J. BIOMED. MATER. RES. SYMPOSIUM
No. 6, pp. 105 I17 (1975)
A Proposed Specification for Acrylic Bone Cement S. S. HAAS, Department of Orthopaedic Surgery, George Washington University, Washington, D.C. 20037, and G . DICKSON and G. M. BRAUER, Dental Research Section, National Bureau of Standards, Washington, D.C. 20234 Summary A proposed specification covering handling characteristics and physical and chemical properties of bone cement composed primarily of methyl methacrylate has been prepared on the basis of data from the authors’ studies and from various other sources. Under handling characteristics, requirements included relate t o dough, handling and setting time, proper plasticity for insertion and temperature rise on setting. Mechanical properties specified include compressive strength and indentation and recovery characteristics. Maximum limits are proposed for water sorption and solubility. Suggested packaging requirements are also included.
INTRODUCTION Concept of Specifications and Standards Much unnecessary confusion and anxiety arises when the terms specification and standardization are raised. Any specification must be a compromise involving three areas: (1) the properties needed for the given application; ( 2 ) the properties attainable with currently available materials; and ( 3 ) the practicality and reproducibility of test methods for evaluating these properties. Testing should be as simple and efficient as possible. Thus, the inclusion of long, involved tests which can only be performed in highly specialized laboratories, greatly limits the usefulness of a specification. The following analogy best explains what we are attempting:
0 1975 by John Wiley & Sons, Inc.
HAAS, DICKSON, A N D B R A U E R
The above diagrammed circle shown is defined by three points. The three points may not be the most significant points of the given circle, but they do provide the means by which one can construct or reproduce the circle. The points on the circle are what we are attempting to set forth in this proposed specification for bone cement. W e are not describing an ideal bone cement. Rather, we are presenting a practical document which we feel is a compilation of the least number of requirements and tests which will identify the satisfactory members of a class of materials which has, so far, shown a high rate of clinical success as a bone cement. The specification does not, and need not, contain tests for all properties required of a bone cement. It should list sufficient requirements and tests to insure that a cement does not have properties inferior to those found in the most satisfactory bone cements now commercially available. Ideally, composition requirements should not be necessary; performance tests should be sufficient for adequate evaluation of a cement. Practically, however, composition requirements are needed to enable one to group candidate products into classes of materials for the purposes of appropriate and efficient selection of test methods. The proposal to be presented is based on studies done at the National Bureau of Standards and at the George Washington University, Department of Orthopaedic Surgery, work of A S T M committees, and on data reported in the literature on the subject. Thus, it is not an attempt on our part to arbitrarily impose rules and regulations on any producer or consumer of the product. Details of the requirements and test methods are given in the proposed specification. I n this introduction we present highlights and comments on some of the more important and some of the less obvious implications of the requirements. Discussion of Proposed Specification
The proposed specification covers handling characteristics and physical and chemical properties of bone cements composed primarily of methyl methacrylate. N o attempt is made to anticipate the characteristics and set requirements for new types of cement which may be developed in the future. Also, this proposal does not deal directly with the problems of toxicity and biocompatibility. The only requirements placed on the liquid and powder components are that they be packaged in the proper mixing ratio, that there be an absence of visually obvious contamination, and that the liquid pass an accelerated storage stability test. T o comply with the stability requirement, the liquid, after being held at 60°C for 48 hr, must not have polymerized to any significant extent as indicated by a change in viscosity.
SPECIFICATION FOR ACRYLIC BONE CEMENT
Several requirements relate t o handling characteristics. First, it is specified that the time to reach the dough state, the condition when the cement will no longer adhere t o a surgical glove, shall not be more than 4 min. The setting time, defined as the time of attainment of the exothermic temperature peak which accompanies hardening, is required t o be between 7 and 13 min after the beginning of mixing. The handling time, the period between dough and setting time, must be 5 to 9 min. A n “intrusion” requirement is included to insure that the cement shall remain in condition for insertion for at least 3 min after the dough state is reached. So that the handling characteristics may not vary erratically from one lot of cement to the next, it is required that the measured dough and handling times shall not deviate more than 1 min, and the setting time shall not deviate more than 2 min from times stated on the product identification insert. The proposed specification restricts the peak temperature on setting, under empirically selected test conditions, to 90°C. While a lower value is desirable, it has not yet been adequately demonstrated that the exotherm can be greatly reduced without deterioration of other properties of the cement. Mechanical properties near the optimum that can be expected of selfcuring methacrylate resing are specified by a compressive strength requirement (a minimum of 80 MPa) and by requirements for indentation and recovery characteristics. The indentation and recovery test provides a measure of both plastic and elastic properties of the cement and has also been shown t o be an indicator of the amount of residual monomer in the material. Limits on water sorption and solubility are also included in the proposed specification. Excessive water sorption would lead to dimensional instability. Excessive solubility would indicate the loss of various components of the cement and possibly a tendency toward release of toxic materials. The proposed specification represents our recommendations based on our own work, and on the work of many others. To return to the analogy used in the introduction, we do not maintain that we have selected the only points that may be used to define the circle. Other workers may select somewhat different requirements and different test methods that will specify the same cement. From our suggestions, and those of others, a specification committee should have no difficulty in assembling a specification for methacrylate bone cement. Such a specification will be of value in determining the consistency of properties of present cements and the relative merits of new cements of this type when they are offered to the orthopedic profession.
HAAS. D I C K S O N , A N D B R A U E R
Proposed Specification for Acrylic Bone Cement
I . Scope 1.1 This specification is for self-curing resins primarily used for the fixation of internal orthopedic prostheses. 1.2 While a variety of copolymers and comonomers may be incorporated, the composition of the cement shall be mainly composed of esters of methacrylic acid. 1.3 This specification does not cover toxicity or biocompatibility of the cement. 2. Definitions 2.1 Dough Time-The time period beginning with the initiation of mixing acrylic monomer (liquid) and polymer (powder) ending when the material does not adhere to the surgeon’s glove and is ready to be used. 2.2 Setting Time-The time period initiated by mixing acrylic monomer and polymer, ending when the material has hardened and attained maximum temperature rise. 2.3 Handling Time-The time period between dough and setting time. This includes the time during which the acrylic material can be shaped and molded by the surgeon, and the subsequent period when it is hardening rapidly and should be held without movement in the desired final position. unit consists of one package or vial of preweighed 2.4 Unit-A powder component and one package or vial of premeasured liquid component. 3. Physical Requirements 3.1 Liquid 3.1.1 Appearance. The liquid shall be clear and free of deposits or sediment. 3.1.2 Stability. The liquid shall not increase in viscosity to such an extent that flow time will be increased by more than 10% when heated 48 hr at 60 i. 2°C in the dark in a closed container and tested as described in 5.4. 3.2 Powder 3.2.1 Appearance. The powder shall be pourable and free from extraneous materials. 3.3 Powder-Liquid Mixture 3.3.1 The material shall conform to the requirements in Table I . 3.3.2 In addition to conforming to the limits in Table I , the dough time and handling time shall be within I min, and the setting time shall be within 2 min of the dough, handling and setting times, respectively, given on the product identification insert (7.5.1).
SPECIFICATION FOR ACRYLIC BONE CEMENT
Ilonyh time (min)
Handling time range (min)
Setting time range (min)
Exotherm max ("C)
I .5 min
3.4 Cured Polymer 3.4.1 The material after setting shall conform to the properties prescribed in Table 11. 4. Test Sampling Sixteen units of powder (of the same lot number) and liquid (of the same lot number) shall be procured to provide sufficient material for all the tests of this specification. The units shall be obtained from regular retail distribution channels. 5. Methods of Test and Sample Size 5.1 All equipment, mixing surfaces and materials shall be maintained at 23 f 2°C at least 3 hr prior to testing, and all tests conducted at 23 f 2°C and 50 f 10% relative humidity, unless otherwise specified. 5.2 Unless otherwise specified, molds, dies and plungers may be made of Teflon, stainless steel or any other substance which will not react with the cement. 5.3 Inspection-Visual inspection shall be used in determining compliance with the requirements outlined in 3.1.1, 3.2.1, 7.1, 7.2, 7.3, 7.4 and 7.5 5.3.1 The liquid component of two separate units shall comply with the requirements of 3.1 . l , 7.1, 7.2, 7.3, 7.4 and 7.5. 5.3.2 The powder component of two separate units shall comply with the requirements of 3.2.1, 7.1, 7.2, 7.3, 7.4 and 7.5. 5.4 Flow Rate Determination-The flow rate of the liquid shall be determined before and after heating to 60°C by timing the flow of the liquid level between 0 and 5 ml marks of a 10 ml measuring pipet. The TABLE 11
Compressive strength (hfPa)
Water solubility (w/cm*)
HAAS, DICKSON, A N D BRAUEK
% change shall be calculated as follows:
where tA is the flow time before heating and tB is the flow time after heating exposure. 5.4.1 The test shall be performed on the liquid component from two separate units. Both shall comply with the requirement. 5.5 Dough Time-All the powder and liquid of a single unit shall be mixed together as directed by the manufacturers instructions (7.5.2). A stopwatch shall be started at the onset of combining the powder with the liquid. Approximately 1.5 min after the onset of mixing, the mixture shall be probed gently with a surgically gloved finger. Visual notice shall be taken as t o the formation of fibers between the surface of the mix and the finger as it leaves the surface. This process of probing is repeated from that time on at 15 sec intervals until the gloved finger separates cleanly from the surface. The time at which this is first observed is denoted as the dough time. 5.5.1 Two separate determinations of the dough time shall be made and the average reported to the nearest 15 sec if the two values agree within 30 sec. 5.5.2 I f the difference between the two determinations is greater than 30 sec, the test shall be repeated on two additional units and the average of all four reported to the nearest 15 sec. 5.6 Setting T i m e a n d Exotherm--Peak t e m p e r a t u r e shall be measured at the center of a cylindrical specimen 15.0 mm in diameter by 15.0 mm in height. The specimen mold shall be made of poly(tetrafluoroethylene) with dimensions as shown in Figure 1 . Temperature shall be measured with a 28 gauge, duplex, stranded (6 strands 0.13 mm diam), glass insulated copper-constantan thermocouple. The thermocouple shall be preformed as shown in Figure 1, to facilitate placement of the junction in the center of the specimen. A poly(tetrafluoroethy1ene) plug shall be placed in the bottom of the mold and the assembly placed on a flat surface. The cement shall be mixed in accordance with the manufacturers’ directions and the mold overfilled; a sheet of polyethylene or cellophane shall be placed over the cement and the excess forced out by pressing the glass plate onto the film. The plate and film shall then be removed, the thermocouple inserted and the glass plate replaced. Temperature shall be recorded until cooling is observed. 5.6.1 The peak temperature shall be the average of two specimens. If the difference between the temperature of the two
SPECIFICATION FOR ACRYLIC BONE C E M E N T
Groove f o r ThemocouDle
' Fig. 1.
A l l dimensions in nun.
Mold and thermocouple for specimen for setting time and exotherm.
specimens is greater then 3.0°C, two additional specimens shall be measured and the average of all four runs shall be reported. 5.6.2 The setting time shall be taken as the time the peak temperature is attained. Two separate determinations of setting time shall be made and the average reported to the nearest 15 sec if the two values agree within 1 min. If the difference between the two determinations is greater than 1 min, the test shall be repeated on two additional units and the average of all four reported to the nearest 15 sec. 5.7 Handling Time-From the results of 5.5 and 5.6, the handling time shall be calculated as the average setting minus the average dough time. It shall be reported to the nearest 15 sec. 5.8 Intrusion-A perforated die (Fig. 2), a glass plate approximately 5 mm thick and 50 mm square and a 2.0 kg weight shall be preheated in an air chamber at 37 & 1°C. One unit of cement shall be mixed in the recommended manner (7.5.2). At 3.0 min after dough time, a 10 to 15 g portion of the mix shall be placed on the die (in the 37°C
HAAS, DICKSON, AND B R A U E R
I I I 50
5 0 MM
-710 MM &M L
chamber). A sheet of polyethylene or cellophane, the glass plate and the 2.0 kg weight shall be placed on the specimen immediately. Ten min later the die and specimen shall be removed from the air chamber and the cement stripped from the die. If necessary, the die and specimen may be soaked in water to facilitate separation. The depth of intrusion shall be measured with a dial gauge or measuring microscope to the nearest 0.1 mm. The depth of intrusion in at least 3 of the holes shall comply with the requirement (3.3.1). 5.9 Compressive Strength-The test specimens shall be cylinders 12 mm high and 6 mm diam. The ends of the specimens shall be flat and smooth and shall be parallel to each other and at right angles to the long axis of the cylinder. An apparatus found convenient for forming these test cylinders is shown in Figure 3. Five cylindrical molds 12 mm high and 6 mm diam shall be placed on a flat glass plate and slightly overfilled from one unit of mixed cement of standard consistency within 1 min after dough time. A second flat glass plate shall be pressed on top of the mold. The mold and plates shall be held firmly together with a small C clamp. All apparatus shall be at 23 f 2°C. One hr later, the ends of the cylinder shall be surfaced plane at right angles to the axis. The ends of the specimens may be ground flat by use of a small amount of 200 mesh silicon carbide powder and water. The molds containing the specimens shall be drawn back and forth across a glass plate coated with the
SPECIFICATION FOR ACRYLIC BONE CEMENT
abrasive and water. After surfacing, the specimens may be removed from the mold by a screw jack (Fig. 3). The time lapse between the start of mixing and the compressive strength testing shall be 24 5 2 hr. Specimens shall be run on any universal testing machine equipped to record load versus deformation. A deformation speed of 0.5 cm/min (0.2 in./min) shall be employed. Specimens shall be tested without the use of any type of pad between the specimen and the platens of the machine. The load at the first maximum on the time-load curve shall be recorded. The compressive strength (average of five specimens) shall be calculated and reported to the nearest megapascal (MPa). 5.10 Indentation and Recovery-Indentation shall be determined with a Rockwell Superficial Hardness Tester. Three specimens 38 mm diam by 2.0 f 0.1 mm thick shall be prepared in the mold shown in Figure 4. At 1 min after dough time, approximately 4 g of the cement shall be placed in the mold, the top plate placed on the mold and a 5.0 kg mass placed on the top plate. Fifteen min later the mass shall be removed and the specimen removed from the mold. Any flash, or
SCREW J A C K F O R SPECIMENS FROM
Fig. 3. Apparatus used in forming compressive strength specimens
H A A S , DICKSON, A N D BRAUER
Mold for indentation and recovery specimens. Dimensions in rnrn
other irregularity which would prevent the specimen from seating firmly on the platen of the hardness tester shall be removed. At 24 + 2 hr after start of mixing, each specimen shall be subjected to a minor force of 29.4 N (3 kgf) employing a steel ball 12.7 mm diam and the dial gauge shall be adjusted to zero. Then a major force of 294 N (30 kgf) which includes the minor force shall be applied for 10 min, at which time the depth of the indentation shall be read from the dial gauge and shall be recorded to the nearest 0.001 mm as Reading A. The major force shall then be released leaving the minor force on the specimen. Ten minutes later the dial gauge shall be read and the value recorded as Reading B. The average of the three specimens for Reading A and Reading B shall be rounded to the nearest 0.01 mm. When the final value falls midway between the two numbers, the even number shall be recorded. 5.10.1 Indentation shall be reported as the average of three specimens. 5.10.2 % recovery shall be calculated as follows:
5.1 1 Water Sorption-The water sorption test shall be made in dupli0.1 mm thick. The cate on disks 50 f 1 mm diam and 0.5 specimens may be made in a mold composed of stainless steel ring 50 mm i.d. by 0.5 f 0.1 mm thick, mounted between two flat glass plates. A sheet of polyethylene or cellophane shall be used between the mix and the glass plate. If desired, the mixture may be obtained simultaneously from the unit being used to provide material for the fabrication of the compressive specimens of 5.9. The mix shall be placed in the mold 1 min after dough time. After setting, the specimens shall be removed from the mold and lightly sanded to dimensions using 240
SPECIFICATION FOR ACRYLIC BONE CEMENT
and 400 grit abrasive papers, successively, backed by a flat, hard surface. The abrasive papers shall be flooded with water during the grinding procedures. All surfaces of the disk shall be smooth and the top and bottom shall be flat. The disks shall be dried in a desiccator containing dry anhydrous calcium sulfate (CaS04) or silica gel (freshly dried at 130°C) at 37 & 2°C for 24 hr, removed to a similar desiccator at room temperature for one hr, then weighed with a precision of 0.2 mg. This cycle shall be repeated until the weight loss of each disk is not more than 0.5 mg in any 24 hr period. This shall be known as the conditioned mass. The disks shall then be immersed in distilled water at 37 & 1°C for 7 days, after which time the disks shall be removed from the water with forceps, wiped with a clean, dry hand towel until free from visible moisture, waved in the air for 15 sec and weighed 1 min after removal from the water. The resulting weight shall be known as the mass after immersion. 5.1 1.1 The value for water sorption shall be calculated as follows for each disk: Sorption
mass after immersion - conditioned mass area
5.11.2 The average of the determined values for the two disks shall be recorded to the nearest 0.1 g/mz (0.01 mg/cm2). The final value (average of two determinations) shall be rounded to the nearest g/mz (0.1 mg/cm2). When the final value falls midway between two numbers, the even number shall be recorded. 5.12 Solubility-After the final weighing (5.1 1) the disks shall be reconditioned to constant weight in the desiccator at 37 f 2°C as was done in 5.1 1. This shall be known as the reconditioned mass. 5.12.1 The value for solubility shall be calculated as follows for each disk: Solubility
conditioned mass (from 5.11) - reconditioned mass area
5.12.2 The final value (average of two determinations) shall be rounded to the nearest 0.1 g/m2 (0.01 mg/cm2). When the final value falls midway between two numbers, the even number shall be recorded. 6. Weights and Permissible Variations The weights and/or volumes of the powder and liquid components shall not deviate by more than 1.0% from those stated on the package (7.4.1).
H A A S , DICKSON, AND BRAUER
6.1 Weight and volume measurements shall be made on the respective powder and liquid components of 5 units. These units may be subsequently utilized in any of the tests of this specification. 6.2 No powder component of a given unit shall deviate by more than f 0.5% from the average for the powder of the 5 units. 6.3 No liquid component of a given unit shall deviate by more than f 0.5% from the average for the liquid of the 5 units. 7. Packaging 7.1 Materials shall be supplied in properly sealed containers made of materials which shall not contaminate or permit contamination of the contents. The containers shall be packaged so as to prevent damage or leakage during shipping and storage. 7.2 The containers shall be easy to open and the contents easily accessible and convenient to mix in the operating room. Entire package contents (both powder and liquid) shall consist of proper amounts of material to achieve recommended proportions. 7.3 The information outlined below must be provided on container labels and inserts accompanying each package of liquid and powder. 7.4 The following information must appear on the label of each component: 7.4.1 Contents. The weight and/or volume of both the liquid and powder component must be stated. In addition, all constituents of the powder and liquid shall be clearly stated in terms of wt or vol %. This shall include all polymers, copolymers, chemical activators, plasticizers, monomers, chemical promoters, stabilizers, crosslinking agents, chain terminators or other ingredients. 7.4.2 Warning to avoid storage above 30°C (86°F) or in direct sunlight to prevent premature polymerization. Warning to avoid usage if premature polymerization should occur. Warning regarding flammability. 7.4.3 Manufacturer and distributor identification. 7.4.4 Batch or lot number must be clearly marked on each individual component of each unit. 7.5 The following information must appear on the product identification insert: 7.5.1 The dough, setting, and handling times measured at 23.0 2.0"C shall be stated. 7.5.2 Adequate and accurate instructions for storing, preparing and handling the resin. Instructions shall include powder-liquid ratio (complete use of units recommended), the time, temperature and procedure required to mix the material along with recommended suitable mixing vessels and mixing tools.
SPECIFICATION FOR ACRYLIC BONE CEMENT
7.5.3 Proper techniques for placing and recommended procedures for using the cement, including any special precautions to be observed such as use of suitable vessels and mixing tools. 7.5.4 Toxic, hazardous or irritating characteristics associated with the handling of the components of the uncured mix shall be indicated. 8. Unit conversion factors 1 kgf = 9.807 N 1 kgf/cm2 = 0.09807 M P a