Implant Technology Unit 2 Flashcards
what does hip joint arthroplasty involve
replacement of the damaged bearing surface of the femoral head and the acetabulum in order to give the patient a new joint which permits pain free motion and a stable hip
what is the basis of hip joint replacements
femoral head is anchored to the femur by a metal stem inserted into the medullary cavity of the femur
the new acetabular cup is made to fit into the existing socket, after reaming
the femoral and acetabular components are held in place either using bone cement of by direct ‘cementless’ contact between the prosthesis and the bone
how does ‘cementless’ contact work
by cutting a reciprocal shape in the bone into which the prosthesis is hammered or occasionally screwed
what does a “press fit” rely on
close surface contact between the stem and the bone - there are no screws or other devices for fixation
obtained when an object of a particular shape is pushed into another object of the same shape but slightly smaller.
what is meant by a total arthroplasty
both bearing surfaces were replaced
Charnley hip replacement is the “gold standard” what did he designed
- smaller femoral head to reduce problems of loosening associated with bearing friction
- introduced bone cement [made from PMMA] between bone and prosthesis to help distribute loads between bone and prosthesis
- introduced high density polyethylene (HDP) as a bearing material, which in combo w/ metal femoral head and lubrication by the body fluids, results in low friction bearing
- produced a system of instrumentation to match his prothesis
why cannot the whole prosthesis be made of ceramic
Bearing surfaces may be made from ceramic (have highly favourable frictional and wear properties)
but are brittle and subject to sudden failure.
what problems does using metal cause
remain very stiff relative to bone and give rise to stress shielding
metals are less brittle however
what is one way to provide a less stiff metal prosthesis
use composite plastic materials, i.e. carbon fibre reinforced polymers
what is the Bombelli hip made of
made of a metal core which gives it strength and a carbon fibre reinforced outer layer
what problems does lowering the stiffness of the stem bring
creates high shear stresses as the load is transferred from stem to the femur
why is getting new licensed bone cement into commercial use difficult
classified as a drug
therefore, huge cost to get onto market
what are the most commonly used implant materials
Cobalt chrome alloy
Stainless steel
Titanium and titanium alloys
High density polyethylene (HDP) Polymethylmethacrylate (PMMA) bone cement.
what is the general criteria for hip joint replacement prothesis
- should be tolerated within the human body w/ no short term and little long term risk of adverse toxic effects
- should give pain relief and restore the activities of daily living to the patient
- should last a reasonable length of time, which ideally should exceed the expected life span of the individual patient w/out the need for revision
- should be able to be inserted by an average surgeon with a predictable outcome guaranteed
- should be cost effective
what is the most effective way of relieving pain and restoring function for OA in the hip
hip arthroplasty
what ROM is needed at the hip for daily activities
[stand, walk, sit down]
extend slightly
flex to min of 30 degrees
abduct when weight bearing
rotate when in full extension
what period of time is a hip replacement expected to work for in 90% of cases
10 years
what is an advantage of cemented total hip arthroplasty
only an approx reciprocal shape of the medullary cavity has to be achieved as remaining space is taken up by PMMA which acts as a grout or filler between bone and prosthesis
what biomechanical factors must be kept in mind when designed ortho implants
load support mechanisms in relation to stress shielding of bone and bone-implant fixation techniques biocompatibility
what are the main forces acting on a normal hip structure
external loads
muscle forces acting at the hip joint
[knowing forces acting on a normal hip means we know how much forces an implant needs to withstand]
what are the 2 ways of estimating the stresses at the hip joint
1 - fixing strain gauges at important locations on the bone, which is the loaded
[time consuming and requires many stresses gauges]
2 - finite element analysis (FEA)
[allows stresses to be determined with relative ease for different prostheses under different loading]
what is important to remember about joint loading
1 - joint loading varies according to the physical activity being undertaken
2 - the magnitude of muscle forces for different activities cannot be determined accurately
why do the joint forces acting at a joint vary in magnitude and direction according to activity undertaken
direction can vary considerably because hip joint has such a wide range of movement
how many muscle and ligaments act across the hip joint
7 groups of muscles and ligaments
different combinations of these active at any one time to balance the external forces and moments acting at the joint so equilibrium can be maintained
why is it not possible to calculate the muscle forces if more than one muscle is active and what is the structure therefore called
as there would be more unknown forces than equations to solve them
- called indeterminate
what does indeterminate mean
means that the forces acting on the femur and the pelvis, and across the joint cannot be calculated precisely and must be approximated
what activity is commonly used when analysing stresses in hip prostheses
standing on one leg, either stationary or during part of the walking cycle
why is standing on one leg used to analyse stress in prostheses
- can estimate muscle forces with some degree of confidence as some muscles are not active at all, leaving mainly the abductor muscle forces to calculate
- also believed to generate high bending stresses in the femur and femoral components of the prosthesis
why is it difficult to determine accurately the stresses in the components of a replacement hip
- Because bone is an anisotropic material and its exact mechanical properties are difficult to determine.
- Because it is difficult to know the true forces acting on the prosthesis due to lack of knowledge about which muscles are active during a particular activity.
what activity generates the largest and smallest joint reaction force - highest to lowest
Ascending stairs
Walking
Descending stairs
Rising from a chair
in what plane in the hip does most movement occur
coronal plane
the hip joint force, J, has what other component
Fc - causes a compressive force in the femur giving rise to a compressive stress
how can compressive stress be calculated
= Fc / A
compressive stress
= compressive force / area
what also affects the compressive stresses
the pull of the muscles at the trochanter and the head and neck of the femur
how is the compressive joint force transferred in a prosthesis
transferred from the stem to the femur as a shear force
passes directly from the stem to the bone in a cementless prosthesis
or via the cement layer in a cemented prosthesis causing shear stresses in the cement
what will happen if the stem bone bond or stem-cement-bone bond is not sufficiently strong
the prosthesis will loosen and sink down the medullary cavity
how can the compressive stress in the stem be found
by dividing the compressive load taken by the stem at any section along its length by the area of the cross section
[compressive load taken by the stem also varies along its length]
what are design ways to prevent the stem from sinking distally in the medullary canal
- tapering the stem
- using a collar at the proximal end of the stem
- fixing the bone to the stem, by means of bone ingrowth or adhesion
- using a cement strong enough to withstand shear stresses
what are design ways to reduce interface shear stress by converting shear loads to compressive loads
- using a support, such as a proximal collar on the stem
- tapering the stem
what are design ways to avoid fracture of the stem
- by selecting a stem w/ a sufficiently large cross section to resist the stresses
- by selecting a high strength material for the stem
what is one of the most important design considerations
- the avoidance of excessive stress shielding of the bone by careful selection of the rigidity of the stem under axial loading
what forces does a joint force acting on a normal hip produce
compressive stress
bending stress
what is the equation for calculating bending stress
[applied bending moment x distance from neutral axis] / second moment of area
My/I
what is the assumption made when calculating the joint force at the hip
assumption that the only active muscle was the abductor group joining the greater trochanter to the pelvic
[the force required by this muscle group was found to be about twice body weight]
what does the bending moment produce on the femur
tension on the lateral side
compression on the medial side
what is the effect of bending stresses on inserting a femoral stem
reduce the stresses in the proximal end of the femur because the stem takes some of the bending load from the bone
what is required in order to keep the stem in static equilibrium
the applied load due to the joint force must be balanced by reaction forces due to contact between the stem and the femur
[the proximal area of the medial side of the femur provides one contact point and the lateral distal side provides another]
what do these contact points prevent
counteracts the tendency for the stem to rotate due to the bending action of the joint force
what is the maximum bending moment, M, equal too
equal to the applied joint force, J, multiplied by its moment arm, d.
when does maximum bending moment reach 0
as the distal end of the stem
[bending stress varies along the length of the stem]
why are modern stems much stronger than older stems
stems are forged rather than cast
what is the main likelihood of stem failure
if it loosens proximally
[bending moment at the distal end would increase drastically and cause failure]
what is important to note about the value of the second moment of area [I]
the value of I for the stem at any point along the stem is smaller that that of the adjacent bone , so it is more highly stressed
because cross sectional dimensions are smaller
what does the magnitude of the bending moment, and hence the bending stress, depend on
the magnitude and direction of the joint force and the abductor muscle force
this depends on the type of activity being undertaken and the angular position of the thigh relative to the pelvis
how is stress shielding at the proximal end of the femur prevented
a substantial proportion of the load is transferred from the bone to the stem proximally
therefore, stem takes less load distally = less stressed
what is loading sharing and stress shielding dependent on
the rigidity of the implant relative to that of the bone
what are ways to ensure that the stem does not fail under a bending load
- by designing it w/ a large enough second moment of area
- by designing its shape to limit the magnitude of the bending moment due to the joint force
what are ways to avoid the stem loosening
- providing sufficiently strong bond between the bone and the stem or cement
- providing a good press fit of the stem in the medullary canal
what are ways to minimise stress shielding of the bone under bending loads
- by selecting a suitable rigidity for the stem
why does the presence of a femoral stem affect the magnitude of the bending stresses in the femur
The stresses are lower because the stem takes some of the load, which means that the bone is less stressed.
what other stresses are generated under the action of a bending load
- radial stresses
- circumferential (hoop) stresses
[radial stresses are directed radially outwards from a central point]
where are radial stresses greatest
at the points of bone-stem contact at the proximal and distal ends
what do radial stresses cause
cause hoop stresses in the bone
[tensile stresses that act in a direction that tends to split the bone]
[found around the circumference]
what will happen if the stem has a loose fit in the bone
will give rise to very high local stresses causing hoop stresses that are high enough to fracture the bone
what are radial stresses inversely proportional to
the square of the length of contact, L, of the stem with the bone
[stems of short length are prone to causing high radial stresses on the bone]
how is radial stresses avoided
ensuring that the stem is long enough
providing a good fit of stem in the medullary cavity
under what circumstances does the femur experience high hoop stresses due to the presence of a femoral stem
- When a loose prosthesis experiences a bending load
- when a tapered stem is loaded axially and presses against the femur.
- when an oversized component is pushed into a cavity, like forcing a large nail into a bamboo cane - there is a tendency to split the cane.
when do torsional loads occur
when one end of the femur rotates axially with respect to the other
restraining the ankle while the upper body is rotated, creates large torsional stresses on the knee that are transferred to the hip
[can lead to loosening of the prothesis which may sink when then subjected to a compressive load]
what are important design factors for hip prosthesis to withstand torsional loads
- use of non circular sections to help resistance to rotational shear forces [reduce shear stress in stem-bone interface]
- shear strength of cement
- good bonding at the bone-cement and cement-implant interfaces
- surface treatments of the stem to improve interface bonding
what are important design factors for hip prosthesis in regards to stress of the acetabulum
- the size and conformity of the replacement joint surfaces - these affect contact areas and hence contact stresses
- ways to maintain the integrity of subchondral cortical bone
- the stiffness and thickness of the cup
- the thickness of the cement layer, if present
- whether or not to use a cup with a metal backing plate
- the technique used to fix the cup to the remaining acetabular bone.
what is used if cement is not used to keep the stem in place
- press fit
- rely on bone to grow into it, unless it is screwed in place
[use of screws not generally acceptable due to high stress conc at bone-screw interface leading to bone reabsorption and screw loosening]
what is bone cement made of and how does it work
PMMA
cement is mixed to a doughy consistency and remains plastic long enough to be inserted into the cavity and for the prosthetic component to be pressed into place
antibiotics and radio-opaque material added to cement
what is bone cement function
acts as a filler or grout [not as an adhesive]
interlocking bond can form between the cement and the small trabeculae of the cancellous bone in the femur
These bonds help to resist shear and tensile forces.
[The surface of a prosthesis component, such as a femoral stem, can also be roughened, or coated with beads or wires to provide a larger surface area for better keying of the cement.]
what are advantages of cemented prostheses
- surfaces of bone and prostheses do not have to be an exact fit because any gaps can be filled with cement
- cement fills all gaps between the bone and prostheses, allowing an even stress distribution thus preventing high stress concentrations
what are disadvantages of cement
- reaction to form polymer is exothermic, temps are high enough to destroy body tissue next to the cement
- PMMA fragments cause an adverse tissue reaction, which can lead to bone resorption; always some monomer left after the chemical reaction which is more toxic than polymer
- the cement is not an adhesive and therefore cannot resist tensile and shear forces [cement strongest in compressive]
what is a known cause for increase surface wear
fragments of cement that fall into replacement joints
what force would cement in prosthesis ideally be kept under but why is this not possible
compressive loads
femur is under tensile loads on its lateral surface
what does tensile loading cause with cement in prosthesis
part the cement from its contact w/ the bone and stem
repetitive loading can lead to cement fatigue failure and eventual loosening
what is the only way to avoid micro-motion under tension in cement
provide a true chemical bone between bone and the prosthesis
[PMMA cannot do]
what is the best technique for hip prosthesis
Charnley cemented “low friction” arthroplasty
cement does not bond chemically to bone, or to implants unless they are coated w/ a cement layer - true or false
true
what is the concern with non-bonded cement interface
- because cement particles will be released into the tissues due to abrasive wear i..e bone rubbing against metal
- because the prosthesis may loosen
what happens is there is not good bonding between bone and metal or cement
layer of fibrous tissue forms which prevent proper ingrowth of bone intro hydroxyapatite coated prostheses
how can the development of the fibrous layer be prevented
keep micro-motion to a minimum after surgery
need an accurate fit of the prosthesis at surgery followed by controlled weight bearing
what are ways to improve the longevity of cemented prostheses
- increase the keying of cement to the prosthesis; roughened prothesis surface etc
- coat metal components with PMMA, which can then bond w/ inserted cement, ensuring an intimate fit and improve resistance to motion
- combine a PMMA surface coating on the prosthesis w/ a cement that bone can bone to i.e. using cement containing hydroxyapatite
- make the stem smooth and allow it to sink down the canal until it forms an interface fit in the cement mantle
what is the difference between monomers and polymers
monomer is a short molecular chain version of a polymer.
Monomers react chemically when activated to join together to produce polymers.
what is the benefit of coating a prosthesis component w/ PMMA during manufacturing
To provide the best possible opportunity for the PMMA inserted during surgery to bond to the stem.
what must the load taken from the stem proximally do and give an example of how this is done [intramedullary stem]
transfer the load to the femur
the stem transfers some of its load proximally, the rest distally, with a central portion of load sharing
how does the rigidity of the stem influence the load taken
the more rigid the stem, the more load it takes proximally
i.e. more rigid stem results in more proximal stress shielding of the femur
what are the benefits and cons of using a low stiffness stem
- reduces stress shielding
- but generates high shear stresses at the proximal bone-stem interface or bone-cement-stem interface and in the cement itself
- can shear apart the interlocking grip at bone-cement and stem-cement interface
- cause loosening
what does Huiskes explanation of load transfer in a cemented femoral stem show
proximal end of femur
- stem takes all the load
- but some load is transferred to the bone in the proximal load transfer region
distal end of femur
- rest of load is transferred to the bone
how is the load taken by the bone in the central load sharing region [Lb] calculated
Lb = Rb / Rt
Rb = rigidity of bone Rt = rigidity of combined bone-stem composite structure respectively
how is the load taken by the stem [Ls] calculated
Ls = Rs / Rt
Rs = rigidity of stem Rt = rigidity of combined bone-stem composite structure respectively
what is the assumption made about load taken by cement
rigidity of the cement is so low that we can assume that it takes a very small proportion of the load in the load sharing region
[so total rigidity is effectively the sum of the rigidity of the bone and the stem only]
if we know the proportion of load taken by both bone and stem in load sharing region, we can determine how much load is transferred from the stem to bone at proximal and distal ends of the stem
what would the proportion be if 40% of the total load was taken by bone in the load sharing region
40% taken by bone»_space;> 60% by stem
this means that only 40% of the total load was transferred from the stem to the femur/bone at proximal end
thus, 60% was transferred distally to the bone/femur
[load transferred as a shear force]
where is the shear stress transferred from stem to bone highest and what effects the magnitude
shear stress is highest at the ends of the stem
magnitude depends on the magnitude of shear force, which in turn depends on how much load is transferred to the bone proximally and distally
e.g. the greater the proximal load transfer from the stem to the bone, the greater the proximal shear stress at the stem-bone interface
what is one way to reduce the proximal shear stress but what are drawbacks of it
using a stiffer material to increase rigidity of the stem [as less load is transferred to bone proximally]
but increases stress shielding of the proximal femur
what are isoelastic stems and why are they not viable
stems with the same stiffness as bone
reduce stress shielding but cause high shear stress at bone-stem or bone-cement-stem interface causing implant failure
what are possible stems that could be seen in the future
computer generated stems based on minimising interface shear stresses in the cement layer
made from stiff material i.e. cobalt chrome or stainless steel
how is load transferred from a femoral stem to the femur and where are the main load transfer regions
In a non-tapered stem, load is transferred solely by shear forces, at the proximal and distal ends of the stem.
The tapered stem does allow load transfer by compression of the stem on the bone; the more the taper, the greater the compressive load transfer.
Explain why the relative rigidities of the stem and femur influence the proximal load transfer stresses.
The relative rigidities of the stem and the femur determine how much load is taken by each in the load sharing region.
A stiff stem takes a large proportion of the total applied load in the load sharing region, so less is transferred to the bone proximally. This means that the proximal shear forces, and hence the shear stresses, associated with load transfer are low.
A low modulus stem takes less load in the load sharing region, so more load is transferred proximally, which means higher shear forces and interface shear stresses.
what does cement allow for in prostheses and what happens if it is too thin or too thick
allows good contact between bone and stem, avoiding high stress concentrations. Should allow good contact along the full length of the prosthesis
magnitude of stress in cement depends upon its thickness [and stiffness]
too thin = very high cement stress and bone reabsorption at proximal end of femur
too thick = high cement stress
what is the optimum cement thickness
layer of 3mm to 7mm proximally
minimum 2mm distally
what is the effect of collar on load transfer
2 sides to the debate; one side thinks they work, the other don’t
However, both collared and collarless stems are used succesfully
For using collar:
- transfers load from stem to the bone proximally, helping to reduce stress shielding and proximal interface shear stress
Against using collar:
- collar acts as a pivot causing fretting wear at the pivot and high stem stresses distally
what is meant by bonded and non-bonded stems
whether the stem is bonded in the first place
non-bonded would let it slide around free
what is the potential mechanism for cement to fail
cement has low shear strength and its interface bonds are weak, particularly with the stem
stem-cement bonds loosens, stresses in cement increases, leads to cracks and eventually cement failure
how is the bond strenghtened
by coating stem with PMMA
by roughening its surface
by applying a porous coating
how does cement-less stems work
relies on good press fit in the bone
promotes hoop stresses in the bone which reduces stress shielding
what are cement-less stems coated in and how does it help
coated with hydroxyapatite
helps with bone ingrowth and potentially eliminates metal debris from bone-metal abrasion
gives the opportunity for the bone to bond to a larger area of stem reducing the chance of failure of the bone under subsequent loading
[however, fully coated stems promote stress shielding of the bone]
what are disadvantages of cement-less stems
lack of distal contact causes thigh pain - can be prevented by distal anchoring of the stem
bone ingrowth might not occur
- caused excessive movement at bone-stem interface after surgery.
- fibrous tissue occurs instead preventing any bony ingrowth
after few years, bone-hydroxyapatite bone breaks down
what is the main advantage of completely coating the surface of a stem w/ hydroxyapatite as opposed to just partial coating
Complete coating gives the best chance for adequate bone ingrowth to hold the prosthesis in place.
what is a general rule of thumb for all stem shapes
all are tapered to prevent subsidence
many have a proximal wedge so that the stem can rest of the bone, allowing transmission of compressive forces as well as shear forces
why is the shape of the stem important in cement-less femoral implants
stem needs to be in contact with a large proportion of the femur
If its outer dimensions at any point along its length are smaller than the corresponding inner dimensions of the medullary canal, there will be a gap that cannot be filled.
Careful stem selection is require to overcome potential shape problems
why are different stem shapes needed for the elderly compared to the young
cortical bone thins and the canal gradually becomes wider
[particularly in women after the age of 50 to 60]
how can the bending moment be reduce in hip protheses to reduce bending stress in the stem
reduce the offset distance from the head to the neutral axis of the stem
can be done by 2 ways:
- reducing the length of the neck of the stem
- increasing the angle between the long axis and the axis of the neck i.e. make it more vertical
[implementing either of these measures does increase joint reaction force, giving rise to greater wear and acetabular bone-implant stress]
why is reducing the bending moment not as big an issue anymore
modern stem materials are much stronger than those of 20 years ago
therefore, fracture of the stem due to bending stresses less of an issue
thus, there is now a trend towards creating a more anatomically accurate stem, which increases the offset distance, reduces the magnitude of J and therefore reduces joint wear.
How does a tapered wedge help proximal load transfer
transfers significant proportion of the load in compression, rather than shear
what are the 2 essential requirements of a replacement hip joint
must have low contact friction between the bearing surfaces
bearing materials used must wear as slowly as possible
what is the equation of friction force
F = μN
μ = coefficient of friction N = normal contact force
what is friction force dependant on
surface properties of the two materials in contact
magnitude of the load pressing them together [N] i.e. magnitude of joint reaction force
how does a synovial joint coefficient of friction compared to HDP on metal beating [i.e. artificial joint]
synovial joint is between about 3 and 100 times less friction than artificial joint
why is HDP used instead of alternative materials with lower coefficient of friction
alternative materials are either toxic or have poorer wear properties
what is one of the consequences of replacement joints having a higher contact friction force than normal joints
much higher shear force transmitted to the acetabulum
- can be high enough to cause loosening at the fixation interface
why do most hip replacements use small heads
interface force for the small head is much less than it would be in a bigger head
[friction force is the same as it is independent of the area of contact]
the Charnley hip replacement are designed with a small offset (lever arm) to reduce bending moment of the hip - what is a consequence of this
higher joint reaction force
this means that the joint friction force, F, increases and so does the interface shear force, F1.
also increases wear in the joint
what are the 2 main types of wear that occur between bearing surfaces and why does it occur
adhesive wear
- occur because two bearing surfaces stick to each other when they are pressed together, and one is torn off by the harder on
abrasive wear
- occurs because surfaces are not perfectly smooth
- hip joints must have polished surfaces so to minimise abrasive wear
how is adhesive wear prevented
bearing surfaces should be made up of materials that have a low level of adhesion
lubricants also provide a layer between the two materials which reduces wear
how is abrasive wear prevented
highly polished hip joint replacement surface
good circulation of lubricant
what is another cause of abrasive wear in hip joint replacements
entry of particulars from outside the bearing
cement particles find their way into replacement joints and accelerate the wear process
3 factors affect the volume of adhesive wear - what is the formula for these factors
v = c.N.s / p
v = volume of wear
c = constant/coefficient of wear N = applied load across bearing surface s = distance the bearing slides
p = hardness of the surface being worn
why does the softer material wear first and more
the softer it is, the more it is loaded, and the more it moves
HDP wear fragments and particles can migrate considerable distances - what effect do they have on the implant
cause intense inflammatory reaction
one of the major problems associated with aseptic (non-infected) loosening of prosthetic components
over a no. of years, can cause bone reabsorption and prosthesis loosening
what are methods of reducing wear
reduce loading on the joint
- i.e. reduce N
keep the sliding distance as small as practically possible
- i.e. reduce s
find alternative materials for the replacement head that reduce wear in the HDP
- i.e. reduce c
how is the sliding distance kept as small as possible
use a femoral head with a small radius
- wears less than one with a large radius as the bearing surface moves less as slides
what ceramic material is being looked at as an alternative material to reduce wear in the HDP
Zirconia
- tougher and more fracture resistant
- harder and more scratch resistant
- produces less that half as much HDP as other ceramics
- some manufacturers now use material for femoral head
what is a disadv to a smaller diameter head (1)
rate of depth of wear is greater than it would be for a larger head as the contact area is less
thus, it will wear through the bearing surface of the cup is less time
joint lose its range of motion as the material wear as the neck of the stem of the femoral component contacts the cup
what is another disadv of a smaller diameter head-socket combo (2)
increased likelihood of dislocation in the post-op period
because of the increased likelihood of neck impingement on the edge of the cup
what affects friction force on bearings
Contact load
Material properties of both surfaces
why are small diameter heads used in hip replacements
reduce cup-bone interface shear stresses, lessening risk of loosening
produce less volume wear of HDP than large diameter heads
disadv of small diameter heads
high contact stresses
rate of depth of wear is greater due to reduced contact area
what is the acetabular component made of
HDP which may or may not have a metal backing between it and its interface with cement or bone
new acetabulum are orientated the same as normal acetabulum with a few degrees margin of error - what is this orientation
about 45 degrees relative to the coronal and sagittal planes of the body and facing slightly backwards
[usually acetabulum is sited first by the surgeon and then the femoral component orientated relative to that acetabular position, making allowances for neck length]
what are features of the acetabular design that can be changed
size of femoral head and acetabular cup
HDP may or may not be lined with a metal backing plate on its outer surface
thickness of HDP layer
outer dimension of the acetabulum
acetabular cup can be placed more centrally by deepening the original socket
wear is also influenced by the design of the acetabular cup and the contact surface area between the femoral head and cup - what two factors should be considered in the design
1 - clearance between femoral head and the cup should be small
[contact pressure between head and cup increases as diameter of head reduces
thus increased diameter head can reduce contact pressure and wear]
2 - radial clearance i.e. difference in the radius between the cup and head, the cup always being slightly bigger
[f the thickness of the HDP cup is reduced or a stiffer material used, then the radial clearance must also be reduced to spread the point contact load over a greater area in order to avoid excessive contact stress on the HDP]
what does the larger outer diameter of the cup matter for the hip prosthesis
larger the outer diameter of the cup the better its anchoarge to the pelvis
and the greater the difference between the frictional torque [twisting motion] at the prosthesis-bone or prosthesis-cement-bone interface
acetabular cups vary in diameter from 40mm to 65mm
how are actebaular cups held in place
cemented
fixed with scews
push fitted
incorporate a threaded stem which is screwed into a thread cut in the bone
what is the disadvantage of threaded cups
shown to result in bone resorption due to high stress concentrations in the region of the sharp threads
why is a metal backing used in acetabular replacements
helps hold the plastic in place and reduces its tendency to creep and distort
thus avoiding high contact stresses and focal wear on the HDP that can occur due to reduced head-socket contact hear
[does not reduce interface stresses]
[the plate eves out the loading on the acetabulum]
what is the disadv to stiff metal backing
stiff metal back to the HDP increases head-cup contact pressure
depends on thickness of the metal layer; thinner layers give higher contact pressure
what is loosening of the cup due to
1 - mechanical overstressing
2 - biological reaction to the ingress of HDP wear particles leading to resoprtion of the trabecular bone at the bone-cement interface
how can stresses at the bone-implant interface be reduced
by retaining the subcondral bone using a thick layer of HDP and a thick layer of cement
also by using a metal backed cup
creates a stiffer structure, thereby reducing areas of high contact stress
how does the loosening of the acetabular cup due to HDP wear particles work
HDP wear particles migrate from the rim of the cup along the bone interface, causing progressive absorption over no. of years
big problem in implants after 15 years
integrity of the bone cement interface is lost
fibrous membrane forms between the materials in the gap where bone is reabsorbed
what happens in the region where the fibrous membrane forms
cement fractures as a secondary effect due to high stress conc associated w/ area of bone loss
[cementless cups are equally prone to loosening via HDP wear particles]
if the acetabular cup is moved nearer to the mid-line in results in lower loads at the joint hip - why is this technique not utilised in practice
1 - effect is probably marginal
2 - In order to move the cup more centrally the strong cortical bone below the joint surface of the acetabulum [subchondral bone plate] has to be breached which means the prosthesis is mounted on softer and weaker bone. The acetabulum is generally deepened only sufficiently to remove any residual articular cartilage.
3 - deepened cup leads to earlier impingement of the femoral neck on the rim of the acetabulum, limiting motion and increasing the risk of the head being prised from the socket and dislocating
List three acetabular design features that affect the contact pressure at the bearing surface of the hip joint.
Diameter of the cup
radial clearance
thickness of the HDP layer
Why is there a minimum recommended thickness to the HDP cup
avoid excessive bearing contact stress
What are the three steps that lead to acetabular component loosening due to HDP fragments
1 - ingress of HDP at the bone interface
2 - bone resorption
3 - migration of HSP along the interface until loosening occurs
what is the failure rate in hip replacements
10% before 10 years
then subsequent estimate failure rate of 1% of the remainder per year
[most prostheses loosen before they wear out]
what is the strategy to deal with failure
remove the failued prosthesis and all surrounding inflammatory tissue, exclude infection and then add a new prosthesis either straight away or after an interval when inflammatory/infective process has resolved
[process termed revision arthroplasty]
[if done in one step called exchange arthroplasty]
what makes a secondary hip replacement difficult
loss of bone due to osteolytic process
failure rate higher than for primary hip replacement
function dimished after second op
