Bone Mechanics

Question 1

Intrinsic Factors that Affect Bone Structure

Answer 1

Location, Function, Loading environment, Genetics, Individual factors (age, health, diet, etc)

Question 2

Viscoelastic behavior

Answer 2

Mechanical properties that vary with strain rate

Loses stiffness without minerals and becomes brittle without collagen

Question 3

Bone porosity

Answer 3

Cortical bone: 5-15%
Trabecular bone: 40-95%

Changes in porosity account for 75% of variation in bone strength

Question 4

Cortical bone

Answer 4

Mechanical behavior is direction-dependent: anisotropic

Stronger in compression and tension
Weaker in radial or circumferential

Different types of loading cause different mechanical response of bone
Strain rates increase tenfold during impact
activities

Question 5

Cortical Bone Mechanics

Answer 5

Compression: High stiffness and strength due to mineral content

Tension: Bi-linear response

Question 6

Trabecular Bone

Answer 6

Anisotropic material properties (recent nano-
studies reveal more complicated properties)

Mechanical properties of trabecular bone are determined by porosity (performance of bone differs by trabecular microstructure)

Greater strength in compression than in tension

Trabecular bone does not exhibit distinct yield point

Question 7

Cortical and Trabecular Bone Locations

Answer 7

Cortical:
Found in diaphysis of long bones
Surrounds trabecular compartment in metaphysis and epiphysis

Trabecular:
Surrounded by cortical shell in long bones
Found in the vertebrae
Flat bones (i.e., pelvis)

Question 8

Bone Fracture Classifications

Answer 8

Severity:
Greenstick (incomplete), Simple, Comminuted, Open/compound

Pattern:
Transverse, Oblique, Spiral

Question 9

Fracture Modes/Mechanisms

Answer 9

Bone fatigues due to micro-damage, which can lead to catastrophic failure

Load exceeds local strength of the bone

Repetitive subcritical loads create fatigue failure

Fatigue loading results in progressive degradation of mechanical properties
of bone

Question 10

Risk Factors of Bone Fracture

Answer 10

Increases with age
Increases with bone porosity
Hormonal changes
Frequent adverse loading (i.e., falls)

Φ= Applied load / Failure load

Estimates probability of fracture
Φ < 1: unlikely to fracture
Φ > 1: fracture predicted