Intro to bone biomechanics

• Bone

  • Living tissue that changes and adapts to requirements of development and activity
  • Bone is ANISOTROPIC

  Comprised of 2 components:

  • 60% mineralized- Hydroxyapatite Ca10 (PO4)6.(OH)2
  • 40% organic proteins- Collagen and Glycosaminoglycans

  Bone is separated into a number of components:

  • Periosteal surface
  • Cortex
  • Endosteal bone
  • Medullary cavity

  the cortical or exosteal surface overlies endosteal bone which is referred to as spongy, cancellous or trabecular bone
• Hierarchy of bone structure

  1st level structures

  • Different types of cortical bone can first be differentiated at the first level structure

  - Woven bone

  - Plexiform bone

  - Primary osteonal cortical bone

  - Secondary osteonal cortical bone (remodelled)
• Hierarchy of bone structure

  2nd level structures

  • Second level cortical bone structure consists of those entities which make up the osteons in primary and secondary bone and the 'bricks' in plexiform bone

  - Osteoblasts (forms)

  - Osteocytes (mature cells)

  - Osteoclasts (destroyers)
• Bone formations

  2 types of bone can be identified microscopically according to the pattern of collagen forming the osteoid:

  • Woven bone ~ characterised by haphazard organization of collagen fibres and is mechanically weak
  • Lamellar bone ~ has a regular parallel alignment of collagen into sheets (lamellae) and is mechanically strong
• Responses of bone to loading

  
• Stress and trauma

  Wolff's Law

  • Mathematical method
  • Bone remodels through life in response to mechanical stress

  Lanyon and Rubin

  • Dynamic rather than static loading promotes remodelling

  Bone functional adaptation

  • Replaced the flawed mathematical models of Wolff
• Forces acting on bone

  • Tension
  • Compression
  • Torsion
  • Bending
  • Shearing

  Speed of fusion:

  -dynamic

  -static

  Focus of force:

  - narrow

  - wide
• Bone strength

  • Bone is elastically anisotropic, i.e. it's properties depend on direction
  • such behaviour is unlike that of steel, aluminium and most plastics, but is similar to that of wood

  Human Femur

  • Strength= load required to fracture skeletal element
  • Compressive longitudinal strength= 205 MPa
  • Compressive transverse strength= 131 MPa
  • Tensile longitudinal strength= 135 MPa
  • Tensile transverse strength= 53 MPa
  • Shear strength= 65-71 MPa
• Biomechanics basics: what is stress

  • Stress is the measure of the forces acting on a body
  • Load is the average force per unit area under which forces act
  • The dimension of stress is that of pressure, therefore SI unit for stress is the Pascal (Pa), which is equivalent to one neutron (force) per square meter (unit area), N/m2
  • Imperial Unit= pound.force per square inches, psi
• Biomechanics basics: what is stress

  • Strain: Deformation of a deformable body under the application of stress
  • couchy strain or engineering strain is expressed as the ratio of the total deformation to the initial dimension of the material body in which the forces are being applied
• Young's Model of Elasticity

  • When load is converted to stress and deformation converted to strain, the relationship follows a curve called the stress-strain curve
  • the slope pf the stress-strain curve within the elastic region is called the elastic or Young's modulus (E)
  • Young's modulus is a measure of the intrinsic stiffness of the material
  • The area under the stress-strained curve is a measure of the amount of energy needed to cause material failure
  • Property= energy absorption or modulus of toughness
• Young's Modulus of Elasticity

  • The maximum stress and strain the bone can sustain are called the ultimate strength and ultimate strain, respectively
  • Strength is defined by the stress-strain curve of an intrinsic property of bone
  • Strength values are independent of the size and shape of the bone
  • the force required to break the bone is different from the intrinsic strength, because ultimate load will vary with bone size