Mechanics of Bone

Question 1

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

Bone Biomechanics Structure

Answer 3

Collagen
Mineral
Ground Substance
Water

Question 4

Collagen

Answer 4

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

Question 5

Mineral

Answer 5

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

Question 6

Ground Substance

Answer 6

(5% dry weight)

Gel like substance surrounding collagen fibers

Question 7

Water

Answer 7

20-25%

Question 8

Geometry

Answer 8

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: ability to resist a twist

Question 9

Polar moment of inertia

Answer 9

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

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

Question 10

Mode of Loading

Answer 10

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 11

Anisotropic

Answer 11

stiffness & strength depend upon mode of loading

Question 12

Rate of Loading

Answer 12

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 13

Viscoelastic

Answer 13

stiffness & strength depend upon speed of applied load

Question 14

↓ loading rate

Answer 14

cracking (no fragments), little or no soft tissue damage

Question 15

↑ loading rate

Answer 15

comminuted (fragments displaced), extensive soft tissue damage

Question 16

Viscoelasticity

Answer 16

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 17

Frequency of Loading: Bone Fatigue

Answer 17

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

Interplay of load, repetition & injury

Question 18

Muscle Activity

Answer 18

Muscles may produce tensile or compressive stresses on bone

Offset mechanical stresses in opposite direction

Question 19

Aging

Answer 19

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

Question 20

Bone Remodeling

Answer 20

Alters size, shape & structure based on the imposed mechanical demands
Wolff’s Law
Affected by activity level and implants (stress shielding)

Question 21

Wolff’s Law

Answer 21

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