Unit 2 - Biomechanics of skeletal tissue Flashcards Preview

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Flashcards in Unit 2 - Biomechanics of skeletal tissue Deck (39)
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1
Q

What are the 4 different categories bones can be grouped into based on structure?

A

Long

Short

Flat

Irregular

2
Q

What are bone cells called?

A

Osteocytes

3
Q

What is the non-cellular organic component of bone made up of?

A

Mostly collagen embedded in ground substance (jelly-like matrix)

4
Q

What is the inorganic component of bone made up of?

A

Mainly calcium phosphate crystals deposited in the matrix

Gives bone majority of its dry weight as well as its strength & rigidity

5
Q

What are the 2 different types of bone tissue?

A

Compact (cortical)

Cancellous (spongy)

6
Q

Describe the structure of compact bone

A

Forms outer layer of bones & has dense structure

Basic structural unit = haversian system (arranged in longitudinal columns

Each unit consists of lamellae (layers) forming concentric cylinders around haversian canals (contain NVB)

Between lamellae, lacunae (cavities) contain osteocytes which are connected to the haversian canals & other lacunae by canaliculi channels (transfer nutrients)

Collagen fibres connect layers of lamellae & each haversian system is surrounded by ground substance (cement)

7
Q

Describe the structure of cancellous bone

A

Forms inner part of short, flat & irregular bone. Lines inner surface of long bones & makes up most of the metaphyses & epiphyses

Mesh like structure - spaces between mesh contains red bone marrow

Basic structural unit = trabeculae (arranged in lattiework of branching sheets & columns

Trabeculae consist of lamellae with lacunae containing osteocytes connected by canaliculi

8
Q

Why does cancellous bone not have haversian canals?

A

Osteocytes get nutrients from blood vessels that pass through marrow filled spaces of lattice

9
Q

Describe tension

A

Load acting to stretch

10
Q

Describe compression

A

Load acting to compress

11
Q

What is stress?

A

Force/Area

12
Q

What is strain?

A

Change in length/Original length

13
Q

What is the elastic region of a stress/strain curve?

A

Linear part of the graph where stress & strain are directly proportional

If a material is not deformed beyond its yield point by a load it will return to original shape & size

14
Q

What is the plastic region of a stress/strain curve?

A

The non-linear part of the graph where the material yields to the force (small increases in stress cause large strains)

When material is deformed beyond yield point it will not completely recover

15
Q

At which point will a material fracture?

A

Ultimate strain & strength

16
Q

What is the relationship between stress & strain?

A

Stress = strain x Young’s modulus

17
Q

Describe shear loading

A

2 forces acting in opposite directions causing layers within a material to slide over each other

Cortical bone weakest in shear (then tension, then compression) however shearing fractures alone are rare (e.g. intraarticular shearing fracture of femoral condyles)

18
Q

Describe the 2 types bending loading

A

Cantilever (1 end fixed & load at the other end)
3-point (each end fixed & load in the middle)

When structure bent 1 side is elongated (tension) and the other is compressed. In the middle there is a neutral axis (that has no deformation)

19
Q

Describe torsional loading

A

Twisting about longitudinal axis. Stress & strain greatest on outer surface

Torsional #s often have spiral appearance (common in tibia of those wearing studded boots - distal tibia more prone due to its smaller CSA)

Long bones resist torsion as they are hollow with strong cortical bone forming outer layer

20
Q

Why may muscles contract even when not causing a movement?

A

To alter the stress distribution on a bone

If a muscle contracts it can eliminate any tensile loading & produce overall compressive loading

Tired athletes more likely to # as the fatigued muscles are unable to distribute the stress as effectively

21
Q

What is Wolff’s law?

A

Bone is:

  • laid down where needed (physical exercise causes increased stress therefore more collagen fibres & mineral salts are laid down)
  • resorbed where not needed (inactivity leads to resorption of unstressed bones)
22
Q

What are the other names for a fatigue fracture and how does it occur?

A

Stress or march fracture

Occurs when a small load is repetitively applied to a bone faster than the bone can remodel, resulting in fracture

23
Q

How is the composition of bone different in children?

A

Bone formation > resorption

Greater proportion collagen which increases flexibility & makes greenstick #s more common (incomplete # where one side is bent & fractures and other side is buckled)

24
Q

At what age does bone begin to be lost?

A

35-40

Cancellous > compact due to thinning & resorption of trabeculae (more brittle bones)

25
Q

Name the 3 types of cartilage & where they would be found

A

Hyaline - covers articular surfaces of bones & forms tip of nose

Elastic - external ear & epiglottis (more elastic than other types)

Fibrocartilage - symphysis pubis & intervertebral discs

26
Q

What is the purpose of articular cartilage?

A

Withstand & distribute large loads

Cushion bones & provide a smooth, lubricating bearing surface with minimal wear

27
Q

What 3 components make up articular cartilage?

A

Organic matrix of non-cellular material - mainly collagen fibrils enmeshed in conc solution of proteoglycans

Chondrocytes - manufacture, secrete & maintain organic matrix

Interstitial fluid - mainly water (makes up 85% of wet weight)

28
Q

Name and describe the 3 zones of articular cartilage?

A

Superficial tangential - fibrils tightly woven into sheets parallel to articular surface. Chondrocytes oblong & also parallel

Middle zone - fibril arranged more randomly & less densely to accommodate high conc of proteoglycans. Chondrocytes circular & randomly distributed

Deep zone - fibrils arranged in large bundles anchored to bone. Chondrocytes in loose columns perpendicular to bone

29
Q

What is beneath the deep zone of articular cartilage?

A

Thin layer of calcified cartilage which merges with bone

Tidemark - interface between articular cartilage & calcified cartilage

30
Q

What mechanical property does articular cartilage exhibit?

A

Viscoelastic

Exhibits both viscous (deforms at a certain rate depending on loading) and elastic (returns to original shape once load removed) behaviour

31
Q

What is creep?

A

The increase in strain (deformation) over time in a material which is constantly loaded

In cartilage - during initial rapid deformation, fluid is rapidly forced out. As amount of fluid in cartilage decreases the rate of expulsion decreases until at equilibrium (load will eventually rest on solid matrix)

32
Q

What is stress relaxation?

A

Reduction is stress in a material which is constantly deformed (constant strain) over a period of time

In cartilage - there is a drag force associated with the flow of fluid through the matrix out of the cartilage during compression. The stress required to maintain deformation decreases as fluid is no longer being forced out

33
Q

Name the 3 types of lubrication

A

Elastohydrodynamic

Boosted

Boundary

(at any moment, 1 of these types may dominate)

34
Q

Describe elastohydrodynamic lubrication

A

Occurs when 2 surfaces (1 deformable) are lubricated by film of fluid as they move relative to each other

2 ways in in which surfaces can move relative to each other:

  • Slide over each other –> hydrodynamic lubrication (as surfaces slide a lifting pressure is generated as motion drags viscous lubricant into narrowing gap of fluid wedge)
  • Move closer together –> squeeze film lubrication (as surfaces forced together fluid will not immediately be squeezed out but cushion & protect)
35
Q

Describe boosted lubrication

A

If 2 lubricated surfaces forced together over period of time eventually lube will become deplete

As gap between articular surfaces decreases resistance to sideways flow increases to greater than the resistance of flow of the small molecules of the lubricant into the cartilage (mostly H2O)

The lubricant that is left is much more concentrated and viscous making it capable of supporting larger loads (keeping the surfaces apart)

36
Q

Describe boundary lubrication

A

Lubricant molecules chemically attach to surfaces creating a boundary layer which has a low shear strength and therefore offers lower friction than bare surface

In synovial joints the surface of cartilage coated with lubricin (protein from fluid)

37
Q

What is the function of tendons & ligaments?

A

Tendons - connect muscle to bone

Ligaments - connect bone to bone

38
Q

What is the difference in structure between tendons & ligaments?

A

Tendons - collagen fibres arranged completely in parallel as they need to withstand large loads in only 1 direction

Ligaments - collagen fibres not completely parallel & have some branching/interweaving as they need to withstand loads in different directions

(elongated fibroblasts are embedded along the direction of matrix of collagen fibres)

39
Q

What mechanical properties do tendons & ligaments exhibit?

A

Viscoelastic (creep & stress relaxation)

Withstand large tensile loads & remain flexible