Biomechanics of cartilage & synovial fluid

Question 1

KINEMATICS

Answer 1

describes the motion of objects with no reference to the mass of those objects nor the forces which cause the motion.
- Combination of rotation and translation occurs in most synovial joints

Question 2

Instantaneous Centre of Rotation (ICR)

Answer 2

“the theoretical axis (centre) of rotation at a specific joint position”
When rotation and translation occur simultaneously, the axis (CoR) is moving.
B indicates that the ICR moves through the range of joint motion.

Question 3

KINETICS

Answer 3

(‘kinesis’ = to move) the study of motion and its causes (forces)
Force (linear)
Moment (angular)
Friction - Frictional forces resist the movement of an object
W = weight of object
N = reaction force of the surface F = applied force
Ff = frictional force
μ = coefficient of friction

• where M = f x d(moment arm)
• compression increases force across the joint surface.
• Co-contraction around joint increases pressure

Question 4

Features of synovial joints

Answer 4

1. Graduated flexibility
   Increased stiffness as move from Articular cartilage
   > Spongy bone > compact bone and diaphysis
2. Variable Bearing Area (contact area where force goes around)
   - at any point in time, differnt contact area that affects stress
3. Articular cartilage
4. Synovial fluid

Question 5

Hyaline

Answer 5

limitted ability to repair
- low metabloic tissue
- low cellular density
avascular
- type 2 collagen

Question 6

DRCTP - achilles tendon

Answer 6

collagen type 1

Question 7

Fibrocartilage

Answer 7

combo of connect tissue proper and hyaline

Question 8

Hyaline Cartilage vs

Fibrocartilage

Answer 8

cells = H:chondrocytes F: chondrocytes & fibroblasts
H2O = H:70-85% F:60-70%
collagen = H:~15-20% by mass Type II (~75% dry weight)
F: ~15-25% by mass Type I (slightly >75% dry)
Fibre orientation:
Hylaine = S – parallel
M – random
D - perpendicular
Fibro =
Circumferential (AF)
Circumferential & radial (meniscus)
proteoglycan = H: < 10% (~25% dry weight)
F: 1-2% (<1% dry weight)

Question 9

Collagen structure:

Answer 9

the ultimate strength of tension-bearing elements depends on the substance they are made of and their cross-sectional area, while the shape of their cross-section is immaterial and is usually the simplest possible.”
“the safety factor increases with subdivision” - occurs in collagen
“flexibility also increases sharply, being directly proportional to the thread number but inversely proportional to the fourth power of the radius of each thread.”

Question 10

Proteoglycans:

Answer 10

*a protein core with side chains of chondroitin sulfate and keratan sulfate.

neg. charged ions attract to pos. ions in synovial fluid > As more move into cartilage, H2O will follow
* resists compression

Question 11

A proteoglycan aggregate =

Answer 11

proteoglycans (green) bound to a hyaluronic acid backbone.

Question 12

Cartilage zones

Answer 12

Superficial = coll. aligned parallel to surface and more coll. than propteogylcans
Middle = more irregular coll. arrangement and increase proteoglycans
Deep = vertical coll. arrangement and collagen anchoring into calcified cartilage and subcondral bone

• Zones reflex different stresses cartilage is exposed to

Question 13

Biomechanical behaviour of cartilage

Answer 13

Biphasic model

1. Fluid phase - water of synovial fluid
2. Solid phase - collagen, proteoglycans

“solids that have time-dependent mechanical behaviors, because of a fluid-like component are viscoelastic” - therefore behaviour is different due to ROA.
• creep
• stressrelaxation

Question 14

Creep (constant load)

Answer 14

behaviour of tissue under constant load

• fluid moves out of hyaline cart. into synovial fluid = see deformation
• initially quite rapid then reaches steady state = balance btwn internal and external fluid.

Question 15

Stress relaxation (constant strain)

Answer 15

stress increase > fluid flow > decrease in stress in fluid as it moves.

Question 16

Compression of the solid phase is resisted by:

Answer 16

1. Viscous drag - fluid moves through proteoglycan and collagen molecules which creases friction
2. Electrostatic repulsion - resist cartilage compression and how much fluid flows
3. Osmotic swelling - maintain water in tissue
   * Want compressive fluid to be resisted by fluid phase

Question 17

Comparison of hyaline cartilage and fibrocartilage mechanical properties:

Answer 17

collagen: H- Type 2 F- T1
proteoglycans: H: <10% F:1-2%
H2O: H:70-85% F: 60-70%

Question 18

Function?

Answer 18

Bearing = a part of a machine that allows one part to rotate or move in contact with another part with as little friction as possible
Binding = the action of fastening or holding together

Question 19

Articular Surfaces in Contact

Answer 19

rough on surface which creates friction

Question 20

Mixed lubrication

- Fluid-film lubrication

Answer 20

= thin film of lubricant between the articular surfaces in contact
> load is supported by the pressure developed in the fluid
> Fluid film needs to be >3x surface roughness
> Viscosity is important (hyalonurate)

Question 21

Mixed lubrication

-Boundary layer lubrication

Answer 21

= surfaces are protected by a boundary lubricant (lubricin) > Prevent contact of articular surfaces

Question 22

friction

Answer 22

“the friction between the articulating cartilage surfaces is mainly dependent on the solid phase interactions and the shearing between the surfaces, the coefficient of friction between the articulating cartilage surfaces can be maintained at a very low level as long as the fluid phase load support is maintained at a high level.”

Question 23

ideal

Answer 23

dynamic loading rather than constant loading conditions & relative motion between articulating surfaces and
>Keeps cartilage in the fluid load support phase