Biomechanics - Skeletal Tissue

Question 1

4 principal types of body tissue?

Answer 1

epithelial
connective
muscle
nervous

Question 2

Function of connective tissue?

Answer 2

Protect and support the body and its organs, connect them together, and transport substances through the body. It is the most abundant and widely distributed bodily tissue

Question 3

4 types of connective tissue?

Answer 3

bone
articular cartilage
tendon
ligament

Question 4

Functions of bone?

Answer 4

Support body structures

Protect delicate structures e.g. heart and lungs

Act as lever arms for movement

Question 5

What are bone cells called?

Answer 5

osteocytes

Question 6

What is the non-cellular organic component of bone composed of?
What percentage of bone does this make up?

Answer 6

Collagen fibres, embedded in a jelly-like matrix called ground substance

Collagen fibres = 95% of non-cellular component, and 25-30% of dry weight of bone

Question 7

Which type of load do collagen fibres resist?

Answer 7

They are flexible, but resist stretching

Question 8

What is the inorganic component of bone composed of?
What benefit does this provide to bone?
What percentage of bone does this make up?

Answer 8

The minerals, calcium and phosphate, in the form of calcium phosphate crystals, which are deposited within the bone matrix

The high content of this inorganic component gives bone its characteristic hardness and rigidity

65-70% if the dry weight of bone

Question 9

2 types of bone and where they are located?

Answer 9

Compact (cortical) - forms the outed layers and has a dense structure

Cancellous (spongy) - forms the inner part of short, flat and irregular bones. In long bones it lines the inner surface and makes up the greater part of the metaphyses and epiphyses. it has a mesh-like structure, between which is red bone marrow

Question 10

Describe the structure of cortical bone? (5)

Answer 10

The basic structural unit is the Haversion System - these are longitudinally organised columns of about 200 micrometers diameter.

In these units, the bone tissue is arranged in lamellae, forming concentric circles around a central Haversion Canal, which contains a neurovascular bundle.

Between lamellae there are small cavities called lacunae, which is where the osteocytes are located.

Each osteocyte is connected to the Haversion Canal and other osteocytes via minute channels called canaliculi, along which nutrients are passed from the blood.

Each Haversion System is surrounded by a cement-like ground substance - the weakest part of the bone’s microstructure, probably because it contains no collagen fibres

Question 11

Describe the structure of cancellous bone?

Answer 11

The basic structural unit is Trabeculae, which are arranged in a latticework of branching sheets and columns.

They consist of layers of tissue arranged in lamellae, with lacunae between these containing osteocytes.

They do not need Haversion Canals, since blood vessels pass through the marrow-filled spaces between the latticework of trabeculae, supplying nutrient to osteocytes through canaliculi

Question 12

What is the ultimate strength of cortical bone in compression, tension and shear?

Answer 12

Compression - 200MN m⁻²

Tension - 130MN m⁻²

Shear - 70MN m⁻²

Question 13

Give an example of a shear fracture?

Answer 13

Shear fracture of the femoral condyle. Shear fractures alone are quire rare

Question 14

Describe shear loading?

Give 2 common examples in orthopaedics

Answer 14

2 forces acting in opposite directions tend to cause layers within the material to slip or shear

A screw being seared by a fixation plate and the bone; and bone cement been sheared by the hip prosthesis and bone

Question 15

Describe bend loading?

2 types?

Answer 15

Loads are applied to a structure that tend to cause the structure to bend

Cantilever - one end of the object is fixed and a load is applied to the opposite end, causing the object to bend

Three-point bending - two forces are applied at each end of the object in the same direction, and another is applied in the middle of the object in a different direction

Question 16

Describe the structure of a bent object?

Answer 16

One side of the object is in compression whilst the other is in tension.

Between the two there is a neutral axis, along which no deformation occurs. In a symmetrical structure this would be in the geometric centre.

Question 17

Give a common bending fracture?

Which side fractures first?

Answer 17

A ‘boot-top’ fracture sustained by skiers - this is the result of three-point bending on the tibia. As the skier falls forward over the top of the ski boot, a force is exerted on the proximal end of the tibia as the distal end of the tibia is fixed in the boot. If the bending force is large enough, the tibia will fracture

The side in tension will fracture first, since bone is stronger in compression than in tension

Question 18

Describe torsional load?

How do torsional fractures occur and what are their appearance?

Answer 18

When the bone is twisted about its longitudinal axis

Usually when a torsional fracture occurs one end of the bone is fixed and the other is twisted. Common in football, rugby and skiing.
Torsional fractures have a characteristic spiral appearance

Question 19

Pattern of stress and strain in an object subject to torsional load?

Answer 19

It is not evenly distributed. There is a neutral axis running through the centre of the object where no distortion occurs. Stress and strain is maximal at the outer surface.

Question 20

How are long bones efficiently designed to resist torsional loads?

Answer 20

They are hollow with strong cortical bone forming the outer layer, which is the most efficient way to distribute the bone tissue. If the same quantity of bone was used to construct a completely solid bone it would be smaller in diameter and less able to resist torsional loads. If the bone was the same diameter but completely solid it would be much heavier. Therefore this maximises the STRENGTH-TO-WEIGHT RATIO

Question 21

Why do torsional fractures of the tibia often occur distally?

Answer 21

The distal cross-section is smaller than the proximal - although the quantity of bone tissue is the same, the distal part is less able to resist torsional loads therefore most liable to fracture

Question 22

What is combined loading?

Why is it more realistic?

Answer 22

When bone is subject to more than one type of load -

This is what happens in reality due to the irregular geometry of bones and the combined action of gravity, muscles and ligaments. Most fractures result from combined loading.

Question 23

Why are tired athletes more likely to suffer fractures than fresh athletes?

Answer 23

When muscles contract they put a load on bones, and they often contract without producing movement in order to neutralise any tensile stress in bones and produce an overall compressive load over the cross-section. This is because bone is stronger in compression than in tension, so by compressing the bone it is less likely to fracture. In tired athletes, the muscles are more fatigued, so they are less able to control the distribution of stress within their bones, making fractures more likely

Question 24

What is Wolff’s Law?

What are the implications?

Answer 24

Bone is laid down where needed and resorbed where not needed

This means bone is constantly remodelled depending on the mechanical demands, with both cortical and cancellous bone constantly being gained or lost

Question 25

What happens to bone in response to exercise?

Answer 25

Bone is subject to increased levels of stress. In response, it lays down more collagen fibres and mineral salts to strengthen the bone

Question 26

What happens to bone in response to sedentary behaviour?

Who is this a particular problem for?

Answer 26

Lack of exercise leads to resorption of bone - bone atrophy - as it is not being subject to stress, so the bone which is surplus to requirements is removed.

This can be a particular problem for those who are wheelchair bound or bedridden for extended periods - once they begin to use their legs again they are prone to easy fractures

Question 27

What is stress shielding?

Answer 27

If a bone is repaired by internal fixation, the plate will carry most of the load rather than that section of bone. If it is not removed soon after the fracture has healed, that area of bone will undergo atrophy and weaken.

Conversely, at points where the screw are inserted, the bone carries a greater load than normal, leading to bone hypertrophy and strengthening

Question 28

How are fatigue fractures caused?

Who does this often occur in?

Answer 28

Bone can either be fractured by application of a single large load too powerful for the bone’s ultimate strength, or several smaller loads less with a lower magnitude than the bone’s ultimate strength - the latter is a fatigue fracture

Bone remodels to match the required load, therefore it is important that the frequency of application of force is well spaced out to allow the bone to recover and remodel If strenuous physical activity is too frequent, this results in fatigue fractures. Muscle fatigue also has a role in this, with the inability to effectively neutralise tensile stress placed on bones

These occur in long-distance runners, fracturing metatarsals, tibia, femoral neck and pubis. Gymnasts can also suffer fatigue fractures of vertebrae

Question 29

Who do greenstick fractures occur in and why?

What is a greenstick fracture?

Answer 29

Children

Their bones contain a greater proportion of collagen, giving them more flexibility than adults’ bones.

It is an incomplete fracture whereby one side of the bone bent, fractured with ruptured periosteum, and the other side is buckled with intact bone and periosteum

Question 30

What happens to bone with age?

Answer 30

In kids, bone tissue formation exceeds resorption as they grow and develop.

Between 35-40y/o bone resorption begins to exceed formation. There is thinning of the compact bone and larger reductions in cancellous - with thinning of the vertical trabeculae and loss of some of the horizontal trabeculae. This results in the bone being more brittle and prone to fracture

Question 31

3 main types of cartilage and main role of each?

Answer 31

Hyaline cartilage - covers articular surfaces of bones in synovial joints (articular cartilage)

Elastic cartilage - forms the auricle and epiglottis among other things

Fibrocartilage - forms pubic symphysis and intervertebral discs

Question 32

2 main roles of articular cartilage?

Answer 32

Cushions bone by absorbing large loads and peaks of shock, and spreading them across larger areas, reducing stress

Provides a smooth, lubricated surface with minimal friction and wear

Question 33

Composition of articular cartilage? (4)

Answer 33

Organic matrix - made of collagen, structured into strong, fine collagen fibrils. This makes up 50-80% of the dry weight and 10-20% of the wet weight of articular cartilage

Collagen fibrils are enmeshed in a solution of proteoglycans - large protein-based molecules. This make up 3-10% of the wet weight of cartilage. Proteoglycans are concentrated in the middle portion of the articular cartilage, and less concentrated in the deep layer adjacent to bone

Cells, called Chondrocytes, are sparsely distributed in cartilage, and account for less than 10% of tissue volume. The cells are more densely packed in the deeper layers, adjacent to bone. Chondrocytes manufacture, secrete and maintain the organic matrix.

Interstitial fluid occupies the spaces in the matrix, and is composed mainly of water. It composes 68-85% of the wet weight of cartilage and 0% of the dry weight

Question 34

Architecture of articular cartilage? (4)

Answer 34

It is divided into 3 zones:

Superficial tangential zone - collagen fibrils are tightly woven into sheets arranged parallel to the articular surface. Chondrocytes are oblong with their longitudinal axes aligned parallel to the articular surface.

Middle zone - Collagen fibrils arranged more randomly but still broadly parallel to articular surface. They are less densely packed to allow high concentration of proteoglycans. Chondrocytes are circular and randomly distributed

Deep zone - collagen fibrils arranged in larger bundles that are anchored to the underlying bone, thereby attaching the articular surface to bone. Chondrocytes are arranged in loose columns aligned perpendicular to the line dividing articular cartilage and the underlying bone

Below the deep zone there is a thin layer of calcified cartilage which gradually merges into the underlying subchondral bone. The interface between the articular cartilage and calcified cartilage beneath it is called the TIDEMARK

Question 35

Mechanical properties of articular cartilage?

Answer 35

It is viscoelastic and displays creep and stress relaxation

Question 36

Coefficient of friction of synovial joints and artificial joints?

Answer 36

Synovial joints: 0.02

Artificial: 0.03-0.1

Question 37

3 types of lubrication in synovial joints?

Answer 37

Elastohydrodynamic lubrication
Boosted lubrication
Boundary lubrication

Question 38

What is elastohydrodynamic lubrication?

What are the 2 subtypes?

Answer 38

This is when 2 surfaces, one of which is deformable, are lubricated by a fluid film as they move relative to one another. The fluid completely separates the surfaces, so they don’t actually touch. The friction, therefore, is largely dependent on the fluid and shape of the the gap between the surfaces.

There are 2 ways in which the surfaces can move relative to one another:

• Hydrodynamic - when they slide over each other
• squeeze film - when they move closer together

Question 39

Describe hydrodynamic lubrication?

Answer 39

When 2 surfaces slide over one another, forming a wedge of fluid. As the surfaces slide, a lifting pressure is generated as the motion drags viscous lubricant into the narrowing gap between the surfaces.

This process causes cars to skid on wet roads

Question 40

What is squeeze film lubrication?

Answer 40

When 2 surfaces are forced together, the viscous lubricant will not instantaneously be squeezed out from the gap, and therefore acts to cushion and protect the surfaces.

However, if loads are maintained, the lubricant will eventually be depleted and the 2 surfaces will come into contact

Question 41

How does the deformable property of articular cartilage relate to lubrication?

Answer 41

When deformation happens, the area over which the load is distributed increases. As a consequence, the pressure is decreased, so the fluid film between the surfaces remains relatively thick, allowing for hydrodynamic and squeeze film lubrication still to occur.

All of this together is called elastohydrodynamic lubrication

Question 42

What is boosted lubrication?

Answer 42

If 2 lubricated surfaces are forced together over a period of time, the lubricant will eventually become depleted.

In synovial joints, the surface of the articular cartilage is only permeable to small molecules, such as water. As the space between the 2 surfaces decreases, the resistance to sideways flow of water increases until it is greater than the resistance to flow into the articular cartilage. Small molecules, including water, which make up the solvent component of synovial fluid then move into the cartilage, leaving behind a thick, viscous gel. This acts as an enriched lubricant, capable of supporting large loads.

Question 43

What is boundary lubrication?

Answer 43

If loads are large enough for a sustained period of time, the fluid film depletes so much that it is not sufficiently thick to prevent contact between the 2 surfaces. Lubricant protein molecules, called Lubricin, then chemically attach to the artuicular surfaces. The molecules have low shear strength, therefore offer lower friction than bare joint surfaces. This is very effective in reducing joint friction.

Question 44

Structure of tendons and ligaments?

Answer 44

They are both dense fibrous connective tissues, containing few cells which are embedded in a matrix composed largely of collagen fibres. The fibroblasts are elongated along the direction of the collagen fibres

In tendons, the fibres are parallel as they need to withstand large loads in one direction only.

In ligaments, the collagen fibres roughly parallel, but not completely. In some ligaments, such as the ACL, they are interwoven and branched. This is because they need to withstand large loads in one main direction, but smaller loads in other directions as well

Question 45

Basic mechanical properties of tendon and ligament?

Answer 45

They are viscoelastic, and therefore display creep and stress relaxation. However, the mechanisms by which this comes about are different than in cartilage.

Tendons are able to withstand large tensile forces exerted by muscles during contraction, and need to be flexible enough to bend round bones as joints move.

Ligaments need to be strong enough to resist the forces which could wrench joints apart, and need to be flexible enough to allow joints to move normally

Question 46

What tensile force is required to rupture the ACL?

How far will it elongate before becoming damaged?

Answer 46

Roughly 1000N

Up to 4mm is normal physiological lengthening. After 4mm there is progressive rupturing of collagen fibres and associated pain.

At 7mm lengthening the ACL will normally rupture

Question 47

How does creep occur in articular cartilage?

Answer 47

Fluid is forced out during the initial rapid deformation. As the fluid within the cartilage diminishes, the rate of expulsion decreases until at equilibrium, when fluid flow ceases and the applied load is borne entirely by the solid matrix.

At equilibrium, the majority of the fluid still remains in the cartilage. Even at the relatively high stress of 1MPa approx 50% of the total fluid content will remain.

In cartilage of 2-4mm it takes 4-16 hours to reach equilibrium

Question 48

How does articular cartilage display stress relaxation?

Answer 48

During the initial deformation, interstitial fluid is forced out as the surface layers are compacted. Because of the large frictional drag assoc w the flow of fluid through the solid matrix, large loads are needed to compress the tissue.

During the stress relaxation phase, the stress required to maintain the deformation is reduced as fluid is no longer being forced out and the fluid within the tissue is redistributed from the least compacted deeper layers to the most compacted surface layers.