Implant technology - unit 2 deck 1 Flashcards
What does a hip joint arthroplasty involve and what is the aim of doing one?
The replacement of the damaged bearing surfaces 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 are the 2 main methods a hip joint arthroplasty could be done and which is the perferred method ?
- Replace the bearing surfaces alone - results in unsatisfactory prosthesis fixation
- Replace the bearing surfaces and some underlying bone - this is the peferred method because it ensures good prothesis fixation (commerical hip joints ustilise this one)
Describe commerical hip joint arthroplastys used nowadays
- The femoral head is anchored to the femur by a metal stem inserted into the femoral medullary cavity.
- The new acetabular cup is made to fit into the exisiting one after reaming.
The femoral and acetabular components are then held in place using either bone cement or by direct cementless contact between the prosthesis and the bone.
(using cementless requires cutting a reciprocal shape into the bone into which the prosthesis is hammered or screwed into.
Why is there tremendous scope for improvement in the development of joint prostheses ?
Because no prosthesis behaves like a normal joint
Attempts to replace damaged joints began as soon as it was possible to do what?
Provide safe and prolonged anaesthesia
Why did early joint replacements fail and what due to?
- Surgeons didnt appreciate the large loads placed across joints
- Didnt consider toxicity or immunology
All efforts failed due to wear, rejection and high infection rates
What method did early joint replacements focus on and specifically within hip joint replacements what was done?
Early attempts concentrated on replacing the surfaces of joints
Most efforts concentrated on the hip and materials were regularly placed between the bearing surfaces known as inter-position arthroplasty
What was the first recorded successful hip replacement and state why?
A hemi-arthroplasty known as the ‘Thompson’ arthroplasty
Because it relied on a press-fit as it involved replacing the femoral head and having a long stem driven down the femoral canal
State what a ‘press-fit’ joint replacement is and what is relies on
- A press-fit is when an object of a particular shape is pushed into another object of the same shape but slightly smaller.
- The then relies on close surface contact between the stem and bone - there are no screws or other devices for fixation
Define what is meant by a total arthroplasty
This is when both bearing surfaces are replaced
What design features make the Charnley hip replacement the ‘gold-standard’ ?
- A smaller femoral head to reduce problems of loosening associated with bearing surfaces
- Introduced bone cement (made from PMMA) to help distribute loads between bone and prosthesis
- Introduced high density polyethylene (HDP) as a bearing material which in combo with a metal femoral head (stainless steel or cobalt chrome) and lubrication by the body fluids, results in a low friction bearing
- Produced a system of instrumentation for inserting and aligning his prosthesis
Why can a whole prosthesis not be made of ceramic ?
Because although the have favourable frictional and wear properties they are brittle and subject to sudden failure
What are the benefits of using metals compared to ceramics?
However what are the problems with using metals?
The are less brittle than ceramics but are very stiff and give rise of stress sheilding
What is one way of providing a less stiff metal prosthesis and therefore reduce stress sheilding?
Give an example of a hip prosthesis which utilises this
- Using composite plastic materials e.g. carbon fibre reinforced polymers
- The Bombelli hip which is made from a metal core which gives it strength and a carbon fibre reinforced outer layer
What are the problems with using the Bombelli hip prosthesis and lowering the stiffness of the stem used ? (by using the Bombelli)
Lowering the stiffness creates high shear stresses as the load is transferred from the stem to the femur
Why is getting new licensed bone cement into commercial use difficult
Because it is classified as a drug and therefore, huge cost to get onto market
List some of the constraints when designing an implant
- Small scope of materials tolerated within the body
- The need to withstand large forces
- The discrepancy between highly structured, heterogeneous bone and homogeneous implant materials
Name the most commonly used implant materials
- Cobalt chrome alloy
- Stainless steel
- Titanium and titanium alloys
- High density polyethylene (HDP)
- Polymethylmethacrylate (PMMA) bone cement
What are the general criteria for hip joint replacements?
- It should be tolerated within the human body with no short term and little long term risk of adverse toxic effects such as carcinogenesis (cancer inducing).
- It should give pain relief and restore the ADL’s to the patient.
- Last a reasonable length of time, ideally exceeding the expected life span of the individual patient without the need for revision.
- Should be able to be inserted by an average surgeon with a predictable outcome guaranteed
- Should be cost effective
What are most hip joints made from ?
Cobalt chrome or titanium, which are fairly corrosion resistant, and high density polyethylene (HDP), which provides a good bearing surface although it does give undesirable tissue reaction when fragmented.
No better alternatives
What is the single most effective long term way of relieving pain and restoring function to the hip joint suffering from OA?
Hip arthroplasty
What is the essential functional range of movement of the hip required for daily living to allow standing, walking and sitting?
- Extend slightly
- Flex to a min of 30 degrees
- Abduct when weight bearing
- Rotate when in full extension.
Excluding the use of hip prostheses in young persons what percentage of hip replacements are expected to last 10 years?
90%
What is an advantage of cemented total hip arthroplasty makes it easier to ensure predictable outcomes even when inserted by a surgeon of average ability?
Only an approximate reciprocal shape of the medullary cavity has to be achieved as the remaining space is taken up by PMMA which acts as a grout or filler between bone and prosthesis. For the average surgeon this is well within his abilities
What biomechanical and biocompatibility factors must be considered when designing orthopaedic implants ?
Load support mechanisms in relation to stress shielding of bone and bone-implant fixation techniques biocompatibility.
What are the forces acting on the normal hip due to ?
- External loads
- The muscle forces acting at the hip
What is the importance of knowing the forces acting on the normal hip ?
The stresses can then be calculated and used to design a replacement which can withstand the stresses without failing.
What are the 2 ways of estimating the stresses acting at the hip joint and describe them?
- Fixing strain gauges at important locations on the bone, which is then loaded. The stress is calculated from the strain, knowing Youngs modulus for the strain gauge material
- Finite element analysis - involves creating two dimensional or three dimensional models of the structure made up of small elements applying joint and muscle loads to the model and letting the computer calculate the stresses.
Which is the better method for calculating the stresses acting at the hip joint - using strain guages or finite element analysis ?
- Finite element analysis because it allows stresses to be determined with relative ease for different prostheses under different loading conditions. Designs can be compared and regions of high stress identified.
- Whereas using strain guages is very time consuming process and requires many strain gauges in order to understand how stresses are distributed in the bone and implant
What is the main drawback of all methods of calculating stresses ?
They are only estimates because the material properties of bone and the bone-implant interface properties are variable and cannot be determined accurately.
What 2 other factors are important to remember which make calculating stresses on the hip only estimates ?
- Joint loading varies according to the physical activity being undertaken.
- 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 the activity being undertaken?
because the hip joint has such a wide range of movement
Why cant the magnitude of muscle forces for different activities cannot be determined accurately at the hip and what is the name given to the hip because you cannot calculate the muscle forces?
- Because there are 7 groups of muscles and ligaments that act across the hip joint and different combinations of these are active at any one time to balance the external forces to maintain equilibrium
- It is not possible to calculate the muscle forces if more than one muscle is active, because there would be more unknown forces than equations to solve them. The structure is therefore called indeterminate.
When the hip is described as indeterminate what does this mean ?
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 the stresses acting at the hip joint and give 2 reasons why?
Standing on one leg - either stationary or during walking gait cycle
Because the muscle forces can be estimated with some degree of confidence as only the abductor muscle forces need to be calculated as the other muscles are not active. Also it is believed to generate high bending stresses in the femur and femoral component of the prosthesis.
Give two reasons why it is so 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.
Which of these activities generates the largest and smallest joint reaction force: walking, ascending stairs, rising from a chair, descending stairs?
- Ascending stairs
- Walking
- Descending stairs
- Rising from a chair
Define what is meant by an indeterminate structure
One where are more unknown forces than there are equations to solve them. The forces can sometimes be estimated quite well by making certain assumptions about the likely proportions that each muscle will contribute, such as minimising total muscle energy consumption. This requires some rather complicated mathematics, called optimisation methods.
In what plane does the highest moments occur at the hip joint ?
The coronal plane
How does compressive stress arise in the femur ?
The hip joint force, J, has a component, FC, which causes a compressive force in the femur, giving rise to a compressive stress.
State the equation to calculate the compressive stress at any section in the bone
The compressive stress, σc, can be calculated by dividing the compressive force (Fc), by the cross sectional area of the bone (A)
Along with the hip joint force what also affects the compressive stresses on the hip?
The pull of the muscles particularly at the trochanter, and the head and the neck of the femur
How is the compressive joint force in the hip transferred to the prosthesis ?
It is transferred from the stem to the femur as a shear force
- Passing 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 in a hip prosthesis is not sufficiently strong?
The prosthesis will loosen and sink down the medullary cavity
How can the compressive stress in the stem of a hip prosthesis be found?
By dividing the compressive load taken by the stem at any section along its length by the area of that cross section.
What does the compressive load taken by the stem of a hip prosthesis vary due to ?
It varies along the length of the stem
What design features help prevent the stem of hip prosthesis from sinking distally in the medullary canal?
- by tapering the stem
- by using a collar at the proximal end of the stem
- by fixing the bone to the stem, by means of bone ingrowth or adhesion
- by using a cement strong enough to withstand the shear stresses
what are design features of hip prosthesis help to reduce interface shear stress by converting shear loads to compressive loads?
- by using a support, such as a proximal collar on the stem
- by tapering the stem
what are design features of hip prosthesis help avoid fractures of the stem?
- by selecting a stem with a sufficiently large cross section to resist the stresses
- by selecting a high strength material for the stem.
How is excessive stress shielding of the bone in hip prostheses avoided and why is this important?
- by careful selection of the rigidity of the stem under axial loading.
The bone forms a structural composite with the stem so there are important load transfer and load sharing mechanisms in play which affect the proportion of the total load carried by each component. If there is excessive stress shielding by the stem, this will result in bone resorption and possible loosening of the implant.
