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As noted, bone cement is an acrylic, and it is similar to other plastics, in that it undergoes relaxation over time. All bone cements creep to some degree;
Dough time: starts from beginning of mixing and ends at the point when the cement will not stick to unpowdered surgical gloves. This occurs approximately 2-3 minutes after the beginning of mixing for most PMMA cements. ●●Working time: this is the time from the end of dough time until the cement is too stiff to manipulate, usually about 5-8 minutes. ●●Setting time: from the beginning of mixing until the time at which the exothermic reaction heats the cement to a temperature that is exactly halfway between the ambient and maximum temperature (i.e., 50% of its maximum value) and is the dough + working times; usually about 8-10 minutes.
Mixing Phase. The mixing phase starts with the addition of the liquid to the powder and ends when the dough is homogenous and stirring becomes effortless. When the liquid and powder components of the cement are mixed together, 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. This dissolution results in a substantial increase in the viscosity of the mixture; however, at this stage the viscosity is still relatively low, compared with the later phases of polymerization. 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. ●●Waiting Phase. The mixing phase is followed by a waiting period to allow further swelling of the beads and to permit polymerization to proceed. This leads to an increase in the viscosity of the mixture. During this phase, 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.” ●●Working Phase. The beginning of the working phase occurs when the cement is no longer sticky, but is of sufficiently low viscosity to enable the surgeon to apply the cement. During this period, polymerization continues and the viscosity continues to increase; in addition, the reaction exotherm associated with polymerization leads to the generation of heat in the cement. In turn, this heat causes thermal expansion of the cement, while there is a competing volumetric shrinkage of the cement as the monomer converts to the denser polymer. During the working phase, the viscosity of the cement must be closely monitored because 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; at this point, an implant can no longer be inserted (Figure 2). Therefore, the prosthesis must be implanted before the end of the working phase.
Bone cement.pptx 2 its science and cementing technique and safe surgical use
• Polymethylmethacrylate remains one of the
most enduring materials in orthopaedic
• It has a central role in the success of total
joint replacement and is also used in newer
techniques such as percutaneous
vertebroplasty and kyphoplasty (tricalcium
• The major breakthrough in the use of PMMA
in total hip replacement (THR) was the work of
• In 1970 who used it to secure fixation of the
acetabular and femoral component and to
transfer loads to bone.
• 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
• Poor tensile strength of 25 Mpa
• Moderate shear strength of 40 Mpa
• Strongest in compression of 90 Mpa
• Brittle, notch sensitive
• Low Young’s modulus of elasticity (E) =2400
• Materials properties vary with rate of
• Behaves like both fluid and solid
• examples – ligaments and cartilage
• Viscoelastic material has three main
characteristics viz. creep, relaxation and
• Time-dependent deformation under constant
load (also known as plastic deformation)
• Creep rate reduces with time
• load of daytime activities causes creep
• Creep, essentially is a mechanical problem that
slowly and steadily can erode the long-term
performance of an implant.
• Cements with higher porosity and viscosity are
less resistant to creep deformation.
2. Stress relaxation
• The change in stress / force with time under
constant strain (deformation) caused by a
change in the structure of the cement
• at night reduced load allows stress relaxation
• The loading and un loading curves are not
• Not all the energy applied to the specimen
during loading is recovered on unloading.
• Gamma radiation shortens the polymer
chains, probably affecting many mechanical
• but this does not occur with ethylene oxide
• 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
• 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.
• 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.
• 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
• This process provides an indication of the end of the
waiting phase when the cement is neither “sticky” nor
• 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;
• Therefore, the prosthesis must be implanted before the end of the
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
• During this phase, the cement continues to
undergo both volumetric and thermal
shrinkage as it cools to body temperature.
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)
typical curing curve for acrylic bone cement where Tmax is the
maximum temperature reached, Tset is the setting temperature
and Tamb is the ambient temperature.
• However, the rates of curing are very sensitive
to environmental factors.
• Low ambient temperatures during storing and
mixing, and high humidity both prolong
Heat production during
• The polymerisation of PMMA is exothermic.
• Can lead to thermal damage to bone.
• Recorded temperatures range between 70°C
• In vitro studies have shown that the
production of heat is increased by-
1. thicker cement mantles,
2. higher ambient temperatures and
3. an increased ratio of monomer to polymer.
• Increases in room temperature shorten both
the dough and setting times by 5% / degree
Factors that Affect Bone Cement
• 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
• 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.
• The powder to liquid ratio
- 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
• First generation
• Original technique of Charnley:
• Hand mixing of the cement
• Finger packing of cement in an unplugged and
uncleaned femoral canal and acetabulum
• No cement restrictor, no cement gun and no
reduction in porosity
• Femoral canal plug
• Cement gun to allow retrograde filling
• Pulsatile lavage
• Cement restrictor
• Pressurization of cement after insertion
• Some form of cement porosity reduction
(vacuum or centrifugation)
• Stem centraliser
Cartridge mixing and delivery
• Latest advancement in bone cement mixing
• It is a simple, universal power mixer that quickly
mixes and then mechanically injects all types of
• This type of device reduces mix times, as it
requires fewer steps to load, mix, and transfer the
• The rotary hand piece reduces variability, which
results in consistent mix times and built-in
charcoal filter reduces harmful fumes.
Antibiotics & Bone Cement
• Ideal antibiotic properties
1. Preparation must be thermally stable
2. Antibiotic properties not affected by heat
3. Must be water soluble for diffusion into tisssues
5. Must be released gradually over an appropriate time
6. Minimal local inflammatory response
7. No resistance
8. Must have action against common pathogens like s.
aureus, s. epidermidis ,coliforms and anaerobes.
9. Must not significantly compromise mechanical integrity
• Gentamycin (most common) and tobramycin
are commonly used
• Vancomycin and ciprofloxacin are also tried
• Ciprofloxacin may inhibit soft tissue healing
• Penicillins and cephalosporins exhibits stability
and good elution properties. But are avoided
due to their potential allerginicity.
• Vancomycin p (ultrafine powder) is used as
lyophilised vancomycin greatly reduces fatigue
• > 4.5 g gentamycin significantly reduces fatigue
• > 10% reduction in fatigue strength is considered
inappropriate for use in arthroplasty fixation
• but weaker cement with higher antibiotic
concentration may be used for spacers and
antibiotic beads ( to deliver high antibiotic
• Vacuum mixing increases fatigue strength
• ↑ed surface area leds to ↑ed drug elution
Antibiotic may be used in low doses or
• Antibiotic content is 0.5 – 1 gm per 40 gm of
• Used in
1. Prophylaxis in revision arthroplasty or
2. In high risk primary arthroplasty
• Contains > 3.6 gm of antibiotic per 40 gm of
• Used in
1. Spacers and beads
2. Second stage of two staged revision ( content
is 1-2 gm per 40 gm cement)
• Cement spacer commonly contains 4 gm
vancomycin and 4.8 gm of tobramycin in 80
gm of cement
(bone cement implantation syndrome)
• BCIS is characterized by hypoxia, hypotension
or both and/or unexpected loss of
consciousness occurring around the time of
cementation, prosthesis insertion, reduction
of the joint or, occasionally, limb tourniquet
deflation in a patient undergoing cemented
• it is characterized by a number of clinical
features that may include-
• cardiac arrhythmias,
• increased pulmonary vascular resistance
• and cardiac arrest.
Proposed severity classification of bone
cement implantation syndrome
• Grade 1: moderate hypoxia (SpO2-94%) or
hypotension [fall in systolic blood pressure
• Grade 2: severe hypoxia (SpO2-88%) or
hypotension (fall in SBP 40%) or unexpected
loss of consciousness.
• Grade 3: cardiovascular collapse requiring
• Several hypothesis
1. Monomer absorption into circulation – but
very low levels of MMA in circulation are
found during cementation.
• Embolization occurs as a result of high
intramedullary pressures developing during
cementation and prosthesis insertion.
• The cement undergoes an exothermic reaction
and expands in the space between the
prosthesis and bone, trapping air and
medullary contents under pressure so that
they are forced into the circulation.
• Embolic showers have been detected using
echocardiography in the right atrium, RV, and
pulmonary artery during surgery.
• Post-mortem studies have demonstrated
pulmonary embolization in animals and man.
• The physiological consequences of embolization
are considered to be the result of both a
mechanical effect and mediator release, which
provokes increased pulmonary vascular tone.
Pulmonary vessel with embolus comprising fat, platelets, fibrin, and marrow
debris. (1) Reflex vasoconstriction and endothelial production of endothelin 1.
(2) Release of vasoconstriction mediators; platelet derived growth factor
(PDGF), serotonin (5-HT), thromboxane A2 (Tx-A2), platelet activating factor
(PAF), adenosine diphosphate (ADP). (3) Vasoconstriction attributable to non-
cellular components of embolus including thrombin.
the increased Pulmonary Vascular Resistance
reduced right-ventricular ejection fraction
the compliant right ventricle (RV) distends
the interventricular septumto bulge into the left
further reducing LV filling, and therefore CO
• When cement is inserted into the femur using
a cement gun, the pressures generated are
almost double those seen when manual
packing is used.
• It has been demonstrated that this debris
• cement particles,
• bone particles,
• and aggregates of platelets and fibrin.
3.Histamine release and
• Anaphylaxis (Type 1 hypersensitivity) was
implicated as a potential cause for a fatal case
of BCIS in 1972.
• The anaphylatoxins C3a and C5a are potent
mediators of vasoconstriction and
• An increase in C3a and C5a levels, suggesting
activation of the complement pathway, has
been demonstrated in cemented
• It is likely that a combination of the above
processes is present in any individual patient
who develops BCIS.
Patient risk factors
Numerous patient-related risk factors have been
implicated in the genesis of BCIS, which are :-
1. old age,
2. poor preexisting physical reserve,
3. impaired cardiopulmonary function,
4. pre-existing pulmonary hypertension
6. bony metastases,
7. and concomitant hip fractures, particularly
pathological or intertrochanteric fractures.
• These latter three factors are associated with
increased or abnormal vascular channels
through which marrow contents can migrate
into the circulation.
Surgical risk factors
• Patients with a previously un-instrumented femoral
canal may be at higher risk of developing the syndrome
than those undergoing revision surgery.
• There are two possible mechanisms.
1. First, there is more potentially embolic material
present in an un-instrumented femur.
2. Second, once the canal has been instrumented and
cemented, the inner surface of the femur becomes
smooth and sclerotic and offers a less permeable
• The use of a long-stem femoral component increases
the likelihood of developing BCIS.
Anaesthetic risk reduction
• In high risk cases discussion should occur
between the surgeon and anaesthetist
regarding the most appropriate anaesthetic
and surgical technique, including the potential
risk-benefit of uncemented compared with
• The avoidance of nitrous oxide should be
considered in high risk patients to avoid
exacerbating air embolism.
• Increasing the inspired oxygen concentration
should be considered in all patients at the
time of cementation.
• Avoiding intravascular volume depletion may
reduce the extent of the haemodynamic
changes in BCIS.
• The use of an intraoperative pulmonary artery
catheter or trans oesophageal
echocardiography has been suggested in high
Surgical risk reduction
• Medullary lavage,
• good haemostasis before cement insertion,
• minimizing the length of the prosthesis,
• using non-cemented prosthesis (especially if
using a long-stem implant),
• and venting the medulla.
• Venting the bone permits the air to escape
from the end of the cement plug and reduces
the risk of an air embolus.
• Can increase the risk of femoral fracture.
• If cement is used, insertion with a cement gun
and retrograde insertion have been suggested as
ways of reducing the incidence of BCIS.
• Cement guns result in more even pressure
distribution in the medullary cavity, and less
reduction in oxygen saturation .
• Paradoxically, it has been demonstrated that
intramedullary pressures are higher when
cementation is performed with a cement gun
rather than finger packing.
• Vacuum mixing of cement reduces mortality
due to BCIS.
• reduction of the prosthetic femoral head is
also a time of increased risk because
previously occluded vessels are re-opened and
accumulated debris may be allowed into the
• During knee arthroplasty, significant venous
emboli are released at the time of tourniquet
deflation and this may also be a high risk
• Early signs of BCIS in the awake patient
undergoing regional anaesthesia include
dyspnoea and altered sensorium.
• If BCIS is suspected, the inspired oxygen
concentration should be increased to 100% and
supplementary oxygen should be continued into
the postoperative period.
• It has been suggested that cardiovascular collapse
in the context of BCIS be treated as RV failure.
• Aggressive resuscitation with i.v. fluids has been
• Haemodynamic instability should be treated
with the potential aetiology in mind.
• Sympathetic a1 agonists should be first-line
agent in the context of right heart dysfunction
• Fluid resuscitation should then be
commenced if there is insufficient pre-load.
Calcium Phosphate Cement
• Mainly used as bone graft substitute
• Is capable of hardening into calcium deficient hydroxyapatite
and remodeling at a similar rate to bone (1-2 years).
• Different forms: monocalcium phosphate monohydrate,
dicalcium phosphate dihydrate, hydroxyapatite, and alpha and
beta tricalcium phosphate (TCP).
• Although beta phase tricalcium phosphate is very
biocompatible, it does not form calcium deficient
hydroxyapatite (CDHA) in the body.
• This is the reason why alpha TCP is preferred to be used as
bone void fillers.
• Bone replacement ( instead of bone grafts) e.
g. osteoporotic bone, bone loss due to trauma
or tumor surgery, kyphoplasty.
•Very high compressive strength once
•Some studies of its use have allowed
earlier weightbearing and range of
• Wet compressive strength slightly less than
• Available as blocks, wedges, and granules
• Numerous tradenames
– Vitoss (Orthovita)
– ChronOS (Synthes)
– Conduit (DePuy)
– Cellplex TCP (Wright Medical)
– Various Theri__ names (Therics)
• The liquid and powder (ratio of 0.34ml/g) undergoes a solid-
state reaction at 370 C forming silicone induced calcium
deficient hydroxyapatite (CDHA+Si).
• New bone formation in silicone doped cement.
• Osteoclasts are also seen in silicon doped cement (bone
• Silicone has excellent osteoblastic activity, enhanced reactivity
with collagen and apatite increasing the rate of bone