Imp Tech U1 Flashcards
What are the 3 factors than need to be considered in implant design?
Structural, kinematic, biocompatibility
What are the components of structural factors that need to be considered in implant design?
Strength, stiffness, lubrication, wear, fatigue
What are the two types of bone and how are they distributed?
Compact; shafts on long bones (prevent deformation under bending/torsional loads)
Cancellous; at the ends of bone for shock absorbing (then cortical underneath to prevent deformation)
What type of loading is bone strongest under (in order of strength)?
Longitudinal loading (the direction bones are naturally loaded) Compressive, tensile, shear
How are trabeculae arranged?
Along lines of greatest stress
What material properties is cortical bone?
Viscoelastic (the faster it is loaded, the stiffer it becomes)
What type of loading can occur at an interface?
Compressive or shear loading
For compressive loading at an interface what happens if the stiffer material is on top, or on the bottom?
On top; the bottom material will bulge out underneath generating shear stresses due to uneven deformation - the load will be evenly distributed across the interface regardless of where it is applied
On bottom; top material will bulge, means that stress will be concentrated at the applied region - more concentrated load means higher interface stress than if stiff material on top
When 2 bars of different stiffness are bonded together and undergo shear loading, how is the stress distributed?
If off different stiffness they deform differently under the load - as joined together gives rise to shear stress
Load transfer occurs at the end regions where the bars join, there is no shear stress in the middle where there is land sharing
What determines how much load is transferred to the implant in the load sharing region?
The relative stiffness of the 2 materials;
- if the bone is stiffer, then less stress is shared so less stress shielding occurs
What geometrical properties affect stiffness?
Length and cross sectional area (S = EA/L)
S= stiffness, E = young’s modulus, A = cross sectional area, L= length
What is used to measure material stiffness under shear or axial loading?
Axial loading = E (Young’s modulus)
Shear (or torsion) = G (Shear modulus)
How is rigidity calculated in axial, bending and torsional loading?
Axial: R = EA
Bending: R = EI
Torsional: R = GJ
How is the amount of load sharing calculated?
The Load on the bone, vs the total load (so the % take by the bone) is calculated by the ratio of the rigidity of the bone vs the total rigidity (of the bone + stem)
Lbone/Ltotal = Rbone / Rbone + Rstem
How are temporary vs permanent implants fixed?
Temporary; screws, nuts, bolts
Permanent; interference fit, bone cement, biological fixation
Why are screws preferable to nuts and bolts for fixing plates?
Screws only need access from one side of the bone (less trauma to tissues)
Nuts and bolts stick out further so cause problems if there isn’t much distance to the skin
What is the purpose of bone cement?
To act as a grout (not an adhesive)
Fills gaps between bone + implant so perfect match is not necessary
What 2 materials are used for biological fixation?
Beads of same material as metallic fixation
OR Hydroxyapatite spray
Aims for bone to grow into surface of implant
What is the downside of biological fixation?
In the long term the coating disappears (1-2 years)
What is galvanic corrosion?
2 electrodes immersed in a liquid which conducts electricity (electrolyte) - electrical current flows between the two electrodes causing a chemical reaction
Causes small areas of material loss (pits or craters)
Is corrosion more or less severe when the electrodes are made of different metals?
More severe, to minimise corrosion use the same metal (some corrosion will still occur due to variation from manufacture)
Alloys (such as steel) are best to use to minimise corrosion
What is fretting corrosion?
When 2 materials rub together and abrade the protective oxide layer
What is crevice corrosion?
Crevices where body fluids become trapped - as lose normal O2 supply become acidic
The high concentration acids corrode metal implant materials
What is a passivisation layer?
A thin layer of metal oxide formed when exposed to a corrosive environment
Seals the underlying material from further corrosion
What can be done to improve corrosion resistance?
- Nitric acid immersion - improves natural passivisation layer
- Titanium nitride coating - decreases corrosion + reduces release of harmful metallic substances into body fluids
What are the 7 body tissue reactions to implanted materials?
- Growth of a thin layer of fibrous tissue
- Local infection
- Body sensitisation to metals
- Inflammation in regions of metal corrosion
- Tissue necrosis
- Immunological reaction
- Tumours
What materials are suitable to make implants (and what aren’t)?
Metals are only suitable material
Ceramics fail in brittle manner
Plastics not used
Does stainless steel have a higher or lower fatigue strength than chrome/titanium?
Lower fatigue strength
also suffers more from local pitting corrosion
How is stainless steel affected by corrosion?
Low carbon content - minimises sensitisation of tissues + corrosion resistance
Prone to crevice corrosion; makes it crack when highly stressed (not suitable for permanent implants)
In what form is stainless steel strongest?
When forged (rather than cast) 4x stronger
Which material is more corrosion resistant; stainless steel or cobalt chrome?
Cobalt chrome (due to chromium)
Less strong but still preferable for permanent implants
What implant surface is cobalt chrome preferred for?
Bearing surface (low coefficient of friction with polyethylene)
Which implant material is most corrosion resistant?
Titanium (also releases less harmful products due to corrosion)
What are the pros and cons of titanium?
Pros:
- lighter than steel/chrome
- less stiff (more compatible with bone)
- higher fatigue strength than steel
Cons
- low wear resistance - not suitable for joint replacements (used for bone fixation plates)
What are fibre reinforced plastics made from?
Stiff, high strength brittle fibres embedded in a flexible resin
What are fibre reinforced plastics mostly used for?
Fracture fixation plates
What is the benefit of fibre reinforced plastics?
Have high fatigue resistance (greater than other metals
