Implant Technology Unit 1 Flashcards
what are ortho implant devices and their function
devices made from non-biological materials to improve structure and/or function
- either provide structural support after an injury (i.e. bone fixators)
=- replace or modify injured, diseased and painful joints
what qualities must an implant have
- be tolerated by human body with no short or little long term risk (e.g. carcinogenesis)
- relieve pain and enable sufficient mobility for ADL
- adequate strength
- function w/out failure for as long as it is required. Ideally should last expected life space.
- practicability of insertion; predictable outcome reasonably guaranteed by competent surgeon
- cost effective
what are most implants made of
metal e.g. stainless steel, titanium alloy
[in joints, bearing surface is made of a plastic material as metal-to-metal causes unsatisfactory result]
what is the main problems associated with implants
infection
- bacteria attracted to metal/cement surfaces
- commonly the normal bacteria found on the skin the culprit
- implant has to be removed
from a structural standpoint, what is the most important factors in the design of an implant
strength
stability
- its fixation to body tissue should be free from movement
- should function in harmony with the natural structures of the body, especially bone
what is important to remember about the cost of the implant
same implants have different prices in different countries
despite low cost of hip replacement it still remains outside the reach of the majority of countries
what are the 3 categories of performance of an implant technology
structural factors
kinematic factors
biocompatibility
what are the factors under ‘structural factors’
strength
- Components must withstand loads acting on them w/out deforming permanently or breaking
stiffness
- components must be rigid enough to bear load without excessive deflection, while not being so stiff that they adversely affect the loading on adjacent tissues
lubrication
- Moving parts must be adequately lubricated or require no lubrication
wear
- The rate of wear of bearing surfaces must not cause failure or generate wear particles which damage body tissues
fatigue
- fatigue life should be greater than the intended life of the implant
what are the factors under ‘kinematics factors’
Motion
- ROM must be sufficient to enable daily living functions, even if it is less than normal joint ROM
- The directions and patterns of motion must be controlled to ensure stability
what are factors under ‘biocompatibility’
biological integration
- Harmful reactions of implant materials w/ body tissues shouldn’t exceed accepted safe levels
- corrosion of materials by the body should not cause the implant to fail
functional integration
- implant should perform such that it does not adversely affect the function of other parts of the body
what are the 2 types of bones
compact bone
- a.k.a cortical bone
cancellous bone
how are bones designed to bear load
- 5 things
- end regions of the bones are shaped so as to accommodate the joint i.e. wider at the ends
- end regions of the bones contain cancellous bone which is more porous and less stiff (more flexible) than cortical bone, giving shock absorbing properties
- in cancellous bone, trabecular are aligned along the directions of greatest stress
- region below articular surface is more dense than the cancellous bone below it, provides a rigid underlying surface for the joint to bear on w/out causing excessive deformation
- shafts of bones contain dense compact bone, more rigid than cancellous bone, provides the necessary resistance to deformation under bending and torsional loads
how is stiffness measure
Young’s modulus (E)
- ratio of stress to strain
how does Young’s modulus change in most material when loaded
remains approximately constant irrespective of the load applied or the rate of loading
What does is mean when a material is said to be isotropic
that their mechanical properties are the same no matter which direction they are loaded
what does anisotropic mean and what materials exhibit this behaviour
it’s young’s modulus depends on the direction in which it is being loaded
bone
what direction is cortical bone stiffest and strongest
when loaded longitudinally
- [main direction in which it is loaded naturally]
what is meant when a bone is described as viscoelastic
the stiffness of bone changes according to the rate at which it is loaded
- faster it is loaded the stiffer it becomes
when is cortical bone strongest and weakest
strongest - under compressive loading
weakest - under shear loading
how are implants designed in terms of loading the bone
try to load the bone in compression
avoid shear stress especially but also try avoid tensile
avoid excessive stress shielding
what is stress shielding/stress protection and what is an example in orthopaedics
when bone is reabsorbed because of reduced loading
i.e. Wolff’s Law
caused in orthopaedics by bone plates; bone around plate gets reabsorbed; can lead to loosening of fixation screws meaning it is no longer effective in supporting the bone
what is the main difference in structure between bone in diaphysis and bone in the region of a joint
Diaphyseal bone
- made from compact bone
- rigid and provides resistance to deformation under loading.
Bone in the region of a joint
- cancellous
- the trabeculae are aligned along directions of greatest stress
- much less rigid than cortical bone and has good shock absorbing properties
- Bones are generally wider at the joints than at the diaphyses.
what is the major role of orthopaedic implants
provide structural support
in an orthopaedic implant:
BONE FIXATOR
- what are the 3 regions of the implant, assuming the bones touch at the fracture site
Region 1 = LOAD TRANSFER
- where the screws fix the plate to the bone
- here, part of the applied load in the bone is transferred to the plate
Region 2 = LOAD SHARING
- the fracture site, where the broken bones are supported by the plate
- here, part of the load is taken by the plate and part by the bone
Region 3 = LOAD TRANSFER
- where the screws fix the plate to the bone at the other side of the fracture
in an orthopaedic implant:
INTRAMEDULLARY STEM OF A CEMENTED JOINT REPLACEMENT
- what are the 3 regions of the implant, assuming the bones touch at the fracture site
Region 1 = LOAD TRANSFER
- where part of applied load is transferred from the stem to the bone
Region 2 = LOAD SHARING
- load sharing between the bone and the stem
Region 3 = LOAD TRANSFER
- remaining part of the load is transferred from the stem to the bone
- the bone below this region, takes all the load
where is the load transferred for bone plate and for the intra-medullary stem
bone plate
- load transferred at bone screw region
intra-medullary stem
- load transferred at end regions of the stem only
what is meant by load sharing and load transfer
Load sharing region
- For an implant attached to bone, the regions where the load is partly taken by the bone and partly taken by the implant
Load transfer region
- where load is transferred from an implant to a bone (or from a bone to an implant)
- load passes across interfaces between them
what does the load generate at the interface
and what serious complication can occur due to this
stresses or relative movement at the interface
stresses = occur when the two materials are bonded together
relative movement = occurs either if they are not bonded or if a bone comes loose
Loosening - serious complication in joint replacement
if there was two materials, one on top of another, and the bottom material was more flexible than the top
what would happen under loading
the bottom half would compressed more under loading and expand laterally more than the top
if there was two materials, one on top of another, and the bottom material was more flexible than the top
what would happen under loading if the two materials were bonded together
any lateral strain at the interface is the same for both materials, so a shear stress is generated at the interface, because one material is trying to expand more than the other one
if there was two materials, one on top of another, and the bottom material was more flexible than the top
what would happen under loading if the two materials were not bonded together
if it is also lubricate
sliding can occur freely so there are no shear stresses
what is the general rule about the difference in the young’s modulus and shear stress
greater the difference in young’s modulus then the greater the shear stress generated
if there was two materials, one on top of another, and the top material is less stiff than the bottom
what is stress at the interface like
stresses at the interface under the region of an applied load from above will be much greater
if there was two materials, one on top of another, and the top material is less stiff than the bottom
what is the advantage of using a stiff tibial component for a knee prosthesis
distributes loads more evenly over the underlying bone than a material with a lower stiffness, such as polyethylene
why does shear stress occur at a bone-implant interface
occurs at a bone-implant interface because the bone and implant each have a different material stiffness i.e. young’s modulus
so they try to deform by different amounts under a load
If joined together they cannot deform separately so a shear stress develops between them along line of interface
what is important to note about shear stress at the interface
shear stress is not constant across the whole length of the interface
[there is no shear stress in the central portion, which is a region of load sharing]
what is the equation for shear stress
shear stress = applied force / area being sheared
what causes stress concentration
sharp corners, notches, holes
what is a consequence of stress shielding
osteopenia due to bone reabsorption
structural stiffness is determined by what 2 factors
material stiffness
- basic property of the material
- i.e. its Youngs Modulus
geometrical stiffness
- shape of the cross section of the structural component
how does metal’s properties compare to bone
10 times stiffer than cortical bone
many more times stiffer than cancellous bone
are isotropic unlike bone
how do you calculate shear modulus
G = shear stress/shear strain
[calculated by applying a twisting load to a material]
what do you measure when you are measuring how stiff something is
measure of how much it deflects under load
defined as = force require to produce a unit deflection
[stiffness = S or k ]
what is the equation for stiffness
S = force / displacement
S = EA/L
E = youngs modulus A = area L = length
how does young’s modulus of the material affect stiffness
becomes stiffer as young’s modulus increases
[becomes stiffer as area increases]
[becomes less stiffer as its length increases]
to summarise what geometrical properties affect the stiffness of a bar under axial loading
cross sectional area
length
when is rigidity used
If we want to compare the stiffnesses of two implants of the same length
as the length is the same it would have no impact so we refer to the RIGIDITY of the 2 rather than stiffness
what is the equation for axial rigidity
R = EA
what is the equation for bending rigidity
R = EI
I = second moment of area
what is the second moment of area
geometrical property of the cross section which is based on the area of material it contains and also on how far it is away from the neutral axis
how does the distribution of the material affect its rigidity whilst bent
the further away a material is placed from the neutral axis the more rigid it is when bent
what are the equations for I
Rectangle:
I = bd^3 / 12
Circle:
I = pieD^4 / 64
Hollow rod:
I = pie/64 (D^4 - d^4)
what does a higher I value mean
the object will be stiffer when bent
i.e. more difficult to bend
what is the equation for rigidity under torsional loading
R = GJ
G = shear modulus J = polar second moment of area
what does the amount of load transfer from bone to implant [or vice versa] depend on
relative loads taken by them in the load sharing region
why does the cement in joint prosthesis inserted into bone take very little of the load
as its rigidity is low due to both a low E and a low cross section of material
in the load sharing region, what is the ratio of the load taken by the bone to that taken by the stem equal to
ratio of their rigidities
Lbone / Lstem = Rbone / Rstem
what is the ratio of rigidity of the bone equal too
total load taken by the bone
Lbone/ Ltotal = Rbone / R total = R bone / R bone + Rstem
[example Q p15]
what is the result is the stem of a prosthesis is less stiff
i.e. more like bone
more load would be transferred proximally and less distally, reducing stress shielding and bone reabsorption
what is the result is the stem of a prosthesis is more stiff
would transfer less load to the bone
more stress shielding and bone reabsorption
what is rigidity
stiffness of the cross section of the material
what is the bone-implant interface and what is essential about this interface
contact area between the fixator of an implant and the bone
must remain fixed and free from movement, otherwise the implant will loosen and probably fail
what holds in fracture fixators
screws which can be undone
allows the fracture fixator to be removed after healing
what is the advantage of screws compared to nuts and bolts
screw attachments only require access from 1 side of a bone only
nuts/bolts need access from both sides
what is the main disadvantages of nuts + bolts
more trauma to the tissue
project more than screws - cause issue when small distance from bone to skin’s surface [i.e. interior part of tibia]
what is the ‘Interference Fit’ dependant on
required no specific fixation device
relies on tight contact between implant and bone, the surface friction between the two prevents movement at the interface
when is the Interference Fit used as a method of implant fixation
when the dimensions of the inner component are slightly larger than those of the outer component
the implant is pressed into the bone to lessen the risk of loosening
used in cementless joint replacements
what is a possible side effect of the Interference Fit being made too tight
the bone can split
what is the function of bone cement and when is it commonly used
fill gaps between a bone and implant
once cement has dried, the bone implant interface should remain free from motion
commonly used in stems of joint replacement
what is the assumption of biological fixation
bone will grow into a porous coating, mesh or roughened area on the surface of an implant, forming an interlock between the two materials
what are the 2 main methods of biological fixation
porous beads
- made from same material as implant
- used mostly w/ titanium protheses stems as titanium is least corrosive and most biocompatible
ceramic coatings
- normally with HAp, the main mineral constituent of bone
why are prostheses stems tapered
so they cannot subside very far into the bone canal
tapered stem forms a tighter git in the bone canal as it sinks
what are 3 important features of an orthopaedic implant
high degree of biocompatibility
suitable structural mechanical properties for the application
ease of manufacture and fabrication of implant devices
what is biocompatibility
interaction between the human body and the implant material
what are 2 factors of biocompatibility
1) the extent to which body fluids and tissues affect a material
- most likely to be corrosion of the material, which can lead to mechanical failure
2) the extent to which a material adversely affects body tissues
- e.g. its tendency to cause abnormal changes to tissue (such as ulceration, allergy or cancer) or tissue death.
what is corrosion and when does it occur
the progressive unwanted removal of material by an electrochemical process
occurs when two electrodes are immersed in a liquid that conducts electricity
what is galvanic corrosion
electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte
what are the components of corrosion in implants and what is the consequence of it
the electrodes are metal or conductive material like carbon
electrolyte is body fluid
causes small areas of loss of material, makes pits and craters
become high stress concentration areas
can lead to failure fatigue
when is the corrosive reaction generally more severe
when the electrodes are different metals
[but can occur still if metals are same material]
what is advised in ortho to prevent corrosion
not to use different implant metals in contact, particularly if one is stainless steel which corrodes when in contact with carbon fibre
e.g. stainless steel screws to fix a carbon fibre reinforced plastic bone plate
what group metals are resistant to corrosion and why are they resistant
alloys - mixture of metals together
passivation layer
- forms on the surface
- layer itself a product of corrosion
- seals underlying layer from further corrosion
what are the 3 alloys used in ortho
stainless steel, cobalt chrome and titanium alloys
what is fretting corrosion
corrosion as a response to the removal of the passivation layer by the repetitive rubbing together under a load of 2 materials
occurs between screws and plates and also morse tapers [rely on the friction between two tapered components to prevent motion]
what can fretting also cause
surface damage to implants
reduces fatigue life
what is crevice corrosion
occurs in crevices between implants, where body fluid can become trapped and lose its normal supply of dissolved oxygen
leads to high conc acid forming which corrodes the metal
what areas are prone to crevice corrosion and how can it be avoided
- edges of bone plates
- between screws and plates
careful surgical assembly of components to ensure good screw-plate contact
what are the 2 methods for improving corrosion resistance
nitric acid immersion
titanium nitride coating
what is nitric acid immersion
improves the natural passivation layer
not entirely sure how it works but thought to be related to the increased amount of chromium in the passivation layer, which improves corrosion resistance
what is titanium nitride coating
significantly decreases corrosion therefore reduces the releases of harmful metallic substances
effective in reducing the release of vanadium and aluminium from titanium alloys
does not decrease the release of titanium but titanium is regarded at the least harmful implant metal
what are tissue reaction to implanted metals
7 things
- growth of thin fibrous tissue layer between implant and body tissue. Fibrous layer is body isolating itself from the foreign body
- local infection
- body sensitisation to metals
- inflammation in regions of metal corrosion, where protective oxide layer is load and small particulars react with body tissues
- tissue necrosis in regions were bone cement is used in joint replacements
- immunological reaction due to wear in the particulars from surface of joint replacement > can lead to cell mediated bone reabsorption
- tumours [rare]
why have ceramic materials not been used in ortho implants
fail in a brittle manner
give no advanced warning of failure
what are the materials used for implants
stainless steel
cobalt chrome alloys
titanium alloys
fibre reinforced plastics
what is the most common stainless steel type used in ortho and what are features of it
316L grade
- low carbon steel content to minimise sensitisation of tissue and make it more resistant to corrosion by the body
- main element is iron
- has high corrosion resistance but can corrode and crack under high stress
- prone to crevice corrosion
what is 316L grade stainless steel used for
temporary implants
- fracture fixation (e.g. screws and plates)
- load on the implant decreases as the bone heals and implant can be removed
what is 316L not ideal for
permanent implants due to it being prone to crevice corrosion
- e.g. hip replacements
what is the strength of stainless steel dependant on
how it is manufactured
- ortho implants normally forged [i.e. heated metal is forced into shape by hammering]
- the work/energy involved in forging process causes metal to harden increasing its yield stress but makes material less ductile
what is forged stainless steel 4 times stronger than
steel produced by casting
what are advantages and disadvantages of using stainless steel
Adv:
- manufacturing costs a relatively low
Disadv:
- suffers from more pitting corrosion [due to the passivation layer failing] than cobalt and titanium
- fatigue strength is lower
what is the main advantage to cobalt chrome alloys and what component gives it this characteristic
more resistant to corrosion in viva than stainless steel
chromium
what ortho procedure is cobalt chrome alloys preferred in and what is the preferred composition
permanent implants [even though it is not as strong]
- hip implants
Stellite 21 - 65% cobalt, 25-30% chromium and 6% molybedum
what are the other commonly used cobalt alloy used in ortho
MP35N
- 35% nickel, 20% cobalt
- used in hip joint stems
CoCrMo
- used as bearing surfaces because of their low coefficient of friction
titanium is used in ortho as either pure metal form or as an alloy - what is the most common alloy form
Ti6Al4V
what happens to pure titanium before it is used in ortho
it is anodised
- process which increases the thickness of an anti-corrosive protective layer on metal’s surface
- makes it very resistance to corrosion within the body
[better corrosion resistance than stainless steel]
what are the mechanical properties of titanium
lighter and half as stiff as steel and cobalt chrome
higher fatigue strength than stainless steel and cobalt chrome alloys
what is titanium commonly used in
fracture fixation plates
[low wear resistance makes it unsuitable for bearing in joint replacements]
what makes fibre reinforced polymers and what properties does it have
very stiff, high strength but brittle fibres embedded in a flexible resin material
- high strength properties
- stiffness can be selected according to the number and type of fibres used
- no longer brittle
what features do carbon fibre reinforced polymers have
most biocompatible
stiffness about one third that of stainless steel, so more mechanically compatible with bone
superior fatigue properties compared to stainless steel
what has carbon fibre reinforced polymers been used for
internal bone fixation plates
fracture plates
[superior fatigue properties means that it can overcome the problem of fatigue in metal plates due to movement at the fracture site]
what metal implants have the best corrosion resistance
titanium and its alloys
in what away is carbon fibre reinforced plastic more like bone than the metals used in ortho 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)]
what qualities must an implant have
strong enough not to break under use
in regions where loads are shared between a bone and an implant, the rigidity of the implant must be such that it minimises stress shielding of the bone [which can lead to bone reabsorption and loosening of a prosthesis]
what are the 2 main problems of implants
corrosion
detrimental effects of the products of corrosion on the body cells, tissues and systems
