implant mechanics & materials Flashcards
An implant must…
…
- be tolerated with no short term and little long term risk of adverse toxic effects
- relieve pain and allow sufficient mobility
- function w/o failure until no longer required
- predictable outcome reasonably guaranteed
- acceptable cost
Why is a plastic material almost always used rather than metal?
metal to metal contact has proved to result in an unsatisfactory bearing surface
what is the stiffness of plastic implant materials similar to
cancellous bone
what does anisotropic mean
different mechanical properties in different directions
structurally what are the 2 most important factors in the design of an implant
strength and stability
what are the 2 requirements for biocompatibility of implants
biological integration - harmful reactions with body tissues must not exceed accepted safe levels and corrosion of materials by the body must not cause it to fail
functional integration - the implant should not adversely affect function of other parts of the body
what are the 5 structural factors important in implants
strength stiffness (not too stiff that it affects loading on adjacent tissues) lubrication wear fatigue
why are most bones wider at the ends
to accommodate the joint
what is the purpose of the end regions of bone containing cancellous bone
shock absorbing properties
what type of load is there on the main body of the femur
considerable bending load due to the joint at the head of femur being displaced laterally from the bone shaft
what can be said about the region of bone directly beneath articular surfaces
more dense than the cancellous bone below it to provide a rigid enough surface for the joint to bear on
why do bone shafts contain dense contact bone
resistance to deformation
what does isotropic mean
mechanical properties are the same no matter which direction they are loaded in
is bone isotropic or anisotropic
anisotropic
what defines the stiffness of a material
youngs modulus
ratio of stress to strain
how does the strength of cortical bone vary from the metaphysis to the diaphysis
only half as strong at the metaphysis
The faster bone is loaded, the ? it becomes
stiffer
which type of stress is bone least tolerant to
shear stress
The greater the difference in youngs modulus between 2 materials, the greater the ?
shear stress
In a join between 2 bars, where would the shear stress be?
at the end regions
there is no shear stress in the central portion because this is an area of load sharing
Why is there a shear stress at a bone-implant interface?
they have different values of Young’s modulus»_space; they try to deform by different amounts but cant do this if they are joined together so a shear stress is generated
In what case is osteopenia most likely to develop
when load sharing is combined with load transfer
what has greater stiffness - material with small or large cross sectional area?
larger cross sec area > stiffer
as length increases does a material become more or less stiff?
less stiff as length increases
what is geometric stiffness dependent upon
cross sectional area and length
diff between material stiffness and geometric stiffness
material stiffness is not dependent on shape
equation for shear modulus (G)
shear stress / shear strain
Axial rigidity = ?
E x A
Bending rigidity = ?
E x I
Torsional rigidity = ?
G x J
G is shear modulus and J is polar 2nd moment of area
what is rigidity?
stiffness of the cross section of a material
What is G (shear modulus)
shear stress / shear strain
will a stiffer implant take more or less load in the load sharing region
stiffer implant will take more load because it is more rigid relative to bone
what is the contact area between a fixator and bone called
bone-implant interface
what causes less trauma to soft tissues - screws or nuts and bolts?
screws - only need access from one side of the bone and the head projects less from the bone to the skin surface
what does interference fit fixation rely on
there is no specific fixation device, instead it relies on tight contact between implant and bone (surface friction prevents movement)
what is necessary practice for the dimensions of the inner component in an interference fit
the inner component should have slightly larger dimensions
what is the assumption of biological fixation
that bone will grow into a porous coating, mesh or roughened area on the surface of an implant, forming an interlock
what are the 2 most common surface coatings used in biological fixation
- porous beads of the same material of the implant
2. a ceramic such as hydroxyapatite (which is the main mineral constituent of bone!)
what technique is used to deposit hydroxyapatite on to the metal surface
plasma spray coating
what is the downside of using beads of metal
increases the surface area»_space; increases the level of corrosion
what metal is most commonly used for biological fixation and why
titanium because its the least corrosive and most biocompatible
what aspect of stems makes them less likely to subside far into the bone canal
they are tapered so that as the begin to subside, the stem forms a tighter fit with the bone
what is corrosion
the progressive unwanted removal of a material by an electrochemical process
what aspect of an implant acts as an electrode
metal or some other conductive material such as carbon in carbon fibre reinforced plastics
what does an area of corrosion of an implant lead to
high stress concentration which can lead to fatigue and failure
is the corrosive reaction most severe if the electrodes are the same metal or 2 different metals?
2 different metals
when can corrosion happen within a single metal component
if there are non homogeneous regions such as impurities
an alloy can increase levels of corrosion - true or false?
false
an alloy rather than a pure metal can reduce the levels of corrosion
which metal does not need to be formed into an alloy
titanium because even in its pure metallic form it is very resistant to corrosion
name the 3 alloys used in implants
stainless steel
cobalt chrome
titanium
what is the property that provides the good corrosion resistance of the metals used?
formation of a thin passivation layer of metal oxide (it forms on the surface of the material when its exposed to a corrosive environment)
what is fretting corrosion?
when abrasion of materials in contact removes the protective metal oxide layer, allowing corrosion to occur
when does crevice corrosion occur
when body fluid becomes trapped in a crevice between implants. the fluid loses its normal supply of dissolved oxygen»_space; high concentration acids form which corrode the metals
name 2 areas particularly prone to crevice corrosion
edges of bone plates and between screws and plates
name 2 methods of improving corrosion resistance
- nitric acid immersion treatment
2. titanium nitride coating
how does nitric acid immersion work
it improves the natural passivation layer
what is titanium nitride coating effective in reducing the release of from the alloy
vanadium and aluminium
what does titanium nitride coating not prevent the release of from titanium alloys
titanium!
what is the most common stainless steel used for implants
316L grade
- a low carbon steel
what is the reason for low carbon content of implants
to minimise sensitisation of tissues and to make it more resistant to corrosion
what is the main element in stainless steel
iron
what type of corrosion is 316L stainless steel prone to and what does this make it less ideal for
crevice corrosion
|»_space; less ideal for permanent implants
what is better for implants - forged or casted steel? why?
forged because it is 4 times as strong
does chromium have good or bad corrosion resistance
good
name a cobalt chrome alloy used in hip prosthesis stems
MP35N
what is the most common titanium alloy used
Ti6A14V
- titanium, aluminium and vanadium
what is anodising
a process which increases the thickness of the anti corrosive layer
why isn’t titanium used in joint replacements
its wear resistance is low
in what way is carbon fibre reinforced plastic more like bone than metals used for implants
it has a lower material stiffness than metals
what may it be possible to simulate with hydroxyapatite fibre reinforced polymers
variations in orientation of trabeculae
