Unit 2: Biomechanics of Skeletal Tissue Flashcards
What .are the principal types of skeletal tissue?
Epithelial tissue
Connective tissue
Muscle tissue
Nervous tissue
What are the 4 types of bone shape?
Long bones
Short bones
Flat bones
Irregular bones
What cells is bone made of?
Osteocytes
What is the non-cellular component of bone?
Strong collagen fibres embedded in ground substance (jelly-like matrix) - flexible but resist stretching
What minerals make of the inorganic component of bone?
Calcium & phosphate (in the form of calcium phosphate crystals) - responsible for hardness & rigidity
What are the 2 types of bone tissue?
Cortical (compact) bone
Cancellous (spongy) bone
What is cortical bone?
The outer dense layer of bone
Where is cancellous bone found?
Inner part of short, flat & irregular bones - also lines inner surfaces and makes up greater part of metaphyses and epiphyses
What is the structure of cancellous bone?
Mesh-like structure (spaces in the mesh contain red bone marrow)
What is the basic structural unit in cortical bone?
Haversian system
What are the layers in cortical bone tissue called?
Lamellae
What is contained within the haversian canals in cortical bone?
Blood vessels & nerve fibres
What are the cavities between lamellae called?
Lacunae
What is contained within lacunae?
Osteocytes
What links haversian canals and lacunae?
Canaliculi
What surrounds each haversian system?
Cement-like ground substance
What is the weakest point of bone’s microstructure and why?
Ground substance because it contains no collagen fibres
What are the basic structural units in cancellous bone?
Trabeculae
What is the main difference between the structural units of cortical and cancellous bone?
Trabeculae (cancellous) do not contain haversian canals
Why do trabeculae not contain haversian canals?
Not needed in cancellous bone as blood vessels pass through the marrow filled spaces between the lattice work of trabeculae
What is the difference between tension & compression?
Tension = acting to stretch material Compression = acting to compress material
Define stress
Force per cross-sectional area
Define strain
The change in length divided by the original length (measure of the deformation a material has undergone)
What are the units of strain?
Strain has no units (it is a ratio)
In what region is stress directly proportional to strain?
Elastic region
What is the point at which a bone fractures called on the stress-strain graph?
Ultimate strength/strain
What does Young’s Modulus describe?
How flexible or stiff a material is
If Young’s Modulus is small does this mean the material is stiff or flexible?
Flexible
How is Young’s Modulus (E) calculated?
E = stress/strain
What are the 5 types of loading?
Tension Compression Bending Shear Torsion
What is shear loading?
When 2 forces at in opposite direction to cause layers within a material to slip or shear
Give 2 examples of shearing in orthopaedics?
- Screw in fracture fixation plate
2. Bone cement being sheared by hip prosthesis & bone
Is human cortical bone weaker or stronger in shear than in tension & compression?
Shear (although shear fractures are rare)
Name the 2 common types of bending
Cantilever bending
Three point bending
Give an example of a bending fracture
‘Boot top’ fracture in skiers
When do torsional loads occur?
When a bone is twisted about its longitudinal axis
What characteristic appearance to fracture have that are caused by torsional loads?
Spiral
In torsional loads on a bar where are stress & strain the greatest?
Outer surface
Which type of load are long bones designed to be resistant to?
Torsional loads (thick outside hollow inside)
Where is the most common site of a fracture caused by a torsional load?
Tibia (distal as smaller cross-sec area)
How do muscles alter the stress distribution in a bone?
Produce overall compressive loading (as bones stronger in compression)
Why are tired athletes more likely to fracture a bone?
Muscles are fatigued and therefore unable to control the distribution of stress within their bones
What is Wolff’s law?
Bone is laid down where needed and resorbed where not needed
What is bone atrophy?
When inactivity leads to the resorption of bone tissue
When can bone remodelling cause a problem in fracture healing?
If plate not removed after fracture has healed bone will weaken (stress shielding)
Points at which screws are inserted there will be bone hypertrophy as these sites carry extra load
What is a fatigue fracture?
Fracture resulting from repeated application of a load that is smaller than the ultimate strength of the bone
In which age group are greenstick fractures more common in and why?
Children - bones are less brittle than adults are they contain greater proportion of collagen
What is a greenstick fracture?
Incomplete fracture whereby one side of the bone is bent and the other side is buckled (caused by bending or torsional loads)
What causes the large reduction in the amount of cancellous bone tissue with age?
Thinning of longitudinal trabeculae
Resorprtion of some transverse trabeculae
What are the 3 types of cartilage?
Hyaline cartilage
Elastic cartilage
Fibrocartilage
Where is hyaline cartilage found?
Covers the articular surfaces of bones in synovial joints also forms tip of nose
Where is elastic cartillage found?
Eternal ear & epiglottis
Where is fibrocartilage found?
Symphysis pubis & intervertebral discs
What is articular cartilage?
Form of hyaline cartilage found on the articulating ends of bones in synovial joints
What is the purpose of articular cartilage?
Cushions bones while providing a mooth lubricated bearing surface that limits contact stress and wear
Describe the structure of articular cartillage
Organic matrix (mainly collagen) fibrils) interspersed with chondrocytes and a conc solution of proteoglycans
Where are chondrocytes most concentrated in articular cartilage?
Deeper layers adjacent to the bone
Where are proteoglycans most concentrated in articular cartilage?
Middle portion
What are the 3 structural zones of articular cartilage?
Superficial tangential zone
Middle zone
Deep zone
What is the structure of the superficial tangential zone?
Collagen fibrils tightly woven into sheets arranged parallel to the articular surface and chondrocytes are are oblong with their longitudinal axis aligned parallel to the articular surface
What is the structure of the middle zone of articular cartilage?
Collagen fibrils are arranged more randomly but still broadly parallel to articular surface. They are less densely packed to accomodate the high conc. of proteoglycans and the chondrocytes are circular and randomly distributed
What is the structure of the deep zone of articular cartillage?
Collagen fibrils arranged in larger fibre bundles that are anchored in underlying bone tissue. Chondrocytes are arranged in loose columns aligned perpendicular to the ine dividing the articular cartilage and underlying bone
What lies below the deep layer in articular cartilage?
Thin layer of calcified cartilage (gradually merges into bone)
What is the interface between the articular cartilage and calcified cartilage beneath called?
Tidemark
What mechanical behaviour does articular cartilage exhibit?
Viscoelastic
What is creep?
When viscoelastic material is subjected to constant load. Initial rapid deformation is followed by a slowly increasinf deformation.
Describe how creep occurs within articular cartilage
Fluid is rapidly forced out of the cartilage then it reaches equilibrium. Fluid flow ceases completely and the applied load is borne entirely by the solid matrix. At equilibrium majority of the fluid still remains in the articular cartilage
What is stress relaxation?
When a viscoelastic material is kept at constant deformation the stress is reduced over time
What is the difference between the coefficients of friction between synovial joints and artificial joints?
Synovial joint = low (0.02)
Artificial joints = higher (0.03-0.1)
What 2 factors is how lubrication is brought about dependent on?
Magnitude of the load
Length of time the load is maintained
What are the 3 main types of lubrication?
Elastohydrodynamic lubrication
Boosted lubrication
Boundary lubrication
What is elastohydrodynamic lubrication?
When 2 surfaces, one of which is deformable, are ubricated by a film of fluid as they move relativ eto one another.
Do the 2 surfaces touch in elastrohydrodynamic lubrication?
No (separated by fluid#)
What is it called when 2 surfaces slide over each other?
Hydrodynamic lubrication
What is it called when 2 surfaces move closer together
Squeeze film lubrication
Describe hydrodynamic lubrication?
When 2 surfaces slide over one another they form a wedge of fluid. As surfaces slide a lifting pressure is generated
Describe squeeze film lubrication
Cushions and protects surfaces but if high loads are maintained lubricant will eventually be depleted
Why does a deformable surface reduce pressure on a joint in elastohydrodynamic lubrication?
Deformation of the surface increases the area over which the load is disributed
What problem does boosted lubrication solve?
That if to lubricated surfaces are forced together over a period of time eventually the lubricant will be completely depleted (squeezed out)
Describe boosted lubrication
As gap between 2 articulating surfaces decreases the resistance to the sideways flow of the lubricant becomes greater than the resistance of flow of the small molecules into articular cartilage. Water molecules move and leave a thick viscose gel (enriched lubricant)
What problem does boundary lubrication solve?
Fluid of film present between two surfaces is not thick enough to prevent contact
Describe boundary lubrication
Lubricant molecules attach themselves chemical to surfaces creating a boundary layer
How does a boundary layer help prevent wear in joints?
Has a low shear strength and therefore lower friction than the bare surfaces
What is the name of the protein from the synovial fluid that coats articular cartilage?
Lubricin
What is a tendon?
Soft connective tissue that connects muscle to bone
What is a ligament?
Soft connective tissue that connects bone to bone
What cells are found in tendons & ligaments?
Fibroblasts
How are collagen fibres arranged in tendons & why?
Arranged completely in parallel as they need to withstand loads in one direction only
How are collagen fibres arranged in ligaments & why?
Not arranged completely in parallel as although need to with large large loads mainly in one direction they also need to withstand small loads in other directions
What mechanical behaviour do tendons & ligaments exhibit?
Viscoelastic behaviour
What is the anatomical position of the ACL?
Centre of the knee joint with one end joined to the femur and the other to the tibia
How far can the ACL be elongated before it fails completely?
Around 7mm
