You are here





  • 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)


The information on this website is intended for orthopaedic surgeons.  It is not intended for the general public. The information on this website may not be complete or accurate.  The eORIF website is not an authoritative reference for orthopaedic surgery or medicine and does not represent the "standard of care".  While the information on this site is about health care issues and sports medicine, it is not medical advice. People seeking specific medical advice or assistance should contact a board certified physician.  See Site Terms / Full Disclaimer