CE ONLINE
Clinical and Economic
Focus on Bone Cement
and Mixing Systems
An Online Continuing Education Activity
Sponsored By
Grant funds provided by
Funds Provided By
Welcome to
Clinical and Economic Focus
on Bone Cement and Mixing
Systems
(An Online Continuing Education Activity)
CONTINUING EDUCATION INSTRUCTIONS
This educational activity is being offered online and may be completed at any time.
Steps for Successful Course Completion
To earn continuing education credit, the participant must complete the following steps:
1. Read the overview and objectives to ensure consistency with your own learning
needs and objectives. At the end of the activity, you will be assessed on the
attainment of each objective.
2. Review the content of the activity, paying particular attention to those areas that
reflect the objectives.
3. Complete the Test Questions. Missed questions will offer the opportunity to reread the question and answer choices. You may also revisit relevant content.
4. For additional information on an issue or topic, consult the references.
5. To receive credit for this activity complete the evaluation and registration form.
6. A certificate of completion will be available for you to print at the conclusion.
Pfiedler Enterprises will maintain a record of your continuing education credits
and provide verification, if necessary, for 7 years. Requests for certificates must
be submitted in writing by the learner.
If you have any questions, please call: 720-748-6144.
CONTACT INFORMATION:
© 2016
All rights reserved
Pfiedler Enterprises, 2170 South Parker Road, Suite 125, Denver, Colorado 80231
www.pfiedlerenterprises.com Phone: 720-748-6144 Fax: 720-748-6196
OVERVIEW
For the past 50 years, polymethylmethacrylate (PMMA) bone cements have been widely
used as the anchoring/grouting agent in total joint replacements of the hip, knee, ankle,
elbow, and shoulder. Good quality cement is essential for long-term implant survival;
therefore, the role of the perioperative nurse in preparing bone cement properly is vitally
important. Strict adherence to good cement mixing and application techniques is a key
factor in reducing the rate of loosening and also in increasing the long-term survival of the
prosthesis. Additionally, in today’s dynamic health care economic environment, the costs
associated with bone cement use must also be considered. The purpose of this continuing
education activity is to provide a review of key concepts regarding composition, properties,
and the various types of bone cements available today, including factors that affect
bone cement polymerization. The potential hazards posed by bone cement and safety
considerations for patients and members of the surgical team will be discussed. Finally,
economic considerations related to the use of bone cement in orthopedics today, including
the current paradigm shift of using products that maintain clinical excellence while being
fiscally responsible, will be outlined.
LEARNER OBJECTIVES
After completing this continuing nursing education activity, the participant should be able to:
1. Review the components of bone cement.
2. Describe the types of bone cement available today.
3. Differentiate the various bone cement mixing systems and application techniques.
4. Identify the safety issues related to the use of bone cement in the perioperative
practice setting.
5. Discuss the economic considerations related to the selections and use of bone
cement and mixing systems.
INTENDED AUDIENCE
This continuing education activity is intended for perioperative registered nurses who are
interested in learning more about the various types of bone cement, the process of bone
cement mixing, and key clinical and economic considerations.
Credit/Credit Information
State Board Approval for Nurses
Pfiedler Enterprises is a provider approved by the California Board of Registered Nursing,
Provider Number CEP14944, for 2.0 contact hour(s).
Obtaining full credit for this offering depends upon attendance, regardless of circumstances,
from beginning to end. Licensees must provide their license numbers for record keeping
purposes.
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The certificate of course completion issued at the conclusion of this course must be
retained in the participant’s records for at least four (4) years as proof of attendance.
Release and Expiration Date
This continuing education activity was planned and provided in accordance with accreditation
criteria. This material was originally produced in June 2016 and can no longer be used after
June 2018 without being updated; therefore, this continuing education activity expires in
June 2018.
Disclaimer
Pfiedler Enterprises does not endorse or promote any commercial product that may be
discussed in this activity
Support
Funds to support this activity were provided by CardinalHealth
Authors/Planning Committee/Reviewer
Rose Moss, MN, RN, CNOR
Nurse Consultant/Author
Moss Enterprises, LLC
Westcliffe, CO
Julia A. Kneedler, EdD, RN
Program Manager/Planning Committee
Pfiedler Enterprises
Aurora, CO
Judith I. Pfister, MBA, RN
Program Manager/Planning Committee
Pfiedler Enterprises
Aurora, CO
Melinda T. Whalen, BSN, RN
Program Manager/Reviewer
Pfiedler Enterprises
Aurora, CO
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for individuals who control content for an educational activity. Information listed below is
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or influences pose a potential bias of content, recommendations or conclusions. The intent
is full disclosure of those in a position to control content, with a goal of objectivity, balance
4
and scientific rigor in the activity. For additional information regarding Pfiedler Enterprises’
disclosure process, visit our website at: http://www.pfiedlerenterprises.com/disclosure.
Disclosure includes relevant financial relationships with commercial interests related to
the subject matter that may be presented in this educational activity. Relevant financial
relationships are those in any amount, occurring within the past 12 months that create a
conflict of interest. A commercial interest is any entity producing, marketing, reselling, or
distributing health care goods or services consumed by, or used on, patients.
Activity Authors/Planning Committee/Reviewer
Rose Moss, MN, RN, CNOR
No conflict of interest
Julia A. Kneedler, EdD, RN
No conflict of interest
Judith I. Pfister, MBA, RN
No conflict of interest
Melinda T. Whalen, BSN, RN
No conflict of interest
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5
INTRODUCTION
Polymethylmethacrylate (PMMA) bone cement is an essential component in many
total joint arthroplasty procedures. In a cemented arthroplasty, the main functions
of the cement are to immobilize the implant, transfer body weight and service
loads from the prosthesis to the bone, and increase the load-carrying capacity
of the prosthesis-to-bone cement-to-bone system. However, the term “cement,”
is misleading since bone cement acts more like a grout, filling in space in order
to create a tight space to hold the implant against bone. Good quality cement is
essential for long-term implant survival, and the role of the perioperative nurse
in preparing that cement is vitally important. Accurate bone cement mixing and
precise application techniques are critical to ensuring the stability and longevity
of the prosthesis. Since bone cement is prepared and used in the operating room
(OR) environment, it is important that all perioperative personnel recognize the
unique safety considerations that are related to its preparation and its use.
As with all aspects of health care today, the costs associated with the selection
and use of bone cement and mixing systems must also be considered. Fiscally
responsible products that provide clinically equivalent performance and offer
clinicians the ability to continue delivering high quality patient care, with no change
in practice or technique, are changing the paradigm in orthopedic product selection.
COMPONENTS OF BONE CEMENT
The PMMA bone cement is usually supplied as two-component systems made up
of a powder and a liquid. These two components are mixed at an approximate ratio
of 2:1 to start a chemical reaction called polymerization, which forms the PMMA
cement.
•
Powder components1:
o Copolymers beads based on the substance polymethylmethacrylate
(PMMA);
o Initiator, such as benzoyl peroxide (BPO), which encourages the
polymer and monomer to polymerize at room temperature;
o Contrast agents such as zirconium dioxide (ZrO2) or barium sulphate
(BaSO4) to make the bone cements radiopaque; and
o Antibiotics (eg, gentamicin, tobramycin).
•
Liquid components2:
o A monomer, methylmethacrylate (MMA);
o Accelerator (N,N-Dimethyl para-toluidine) (DMPT);
o Stabilizers (or inhibitors) to prevent premature polymerization from
exposure to light or high temperature during storage; and
o Chlorophyll or artificial pigment; sometimes added to cements for
easier visualization in case of revision.
6
There is a difference between PMMA bone cement and PMMA; however, many health
care personnel use the terms interchangeably and PMMA has become shorthand for
“bone cement.” It is important to understand that PMMA is the substance from which
copolymers are derived for the powder component. When the copolymer powder is mixed
with the MMA monomer liquid, polymerization occurs and PMMA bone cement is created.
POLYMERIZATION
Polymerization is a chemical reaction in which two or more small molecules combine to
form larger molecules that contain repeating structural units of the original molecules. In
the case of bone cement, the polymerization process starts when the copolymer powder
and monomer liquid meet, reacting together to produce an initiation reaction creating free
radicals that cause the polymerization of the monomer molecules. The original polymer
beads of the powder are bonded into a dough-like mass, which eventually hardens into
hard cement.
The polymerization process is an exothermic reaction, which means it produces
heat. With a maximum in vivo temperature of 40°C to 47°C, this thermal energy is
dissipated into the circulating blood, the prosthesis, and the surrounding tissue. Once
polymerization ends, the temperature decreases and the cement starts to shrink.
Phases and Times
The polymerization process can be divided into four different phases: mixing, waiting,
working, and setting. Package inserts that come with the products often refer to dough
time, working time, and setting time. Dough time and setting time are measured from the
beginning of mixing; working time is the interval between dough time and setting time.
Both the phases and corresponding times are described below.
Mixing Phase
The mixing phase represents the time taken to fully integrate the powder and liquid. As
the monomer starts to dissolve the polymer powder, the BPO is released into the mixture.
This release of the initiator BPO and the accelerator DMPT is actually what causes the
cement to begin the polymerization process. It is important for the cement to be mixed
homogeneously, thus minimizing the number of pores.
Waiting Phase/Dough Time
During this phase, typically lasting several minutes, the cement achieves a suitable
viscosity for handling (ie, can be handled without sticking to gloves). The cement is a
sticky dough for most of this phase.
Dough time is the time point measured from the beginning of mixing to the point when the
cement no longer sticks to surgical gloves. Under typical conditions (23°C to 25°C, 65%
relative humidity), dough time is 2 to 3 minutes after beginning of mixing for most bone
cements. Before this time point, after the components are well mixed, the bone cement
may be loaded into a syringe, cartridge, or injection gun for assisted application.3
7
Working Phase/Working Time
The working phase is the period during which the cement can be manipulated and the
prosthesis can be inserted. The working phase results in an increase in viscosity and the
generation of heat from the cement. The implant must be implanted before the end of the
working phase.
Working time is the interval between the dough and setting times, typically 5 to 8
minutes. Previously, this represented the full time interval available for use of a particular
mix of bone cement. The use of mechanical introduction tools, such as syringes and
cartridges, extends this time by 1 to 1.5 minutes.4
Setting Phase/Setting Time
During this phase, the cement hardens (cures) and sets completely, and the temperature
reaches its peak. The cement continues to undergo both volumetric and thermal
shrinkage as it cools to body temperature. Hardening is influenced by the cement
temperature, the OR temperature, and the body temperature of the patient.
Setting time is the time point measured 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 (ie, 50% of its maximum value), usually
about 8-10 minutes. The temperature increase is due to conversion of chemical to
thermal energy as polymerization takes place.5
Factors that Affect Dough, Working, and Setting Times
Factors that affect dough, working, and setting times include the following6:
• Mixing Process – Mixing that is too rapid can accelerate dough time and is not
desirable since it may produce a weaker, more porous bone cement.
• Ambient Temperature – Increased temperature reduces both dough and
setting times approximately 5% per degree Centigrade, whereas decreased
temperature in the OR increases them at essentially the same rate.
• Humidity – High humidity accelerates setting time whereas low humidity
retards it.
The combination of these factors is such that in a cold OR on a very dry winter day,
setting time may stretch out and raise concerns as to whether there is something wrong
with the bone cement kit in use. There usually is not, but patience is required under these
conditions. Water (or anything else) should never be added to bone cement in an attempt
to modify its curing behavior.
Why Don’t All Cements Behave the Same?
Despite the fact that basic PMMA bone cement materials are the same, the behavior of
various cement products can be significantly different when they are mixed under similar
conditions. Several reasons for these differences are included below.
8
•
•
•
•
•
The polymer component of a number of cements is not purely PMMA. Some
cement may contain PMMA copolymers such as methyl acrylate and styrene
in the powder and additional polymers such as butyl methacrylate. All cements
are labelled to show their ingredients.
The ratio of the components and the overall powder-to-liquid ratio may differ
between cements.
The size, shape and weight of the polymer molecules can vary considerably.
Manufacturing processes may differ.
Sterilization method may differ (eg, gamma, ethylene oxide gas).
CEMENT PROPERTIES
Cement properties (eg, viscosity change, setting time, cement temperature, mechanical
strength, shrinkage, residual monomer) critical for operating procedures are determined
during polymerization. These properties will influence cement handling, penetration, and
interaction with the prosthesis. The most important cement properties include porosity,
viscosity, and temperature as discussed below.
Cement Porosity
Porosity is the fraction of the volume of an apparent solid that is actually empty space.
High bone cement porosity compromises the cement’s mechanical strength and
decreases its fatigue life. This may lead to aseptic loosening. The source of porosity in
cured bone cement is usually trapped air that can occur:
• between the powder beads as the powder is wetted,
• during mixing, or
• during transfer from mixing container to application device.
Hand mixing bone cement in an open bowl may introduce the greatest possibility of
these occurrences, which is why hand-mixed cement can contain a substantial number of
pores. Centrifugation and vacuum mixing methods, and pressurized cement application
can decrease the porosity of bone cement.
Cement Viscosity
Viscosity is a measure of the resistance of a fluid to deformation under shear forces and
is commonly described as “thickness” of a fluid. Viscosity also represents the resistance
to flow and is thought to be a measure of fluid friction. Cement viscosity determines the
handling and working properties of the cement.
Mixing together the powder and the liquid components marks the start of the
polymerization process. During the reaction, the cement viscosity increases, slowly at
first, then later more rapidly. During the working phase, there are two requirements for
bone cement viscosity – it must be sufficiently low to facilitate the delivery of the cement
dough to the bone site, and it must penetrate into the interstices of the bone.7 On the
other hand, the viscosity of the bone cement should be sufficiently high to withstand
the back-bleeding pressure, thus avoiding the risk of inclusion of blood into the cement
9
because this could significantly reduce the stability of the bone cement. It is important
that the cement retains an optimized viscosity for an adequate duration to allow a
“comfortable” working time.8
Viscosity can affect the setting time, cement penetration into cancellous bone, and the
timing of the prosthesis insertion.9 For example, high viscosity cements are sometimes
pre-chilled for use with mixing systems to facilitate mixing and prolong the working
phase, which will also increase the setting time. In general, insertion of the prosthesis
should be delayed until the cement has developed a sufficient degree of viscosity to
resist excessive displacement by the prosthesis; however, insertion of the prosthesis
should not be delayed to the point that it increases the risk of not completing the
procedure because of cement hardening.
Cement Temperature
To achieve optimal cement properties, it is important to adhere to the time schedules
indicating the correlation of temperature to handling time. These time schedules are
usually included in the manufacturer’s instructions for the bone cement.
Effects of Temperature
Temperature affects mixing time, delivery of the cement, prosthesis insertion, and other
aspects of the cementing process. Storage temperature can also affect the cement
times – not just the temperature at which it is mixed as described in Table 1. In general,
all the phases except for the mixing phase will be prolonged when cement is stored in
a cold environment and all phases will be shorter when cement is stored in a warmer
environment. Issues related to high temperatures include difficulty integrating the powder
and liquid, as well as extrusion from the delivery gun. With high temperatures, there
is also the potential for inserting the cement during the setting phase and decreasing
cement strength due to the formation of laminations.10
10
Table 1 – Examples of How Temperature Effects Bone Cement
Cement Stored in Cold
Environment
Cement Stored in a Warmer
Environment
Issues created by high
temperatures1
All the phases apart from
the mixing phase will be
prolonged.
All phases will be shorter.
Integration of the powder and
liquid can be difficult.
High-viscosity cements are
sometimes pre-chilled for use
with mixing systems for easier
mixing and prolonged working
phase.
Extrusion from a delivery gun
can become difficult and may
reduce delivery pressures.
Pre-chilling can also increase
the setting time.
Potential exists for cement to
be inserted during the setting
phase.
Laminations can form
between 3.5 and 6.5 minutes
and reduce cement strength
by up to 54%.
1.
Gruen TA, Markolf KL, Amstutz HC. Effects of laminations and blood entrapment on the
strength of acrylic bone cement. Clin Orthop Relat Res. 1976;(119):250-255.
Mechanical Properties
The aim of a good cement mix is to produce bone cement that has the best mechanical
properties possible so that it can carry out its load transfer role successfully over the
lifetime of the implant. Once positioned within the hip or knee replacement, the cement
around the prosthesis is subjected to a series of physical forces that will have an effect on
the lifespan of the cement. These physical forces subject the cement to fatigue, creep, and
increased stress. The mechanical properties of the cement should be enhanced as much
as possible to increase the resistance to fatigue and creep and decrease stress on the
implant and cement to avoid exceeding the strength of the cement.
Fatigue
Fatigue is the failure of a component after it is subjected to a large number of alternating,
fluctuating loads; fatigue strength is a measure of a bone cement’s durability. If applied
only once, these loads would not be large enough to cause failure. A good example of this
is a paper clip, which when bent once will not break, but after it has been bent a number of
times, it will break easily.
As the cemented implant is subjected to not only static load but also dynamically
alternating loads, the fatigue properties of the cement affect survival of the implant.
Cement will have a natural lifespan, and the repeated loads it is subjected to over time will
cause it to break down and fail. The quality of the cement mix will determine its lifespan. A
well-mixed cement will be better equipped to deal with the loads placed upon it.
11
The ability of bone cement to resist fatigue is critical given the loads to which it will be
subjected. Clinical evidence has documented the existence of fatigue cracks in revisionretrieved cement11,12 and in postmortem retrieved stem/cement/bone constructs.13 This
suggests that the fatigue resistance of bone cement should be optimized to prevent
fatigue failure.
Creep
Creep is the deformation of a material under constant load. Under constant load, a
material capable of creep will deform by an amount dependent on the size of the load
and the length of time it is applied. Creep generally increases with temperature. Creep
essentially is a mechanical problem that slowly and steadily can erode the long-term
performance of an implant. Cements with higher porosity are less resistant to creep
deformation.
Polymers are particularly susceptible to creep because of their molecular structure.
Therefore, bone cement, as a polymer, is likely to exhibit creep as it is under a load and
is at 37°C in the body.
Significant bone cement creep will lead to implant subsidence, which, in turn, may lead to
failure.14 In the 1990s, a new formulation of bone cement had to be withdrawn after it was
found to significantly creep, leading to implant subsidence, aseptic loosening, and high
revision rates.15,16
Interestingly, a small degree of creep may in fact be advantageous in the early
postoperative stages with some implant designs. A polished, tapered stem without a
collar relies on some subsidence so that it becomes “wedged” in the bone cement,
thereby improving the load transfer mechanism.17
Stress
Stress is the load applied to a material over a given area. Stresses in the hip joint
are a combination of compression, bending, and torsional (twisting) forces. As load is
transferred during walking, the new joint and cement will be subjected to high stresses.
If these high stresses exceed the strength of the cement, it will deform permanently and
then, possibly, fail.
TYPES OF BONE CEMENT
Cements can be grouped as those with high, medium, or low viscosity and those with or
without antibiotics.
The viscosity designation refers to the viscosity of the powder and liquid during the
mixing phase: high-viscosity cement is dough-like, while low-viscosity cement is more
like a liquid. The handling phases of different viscosity cements also vary considerably,
and the choice of which cement to use is often surgeon preference. For example, a 2006
national survey of 587 surgeons in the United Kingdom found that high-viscosity cement
12
was used in total hip arthroplasty by 82% of the surgeons, medium-viscosity cement by
12%, and low-viscosity cement was used by 6%.18 The characteristics of the different
viscosities are described below and summarized in Table 2.
High Viscosity
High-viscosity bone cements have a short mixing phase and lose their stickiness quickly.
This makes for a longer working phase. The viscosity remains constant until the end of
the working phase. The setting phase lasts between one minute 30 seconds and two
minutes.19 High-viscosity cements are associated with reduced revision rates for total hip
arthroplasty.20
Medium Viscosity
These cements typically have a long waiting phase of three minutes, but during the
working phase, the viscosity only increases slowly. Setting takes between one minute 30
seconds, and two minutes 30 seconds.21
Low Viscosity
Low-viscosity cements have a long waiting phase of three minutes and the viscosity
rapidly increases during the working phase, making for a short working phase. As
a consequence, application of low-viscosity cements requires strict adherence to
application times. The setting phase is one to two minutes long.22
Table 2 – Characteristics of Bone Cement with Different Viscosities
High Viscosity
Medium Viscosity
Low Viscosity
Short mixing phase.
Long waiting phase of three
minutes.
Long waiting phase of three
minutes.
Viscosity increases slowly
during the working phase.
Viscosity rapidly increases
during the working phase.
Lose their stickiness quickly.
Viscosity remains constant
until the end of the working
phase.
Short working phase.
Longer working phase.
Setting phase lasts between
one minute 30 seconds and
two minutes.
Setting takes between one
minute 30 seconds, and two
minutes 30 seconds.
Associated with reduced
revision rates for total hip
arthroplasty.
The setting phase is one to
two minutes long.
Application of low-viscosity
cements requires strict
adherence to application
times.
Antibiotic Cements
Periprosthetic infection is the most feared complication in total hip and knee replacement.
The infection usually leads to a complete failure of the joint replacement, resulting in
13
a long series of operative procedures, great discomfort for the patient, and significant
costs.
The use of antibiotic-impregnated bone cement to treat musculoskeletal infection has
been reported in the literature for more than three decades despite the fact that it
wasn’t until 2003 that the first pre-blended bone cement containing an antibiotic (ie,
tobramycin) became available for sale in the United States, specifically for the treatment
and reimplantation of infected arthroplasties.23,24 Prior to 2003, United States surgeons
prepared antibiotic cement in the OR by adding antibiotic powder to the powdered bone
cement prior to the addition of the liquid monomer. In Europe, however, pre-blended
antibiotic bone cements have been available since the 1970s and the indications
and scientific evidence for its use have expanded to primary arthroplasty to minimize
postoperative infection. Use of antibiotic cements for primary arthroplasty, however,
remains controversial in the United States. The primary arguments against the routine
use of antibiotic bone cement are lack of efficacy, adverse effects on mechanical
properties, increased costs, bacterial resistance, and systemic toxicity.25,26 However, there
is significant evidence to refute these arguments.27,28,29
The elution of antibiotics from PMMA bone cement can be affected by certain factors
including the type of cement used, preparation methods, surface characteristics, porosity
of the cement, and the amount and/or type of antibiotics used.30
Not all antibiotics are suitable for use in bone cements. The first consideration is that
it must be available in a powdered form so that it can be added to the cement.31 The
antibiotic also should have a broad antimicrobial spectrum, a low rate of bacterial
resistance, thermal stability, and high water solubility, as well as offer a low risk for
allergic reactions or toxicity and availability at a reasonable cost.32 It should also:
• not compromise the structural integrity of the cement,33
• have good release from the cured/polymerized bone cement,34 and
• reach appropriate antibiotic concentrations in both the tissues and bone while
avoiding toxicity.35
Gentamicin and tobramycin are the only antibiotics available in United States commercial
antibiotic bone cement products; tobramycin is the most often used and studied antibiotic
added to cement worldwide, but gentamicin is more common in the United States.36
Other antibiotics (singly or in combination with other antibiotics) that have been studied
include vancomycin, cephalothin, clindamycin, meropenem, teicoplanin, ceftazidime,
imipenem, piperacillin, and ciprofloxacin.37,38,39
BONE CEMENT MIXING AND APPLICATION
Mixing
In the 1980s, mixing under vacuum was introduced to reduce exposure to fumes while
also improving tensile strength and fatigue life of bone cement.40,41,42,43 After some
refining, it produced better results than centrifugation, which was soon thereafter retired
14
in favor of vacuum mixing44 and quickly became the preferred method of mixing. For
example, a 2006 national survey of 587 surgeons in the United Kingdom found that
94% were using vacuum mixing systems for bone cement preparation with total hip
arthroplasty.45
In most ORs today, bone cement is mixed under a vacuum, which results in a low
porosity cement with increased strength and resistance to cement fatigue and creep.
Trying to eliminate all of the porosity by using a very high vacuum level can promote
excessive shrinkage and cracking.
With a vacuum mixing system, the cement is mixed in a syringe, bowl, or cartridge. All of
these systems consist of an enclosed chamber connected to a vacuum source (eg, wall
suction or a dedicated vacuum pump). All ingredients are added and mixed while the
system is closed.
In the last 20 years, technological advancements in bone cement closed vacuum bowl
mixing systems have led to those with a rotational axis design that include charcoal
filtration (Figure 1). This type of bowl can mix reproducible, high-quality cement for all
cement viscosities. Moreover, compared to a fixed axis bowl, a rotational axis bowl
produces cement with significantly fewer voids greater than 1 mm and fewer instances of
unmixed powder.46
Figure 1 – Closed Vacuum Bowl Mixing System
Advanced bone cement closed vacuum cartridge mixing systems have also been in use
for more than 19 years. In addition to an injection gun (Figure 2) that facilitates cement
delivery, these types of systems typically also include an extruder and scraper that help
to maximize the amount of cement used, thereby reducing cement waste. A nozzle that
converts to a shorter length is also available, helping to reduce the number of nozzles
needed for a procedure. It has been reported that both the fatigue life and reproducibility
of bone cement are influenced by mixing technique and vacuum level.47
15
It should be noted that no change in practice or technique is needed when using either
the vacuum cartridge systems or a rotational axis bowl.
Figure 2 – Closed Vacuum Cartridge Mixing System with Injection Gun
Application
The methods for application of bone cement include hand packing, injection, and gun
pressurization.
•
•
•
Hand Packing: The original method for hip arthroplasty was hand packing,
where surgeons packed the proximal end of the femoral canal with cement
by pressing with their fingers or thumbs; this pressurization forced the cement
into the bone interstices. It is still common for surgeons to hand pack cement
during total knee arthroplasty procedures because they can readily identify the
surfaces, making it more feasible to apply pressure by hand.
Injection: Surgeons use syringes to apply, or inject, the cement.
Gun Pressurization: When surgeons inject the cement with a gun, it offers
a mechanical advantage that allows them to force more cement into the
interstices of the bone via higher pressurization. The pressurization tips of
these devices allow more cement to be forced tightly into the bone while also
preventing overflow.
SAFETY ISSUES RELATED TO BONE CEMENT
The components of PMMA bone cement (ie, powder and liquid MMA monomer) are toxic
and highly flammable. As a consequence, perioperative personnel must be aware of the
potential hazards for both personnel and patients in the OR environment. Appropriate
16
safety precautions must be implemented to reduce the risk of exposure and to monitor
patient reactions closely. The specific hazards associated with the use of PMMA bone
cement are described below.
Flammability/Combustion Hazards
As packaged, the product is considered stable. Nevertheless, the powder component
is combustible and sensitive to static discharge. The liquid component is a volatile
flammable liquid that slowly attacks rubber. The liquid will polymerize very readily and
contamination must be avoided, particularly organic peroxides, catalysts, free radicals
generators and multivalent metal oxides, especially when wet. Heat and strong light,
particularly fluorescent or ultraviolet, could cause polymerization.48 The OR should be
adequately ventilated to eliminate monomer vapors. Ignition of monomer vapors caused
by the use of electrocautery devices in surgical sites near freshly implanted bone cement
has been reported.49
Health Risks to Personnel50
Caution should be exercised during the mixing of the liquid and powder components of
the PMMA bone cement to prevent excessive exposure to the concentrated vapors of
the liquid MMA monomer because it may produce irritation of the respiratory tract, eyes,
and possibly the liver. The MMA fumes that are emitted during preparation of PMMA
bone cement have been shown to have toxic side effects ranging from allergic reactions
to neurological disorders. Although there is no evidence for potential carcinogenicity of
the substance, all efforts should be made to reduce the exposure.51 The permissible
exposure limit (PEL) value established by OSHA is a time-weighted average limit of 100
parts of MMA per million (ppm) of air or a time-weighted average of 410 milligrams of
MMA per cubic meter of air during any 8-hour work shift in a 40-hour work week.52
Skin contact with the liquid monomer can cause contact dermatitis and hypersensitivity
reactions. The MMA monomer is a powerful lipid solvent. It should not contact rubber or
latex gloves. Double gloving or use of special gloves resistant to the monomer, and strict
adherence to the mixing instructions may diminish the possibility of contact dermatitis
and hypersensitivity reactions. The mixed PMMA bone cement should not contact the
gloved hand until the cement has acquired the consistency of dough.
Eye contact with the liquid can be quite serious, causing considerable irritation or
burns to the eyes. Soft contact lenses are very permeable and should not be worn
where methylmethacrylate is being mixed because the lenses are subject to pitting and
penetration by the vapors. Personnel wearing soft contact lenses should not mix PMMA
bone cement or be nearby.
Health Risks to Patients
According to the United States Food and Drug Administration:
“Serious adverse events, some with fatal outcome, associated
with the use of acrylic bone cements include myocardial infarction,
cardiac arrest, cerebrovascular accident, and pulmonary embolism.
17
The most frequent adverse reactions reported with acrylic bone
cements are a transitory fall in blood pressure, thrombophlebitis,
hemorrhage and hematoma, loosening or displacement of the
prosthesis, superficial or deep wound infection, trochanteric
bursitis, and short-term cardiac conduction irregularities. Other
reported adverse reactions include heterotopic new bone formation
and trochanteric separation. Other reported adverse events for
acrylic bone cements include pyrexia due to an allergy to the bone
cement, hematuria, dysuria, bladder fistula, delayed sciatic nerve
entrapment due to extrusion of the bone cement beyond the region
of its intended application, and adhesions and stricture of the ileum
due to the heat released during polymerization.”53
Hypotensive reactions can occur between 10 and 165 seconds after application of
the PMMA bone cement and can last for 30 seconds to 5 or more minutes. Some
hypotensive reactions have progressed to cardiac arrest. The blood pressure of patients
should be monitored carefully during and immediately following the application of the
PMMA bone cement. In addition, overpressurization of the PMMA bone cement should
be avoided during insertion of the PMMA bone cement and implant in order to minimize
the occurrence of pulmonary embolism.54
Bone cement implantation syndrome (BCIS) is a poorly defined, poorly understood,
rare, and potentially fatal intraoperative complication occurring in patients undergoing
cemented orthopaedic surgeries.55,56 It can occur within minutes of the procedure; it
also may be seen in the postoperative period in a milder form causing hypoxia and
confusion. Experts have not agreed upon a precise definition for BCIS; although it is
characterized by a number of clinical features that may include hypoxia, hypotension,
cardiac arrhythmias, increased pulmonary vascular resistance, and cardiac arrest. It is
most commonly associated with, but is not restricted to, hip arthroplasty. It usually occurs
at one of the following stages in the surgical procedure: femoral reaming, acetabular or
femoral cement implantation, insertion of the prosthesis, or joint reduction.57
GUIDELINES FOR SAFE USE OF PMMA BONE CEMENT
The Association of periOperative Registered Nurses (AORN) Guideline for a Safe
Environment of Care, Part 1 states that precautions must be taken to minimize the risks
and potential hazards associated with the use of MMA in the practice setting and safe
practices should be established. Safe practices include the following measures58:
• Material safety data sheet (MSDS) or safety data sheet (SDS) information
for MMA must be readily accessible to employees within the practice setting.
This information includes identification of hazards, precautions or special
handling recommendations, signs and symptoms of toxic exposure, and first
aid treatments for exposure. Perioperative personnel should handle MMA
according to its respective MSDS or SDS and the manufacturer’s written
instructions for use.
18
•
•
•
•
•
•
•
•
Eye protection must be worn during mixing and when inserting bone cement to
prevent contact with eyes, as MMA fumes may irritate the eyes.
The mixed cement should not be permitted to come into contact with gloves until
the dough stage. A second pair of gloves should be worn when handling MMA
and should be discarded after contact with the cement. The manufacturer’s
instructions should be followed regarding the composition of the second pair of
gloves. The MMA can penetrate many plastic and latex compounds and may be
absorbed through the skin, leading to contact dermatitis.
Personnel should avoid direct contact with the MMA monomer; the liquid
component is a mild skin irritant and may cause skin sensitization.
The manufacturer’s recommendations should be followed for mixing the bone
cement and wearing the required personal protective equipment (PPE) when
handling and managing spills.
A closed mixing system or mixing gun should be used to mix the cement to
decrease handling of the product. When compared to open mixing systems,
closed mixing systems, with or without a vacuum, release less vapor into the
breathing zone of the perioperative team members.
Any bone cement that will be discarded should not remain in contact with the
patient’s skin. As the cement cures, it releases heat and has been reported to
result in patient burns.
The MMA monomer is hazardous waste and therefore must be disposed of
according to local, state, and federal requirements.
For MMA spills:
o the area of the spill should be ventilated until the odor has dissipated;
o all sources of ignition should be removed;
o appropriate PPE should be worn during the clean-up, as outlined by the
MSDS, SDS, or manufacturer’s instructions;
o the area of the spill should be isolated;
o the liquid component should be covered with an activated charcoal
absorbent; and
o the waste product should be disposed of in a hazardous waste container.
ECONOMIC CONSIDERATIONS OF BONE CEMENT AND MIXING
SYSTEMS
As noted above, advances in bone cement and mixing systems offer clinicians improved
performance and the ability to continue delivering high quality patient care, with no change
in practice or technique. But what about cost? In today’s dynamic health care economic
environment, in addition to quality of care, costs must also be considered.
The AORN Guideline for Product Selection recommends that health care organizations
develop a product-specific evaluation tool to use in the product selection process; the
product-specific criteria may include, but is not limited to59:
• safety;
• quality;
• performance;
19
•
•
•
•
•
efficiency;
ease of use;
impact on quality patient care and clinical outcomes;
evidence-based efficacy; and
a financial impact analysis that should include the following:
o direct costs (eg, cost of the product and related equipment);
o indirect costs (eg, waste disposal, storage, training);
o reimbursement; and
o pricing as outlined by the group purchasing organization (GPO).
Orthopedics is considered to be one of the largest specialties in many health care
organizations but there is no guarantee that it will be an automatic profit center. Health
care facilities, as well as providers, continue to face cost pressures as product pricing has
increased at an average rate of 5% to 15% annually without simultaneous increases in
reimbursement.60 While these increases may affect implants or the cost of some specialty
instruments or equipment, bone cement is an example of stabilized technology, or a basic
product that should not have an annual price increase because no new technology has
proven improved clinical outcomes.
In addition to the steady increase in the cost of providing care in the face of declining
reimbursement, the following key issues are related to orthopedics.
• Orthopedic devices are typically subject to annual price increases.
• The orthopedic supply chain is full of excess inventory and inefficiency.
• Excessive reliance on sales representatives contributes to higher product
cost.
As a result of these issues, health care providers are forced to find ways to reduce
expenses while maintaining high quality care. The following three strategies can help
perioperative leaders accomplish their fiscal goals.
• Acquire knowledge to differentiate new technologies versus stabilized
technologies to better understand whether price increases are justifiable.
• Ensure high quality by using a manufacturing process that is International
Organization for Standardization (ISO) Certified and conforms to American
Society for Testing and Materials (ASTM) standards.
• Achieve cost savings by identifying standard products based on proven and
accepted designs (eg, a high quality vacuum cartridge mixing and delivery
system that is 20% to 40% lower than the average market price of the leading
manufacturer’s products).
A new business model should also be considered that makes the facility leaders less
dependent on the health care industry representative in the OR. By changing the orthopedic
model, health care facilities and providers are empowered to regain control of their
purchasing decisions, allowing them to focus on what really matters: delivering excellent
patient care, while benefitting from significant cost savings. This type of solution benefits
everyone, as outlined below.
20
Surgical Services/OR Nurse Leaders:
o Allows the focus to be on the patients’ needs, as well as those of the
perioperative team.
o Eases the stress of meeting the demands of surgeons and the facility for
more effective and efficient management of the OR.
o Ensures the right products, at the right prices, are available at the right
time.
• Orthopedic surgeons:
o Allows high clinical standards to be achieved/maintained, while addressing
the mounting pressures to reduce costs in today’s changing health care
environment.
o Enables surgeons to become a pioneer for change in the new orthopedic
paradigm.
• Supply Chain Directors:
o Enhances the ability to be an expert problem solver in regards to vendor
management of orthopedic products.
o Helps to drive towards significant savings and orthopedic product
standardization.
o Provides the confidence to explore cost-saving options in physician
preference products.
• Facility Chief Financial Officers:
o Enables a stronger position to ensure the financial health of the institution
while maintaining a high quality of care.
o Helps to build consensus among orthopedic surgeons so they become
partners with the facility in adopting new approaches.
o Facilitates a balance in the competing interests faced daily, from financial
targets to quality and patient satisfaction metrics.
•
21
SUMMARY
The PMMA bone cement has been used in cemented arthroplasty procedures for more
than 50 years. Good quality cement is essential for long-term implant survival and the
role of the perioperative nurse in preparing that cement is vitally important. The quality
of bone cement is determined by several factors, including the type of cement selected,
(ie, viscosity, presence of antibiotics) and strict adherence to instructions provided by
the manufacturer. Its effectiveness is highly dependent upon the use of optimal mixing
and application techniques. The components of PMMA bone cement (powder and
liquid MMA monomer) are toxic and highly flammable. As a consequence, perioperative
personnel must be aware of the potential hazards for both personnel and patients in
the OR environment. Appropriate safety precautions must be implemented to reduce
the risk of exposure and to monitor patient reactions closely. Today, the use of fiscally
responsible products that provide clinically equivalent performance and the ability to
continue delivering high quality patient care, with no change in practice or technique,
are changing the paradigm in orthopedic product selection.
22
GLOSSARY
Accelerator
A catalytic agent used to hasten a chemical reaction.
Ambient Temperature
The temperature of the air in the surrounding
environment. In the OR, the ambient temperature may
be less than in the storage areas or in unrestricted
areas within the perioperative suite.
Bone Cement Implantation
Syndrome (BCIS) A rare complex of sudden physiologic changes
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 bone
surgery. Symptoms may occur within minutes of the
use of PMMA cement.
Compressive Strength
The measure of bone cement’s durability during
weight bearing.
Copolymer
A polymer derived from two or more monomers.
Creep
The measure of bone cement’s reaction to a
combination of compressive and shear forces that
occur during a variety of normal activities of daily living
over time.
Dough Time
The time point measured from the beginning of mixing
to the point when the cement no longer sticks to
surgical gloves.
Elution
To extract one substance from another, sometimes
using a solvent or water to separate the heavier
material.
Exothermic Reaction
A chemical reaction that produces heat.
Fatigue
The failure of a component after it is subjected to a
large number of alternating, fluctuating loads.
High-Viscosity Cements
Cements that have a short waiting/sticky phase and
a long working phase. The viscosity remains constant
until the end of the working phase.
Implant Subsidence
A shift in the prosthesis after the time it was originally
implanted.
23
Laminations
Faults or folds in the bone cement, which may be
caused by high temperature or “intrusions” such as
bone, water, blood. Laminations create potential areas
of weakness in the cement mantle where a failure can
occur.
Low-Viscosity Cements
Cements that have a long waiting phase of about 3
minutes; the viscosity rapidly increases during the
working phase, making for a short working phase.
Medium-Viscosity Cements
Cements that have a long waiting phase of
approximately 3 minutes, but during the working
phase, the viscosity only increases slowly.
Methylmethacrylate (MMA)
The liquid component of bone cement; MMA is a
monomer.
Mixing Phase
The phase in which the monomer is thoroughly mixed
throughout the powder bed and polymerization is
initiated.
Monomer
A molecule of low molecular weight capable of
reacting with identical or different molecules of
low molecular weight to form a polymer. For bone
cement, the monomer MMA (a liquid) reacts with the
copolymers based on PMMA to form PMMA bone
cement.
Parts per Million (ppm)
Refers to a substance per million parts of air; it is a
measure of the substance’s concentration of volume
in air.
Permissible Exposure Limit
(PEL) The permissible exposure limit of a hazardous
substance, which is enforceable by OSHA.
Polymerization
The formation of a compound, usually of high
molecular weight, by the combination of several
low molecular weight compounds (eg, monomers,
copolymers).
Polymethylmethacrylate
(PMMA) A synthetic acrylic resin used as the basis for PMMA
bone cement. Bone cement consists of two primary
components: a powder consisting of copolymers
based on polymethylmethacrylate (PMMA), and a
liquid monomer, methylmethacrylate (MMA).
Pores
Empty space in an apparent solid; may be caused by
trapped air.
24
Porosity
The presence of entrapped air in bone cement. High
bone cement porosity compromises the cement’s
mechanical strength and decreases its fatigue life.
Centrifugation and vacuum mixing methods, and
pressurized cement application can decrease the
porosity of bone cement.
Setting Phase
The final curing state of the polymerization process of
bone cement; the implant should already be in its final
position.
Setting Time
Time point 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 (ie, 50% of its
maximum value), usually about 8-10 minutes.
Stress
The load applied to a material over a given area.
Viscosity
A measure of the resistance of a fluid to deformation
under shear forces and is commonly described as
“thickness” of a fluid. The viscosity of bone cement
affects its handling characteristics, handling time, and
penetration of the cement into the cancellous bone.
Waiting Phase
The phase of the polymerization process where
bone cement begins to swell and viscosity begins to
increase, creating a sticky dough. By the end of the
waiting phase, the doughy cement will not stick to
surgical gloves.
Working Phase
The time during the polymerization process at which
bone cement is ready for application; the implant must
be implanted before the end of the working phase.
Working Time
The interval between the dough and setting times,
typically 5-8 minutes.
25
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Post-Test and Evaluation
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