Week 4: Bone Biomechanics Notes

Week 4 Fundamentals of Biomechanics: Bone Biomechanics

Basic Biomechanics of the Musculoskeletal System

• This section covers the biomechanics of bone, including its behavior under various loads and the influence of muscle activity.
• It also addresses bone remodeling and degenerative changes associated with aging.

Stress-Strain

• Stress:
  • Defined as force per area, which is also known as pressure.
  • Measured in units such as atmospheres, Pascals (Pa), and pounds per square inch (PSI).
  • Stress is a tensor quantity.
• Strain:
  • Represents deformation in response to stress.
  • It is the ratio of the change in an object's dimensions to its original dimensions.

Stiffness

• Stiffness:
  • Ratio of stress to strain in the elastic region of a material.
• Young’s Modulus:
  • A measure of stiffness.
• Elasticity:
  • The ability of a material to return to its original shape after stress is removed.
  • In the elastic region, there's a linear relationship between stress and strain.
  • Deformation force is equal in loading and unloading.
• Hysteresis:
  • Observed in viscoelastic materials where the unloading response differs from the loading response.
  • Measures energy lost in materials that are not perfectly elastic.

Strength

• Mechanical Strength:
  • The maximum force or energy a material can absorb before failure.
• Anisotropic:
  • Materials exhibiting varying strength depending on the direction of loading.

Creep

• Viscoelastic:
  • Stress and strain are dependent on the rate of loading.
• Creep:
  • Gradual elongation of a material over time under constant tensile stress.

Stress Relaxation

• Stress Relaxation:
  • Decrease in stress over time in a fixed lengthening.
• Hysteresis:
  • Loading and unloading responses are not equal in viscoelastic materials.

Bone Loading

• Compression:
  • An external force that squeezes the molecules of a material together.
  • Bone can withstand approximately 200 MPa or 29,000 lbs/in^2 in compression.
• Tension:
  • A load that acts to stretch or pull apart a material.
  • Bone can withstand approximately 125 MPa or 18,000 lbs/in^2 in tension.
• Shear:
  • Loading at a right angle, acting in opposite directions.
  • Bone can withstand approximately 65 MPa or 9,500 lbs/in^2 in shear.
• Torsion and Bending:
  • Multiple forces acting on a body combine to create torsion or bending.
  • Torsion: Involves rotation and counter-rotation.
  • Bending: Combination of compression and tension.

Bone Remodeling

• Wolff’s Law:
  • Bone deposition and resorption are dependent on the stresses applied to the bone.
  • Influenced by weight bearing, space travel, and metabolism.