Unit 2 - Hip Joint Replacement Flashcards
Name 3 contributions to joint replacement design made by John Charnley?
- Designed the low friction hip, which has a small femoral head (and reduces interface shear forces)
- A low friction metal-HDP bearing and bone cement
- First to produce a system of instrumentation for inserting and aligning the prostheses
For daily living it is essential to be able to stand, walk and sit down. To do these tasks efficiently the hip must:
- Extend slightly
- Flex to a minimum of 30 degrees
- Abduct when weight bearing
- Rotate when in full extension
What are the 2 important points to remember concerning the biomechanics of normal and replacement hips?
- Loading varies according to physical activity being undertaken
- The magnitude of muscle forces for different activities cannot be determined accurately
Which of these activities generates the largest and smallest joint reaction force: walking, ascending stairs, rising from a chair, descending stairs?
In order, highest first:
- Ascending stairs
- Walking
- Descending stairs
- Rising from a chair
The highest moments occur in which plane?
Coronal
In a hip prosthesis, compressive joint force is transferred from the stem to the femur as what?
Shear force
Name 4 ways of preventing the stem 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
Name 2 ways to reduce interface shear stresses by converting shear loads to compressive loads?
- By using a support, such as a proximal collar on the stem
2. By tapering the stem
Name 2 ways to avoid fracture 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
Name a way to avoid excessive stress shielding of the bone?
By careful selection of the rigidity of the stem under axial loading
What is the formula for being stress?
Bending stress = bending moment x distance from neutral axis/second moment of area
What are the 3 quantities that influence the maximum bending stress in a structure?
- Magnitudes of the bending moment
- Distance from the neutral axis to the outer edge of the cross section
- Second moment of area
Name 2 ways to ensure that the stem does not fail under a bending load?
- By designing it with a large enough second moment of area
- By designing its shape to limit the magnitude of the bending moment due to the
joint force
Name 2 way stop avoid the stem loosening?
- By providing a sufficiently strong bond between the bone and the stem or cement
OR - By providing a good press fit of the stem in the medullary canal
Name a way to minimise stress shielding of the bone under bending loads?
By selecting a suitable rigidity for the stem
Name 2 ways to avoid excessive hoop stresses?
- By ensuring that the stem is long enough
2. By providing a good fit of the stem in the medullary cavity
Why is it desirable to use non-circular sections for the stem of a femoral component?
To reduce the shear stresses in the stem-bone interface
State 3 important factors to consider in the design of a replacement acetabulum?
- The stiffness and thickness of the cup
- Whether or not to use a cup with a metal backing plate
- The method used for fixing the cup into the acetabular socket
What is bone cement made from?
Polymethyl methacrylate (PMMA)
Name 3 problems of cement?
- When setting, temperature are high enough to destroy body tissue
- Small fragments cause intense inflammatory reactions
- Repetitive tensile loading can part the cement from its contact with the bone and stem which can lead to fatigue failure and eventual loosening
Name 4 ways to try and improve the longevity of cemented prostheses?
- Increase the keying of the cement to the prosthesis
- Coat metal components with PMMA
- Combine a PMMA surface coating on the prosthesis with a cement that bone can bond to
- Make the stem smooth and allow it to sink down the canal
Name 3 ways to strengthen a stem-cement bond?
- Coating the stem with PMMA
- Roughening its surface
- Applying a porous coating
Cementless stems are surface coated using what?
Hydroxyapatite
What is a disadvantage of fully coated stems?
Promotes stress shielding of the bone
The bending moment can be reduced by reducing the offset distance from the head to the neutral axis of the stem through 2 methods, what are they?
- By reducing the length of the neck of the stem
2. By increasing the angle between the long axis and the axis of the neck
How does a tapered “wedge” help proximal load transfer?
By transferring a significant proportion of the load in compression, rather than shear
What are the 2 main types of wear that occur between bearing surfaces?
- Abrasion wear
2. Adhesion wear
Why does abrasion wear happen?
Happens because surfaces are not perfectly smooth
Why does adhesion wear happen?
Happens because the 2 bearing surfaces stick to each other and usually the soft surface is torn off by the harder one
Name 3 methods of reducing the amount of wear debris?
- Reduce loading on the joint
- Keep the sliding distance as small as practically possible
- Find alternative materials for the replacement head that reduce wear in the HDP
Name 2 disadvantages of a small diameter femoral head?
- Depth of wear is greater
2. Increased likelihood of dislocation in the post-operative period
Name 3 ways to reduce stress at the bone-implant interface and therefore prevent loosening?
- Using a thick layer of HDP
- Using a thick layer of cement
- Using a metal backed cup
Name 2 disadvantages of the cup moving nearer to the mid-line of the body?
- Prosthesis is mounted on softer and weaker bone
2. The deepened cup leads to earlier impingement of the femoral neck on the rim of the acetabulum
