Biomechanics

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Biomechanics

• The objective of these lectures is to give an
  introduction to some of the important
  biomechanical relationships of different systems
  of the human body to facilitate the design of
  prosthetic devices.

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Stress Shielding

• Stress shielding is defined as
• Osteopenia occurring in bone as the result of
  removal of
• normal stress from the bone by an implant.
• This phenomenon has been attributed to Wolfes
  Law, which states that bone remodels in response
  to the loads that it is subjected to.
• Thus, if the normal loading on a particular bone
  decreases (due to the presence of an implant),
  the bone will remodel and become less dense as
  less bone mass is required to support the new
  loading condition.

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Stress Shielding

• So, by what degree does the presence of an
  implant actually affect bone loading?

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Stress Shielding
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Contact Mechanics

• Contact mechanics is the study of the deformation
  of solids that touch each other at one or more
  points. This field of particular importance to
  the design of prosthetic joint replacements as
  the area of contact between the bearing surfaces
  dictates the magnitude of the applied stresses of
  the bearing materials.

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Contact Mechanics
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Viscoelasticity

• Viscoelasticity is the property of a material
  that exhibits both viscous and elastic responses
  when undergoing deformation.
• Elastic materials strain instantaneously when
  deformed and also instantaneously return to their
  original state when the stress is removed.
• Alternatively, viscous materials resist shear
  flow and strain linearly with time when a stress
  is applied.

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Viscoelasticity

• Viscoelastic materials display both of these
  properties and, as such, are characterized by
  their
• Time dependence
• Loading history dependence
• Lack of a one-to-one correspondence between
  stress and strain
• Viscoelasticity is important as many materials
  exhibit this type of behaviour
• Biological materials (cartilage, skin, ligament,
  etc)
• Most polymers
• Some metals

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Viscoelastic Phenomena
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Viscoelastic Phenomena

• Another observed characteristic is hysteresis
  (energy lost during deformation) as the loading
  and unloading curves are not superimposed on one
  another as in the case of elastic materials.

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Viscoelastic Phenomena

• This effect is also observed under dynamic
  loading. As some of the energy is lost during
  each cycle, the strain response will always lag
  behind the stress.

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Linear Viscoelasticity