Biomechanics of Bone

Question 1

Bone Biomechanics - Functions

Answer 1

Protect vital organs
Support soft tissue
Produce RBC’s
Reservoir for minerals
Provide attachment site for skeletal muscle
Act as a system of machines to receive muscle torques and make movement possible

Question 2

Factors Influencing Mechanical Properties

Answer 2

Structure
Geometry
Mode of loading
Rate of loading
Frequency of loading
Muscle activity
Age

Question 3

Structure of Bone

Answer 3

Collagen (25-30% dry weight)
Tensile strength
Provides some flexibility: however, relatively little

Mineral (60-70% dry weight)
Calcium & phosphate
Compressive strength

Ground substance (5% dry weight)
Gel like substance surrounding collagen fibers
Compressive strength

Water (20-25%)

Question 4

Geometry

Answer 4

Cross sectional area is proportional to ultimate failure point
↑ area = ↑ ultimate strength

Distribution of bone tissue around it’s neutral axis
Polar moment of inertia

Question 5

Geometry: Polar Moment of Inertia

Answer 5

Mass distributed away from neutral axis = ↑ polar moment of inertia

Mass distributed close to neutral axis = ↓ polar moment of inertia

Question 6

Mode of Loading

Answer 6

Anisotropic: stiffness & strength depend upon mode of loading
Failure point highest during compression (193 MPa)

Failure point second highest during tension (133 MPa)

Failure point lowest during shear (68 MPa)

Compression > Tension > Shear

Question 7

Rate of Loading

Answer 7

Viscoelastic: stiffness & strength depend upon speed of applied load

High loading rate:
↑ stiffness
↑ ultimate failure point
↑ energy storage prior to failure

Loading rate influences fracture patterns and soft tissue damage → energy release
↓ loading rate = cracking (no fragments), little or no soft tissue damage
↑ loading rate = comminuted (fragments displaced), extensive soft tissue damage

Question 8

Viscoelasticity

Answer 8

Materials may behave in both elastic and viscous manners with different rates of loading
Rate-sensitive loading
Higher loading rate = increased stiffness
Lower loading rate = decreased stiffness

Question 9

Frequency of Loading

Answer 9

Bone fatigue
Weakening of bone during repeated loading
Factors:
Magnitude of load
Number of load applications
Number of load applications in a given time***
Fatigue process out paces the repair-remodeling process

Fatigue fracture (stress fracture)
High load, few repetitions
Low load, many repetitions

“Fatigue-Injury Curve”
Interplay of load, repetition & injury

Question 10

Muscle Activity

Answer 10

Muscles may produce tensile or compressive stresses on bone

Offset mechanical stresses in opposite direction

Question 11

Aging

Answer 11

Progressive ↓ bone density with age
↓ collagen & mineral content
↓ bone mass-stiffness and strength
↓ bone size-stiffness and strength

Question 12

Bone Remodeling

Answer 12

Alters size, shape & structure based on the imposed mechanical demands

Wolff’s Law
Bone tissue is gained or lost depending upon the level of stress sustained
↑ mechanical stress → ↑ bone tissue production
Depends on rate of loading
↓ mechanical stress → ↓ bone tissue production

Affected by activity level and implants (stress shielding)

Question 13

Wolffs Law

Answer 13

Bone tissue is gained or lost depending upon the level of stress sustained
↑ mechanical stress → ↑ bone tissue production
Depends on rate of loading
↓ mechanical stress → ↓ bone tissue production