Unit 2 - Hip Replacement Flashcards
Who designed the first successful hip prosthesis?
John Charnley
What features does the Charnley hip have that other prior prostheses didnt?
smaller femoral head
bone cement
HDP bearing surface
matching instrumentation
What kind of fit does a cementless hip rely on?
interferene (press) fit
Why is it difficult to produce new bone cements?
they are classed as drugs by the FDA
What is the general criteria for hip replacements?
tolerated in the body with no short term and little long term risk of adverse effects
give pain relief and restore activities of daily living
should exceed the expected life span of the individual without need for revision
insertable by a competent surgeon of average ability
acceptable cost inc. hospital stay and country economy
what is the essential functional movement required at the hip?
extend slightly
flex to a minimum of 30 degrees
abduct when weight bearing
rotate when in full extension
Name a computational method of stress analysis
finite element analysis
Name a traditional method of stress measurement
strain gauge attached to the surface of the bone
Which activity of daily living involving hip movement has the largest resultant force?
ascending stairs
Which activity of daily living has the lowest resultant force on the hip?
rising from a chair
Why is it difficult to calculate exact loads on the hip joint?
there are 7 sets of ligaments and muscles
What is it called when the exact forces in a joint cannot be calculated?
indeterminate structure
give two reasons why the loads on the hip joint cannot be determined accurately
magnitude of muscle forces cannot be determined accurately
joint load varies according to physical activity being undertaken
What is the equation for compressive stress?
Compressive force/area compressed
What design features can prevent the stem sinking distally into the medullary canal?
tapering the stem
proximal collar
fixing bone to the stem by ingrowth/adhesion
strong cements to withstand shear
How can interface shear stress be reduced?
proximal collar
tapering
(convert shear to compression)
How can fracture of the stem be avoided?
sufficiently large cross-section
high strength material
How can stress shielding be avoided?
careful selection of the stem rigidity
What is the equation for bending stress>
Bending moment * distance from neutral axis/ the second moment of area
What design features can be implemented to ensure the stem doesn’t fail under bending load?
large enough I
shape design to limit bending moment due to joint force
How can stem loosening be avoided from bending stresses?
strong bond between bone an stem/cement
good press fit in the medullar canal
How can stress shielding of the bone from bending loads be minimised?
suitable rigidity of stem
What other two stresses are generated under bending load?
radial and circumferential
What is another word for circumferential stress?
hoop stress
Where are radial stresses highest?
points of bone-stem contact
What causes hoop stresses?
radial stresses which act in a direction to tend to split the bone
What kind of stems cause high radial stresses?
short stems
how are hoop stresses avoided?
long enough stem
ensure good fit i the medullary canal
What design factors help to reduce torsional stresses?
non-circular stem sections
shear strength of cement
good bone-cement and cement-implant bonding
surface treatments of the stem to improve interface bonding
Why are non-circular sections for the stem used?
allow some of the torsional stress to be taken as compression
Name some important factors in design of acetabular replacement
size and conformity of joint surfaces
ways to maintain the integrity of the subchondral bone
thickness of cement layer
use of a backing plate
technique used to fix the cup into the remaining acetabular bone
