Lecture 17 - Biomechanics of Hard and Soft Tissue

Question 1

Describe the structure of bone

Answer 1

Cells:
• Osteocytes
• Osteoclasts
• Osteoblasts

Organic matrix - 10% adult bone mass
• 90% collagen (90% of organic matrix)
• Other proteins that help bind the minerals

Mineral component - 65% A.B.M.
• Calcium phosphate (85%)
• Calcium carbonate (10%)

Water - 25% A.B.M.

Question 2

List the main structural properties of bone

Interpret these properties

Answer 2

1. Anisotropic
   • Responds differently to forces going in different directions
   • i.e. mechanical properties are different in different directions
2. Non-homogenous
   • Consists of various cell types and organic and inorganic materials
3. Non linear
4. Thermorheologically complex
5. Viscoelastic (time-dependent)
   • It responds to different loads in different ways, depending on the time frame that the load is experienced
   • Bone resists rapidly applied loads better than slowly applied loads

Question 3

What are the different tissue types in bone?

Answer 3

Two tissue types:
1. Cortical / compact bone
• Dense
• Forms outer layer and diaphysial regions

2. Cancellous / trabecular / spongy bone
   • Consists of trabeculae
   • Loose mesh
   • Surrounded by cortical bone

Periosteum:
• Dense fibrous membrane
• Surrounds the entire bone except the articular surfaces

Question 4

What is the diaphysis?

Answer 4

The shaft / central part of a long bone

Question 5

Describe the stress-strain curve for bone

Answer 5

Three distinct regions of the curve:

1. Initial region:
   • Curve is almost linear
   • Modulus is 17 GPa
2. Intermediate region:
   • Bone exhibits non-linear elastoplastic material behaviour
   • Yielding occurs in this region
   • Yield strength of bone is 110 MPa
3. Final region:
   • Bone exhibits plastic material behaviour
   • Bone fractures at about 128 MPa

Question 6

What is meant by a high strain rate of bone?

Answer 6

High strain rate = rapid loading of force on bone

Low strain rate = slow loading of force

Question 7

What is meant by the term ‘anisotropic’?

Answer 7

Mechanical properties are different in different directions

Question 8

Compare how bone responds to forces applied in longitudinal and transverse directions

Answer 8

Bone has greater strength elastic modulus during longitudinal loading, compare to transverse loading

Question 9

How are cortical and cancellous bone differentiated?

Answer 9

The major difference is porosity

They are differentiated by their apparent density

Question 10

Describe how cancellous bone fractures

Answer 10

Initial linearly elastic region

Eventually yields as the trabeculae break

Question 11

Describe how cortical bone fractures

What sort of material behaviour is this?

Answer 11

**

= brittle material behaviour

Question 12

Which things determine the distribution of internal forces in bone when force is applied?

Answer 12

Size and geometry of the bone

Changes to the geometry:
 • Surgical procedures
 • Bone defects
 • Age
 • Gender
 • Type of bone
 • Bone disease
 • Dry / wet bone

These things can all change the way bone responds to forces

Question 13

Which two proteins are important for mechanical strength in soft tissues?

Answer 13

Collagen:
• Endows tissue with tensile strength
• Can withstand far greater pressure before breaking (compared to elastin)

Elastin:
• Helps tissues resume their shape after stretching

Question 14

Compare presence of collagen and elastin in the following tissues:
• Skin
• Tendon
• Ligaments

Answer 14

Skin:
 • 70% collagen
 • 0.6-2.1% elastin
 • Reticulin fibres
 • Mesh

Tendons:
 • High proportion of collagen fibres
- Aligned with tensile stresses that the tendons experience physiologically
 • Elastin
 • Forms mesh

Ligaments:
• Parallel elastin and collagen fibres

Question 15

Compare tendons and ligaments

Answer 15

Tendons: connect bone to muscle

Ligaments: connect bone to bone

Question 16

Compare how skin and tendons react to forces

Answer 16

They react differently because of their different arrangements of collagen fibres

Question 17

What is ‘crimping’?

Answer 17

Collagen fibres in tendons straightening out due to loading

Question 18

In soft tissues, what does the stress on the tissue depend on?

Answer 18

Depends on:
• Stress at that time
• Stress history

Question 19

Describe how stress on soft tissue is time dependent

Answer 19

The load-deformation rate of the tissue changes when the load is applied at different time rates

‘Creep’:
• When a load is applied an maintained at a constant level, the deformation continues to increase as time elapses

‘Preconditioning’:
• Repeated loading and unloading leads to increasing deformation
• Eventually the tissue stabilises, this is called pre-conditioning

Question 20

What is hysteresis?

Answer 20

The dependence of the output of a system not only on its current input, but also on its history of past input

Question 21

Differentiate between compressive and tensile forces

Answer 21

Compressive: pushing in
Tensile: pulling out

Question 22

What is pre-stress?

Answer 22

Stress in the body that pulls soft tissue (e.g. skin) so it looks taut

Langer described this:
Circular cuts taken out of the skin
The wound was not circular, but rather oval
This implies there is some prestress pulling on the tissue

This has implications for plastic surgeons and wound healing

The whole body is under stress

Question 23

What is meant by ‘strain’ in the stress/strain curves?

Answer 23

Strain is the displacement of the tissue, i.e. how much the tissue stretches