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Physical properties of plastic teeth
Physical properties of plastic teeth W . T . Sw eeney,* A .B .; E. L . Y o st,f B.S., and J. G. Fee,% A .B ., W ashington, D . C.
used are polym ethyl methacrylate and copolymers in w hich methyl methacrylate is a m ajor constituent. T h e chemical properties are very satisfactory. M ethyl methacrylate polymers are nonpoisonous, are insoluble in the fluids fou n d in the oral cavity, and the m onom er is suffi ciently chem ically reactive to other methacrylates under heat and pressure to form a chem ical bond. W hen mixtures o f methacrylate polym er and m onom er are subjected to heat and pressure in processing dentures, a chem ical union re sults. Thus, the reaction o f the m onom er in the uncured denture base with the plastic teeth also form s a chem ical bond between the denture base and the tooth. H ow ever, polym ethyl methacrylate is soluble to some extent in m ost organic solvents such as ketones, arom atic hydro carbons, and so forth, and it is important, therefore, that the denture not be cleansed in an organic solvent. In general, m echanical properties of these resins are low in com parison with those o f other dental restorative mate rials such as am algam , gold alloy and so forth. T h e low m echanical properties such as strength, abrasion resistance, modulus o f elasticity and hardness, h ow ever, have some advantages. T h e lower modulus* o f elasticity contributes to the
Plastic teeth have been used by the den tal profession for approxim ately 15 years. During this period, considerable clinical evaluation has been m ade. A lthough den tists are not in total agreement as to the relative value o f plastic as com pared to porcelain teeth, the continued widespread sale o f plastic teeth indicates that the dental profession has fou n d them useful in restorative dentistry. A survey o f prosthodontists indicated that over 90 per cent o f them had used or were using plastic teeth. O n e tenth used plastic teeth exclusively whereas others used them in varying degrees. T he m ajority o f the dentists answering this questionnaire were users o f plastic teeth in particular situations where very short teeth were required. T h e replies o f this group indicated that plastic teeth are deficient in some physical properties and that other physical p rop erties are satisfactory and desirable for dental use. It is also evident that differ ent com m ercial brands o f plastic teeth vary widely in some properties. Since d if ferences have been noted, it is necessary to establish laboratory tests and data fo r the evaluation o f plastic teeth. This re port includes data obtained on several brands o f plastic teeth. P R O P E R T IE S O F D E N T A L S IG N IF IC A N C E
*Chief, dental research section, National Bureau of Standards. fG uest worker, U. S. Arm y, dental research section, National Bureau of Standards.
Plastics have many properties that make them attractive materials fo r production o f teeth. T h e resins that have been most
iPhysicist, dental of Standards.
833
research
section,
National
Bureau
834 • T H E JO U R N A L O F T H E A M E R IC A N DEN TA L A S S O C IA T IO N
elimination o f a “ clicking” sound such as is produ ced when porcelain teeth com e in contact with each other. T h e low hard ness and abrasion resistance facilitate the adjustm ent of the occlusion. A lso plastic teeth have a high degree o f toughness; therefore, less shattering or breaking o c curs. Plastic teeth have a desirable es thetic appearance. T h eir optical proper ties make it possible to m atch shades and colors with those of natural teeth. A serious drawback o f some o f the early brands o f plastic teeth was crazing in service. T h e mechanism o f crazing is not com pletely understood but is generally thought to be caused by the relieving o f localized strain.1 Crazing was com m only noticed after repair or rebasing o f a den ture. Formulations o f the currently m anu factured teeth are such that crazing is rarely observed. T h e use o f cross-linked polymers is the main cause fo r this im provement. T E E T H IN V E S T IG A T E D
T h e teeth included in this investigation were procured and tested from 1950 to 1952. T hey were obtained from the fo l low ing sources: Trade Nam e
Manufacturer
M icromold
Austenal Laboratories, Inc.
Vitalon
Austenal Laboratories, Inc.
Trubyte Acrylic
T h e Dentists’ Supply Co. of New York
Trubyte Dentron
T he Dentists’ Supply Co. of New York
Denta Pearl
H . D . Justi & Son, Inc.
S -R
H . D . Justi & Son, Inc.
Luxene
Luxene, Inc.
Dura-Blend
Myerson Tooth Corporation
Plastic Five-Phase
Universal Dental Co.
Verident
Universal Dental Co.
Biocryl
Biodent M fg . Co. Heidelberg, Germany
E X P E R IM E N T A L PR O C E D U R E AND R ESU LTS
Crazing • Craze tests were m ade on plastic teeth in three conditions: (1 ) as received from the manufacturer, (2 ) after a curing cycle such as dentures normally receive in dental laboratories and (3) after a curing cycle in which special pre cautions were taken to prevent water from com ing into contact with the tooth. It was fou n d that it was unnecessary to process the teeth in denture base m ate rial fo r the curing cycle. T h e technic used was as follow s: 1. T he teeth were invested in gyp sum in a dental flask. 2. T he invested teeth were heated in a tem perature-controlled water bath. 3. T he flask was ben ch-cooled to room temperature. 4. T h e teeth were deflasked and were allowed to dry to room temperature for several hours or longer. T h e craze test consisted o f immersing the teeth in a solvent (m ethyl methacry late m onom er o r ethyl alcoh ol) for three periods o f five seconds each, with an in terval o f one m inute between each im mersion. A t first several solvents were used. M ethyl methacrylate m onom er and alcohol were finally chosen fo r test sol vents as they w ou ld m ore likely be en countered in practical u se: the m onomer, when a denture is repaired or rebased, and the alcohol when beverages are taken orally. T h e crazing was observed by visual inspection and also by the use o f a b in oc ular m icroscope. T h e specimens were given a second treatment after a day of storage if they did not show crazing im mediately. T able 1 shows that five brands did not craze, one brand exhibited crazing on some specimens and no crazing on other
I. Sweeney, W . T.; Brauer, G . M., and Schoonover, I. C . Crazing of acrylic resins. J . D. Res. 34:306 Jun e 1955.
S W E E N E Y — YOST— FE E . . . V O L U M E 56, J U N E 1958 • 835
specimens, and five brands gave very definite crazing by the solvent test. Craz ing was observed to be approxim ately the same for alcohol or methyl methacrylate m onom er. In some instances the m on om er caused slightly m ore crazing. T h e types o f crazing observed were quite varied as sh ow n . by examples in Figure 1. It was not possible to judge accurately the degree o f crazing because some specimens contained many fine cracks whereas others contained few er but larger cracks. Examination o f sections o f crazed teeth, under a m icroscope, indicated that the depths o f the cracks were rather varied, ranging from 0.2 to 1 mm. Teeth heated to 212° F. in water after investing showed m ore crazing than those heated to 160° F. in water. Experiments were conducted to deter mine whether teeth susceptible to crazing w ould craze when heated under con d i tions such that no water cam e in contact with them during heating. Several p ro cedures were used for this, such as (1 ) Table 1
drying out free water from the gypsum by heating or reduced pressure, (2 ) sur rounding the teeth with tin foil and (3 ) coating the teeth with a thick layer o f w ax before investing. In all instances in which water in both liquid and vapor phase was prevented from com ing in contact with the teeth, no crazing was produced by the solvent tests. This leads to the conclusion that water plays an im portant role in the craz ing phenom enon. Dentures worn by patients exhibit this same crazing but usually to a m uch less degree. Figure 2 shows a practical im mediate denture w hich was rebased after eight months. T he crazing appeared in the plastic teeth after rebasing; all six o f the anterior teeth showed severe crazing and had to be replaced. M echanical Properties • D ata were o b tained on hardness (K n o o p ), indenta tion resistance, strength, m odulus o f elas ticity and elastic recovery o f the plastic teeth investigated.
• Physical p ro p ertie s o f plastic teeth Indentation resistance C/2 inch ball)
K n o op indentation hardness number (100 Gm . load) Brand
C ra z e * N o. of o b ser vations
1 2 3 4 5
6 7 8 9 10 11
Yes No No No No Yes Yes Yes + + Yes No
A v e ra g e
kg./mm.2 19.9 19.8 19.1 19.7 18.7 18.2 19.4 18.5 18.1 18.2 14.6
7 40 20 14 IS 9 30 6 19 19 6
S .D .f
0.3 1.2 0.6 2.9 0.9 0.4 0.7 0.6 0.6 0.7 0.5
Depth (30 kg. lo a d = 10 min.)
R e co v e ry (3 kg. lo a d — 10 min.)
mm. 0.099 0.090 0.098 0.090 0.103 0.106 0.098 0.104 0.098 0.148 0.099
% 87 88 89 91 88 82 88 87 91 70 82
Ratio o f lo a d to pro jected a re a o f inden tation
psi 9,800 10,800 9,900 10,800 9,400 9,200 9,900 9,300 9,900 6,400 9,800
Com pressive properties
Modulus, o f e la s ticity
105 psi 3.9 ± 0 .2 3.8 + 0.3 3.4 + 0.3 3 .8 + 0.1 4 .2 + 0.4 3 .6 + 0.1 3 .1 + 0 .3 3 .6 + 0.2 3.8 ± 0 .3 2 .8 + 0.5 4.1 +0.1
•Solvents used were methyl m ethacrylate monomer and ethyl alcohol, after normal curing cycle.
{Standard
deviation
2 (xi —x) 2 n— 1
JSom e specimens crazed, others did not.
Y ield strength
psi 8,600 8,100 6,700 8,600 8,400 7,100 8,100 7,600 8,600 6,400 8,400
836
THE JOURNAL OF THE AMERICAN DENTAL ASSOCIATION
Hardness Hardness was determined with a Kentron tester with a Knoop indenter at a temperature of 70° k 2' F., using a 100 Gm. load with a 10 second descent, and 20 second contact. The teeth were cut and polished to a smooth flat surface about 2 mm. from the tip of the cusps. The final polish was made with fine polishing paper (no. 000) followed by fine whiting. The teeth were measured in the "as received" condition, that is, not dried or stored in water before testing. The Knoop indentation numbers are given in Table 1. Indentation Resistance and Recovery Flat specimens 2 to 3 mm. thick were cut from plastic teeth. The surfaces were finished with no. 1 G sandpaper. The specimen was placed in a Rockwell superficial hardness tester and subjected to a 30 kg. major load with a T/r inch ball for ten minutes. At the end of this period the d e ~ t hof indentation was observed. the major load was removed, and the minor load of 3 kg. was applied for ten additional minutes. At this time the depth of indentation was again observed. he recovery value is expressed in percentage of the depth of indentation. The results
Fig. I
Examples o f crazing produced
shown in Table 1 are averages of six specimens. The results are compared in Figure 3 with observations made on direct filling resins and dental amalgams. Ratio of Load to Projected Area of Indentation Since in the procedure described previously the indenter sphere continues to penetrate the resin until the area of contact is sufficient to resist the force exerted on the specimen, the ratio of the load to the projected area of the indentation gives a basis for comparing strength of similar materials. The projected area of the indentation can be calculated from the depth reading on the Rockwell superficial hardness tester with a correction for the slight indentation of the minor load.2 The load divided by this area gives an indication of the strength in the same units as stress. The data obtained for this ratio for specimens cut from plastic teeth are shown in Table 1. Stress-Strain Relationship Cylindrical specimens 3 mm. in diameter by 6 mm. in length were machined from the 2. Sweeney W. T : Mechanical ;ropertiis
49:513 Nov. 1954.
Sheehan W. D of d i r e b fillin;
and Yost E. L. resins. J.A.D.A.
by immersion of plastic tooth in solvent
S W E E N E Y — YO ST— F E E . . . V O L U M E 56, J U N E 1958 • 837
plastic teeth. T he specimens were placed in a Tinius Olsen testing m achine and loaded in compression at an average strain rate of approxim ately 0.003 inches per inch per minute. T h e specimens were mounted between two steel bearing blocks Ys inch in diameter. T w o 1-inch Tuckerman strain gauges were m ounted on the bearing blocks to measure the axial d e form ation o f the specimen. T h e deform a tion o f the portions o f the bearing blocks within the gauge length was considered to be negligible. T h e strain was recorded at 2Yz pound load increments. T h e modulus o f elasticity was calcu lated from the straight portion o f the stress-strain curve below 7,000 psi (Fig. 4 -6 ). T h e compressive “ yield strength” for the purpose o f this paper is defined as the stress (load per unit o f cross-sectional area) at which the deform ation produced by a stress increment equals or exceeds 1.25 times that observed in the first few increments o f stress starting at approximately 1,000 psi.3 T h e values for the modulus o f elasticity and “ yield strength” shown in T able 1 are averages o f fou r to eight specimens. Abrasion or W ear Resistance • T h e data obtained on abrasion o f teeth are o f a very preliminary nature and are reported at the present time for inform ation on the ball mill type o f test. T h e m ethod used involves abrading the teeth in a ball mill
and observing the loss o f weight. T he mill was a one quart A bbe porcelain jar mill rotating at 69 revolutions per minute. T o com pare the wear o f different brands o f teeth the mill was charged with the abrasive material and a total o f 40 to 50 teeth o f various brands, w hich had been saturated in water, and was run for periods o f time ranging from tw o hours to several hundred hours. T he relative effects o f several different abrasive materials and o f different tooth shapes are shown in Tables 2 and 3, re spectively. T h e relative wear o f eight brands o f teeth when placed in the mill with steel balls with and w ithout addi tional abrasives is given in T a b le 4. M ost consistent results were obtained when 54 inch steel balls with no. 200 A loxite p o w der in water was used as the abrasive material according to the follow ing tech n ic: 1. O n e thousand three hundred and fifty grams o f Y\ inch carbon steel balls were washed with distilled water and allowed to drain.
3. Paffenbarger, G . C .; Sweeney, W . T., and Isaacs, A . W ro ug ht gold wire alloys: physical properties and a specification. J.A .D .A . 19:2061 Dec. 1932.
TIM E-M IN UTES
Fig. 3 • Indentation and re co ve ry of plastic teeth (a v e ra g e of ten b rand s), den ture base resin, d i rect filling resins and am algam in Rockwell super ficial hardness tester with 1/2 inch ball and ten minute load of 30 kg. and ten minute load of 3 kg.
838 • T H E JO U R N A L O F T H E A M E R IC A N D EN T A L A S S O C IA T IO N
relatively constant fo r any run made with the same abrasive and conditions. A lthough the data obtained from the ball mill tests show differences between the various brands o f teeth, no data have yet been obtained indicating whether or not the test results correlate with the wear that occurs in service. M u ch m ore re search will be needed before any state ment can be m ade on the relative clinical wear resistance of various brands o f teeth. D IS C U S S IO N O F R E S U L T S
Fig . 4 • Stress-strain curves fo r five brands of p lastic teeth
2. T eeth were selected, marked and individually weighed. T h e teeth used should be fairly large, from 0.3 to 0.5 G m . except in teeth with m echanical retention holes; fo r the latter, teeth weighing 0.25 to 0.45 G m . will be about the same size as the others. T o mark the teeth fo r identification, shallow holes were bored in recessed parts o f the teeth and these were filled with drawing inks o f various colors. 3. T h e balls, 20 Gm . o f no. 200 A loxite pow der and 100 Gm . o f water were placed in the one quart jar, w hich was centered on the mill and run fo r two hours. 4. T h e used A loxite pow der was cleaned out, the ja r and balls rinsed with distilled water and im mediately 20 Gm. o f fresh no. 200 A loxite pow der, 100 G m . o f distilled water and from 40 to 50 teeth were added. T h e test was run immediately after the cleaning charge and fo r a defi nite time. 5. A t the conclusion o f the test the teeth were rem oved from the mill and weighed individually. T h e percentage loss in weight was calculated. For com parison o f brands the average loss fo r a given brand was divided by the loss fo r one brand selected as a standard. This gives a num ber that seems to be
T h e craze resistance seems to be a fu n c tion o f the processing conditions. During curing o f the denture, the water absorbed by the plastic teeth that are susceptible to crazing evidently plays an im portant role, since it was shown that w hen water was prevented from contacting a tooth the tooth so treated was craze resistant. Also, teeth invested in dental stone and heated in boiling water exhibited m ore crazing by the solvent test than did those heated to 160° F. in water. This m ay have been caused by the greater mutual solubility o f resin and water at the higher tempera ture. Although, the mechanism o f crazing is not established, the theory has been p ro posed that the relative solubility o f water
Fig . 5 • Stress-strain curves fo r four brands of plastic teeth
S W E E N E Y — YOST— FE E . . . V O L U M E 56, J U N E 1958 • 839
and resin changes with temperature and that the dimensional change caused by absorption o f water during curing and separation during cooling is the basic cause o f the residual strain in the resin.1 So far as the present experimental data are concerned, the facts correlate satis factorily, but m ore observations o f a pre cise nature are necessary to prove the validity o f this theory. N o correlation be tween com plete craze resistance and the amount o f water sorption was fou n d in this investigation. U nfortunately there is no practical m ethod for fabricating dentures under water-free conditions, as an excess o f free water is present in the set dental stone or plaster used to invest dentures fo r p ro c essing. It is considered best, therefore, to use teeth that d o not exhibit crazing if this difficulty is to be overcom e. T h e indentation hardness values de termined by the K n o op m ethod show that the range in hardness is small. This is to be expected because the teeth are essen tially methyl methacrylate polymers with the exception o f one brand that contains considerable vinyl chloride-acetate and has a low er K n o o p indentation number. T h e ability o f the tooth to resist d e form ation from pressure applied to a local area such as biting on a hard par-
Fig. 6 • Stress-strain curves for two brands of plastic teeth
Table 2 • Effect o f vario us ab rasives on w eight loss o f teeth in ball mill test* W e ig h t loss per hour, % A brasive D ry
I
W et
O x fo rd crystal feldspar
0.002
-
N o . 18 glass b e a d s
0.002
—
Flint pebbles
0.01
0.04
—
0.02
—
0.1
—
0.2
Ste el balls /a inch Steel balls
Zi
inch
Flint pebbles roughened with carborundum N o . 200 A loxite, % steel balls
inch
Fine carborundum , !4 inch steel balls
0.2
0.7
0.3
—
*Time of test varied from a few hours for the more abrasive to several days for the less abrasive charges.
tid e o f food , bone and so forth is im portant. T o evaluate this property the rate o f indentation by a sphere under constant load was used. T h e rate o f in dentation is dependent on the m echanical properties o f the sample. T h e rate o f penetration o f the ball into the specimen is fast fo r the first few seconds and then gradually decreases until the ball comes to rest. F or practical procedures a 10 minute load application was fou n d to ap proach very closely a static condition. This provided a simple m ethod for o b serving the flow properties and resistance to indentation. T h e probable error of the depth o f indentation under the best co n dition is o f the order o f 2 per cent. T h e depth o f indentation fo r 10 of the 11 brands o f *teeth studied varied from 0.090 mm. to 0.106 m m . T h e other brand (no. 10) had an extremely high value, 0.148 m m ., and is not included in the average because o f the excessive amount o f porosity observed. T h e average inden tation o f 0.098 mm . fo r the ten brands is between those fo r denture base resin (0.096 m m .) and the direct filling resins (0.108 m m .), and is m uch greater than that obtained fo r dental am algam (0.043 m m .) under the same test conditions. T he
840 • T H E JO U R N A L O F T H E A M E R IC A N D EN T A L A S S O C IA T IO N
Table 3
Cuspid Brand
W e ig h t Gm.
Gm .
0.3549 0.3936 0.3296
W e ig h t Gm .
%
0.0083 0.0065 0.0074
0.3764 0.4033 0.3509
0.0014 0.0003 0.0013
2.34 1.65 2.24
0.2277 0.2794 0.2131
0.37 0.07 0.37
0.0082 0.0064 0.0058
W e ig h t loss
W e ig h t Gm .
%
Gm .
%
3.60 2.29 2.72
0.3734 0.4254 0.3296
0.0088 0.0071 0.0084
2.36 1.67 2.55
0.0013 0.0009 0.0016
0.48 0.19 0.44
Time: 20 h o i rs
0.0014
0.2268
0.0017
0.50
----
0.75
0.2697 0.4755 0.3670
ranged from 82 to 91 per cent fo r ten of the brands tested. T h e eleventh brand, which also had a high indentation value, recovered only 70 per cent. T h e m odulus o f elasticity in com pres sion com puted over stress ranges o f 1,000 to 7,000 psi varied from 280,000 to 420,000 psi fo r the different brands. This property is significant as it indicates the stiffness o f these materials. T h e yield stress o f plastic teeth is low . T h e “ yield strengths” as calculated from the stressstrain curves ranged from 6,400 to 8,600
• Relative w e a r o f v ario us brands o f teeth in ball mill test
W e t A lox ite p o w d e r and Va " steel ba ils* Brand
4
test
Time: 25 hou rs
0.2791
-------
average results for the three types o f materials are shown in Figure 3. O n e of the fundam ental properties of resins is the property o f absorbing energy by deform ation and returning closely to the original form . This is a fa ctor in den ture prosthesis, as the energy absorbed by oral structures probably plays an im por tant role in resorption o f tissue. T h e re covery o f the plastic teeth as measured by the decrease in depth o f the indenta tion o f a spherical ball when the load on the ball was reduced from 30 to 3 kg. Table 4
mill
C en tra l incisor
W e ig h t loss Gm.
A b ra sive: W et Va " balls 8 11 1
ball
Lateral incisor
W e ig h t loss
A b ra sive: W e t Z7 " balls 8 11 1
in
• Effect o f tooth shape on w eigh t loss o f teeth
D ry A lox ite Va '' steel balls
D ry fine ca rb o ru n dum Va " steel balls
W et w steel balls
/a " steel balls
W et
8 hr.
4 hr.
8 hr.
4 hr.
4 hr.
25 hr.
2 hr.
1.92
*1.69
1.74
1.95
1.93
—
1.17
2
—
1.21
1.17
1.28
1.17
—
.85
6
1.05
1.12
1.04
1.49
1.08
—
.96
5
1.07
1.06
1.14
1.28
1.13
—
.83
8
1.02
1.04
1.04
1.15
1.04
1.10
.85
1
1.00
1.00
1.00
1.00
1.00
1.00
1.00
9
.88
.92
.96
1.16
.99
—
.83
11
.53
.46
.54
.36
.53
.75
.51
10
—
—
—
—
—
.88
•Values for three runs are given to show precision of results. Brand I is used as reference value.
S W E E N E Y — YOST— FE E . . . V O L U M E 56, J U N E 1958 • 841
psi. These values are considerably lower than the masticating stress o f 17,000 psi on cuspids as calculated by Skinner.4 T he fact that plastic teeth used in clinical dentures d o not exhibit m uch deform a tion is explained partially by the high recovery value (Fig. 3 ). Abrasion or wear resistance o f plastic teeth is one o f their weakest properties. A t present the work on abrasion meas urement is only in the developm ent stage. It is considered essential that new and m ore practical test methods be d e veloped to evaluate this factor. SUM M ARY
T h e m echanical properties o f eleven brands o f plastic teeth were investigated and the follow ing observations were m ade:
1. T h e m echanical properties were low in com parison with those o f other materials used fo r restorative purposes in the oral cavity. 2. Som e brands o f teeth were found to be susceptible to crazing after a p ro c essing cycle similar to the curing o f a denture. Five brands crazed consistently, five brands did not craze and one brand crazed in some instances. 3. Crazing was not observed on any teeth if water was prevented from co n tacting them during heating. 4. T h e resistance to crazing was found to be the most outstanding difference be tween the com m ercial brands studied. 5. N o definite correlation was found between the physical properties studied and crazing. 4. Skinner, E. W . Science of dental materials, ed. 3. Philadelphia, W . B. Saunders Co., 1946, p. 17.
Preventive Health Service • T h e emphasis in preventive health service has shifted from the lowering of hazard to the raising of im m unity; and in doing so it has shifted from things which can be done for people to things which people must do for themselves. This change is sig nificant; it reversed a long accepted trend. In equating himself with his environment, it has been the pride of progressive man to make his environment fit him. T o accept the environment and learn to be equal to it is by no means orthodox but it is refreshingly sane. But there still lies far in the background the concept of health as a dynamic condition, of which immunity to disease is only one reflection. Our knowledge of constitutional types is still slight, still slighter is our knowledge of what determines the strange variations in apparent vitality between man and man or how far this elusive quality can be developed. Occasional innovations in teaching and training assert and demonstrate that we might make more of ourselves than in fact we d o; but, by and large, only the mountaineer and the athlete try to get fit, rather than merely to keep well. People seem to think that the role of science is to remove hazards, rather than to fit us to face them. This is a dangerous fallacy. Neither physical science nor medicine or dentistry can obtain that— except at a price which mankind will be unwilling to pay. G eoffrey Vickers.
used are polym ethyl methacrylate and copolymers in w hich methyl methacrylate is a m ajor constituent. T h e chemical properties are very satisfactory. M ethyl methacrylate polymers are nonpoisonous, are insoluble in the fluids fou n d in the oral cavity, and the m onom er is suffi ciently chem ically reactive to other methacrylates under heat and pressure to form a chem ical bond. W hen mixtures o f methacrylate polym er and m onom er are subjected to heat and pressure in processing dentures, a chem ical union re sults. Thus, the reaction o f the m onom er in the uncured denture base with the plastic teeth also form s a chem ical bond between the denture base and the tooth. H ow ever, polym ethyl methacrylate is soluble to some extent in m ost organic solvents such as ketones, arom atic hydro carbons, and so forth, and it is important, therefore, that the denture not be cleansed in an organic solvent. In general, m echanical properties of these resins are low in com parison with those o f other dental restorative mate rials such as am algam , gold alloy and so forth. T h e low m echanical properties such as strength, abrasion resistance, modulus o f elasticity and hardness, h ow ever, have some advantages. T h e lower modulus* o f elasticity contributes to the
Plastic teeth have been used by the den tal profession for approxim ately 15 years. During this period, considerable clinical evaluation has been m ade. A lthough den tists are not in total agreement as to the relative value o f plastic as com pared to porcelain teeth, the continued widespread sale o f plastic teeth indicates that the dental profession has fou n d them useful in restorative dentistry. A survey o f prosthodontists indicated that over 90 per cent o f them had used or were using plastic teeth. O n e tenth used plastic teeth exclusively whereas others used them in varying degrees. T he m ajority o f the dentists answering this questionnaire were users o f plastic teeth in particular situations where very short teeth were required. T h e replies o f this group indicated that plastic teeth are deficient in some physical properties and that other physical p rop erties are satisfactory and desirable for dental use. It is also evident that differ ent com m ercial brands o f plastic teeth vary widely in some properties. Since d if ferences have been noted, it is necessary to establish laboratory tests and data fo r the evaluation o f plastic teeth. This re port includes data obtained on several brands o f plastic teeth. P R O P E R T IE S O F D E N T A L S IG N IF IC A N C E
*Chief, dental research section, National Bureau of Standards. fG uest worker, U. S. Arm y, dental research section, National Bureau of Standards.
Plastics have many properties that make them attractive materials fo r production o f teeth. T h e resins that have been most
iPhysicist, dental of Standards.
833
research
section,
National
Bureau
834 • T H E JO U R N A L O F T H E A M E R IC A N DEN TA L A S S O C IA T IO N
elimination o f a “ clicking” sound such as is produ ced when porcelain teeth com e in contact with each other. T h e low hard ness and abrasion resistance facilitate the adjustm ent of the occlusion. A lso plastic teeth have a high degree o f toughness; therefore, less shattering or breaking o c curs. Plastic teeth have a desirable es thetic appearance. T h eir optical proper ties make it possible to m atch shades and colors with those of natural teeth. A serious drawback o f some o f the early brands o f plastic teeth was crazing in service. T h e mechanism o f crazing is not com pletely understood but is generally thought to be caused by the relieving o f localized strain.1 Crazing was com m only noticed after repair or rebasing o f a den ture. Formulations o f the currently m anu factured teeth are such that crazing is rarely observed. T h e use o f cross-linked polymers is the main cause fo r this im provement. T E E T H IN V E S T IG A T E D
T h e teeth included in this investigation were procured and tested from 1950 to 1952. T hey were obtained from the fo l low ing sources: Trade Nam e
Manufacturer
M icromold
Austenal Laboratories, Inc.
Vitalon
Austenal Laboratories, Inc.
Trubyte Acrylic
T h e Dentists’ Supply Co. of New York
Trubyte Dentron
T he Dentists’ Supply Co. of New York
Denta Pearl
H . D . Justi & Son, Inc.
S -R
H . D . Justi & Son, Inc.
Luxene
Luxene, Inc.
Dura-Blend
Myerson Tooth Corporation
Plastic Five-Phase
Universal Dental Co.
Verident
Universal Dental Co.
Biocryl
Biodent M fg . Co. Heidelberg, Germany
E X P E R IM E N T A L PR O C E D U R E AND R ESU LTS
Crazing • Craze tests were m ade on plastic teeth in three conditions: (1 ) as received from the manufacturer, (2 ) after a curing cycle such as dentures normally receive in dental laboratories and (3) after a curing cycle in which special pre cautions were taken to prevent water from com ing into contact with the tooth. It was fou n d that it was unnecessary to process the teeth in denture base m ate rial fo r the curing cycle. T h e technic used was as follow s: 1. T he teeth were invested in gyp sum in a dental flask. 2. T he invested teeth were heated in a tem perature-controlled water bath. 3. T he flask was ben ch-cooled to room temperature. 4. T h e teeth were deflasked and were allowed to dry to room temperature for several hours or longer. T h e craze test consisted o f immersing the teeth in a solvent (m ethyl methacry late m onom er o r ethyl alcoh ol) for three periods o f five seconds each, with an in terval o f one m inute between each im mersion. A t first several solvents were used. M ethyl methacrylate m onom er and alcohol were finally chosen fo r test sol vents as they w ou ld m ore likely be en countered in practical u se: the m onomer, when a denture is repaired or rebased, and the alcohol when beverages are taken orally. T h e crazing was observed by visual inspection and also by the use o f a b in oc ular m icroscope. T h e specimens were given a second treatment after a day of storage if they did not show crazing im mediately. T able 1 shows that five brands did not craze, one brand exhibited crazing on some specimens and no crazing on other
I. Sweeney, W . T.; Brauer, G . M., and Schoonover, I. C . Crazing of acrylic resins. J . D. Res. 34:306 Jun e 1955.
S W E E N E Y — YOST— FE E . . . V O L U M E 56, J U N E 1958 • 835
specimens, and five brands gave very definite crazing by the solvent test. Craz ing was observed to be approxim ately the same for alcohol or methyl methacrylate m onom er. In some instances the m on om er caused slightly m ore crazing. T h e types o f crazing observed were quite varied as sh ow n . by examples in Figure 1. It was not possible to judge accurately the degree o f crazing because some specimens contained many fine cracks whereas others contained few er but larger cracks. Examination o f sections o f crazed teeth, under a m icroscope, indicated that the depths o f the cracks were rather varied, ranging from 0.2 to 1 mm. Teeth heated to 212° F. in water after investing showed m ore crazing than those heated to 160° F. in water. Experiments were conducted to deter mine whether teeth susceptible to crazing w ould craze when heated under con d i tions such that no water cam e in contact with them during heating. Several p ro cedures were used for this, such as (1 ) Table 1
drying out free water from the gypsum by heating or reduced pressure, (2 ) sur rounding the teeth with tin foil and (3 ) coating the teeth with a thick layer o f w ax before investing. In all instances in which water in both liquid and vapor phase was prevented from com ing in contact with the teeth, no crazing was produced by the solvent tests. This leads to the conclusion that water plays an im portant role in the craz ing phenom enon. Dentures worn by patients exhibit this same crazing but usually to a m uch less degree. Figure 2 shows a practical im mediate denture w hich was rebased after eight months. T he crazing appeared in the plastic teeth after rebasing; all six o f the anterior teeth showed severe crazing and had to be replaced. M echanical Properties • D ata were o b tained on hardness (K n o o p ), indenta tion resistance, strength, m odulus o f elas ticity and elastic recovery o f the plastic teeth investigated.
• Physical p ro p ertie s o f plastic teeth Indentation resistance C/2 inch ball)
K n o op indentation hardness number (100 Gm . load) Brand
C ra z e * N o. of o b ser vations
1 2 3 4 5
6 7 8 9 10 11
Yes No No No No Yes Yes Yes + + Yes No
A v e ra g e
kg./mm.2 19.9 19.8 19.1 19.7 18.7 18.2 19.4 18.5 18.1 18.2 14.6
7 40 20 14 IS 9 30 6 19 19 6
S .D .f
0.3 1.2 0.6 2.9 0.9 0.4 0.7 0.6 0.6 0.7 0.5
Depth (30 kg. lo a d = 10 min.)
R e co v e ry (3 kg. lo a d — 10 min.)
mm. 0.099 0.090 0.098 0.090 0.103 0.106 0.098 0.104 0.098 0.148 0.099
% 87 88 89 91 88 82 88 87 91 70 82
Ratio o f lo a d to pro jected a re a o f inden tation
psi 9,800 10,800 9,900 10,800 9,400 9,200 9,900 9,300 9,900 6,400 9,800
Com pressive properties
Modulus, o f e la s ticity
105 psi 3.9 ± 0 .2 3.8 + 0.3 3.4 + 0.3 3 .8 + 0.1 4 .2 + 0.4 3 .6 + 0.1 3 .1 + 0 .3 3 .6 + 0.2 3.8 ± 0 .3 2 .8 + 0.5 4.1 +0.1
•Solvents used were methyl m ethacrylate monomer and ethyl alcohol, after normal curing cycle.
{Standard
deviation
2 (xi —x) 2 n— 1
JSom e specimens crazed, others did not.
Y ield strength
psi 8,600 8,100 6,700 8,600 8,400 7,100 8,100 7,600 8,600 6,400 8,400
836
THE JOURNAL OF THE AMERICAN DENTAL ASSOCIATION
Hardness Hardness was determined with a Kentron tester with a Knoop indenter at a temperature of 70° k 2' F., using a 100 Gm. load with a 10 second descent, and 20 second contact. The teeth were cut and polished to a smooth flat surface about 2 mm. from the tip of the cusps. The final polish was made with fine polishing paper (no. 000) followed by fine whiting. The teeth were measured in the "as received" condition, that is, not dried or stored in water before testing. The Knoop indentation numbers are given in Table 1. Indentation Resistance and Recovery Flat specimens 2 to 3 mm. thick were cut from plastic teeth. The surfaces were finished with no. 1 G sandpaper. The specimen was placed in a Rockwell superficial hardness tester and subjected to a 30 kg. major load with a T/r inch ball for ten minutes. At the end of this period the d e ~ t hof indentation was observed. the major load was removed, and the minor load of 3 kg. was applied for ten additional minutes. At this time the depth of indentation was again observed. he recovery value is expressed in percentage of the depth of indentation. The results
Fig. I
Examples o f crazing produced
shown in Table 1 are averages of six specimens. The results are compared in Figure 3 with observations made on direct filling resins and dental amalgams. Ratio of Load to Projected Area of Indentation Since in the procedure described previously the indenter sphere continues to penetrate the resin until the area of contact is sufficient to resist the force exerted on the specimen, the ratio of the load to the projected area of the indentation gives a basis for comparing strength of similar materials. The projected area of the indentation can be calculated from the depth reading on the Rockwell superficial hardness tester with a correction for the slight indentation of the minor load.2 The load divided by this area gives an indication of the strength in the same units as stress. The data obtained for this ratio for specimens cut from plastic teeth are shown in Table 1. Stress-Strain Relationship Cylindrical specimens 3 mm. in diameter by 6 mm. in length were machined from the 2. Sweeney W. T : Mechanical ;ropertiis
49:513 Nov. 1954.
Sheehan W. D of d i r e b fillin;
and Yost E. L. resins. J.A.D.A.
by immersion of plastic tooth in solvent
S W E E N E Y — YO ST— F E E . . . V O L U M E 56, J U N E 1958 • 837
plastic teeth. T he specimens were placed in a Tinius Olsen testing m achine and loaded in compression at an average strain rate of approxim ately 0.003 inches per inch per minute. T h e specimens were mounted between two steel bearing blocks Ys inch in diameter. T w o 1-inch Tuckerman strain gauges were m ounted on the bearing blocks to measure the axial d e form ation o f the specimen. T h e deform a tion o f the portions o f the bearing blocks within the gauge length was considered to be negligible. T h e strain was recorded at 2Yz pound load increments. T h e modulus o f elasticity was calcu lated from the straight portion o f the stress-strain curve below 7,000 psi (Fig. 4 -6 ). T h e compressive “ yield strength” for the purpose o f this paper is defined as the stress (load per unit o f cross-sectional area) at which the deform ation produced by a stress increment equals or exceeds 1.25 times that observed in the first few increments o f stress starting at approximately 1,000 psi.3 T h e values for the modulus o f elasticity and “ yield strength” shown in T able 1 are averages o f fou r to eight specimens. Abrasion or W ear Resistance • T h e data obtained on abrasion o f teeth are o f a very preliminary nature and are reported at the present time for inform ation on the ball mill type o f test. T h e m ethod used involves abrading the teeth in a ball mill
and observing the loss o f weight. T he mill was a one quart A bbe porcelain jar mill rotating at 69 revolutions per minute. T o com pare the wear o f different brands o f teeth the mill was charged with the abrasive material and a total o f 40 to 50 teeth o f various brands, w hich had been saturated in water, and was run for periods o f time ranging from tw o hours to several hundred hours. T he relative effects o f several different abrasive materials and o f different tooth shapes are shown in Tables 2 and 3, re spectively. T h e relative wear o f eight brands o f teeth when placed in the mill with steel balls with and w ithout addi tional abrasives is given in T a b le 4. M ost consistent results were obtained when 54 inch steel balls with no. 200 A loxite p o w der in water was used as the abrasive material according to the follow ing tech n ic: 1. O n e thousand three hundred and fifty grams o f Y\ inch carbon steel balls were washed with distilled water and allowed to drain.
3. Paffenbarger, G . C .; Sweeney, W . T., and Isaacs, A . W ro ug ht gold wire alloys: physical properties and a specification. J.A .D .A . 19:2061 Dec. 1932.
TIM E-M IN UTES
Fig. 3 • Indentation and re co ve ry of plastic teeth (a v e ra g e of ten b rand s), den ture base resin, d i rect filling resins and am algam in Rockwell super ficial hardness tester with 1/2 inch ball and ten minute load of 30 kg. and ten minute load of 3 kg.
838 • T H E JO U R N A L O F T H E A M E R IC A N D EN T A L A S S O C IA T IO N
relatively constant fo r any run made with the same abrasive and conditions. A lthough the data obtained from the ball mill tests show differences between the various brands o f teeth, no data have yet been obtained indicating whether or not the test results correlate with the wear that occurs in service. M u ch m ore re search will be needed before any state ment can be m ade on the relative clinical wear resistance of various brands o f teeth. D IS C U S S IO N O F R E S U L T S
Fig . 4 • Stress-strain curves fo r five brands of p lastic teeth
2. T eeth were selected, marked and individually weighed. T h e teeth used should be fairly large, from 0.3 to 0.5 G m . except in teeth with m echanical retention holes; fo r the latter, teeth weighing 0.25 to 0.45 G m . will be about the same size as the others. T o mark the teeth fo r identification, shallow holes were bored in recessed parts o f the teeth and these were filled with drawing inks o f various colors. 3. T h e balls, 20 Gm . o f no. 200 A loxite pow der and 100 Gm . o f water were placed in the one quart jar, w hich was centered on the mill and run fo r two hours. 4. T h e used A loxite pow der was cleaned out, the ja r and balls rinsed with distilled water and im mediately 20 Gm. o f fresh no. 200 A loxite pow der, 100 G m . o f distilled water and from 40 to 50 teeth were added. T h e test was run immediately after the cleaning charge and fo r a defi nite time. 5. A t the conclusion o f the test the teeth were rem oved from the mill and weighed individually. T h e percentage loss in weight was calculated. For com parison o f brands the average loss fo r a given brand was divided by the loss fo r one brand selected as a standard. This gives a num ber that seems to be
T h e craze resistance seems to be a fu n c tion o f the processing conditions. During curing o f the denture, the water absorbed by the plastic teeth that are susceptible to crazing evidently plays an im portant role, since it was shown that w hen water was prevented from contacting a tooth the tooth so treated was craze resistant. Also, teeth invested in dental stone and heated in boiling water exhibited m ore crazing by the solvent test than did those heated to 160° F. in water. This m ay have been caused by the greater mutual solubility o f resin and water at the higher tempera ture. Although, the mechanism o f crazing is not established, the theory has been p ro posed that the relative solubility o f water
Fig . 5 • Stress-strain curves fo r four brands of plastic teeth
S W E E N E Y — YOST— FE E . . . V O L U M E 56, J U N E 1958 • 839
and resin changes with temperature and that the dimensional change caused by absorption o f water during curing and separation during cooling is the basic cause o f the residual strain in the resin.1 So far as the present experimental data are concerned, the facts correlate satis factorily, but m ore observations o f a pre cise nature are necessary to prove the validity o f this theory. N o correlation be tween com plete craze resistance and the amount o f water sorption was fou n d in this investigation. U nfortunately there is no practical m ethod for fabricating dentures under water-free conditions, as an excess o f free water is present in the set dental stone or plaster used to invest dentures fo r p ro c essing. It is considered best, therefore, to use teeth that d o not exhibit crazing if this difficulty is to be overcom e. T h e indentation hardness values de termined by the K n o op m ethod show that the range in hardness is small. This is to be expected because the teeth are essen tially methyl methacrylate polymers with the exception o f one brand that contains considerable vinyl chloride-acetate and has a low er K n o o p indentation number. T h e ability o f the tooth to resist d e form ation from pressure applied to a local area such as biting on a hard par-
Fig. 6 • Stress-strain curves for two brands of plastic teeth
Table 2 • Effect o f vario us ab rasives on w eight loss o f teeth in ball mill test* W e ig h t loss per hour, % A brasive D ry
I
W et
O x fo rd crystal feldspar
0.002
-
N o . 18 glass b e a d s
0.002
—
Flint pebbles
0.01
0.04
—
0.02
—
0.1
—
0.2
Ste el balls /a inch Steel balls
Zi
inch
Flint pebbles roughened with carborundum N o . 200 A loxite, % steel balls
inch
Fine carborundum , !4 inch steel balls
0.2
0.7
0.3
—
*Time of test varied from a few hours for the more abrasive to several days for the less abrasive charges.
tid e o f food , bone and so forth is im portant. T o evaluate this property the rate o f indentation by a sphere under constant load was used. T h e rate o f in dentation is dependent on the m echanical properties o f the sample. T h e rate o f penetration o f the ball into the specimen is fast fo r the first few seconds and then gradually decreases until the ball comes to rest. F or practical procedures a 10 minute load application was fou n d to ap proach very closely a static condition. This provided a simple m ethod for o b serving the flow properties and resistance to indentation. T h e probable error of the depth o f indentation under the best co n dition is o f the order o f 2 per cent. T h e depth o f indentation fo r 10 of the 11 brands o f *teeth studied varied from 0.090 mm. to 0.106 m m . T h e other brand (no. 10) had an extremely high value, 0.148 m m ., and is not included in the average because o f the excessive amount o f porosity observed. T h e average inden tation o f 0.098 mm . fo r the ten brands is between those fo r denture base resin (0.096 m m .) and the direct filling resins (0.108 m m .), and is m uch greater than that obtained fo r dental am algam (0.043 m m .) under the same test conditions. T he
840 • T H E JO U R N A L O F T H E A M E R IC A N D EN T A L A S S O C IA T IO N
Table 3
Cuspid Brand
W e ig h t Gm.
Gm .
0.3549 0.3936 0.3296
W e ig h t Gm .
%
0.0083 0.0065 0.0074
0.3764 0.4033 0.3509
0.0014 0.0003 0.0013
2.34 1.65 2.24
0.2277 0.2794 0.2131
0.37 0.07 0.37
0.0082 0.0064 0.0058
W e ig h t loss
W e ig h t Gm .
%
Gm .
%
3.60 2.29 2.72
0.3734 0.4254 0.3296
0.0088 0.0071 0.0084
2.36 1.67 2.55
0.0013 0.0009 0.0016
0.48 0.19 0.44
Time: 20 h o i rs
0.0014
0.2268
0.0017
0.50
----
0.75
0.2697 0.4755 0.3670
ranged from 82 to 91 per cent fo r ten of the brands tested. T h e eleventh brand, which also had a high indentation value, recovered only 70 per cent. T h e m odulus o f elasticity in com pres sion com puted over stress ranges o f 1,000 to 7,000 psi varied from 280,000 to 420,000 psi fo r the different brands. This property is significant as it indicates the stiffness o f these materials. T h e yield stress o f plastic teeth is low . T h e “ yield strengths” as calculated from the stressstrain curves ranged from 6,400 to 8,600
• Relative w e a r o f v ario us brands o f teeth in ball mill test
W e t A lox ite p o w d e r and Va " steel ba ils* Brand
4
test
Time: 25 hou rs
0.2791
-------
average results for the three types o f materials are shown in Figure 3. O n e of the fundam ental properties of resins is the property o f absorbing energy by deform ation and returning closely to the original form . This is a fa ctor in den ture prosthesis, as the energy absorbed by oral structures probably plays an im por tant role in resorption o f tissue. T h e re covery o f the plastic teeth as measured by the decrease in depth o f the indenta tion o f a spherical ball when the load on the ball was reduced from 30 to 3 kg. Table 4
mill
C en tra l incisor
W e ig h t loss Gm.
A b ra sive: W et Va " balls 8 11 1
ball
Lateral incisor
W e ig h t loss
A b ra sive: W e t Z7 " balls 8 11 1
in
• Effect o f tooth shape on w eigh t loss o f teeth
D ry A lox ite Va '' steel balls
D ry fine ca rb o ru n dum Va " steel balls
W et w steel balls
/a " steel balls
W et
8 hr.
4 hr.
8 hr.
4 hr.
4 hr.
25 hr.
2 hr.
1.92
*1.69
1.74
1.95
1.93
—
1.17
2
—
1.21
1.17
1.28
1.17
—
.85
6
1.05
1.12
1.04
1.49
1.08
—
.96
5
1.07
1.06
1.14
1.28
1.13
—
.83
8
1.02
1.04
1.04
1.15
1.04
1.10
.85
1
1.00
1.00
1.00
1.00
1.00
1.00
1.00
9
.88
.92
.96
1.16
.99
—
.83
11
.53
.46
.54
.36
.53
.75
.51
10
—
—
—
—
—
.88
•Values for three runs are given to show precision of results. Brand I is used as reference value.
S W E E N E Y — YOST— FE E . . . V O L U M E 56, J U N E 1958 • 841
psi. These values are considerably lower than the masticating stress o f 17,000 psi on cuspids as calculated by Skinner.4 T he fact that plastic teeth used in clinical dentures d o not exhibit m uch deform a tion is explained partially by the high recovery value (Fig. 3 ). Abrasion or wear resistance o f plastic teeth is one o f their weakest properties. A t present the work on abrasion meas urement is only in the developm ent stage. It is considered essential that new and m ore practical test methods be d e veloped to evaluate this factor. SUM M ARY
T h e m echanical properties o f eleven brands o f plastic teeth were investigated and the follow ing observations were m ade:
1. T h e m echanical properties were low in com parison with those o f other materials used fo r restorative purposes in the oral cavity. 2. Som e brands o f teeth were found to be susceptible to crazing after a p ro c essing cycle similar to the curing o f a denture. Five brands crazed consistently, five brands did not craze and one brand crazed in some instances. 3. Crazing was not observed on any teeth if water was prevented from co n tacting them during heating. 4. T h e resistance to crazing was found to be the most outstanding difference be tween the com m ercial brands studied. 5. N o definite correlation was found between the physical properties studied and crazing. 4. Skinner, E. W . Science of dental materials, ed. 3. Philadelphia, W . B. Saunders Co., 1946, p. 17.
Preventive Health Service • T h e emphasis in preventive health service has shifted from the lowering of hazard to the raising of im m unity; and in doing so it has shifted from things which can be done for people to things which people must do for themselves. This change is sig nificant; it reversed a long accepted trend. In equating himself with his environment, it has been the pride of progressive man to make his environment fit him. T o accept the environment and learn to be equal to it is by no means orthodox but it is refreshingly sane. But there still lies far in the background the concept of health as a dynamic condition, of which immunity to disease is only one reflection. Our knowledge of constitutional types is still slight, still slighter is our knowledge of what determines the strange variations in apparent vitality between man and man or how far this elusive quality can be developed. Occasional innovations in teaching and training assert and demonstrate that we might make more of ourselves than in fact we d o; but, by and large, only the mountaineer and the athlete try to get fit, rather than merely to keep well. People seem to think that the role of science is to remove hazards, rather than to fit us to face them. This is a dangerous fallacy. Neither physical science nor medicine or dentistry can obtain that— except at a price which mankind will be unwilling to pay. G eoffrey Vickers.