Biomechanics Definitions

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BIOMATERIAL : A non-viable material used in a medical device, intended to interact with biological systems.

BIOCOMPATIBILITY : The ability of a material to perform with an appropriate host response in a specific application.

BIOINERTNo host response to the material.

IMPLANT– An object made from non living material that is inserted into the human body where it is intended to remain for a significant period of time in order to perform a specific function.

  • Uses:
  1. Replacement of damaged or diseased part of the anatomy- eg. Total joint replacement
  2. To aid in healing of tissue-eg. fracture plate
  3. To correct deformity- eg a plate used after osteotomy.

STRESS: Force applied ¸area (N/M2 or Pascals)

  • A measure of how hard the atoms and molecules in a material are being pushed together or pulled apart.

STRAIN: Change in length ¸ original length (no units)

  • A measure of how far the the atoms at any point in a solid are being pulled apart.

TENSILE STRENGTH/ ULTIMATE STRESS :  The maximum stress on the curve before breakage (N/M2)

YIELD STRESS: Point at which elastic behaviour changes to plastic behaviour. Not always a sudden change. In these cases a nominal stress necessary to produce a specified amount of strain is referred to i.e. proof stress.

STIFFNESS: the ability of a material to resist deformation (or a measure of how hard it is to deform a material a certain amount)

  • Measured as Young’s modulus E = Stress ¸Strain  for elastic part of curve or the slope of the elastic part of the curve.
  • Stiffness is a property of the material and its shape.



    • For example. bending stiffness is given by E.I, where E is Young’s modulus of the material and I is the second moment of area of the cross section – a geometrical factor that accounts for the shape of the device.
  • If there is no definite change from elastic to plastic behaviour, the Secant modulus at 0.2% strain can be used (see below)

DUCTILITY/ BRITTLENESS: The amount by which a material deforms (i.e. the strain that occurs) before it breaks. Represented by percentage elongation or percentage reduction in cross section.

HARDNESS: The ability of the surface of a material to withstand forces.

  • Translates to wear properties.
  • Measured by Mohrs scale or by indentation tests.
  • Hardness is indirectly related to strength, therefore indentation tests can be used to measure strength without destroying the material.

TOUGHNESS : The amount of energy absorbed by a material before breakage (before necking occurs), represented by the area under the stress strain graph.

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