Biomech unit 2

Question 1

What materials is bone made from?

Answer 1

Osteocytes (cells), collagen fibres (non-cellular component), minerals (organic component)

Question 2

What are the 2 types of bone and what are the gross differences between them?

Answer 2

Compact/cortical: dense structure, outer layer of bone Cancellous: mesh like structure (spongy bone), inside surface of bones

Question 3

Describe the arrangement of cortical vs cancellous bone tissue?

Answer 3

Compact: - columns of haversian systems - bone arranged in lamellae (layers) around a central canal (haversian canal - contains blood vessels and nervous tissue) - lamellae connected with collagen - osteocytes in lacunae (small cavities between lamellae) - lacunae connected by canaliculi Cancellous bone: - arranged in trabeculae (latticework of branching sheets and columns) - layers of lamellae containing lacunae for osteocytes - canaliculi to connect lacunae - (no haversian canals - blood vessels travel in bone marrow)

Question 4

What are the 5 types of loading?

Answer 4

Tension, Compression, Shear, Bending, Torsional

Question 5

What are the stress and strain? (and how to calculate/units)

Answer 5

Stress: the force applied per cross sectional area Stress = Force/Area (N/m2, Pa) Strain: the change in length of a material Strain = change in length/original length (no units)

Question 6

What are elastic and plastic behaviour in bones?

Answer 6

Elastic: stress is directly proportional to strain (linear), when stressed the bone will return to its original shape when the load is removed Plastic: there is a large amount of strain for a small amount of stress (bone deforms a lot for a small amount of load), when load is removed the bone will not return to its original shape (permanently deformed)

Question 7

What are the yield point and the fracture point?

Answer 7

Yield point: the stress at which the material will return to its original shape (permanently deformed Fracture point: the ultimate strain and stress point where the bone will fracture

Question 8

What is Youngs modulus?

Answer 8

The ratio of stress to strain which describes how flexible or stiff a material is Youngs modulus = stress/strain Flexible material: requires a small amount of stress to cause a large amount of strain (low youngs modulus) Stiff material: requires a large amount of stress to cause a small amount of strain (large youngs modulus)

Question 9

In bending loading, which surface will fracture first?

Answer 9

The elongated surface (bone is stronger under compression than tension)

Question 10

In torsional loading, where will a fracture start and why?

Answer 10

Starts on the outer surface The centre is a neutral axis where there is no stress or strain, this increases as move outwards

Question 11

What is the purpose of muscle activity in reducing the stress on bones? Give an example

Answer 11

Muscles contract to alter the stress distribution - they usually contract to cause compression and prevent any tensile loading as bones are stronger under compression then tension Ie when there is a bending load on the tibia, the soleus contracts to put the whole bone under compression and prevent bending (increases the compressive load but eliminates the tensile load which reduces fracture risk) Athletes are more likely to fracture a bone as their muscles are fatigues and they are unable to control the stress distribution

Question 12

What is Wolff’s law?

Answer 12

Bone is laid down where needed and resorbed where not needed

Question 13

How to bones respond to physical exercise or inactivity?

Answer 13

Physical exercise: bones subjected to increased levels of stress, lay down more collagen + minerals to strengthen bones Inactivity: resorption of bones (atrophy) - if spend a period of time bedridden or in a wheelchair, when use legs again are more likely to fracture

Question 14

What bone remodelling changes occur when a fracture fixation plate is used?

Answer 14

Plate will carry most of the load of the limb - if not removed soon after healing the bone will weaken as unstressed tissue is resorbed (stress shielding) Where screws are inserted the bone is carrying a greater load than normal so will increase in bone tissue (bone hypertrophy)

Question 15

How does a fatigue fracture/stress fracture occur?

Answer 15

Repeated loading of the bone that is less than the ultimate stress of the bone, occurs when the frequency is too fast for the remodelling process (number of repetitions depends upon the force; higher force = less repetitions, lower force = more repetitions)

Question 16

When are fatigue fractures most likely to occur and why?

Answer 16

During a period of sustained physical activity, as muscles become fatigued they are less able to neutralise tensile forces

Question 17

What are the age related bone remodelling changes in children vs adults?

Answer 17

Children: there is more bone formation than resorption as grow/develop Young adults: balanced amount of formation + resorption (total tissue doesn’t alter) After 35/40: bone resorption exceeds formation so bone tissue is lost

Question 18

What are the specific changes in bone remodelling of an older adult?

Answer 18

Large loss of cancellous bone, thinning of longitudinal trabeculae + resorption of transverse trabeculae Slightly less resorption of compact bone (ultimate strength of bone is lower so more likely to fracture when subjected to the same loading as a younger person)

Question 19

What are the 3 types of cartilage and where are they found?

Answer 19

Hyaline cartilage; articular cartilage, covers lining of joints (articular surfaces) Elastic cartilage; epiglottis/external ear Fibrocartilage: intervertebral discs/pubic symphysis

Question 20

What is the function of articular cartilage and where is it found?

Answer 20

Covers the articular surfaces of synovial joints Acts as a shock absorbed to distribute load across a larger area and provide a smooth/lubricated surface for movement (very strong - can withstand forces several times bodyweight)

Question 21

What are the components of articular cartilage and what do they each do?

Answer 21

Collagen fibrils; provide strength Proteoglycans; mechanical properties (proteins) Chondrocytes; manufacture, secrete + maintain matrix Interstitial fluid; fills space in the matrix

Question 22

Describe the layers of articular cartilage?

Answer 22

Superficial tangential; tightly woven collagen parallel to articular surface, oblong chondrocytes between collagen sheets

Middle: less densely packed randomly arranged collagen, lots of proteoglycans, round chondrocytes (more space)

Deep: collagen arranged into bundles anchored to underlying bone tissue, chondrocytes in columns perpendicular to bone surface

Question 23

What is the tidemark?

Answer 23

Interface between the articular cartilage and the calcified cartilage underneath which merges into bone

Question 24

What type of behaviour is viscoelastic?

Answer 24

Time dependent (response to a load varies depending upon the amount of time exposed to it) Exhibits a combination of viscous + elastic behaviour

Question 25

What is creep? Draw a graph demonstrating creep

Answer 25

As a load is applied to a material, the material continues to deform over time (decreasing rate)

Question 26

What is stress relaxation?

Answer 26

When held at a constant deformation, the stress required to maintain decreases over time (as there is molecular rearrangement)

Question 27

Why Does creep occur in articular cartilage? (what is the process)

Answer 27

Initial rapid deformation due to fluid being forced out of cartilage, rate of fluid being expelled decreases and the volume of the fluid remaining decreases (at the final point there is no fluid remaining and stress if held entirely by the solid matrix)

Question 28

Why does articular cartilage exhibit stress relaxation? (what is the process)

Answer 28

Once the fluid is forced out, less load is requried to maintain the same deformation

Question 29

What are the three types of lubrication in synovial joints? Describe each

Answer 29

Elastohydrodynamic lubrication

Boosted lubrication

Boundary lubrication

Question 30

Describe elastohydrodynamic lubrication

Answer 30

A film of fluid between two surfaces lubricates them to slide over each other, the fluid completely separates them so they don’t touch

Hydrodynamic lubrication: when 2 surfaces slide over one another a wedge is formed from the rear end due to the lifting pressure (as the motion drags fluid with it into the gap)

Squeeze film lubrication: if pressure is applied directly from above the fluid is not immediately forced out from between the two surfaces (acts as a cushion to protect both surfaces) - will eventually be forced out if load maintained

Question 31

Describe boosted lubrication

Answer 31

Occurs in artilcular cartilage when 2 surfaces pressed together

When 2 surfaces are pressed together the resistance to sideways flow of the lubricant increases until the lowest path of resistance is for water molecules ot permeate into the articular cartilage

Leaves behind a thick viscous gel which acts as an enriched lubrcant and can support large loads (keeps the articular surfaces apart)

Question 32

Describe boundary lubrication

Answer 32

Occurs if there is a very large load over a sustained period of time which completely depletes the fluid film, the surfaces are protected by boundary lubrication

Lubricant proteins (lubricin protein) chemically bind to the surfaces creating a boundry layer which has a low shear strength (so lower friction than the bare surfaces)

Question 33

Describe the difference between the structure of tendons and ligaments

Answer 33

Both formed of mostly collagen fibres with a few fibroblast cells

Tendons: collagen completely in parallel as only need to withstand forces in one direction

Ligaments: collagen fibres branched/interwoven as mostly withstand forces in one direction but also smaller loads in other directions

Question 34

Describe the mechanical properties of the ACL

Answer 34

Connects the femur to the tibia (so is stretched when there is a tensile load applied)

Viscoelastic behaviour: withstand large tensile forces but also be highly flexible to allow movement

Up to 4mm stretch is normal however beyond this collagen fibres rupture and there is pain + joint instability (snaps completely at 7mm)

Question 35

What are the cells in tendons/ligaments called?

Answer 35

Fibroblasts