Principles Of Bone CE ONLINE Of Bone Cement Mixing Cement .

POLYMERIZATIONPolymerization is a chemical reaction in which two or more small molecules combine toform larger molecules that contain repeating structural units of the original molecules. Inthe case of bone cement, the polymerization process starts when the copolymer powderand monomer liquid meet, reacting together to produce an initiation reaction creating freeradicals that cause the polymerization of the monomer molecules. The original polymerbeads of the powder are bonded into a dough-like mass, which eventually hardens intohard cement.The polymerization process is an exothermic reaction, which means it producesheat. With a maximum in vivo temperature of 40 C to 47 C, this thermal energy isdissipated into the circulating blood, the prosthesis, and the surrounding tissue. Oncepolymerization ends, the temperature decreases and the cement starts to shrink.Phases and TimesThe polymerization process can be divided into four different phases: mixing, waiting,working, and setting. Package inserts that come with the products often refer to DoughTime, Working Time, and Setting Time. Dough Time and Setting Time are measured fromthe beginning of mixing; Working Time is the interval between Dough Time and SettingTime. Both the Phases and corresponding Times are described below.Mixing PhaseThe mixing phase represents the time taken to fully integrate the powder and liquid. Asthe monomer starts to dissolve the polymer powder, the benzoyl peroxide is released intothe mixture. This release of the initiator benzoyl peroxide and the accelerator DMPT isactually what causes the cement to begin the polymerization process. It is important forthe cement to be mixed homogeneously, thus minimizing the number of pores.Waiting Phase/Dough TimeDuring this phase, typically lasting several minutes, the cement achieves a suitableviscosity for handling (ie, can be handled without sticking to gloves). The cement is asticky dough for most of this phase.Dough time is the time point measured from the beginning of mixing to the point whenthe cement no longer sticks to surgical gloves. Under typical conditions (23 C-25 C,65% relative humidity), dough time is 2-3 minutes after beginning of mixing for most bonecements. Before this time point, after the components are well mixed, the bone cementmay be loaded into a syringe, cartridge, or injection gun for assisted application.3Working Phase/Working TimeThe working phase is the period during which the cement can be manipulated and theprosthesis can be inserted. The working phase results in an increase in viscosity and thegeneration of heat from the cement. The implant must be implanted before the end of theworking phase.8

Working time is the interval between the dough and setting times, typically 5-8 minutes.Previously, this represented the full time interval available for use of a particular mix ofbone cement. The use of mechanical introduction tools, such as syringes and cartridges,extends this time by 1 to 1.5 min.4Setting Phase/Setting TimeDuring this phase, the cement hardens (cures) and sets completely, and the temperaturereaches its peak. The cement continues to undergo both volumetric and thermalshrinkage as it cools to body temperature. Hardening is influenced by the cementtemperature, 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 atwhich the exothermic reaction heats the cement to a temperature that is exactly halfwaybetween the ambient and maximum temperature (ie, 50% of its maximum value), usuallyabout 8-10 minutes. The temperature increase is due to conversion of chemical tothermal energy as polymerization takes place.5Factors that Affect Dough, Working, and Setting TimesFactors that affect dough, working, and setting times include the following6: Mixing Process – Mixing that is too rapid can accelerate dough time and is notdesirable since it may produce a weaker, more porous bone cement. Ambient Temperature – Increased temperature reduces both dough and settingtimes approximately 5% per degree Centigrade, whereas decreased temperatureincreases them at essentially the same rate. Humidity – High humidity accelerates setting time whereas low humidity retardsit.The combination of these factors is such that in a cold operating room on a very drywinter day, setting time may stretch out and raise concerns as to whether there issomething wrong with the bone cement kit in use. There usually is not, but patience isrequired under these conditions. Water (or anything else) should never be added to bonecement 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 ofvarious cement products can be significantly different when they are mixed under similarconditions. There are several reasons for these differences: The polymer component of a number of cements is not purely PMMA. Somecement may contain PMMA copolymers such as methyl acrylate and styrene inthe powder and additional polymers such as butyl methacrylate. All cements arelabelled to show their ingredients. The ratio of the components and the overall powder-to-liquid ratio may differbetween cements.9

The size, shape and weight of the polymer molecules can vary considerably. Manufacturing processes may differ. Sterilisation method may differ (eg, gamma, and ethylene oxide gas sterilisation).CEMENT PROPERTIESCement properties critical for operating procedures, such as viscosity change, settingtime, cement temperature, mechanical strength, shrinkage, and residual monomer, aredetermined during polymerization. These properties will influence cement handling,penetration, and interaction with the prosthesis. The most important properties arediscussed below.Cement PorosityPorosity 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 anddecreases its fatigue life. This may lead to aseptic loosening. Sources of porosity incured bone cement include: Trapped air between the powder beads as the powder is wetted. Trapped air in the cement during mixing. Trapped air in the cement during transfer from mixing container to applicationdevice.Hand mixing bone cement in an open bowl may introduce the greatest possibility ofthese occurrences, which is why hand-mixed cement can contain a substantial number ofpores. Centrifugation and vacuum mixing methods, and pressurized cement applicationcan decrease the porosity of bone cement.Cement ViscosityViscosity is a measure of the resistance of a fluid to deformation under shear forces andis commonly described as “thickness” of a fluid. Viscosity also represents the resistanceto flow and is thought to be a measure of fluid friction. Cement viscosity determines thehandling and working properties of the cement.Mixing together the powder and the liquid components marks the start of thepolymerization process. During the reaction, the cement viscosity increases, slowly atfirst, then later more rapidly. During the working phase, there are two requirements forbone cement viscosity – it must be sufficiently low to facilitate the delivery of the cementdough to the bone site, and it must penetrate into the interstices of the bone.7 On theother hand, the viscosity of the bone cement should be sufficiently high to withstandthe back-bleeding pressure, thus avoiding the risk of inclusion of blood into the cementbecause this could significantly reduce the stability of the bone cement. It is importantthat the cement retains an optimized viscosity for an adequate duration to allow a“comfortable” working time.810

Viscosity affects the following9: Mixing behaviour; Penetration into cancellous bone; Resistance against bleeding; and Insertion of the prosthesis.Cement TemperatureTo achieve optimal cement properties, it is important to adhere to the time schedulesindicating the correlation of temperature to handling time. These time schedules areusually included in the manufacturer’s instructions for the bone cement.Effects of Temperature: Temperature affects mixing time, delivery of the cement, prosthesis insertion, andother aspects of the cementing process. Storage temperature will affect the cement times – not just the temperature atwhich it is mixed. If cement has been stored in a cold environment, all the phases apart from themixing phase will be prolonged. High-viscosity cements are sometimes pre-chilledfor use with mixing systems for easier mixing and prolonged working phase. Thiswill also increase the setting time. If cement has been stored in a warmer environment, all phases will be shorter. Issues created by high temperatures: Integration of the powder and liquid can be difficult. Extrusion from a delivery gun can become difficult and may reduce deliverypressures. Potential exists for cement to be inserted during the setting phase. Laminations can form between 3.5 and 6.5 minutes and reduce cementstrength by up to 54%.10Mechanical PropertiesThe aim of a good cement mix is to produce bone cement that has the best mechanicalproperties possible so that it can carry out its load transfer role successfully over thelifetime of the implant. Once positioned within the hip or knee replacement, the cementaround the prosthesis is subjected to a series of physical forces that will have an effecton the lifespan of the cement. These physical forces subject the cement to fatigue, creep,and high stresses. The mechanical properties of the cement (eg, resistance to fatigueand creep, and strength) should be enhanced as much as possible.11

FatigueFatigue 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 appliedonly once, these loads would not be large enough to cause failure. A good example ofthis is a paper clip, which when bent once will not break, but after it has been bent anumber of times, it will break easily.As the cemented implant is subjected to not only static load but also dynamicallyalternating 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 will, overtime, cause it to break down and fail. It is the quality of the cement mix that will determineits lifespan. A well-mixed cement will be better equipped to deal with the loads placedupon it.The ability of bone cement to resist fatigue is critical given the loads to which it will besubjected. Clinical evidence has documented the existence of fatigue cracks in revisionretrieved cement11,12 and in postmortem retrieved stem/cement/bone constructs.13 Thissuggests that the fatigue resistance of bone cement should be optimized to preventfatigue failure.CreepCreep is the deformation of a material under constant load. Under constant load, amaterial capable of creep will deform by an amount dependent on the size of the loadand the length of time it is applied. Creep generally increases with temperature. Creepessentially is a mechanical problem that slowly and steadily can erode the long-termperformance of an implant. Cements with higher porosity are less resistant to creepdeformation.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 andis at 37 C in the body.Significant bone cement creep will lead to implant subsidence, which, in turn, may lead tofailure.14 In the 1990s, a new formulation of bone cement had to be withdrawn after it wasfound to significantly creep, leading to implant subsidence, aseptic loosening, and highrevision rates.15,16Interestingly, a small degree of creep may in fact be advantageous in the earlypostoperative stages with some implant designs. A polished, tapered stem without acollar relies on some subsidence so that it becomes “wedged” in the bone cement,thereby improving the load transfer mechanism.17StressStress is the load applied to a material over a given area. Stresses in the hip jointare a combination of compression, bending, and torsional (twisting) forces. As load istransferred 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 andthen, possibly, fail.12

TYPES OF BONE CEMENTCements can be grouped as high, medium, or low viscosity, with or without antibiotics.The viscosity designation refers to the viscosity of the powder and liquid during themixing phase: high-viscosity cement is dough-like, while low-viscosity cement is morelike a liquid. The handling phases of different viscosity cements also vary considerablyand the choice of which cement to use is often surgeon preference. For example, a 2006national survey of 587 surgeons in the UK found that high-viscosity cement was usedin total hip arthroplasty by 82% of the surgeons, medium-viscosity cement by 12%, andlow-viscosity cement was used by 6%.18High ViscosityHigh-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 ofthe working phase. The setting phase lasts between one minute 30 seconds and twominutes.19 High-viscosity cements are associated with reduced revision rates for total hiparthroplasty.20Medium ViscosityThese cements typically have a long waiting phase of three minutes, but during theworking phase, the viscosity only increases slowly. Setting takes between one minute 30seconds, and two minutes 30 seconds.21Low ViscosityLow-viscosity cements have a long waiting phase of three minutes and the viscosityrapidly increases during the working phase, making for a short working phase. Asa consequence, application of low-viscosity cements requires strict adherence toapplication times. The setting phase is one to two minutes long.22Antibiotic CementsPeriprosthetic 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 ina long series of operative procedures, great discomfort for the patient, and significantcosts.The use of antibiotic-impregnated bone cement to treat musculoskeletal infection hasbeen reported in the literature for more than three decades despite the fact that it wasn’tuntil 2003 that the first pre-blended bone cement containing an antibiotic (tobramycin)became available for sale in the United States, specifically for the treatment andreimplantation of infected arthroplasties.23,24 Prior to 2003, U.S. surgeons preparedantibiotic cement on-site (ie, in the operating room) by adding antibiotic powder to thepowdered 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 theindications and scientific evidence for its use have expanded to primary arthroplastyto minimize postoperative infection. Use of antibiotic cements for primary arthroplasty,13

however, remains controversial in the United States. The primary arguments profferedagainst the routine use of antibiotic bone cement are lack of efficacy, adverse effects onmechanical properties, increased costs, bacterial resistance, and systemic toxicity.25,26However, there is significant evidence to refute these arguments.27,28,29The elution of antibiotics from PMMA bone cement can be affected by certain factorsincluding the type of cement used, preparation methods, surface characteristics, porosityof the cement, and the amount and/or type of antibiotics used.30Not all antibiotics are suitable for use in bone cements. The following bacteriologic andphysical and chemical factors should be considered in the choice of an antibiotic31: Preparation must be thermally stable and able to withstand the exothermictemperature of polymerization. Must have broad antimicrobial coverage. Must be available as a powder. Must have a low incidence of allergy. Must not significantly compromise mechanical integrity. Must elute from the cement over an appropriate period of time.Gentamicin and tobramycin are the only antibiotics available in U.S. commercial antibioticbone cement products; tobramycin is the most often used and studied antibiotic addedto cement worldwide, but gentamicin is more common in the United States.32 Otherantibiotics (singly or in combination with other antibiotics) that have been studied includevancomycin, cephalothin, clindamycin, meropenem, teicoplanin, ceftazidime, imipenem,piperacillin, and ciprofloxacin.33,34,35HISTORY OF BONE CEMENT MIXING SYSTEMSManual MixingUntil the 1980s, the composition and preparation of bone cement did not stray much fromthe standards introduced in 1959 by Sir John Charnley, a British orthopaedic surgeonwho pioneered the hip replacement operation.36 Techniques for improving cementstrength were not extensively tried.Original mixing techniques were either hand- or bag-mixing. The liquid was injected intoa powder bag and the two components were mixed by kneading. As mixing techniquesevolved, an open bowl was used to mix the cement. The liquid and powder were pouredinto a plastic or stainless steel bowl and then mixed together with a spatula. A 1988study by Linden of 46 samples of acrylic cement mixed by seven nurses found that amanual mixing technique lacks reproducibility and produces cements with uncontrollableporosity.37Early in the use of open bowl mixing, exposure to the resulting noxious fumes createdserious safety concerns. A certain amount of porosity in the final material remainsunavoidable with conventional hand mixing techniques today, due to the air introduced14

by stirring during hand spatulation. In order to reduce both the harmful fumes as well asthe introduction of air into the cement mixture, the closed bowl technique, using a paddlemixing system and wall suction to evacuate the fumes, was developed.VibrationDuring the 1980s, a vibrating mixing technique was introduced in hopes of improvingbone cement properties. The results, however, were not convincing.38CentrifugationIn this technique, cement was first mixed manually and then subjected to centrifugationto eliminate any air inclusions introduced during mixing and thus reduce porosity inhopes of improving compressing strength and handling properties. The techniquerequired chilling the liquid monomer prior to mixing in order to negate the shorteningeffect of centrifugation on setting time. The resulting low-viscosity mixture then wasintroduced into a cement syringe, which was centrifuged at high speed for a short periodof time. The method succeeded in reducing porosity but procedures varied significantlydepending on the type of centrifugation and cement used.Vacuum MixingAlso in the 1980s, mixing under vacuum was introduced to reduce exposure to fumeswhile also improving tensile strength and fatigue life of bone cement.39,40,41,42 After somerefining, it produced better results than centrifugation, which was soon thereafter retiredin favor of vacuum mixing43 and quickly became the preferred method of mixing. Forexample, a 2006 national survey of 587 surgeons in the UK found that 94% were usingvacuum mixing systems for bone cement preparation with total hip arthroplasty.44In most operating rooms today, bone cement is mixed under a vacuum, which results in alow 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 promoteexcessive shrinkage and cracking.With a vacuum mixing system, the cement is mixed in a syringe, bowl, or cartridge. All ofthese systems consist of an enclosed chamber connected to a vacuum source (eg, wallsuction or a dedica

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