Unit 1: Mechanics and Materials Flashcards
List five criteria for a successful orthopaedic implant
be tolerated by the human body with no short term and little long term risk of adverse effects
relieve any pain and enable the patient to achieve sufficient mobility to perform everyday activities
function without failure until it is no longer required
be designed to be implanted successfully by a surgeon of average ability
be of acceptable cost to the purchaser
What are the two main structural requirements for an orthopaedic implant?
strength and stability
What are the three main factors affecting implant design?
structure
kinematics
biocompatibility
What are the structural factors that must be considered in the design of all orthopaedic implants?
strength stiffness lubrication wear fatigue
What are the kinematic factors that must be considered in the design of all orthopaedic implants ?
range of motion must allow for activities of daily living but direction and pattern must be controlled in order to ensure its stable
What are the important requirements of an orthopaedic implant that are essential for biocompatibility?
biological and functional integration
What is biological integration?
any harmful reaction between body tissues and a material must be below the accepted level
corrosion shouldnt cause failure
What is functional integration?
implant performance should not adversely effect other body parts
What does anisotropic mean?
it has different mechanical properties in different directions
What is a composite structure?
composed of more than one material
What are the main differences in structure between bone in a diaphysis and bone in the region of a joint?
Diaphyseal bone
- compact bone
- rigid
- resists deformation under loading
Joint bone
- cancellous
- trabeculae align along directions of stress
- less rigid
- shock absorbing
- wider at joints
Under which type of loading (compressive, tensile or shear) is bone strongest and weakest?
Bone is strongest under compressive loading and weakest under shear loading.
What does isotropic mean?
mechanical properties are the same no matter what direction
Give an example of an
a) anisotropic material?
b) isotropic material?
a) bone
b) most non-biological materials
In what ways is bone viscoelastic?
stiffness increases with increasing rate of loading
In what direction of loading is cortical bone stiffest and strongest?
longitudinal loading
What is the meaning of stress shielding and how can it adversely affect bone?
Stress shielding is the reduction in the load (and therefore stress) that would normally be taken by a bone, due to the presence of an implant. This occurs because some of the load is taken by the implant. Bone resorbs when understressed and this can lead to loosening of the implant.
What is the major role of all orthopaedic implants?
to provide structural support
What is load sharing?
the regions where the load is partly taken by the bone and partly taken by the implant are called regions of load sharing.
What is load transfer?
The regions where load is transferred from an implant to a bone (or from a bone to an implant) are called regions of load transfer
In a bone fixation plate how does load transfer and share between bone and plate?
above the fracture: screws fix plate to bone and load transfers from bone to plate
at fracture: broken bones supported by the plate so load is shared between plate and bone
below fracture: screws fix plate to bone and load transfers back to the bone
In a joint replacement stem how does load transfer and share between bone and stem?
the load transfers from the stem to the bone
there is a midpoint where stem and bone share this load
then load transfers more on to the bone
below the stem the bone takes all the load
Where does load transfer take place for a bone plate?
at bone screw regions
Where does load transfer take place for an intramedullary stem?
at end regions of the stem
In a composite material, where does load pass from one material to another?
at the interface between them
What can interface loading generate if
a) the materials are bonded ?
b) the materials are not bonded?
a) interface stresses
b) relative movement which can cause loosening
What is load transfer across 2 materials under compressive stress dependent on?
the stiffness aka young’s modulus
If two materials are subjected to a compressive strength and the youngs modulus of the top material is greater what happens to the materials?
bottom is more flexible so it expands laterally
even stress pattern at interface
If two materials are subjected to a compressive strength and the young’s modulus of the top material is greater
what is the stress at the interface
a) if they are bonded
b) if they are not + lubricated ?
a) even lateral stress pattern at interface but sheer stress generates
b) sliding occurs so no sheer stress
How does youngs modulus affect shear stress?
the bigger difference in E for the two materials the greater the shear stress
Are interface pressures higher when
a) top material has a greater young’s modulus than bottom?
b) bottom material has a greater young’s modulus than top?
B
How do we calculate stress?
force/area
What is shear stress?
slippage of surfaces or planes within a material
Why is there a shear stress at a bone-implant interface?
the bone and implant each have a different material stiffness (Young’s modulus) so they try to deform by different amounts under the action of a load. If joined together they cannot deform separately so a shear stress develops between them - along the line of the interface.
How can shear stress at bone-implant interface be calculated?
normally shear stress= force/area but in reality the load transfer occurs more at the end of the interface so values are higher than theoretical values
Give 3 reasons that stresses between two materials can be higher in reality than in calculation
- nonuniform contact
- nonuniform mechanical properties eg. stiffness and
variations in surface contours - inc stress concentration due to sharp corners, notches
and holes
How does length of interface alter shear stress in reality?
smaller area being shared, higher stress but closer to the expected value
What is the approximate ratio of the material stiffness of stainless steel to that of cortical bone?
200/20 = 10
What is material stiffness ?
Material stiffness is the same as Young’s modulus and is a constant value for a particular material irrespective of its shape.
What is geometric stiffness?
Geometrical stiffness is associated with the stiffness of a particular structural component and depends on its cross sectional shape and its length.
What are the geometrical factors that affect the axial stiffness of a structural component?
cross sectional area and length.
what are the two components of structural stiffness?
material stiffness
geometrical stiffness
in which type of loading do metals not fail?
compression
Metals have high … stiffness but low …. stiffnes
structural is high
geometric is low
What represents material stiffness under shear loading?
shear modulus (G) = shear stress/shear strain
What term is used to compare the stiffness of two implants of the same length?
rigidity
What is meant by rigidity?
the stiffness of the cross section of a material. It is similar to structural stiffness but ignores the length of the structure. It is a particularly useful quantity for calculating the loads taken by a bone and an implant in a load sharing region
What are the two factors affecting the rigidity of a structure?
Material stiffness and a geometrical property of the cross section
How do we calculate
a) axial rigidity?
b) bending rigidity?
c) torsional rigidity?
a) young’s modulus x cross sectional area
EA
b) young’s modulus x 2nd moment of area
EI
c) shear modulus x polar 2nd moment of area
GJ
What is the polar second moment of area?
2I
In a load sharing region the ratio of loads taken by bone and stem is what?
a ratio of their rigidities
will a stiff stem give more or less stress shielding of bone in Region 1, than a less stiff stem?
The stiffer stem gives a greater degree of stress shielding in Region 1. This can be explained as follows.
In the load sharing region, Region 2, the stiffer implant takes more load than the less stiff one because it is more rigid relative to bone. This means that less load will be transferred from the stem to the bone in Region 1 for the stiff implant, i.e. the bone in Region 1 takes less load and is therefore more stress shielded.
What is the proportion of the total load taken by the bone equal to?
the ratio of the rigidity of the bone to the total rigidity of the section
Why are orthopaedic implants fixated to bone?
to prevent loosening and failure
What are the main advantages of using screws instead of nuts and bolts in the fixation of implants to bone?
access from one side reduces tissue damage
project less
What is an interference fit?
dimensions of inner component larger than those of outer component so it is pressed into the bone to prevent loosening
Why are prostheses stems tapered?
cannot subside very far into the bone canal. Subsidence stops because the tapered stem forms a tighter fit in the bone canal as it sinks
When is interference fit with tapered stem used?
cementless joint replacement
What is the function of bone cement?
it fills the gaps between the bone and implant to reduce movement and loosening
What are the two main methods of biological fixation?
Porous beads
ceramic coatings
What are biomaterials?
non-biological materials used within the body to repair or replace body parts that have failed
Name three important features required of an orthopaedic material
biocompatible
suitable structural properties
easy to manufacture
What is biocompatibility?
the interaction between body tissue and implant
- how body fluids affect the material
- how the material adversely affects the body
What is corrosion?
when 2 electrodes are immersed in an electrolyte fluid and a current flows between them, allowing a chemical reaction where there is progressive removal of material
In an implant what is the
a) electrode?
b) electrolyte?
a) metal or carbon
b) body fluid inc. salt
How does corrosion result in fatigue failure?
loss of material shows up as pits and craters that have high stress concentrations making them more likely to fatigue fail
When are corrosive reactions most severe?
different metals
Under what circumstances can an implant made of one alloy behave as two electrodes?
manufacturing problems
- contains impurities
- not mixed so non-homogenous regions
What materials can reduce corrosion?
alloys eg stainless steel, cobalt chrome, titanium alloys
pure titanium
What is fretting corrosion?
when abrasion of materials in contact removes protective metal oxide leading to surface damage and reduced fatigue life
Give examples of when fretting corrosion may occur
between screws and plates
when interference fits used
in modular prostheses
What is crevice corrosion?
when body fluid gets trapped in the crevices between implants it loses its dissolved oxygen and becomes a high concentration acid that corrodes the implant
Give examples of where crevice corrosion occurs
edges of bone plates
between screws and plates
Give two ways to improve corrosion resistance
nitric acid immersion
titanium nitride coating
What are the adverse effects of implanted materials on the human body.
growth of a fibrous layer between the bone and the implant
local infection
body sensitisation to metals
tissue inflammation in regions of implant corrosion
tissue necrosis in the region of bone cement immunological reaction to wear particles
tumours
Which metal implant materials are thought to have the best corrosion resistance?
titanium and its alloys
What is the main element in stainless steel?
iron
What is the most commonly used stainless steel? What is it used for?
316L grade
temporary implants eg. screws and plates
What type of manufacturing process makes stainless steel stronger? What does this sacrifice?
forged
less ductle
What negative quality does stainless steel have?
corrodes and cracks
crevice corrosion
local pitting corrosion
low fatigue strength
Is cobalt chrome or stainless steel more corrosion resistant?
cobalt chrome
Is cobalt chrome or stainless steel stronger?
stainless steel
What is the original cobaly chrome alloy?
stellite 21 CoCrMo cobalt chromium molybedum
What are the differences between forged and casted cobalt chrome alloys?
forged are stronger but less corrosion resistant
Where are cobalt chrome alloys used? Why?
hip joint because strength from the size
bearing surface as low coefficient of friction with polyethylene
What property makes pure titanium very corrosion resistant?
anodised
ie. more of a protective layer
How does titanium compare to stainless steel in its
a) density?
b) stiffness?
c) fatigue strength?
a) lower
b) 1/2
c) higher
What is titanium used for?
bone fixation plates
Why isnt titanium used in bearing joint surfaces?
low wear resistance
In what way is carbon fibre reinforced plastic more like bone than the metals used for orthopaedic implants?
It has lower material stiffness than metals - about three times that of cortical bone rather than ten times (steel and cobalt chrome) or five times (titanium and its alloys).
