Biomechanics of Bone

The Biological Components of Bone
1. What basic materials is bone made of?
  1. Collagen
    1. Organic, makes up about 40% of bones' dry weight, primarily type I
  2. Hydroxyapatite (HA)
    1. Inorganic, makes up about 60% of bones' dry weight. Ceramic-like.
      1. strong in compression, weak in tension, generally brittle
2. What are the two types of mature bone?
  1. Cortical
    1. AKA compact bone
      1. hard, dense bone
      2. found in midshaft of femur
  2. Cancellous
    1. AKA trabecular bone
      1. not as dense, and filled with spaces
      2. found in interior of femoral head
Parts of a Bone
1. Basic unit of bone--an osteon.
  1. Also called haversian canal. A hollow tube of bone with blood vessels and nerves inside.
    1. Surrounded by concentric circles of bone called lamellae.
      1. Runs in direction of load bearing for bone.
2. The three cells that build bones
  1. Osteoclasts
    1. Dig tunnels in the bone as it grows.
  2. Osteoblasts
    1. Line the tunnels made by osteoclasts with collagen.
  3. Osteocytes
    1. Calcify the tunnels made by osteoblasts/clasts into hard bone.
Useful Terms for This Chapter
1. Wolff's Law
  1. Formerly: "The shape of bone is determined only by static loading.
    1. Currently: "Physical laws are a major factor influencing bone modeling and remodeling."
2. Young's Modulus
  1. Definition from page 28: "Young’s modulus indicates either a) how much a material stretches or strains when it is subjected to a certain stress or b) how much stress builds up in a material when it is stretched or strained by a certain amount. Young's modulus also is sometimes called the material's stiffness."
3. Constitutive equation
  1. The relationship between stress and strain, AKA the effects of both stress and strain added together to measure total amount of force applied to a bone.
Material Properties of Bone
1. BONE IS:
  1. Anisotropic
    1. Meaning: it has different properties in different directions, i.e. the compressive strength of compact bone is greater than its tensile strength.
      1. Anisotropic Behavior from Oatis, Chapter 3
  2. Transversely orthtropic
    1. Meaning: the properties of bone are nearly the same in any direction within the transverse plane.
      1. modulus of elasticity and ultimate strength are the same in x and y directions and it makes no difference where those directions are assigned, but differ in the z direction
      2. Transversely orthotropic diagram from Oatis, Chapter 3
  3. Viscoelastic
    1. Meaning: Properties of bone vary depending upon how fast the bone is loaded. The modulus of elasticity and its strength increase with loading rate in bone.
      1. collagen imparts, behavior of bone differs from standard structural ceramics
  4. Cylindrical
    1. Cylindrical shape of bone allows:
      1. Bone to be stronger along the longitudinal axis than the transverse axis.
      2. Any point on the transverse axis of a bone to be approximately equally as strong as any other point on the transverse axis.
How do fractures heal?
1. Step 1: Blood clot forms at fracture.
  1. Step 2: Cells to repair fracture gather at fracture site.
    1. Step 3: A callus forms 2 weeks after fracture.
      1. Step 4: The callus calcifies into woven bone.
      2. Step 5: The woven callus is remodeled by osteoclasts, osteoblasts, and osteocytes into mature bone.
Effects of Age on Bone
1. Special aspects of immature bone
  1. Children's bones are made of woven bone with randomly distributed collagen fibers, making it more flexible.
  2. Children's bones grow from the ends out--at the metaphysis and epiphysis.These contain special growth plates.
    1. if growth plate is damaged, bone may stop growing and will not reach normal length
2. Special aspects of aging bone
  1. bone geometry and fundamental material properties change, mechanical properties decrease
  2. Young's modulus, fracture toughness, and bending strength of bone all begin to decrease after age 35.
  3. Osteoporosis can occur in older adults, brought on by decreased activity and by hormonal changes in menopausal women.
    1. from braceability.com
Example Bone: The Femur
1. Important Function
  1. Carries all of the body's weight with each walking step.
    1. Both type/location of bone and the way an external load is applied to a bone will affect the type of fracture that may occur
2. Traits of the Femur
  1. Stiff (high Young's modulus)
    1. Why? So that the bone does not compress like a spring.
  2. Strong in compression
    1. Why? So that the bone does not fracture or crush.
  3. Strong against tensile loads
    1. Why? So that the bone does not bend with the off-center forces of the muscles.
START HERE, GO CLOCKWISE
3 Different Measures of Bone Durability
1. Young's Modulus (stiffness)
  1. How well the bone resists compression.
2. Strength
  1. How much compression or tension a bone can take without breaking.
    1. not represented by single value
      1. compressive strength in longitudinal direction is greater than tensile strength
3. Fracture Toughness
  1. Measure of a material's ability to resist crack growth if a crack has already initiated.
    1. cortical bone has relatively low fracture toughness because it's brittle and does not easily absorb strain energy
Strain Rate
1. How fast a bone is loaded affects how much force it can take.
  1. protective mechanism: bone is able to withstand greater stresses during traumatic, rapid loading when needed
  2. has an effect on mechanical response: slow loading bone may show ductile behavior, while at fast loading bone may be brittle
Basic Structure of a Bone From Oatis, Chapter 3
Fracture Healing from Orthotips
Mature Bone from anatomyinnerbody.com