Unit 2 - Biomechanics of skeletal tissue

Question 1

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

Answer 1

Long

Short

Flat

Irregular

Question 2

What are bone cells called?

Answer 2

Osteocytes

Question 3

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

Answer 3

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

Question 4

What is the inorganic component of bone made up of?

Answer 4

Mainly calcium phosphate crystals deposited in the matrix

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

Question 5

What are the 2 different types of bone tissue?

Answer 5

Compact (cortical)

Cancellous (spongy)

Question 6

Describe the structure of compact bone

Answer 6

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)

Question 7

Describe the structure of cancellous bone

Answer 7

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

Question 8

Why does cancellous bone not have haversian canals?

Answer 8

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

Question 9

Describe tension

Answer 9

Load acting to stretch

Question 10

Describe compression

Answer 10

Load acting to compress

Question 11

What is stress?

Answer 11

Force/Area

Question 12

What is strain?

Answer 12

Change in length/Original length

Question 13

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

Answer 13

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

Question 14

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

Answer 14

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

Question 15

At which point will a material fracture?

Answer 15

Ultimate strain & strength

Question 16

What is the relationship between stress & strain?

Answer 16

Stress = strain x Young’s modulus

Question 17

Describe shear loading

Answer 17

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)

Question 18

Describe the 2 types bending loading

Answer 18

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)

Question 19

Describe torsional loading

Answer 19

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

Question 20

Why may muscles contract even when not causing a movement?

Answer 20

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

Question 21

What is Wolff’s law?

Answer 21

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)

Question 22

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

Answer 22

Stress or march fracture

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

Question 23

How is the composition of bone different in children?

Answer 23

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)

Question 24

At what age does bone begin to be lost?

Answer 24

35-40

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

Question 25

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

Answer 25

Hyaline - covers articular surfaces of bones & forms tip of nose Elastic - external ear & epiglottis (more elastic than other types) Fibrocartilage - symphysis pubis & intervertebral discs

Question 26

What is the purpose of articular cartilage?

Answer 26

Withstand & distribute large loads Cushion bones & provide a smooth, lubricating bearing surface with minimal wear

Question 27

What 3 components make up articular cartilage?

Answer 27

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)

Question 28

Name and describe the 3 zones of articular cartilage?

Answer 28

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

Question 29

What is beneath the deep zone of articular cartilage?

Answer 29

Thin layer of calcified cartilage which merges with bone Tidemark - interface between articular cartilage & calcified cartilage

Question 30

What mechanical property does articular cartilage exhibit?

Answer 30

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

Question 31

What is creep?

Answer 31

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)

Question 32

What is stress relaxation?

Answer 32

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

Question 33

Name the 3 types of lubrication

Answer 33

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

Question 34

Describe elastohydrodynamic lubrication

Answer 34

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)

Question 35

Describe boosted lubrication

Answer 35

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)

Question 36

Describe boundary lubrication

Answer 36

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)

Question 37

What is the function of tendons & ligaments?

Answer 37

Tendons - connect muscle to bone Ligaments - connect bone to bone

Question 38

What is the difference in structure between tendons & ligaments?

Answer 38

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)

Question 39

What mechanical properties do tendons & ligaments exhibit?

Answer 39

Viscoelastic (creep & stress relaxation) Withstand large tensile loads & remain flexible