Unit 1: Orthopaedic implant materials Flashcards
What are 6 requirements of an implant?
Biocompatibility Relieve pain and allow adequate movement Adequate strength and lifespan Cost effective manufacture Practicability of insertion Safety for the surgeon and the patient
How does the stiffness of HDP compare to bone?
Similar stiffness to that of cancellous bone
How does the stiffness of metal compare to bone?
Metals are stiffer
What is the main associated problem with the insertion of orthopaedic impants?
Infection
What structural factors are important in implant design? (5)
Strength Stiffness Lubrication Wear Fatigue
What is the structure of the end regions of bone?
Shaped to accomodate the joint (wider at the end)
Contains cancellous bone
Why is is desirable for the end regions of bone to contain cancellous bone?
Shock absorbing properties (more porous and less stiff)
Describe the structure of cancellous bone in the knee joint
Trabeculae are alligned along direction of greatest stress
In femur - horizontal to stop lateral displacement
In tibia - vertical to resist compressive loads
Why is the region directly beneath the articular surfaces more dense than the cancellous bone below it?
To provide a fairly rigid underlying surface for the joint to bear on without causing excessive deformation of the bearing surfaces
What is the structure in the shafts of bones?
Contain dense compact bone (more rigid than cancellous so provides resistance under bending and torsional loads)
How is the stiffness of a material described?
By its Young’s Modulus (stress/strain)
Most biological materials are isotropic. What does this mean?
Their mechanical properties are the same no matter which direction they are loaded
Bone is anisotropic. What does this mean?
It’s Young’s modulus depends on the direction in which it is being loaded
In which direction is cortical bone stiffest and strongest?
When loaded longituidinally
How does the strength of metaphyseal bone compare to that of diaphyseal bone?
Metaphyseal bone (near the ends) is only about half as strong as diaphyseal bone
Describe how bone is viscoelastic
The stiffness of bone changes according to the rate at which it is loaded.
The faster it is loaded the stiffer it becomes.
Quantify the difference in the ultimate stress of cortical bone under different types of loading
Cortical bone is twice as strong under tensile and three times as strong under compressive than shear loading
How does the varying ultimate stress of cortical bone affect implant design?
Need to find ways to load bone under compression and avoid shear stresses especially but also tensile stresses as much as possible
Why might designing a mesh like material to act like cancellous bone not be desirable in an implant?
Increased risk of infection due to large surface area
Also this type of structure might not be firm enough for attaching or bonding an artificial joint
What is stress shielding?
When reduction in loading and stressing of a bone, due to an implant, leads to bone resorption
What is load transfer and when does it occur?
Load transfer where part of the applied load is transferred between bone and implant. It happens at specific regions depending on the implant
What causes interface stresses?
Movement at the interface when two materials are not bonded or if a bond comes loose
What can induce stress concentrations?
Sharp corners
Notches
Holes
What affects how loads are shared in the load sharing region?
Relative stiffness of the two components
How is material stiffness defined under axial and bending loads?
Young’s Modulus
How is the material stiffness of a material defined under shear loading (including torsion)?
Shear Modulus (G = shear stress/shear strain)
How is stiffness defined mathematically?
Force required to produce a unit deflection
How do Young’s modulus and cross-sec area affect the stiffness of a structure?
As E and A increase the material becomes stiffer
How does length affect the stiffness of a structure?
As length increases stiffness decreases
How is the stiffness of two implants with the same length compared?
By their rigidity
How is axial rigidity calculated?
R = EA
How is bending rigidity calculated?
R = EI
How is the second moment of area calculated for a rectangular structure?
I = bd3 / 12
How is the second moment of area fro a circular structure calculated?
I = πd4 / 64
How is torsional rigidity calculated?
R - GJ
G = shear modulus, J = polar second moment of area
For circular sections how is J related to I?
J = 2I
In a load sharing region, how is the ratio of load taken by the bone to that taken by the stem calculated?
It is the ratio of their rigidities
Lb/Ls = Rb/Rs
In a load sharing region, how is the proportion of the total load taken by the bone calculated?
Ratio of the rigidity of the bone to the total rigidity of the section
Lb/Lt = Lb/Rt = Rb/Rb + Rs
If the stem is less stiff how does this affect load transfer?
More load transferred proximally and less distally
What is the benefit of a less stiff stem?
Reduces stress shielding and bone resorption
What dictates how a bone is fixated?
Whether or not the implant is intended to be removed at a later date
What are the advantages of screws over nuts and bolts?
Requires access from one side of a bone only
Less trauma to tissues
What is interference fit?
Relies on tight contact between implant and bone - surface friction between the two materials prevents movement at the interface
What shape of stem is particularly useful in interference fit?
Tapered stem
What is the purpose of bone cement?
Acts as a filling material to fill gaps between a bone and implant so a perfect geometrical match is not required
Why would it be extremely difficult to apply an adhesive from a practical point of view?
Bones are wet and difficult to access for cleaning and preparation
What assumption does biological fixation work under?
That bone will grow into a porous coating, mesh or roughened area on the surface of an implant
What are the two most common surface coatings used in biological fixation?
Beads of the same material as the metallic implant or a ceramic
Beading of the surface for biological fixation is ost commonly used for which metal and why?
Titanium - exposing large surface area of metal increases corrosion (particularly crevice) and titanium is least corrosive and most biocompatible
What is the main mineral constituent of bone?
Hydroxyapatite
What is plasma spray coating?
A technique to deposit HAp directly on to the metal surface
What are the current uses of HAp coating?
Good short term bonding but loosens after a year or two
Can be applied to orous metal coating which may produce better long term results
What are the 3 important features required of a orthopaedic implant material?
High biocompatibility
Suitable mechanical properties
Ease of manufacture
What 2 factors are important to consider when evaluating biocompatibility?
The xtent to which body fluids and tissues affect a material and the extent to which a material adversely affects body tissues
Define corrosion
The progressive unwanted removal of a material by an electrochemical process
Describe galvanic corrosion
2 electrodes immersed in an electrolyte - current can flow from one electrode to the other allowing a chemical reaction between the electrodes and electrolyte
What comprises the electrodes & electrolyte in implants?
Electrode - metal or conductive material
Electrolyte - body fluids (contain salts)
How does corrosion affect implants?
Causes small areas of loss of material (often show up as small pits and craters) - stress concentrations that lead to fatigue failure
How can corrosion be minimised?
Is more severe between different metals
Can occur in single metal component so important to reduce impurities
Using an alloy
What are the only three alloys used in implants?
Stainless steel
Cobalt chrome
Titanium alloys
Why do metal alloys and titanium have good corrosion resistance?
Passivation of metal oxide forms on the surface of the material when it is 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 fretting corrosion tend to occur?
Between screws and plates
Interference fits
What is crevice corrosion?
Occurs in crevices between implants where body fluid can become trapped and lose its normal supply of dissolved oxygen - high conc acids form which corrodes metals
Where is prone to crevice corrosion?
Edges of bone plates
Between screws and plates
What two methods can be used to improve corrosion resistance?
Nitric acid immersion
Titanium nitride coating
How does nitric acid immersion work?
Improves the natural passivation later (in stainless steel and CoCr is though to be related to increased amount of chromium)
Why is titanium nitride coating good?
Reduces release of harmful metallic substances in the alloys into the body fluids (in particular vanadium and aluminium from titanium alloys)
Where do the products of corrosion of implants appear?
In small quantities in the blood, urine, some tissues, storage organs (liver) and in the nails and hair
What are the 7 main biological reactions to implant materials?
Growth of fibrous layer Local infection Body sensitisation to metals Inflammation in regions of corrosion Tissue necrosis from bone cement Immune reaction to wear particles Tumours
Why does a thin fibrous layer form between the body and implant and why is this bad?
If there is micromotion at the interface
Stops fixation so bone and implant can’t be true composite structure
Why does infection occur in implants?
Ingress of bateria before or during surgery - implants tend to suppress body’s defence mechanisms to infection
Why does inflammation in regions of metal corrosion occur?
Protective oxide layer is lost and small wear particles of the material react with body tissues
What percentage of patients may develop sensitivity to cobalt, chromium or nickel?
50%
What is the name of the most common stainless steel used for implants?
316L grade
Why does 316L stainless steel have a low carbon content?
Minimise sensitisation of tissues
More corrosion resistant
Which type of corrosion is stainless steel prone to?
Crevice corrosion
What implants is stainless steel most appropriate for?
Temporary implants such as fracture fixation
How is stainless steel for implants made? why?
Forged - energy involved increases yield stress it is less ductile than cast steel but 4 times as strong
How does the fatigue strength of stainless steel compare to chrome and titanium?
Lower
Which part of cobalt chrome provides corrosion resistance?
Chromium
What are the 3 main components of a cobalt chrome alloy?
Cobalt
Chromium
Molybedum
Comment on the strength of cobalt chrome
Cast CoCr is not as strong as stainless steel but stiffness is similar
Used in joints where replacement part is big enough to have sufficient strength
What is the advantage of cast CoCrMo?
Complex shapes can be cast more easily than forged
What are cast CoCrMo alloys useful for and why?
Bearing surfaces due to low coefficient of friction with polyethylene
What is anodising?
A process which increases the thickness of an anti-corrosive protective layer on a metal’s surface (increases corrosion resistance)
How do the corrosion products of titanium compare to those from stainless steel and cobalt chrome alloys?
Less harmful to the body
How do the mechanical properties of titanium compare to stainless steel and cobalt chrome?
Titanium is less dense (lighter) and about half as stiff
Has higher fatigue strength than stainless steel
Why is titanium not suitable for bearings in joint replacements?
Low wear resistance
What are the properties of fibre reinforced polymers?
Very stiff, high strength but brittle fibres embedded in a much more flexible resin material
Why are fibre reinforced polymers good?
High strength properties Stiffness can be selected
Mechanically more compatible with bone
Superior fatigue properties to many metals
