Biomechanics of normal & replacement hip joint Flashcards
What is “press-fit”?
Long stem driven down the femoral canal (relies on close surface contact - no screws or other devices used)
What were the 4 major contributions of Charnley to hip joint replacement?
Smaller head (reduce problems of loosening asoc eith bearing friction)
Introduced bone cement (to help distribute loads)
Introduced HDP as bearing material (low friction)
Produced a system of instrumentation
What are the advantages and disadvantages of using a ceramic as a bearing material?
Adv - frictional and wear properties
Disadv - brittle, subject to sudden failure
What is the single most effective way of relieving pain and restoring function in sufferers of arthritis of the hip?
Hip arthroplasty
What are the essential functional movements of the hip?
Extend slightly
Flex to a min of 30 degree
Abduct when weight bearing
Rotate when in full extension
What are the two methods of estimating the stresses on the hip joint?
Measuring - strain gauges (use E to calculate stress)
Finite Element Analysis
How many groups of muscles and ligaments cross the hip joint?
7
Why is the hip joint indeterminate as a structure?
Not possible to calculate muscle forces if more than one muscle is active - must be approximated
In which plane of the hip joint do the highest moments occur?
Coronal plane
how can the compressive stress be calculated at any point in the femur?
Divide compressive force component by cross-sec area
How is force transmitted from the stem ot the femur?
As a shear force
What 4 methods exist for preventing the stem from sinking distally in the medullary canal?
Tapering the stem
Using a proximal collar
Fixing the bone ot the stem (bone ingrowth or adhesion)
Using a cement strong enough to withstand the shear stress
What are the 2 ways of reducing interface shear stresses by converting shear loads to compressive loads?
Using a support (e.g. proximal collar)
Tapering the stem
What are the 2 ways of avoiding fracture of the stem?
Use a stem with sufficiently large cross-sec to resist the stresses
Use high strength material for the stem
What equation is used to calculate bending stress?
Ϭ = My / I
M = applied bending moment (
y = distance from neutral axis to section of interest
I -= second moment of area
When calculating the hip joint force which group of muscles are presumed only to be acting?
Abductor
What is the force required from the abductor muscle group to maintain equilibrium?
2 x body weight
In terms of bending stress where is failure most likely to occur/
If stem loosens proximally - beinding moment at distal end increases - failure can occur
Why is the value for I for the stem at any point along the stem smaller than that of adjacent bone?
Because its cross-sectional dimensions are smaller
What are the 2 design factors to ensure that the stem does not fail under a bending load?
Designing it with a large enough seond moment of area
Dseigning its shape to limit the magnitude of the bending moment due to the joint force
What are radial stresses?
Stresses that are directed radially outwards from a central point
Where are radial stresses in the femur greatest?
At the points of bone-stem contact and the proximal and distl ends
What are hoop stresses?
Primarily tensile stresses that act in a direction that tends to split the bone
How does radial stress relate to the length of contact of the stem with the bone?
Inversely proportional to the square of the length of contact (stems of short length are prone to cause higher radial stresses in the bone)
