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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

- 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


- 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


what are the factors under 'structural factors'

- Components must withstand loads acting on them w/out deforming permanently or breaking

- 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

- Moving parts must be adequately lubricated or require no lubrication

- The rate of wear of bearing surfaces must not cause failure or generate wear particles which damage body tissues

- fatigue life should be greater than the intended life of the implant


what are the factors under 'kinematics factors'

- 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



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:


- what are the 3 regions of the implant, assuming the bones touch at the fracture site

- where the screws fix the plate to the bone
- here, part of the applied load in the bone is transferred to the plate

- 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

- where the screws fix the plate to the bone at the other side of the fracture


in an orthopaedic implant:


- what are the 3 regions of the implant, assuming the bones touch at the fracture site

- where part of applied load is transferred from the stem to the bone

- load sharing between the bone and the stem

- 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


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

I = bd^3 / 12

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

- manufacturing costs a relatively low

- 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



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

- 35% nickel, 20% cobalt
- used in hip joint stems

- 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



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


detrimental effects of the products of corrosion on the body cells, tissues and systems