Biomechanics unit 2 - biomech of skeletal muscle tissue deck 5

Question 1

When does elastrohydrodynamic lubircation occur ?

Answer 1

When two surfaces, one of which is deformable, are lubricated by a film of fluid as they move relative to one another.

Question 2

What does the fluid film do in elastrohydrodynamic lubrication ?

Answer 2

It completely separates the two surfaces so that they do not actually touch.

Question 3

In elastrohydrodynamic lubrication what does the friction between the articular surfaces depend on ?

Answer 3

The fluid and the shape of the gap between the two surfaces.

Question 4

What are the two ways in which surfaces can move relative to one another in elastrohydrodynamic lubrication ?

Answer 4

They can slide over each over - this is called hydrodynamic lubrication (Fig. B)

They can move closer together - this is called squeeze film lubrication (Fig. A).

(The articular surfaces have been simplified to two rigid surfaces in the pic (for true elastohydrodynamic lubrication one surface is deformable). The solid arrows show the direction of the load bearing down on the top surface, and the hollow arrows show the relative movement of the two surfaces. The series of parallel arrows above the top surface show how the load is distributed over the surface - the pressure or stress distribution.)

Question 5

Where is the pressure or stress distribution greatest in elastohydrodynamic lubrication (both squeeze and hydrodyanmic)?

Answer 5

In the centre below where the load is applied.

Question 6

Describe how hydrodyanmic lubrication occurs

Answer 6

• It occurs when two surfaces slide over one another, forming a wedge of fluid.
• As the surfaces slide, a lifting pressure is generated as the motion drags the viscous lubricant into the narrowing gap between the surfaces. (shown in fig.B)

e.g. This process causes cars to skid on wet roads.

Question 7

Describe how Squeeze film lubrication occurs and its function

Answer 7

• It occurs when two surfaces are forced together. The viscous lubricant will not instantaneously be squeezed out from the gap between the two surfaces (fig.A)
• Squeeze film lubrication therefore acts to cushion and so protect the surfaces

Question 8

If the high loads are maintained, what will eventually happen to the fluid film between the two surfaces in squeeze film lubrication ?

Answer 8

The lubricant will eventually be depleted and the two surfaces will come into contact.

Question 9

In elastrohydrodyanmic lubrication - When one or both of the surfaces are relatively soft then what will happen to the surfaces when a load is applied and give an example of surfaces this would occur in ?

Answer 9

• They will deform
• This occurs in synovial joints

Question 10

What does deformation of soft surfaces in elastohydrodynamic lubrication result in ?

Answer 10

It increases the area over which the load is distributed (P = F / A)

Question 11

Describe how the surfaces of synovial joints deform when a load is applied, in elastohydrodyanmic lubrication

Answer 11

The relatively soft articular surfaces deform as they are moved over one another as the joint is flexed or extended or as they are forced together when the joint is fixed

Question 12

Describe the process of hydrodynamic and squeeze film action between a flat surface and a curved bearing for both rigid and deformable bearings.

Answer 12

For the rigid surfaces the pressure distribution is small (area) and high pressures are present (Fig A&B).

For the softer surfaces (deformable) the pressure distribution is increased (area) over the deformed surfaces (Fig C&D). As a consequence the magnitude of the pressure is decreased and the film remains relatively thick - this is elastohydrodynamic lubrication.

Question 13

If two lubricated surfaces are forced together over a period of time eventually the lubricant will be completed depleted, it will be squeezed out what mechanism in synovial joints combats this problem?

Answer 13

Boosted lubrication

Question 14

What does the mechanism of boosted lubrication rely on ?

Answer 14

The permeability of articular cartilage

Question 15

Describe how boosted lubrication occurs

Answer 15

The surface of articular cartilage is only permeable to molecules below a certain size, such as water and other small molecules.

As the size of the gap between the two articulating surfaces decreases, the resistance to the sideways flow of the lubricant (i.e. L& R spreading across the surfaces) eventually becomes greater than the resistance of flow of the small molecules into the articular cartilage (Figure 32). ==> flow of these small molecules into the articular cartilage occurs.

These small molecules include water molecules that make up the solvent component of the synovial fluid. With this solvent component removed a thick viscose gel is left behind. This acts as an enriched lubricant which is capable of supporting large loads and thus acts to keep the two articular surfaces apart.

Question 16

If loads are large enough and sustained for long enough to deplete the fluid film completely then what mechanism is still present to protect the articular surfaces ?

Answer 16

Boundary lubrication

Question 17

In boundary lubrication the film of fluid present between two surfaces is not sufficiently thick to prevent contact between the two surfaces - T or F ?

Answer 17

True - it instead forms a boundary layer, which prevents direct contact

Question 18

Describe boundary lubrication

Answer 18

• This is where lubricant molecules attach themselves chemically to the surfaces, creating a boundary layer.
• The boundary layer has a low shear strength and therefore offers a lower friction than the bare surfaces. This is akin to waxed floors and Teflon frying pans.

Question 19

What forms the boundary layer in synovial joints?

Answer 19

• In human synovial joints, the surface of articular cartilage is coated with protein from the synovial fluid called, appropriately, lubricin (Fig).
• This is very effective in reducing joint friction when loads are sustained for long enough to deplete the fluid film.

Question 20

Define what tendons and ligaments are

Answer 20

They are dense fibrous connective tissues

Question 21

What do tendons and ligaments connect together ?

Answer 21

• Tendons connect muscles to bones
• Ligaments connect bones to bones

e.g. in the knee joint the patellar tendon (also known as the quadriceps tendon) connects the quadriceps muscles to the patella and the patellar ligament connects the patella to the tibia

Question 22

What are tendons and ligaments comprised of ?

Answer 22

Like other connective tissues, they contain relatively few cells, called fibroblasts, which are embedded in a matrix composed largely of collagen fibres.

Question 23

Describe the arrangement of collagen fibres in tendons and explain why they are arranged this way

Answer 23

The collagen fibres are arranged completely in parallel (Fig. A) as they need to withstand large loads in one direction only.

Question 24

Describe the arrangement of collagen fibres in ligaments and explain why they are arranged this way

Answer 24

• In ligaments the collagen fibres are not arranged completely in parallel (they are arranged almost parallel) (Fig. B) and in some ligaments, such as the cruciate ligaments in the knee, the fibres are branched and interwoven.
• This is because, although ligaments need to withstand large loads mainly in one direction, they also need to withstand smaller loads in other directions

Question 25

How are the fibroblasts arranged in tendons and ligaments ?

Answer 25

They are elongated along the direction of the collagen fibre

Question 26

What are the overall basic mechanical properties of tendons and ligaments ?

Answer 26

• They are viscoelastic - just like articular cartilage
• They have to be able to withstand large tensile forces and be very flexible

Question 27

Describe why tendons need to be able to withstand large tensile forces and also be very flexible

Answer 27

They need to withstand the large tensile forces exerted by muscles during contraction and they need to be flexible enough to bend around the surfaces of the bones as joints move

Question 28

Describe why ligaments need to be able to withstand large tensile forces and also be very flexible

Answer 28

They need to be strong enough to resist the forces that could wrench joints apart and they need to be flexible enough to allow joints to move normally.

Question 29

Describe the difference in the forces tendons and ligaments experience in contrast to articular cartilage

Answer 29

Tendons and ligaments need to withstand large tensile forces whereas articular cartilage must withstand high compressive loads as the bones in the joints are forced together

Question 30

Because tendons and ligaments are viscoelastic what 2 behaviours do they display because of this?

Answer 30

• Creep and stress relaxation
• However, the mechanisms that result in these properties are quite different from those in articular cartilage.

Question 31

Appreciate this:

The most interesting mechanical behaviours of tendons and ligaments are those that are observed with the tendon or ligament still in position. In particular, injury mechanisms are very important. The mechanical behaviour of all the different tendons and ligaments in the body can not be covered here, so as an illustrative example we will look in detail at the anterior cruciate ligament.

Question 32

What is the function of the ACL ?

Answer 32

It helps keep the knee joint together

Question 33

What is the position of the ACL within the knee joint ?

Answer 33

It is positioned in the centre of the knee joint with one end joined to the femur and the other to the tibia.

Question 34

If a tensile load is applied to pull the femur and tibia apart, by how much can the ACL be elongated (joint displacement - the gap between the articular surfaces) before the ACL fails completely ?

Answer 34

7mm

Question 35

Roughly what is the force required to break the ACL ?

Answer 35

Around 1000N - roughly equivalent to one and a quarter to one and a half times body weight

Question 36

Up to what joint displacement is within the normal physiological limit in terms of the ACL ?

Answer 36

4mm - within this range the ligament remains undamaged

Question 37

Beyond 4mm of joint displacement what happens to the ACL ligament ?

Answer 37

The collagen fibres are progressively ruptured (up to around 7 mm joint displacement where the ligament completely fails) resulting in progressively more pain and joint instability.