3. The major breakthrough in the use of PMMA in
total hip replacement (THR) was the work of
Charnley
In 1970 who used it to secure fixation of the
acetabular and femoral component and to
transfer loads to bone.
6. IN ARTHROPLASTY
Allows secure fixation of implant to bone
It’s not glue – has no adhesive properties
Mechanical interlock and space filling
Load transferring material (from the component
into bone)
Maintenance/restoration of bone stock
7. MECHANICAL PROPERTIES
Poor tensile strength of 25 Mpa
Moderate shear strength of 40 Mpa
Strongest in compression of 90 Mpa
8. Cement - Basics
Two primary component – PMMA powder and a
liquid monomer – MMA.
Mixed in ratio of 2 :1 to form PMMA cement.
To make it radioopaque: zirconium dioxide (ZrO2)
or barium sulphate (BaSO4)
10. Polymerization process (curing)
carbon-to-carbon double bonds broken
new carbon single bonds form
Linear long-chain polymers
free of cross-linking
Volume shrinkage (7%)
12. Mixing Phase
Starts with the addition of the liquid to the powder
and ends when the dough is homogenous and
stirring becomes effortless.
The liquid wets the surface of the prepolymerized
powder.
Because PMMA is a polymer that dissolves in its
monomer (which is not the case for all polymers), the
prepolymerized beads swell and some of them
dissolve completely during mixing. 會溶
At the end of the mixing phase, the mixture is a
homogenous mass and the cement is sticky and has
a consistency similar to toothpaste.
13. Waiting Phase
Allows further swelling of the beads and to permit
polymerization to proceed.
This leads to an increase in the viscosity of the
mixture.
The cement turns into sticky dough.
This dough is subsequently tested with gloved
fingers every 5 seconds, using a different part of
the glove on another part of the cement surface
on each testing occasion.
This process provides an indication of the end of
the waiting phase when the cement is neither
“sticky” nor “hairy.”
14. Working Phase
The cement is no longer sticky, but is of sufficiently low
viscosity to enable the surgeon to apply the cement.
Polymerization continues and the viscosity continues to
increase;
Heat of polymerization causes thermal expansion of the
cement, while there is a competing volumetric shrinkage of the
cement as the monomer converts to the denser polymer.
With a very low viscosity, the cement
would not be able to withstand
bleeding pressure. This would result
in blood lamination in the cement,
which causes the cement to weaken.
This phase is completed when the cement does not join
without folds during continuous kneading by hand
15. Hardening or Setting phase
The polymerization stops and the cement cures
to a hard consistency.
The temperature of the cement continues to be
elevated, but then slowly decreases to body
temperature.
During this phase, the cement continues to
undergo both volumetric and thermal shrinkage
as it cools to body temperature.
16. Curing process time periods
Dough time: mixing >> non sticky (approximately
2-3 minutes)
Working time: difference between dough time and
setting time (end of dough time until the cement is
too stiff to manipulate, usually about 5-8 minutes)
Setting time: mixing >> surface temperature is
half maximum (usually about 8-10 minutes)
17. Factors that Affect Bone Cement
Preparation
The ambient temperature - higher the temperature, the
shorter the phase and the colder the temperature, the
longer the phases.
The mixing process - Mixing cement too quickly or too
aggressively can hasten the polymerization reaction
resulting in a reduced setting time.
In general, the lower the heat of polymerization, the longer
the setting time, and the greater the heat of polymerization,
the shorter the setting time. (用熱水的目的)
The powder to liquid ratio (粉越多 越快硬)
- If more liquid, or less powder, than required is used,
setting time
will be prolonged;
- on the other hand, if less liquid, or more powder is used,
setting
time will be shortened
19. Different Types
Low viscosity: Long-lasting liquid, or mixing
phase, which makes for a short working phase.
Requires strict adherence to application times
but gives time for filling Delivery syringe
High viscosity: Short mixing phase, loose
their stickiness quickly. Longer working phase,
giving the surgeon more time for
application.(Depuy)
23. Bone Bed Preparation
Effective micro-interlock between the bone -
cement is essential.
Use of Pressure Pulse lavage and brush have
shown to reduce the incidence of aseptic
loosening.
Curettes, knives can be used but not as effective.
24. Bone Bed Preparation
Reaming
Brushing
Lavage
Cement Restrictor
Anchorage hole
Key – To preserve the cancellous bone.
Aim – Good microlock
25. Reaming
Shape the femoral canal and acetabular cavity for
even cement mantle.
Preoperative planning helps.
Preserve the cancellous bone.
Preserve the transverse acetabular ligament.
27. Brushing and lavage
Brushing – both acetabular and femoral reduce
the loose cancellous bone and soft tissue.
High pressure pulse lavage produce clean
surface. Also prevents circulatory changes.
Both reduce the risk of lamination in the cement
that can decrease the strength of mantle by 8 -
16%
31. Delivery
Hand packing – made into rough cylindrical mass
and then packed.
Gun: ensures delivery under pressure.
Inject the cement in retrograde fashion, letting the
cement gun work its own way out of the femur.
Swedish Hip Registry has shown that retrograde
cement filling using a cement gun in the femur
reduces risk of revision.
33. Pressurization
Involves use of Cement Gun, special pressurizers,
and centralizers.
Afford greater penetration into cancellous bone
ensuring micro – lock.
Optimal viscosity is key to good pressurizing.
Femur -Marrow extrusion in the greater trochanter
(the so-called sweating trochanter sign).
37. Safety First!
Liquid monomer is highly volatile, flammable,
powerful lipid solvent.
Avoid contact of the liquid with surgical gloves,
skin or mucous membranes.
Fumes -irritation of the respiratory tract, eyes,
liver; hypersensitivity, contact dermatitis.
Contact Lenses: reactions between monomer
vapors and soft contact lenses.
38. Word of Caution!
Premature insertion of bone cement -> drop in BP,
arrhythmias or ischemia.
Expulsion of bone marrow –PE; esp
Osteoporotic, overpressurization.
Inform Anaesthetist prior to cementing –
Hypotensive episode lasts between 30 sec to 5
min.
