Biomechanics unit 2 SAQ's

Question 1

What are bone cells called

Answer 1

Osteocytes

Question 2

Which component of bone gives it its characteristic hardness and rigidity?

Answer 2

The inorganic component, which consists mainly of crystals of calcium phosphate, gives bone its characteristic hardness and rigidity

Question 3

Which type of bone tissue contains haversian canals and why are they needed?

Answer 3

Compact bone tissue contains haversian canals. These contain blood vessels which are needed to supply the bone tissue with nutrients.

Question 4

What happens to a material under a tensile load?

Answer 4

When a material is under a tensile load it will elongate.

Question 5

What is the definition of strain?

Answer 5

Strain is defined as the change in length divided by the original length.

Question 6

What are the SI units of stress?

Answer 6

The SI units of stress are newtons per metre squared (N m^-2).

Question 7

The pic shows the initial part of the stress-strain curves for several materials along with corresponding Young’s modulus. How stiff or flexible is cortical bone compared to the other materials?

Answer 7

Use equations to think about it:

Strain = stress / youngs modulus

• When you have a large ym then you need a large stress to produce a small strain (stiff)
• When you have a small ym you need a small stress to produce a large strain (flexible)

Cortical bone is stiffer than wood but more flexible than glass, aluminium and steel.

Question 8

Is cortical bone stronger in shear or tension?

Answer 8

Cortical bone is stronger in tension than in shear.

Question 9

Name two types of bending.

Answer 9

Three point bending and cantilever bending are two types of bending loading.

Question 10

What is the neutral axis?

Answer 10

The neutral axis is the line through a structure along which no distortion occurs.

Question 11

Along which surface will a fracture most likely occur when an excessive bending load is applied to a long bone?

Answer 11

When an excessive bending force is applied to a long bone the fracture will more likely occur on the elongated surface of the bone

Question 12

When considering torsional loads, do hollow or solid bars have the best strength-to-weight ratio?

Answer 12

A hollow bar has a better strength-to-weight ratio than a solid bar

Question 13

Name five types of loadings.

Answer 13

Five types of loading are: tensile, compressive, bending, shearing and torsional.

Question 14

What is a combined loading?

Answer 14

Combined loading is when more than one type of loading is present.

Question 15

Can you think of any problems astronauts may suffer from if they spend long periods in the weightless environment of outer space?

Answer 15

If astronauts spent long periods in the weightless environment of outer space then they may suffer from bone atrophy if they do not undertake exercises that will help them to stress their bones

Question 16

What is the difference between bone hypertrophy and bone atrophy?

Answer 16

Bone hypertrophy means an increase in bone tissue and bone atrophy means a decrease in bone tissue.

Question 17

How is a fatigue fracture caused?

Answer 17

A fatigue fracture is caused by a load being applied repeatedly over a short period of time, where the frequency of repetition is too fast for the remodelling process

Question 18

How would you recognise a greenstick fracture?

Answer 18

A greenstick fracture is characterised by an incomplete fracture with one side bent and the other buckled.

Question 19

Why are children prone to greenstick fractures but not adults?

Answer 19

Children are prone to greenstick fractures because their bones contain a greater proportion of collagen than adult bones which gives their bones a greater amount of flexibility.

Question 20

Why is it sometimes desirable for muscles to contract even when it is not to produce movement?

Answer 20

It is sometimes desirable for muscles to contract even when it is not to produce movement as they can alter the distribution of stress in bones to reduce the amount of tensile stress and in doing so increase compression stress therefore potentially preventing any damage to the bones as the bones are stronger in compression than tension

Question 21

How is cancellous bone tissue affected by ageing?

Answer 21

The amount of cancellous bone is reduced with ageing.

Question 22

In what type of joints is articular cartilage found?

Answer 22

Articular cartilage is found in synovial joints.

Question 23

What are the two main roles of articular cartilage?

Answer 23

The two main roles of articular cartilage are to cushion the bones and to provide a smooth, lubricated, bearing surface.

Question 24

What is the role of the chondrocytes in articular cartilage?

Answer 24

Chondrocytes manufacture, secrete and maintain the organic matrix

Question 25

How many layers (or zones) does articular cartilage have?

Answer 25

There are three main layers in articular cartilage.

Question 26

Name the different layers of articular cartilage

Answer 26

The main layers are: superficial tangential, middle and deep zones

Question 27

How is articular cartilage attached to bone?

Answer 27

Articular cartilage is attached to bone by means of the collagen fibrils that extend from the deep zone into the underlying bone tissue.

Question 28

How does articular cartilage behave mechanically?

Answer 28

Articular cartilage demonstrates viscoelastic mechanical behaviour

Question 29

What is the difference between creep and stress relaxation?

Answer 29

Creep is an increase in strain under a constant stress whilst stress relaxation is a reduction in stress under a constant strain.

Question 30

Summarise the differences between hydrodynamic and squeeze film lubrication in terms of when they occur.

Answer 30

Hydrodynamic lubrication occurs when the two surfaces are moving over one another whilst squeeze film lubrication occurs as the two surfaces are forced together

Question 31

In boosted lubrication which type of molecules are forced into the articular cartilage?

Answer 31

The small molecules including water are forced into the articular cartilage.

Question 32

Give an example of where boundary lubrication occurs.

Answer 32

Examples of where boundary lubrication occurs are Teflon frying pans, waxed floors and synovial joints when the fluid film is completely depleted.

Question 33

Do tendons connect muscles to bone or bone to bone?

Answer 33

Tendons connect muscles to bone.

Question 34

What are tendon and ligament cells called?

Answer 34

Fibroblasts

Question 35

How are the collagen fibres arranged in tendons and ligaments?

Answer 35

• In tendons collagen fibres are aligned in parallel
• In ligaments collagen fibres are aligned nearly in parallel.

Question 36

What is the basic mechanical behaviour of tendons and ligaments?

Answer 36

The basic mechanical behaviour of tendons and ligaments is viscoelastic.

Question 37

Approximately how far can the anterior cruciate ligament be elongated before rupturing completely?

Answer 37

The anterior cruciate ligament can be elongated by approximately 7 mm before rupturing.