Chapter 39: Bone Biomechanics and Fracture Biology

Question 1

What is a moment arm?

Answer 1

Distance between the line of action of a force and the axis of rotation. When a moment is applied to a body, a tendency to rotate is induced - if it cannot rotate you get bending/twisting.

Question 2

Formula for stress?

Answer 2

Force / area

Question 3

Cauchy described what?

Answer 3

Stress distribution within a body. (Cauchy stress)

Question 4

Formula for strain?

Answer 4

Displacement (change in gap) / length of total bone

Measure of deformation of a structure in response to stress.

Question 5

Area under the under the linear region of the stress/strain curve up to the proportional limit is what?

Answer 5

Resilience:
Measure of a materials ability to absorb energy when loaded and then fully recover, releasing the stored energy when load is removed

Question 6

Area under the entire stress-strain curve up to failure point is called the?

Answer 6

toughness - measure of the energy absorbed by the material prior to failure

Question 7

What is ductility?

Answer 7

The strain that remains if load is removed just prior to fracture/failure. It is the length on the X axis under the stress/strain curve (the x axis is measured in strain)

Question 8

Peak stress prior to failure is called the ?

Answer 8

Ultimate strength of the material - the peak/high point on the stress/strain curve

Question 9

What is happening in the ‘toe region’ of a stress/strain curve for a tendon/non-linear material?

Answer 9

The toe region (red on graph) represents “un-crimping” of the collagen fibrils; toe region ends at about 2% of strain when all crimpled fibers straighten.

Once they are straight, the material stiffens and behaves in a more linear/elastic fashion (blue on graph) until it reaches the yield point.

Question 10

Most flat bones are formed by?

Answer 10

Intramembranous ossification (Skull or scapula)

Question 11

What pathway is essential for osteogenic differentiation?

Answer 11

The canonical Wnt signalling pathway affects the proliferation and differentiation of mesenchymal stem cells (MSCs) and osteoblast progenitor cells, as well as the bone resorption of osteoclasts, contributing to bone formation and playing an indispensable role in the maintenance of bone homeostasis.

It came from Wnt (wingless) in drosophila

Question 12

What are the 5 zones of endochondral ossification?

Answer 12

Resting (identical to hyaline cartilage)
Proliferation (mitosis in a type II collagen matrix)
hypertrophy (produces type X collagen)
calcification (Ca-Phos aggregates and matrix mineralization)
ossification (osteoblasts make woven bone and -clasts remodel it to lamellar)

Question 13

Proliferation chondrocytes have which appearance?

Answer 13

Stacked coin appearance - matrix collagen type 2

Question 14

Hypertrophic zone chondrocytes have what appearance?

Answer 14

Hypertrophied

Question 15

Which type of collagen predominates in hypertrophy stage?

Answer 15

Collagen X (10) - appearance leads to local tissue hypoxia, VEGF expression by hypertrophied chondrocytes and vascular invasion via angiogenesis

Question 16

Why are Salter-Harris fractures so common in the hypertrophic zone?

Answer 16

Zone contains little matrix and is structurally the weakest component of the epiphyseal plate - Predisposing this region to fracture in young animals

Question 17

What happens in the zone of calcification?

Answer 17

Chondrocytes continue to undergo apoptosis due to persistent hypoxia - chondroclasts arrive, vascular invasion continues, chondrocytes release vesicles containing ALP and other enzymes that scavenge calcium and phosphate from adjacent tissues

Question 18

What happens in the zone of ossiciation?

Answer 18

Bone is finally deposited, osteoblasts derived from perivascular mesenchymal stem cells follow vascular networks and produce woven bone on calcified tissue surface - osteoclasts then remodel woven bone to lamellar bone over time

Question 19

What is formation modelling and where does it occur?

Answer 19

Laying down of new bone where it was not present before by osteoblasts
At the epiphyseal region of the bone and along the endosteal surface

Question 20

What is resorptive modelling and where does it occur?

Answer 20

Bone is removed to alter the shape of primary bone by osteoclasts
On the periosteal surface as well as in the region previously occupied by epiphyseal cortex

Question 21

What is the Poisson effect in bone?

Answer 21

The Poisson effect is defined as a material’s tendency to expand in a direction perpendicular to the compression direction.

As bone shortens, the Poisson effect can be seen as it is strained and bulges in the transverse direction, implying the presence of tensile stresses about the circumference of the cylindrical long bone.

Question 22

Bone is optimised to accommodate strain rates of approx?

Answer 22

0.1/s (which is about 10x the rate induced by brisk walking)

Question 23

Does a smaller fracture gap or larger fracture gap have greater strain potential?

Answer 23

Smaller fracture gap - these concentrate strain, whereas large fracture gaps result in lower strain environment within the fracture.

Question 24

Granulation tissue can withstand what % of deformation?
- fibrocartilage %?
- Bone?

Answer 24

Granulation: 100%
Fibrocartilage: 10-15%
Bone: 2%

Question 25

There are two approaches to interfragmentary stain in fracture repair, how do they differ?

Answer 25

Goal 1: Eliminate interfragmentary strain through anatomic reduction, compression and absolute stability
OR
Goal 2: Maintain a low strain environment through bridging techniques and implants that allow relative stability - maintain a somewhat large interfragmentary gap to distribute strain among the fracture fragments

Question 26

What are the four functional bone envelopes?

Answer 26

cancellous, endocortical, intracortical, and periosteal

Question 27

Define buckling?

Answer 27

Bending induced by axial compressive loading.

Question 28

What is Newton’s first law of motion?

Answer 28

An object will continue in the current state of motion unless a force is applied

Question 29

What is Newton’s second law of motion?

Answer 29

F=ma (a force acting on a body will produce acceleration that is inversely proportionate to mass)

Question 30

What is Newton’s third law of motion?

Answer 30

Every force that one body applies to a second body has an equal but opposite reaction force applied to the first body

Question 31

Define resultant force:

Answer 31

Net force acting on a body (magnitude and direction)

Question 32

A change in momentum is called what?

Answer 32

An impulse

Question 33

Contact primary bone healing occurs when what two conditions are met?

Answer 33

Fracture gap < 0.01mm
Strain <2%

Question 34

Gap primary bone healing occurs when what two conditions are met?

Answer 34

Gap <1mm
<2% strain

Question 35

Secondary bone healing’s hallmark is what?

Answer 35

Callus
5 steps:
inflammation -> intramembranous ossification -> soft callus formation -> hard callus formation ->bone remodelling

Question 36

What is Wolff’s law?

Answer 36

Bone in a healthy animal will adapt to the loads under which it was placed

Mechanotransduction via piezoelectricity drives resorption with less stress and bone formation with more.