graph (N/mm).
  • Depends on the geometry of the stucture and modulus of elasticity.


  • Strength = The highest stress on a stress-strain curve to failure. Strength and stiffness are not the same thing, and the difference is important.
  • Strength = maximum load a structure can withstand without causing material failure.

Load to failure = The force that is measured at the time that a specimen breaks in a test involving a continuous application of force.(OKU-11)

Fatigue failure = Cyclical subfailure loading may result in failure caused by fatigue. Load-to-failure testing data are most often reported in the orthopaedic literature; however, clinical failure of fracture fixation constructs or implants is often caused by fatigue failure.(OKU-11)

Elastic modulus =The ratio of the stress to strain over a region for which the deformation is elastic, which is analogous to stiffness for force displacement. The units are the same as for stress. (OKU-11)

Yield point =The point on the stress-strain curve where deformation goes from elastic (fully recoverable on unloading) to plastic (unrecoverable).(OKU-11)

Anisotropy = The concept that properties vary along different directions. Isotropy (in contrast) implies that the properties are invariant with direction.(OKU-11)

Brittle = A material that experiences little plastic deformation (strain) before it fails is said to be brittle (for example, glass or cortical bone).(OKU-11)

Ductile = If a material has a large plastic deformation region before it fails, it is said to be ductile (for example, copper).

Toughness = A material that can absorb more energy before failure (large area under the stress-strain curve) is said to be tougher. Units for toughness are (N/m2)(m/m) or joules/m3.(OKU-11)