Biomechanics - Skeletal Tissue Flashcards

Question

What happens to bone in response to exercise?

Answer 1

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

Answer 2

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

Answer 3

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

Answer 4

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

Answer 5

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

Answer 6

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

Answer 7

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

Answer 8

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

Answer 9

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

Answer 10

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

Answer 11

It is viscoelastic and displays creep and stress relaxation

Answer 12

Synovial joints: 0.02 Artificial: 0.03-0.1

Answer 13

Elastohydrodynamic lubrication Boosted lubrication Boundary lubrication

Answer 14

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

Answer 15

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

Answer 16

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

Answer 17

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

Answer 18

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.

Answer 19

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.

Answer 20

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

Answer 21

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

Answer 22

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

Answer 23

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

Answer 24

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.