Lets Go Flashcards
(25 cards)
Hip replacement
- structure of hip replacement
- material selection
- femoral stem, femoral head, acetabular cup
Femoral stem: titanium - easy to manufacture
- strong so can withstand load of bone
- ductile so avoids catastrophic fracture
Femoral head: Aluminium oxide - low friction coefficient allows smooth movement of joint
- lightweight
- high compressive strength
Acetabular cup: UHMWPE - flexible and tough
- lightweight
- cheap and easy to manufacture
Thermoplastic vs thermoset
- vulcanisation
Thermoplastic: consisting of long polymer chains not chemically bonded to each other, melts when heat applied
Thermoset: consisting of polymer chains chemically bonded to each other through cross linking, does not melt when heat applied
Vulcanisation is the process by which thermoplastics can become thermoset through cross linking polymers. This creates elastomers.
Four types of materials body reacts with, 2 examples of each
Biotoxic - cadmium, carbon steel
Biointert - tantalum, titanium
Bioactive - high density hydroxyapatite
Bioresorbable - porous hydroxyapatite
Structure of teeth (inner to outer)
- pulp: nutrition, sensory information
- dentine: core structure, tough and resilient
- enamel: wear resistant
- cementium: anchorage site for periodontal ligament
- periodontal ligament: absorbs and distributes loads
Four types of teeth fillings and their processes
Golden crowns:
- cast made of teeth, golden crown made in dental lab, crown cemented onto tooth
Amalgams
- metal particles mixed with mercury and packed into teeth, amalgam sets
Composite resin
- photo curable polymer mixed with a filler and applied to teeth, resin is cured with a coupling agent (UV light)
Glass ionomer cement
- polyacrylate mixed with glass particles, mixture applied to teeth and sets
Positives and negatives of each type of filling
Golden crowns
+ves: corrosion resistant, strong
-ves: expensive, takes a long time
Amalgams:
+ves: cheap, strong
-ves: black staining of teeth, more corrosive than gold
Composite resin and glass ionomer cement:
+ves: colour matched to teeth, will not corrode
-ves: less durable than metal fillings
Corrosion
- redox reaction
- electrochemical series
- corrosion in the body
- process by which a metal, the anode, is oxidised, and oxygen, the cathode, is reduced.
- materials want to reach the lowest possible free energy, alloys, hydroxides, and sulphides have negative free energy so are thermodynamically favoured
- tissue fluid provides environment for the corrosion of metal as it contains oxygen, proteins, ions, chlorides and hydroxides
Tissue engineering:
- definition
- aim
- procedure
- the repair, regeneration or replacement of diseased tissues to their original state and function
- aims to generate natural tissues in sufficient quantity and of desired shape that are structurally and functionally equal to the original tissue
- stem cells are taken from patient, cells are differentiated and proliferated, cells are placed on a scaffold, and extracellular matrix is layed, the tissue is implanted in the body
Hydrolysis
- reaction
- biodegradable materials
Ester + water <> acid + alcohol
Examples of esters:
- polyglycolide - gives glycolic acid
- polylactide - gives lactic acid
Skin structure and four functions
Roles: protection, thermal control, maintain shape, sensor system
Epidermis:
- protective layer
- contains keratinocytes and keratin
- avascular
Dermis
- contains collagen and fibroblasts
- vascularised
Hypodermis;
- contains fibrous and fatty (adipose) tissue
Four ways body looses heat
- conduction
- convection
- radiation
- evaporation
Phases of wound healing
Inflammatory phase: blood vessels contract and clots form
Proliferative phase: fibroblasts lay collagen bed forming new capillaries and epithelial layers
Remodelling: new collagen forms and tensile strength increases, scar tissue lacks elastin
Degrees of burns
First degree: destroys epidermis
Second degree: destroyed epidermis and some of dermis
Third degree: all layers of skin destroyed
Fourth degree: underlying tissue damaged
Skin grafts
- when it is needed
- procedure
- negatives
Needed when full thickness of skin is lost in an area grater than 4cm diameter
- healthy skin is cut from body
- skin is meshed to cover a larger area
- skin is placed on burnt area
-ves: can cause infection, can only take a limited amount of skin
Skin engineering
- process
- negatives
- products on the market
- patient has biopsy
- skin is peeled and separated into epidermis and dermis
- keratinocytes and fibroblasts are separated
- cells are transferred onto a scaffold
- extracellular matrix is laid down
- skin is applied to wound
-ves: skin engineering takes 3 to 4 weeks - open wound is prone to infection
Integra: plastic film to protect wound with collagen layer for cells to grow on to - victims cells grow new dermis, top layer is removed and sheet of epithelial cells is applied so new epidermis forms
Material properties definitions:
- stiffness
- strength
- toughness
- hardness
Stiffness: measures resistance to elastic deformation
Strength:
- yield: stress beyond which hooks law fails
- fracture: stress where material breaks
Toughness: total energy a material can absorb before fracture
Hardness: ability to resist penetration
Collagen-elastin model
- J shaped curve explanation
- anisotropic definition
- viscoelastic definition
- at low stress, elastic modules is small but at high stresses elastic modulus is large
- this means material is flexible at low stresses but not at high stresses so that it can retain its shape. Also elastic energy energy stored is less than a Hookean material so less energy for cracking
- when skin is stretched, elastin supports low stresses while collagen unfolds, at high stresses collagen is unfolded so supports material with its high elastic modulus
- anisotropic: elastic behaviour depends on what direction you pull fibres
- viscoelastic: material deforms but returns to original shape with time
How is fracture prevented in materials
- increasing toughness
- preventing strain energy from reaching crack tip by viscous effects
- preventing stress concentrations by convoluted fracture areas
- extending fibres across fracture
- keeping structure small so sufficient crack energy cannot be stored
Bone functions
- protection: skull
- sound transmission: auditory ossicles
- blood production: marrow
- mineral store: calcium and phosphorous
- motion and support
-Types of bone
-Types of fracture
- remodelling
- compact (cortical), trabecular (cancellous), woven and lamellar
- complete or incomplete, linear, transverse or oblique, spiral, comminuted
- Wolffs law: the re-arrangement of trabecular bone in response to load applied; more bone is produced along stress lines of loading
Process of healing fractured bone
<5 days
- blood clots at site, fibroblasts lay scaffold
4 days - 3 weeks
- callus of cartilage fills gaps, fibroblasts and osteoblasts begin remaking bone
3 - 12 weeks
- bony callus forms (woven bone)
12 weeks - years
- woven bone remodels into lamellar bone
Factors affecting properties of bone
- composition: high mineral content increases stiffness but lowers toughness
- humidity: dry bone is stiff and tougher
- type of load: stronger in tension than compression, weak in torsion
- rate of loading
Describe the collagen helix
- collagen is a coil which itself is made up of coils
- collagen twist reverses at a higher level to give a tighter structure and greater tensile strength
- five collagen molecules wind in third level helix to form a microfibril
- microfibrils wind together to form fibrils, made stronger by cross-linking
-fibrils pack together to form fibres
-fibres pack together to give fascicles - fascicles form tendons
Describe the stress-strain curve of tendon
- toe region shows stiffness increases as collagen crimps straighten out
- linear, elastic region shows collagens elastic behaviour until yield point
- plastic region shows irreversible extension as fibrillar structure is disrupted
- tendon fails
