Hip Replacement 2 Flashcards
What is bone cement made from?
PMMA - polymethyl methacrylate
How is bone cement made?
monomer powder mixed with a volatile agent containing catalyst which makes a doughy substance
What additives can be put into bone cement?
antibiotics
radio-opaque materials
Is PMMA an adhesive?
No - a filler like grout
What can be done to increase cement keying?
roughened prosthesis
beaded coatings
wire coating
Name advantages of cemented prostheses
no need for exact fit as bone cement will fill the holes = less rasping and less bone loss
provides an even stress distribution as cement fills all holes
Name some problems with cement
exothermic curing reaction can damage tissues
small fragments can cause intense inflammation
wear particles in the joint bearings rapidly increase wear
bone cement is weaker in tension and shear and such stress can cause loosening
why is hydroxyapatite coating of interest?
could mean direct bonding between the bone and prostheses
Why does a fibrous layer form between the bone and the prostheses?
micromotion - accurate fit and controlled weight bearing are important in prevention
How can the longevity cemented protheses be increased?
techniques to increase keying of cement
coat metal components with PMMA which can then bond with inserted cement
combine PMMA coating with a cement that bone can bond to
remove cement altogether and allow an interference fit
Why is the non-bonded interface a problem?
high amounts of motion releases wear particles which cause biological reaction
also increases wear at the joint bearings
fibrous layers can form
Why can cement be removed from load calculations?
it takes such little load it can be ignored
What is the advantage of an isoelastic stem?
they have the same stiffness as bone
What is the main disadvantage of isoelastic stems?
high proximal load transfer leads to failure of the interlocking and chemical bonds at the interface
What effect does cement have on load transfer?
allows good contact between the stem and bone therefore reducing stress concentrations
What arguments are presented by Harris, Boston, USA?
proximal collar allows compressive load transfer. The collar prevents sinkage and the bone-cement interface is usually well interlocked so no-need to have slip
Why does Ling disagree with Harris?
Ling says the collar-calc area acts as a pivot for the stem to rotate causing high stress concentrations distally and failure. Also debris produced by fretting at the collar. Cement metal interface is weak and therefore it should be free to slide
Which of Harris and Ling have prevailed?
Neither - and both cement and cementless stems are used successfully
What fit do cementless stems rely on?
press/interference fit
What can be used to coat cementless stems?
HaP
What causes thigh pain in cementless stems?
lack of distal contact
Why does bone ingrowth into cementless stems sometimes not occur?
excessive movement - can cause growth of a fibrous layer
Why is it useful to completely coat the stem in HaP?
helps bone ingrowth and potentially eliminates metal debris from bone- metal abrasion. It also gives the opportunity for the bone to bond to a larger area of the stem which lessens the chance of failure of the bond under subsequent loading.
Why is the shape of the stem important in cementless stems?
there needs to be excellent bone-stem contact
What effect does head offset have on load transfer?
reduces the bending moment and increases compression
how can head offset be reduced?
decreasing neck length
increase offset angle
What is the main problem with reducing femoral head offset?
increases the joint reaction force giving rise to greater wear and acetabular-bone-implant stesses.
what equation explains the increase in J as a result of reduced femoral head offset?
J = W*d/x
What two factors influence friction?
surface properties of the two materials in contact
the magnitude of the load pressing them together
How many times less friction does a synovial joint have over a HDP-metal bearing?
3-100x
Why are small femoral heads used?
smaller surface area = less friction = less shear force
What is adhesive wear?
when two bearing surface stick to each other when they are pressed together, and, the softer one is torn off by the harder one
What is abrasive wear?
occurs when surfaces are not perfectly smooth
How can adhesive wear be reduced?
use bearing surfaces with low adhesion and provide lubricants
How can abrasive wear be reduced?
good circulation of lubricant
highly polished surfaces
What is the problem with HDP wear particles?
they cause intense inflammatory reactions
What is the main cause of aseptic loosening?
HDP wear particles
