Chapter 39: Bone Biomechanics and Fracture Biology Flashcards
What is a moment arm?
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.
Formula for stress?
Force / area
Cauchy described what?
Stress distribution within a body. (Cauchy stress)
Formula for strain?
Displacement (change in gap) / length of total bone
Measure of deformation of a structure in response to stress.
Area under the under the linear region of the stress/strain curve up to the proportional limit is what?
Resilience:
Measure of a materials ability to absorb energy when loaded and then fully recover, releasing the stored energy when load is removed
Area under the entire stress-strain curve up to failure point is called the?
toughness - measure of the energy absorbed by the material prior to failure
What is ductility?
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)
Peak stress prior to failure is called the ?
Ultimate strength of the material - the peak/high point on the stress/strain curve
What is happening in the ‘toe region’ of a stress/strain curve for a tendon/non-linear material?
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.
Most flat bones are formed by?
Intramembranous ossification (Skull or scapula)
What pathway is essential for osteogenic differentiation?
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
What are the 5 zones of endochondral ossification?
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)
Proliferation chondrocytes have which appearance?
Stacked coin appearance - matrix collagen type 2
Hypertrophic zone chondrocytes have what appearance?
Hypertrophied
Which type of collagen predominates in hypertrophy stage?
Collagen X (10) - appearance leads to local tissue hypoxia, VEGF expression by hypertrophied chondrocytes and vascular invasion via angiogenesis
Why are Salter-Harris fractures so common in the hypertrophic zone?
Zone contains little matrix and is structurally the weakest component of the epiphyseal plate - Predisposing this region to fracture in young animals
What happens in the zone of calcification?
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
What happens in the zone of ossiciation?
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
What is formation modelling and where does it occur?
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
What is resorptive modelling and where does it occur?
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
What is the Poisson effect in bone?
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.
Bone is optimised to accommodate strain rates of approx?
0.1/s (which is about 10x the rate induced by brisk walking)
Does a smaller fracture gap or larger fracture gap have greater strain potential?
Smaller fracture gap - these concentrate strain, whereas large fracture gaps result in lower strain environment within the fracture.
Granulation tissue can withstand what % of deformation?
- fibrocartilage %?
- Bone?
Granulation: 100%
Fibrocartilage: 10-15%
Bone: 2%
There are two approaches to interfragmentary stain in fracture repair, how do they differ?
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
What are the four functional bone envelopes?
cancellous, endocortical, intracortical, and periosteal
Define buckling?
Bending induced by axial compressive loading.
What is Newton’s first law of motion?
An object will continue in the current state of motion unless a force is applied
What is Newton’s second law of motion?
F=ma (a force acting on a body will produce acceleration that is inversely proportionate to mass)
What is Newton’s third law of motion?
Every force that one body applies to a second body has an equal but opposite reaction force applied to the first body
Define resultant force:
Net force acting on a body (magnitude and direction)
A change in momentum is called what?
An impulse
Contact primary bone healing occurs when what two conditions are met?
Fracture gap < 0.01mm
Strain <2%
Gap primary bone healing occurs when what two conditions are met?
Gap <1mm
<2% strain
Secondary bone healing’s hallmark is what?
Callus
5 steps:
inflammation -> intramembranous ossification -> soft callus formation -> hard callus formation ->bone remodelling
What is Wolff’s law?
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.
