Implant Technology Unit 3 Flashcards
what are the main design difficulty for knee prostheses
- needs to have an acceptable replication of the motion of the natural joint
- sufficient stability w.out being so rigidly constratined in its motion that it resulst in high stresses at the bone-implant interfaces under lateral and twisting loads
what is the most successful knee protheses design to date
“total condylar” design
what keeps the knee joint stable
ligaments, posterior joint capsule and good musculature
soft tissues act together to hold the knee in place throughtout its range of motion
[knee prostheses must take in to account the ligaments]
what do the collateral and cruciate ligaments work together to prevent
subluxation
what are the main ligaments of the knee and their function
Anterior cruciate ligament (ACL): resists posterior subluxation of the femur
Posterior cruciate ligament (PCL): resists anterior subluxation of the femur
Lateral collateral ligament (LCL): resists adduction of the joint
Medial collateral ligament (MCL): resists abduction of the joint
All the ligaments act together to limit distraction of the knee
All the ligaments act together to limit long axis rotation of the joint
what is the function of the posterior capsule
resist hyper-extension
what are the ACL and PCL named in relation too
their attachment to the tibia
what would happen if there was no ACL and no PCL
no ACL = femur can slide backwards over tibia
no PCL = femur can slide excessively forward
what is important for the surgeon to do in a knee replacement
correct ligament imbalance and looseness
if ligaments are damaged or removed during knee replacements surgery, the resulsting loss of stability must be compensated for in the design of the prosthetic knee
what does subluxation of a joint mean
partial or complete dislocation of a joint
what type of knee subluxation does ACL prevent
anterior subluxation of the tibia
[posterior subluxation of the femur = same thing]
knee ligaments move isometrially, what does this mean
they keep the same lenght as they move and do not lengthen or shorten
what happens to the axis of rotation in the knee as it flexes
axis of rotation changes
known as instantaneous centre of rotation as it changes as every instant of motion
moves posteriorly as knee rotates
[screw-home mechanism which follows a spiral motion]
what is the shape of the tibia plateau
medial compartment is slightly concave [lower at the centre than at the edges]
lateral compartment is convex
what is the motion at the knee joint as it flexes and extends
knee extends - tibia rotates externally
knee flexes - tibia rotates internally
[at full extension rotation is restricted by interlocking femoral and tibial condyles]
what is the name of the mechanism that desribes the movement of the knee
screw home mechanism
what does the four bar linkage cruciate mechanism do
constrains the motion of the femur on the tibia so that there is a combo of rolling and sliding motion
if the radius of the posterior part of the femoral condule is 22mm and the knee flexion is 140 degrees - calculate the lenght of the arc
s = [2 x pie x radius] x 140/360
radius in this case would be 22mm
s = 54mm
what does the limit to rolling distance in the knee prevent
[Why does the femur not roll off the tibia as the knee flexes?]
controls the position of the most posterior point of the centre of rotation
so enabling the knee to flex fully w/out rolling up against the posterior capsule
[cruciate ligamenst and joint capsule prevents it from doing so]
How does the position of the instantaneous centre of rotation change as the knee moves from extension to flexion?
It moves posteriorly by upwards of 10 mm and distally by a few mm
what force is the knee joint normally under and why is the magnitude greater than that of the body weight
compression
forces much higher than the body weight due to the combined effect of these gravitational forces, the contracting forces of the muscle and the balancing loads of the ligaments
joint force ranges from 2 to 6 times body weight under normal daily activity
since the knee is under mainly compressive load, what does the mean for designs of prostheses
that cement is a good option as it is very effective in compression
during the STANCE phase of gait what forces are seen at the knee
[GRF = ground reaction force]
[BW = body weight]
- vertical component of GRF just exceeds body weight during stance phase
- transmitted to the knee
- compressive force due to the action of quadriceps acting via patellar ligament generates max force of about 3 x BW
- GRF is about 1 X BW
- Resultant joint reaction force = 4 x BW
there is a fore-aft ground reaction force component of up to 20% body weight which is also transmitted to the joint - what has to work to counter act this
the cruciate ligaments
[where the forward component of the load acting on the femur tends to push it forwards over the tibia and the PCL restrains this movement]
what is the other component of GRF whilst walking
a horizontal component directed medially causing an adduction moment
this generates a turning moment on the knee
typically about 5% of body weight
[must be balanced by the muscles and ligaments]
what happens to the load distribution due to an adduction moment caused by medially acting horizontal GRF
the load distribution shifts such that the greater the horizontal force, the greater the load transferred from the lateral compartment to the medial compartment of the joint
what happens when it is a low magnitude sideways medial reaction force [like those that occur during gait]
the quadriceps muscle acting via the patellar tendon ligament, can pull the joint together hard enough to keep both condylar surfaces in contact with the tibial plateau
what happens when the horizontal force increases due to more strenuous activities
becomes necessary to use hamstrings as well, thus increasing joint reaction force
what happens as the load continues to increase
the muscles do not have strenght to maintain contact at both condylar surfaces
lateral side loses contact and all the load is taken by the MEDIAL condyle
stability of the joint relies on the LATERAL collateral ligament which is required to balance the turning moment due to sideways acting force
what implications does the high loads acting on the medial compartment of the knee have on joint replacement designs
the tibial component needs to be able to transfer high medial compartment loads on its upper surface to the underlying bone w/out causing high compressive stresses which could cause the bone to fail
what would be required if the collateral ligaments where absent or cannot be retained during surgery
the replacement joint would be required to provide all the lateral stability
linked prosthesis, such as a hinge, would be required
what other forces is the knee require to resist
axially generated torques which try to twist the knee axially and, if excessive can cause a meniscus to tear
[stability once again relies on ligaments, replacement joint would have to do the same]
why does the joint reaction force at the knee increases as the sideways horizontal component of the ground reaction force increases
as force increases, a greater patella tendon force and a greater hamstring force are required to balance its effects which adds to the joint reaction force
What adverse effect could a high contact force in the medial compartment of the knee have on a joint replacement
cause high local stresses medially which could cause the underlying cancellous bone to fail
what is the general criteria for knee implants
- Be tolerated within the human body with no short term and little long term risk of adverse toxic effects such as carcinogenesis
- Achieve its aim of relieving pain and restoring the activities of daily living.
- Last a reasonable length of time which ideally should extend beyond the expected life span of the individual patient without the need for revision
- Be insertable by a competent surgeon of average ability such that a predictable outcome can reasonably be guaranteed.
- cost-effective
what are most commerically avaliable knee replacements made of
femoral component - cobalt chrome
tibial component - HDP
what is the minimum functional kinematic requirements of a knee replacement
- should fully extend to 180° at which point the patient should be able to stand without the need for muscular effort by the quadriceps.
- collateral ligaments and posterior capsule must be intact to enable screw home mechanism or be designed with alternative stabilising mechanism
- should flex to 90 degrees [allows person to walk up/down stairs]
- should permit slight axial rotation as the knee extends to maintain natural ligamet tension throughout flexion and extension
what is essential for the surgeon to do in a knee replacement
[apart from balance the ligaments]
essential that the two bearing surfaces are cut parallel
means the tibial surface is maintained at right angles to the tibial shaft, parallel w/ the ground when weight bearing
femoral cut will have to be at an angle to compensate for the natural angulation of the femur relative to the tibia
[cut needs to be 6 or 7 degrees relative to axis of femur]
what needs to be removed to ensure that the replacement knee can fully extend
posterior capsule of the knee off the back of the femur
how should collateral ligaments be balanced
balanced in tension so that the bony cuts are parallel when the bones are stretched apart by the new joint and there is no tendency of the joint to open more medially than laterally or vice versa
what is the easiest method to balance the ligaments
lenghten tightened ligaments to match slack ones
what is the controversy of the cost of knee replacements compared to hips
they cost on average 5 times as much as hips
what is meant by the word “constraint” in context of modern knee replacement
relationship between tibial and femoral bearing surface geometrics
what are the functional design features of knee replacements
- to provide an acceptable ROM of joint combined with good stability under loading
- screw home mechanism or some equilavent that allows standing up straight w/out the need to apply the quad muscle
- if 1 or more ligaments cannot be used, the prosthesis must be designed to compensate for the functional loss
what knee implant design is used if there is no ligaments intact
hinged prosthesis
- constrains the motion of the knee to a single axis of rotation with total stability
what is the problem with hinged joint prosthesis
has no give under lateral and long axis rotational loading
transmits the high shear forces associated w/ these loadings to the implant-cement and cement-bone interfaces
what condition often causes the destruction or degradation of the ACL
OA
- PCL is preserved more often that not
how does the preservation of the PCL influence the knee replacement design used
PCL controls rolling motion of the tibia
implant depends of whether PCL is retained or removed
if not retained it is necessary to substitute a mechanism within the prosthesis
why is it important to find some mechanism to replace PCL
enables the femur to rotate on the tibial plateau w/out sliding too far posteriorly
thus allowing a good range of knee flexion w/out restriction of movement due to soft tissue
what are the theoretical advantages of retaining the PCL
provides some degree of A-P knee stability and may preserve some proprioceptic activity
normal gait unaffected w/out PCL but walking on stairs more stable w/ PCL
what are disadv of retaining PCL
constricts a free surgical dissection of posterior capsule
- may limit full exntesion
- enourages femoral component to slide over tibial bearing which may have deterimental surface wear effects
removal of PCL allows the use of more congruent joint surfaces
- reduces HDP wear
removal may correct deformity also
[some surgeons prefer to remove PCL]
what is the design of prostheses that retain the PCL and what aspect of the surgery is difficult
have fairly flat tibial plateau like that on natural tibia
need to position tibial plataeu accurately to get the PCL to work as it should
if PCL is too loose - allows forward movement of the femur on the tibia so that normal rolling back motion no longer works
if PCL is too tight - there will be restricted degree of flexion, excessive rolling back of the femur on the tibia. Also compression of the 2 prothetic joint surfaces together posteriorly, causing high contact stresses
what problems are associated with PCL retaining prosthesis designs
HDP wear problems
Fatigue problems
Why does a replacement knee need to have a fairly flat tibial plateau when the PCL is retained
Because the PCL could otherwise become lax or too tight during flexion- extension movement.
what are the 3 important mechanical factors affecting surface shape and degree of motion
- effect of constraint on load transmission and generation of high shear stresses
- effect of surface contact on WEAR of HDP tibial component
- effect of surface contact area on the STRESSES of HDP tibial component
HDP is known for its adverse affects of its wear debris on bone tissue, leading to bone resorption - what is another of its side effects
surface becomes stiffer due to increase in density after sterilisation w/ gamma radiation and over time after implantation due to oxidisation
greater stiffness increases joint contact stress under loading therefore makes the HDP tibial component more prone to wear
how does the HDP (tibial component) and CoCr (femoral component) wear related to each other
HDP being softer material wears out first while CoCr is hardly affected by wear
HDP component is also prone to fatigue failure under loading»_space; eventually results in failure of the joint
HDP fatigue and wear are more of a problem in the knee than in the hip due to the smaller bearing surface contact area, so stresses in the material are higher
effect of degree of constraint on load transmission
Unit 3 page 15
what knee replacment joint designs are most representative of a natural knee joint
The femoral component should be fairly circular and the joint should only partially constrained.
if the prosthesis does not loosen or the components break, and there are no medical problems i.e. infection - then what determines the useful life of a prothesis
rate of wear of the HDP component
what is the equation for volume of wear
v = c.N.s / p
v = volume of wear c = coefficient of wear/constant N = applied load across bearing surface (normal load) s = distance that the bearing slides p = hardness of the surface being worn
what components of the equation for volume of wear in the cobalt chrome femoral component constant
c = coefficient of wear/constant
p = hardness of the surface being worn
for a given activity N is defined, so how can the formula be simplified for the purpose of comparing the wear rate of different surface geometries
v = K x s
K = some constant
thus, volume of wear material produced is proportional to the sliding distance moved
what is the sliding distance to achieve a rotation of theta degrees proportional to
the radius of the bearing,
as s = r x theta
[r = d / 2]
thus v = K x d/2 x theta
what does this relationship between radius and sliding distance mean for volume of wear
that a smaller diameter of bearing will reduce the volume of wear material
what value determines the life of the HDP bearing
the rate of DEPTH of wear
[not the rate of volume of wear]
what is the equation for depth of wear
v = A x t
v = volume of wear A = area of contact t = depth of wear
thus depth of wear can be reduced by having a large surface area of contact
what is the area of contact equal too
lenght of the arc x width of bearing
s x W
[s = d x theta]
why is the width of the bearing increased to increase area of contact rather than the lenght of the arc
increasing d [to increase s] increases the rate of wear so it is preferable to increase W
how is volume of rate of wear reduced and how is depth of wear reduced
to minimise rate of production of wear particles i.e. volume rate of wear, the SLIDING distance of the bearing surfaces is minimised
to reduce rate of depth of wear the contact area should be increased, by having a wide bearing
what can HDP fatigue cause
premature wear and subsurface lamination cracking at the joint
if a model knee prosthesis had a lower contact area than another, what would it be more at risk too
contact stresses much higher and therefore will be more prone to fatigue failure under the cyclical loading that occurs during flexion and extension of a loaded knee
[thus larger contact area superior]
the moment due to lateral GRF can cause one of the condyles to lift off and place all the load on the other one - what prostheses design is suited to deal with this
one with a curved edge to spread out the load
the curved surface maintains a higher contact area when horiztonal and tilted
why is the HDP tibial component prone to fatigue failure
Because it experiences repetitive loading / unloading and tensile / compressive loading cycles.
what are the important design features that influence prosthesis contact stress and load transfer [affecting the interface stresses between the tibial component and the underlying bone]
- HDP surface shape
- thickness of HDP component
- whether or not HDP has a metal backing plate
- whether the tibial component has a stem
- stiffness of the HDP material
how does the thickness of the HDP component and stress relate
the thinner the HDP component, the greater it is stressed
due to the stress being unable to be distribute evenly in the material
what is the minimum thickness of HDP tibial component w/out metal backing plate
8mm thickness
if a metal tray is used, HDP tibial component can be thinner to limit total thickness
when is metal backing of value
when the soft tissue tightness forces the choice of a prosthesis of minimum thickness upon the surgeon
why is the ability to vary the tibial height in a prosthesis important
essential to ligament balancing
[If the joint is to be compressed its ligaments must be tight, or at least maintained at a length that does not allow the joint to open up medially or laterally when loaded with a lateral turning moment]
how does the natural tibial plateau distribute load and how does this compare to the replacement tibial plateau
- takes most of the load on the medial side (at least 60% but sometimes more)
- replacement tibial plateau will be similarly loaded
- medial edge of the plateau will be more highly stressed thus transferring a higher load and stress to underlying bone than on the lateral side
- load is taken proximally by cancellous bone only and is transferred to cortical bone more distally
what is the tibial backing tray under the HDP tibial component function
to distribute high contact stresses under the condyles in order to provide an even loading on the bone beneath it
conc of load on the top surface of the HDP is spread out over the contact area w/ bone, due to the high stiffness of the metal plate
what is the main disadvantage seen in tibial backing metal trays
- occurs if knee is loaded unevenly
[normally it is the medial side that takes largest proportion of the load]
- stress concentration will be greater in the underlying medial bone
- also tensile stresses arising between the plate and the bone laterally [higher tensile stresses than using an all-HDP component]
- the bone cement will not tolerate the tensile stresses well
most tibial components have a small peg between 30-50mm long - what is the function of this
found that tibial components loosen mainly from sinkage in the bone, probably due to gradual done failure from high localised stresses
providing central peg reduces the incidence of loosening
[peg helps reduce the high contact stresses due to uneven loading that could cause the bone to fail and subside]
how does the young’s modulus of the HDP component affect the contact stress
higher the young’s modulus of HDP the greater the contact stress
4 design features of a tibial component that affect the magnitude of the stresses on the underlying bone.
thickness of HDP component
whether or not HDP has a metal backing plate
whether tibial component has a stem
stiffness of a material
advantage of a modular tibial component that can take different thicknesses of tibial insert.
Ligament tension can be set correctly be selecting the right thickness of insert
advantage and one possible disadvantage of using a metal backed tibial component.
adv
- provides a more even loading distribution on the underlyning bone
disadv
- may cause excessively high stresses on the medial bone if the knee is unevenly loaded as the metal plate is stiff
- thus does not spread out a high contact load as the less stiff HDP
what are the headings for the priniciple features of knee prostheses
femoral component shape
tibial surface shape
methods of anchorage of components
how does the femoral component shape of the prostheses vary from the shape of the normal knee
left and right side of the prostheses are symmetrical
in a normal knee, the medial condyle is larger than the lateral condyle
what is the main disadvantage of trying to copy the knee’s natural asymmetry
doubles the size of the required inventory of components required to carry out knee joint replacement
what does the anterior part of the femoral component curvature accommodate
the movement of the patella during flexion and extension
what largely determines the degree of constraint of a knee prosthesis
the surface shape design of the tibial component
what shape of tibial component is favoured and why
partially constrained shapes, used in total condylar prostheses
they provide the required degree of functional movement and do not suffer greatly from loosening due to overstressing and limit the range of sliding motion to help reduce wear
what tibial component shape is needed if the PCL is retained
flattish surface profile
what tibial component shape is required if the PCL is not retained
surface shape which is dished in all directions
what does the posterior stabilised design mechanism need to be able to do in knee prostheses
- prevent posterior femoral subluxation of the femur over the tibia
- cause the femur to “roll back” as it flexes
what is an example of a knee prostheses which has a posterior stabilised design mechanism
“cam” shaped designs
- employ more of a gradual curve on the tibial component
- produces a smoother transition to the roll back position in flexion
if a knee joint replacement is properly aligned and the ligaments balanced, how should the femoral and tibial component rest
the femoral and tibial components should be maintained in compression throughtout the range of joint motion
[if tibial component is loaded evenly]
[therefore use of PMMA cement should provide good anchorage]
what will happen to the tibial component if only one condyle is loaded
due to a lateral turning moment, the other side will tend to lift off and give rise to tensile stress in any bonding material between tibial tray and underlying bone
what can cause uneven loading in the tibial component
imbalance in the ligaments will result in uneven loading and greater stresses
what can be added to the undersurface of the tibial component to give additional rotatory control
projections built into the undersurface
when are cementless knee bone protheses used
PCL-retaining knee protheses
What is the difference between a standard total condylar prosthesis and a posterior stabilised prosthesis?
posterior stabilised version has a stability device to substitute for the lost PCL.
What features of a knee prosthesis provide rotational stability
Projections on the undersurface of the tibial or femoral components, the stem (if present), screws (if present).
What additional fixation methods are required for cementless knee prostheses
Tibial component
- requires screw fixation to a stem
Femoral Component
- press fit
- use of pegs
what part of the knee can be the principal source of pain in OA in the knee and how can this be solved in a TKR
patello-femoral joint
- a total knee replacement w/out specifically resurfacing patella can cause total pain relief
- in a TKR the patellar bearing surface of the femur is replaced, so may be regarded as a form of hemi-arthroplasty
what is the biomechanic function of the patella
provides a better leverage for patellar tendon, so flexion movement can be provided by lower patellar force than w/out
thus, causes lower joint reaction force, so component wear and loading are reduced
what needs to be taken in to account when designing a replacement patella
reaction force of the patella against the femur can be as high as 4-5 times body weight
fractures of replacement patella are not common but can occur in more active patients
how is the femoral component designed to accomodate the patella
anterior part is grooved in the frontal plane to better accommodate the patella and to encourage patellar tracking
what is the replacement patellar bearing surface made from
HDP
- so excessive wear needs to be taken into account
what can affect the wear rate
shape of the contact surface of the patella
conforming shapes that match the femur wear less than convex (non-conforming) shapes
why is wear shown to be worse in metal backed patella
as HDP is insufficiently thick to distribute the loads and is prone to higher contact stresses than the less rigid all-HDP component
some studies have shown that patella replacements have what benefits
patients complained of less pain
also found the knee to be stronger in flexion for demanding activities such as stair climbing
why would the patella be made thinner in some cases
to achieve closure of the surgical wound after TKR
use of an HDP patellar replacement component can help make it thinner
how might RA and OA differ in regards to the whether to replace the patella
RA preferable to replace all joint surfaces due to involvement of whole joint
OA, patella resurface may be appropriate for some stiff OA knees with deformed patellae
how does absence of patella affect knee extension force
The patella tendon lever arm is reduced, requiring a greater quadriceps force. This adds to the patello-femoral joint reaction force.
why is metal backed patella replacement not used
The HDP is too thin to prevent high stresses in the material, which can result in extremely fast wear.
what do replacement mescius bearing consist of
a metal femoral component, a metal tibial component and an HDP meniscus
why is the metal meniscal bearing designed the way it is
large contact area to reduce contact stresses
low degree of constraint to avoid high stresses, other than compression, during load transfer
[actual designs of meniscal bearings vary]
how does the meniscus move during flexion/extension and long axis rotation/lateral movement
extension
- moves forward
flexion
- moves backwards
long axis rotation and lateral movement
- meniscus slides at the lower bearing surface
what is the principle disadvantage of meniscal designs
increased technical difficulty in achieving ligamentous balance and overall alignment w/out risking dislocation of the moving bearing
[newer designs reduce the risk of dislocation by limiting sliding motion of the menisci]
what does a hemi-arthroplasty consist of in the knee
replacing only one side of the tibio-femoral joint - either medial or lateral side
when is a hemi-athroplasty done
as an alternative to osteotomy in younger patients who have painful and deformed joints that is not severe enough to warrant TKR but too advanced disease process to permit osteotomy
what are both osteotomy and hemi-athroplasty surgeries designed to do
restore joint aligment to normal so to balance the forces on the medial and lateral sides of the joint
what is the design of a hemi-athroplasty
1) femoral component
- must be broad enough to cap damaged condyle
- small pegs or lugs are used to limit loss of bone stock in case of need of revision op
2) two lugs are required to counter any tendency to rotate
3) unicompartmental meniscal bearings
- require removal of more bone in order to fit
4) HDP thickness of at least 8mm to prevent excessive wear
TKR fail through wear, loosening and infection - why are revision knee surgeries difficult
same problem as seen in hip revision surgeries
- loss of bone stock for anchoring the revision prosthesis
problems trying to achieve ligamentous balance especially when ligaments may have been destroyed in loosening process
what is the design of revision knee prostheses
bearing surface w/ high degree of constraint built into the tibio-femoral bearing
can be provided by having high peg projection above tibial plateau which is partially captured by a central groove in the femoral component
why are linked hinged designs unsatisfactory
their inability to accommodaet axially generated torque w/out putting great strain on the bone prosthesis interface
what is the function of the central peg in revision knee prostheses
secondary constraint - fail safe mechanism - to offer stability only if the joint is loaded heavily either medially or laterally
if there is considerbale bone loss, what is needed in knee revision prostheses
modular components which are flat or wedge shaped that can be added to prosthesis to fill the gaps and allow prosthesis to rest on the bone
- called augmentation blocks
what can be done for knee revision prostheses if there is gross bone loss
intramedullary stems may be needed to get some degree of anchorage
stems need to be fixed initimately within the long bone medullae of the femur and tibia
one advantage and one disadvantage of using meniscal bearing knee prostheses.
Advantage
- lowers contact stresses.
Disadvantages
- requires removal of more bone, more difficult operation to perform.
why are two pegs commonly used on unicompartmental surface replacement
resist the tendency to rotate under torsional loading.
what is the main problem with knee replacements
HDP wear particles limit the knee replacement longevity
what does anchorage depend on
converting the axial loads into hoop stresses at the stem-bone interface
why is HDP fatigue and wear more of a problem in the knee than in the hip
smaller bearing surface contact area
so stresses in the material are higher
