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A Method for Determining the Extent of Polymerization of Acrylic Resins and its Applications for Dentures
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A M E T H O D FO R D E T E R M IN IN G T H E E X T E N T O F P O L Y M E R IZ A T IO N O F A C R Y L IC RESINS A N D ITS A P P L IC A T IO N S F O R D E N TU R E S
Harold J. C au l,* B.S., and Irl C . Schoonover,f Ph.D., W ashington, D. C .
molecule also containing a double bond to form a chain of monomer units, with loss of one double bond each time two units join. This chain formation, or poly merization, is the process by which the plastic mixture of powder and liquid is converted into a solid. It is believed that variations in physical properties and some of the other difficul ties observed in acrylic dentures are as sociated with the nature and complete ness of polymerization and the possible retention of monomer. Any improvement, therefore, in the resin as a denture ma terial will be derived from increased knowledge and the application of ele ments which influence polymerization. A chemical method for determining the degree of polymerization has been de veloped, and has been used to show that
resin has b e e n used by the den tal profession for several years. In this resin and its copolymers dentists have potentially some of their best prosthetic materials. In spite of their excellence, many difficulties have been reported, among them water absorption, dimen sional instability, brittleness, discolora tion, difficulty of repair and bubble for mation. These difficulties for the most part are not inherent properties of acrylic resin but are defects induced in it by im proper processing. Acrylic resin is supplied for dental use in two forms. In one, a powder (poly mer) and a liquid (monomer) are fur nished and mixed at the time of proces sing, and in the other, the material is sold as a gel, the mixing of the polymer and the monomer having been done by the manufacturer. The mechanism of polymerization (curing) in both is iden tical. The liquid (monomer) contains a double bond in each molecule, which is capable of reacting with any similar c r y lic
* Research associate of the Am erican Dental Associa tion a t the N ational Bureau of Standards. f C h ief, Dental M aterials Section, N ational Bureau of Slandards.
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The Journal of the American Dental Association
the variations in physical properties are related to the degree of polymerization, which is affected by time, temperature and pressure, as well as by the presence of impurities. The method is applicable to the investigation of irregularly shaped specimens, which cannot be studied by the usual physical methods. It is the purpose of this paper to describe this chemical method and its application to some dental problems en countered in curing acrylic dentures. Method Wijs1 demonstrated that a solution of iodine monochloride reacts with double bonds of some unsaturated organic com pounds. Experiments performed at the National Bureau of Standards have shown that this solution may be used to determine the number of double bonds in solutions of monomer or polymerized acrylic resin. The term “ acrylic resin” refers to the family of acrylic ester resins used for making dentures. Methyl metha crylate is a member of this family. In these experiments weighed samples of methyl methacrylate of not more than 5 Gm. polymer or 90 mg. monomer were dissolved in 40 ml. chloroform in glass stoppered 300 ml. Erlenmeyer flasks. Twenty milliliters of approximately 0.2 normal Wijs solution2 (iodine mono chloride) was transferred with a pipet into each flask. The stoppered flasks were placed in the dark at room temperature for 16 to 24 hours, after which the excess Wijs solution was determined by titra tion with tenth-normal standard sodium thiosulfate solution in the following man ner: One hundred milliliters of a freshly prepared 10 per cent potassium iodide solution3 is added to the reacted solution to be tested. Iodine is liberated in ac cordance with the following reaction : K l + ICI -» KC1 + Ii The mixture of the two solutions sepa rates into two dark reddish brown layers,
an aqueous layer on top and a chloroform layer on the bottom. Tenth-normal stan dard thiosulfate is added from a buret while the flask is constantly swirled until the top, or aqueous, layer becomes color less and the chloroform layer pink. A t this time sufficient thiosulfate has been added to react with most of the iodine. Five milliliters of a starch solution4 is added, and if the aqueous layer turns dark, thiosulfate is added dropwise while the flask is swirled until the top layer again becomes colorless. The flask is then stoppered and shaken vigorously in order to extract the remaining iodine from the chloroform layer. The aqueous layer again takes on color. Thiosulfate is again added dropwise while the flask is swirled or shaken, until 1 drop of thiosulfate solution makes both layers colorless when the monomer is being titrated. With the polymer, however, the tendency is for the chloroform layer to become white and opaque. The aqueous layer also becomes white, but the color is not nearly so dense. The thiosulfate solution is added until there is no change in color. Titrations containing either polymer or monomer usually become colored again after stand ing a few minutes. The end point is con sidered to have been reached when there is no return of color for at least one minute. Blanks containing volumes of chloro form and Wijs solution equal to those in the samples are tested along with the samples. Both samples and blanks are determined in duplicate. The quantity of Wijs solution (iodine monochloride) ab sorbed by the resin sample is obtained by
1. Wijs, J . J . A ., Zur Jod-Additionsmethode. B er. d. deutsch. cnem . Gesellsch. 3 1 :7 5 0 , 1898. 2. ' All the reagents used except the potassium iodide solution were prepared according to the federal specifi cation for varnish and shellac, No. T T -V -g ia (Wash ington: U . S. Government Printing Office). 3. Potassium iodide free from iodate, 100 G m ., dis solved in distilled water and diluted to make 1 liter. Fresh solution must be employed, since iodide solutions are readily oxidized on standing to form free iodine. 4. Soluble starch, 5 G m .; dissolved in 500 ml. hot water containing 1 Gm. salicylic acid.
J .A .D .A ., Vol. 39, July 1949 . . .
Caul-Schoonover
correcting the titer for the quantity used by the blank. This corrected quantity of Wijs solution is used to calculate the number average molecular weight, which is an indication of the number of double bonds in the monomer or polymer sample. Assuming that each molecule contains one double bond, the number of double bonds or molecules present is a measure of the degree of polymerization. For the purpose of this discussion the degree of polymerization has been expressed as the number average molecular weight (M n), which is an average in which each mole cule, whether polymer or monomer, car ries the same weight in the resulting average. It is an excellent measure of the completeness of polymerization. This value may be calculated from the quan tity of tenth-normal Wijs solution which reacts with the methyl methacrylate sample and the weight of the sample in milligrams. W
M, = — D
U)
where W = milligrams of sample n = millimols of polymer or monomer 5 IV
but
“ = W l = 0051V
(2)
where V = milliliters of tenth-normal Wijs solution absorbed by the reaction 0.051 = millimols of monomer or polymer per milliliter of tenth-normal Wijs solution 5.1 = milligrams of monomer per mil liliter of Wijs solution (Fig. 1) 100.1 = molecular weight of methyl methacrylate monomer Substituting the value of n in equation 1 gives M0 =
W 0.051V
(3)
Since W and F are measured quantities, a value of Mnmay be obtained. The value for the number average molecular weight obtained by this method would be affected by the presence in the resin of plasticizers, fillers and resins other than acrylic. These additions would give
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a false value for the number average mol ecular weight. However, this does not interfere with the usefulness of the method for investigating the factors which effect polymerization for any one brand of resin. Mixtures of polymer and monomer were made, and the number average molecular weights were determined and also calculated (Table i) . The number average molecular weight may be defined as S n jM j
Siij
Sfi/M ,
where ni is the number of molecules of Mi, and fi is the weight fraction of these molecules. This fraction may bg reduced to M , =*
U + f.
(4)
M,
for Table i, in which it is assumed that all the polymer has a molecular weight (M p) of 22400, and the molecular weight of the monomer (M m) is 100. The sym bols fp and fm designate weight fractions of polymer and monomer. From Table 1 it can be seen that if 0.9 per cent of residual monomer was in a sample of resin having an original num ber average molecular weight of 22400, the resultant resin had a number average molecular weight of about 7300. The number average molecular weights of ten brands of acrylic resin powder and of specimens polymerized from these powders in accordance with the manu facturers’ recommendations were deter mined (Table 2). The number average molecular weights of the polymeric powders prior to curing varied between 3500 and 36000. These same products after curing had number average molecu lar weights from 8000 to 39000. In all but one product, the molecular weight increased on curing. It is not known at the present time how the wide variation
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The Journal of the American Dental Association
Table I.— Lowering of the num ber average m olecular w eight ( M „ ) of methyl methacrylate b y additions of small quantities of monomer
M on om er, p e rce n t
0.1 N W ijs solution absorbed, ml.
M on om er, m g.
P olym er, m g.
0.0
535
0.00
0.47
22400
-------
4.7
535
0.87
1.45
7300
7600
14.2
535
2.59
3.28
3300
3300
37.8
535
6.60
8.00
1400
1400
in number average molecular weight in both the powder and the cured resin af fects the chemical and physical properties of a denture prepared from the material. Comment Preparation of Sam ple.— Inhibitor.— The acrylic resin used by the dental profes sion is usually purchased in the “ powderliquid” form. The liquid contains an in hibitor, usually hydroquinone, to prevent polymerization at room temperature. In order that the effect of the inhibitor on the Wijs reaction might be deter mined, 5 ml. portions of inhibited mono mer were placed in flasks and evaporated to dryness with a gentle stream of air
Table 2.— N u m be r average molecular w eight of several specim ens of acrylic resin before and after being cured
Sam ple A B C D E F G H I
J
P ow der
C ured specim en*
3500 7000 22000 11500 29000 23000 31000 27000 36000 11000
10000 8000 36000 9000 32000 35000 36000 32000 39000 17000
* C ured according to the recommendations of the manufacturers.
Mn (equation 3)
Mn (e q u a tio n 4)
blown into the flask. Then chloroform was added, followed by Wijs solution. The solution was titrated. None of the Wijs solution was used by the residue from the inhibited monomer. Therefore, the inhibitor, either because a small quantity was present or because it did not react with the Wijs solution, had no ef fect on the reaction. Size of sample.— The quantity of 0.2 normal Wijs solution to be added to the chloroform solution of monomer or resin for consistent results was determined by a series of experiments with various quan tities (4 to 142 mg.) of purified mono mer5 dissolved in chloroform solution. Ten, 20 and 40 ml. of 0.2 normal Wijs solution were added to the chloroform solutions. Titrations with thiosulfate neu tralized the excess Wijs solution, as pre viously described. The excess Wijs solu tion, subtracted from the amount added, which had been corrected for the blank, gave the amount of Wijs solution ab sorbed. Values have been plotted in Figure 1 as tenth-normal Wijs solution absorbed. A plot of the various quantities of monomer and the volume of Wijs solution absorbed by each quantity must
5. Inhibitor-free monomer was prepared for these experiments by distillation of the inhibited monomer at pressure of 9 to 10 cm. mercury. T h e middle fraction was used for the sample. Five milliliters of the middle fraction was pipetted into chloroform #and diluted to make 1 liter. This solution was stored in a refrigerator to avoid any possibility of polymerization.
J .A .D .A ., Vol. 39, July 1949 . . .
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W Mn = 0.051 V ’
ume of Wijs solution absorbed will vary with the size of sample used. If an error of ± 0 .1 ml. is assumed for the volume of Wijs solution absorbed because of titration errors, it is possible to calculate the error in number average molecular weight. For example, 400 mg. of 19600molecular-weight methyl methacrylate resin absorbs 0.4 ml. of tenth-normal Wijs solution. In this case the volume of Wijs solution would be o.41+10.1 ml. Then the number average molecular weight would be 19600 igg“ , or an error of 20 to 33 per cent. If the size of sample is increased tenfold, to 4,000 mg., the Wijs solution absorbed would be 4 .0 + 0 .1 ml. Then the number average molecular weight would equal 19600 ± 5 0 0 , or an error of about 2 / 2 per cent. By the use of this method the error in number av erage molecular weight can be calculated and the size of sample adjusted to de crease the error. A t the present time it has been found somewhat impractical to use samples weighing more than 5 Gm. in 40 ml. of chloroform. This is due primarily to the viscous character of solutions containing larger percentages of polymer. Difficulties of titration are encountered, and the re sults become less accurate. The use of larger volumes of chloroform solution should overcome this difficulty.
two important facts may be found: 1. When the errors are not large, the per centage of error in number average mol ecular weight is approximately equal to the percentage of error in the volume of Wijs solution absorbed by the reaction if the errors in the weight of sample and the factor 0.051 can be neglected. 2. The volume of Wijs solution absorbed is pro portional to the weight of the sample of resin. Thus, if the number average molecu lar weight of a sample of methyl metha crylate resin is determined, the error in volved will be dependent on the volume of Wijs solution absorbed, and the vol
Time and Temperature.— The minimum time for the reaction between monomer and iodine monochloridc to reach com pletion was determined by a set of ex periments in which the reaction solutions of monomer and iodine monochloridc were allowed to react for definite periods up to 72 hours. After 16 hours the reac tion appeared to be complete, because the quantity of iodine monochloride ab sorbed by the monomer solution was practically constant after this time. Figure 2 shows the results of this set of experi ments during the first 24 hours of reac tion. The reaction time used was based on these results.
result in a straight line if the reaction goes to completion. The addition of io ml. of 0.2 normal Wijs solution was insufficient to complete reactions in which less than 50 per cent of the added solution was absorbed (Fig. 1). Twenty milliliters of 0.2 normal Wijs solution is sufficient to react completely with 90 mg. or less of methyl methacry late monomer. The weight of polymer must be adjusted so that the quantity of 0.2 normal Wijs solution absorbed in the reaction will always be less than 8 / 2 ml. when 20 ml. of 0.2 normal Wijs solu tion is added. In Figure 1, curve 20, 5 1 mg. of mono mer is shown to absorb 10 ml. of tenthnormal Wijs solution, or 1 milliequivaIent of Wijs solution. Since the molecular weight of methyl methacrylate is 100.1, the equivalent weight for this reaction would be 50.05. The discrepancy between the value of 50.05 and the graph value of 51 may indicate one or more of three things : ( 1 ) that the values agree within the experimental error of the method, (2) that the reaction is not complete or (3) that the methyl methacrylate mono mer used was not pure. Errors in Determination.— In equation 3,
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The Journal of the American Dental Association
2. The high percentage of potassium iodide makes the free iodine more soluble in the aqueous layer, so that less iodine is retained by the chloroform layer. The actual titration becomes easier as the end point is approached before the addition of the starch solution. 3. The large quantity of potassium iodide prevents volatilization of iodine. Applications
2
4
6
e
10
12
14
16
18
20
22
24
26
28
30
UU OF 0-' N WtJS SOLUTION ABSORBED
Fig. ! .— R elation of amount of Wijs solution consumed to the quantity of monomer present. T h e numerals io , 20 and 40 on the curves indicate that 10 , 20 and 40 ml. of 0.2 normal Wijs solution were a d d ed to the solutions of monomer. T h e quantities of Wijs solution used by the monomer were calculated to tenthnormal because the volumes in the titrations were read as volumes of approxim ately tenthnormal standard sodium thiosulfate solution and corrected to 0 .1000 normal
Because the temperature at which a chemical reaction occurs often has an im portant effect on the completeness of the reaction, another group of experiments was performed to determine the effect of temperature. One series of reactions was kept at a temperature of 33 C. (91.4 F.) and a second series at 8 C. (46.4 F . ) . No differences in the quantity of Wijs solution absorbed by the monomer solu tion were observed. Potassium Iodide Solution.— A large quan tity (100 ml.) of 10 per cent aqueous potassium iodide solution was used for several reasons: 1. Potassium iodide reacts with the chlorine present in the excess Wijs solu tion to form iodine: K l + IC I - * KG1 +
U
This in turn is titrated with the thiosul fate solution.
Polymerization of acrylic resins is an exothermic reaction, one that produces heat. Studies by the National Bureau of Standards and other laboratories showed that during curing the thick ridge sections of dentures reached temperatures as high as 140C . (2 8 4 F .), while in the thin palatal regions temperatures seldom were above 110 C . (2 3 0 F .). Where lower tem peratures exist, the speed of reaction is less, and polymerization is likely to be incomplete. Where higher temperatures result, the speed of the reaction is greater, and polymerization is more nearly com plete. The method has been used to investi gate the effects of curing temperature, thickness, additional cure (repair) and impurity of specimens.
J h -u u 11 I n 1 1m
111
ili. 1 '
"
1" 11' "
j . " 1 h 1 i j 1 1 1_
RE4CTKJN TW£ (HOURS)
Fig. 2 .— Effect of time on the completeness of reaction of Wijs solution and monomer
Caul-Schoonover
Coring Temperature.— Experiments were made in which samples of “ powderliquid” resin were cured by two different technics. One sample was cured by im mersion of the flask in a water bath at 75 C. (16 7 F .) for two and a half hours. The flask containing the other sample was placed in water at room temperature, gradually warmed to 100C. ( 2 1 2 F.) in one and a half hours and kept at this temperature for 45 minutes. Transverse strength tests were made of specimens cut from the cakes of resin prepared by these methods. The speci mens that received the boiling water cure were stronger and stiffer than the speci mens cured at 7 5 C. (16 7 F .) (Fig. 3). The number average molecular weights were determined on the broken speci mens. For the specimen cured in boiling water, the value was 35000; for the speci men cured at 75 C. (16 7 F .), the value was 5600.
Thickness.— Harman6 observed that speci mens cut from different sections of den ture have different physical properties. Specimens from the thin, or palatal, re gions are weaker, softer and dimensionally less stable than specimens from the thicker, or ridge, regions of the same denture. In a series of experiments a specimen having thick and thin sections similar to those in a denture was cured in a water bath at 75 C. (16 7 F .) for two and a half hours. The dimensions of the thin section were 48 by 54 by 2 mm., and those of the thick section were 48 by 12 by 14 mm. Thermocouples were placed in the two sections of the specimen. The highest temperature obtained in the thin section was 79 C . ( 1 7 4 F .) , and in the thick sec tion, 1 1 2 C . (2 3 4 F .). Number average molecular weights were found to be 6000 for the thin section and 25000 for the thick. These data indicate that the thick section will be stronger than the thin sec tion because of the more complete cure which occurred in the thick section,
J .A .D .A ., Voi. 39, July 1949 . . .
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where the higher temperature was ob served. The thin section, because of low molecular weight material or residual monomer, will be weaker. Further evi dence of the same nature was obtained by Beall and Caul,7 who reported that the introduction of a small amount of solvent into fully polymerized acrylic resin lowers the transverse strength and hardness. This explains the observations made by Harman6 that specimens made from thin sections are weaker than speci mens cut from thick sections. Additional Cure (Repair).— There is some possibility that difficulties in repairing dentures may occur from further poly merization during recuring. Tw o speci mens were prepared for investigation of this possibility. One was cured at 75 C. (16 7F .) for two and a half hours and the other in boiling water for 45 minutes after a preliminary heating from room temperature to the boiling point in one and a half hours. A portion of each speci men was reserved for determinations of number average molecular weight, and the remainder was recured twice by being “ flasked” and placed in boiling water each time for one hour. The number average molecular weight of the speci men cured at 75 C. (16 7 F .) increased from 5400 to 29000 when the specimen was recured at 100C. ( 2 1 2 F .). The number average molecular weight of the specimen initially cured in boiling water did not change when it was recured in boiling water. The value remained at 29000. One would expect that specimens pre pared from the resin cured at 75 C. (16 7 F .) would be dimensionally less stable, since shrinkage invariably accom panies further polymerization. This may account for part of the shrinkage which
6. Harman, 1. M ., Effects of Time and Tempera ture on Polymerization of a Methacrylate Resin Den ture Base. J . A .D .A . 38:¡88 ( F e b .) 1949. 7. Beall, J . R ., and Caul, H . J ., “ Liners” for Den tures. J.A .D .A . 3 3 :3 0 4 (March 1) 1946.
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The Journal of the American Dental Association
LOAD IN 1000 GRAMS'
Pig- 3-— E ffect of different curing tem pera tures on the transverse strength of acrylic resin. C urve A , average of four specimens cured at 75 C. ( 16 7 F .) fo r two and a half hours; curve B , average of four specimens cured by being heated from room tem perature to 10 0 C. ( 2 1 2 F .) in one and a half hours and held at 100 C. for 4.5 minutes
has been observed on repair, particu larly if the palatal, or thin, sections of the denture are incompletely polymerized during the initial cure of the denture. Impurities.— Data obtained at the N a tional Bureau of Standards show that the addition of small quantities of impurities, known to affect polymerization, definitely
changed the physical properties of the resin cured by a standard dental technic. Ferguson, PafTenbarger and Schoon over3 observed that clear resins cured against some brands of rubber dam turn red and that the transverse strengths of specimens cut from these resins are lower than the values obtained from specimens cured against tinfoil. Determinations of the number average molecular weight of these specimens were made. A value of 14000 was found for the specimens cured against the rubber dam, and a value of 30000 was obtained for the specimens cured against tinfoil. The effect on trans verse strength is shown in Figure 4. The decrease in number average molecular weight when the rubber dam was used has been traced to the presence of some material in the rubber which was dis solved by the monomer and was the cause of the retarded polymerization. The effect of another impurity on the process of polymerization was illustrated further by a series of experiments in which sulfur was intentionally added to the resin prior to curing. Six rods (6 mm. in diameter and 50 mm. long) were pre pared and cured in the same flask with the same brand of powder and liquid. Three of the rods were intentionally con taminated by using monomer saturated with sulfur. After being cured, the speci mens without sulfur were normal, but the ones containing sulfur were rubbery. Number average molecular weights were 1400 for sample containing sulfur and 17000 for samples without sulfur. These results indicate that a small amount of sulfur has a decided inhibiting action on this type of polymerization. Sum m ary
LOAD IN 1000 GRAMS
Fig. 4.— E ffect of curing against some brands of rubber dam and tinfoil on the transverse strength of acrylic resin. C urve R , average of five specimens cured against rubber dam ; curve T , average of five specimens cured against tinfoil in same manner as in R
A chemical method has been developed to determine the completeness of poly-
8. Ferguson, G . VV.- PafTenbarger, G . C ., and Schoonover, I. C .. Deficiencies of Tinfoil Substitutes in the Processing of Acrylic Resin. J.A .D .A . 3 8 :5 7 3 (May) 1949.
J.A .D .A ., Vol. 39, July 1949 . . .
Murphey-Newton-Schmid
merization of cured acrylic resin. The method is based on a determination of number average molecular weight or of the number of double bonds. Number average molecular weight may be used to correlate some of the chemical and physical properties with conditions of cure (time, temperature and presence of impurities). The method is applicable to the inves tigation of irregularly shaped specimens which cannot be tested by conventional methods, and is useful in evaluating the uniformity of a manufacturer’s product. Data have been obtained which indi cate that the lower transverse strength of thin sections of dentures as compared with thick sections is the result of less
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complete polymerization in the thin sec tions. The low transverse strength of speci mens cured at 75 C. ( 16 7 F .) for two and a half hours appears to be the result of incomplete polymerization. Specimens polymerized at 100C, (2 12 F .) for 45 minutes are stronger. Impurities introduced into resins proc essed against some brands of rubber dam prevent complete curing of the resin. The number average molecular weights of several different brands of acrylic powder prior to curing vary between 3500 and 36000; after curing in accord ance with the manufacturer’s recom mendations, they vary from 8000 to 39000.
THE EVOLVEMENT OF OPHTHALMOPROSTHESIS TO CIVILIAN REQUIREMENTS Phelps J . Murphey,* D.D.S., F. H. N ew fon.f M.D., and Dorothy Ja n e Schmid.t B.A., B.S.
science of eye replacement has its derivation from China, where jade was inserted over the sunken eyeballs of the deceased more than two thous and years ago.1 In a replica of the bust h e
Presented before the American Dental Society of Europe, London, England, July 27 to 30, 1948; and before the eighty-ninth annual session of the American Dental Association, Chicago, September 1948. * Chief of the Maxillofacial Prosthesis Department, the Richmond Freeman Clinic, Children’s Medical Center, Dallas, Tex. t C h ie f o f the O phthalm ology D epartm ent, T h e R ich mond Freem an C lin ic, Children s M edical C enter, Dallas, T e x . $ M edical illustrator, T h e R ichm ond F reem an C lin ic, C hild ren’s M edical C en ter, Dallas, T e x . Affiliated w ith the Southwestern M edical College, Southwestern M e d ical Foundation, Dallas, T ex .
of Caligula, third emperor of Rome, about 53 B.C., anatomic prostheses were made to replace the original of ivory and glass.2 Paramount among the advances in medicine resulting from World W ar II are plastic, maxillofacial and rehabilita tion surgery.3 These words are from an
1. Chan, Eugene, Personal correspondence, E .E .N .T . Hospital, United Hospitals, W . China Union Uni versity, Changtu Sze, China. 2. Bruce, G . M ., Ancient Origins of Artificial Eyes. A nn . M . H ist. 2 :io (Jan.) 1940. 3. Zieman, Stephen, Acrylic Ocular Prosthesis. (Editorial) U . S. N avy M e d . B u ll. 4 3 :1 2 5 8 (Dec.) »944*
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DENTAL
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A M E T H O D FO R D E T E R M IN IN G T H E E X T E N T O F P O L Y M E R IZ A T IO N O F A C R Y L IC RESINS A N D ITS A P P L IC A T IO N S F O R D E N TU R E S
Harold J. C au l,* B.S., and Irl C . Schoonover,f Ph.D., W ashington, D. C .
molecule also containing a double bond to form a chain of monomer units, with loss of one double bond each time two units join. This chain formation, or poly merization, is the process by which the plastic mixture of powder and liquid is converted into a solid. It is believed that variations in physical properties and some of the other difficul ties observed in acrylic dentures are as sociated with the nature and complete ness of polymerization and the possible retention of monomer. Any improvement, therefore, in the resin as a denture ma terial will be derived from increased knowledge and the application of ele ments which influence polymerization. A chemical method for determining the degree of polymerization has been de veloped, and has been used to show that
resin has b e e n used by the den tal profession for several years. In this resin and its copolymers dentists have potentially some of their best prosthetic materials. In spite of their excellence, many difficulties have been reported, among them water absorption, dimen sional instability, brittleness, discolora tion, difficulty of repair and bubble for mation. These difficulties for the most part are not inherent properties of acrylic resin but are defects induced in it by im proper processing. Acrylic resin is supplied for dental use in two forms. In one, a powder (poly mer) and a liquid (monomer) are fur nished and mixed at the time of proces sing, and in the other, the material is sold as a gel, the mixing of the polymer and the monomer having been done by the manufacturer. The mechanism of polymerization (curing) in both is iden tical. The liquid (monomer) contains a double bond in each molecule, which is capable of reacting with any similar c r y lic
* Research associate of the Am erican Dental Associa tion a t the N ational Bureau of Standards. f C h ief, Dental M aterials Section, N ational Bureau of Slandards.
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The Journal of the American Dental Association
the variations in physical properties are related to the degree of polymerization, which is affected by time, temperature and pressure, as well as by the presence of impurities. The method is applicable to the investigation of irregularly shaped specimens, which cannot be studied by the usual physical methods. It is the purpose of this paper to describe this chemical method and its application to some dental problems en countered in curing acrylic dentures. Method Wijs1 demonstrated that a solution of iodine monochloride reacts with double bonds of some unsaturated organic com pounds. Experiments performed at the National Bureau of Standards have shown that this solution may be used to determine the number of double bonds in solutions of monomer or polymerized acrylic resin. The term “ acrylic resin” refers to the family of acrylic ester resins used for making dentures. Methyl metha crylate is a member of this family. In these experiments weighed samples of methyl methacrylate of not more than 5 Gm. polymer or 90 mg. monomer were dissolved in 40 ml. chloroform in glass stoppered 300 ml. Erlenmeyer flasks. Twenty milliliters of approximately 0.2 normal Wijs solution2 (iodine mono chloride) was transferred with a pipet into each flask. The stoppered flasks were placed in the dark at room temperature for 16 to 24 hours, after which the excess Wijs solution was determined by titra tion with tenth-normal standard sodium thiosulfate solution in the following man ner: One hundred milliliters of a freshly prepared 10 per cent potassium iodide solution3 is added to the reacted solution to be tested. Iodine is liberated in ac cordance with the following reaction : K l + ICI -» KC1 + Ii The mixture of the two solutions sepa rates into two dark reddish brown layers,
an aqueous layer on top and a chloroform layer on the bottom. Tenth-normal stan dard thiosulfate is added from a buret while the flask is constantly swirled until the top, or aqueous, layer becomes color less and the chloroform layer pink. A t this time sufficient thiosulfate has been added to react with most of the iodine. Five milliliters of a starch solution4 is added, and if the aqueous layer turns dark, thiosulfate is added dropwise while the flask is swirled until the top layer again becomes colorless. The flask is then stoppered and shaken vigorously in order to extract the remaining iodine from the chloroform layer. The aqueous layer again takes on color. Thiosulfate is again added dropwise while the flask is swirled or shaken, until 1 drop of thiosulfate solution makes both layers colorless when the monomer is being titrated. With the polymer, however, the tendency is for the chloroform layer to become white and opaque. The aqueous layer also becomes white, but the color is not nearly so dense. The thiosulfate solution is added until there is no change in color. Titrations containing either polymer or monomer usually become colored again after stand ing a few minutes. The end point is con sidered to have been reached when there is no return of color for at least one minute. Blanks containing volumes of chloro form and Wijs solution equal to those in the samples are tested along with the samples. Both samples and blanks are determined in duplicate. The quantity of Wijs solution (iodine monochloride) ab sorbed by the resin sample is obtained by
1. Wijs, J . J . A ., Zur Jod-Additionsmethode. B er. d. deutsch. cnem . Gesellsch. 3 1 :7 5 0 , 1898. 2. ' All the reagents used except the potassium iodide solution were prepared according to the federal specifi cation for varnish and shellac, No. T T -V -g ia (Wash ington: U . S. Government Printing Office). 3. Potassium iodide free from iodate, 100 G m ., dis solved in distilled water and diluted to make 1 liter. Fresh solution must be employed, since iodide solutions are readily oxidized on standing to form free iodine. 4. Soluble starch, 5 G m .; dissolved in 500 ml. hot water containing 1 Gm. salicylic acid.
J .A .D .A ., Vol. 39, July 1949 . . .
Caul-Schoonover
correcting the titer for the quantity used by the blank. This corrected quantity of Wijs solution is used to calculate the number average molecular weight, which is an indication of the number of double bonds in the monomer or polymer sample. Assuming that each molecule contains one double bond, the number of double bonds or molecules present is a measure of the degree of polymerization. For the purpose of this discussion the degree of polymerization has been expressed as the number average molecular weight (M n), which is an average in which each mole cule, whether polymer or monomer, car ries the same weight in the resulting average. It is an excellent measure of the completeness of polymerization. This value may be calculated from the quan tity of tenth-normal Wijs solution which reacts with the methyl methacrylate sample and the weight of the sample in milligrams. W
M, = — D
U)
where W = milligrams of sample n = millimols of polymer or monomer 5 IV
but
“ = W l = 0051V
(2)
where V = milliliters of tenth-normal Wijs solution absorbed by the reaction 0.051 = millimols of monomer or polymer per milliliter of tenth-normal Wijs solution 5.1 = milligrams of monomer per mil liliter of Wijs solution (Fig. 1) 100.1 = molecular weight of methyl methacrylate monomer Substituting the value of n in equation 1 gives M0 =
W 0.051V
(3)
Since W and F are measured quantities, a value of Mnmay be obtained. The value for the number average molecular weight obtained by this method would be affected by the presence in the resin of plasticizers, fillers and resins other than acrylic. These additions would give
3
a false value for the number average mol ecular weight. However, this does not interfere with the usefulness of the method for investigating the factors which effect polymerization for any one brand of resin. Mixtures of polymer and monomer were made, and the number average molecular weights were determined and also calculated (Table i) . The number average molecular weight may be defined as S n jM j
Siij
Sfi/M ,
where ni is the number of molecules of Mi, and fi is the weight fraction of these molecules. This fraction may bg reduced to M , =*
U + f.
(4)
M,
for Table i, in which it is assumed that all the polymer has a molecular weight (M p) of 22400, and the molecular weight of the monomer (M m) is 100. The sym bols fp and fm designate weight fractions of polymer and monomer. From Table 1 it can be seen that if 0.9 per cent of residual monomer was in a sample of resin having an original num ber average molecular weight of 22400, the resultant resin had a number average molecular weight of about 7300. The number average molecular weights of ten brands of acrylic resin powder and of specimens polymerized from these powders in accordance with the manu facturers’ recommendations were deter mined (Table 2). The number average molecular weights of the polymeric powders prior to curing varied between 3500 and 36000. These same products after curing had number average molecu lar weights from 8000 to 39000. In all but one product, the molecular weight increased on curing. It is not known at the present time how the wide variation
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The Journal of the American Dental Association
Table I.— Lowering of the num ber average m olecular w eight ( M „ ) of methyl methacrylate b y additions of small quantities of monomer
M on om er, p e rce n t
0.1 N W ijs solution absorbed, ml.
M on om er, m g.
P olym er, m g.
0.0
535
0.00
0.47
22400
-------
4.7
535
0.87
1.45
7300
7600
14.2
535
2.59
3.28
3300
3300
37.8
535
6.60
8.00
1400
1400
in number average molecular weight in both the powder and the cured resin af fects the chemical and physical properties of a denture prepared from the material. Comment Preparation of Sam ple.— Inhibitor.— The acrylic resin used by the dental profes sion is usually purchased in the “ powderliquid” form. The liquid contains an in hibitor, usually hydroquinone, to prevent polymerization at room temperature. In order that the effect of the inhibitor on the Wijs reaction might be deter mined, 5 ml. portions of inhibited mono mer were placed in flasks and evaporated to dryness with a gentle stream of air
Table 2.— N u m be r average molecular w eight of several specim ens of acrylic resin before and after being cured
Sam ple A B C D E F G H I
J
P ow der
C ured specim en*
3500 7000 22000 11500 29000 23000 31000 27000 36000 11000
10000 8000 36000 9000 32000 35000 36000 32000 39000 17000
* C ured according to the recommendations of the manufacturers.
Mn (equation 3)
Mn (e q u a tio n 4)
blown into the flask. Then chloroform was added, followed by Wijs solution. The solution was titrated. None of the Wijs solution was used by the residue from the inhibited monomer. Therefore, the inhibitor, either because a small quantity was present or because it did not react with the Wijs solution, had no ef fect on the reaction. Size of sample.— The quantity of 0.2 normal Wijs solution to be added to the chloroform solution of monomer or resin for consistent results was determined by a series of experiments with various quan tities (4 to 142 mg.) of purified mono mer5 dissolved in chloroform solution. Ten, 20 and 40 ml. of 0.2 normal Wijs solution were added to the chloroform solutions. Titrations with thiosulfate neu tralized the excess Wijs solution, as pre viously described. The excess Wijs solu tion, subtracted from the amount added, which had been corrected for the blank, gave the amount of Wijs solution ab sorbed. Values have been plotted in Figure 1 as tenth-normal Wijs solution absorbed. A plot of the various quantities of monomer and the volume of Wijs solution absorbed by each quantity must
5. Inhibitor-free monomer was prepared for these experiments by distillation of the inhibited monomer at pressure of 9 to 10 cm. mercury. T h e middle fraction was used for the sample. Five milliliters of the middle fraction was pipetted into chloroform #and diluted to make 1 liter. This solution was stored in a refrigerator to avoid any possibility of polymerization.
J .A .D .A ., Vol. 39, July 1949 . . .
Caul-Schoonover
5
W Mn = 0.051 V ’
ume of Wijs solution absorbed will vary with the size of sample used. If an error of ± 0 .1 ml. is assumed for the volume of Wijs solution absorbed because of titration errors, it is possible to calculate the error in number average molecular weight. For example, 400 mg. of 19600molecular-weight methyl methacrylate resin absorbs 0.4 ml. of tenth-normal Wijs solution. In this case the volume of Wijs solution would be o.41+10.1 ml. Then the number average molecular weight would be 19600 igg“ , or an error of 20 to 33 per cent. If the size of sample is increased tenfold, to 4,000 mg., the Wijs solution absorbed would be 4 .0 + 0 .1 ml. Then the number average molecular weight would equal 19600 ± 5 0 0 , or an error of about 2 / 2 per cent. By the use of this method the error in number av erage molecular weight can be calculated and the size of sample adjusted to de crease the error. A t the present time it has been found somewhat impractical to use samples weighing more than 5 Gm. in 40 ml. of chloroform. This is due primarily to the viscous character of solutions containing larger percentages of polymer. Difficulties of titration are encountered, and the re sults become less accurate. The use of larger volumes of chloroform solution should overcome this difficulty.
two important facts may be found: 1. When the errors are not large, the per centage of error in number average mol ecular weight is approximately equal to the percentage of error in the volume of Wijs solution absorbed by the reaction if the errors in the weight of sample and the factor 0.051 can be neglected. 2. The volume of Wijs solution absorbed is pro portional to the weight of the sample of resin. Thus, if the number average molecu lar weight of a sample of methyl metha crylate resin is determined, the error in volved will be dependent on the volume of Wijs solution absorbed, and the vol
Time and Temperature.— The minimum time for the reaction between monomer and iodine monochloridc to reach com pletion was determined by a set of ex periments in which the reaction solutions of monomer and iodine monochloridc were allowed to react for definite periods up to 72 hours. After 16 hours the reac tion appeared to be complete, because the quantity of iodine monochloride ab sorbed by the monomer solution was practically constant after this time. Figure 2 shows the results of this set of experi ments during the first 24 hours of reac tion. The reaction time used was based on these results.
result in a straight line if the reaction goes to completion. The addition of io ml. of 0.2 normal Wijs solution was insufficient to complete reactions in which less than 50 per cent of the added solution was absorbed (Fig. 1). Twenty milliliters of 0.2 normal Wijs solution is sufficient to react completely with 90 mg. or less of methyl methacry late monomer. The weight of polymer must be adjusted so that the quantity of 0.2 normal Wijs solution absorbed in the reaction will always be less than 8 / 2 ml. when 20 ml. of 0.2 normal Wijs solu tion is added. In Figure 1, curve 20, 5 1 mg. of mono mer is shown to absorb 10 ml. of tenthnormal Wijs solution, or 1 milliequivaIent of Wijs solution. Since the molecular weight of methyl methacrylate is 100.1, the equivalent weight for this reaction would be 50.05. The discrepancy between the value of 50.05 and the graph value of 51 may indicate one or more of three things : ( 1 ) that the values agree within the experimental error of the method, (2) that the reaction is not complete or (3) that the methyl methacrylate mono mer used was not pure. Errors in Determination.— In equation 3,
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The Journal of the American Dental Association
2. The high percentage of potassium iodide makes the free iodine more soluble in the aqueous layer, so that less iodine is retained by the chloroform layer. The actual titration becomes easier as the end point is approached before the addition of the starch solution. 3. The large quantity of potassium iodide prevents volatilization of iodine. Applications
2
4
6
e
10
12
14
16
18
20
22
24
26
28
30
UU OF 0-' N WtJS SOLUTION ABSORBED
Fig. ! .— R elation of amount of Wijs solution consumed to the quantity of monomer present. T h e numerals io , 20 and 40 on the curves indicate that 10 , 20 and 40 ml. of 0.2 normal Wijs solution were a d d ed to the solutions of monomer. T h e quantities of Wijs solution used by the monomer were calculated to tenthnormal because the volumes in the titrations were read as volumes of approxim ately tenthnormal standard sodium thiosulfate solution and corrected to 0 .1000 normal
Because the temperature at which a chemical reaction occurs often has an im portant effect on the completeness of the reaction, another group of experiments was performed to determine the effect of temperature. One series of reactions was kept at a temperature of 33 C. (91.4 F.) and a second series at 8 C. (46.4 F . ) . No differences in the quantity of Wijs solution absorbed by the monomer solu tion were observed. Potassium Iodide Solution.— A large quan tity (100 ml.) of 10 per cent aqueous potassium iodide solution was used for several reasons: 1. Potassium iodide reacts with the chlorine present in the excess Wijs solu tion to form iodine: K l + IC I - * KG1 +
U
This in turn is titrated with the thiosul fate solution.
Polymerization of acrylic resins is an exothermic reaction, one that produces heat. Studies by the National Bureau of Standards and other laboratories showed that during curing the thick ridge sections of dentures reached temperatures as high as 140C . (2 8 4 F .), while in the thin palatal regions temperatures seldom were above 110 C . (2 3 0 F .). Where lower tem peratures exist, the speed of reaction is less, and polymerization is likely to be incomplete. Where higher temperatures result, the speed of the reaction is greater, and polymerization is more nearly com plete. The method has been used to investi gate the effects of curing temperature, thickness, additional cure (repair) and impurity of specimens.
J h -u u 11 I n 1 1m
111
ili. 1 '
"
1" 11' "
j . " 1 h 1 i j 1 1 1_
RE4CTKJN TW£ (HOURS)
Fig. 2 .— Effect of time on the completeness of reaction of Wijs solution and monomer
Caul-Schoonover
Coring Temperature.— Experiments were made in which samples of “ powderliquid” resin were cured by two different technics. One sample was cured by im mersion of the flask in a water bath at 75 C. (16 7 F .) for two and a half hours. The flask containing the other sample was placed in water at room temperature, gradually warmed to 100C. ( 2 1 2 F.) in one and a half hours and kept at this temperature for 45 minutes. Transverse strength tests were made of specimens cut from the cakes of resin prepared by these methods. The speci mens that received the boiling water cure were stronger and stiffer than the speci mens cured at 7 5 C. (16 7 F .) (Fig. 3). The number average molecular weights were determined on the broken speci mens. For the specimen cured in boiling water, the value was 35000; for the speci men cured at 75 C. (16 7 F .), the value was 5600.
Thickness.— Harman6 observed that speci mens cut from different sections of den ture have different physical properties. Specimens from the thin, or palatal, re gions are weaker, softer and dimensionally less stable than specimens from the thicker, or ridge, regions of the same denture. In a series of experiments a specimen having thick and thin sections similar to those in a denture was cured in a water bath at 75 C. (16 7 F .) for two and a half hours. The dimensions of the thin section were 48 by 54 by 2 mm., and those of the thick section were 48 by 12 by 14 mm. Thermocouples were placed in the two sections of the specimen. The highest temperature obtained in the thin section was 79 C . ( 1 7 4 F .) , and in the thick sec tion, 1 1 2 C . (2 3 4 F .). Number average molecular weights were found to be 6000 for the thin section and 25000 for the thick. These data indicate that the thick section will be stronger than the thin sec tion because of the more complete cure which occurred in the thick section,
J .A .D .A ., Voi. 39, July 1949 . . .
7
where the higher temperature was ob served. The thin section, because of low molecular weight material or residual monomer, will be weaker. Further evi dence of the same nature was obtained by Beall and Caul,7 who reported that the introduction of a small amount of solvent into fully polymerized acrylic resin lowers the transverse strength and hardness. This explains the observations made by Harman6 that specimens made from thin sections are weaker than speci mens cut from thick sections. Additional Cure (Repair).— There is some possibility that difficulties in repairing dentures may occur from further poly merization during recuring. Tw o speci mens were prepared for investigation of this possibility. One was cured at 75 C. (16 7F .) for two and a half hours and the other in boiling water for 45 minutes after a preliminary heating from room temperature to the boiling point in one and a half hours. A portion of each speci men was reserved for determinations of number average molecular weight, and the remainder was recured twice by being “ flasked” and placed in boiling water each time for one hour. The number average molecular weight of the speci men cured at 75 C. (16 7 F .) increased from 5400 to 29000 when the specimen was recured at 100C. ( 2 1 2 F .). The number average molecular weight of the specimen initially cured in boiling water did not change when it was recured in boiling water. The value remained at 29000. One would expect that specimens pre pared from the resin cured at 75 C. (16 7 F .) would be dimensionally less stable, since shrinkage invariably accom panies further polymerization. This may account for part of the shrinkage which
6. Harman, 1. M ., Effects of Time and Tempera ture on Polymerization of a Methacrylate Resin Den ture Base. J . A .D .A . 38:¡88 ( F e b .) 1949. 7. Beall, J . R ., and Caul, H . J ., “ Liners” for Den tures. J.A .D .A . 3 3 :3 0 4 (March 1) 1946.
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The Journal of the American Dental Association
LOAD IN 1000 GRAMS'
Pig- 3-— E ffect of different curing tem pera tures on the transverse strength of acrylic resin. C urve A , average of four specimens cured at 75 C. ( 16 7 F .) fo r two and a half hours; curve B , average of four specimens cured by being heated from room tem perature to 10 0 C. ( 2 1 2 F .) in one and a half hours and held at 100 C. for 4.5 minutes
has been observed on repair, particu larly if the palatal, or thin, sections of the denture are incompletely polymerized during the initial cure of the denture. Impurities.— Data obtained at the N a tional Bureau of Standards show that the addition of small quantities of impurities, known to affect polymerization, definitely
changed the physical properties of the resin cured by a standard dental technic. Ferguson, PafTenbarger and Schoon over3 observed that clear resins cured against some brands of rubber dam turn red and that the transverse strengths of specimens cut from these resins are lower than the values obtained from specimens cured against tinfoil. Determinations of the number average molecular weight of these specimens were made. A value of 14000 was found for the specimens cured against the rubber dam, and a value of 30000 was obtained for the specimens cured against tinfoil. The effect on trans verse strength is shown in Figure 4. The decrease in number average molecular weight when the rubber dam was used has been traced to the presence of some material in the rubber which was dis solved by the monomer and was the cause of the retarded polymerization. The effect of another impurity on the process of polymerization was illustrated further by a series of experiments in which sulfur was intentionally added to the resin prior to curing. Six rods (6 mm. in diameter and 50 mm. long) were pre pared and cured in the same flask with the same brand of powder and liquid. Three of the rods were intentionally con taminated by using monomer saturated with sulfur. After being cured, the speci mens without sulfur were normal, but the ones containing sulfur were rubbery. Number average molecular weights were 1400 for sample containing sulfur and 17000 for samples without sulfur. These results indicate that a small amount of sulfur has a decided inhibiting action on this type of polymerization. Sum m ary
LOAD IN 1000 GRAMS
Fig. 4.— E ffect of curing against some brands of rubber dam and tinfoil on the transverse strength of acrylic resin. C urve R , average of five specimens cured against rubber dam ; curve T , average of five specimens cured against tinfoil in same manner as in R
A chemical method has been developed to determine the completeness of poly-
8. Ferguson, G . VV.- PafTenbarger, G . C ., and Schoonover, I. C .. Deficiencies of Tinfoil Substitutes in the Processing of Acrylic Resin. J.A .D .A . 3 8 :5 7 3 (May) 1949.
J.A .D .A ., Vol. 39, July 1949 . . .
Murphey-Newton-Schmid
merization of cured acrylic resin. The method is based on a determination of number average molecular weight or of the number of double bonds. Number average molecular weight may be used to correlate some of the chemical and physical properties with conditions of cure (time, temperature and presence of impurities). The method is applicable to the inves tigation of irregularly shaped specimens which cannot be tested by conventional methods, and is useful in evaluating the uniformity of a manufacturer’s product. Data have been obtained which indi cate that the lower transverse strength of thin sections of dentures as compared with thick sections is the result of less
9
complete polymerization in the thin sec tions. The low transverse strength of speci mens cured at 75 C. ( 16 7 F .) for two and a half hours appears to be the result of incomplete polymerization. Specimens polymerized at 100C, (2 12 F .) for 45 minutes are stronger. Impurities introduced into resins proc essed against some brands of rubber dam prevent complete curing of the resin. The number average molecular weights of several different brands of acrylic powder prior to curing vary between 3500 and 36000; after curing in accord ance with the manufacturer’s recom mendations, they vary from 8000 to 39000.
THE EVOLVEMENT OF OPHTHALMOPROSTHESIS TO CIVILIAN REQUIREMENTS Phelps J . Murphey,* D.D.S., F. H. N ew fon.f M.D., and Dorothy Ja n e Schmid.t B.A., B.S.
science of eye replacement has its derivation from China, where jade was inserted over the sunken eyeballs of the deceased more than two thous and years ago.1 In a replica of the bust h e
Presented before the American Dental Society of Europe, London, England, July 27 to 30, 1948; and before the eighty-ninth annual session of the American Dental Association, Chicago, September 1948. * Chief of the Maxillofacial Prosthesis Department, the Richmond Freeman Clinic, Children’s Medical Center, Dallas, Tex. t C h ie f o f the O phthalm ology D epartm ent, T h e R ich mond Freem an C lin ic, Children s M edical C enter, Dallas, T e x . $ M edical illustrator, T h e R ichm ond F reem an C lin ic, C hild ren’s M edical C en ter, Dallas, T e x . Affiliated w ith the Southwestern M edical College, Southwestern M e d ical Foundation, Dallas, T ex .
of Caligula, third emperor of Rome, about 53 B.C., anatomic prostheses were made to replace the original of ivory and glass.2 Paramount among the advances in medicine resulting from World W ar II are plastic, maxillofacial and rehabilita tion surgery.3 These words are from an
1. Chan, Eugene, Personal correspondence, E .E .N .T . Hospital, United Hospitals, W . China Union Uni versity, Changtu Sze, China. 2. Bruce, G . M ., Ancient Origins of Artificial Eyes. A nn . M . H ist. 2 :io (Jan.) 1940. 3. Zieman, Stephen, Acrylic Ocular Prosthesis. (Editorial) U . S. N avy M e d . B u ll. 4 3 :1 2 5 8 (Dec.) »944*