NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

3 11.05-1 1 -31 1056 1 January 20, 197 1 10 522

P rogress Report

on DIMENSIONAL CHANGES IN AMALGAM

George C. Paffenbarger, D.D.S.*

and

Nelson W. Rupp, A.B., D.D.S., M.S.**

* Senior Research Associate Emeritus, American Dental

Association Research Unit, National Bureau of Standards,

Washington, D. C. 20234.

** Research Associate for the American Dental Association Research Unit, National Bureau of Standards,

Washington, D. C. 20234.

This investigation was supported in part by research

grant D E —027 42 to the American Dental Association from

the N ational Institute of Dental Research and is part

of the dental research program conducted by the

National Bureau of Standards, in cooperation with the Council on Dental Research of the American Dental

Association; the United States Medical Research and

Development Command; the Dental Sciences Division of

the School of Aerospace Medicine, USAF; the National

Institute of Dental Research and the Veterans Administration.

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U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
IIIIIIIII
Dimensional Changes in Amalgam

Dimensional change during hardening

Importance . The change in dimension during hardening

has long been considered a critical factor in evaluating

dental amalgam as a filling material. G. Vi Black rightly

concluded that an amalgam with zero change would ideally

be best, and that shrinkage and expansion should be kept

( 1 ) to the lowest possible minimum. This excellent advice

is as good now as it was when he gave it seven or more

decades ago. It has been the general impression among

dentists and researchers that a slightly expanding alloy

was much to be preferred over one with a slight shrinkage

on hardening. In fact, most believe that no terminal

shrinkage should be permitted. These concepts were

introduced in the first specification for amalgam as

( 2

)

proposed by Souder who believed that it was essential

to have a linear expansion of 0.04 to 0.05% to compress

the dentin (of course below its elastic limit) and to

thus ensure sealing the cavity-restoration interface

(especially the margin) . No one knows the force or thrust
- 2 -

exerted by amalgam as it expands during hardening.

( 2 ) Then, too, it was postulated that this slight expansion

would compensate, at leadt in part, for the effect of the

differential thermal expansion of the tooth (average

( 3 ) value 8 x 10“ to /°C) and of the amalgam (average value

13 ) of 25 x 10“ 6 /°C) ^ ' so the first specification for alloy

for dental amalgam (November 30, 1926) had the following

requirement: "4. Setting changes .-Twenty- four hours

after amalgamation the length shall have increased

between one and ten microns per centimeter . If at any

time during the test a contraction of more than four

microns per centimeter is detected, the alloy shall

(4) be rejected." identical wording was used in the

(5) first ADA specification. Subsequently this expansion

value in the ADA specification was changed from +0.01 to

( 6 ) +0.10% to +0.03 to +0.13% range so that "Certain alloys

which are now on the border line in setting changes would

be rejected as a result of harsh treatment in mixing and

packing." Also, the former requirement which permitted

I
3-

amalgams no more than 0.04% initial contraction was

deleted as unnecessary with the new dimensional change

( 6 ) range of +0.03 to +0.13%. All of the foregoing

values were on unrestricted specimens at room temperature with a fiducial reading when the test specimen was

15 minutes old as timed from beginning the mix. It is

known that a restricted specimen has less expansion than

(7)

an unrestricted one. As an example, an unrestricted

cylindrical specimen had an expansion of 0.12%, the same

size specimen in a tube with both ends open +0.07%, and

(7) if one end of the tube was closed, about +0.03%. With

the complex shape of cavities in practice, with varying

degrees of roughness of cavity surfaces, and with the

usual undercutting, one could obtain an infinite number

of values for dimensional change during hardening because

of variables that cannot be controlled clinically. Therefore, in specification testing it is desirable to use an

unrestricted specimen. Even then values to within ±15%

are considered good.
-4-

It is true that workers have shown that amalgam

that shrank from 0.02 to 0.04% when measured on an

unrestricted specimen did not present any clinical

evidence of failure during the three years the 34

( 8 ) restorations were observed. Additional tests on 60

clinical restorations made with a high, tin-content, alloy

which shrank about 0.31 ± 0.03% on an unrestricted specimen presented no clinical evidence of open margins or

recurrent caries during three years (34 restorations)

(9)

or for shorter periods (20 restorations). Other

workers have shown by observation a marginal defect of

50 micrometers or more induces recurrent caries.

Such a discrepancy is caused by a marginal excess of

amalgam which breaks away or by marginal fracture caused

(10) by corrosion of the amalgam, or both. Certainly

such large dimensional changes that would give a margin

defect of 50 micrometers are not occurring in amalgam

during hardening.

In 1957 the Federation Dentaire Internationale (FDI)

adopted an international specification for alloy for dental
-5-

amalgam in which the linear setting change during

hardening was restricted to 0 to +0.20% at 37 °C on an

unrestricted specimen with a fiducial reading when the

( 11 ) specimen is 15 minutes old. This was supposed to

allow for the effects for mechanical amalgamation,

testing at mouth temperature, and restriction of expansion in clinical restorations. The latest change,

(12) effective June 1, 1970 is 0 ± .20% on an unrestricted specimen with a fiducial reading when the specimen

is 5 minutes old and the amalgam is mixed and packed

mechanically

.

From the foregoing it can be seen that current

laboratory practice in specification requirements is

mostly theory derived on testing procedures that are

perhaps too conventional to attempt to relate them to

dimensional changes during hardening on clinical

restorations. In fact, the actual clinical effects

of the dimensional changes during hardening of the amalgam

are unknown.
- 6 -

What one really would like to know are the actual

dimensional changes during hardening in dental restorations , and the effect of such changes on the restorationtooth interface. It is the authors' opinion that clinical

findings will not reveal this because of the impossibility

of setting up quantitative procedures in the mouth to

measure dimensional change on restorations or the

quantitative adaption of the set amalgam to the surfaces

of the cavity. Some preliminary laboratory testing is

being started at the National Bureau of Standards by the

authors to quantitate adaption of amalgam to the cavity

and to relate dimensional changes occurring on unrestricted laboratory specimens to such quantitative adaption

measurements

.

Effect of Composition . Black showed the critical

(1) effect of composition. Gayler showed that the tin

content of the powdered alloy was critical and set the

(13) limits of not less than 25 nor more than 26.8% tin.

Less than 25% caused expansion, more than 26.8% caused

shrinkage in the binary silver-tin system. Her measurements were made at 37 °C on restricted specimens in
-7-

steel or composition "tooth" cavities. In 1965 an

analysis of 83 alloys on the List of Certified Dental

Materials as established by the American Dental Association showed tin contents ranging from only 25.3 to 27.0%.^

However, several years ago in the routine testing of alloys

for amalgam certified by their respective manufacturers

to the American Dental Association, one of the authors

tested an alloy - Ag 62.2, Sn 32.7, Cu 4.9 and ZnO.2%

that complied with the flow and dimensional change requirements of the ADA specification which, because of the large

tin content of about 33%, is at variance with the fore-

(12) (14) going

.

A number of years ago an alloy was made in the USA

with atomic proportions Ag g SnCu (weight percentage

approximately Ag 54, Sn 30, Cu 16, Zn none) . This alloy

(14) had a linear setting expansion of from 0.02 to 0.03%.

Here again the tin content was more than Gayler's maximum

limit of 27.5%.

Heat treatment of the ingot changes the distribution

of the silver-tin phases in the alloy and this may cause
- 8-

differences in expansion during hardening but the authors

are not familiar with such data in the literature. Annealing of freshly cut particles reduces the amount of dimen-

( 1 ) sional changes during hardening of amalgams. One must

also realize that surface treatment of the alloy particles

may alter the composition of the surface which is not

reflected in the overall composition.

Effect of specimen preparation technic . Strader showed that

the composition of a test specimen of dental amalgam varied

in mercury content from one end to the other with an accompanying variation of dimensional change in different parts

(15)

of the specimen. He quoted values of +0.19% for the

bottom one third of the cylindrical mold to +0.06% for the

top one third. Thus, the value one usually obtains is an

average of the linear changes occurring in different

sections of the specimen.

This presents another excellent reason for using the

lowest amount of mercury that will give a packable mix

with the removal of little or no excess mercury during

condensation

.

Of course, almost everyone who has tested dental

amalgam is familiar with the wide variation of values for
-9-

dimensional changes during hardening that occurs with

variations in the work done on the alloy-mercury mass

during trituration and condensing and many other variables

associated with the preparation of restorations. Gray was

(16)

among the first to show this. For some reason G. V.

Black believed that "Shrinkage and expansion of the amalgam

used is not under the control of manipulation by the

operator. This can be controlled only in the selection of

( 1 ) alloy to be used."

Influence of the direction of condensation . Setting expansion is slightly greater (0.03 to 0.05%) parallel to the

direction of pressure during condensation than at right

angles to the direction of pressure when condensation by a

piston is used. Evidently the orientation of the Ag 2 Hg3

(Y x ) and Sn7 Hg or Sn 8 Hg (y g ) phases are affected. No effect

(17)

was noted when using hand condensation.

Effect of length of specimen . As cylindrical specimen

length (4x6, 4x4, 4x2 mm) decreased, the linear

(18) change increased

.

(16 19) Effect of particle size. Gray ' was possibly the

first to show that excessive expansion occurred with
- 10-

coarse sized particles of alloy and with real fine

particles there was shrinkage and that with the proper

blend of sizes or surface area the "proper" expansion

was obtained. Many other investigators have since

confirmed this

.

Effect of temperature at which test is conducted . Gray

showed less expansion at 25 °C than at 37.5°C while

Mitchell et al showed more.

Amount of dimensional change desired . As previously

stated, zero dimensional change or as close as one can

get to it is the most desirable--at least theoretically

There is a point at which expansion or shrinkage would

be undesirable and that point would probably be where

the magnitude of the dimensional change interferes with

the adaption of the amalgam to the cavity surfaces.

As previously stated, the authors are beginning

experiments with actual restorations in porcelain teeth

in an attempt to determine the effects of various dimensional changes on hardening on the amount of space

(16)

between the amalgam and cavity wall. Dimensional changes
- 11 -

will be determined on unrestricted specimens then the

various amalgams with linear dimensional changes from

+0.60 to -0.60% will be placed in the Class I, Class V

and MOD cavity preparations in porcelain teeth. The

quantitative adaption of the amalgam restorations to

the cavity walls will be measured by air under pressure.

As regards such effects consider the following

outline drawing of the occlusal view of an MOD restoration in a lower molar.

The approximal and gingival surfaces indicated by

arrow A are most frequently involved in marginal leakage

(10,21)

and recurrent caries. Would a contracting

restricted restoration tend to tighten the apposition

in A areas? Would an expanding amalgam tend to lessen

the apposition? What happens at B with a contracting
- 12-

or an expanding amalgam? To the best of the authors

knowledge it has never been determined whether these

vulnerable areas are influenced by dimensional change

values on an unrestricted expanding or shrinking amalgam.

Explanation of the dilation curve of amalgam during

hardening . There has been much speculation on the

physical and chemical changes occurring during the

hardening of amalgam and the shape of the resultant

dilation curve. Winterhager, using quantitative x-ray

diffraction methods on silver amalgams, with a varying

silver content, showed a correlation between the amount

of, and the time of, forming of and y 1 phases and the

( 22 ) shape of the dilation curve.

Dimensional changes caused by corrosion

Moisture and contamination. Schoonover and associates ( 23 )

showed that the mechanism of the large delayed expansion

of moisture-contaminated amalgam made from alloys containing zinc was caused by the electrolytic dissociation of

water by the zinc-containing amalgam into hydrogen and

oxygen. As the hydrogen accumulates it builds up pressure
- 13 -

which caused the amalgam to flow out of the cavity or

to elongate the unrestricted test specimen by several

percent. Excessive corrosion as well as excessive

expansion occurs . If mercury is released by the corrosion it will probably combine with the residual Ag3 Sn

and matrix to form additional matrix with subsequent

dimensional changes. In any event the delayed expansion

of moisture-contaminated, zinc-containing amalgam is of

sufficient magnitude to be manifested clinically by post-

( 24 , 25 ) operative pain.

How much delayed expansion occurs in amalgam restorations and the percentage of amalgam restorations

that are so affected are not known. It is known that

most of the alloys sold contain zinc. It is known that

when one transfers a mouth mirror from room temperature

and humidity conditions to mouth temperature and humidity

that water condenses on the mirror until it rises above

the dewpoint. In all probability this condensation of

water occurs on amalgam increments as they are passed

from the room to mouth environment unless the rubber dam

is used. As very little rubber dam is used it may be

that some of the extrusion of amalgam restorations from
-14-

the cavity is caused by such moisture contamination of

zinc-containing amalgam. The authors are investigating

this aspect under oral conditions.

Gamma 2 corrosion . The tin-mercury phase of amalgams

(

y

2 ) is chiefly responsible for the corrosion of dental

(26,27) amalgam. The free mercury released by the degradation of Y @ recombines with the residual alloy Ag3 Sn

(20,26) particles (Y phase) to cause additional expansion.

The amount of this expansion is great and depends upon the

amount of free mercury released, that is, the extent of

the corrosion of the Y g phase. Jorgensen's data show

about 5% linear expansion per one percentage weight of

released mercury. Obviously the way to rectify this

situation is to eliminate the Y 8 phase without impairing

the desirable properties of amalgam. It would appear that

Johnson of the University of Virginia has accomplished

this. ( 28 >

Thermal effects

Differential thermal expansion of hard tooth tissue and

amalgam . Recently the coefficient of thermal expansion
-15-

of human enamel was found to be 12 x 10” S /°C (25-50 °C)

(29)

and for dentin 7.5 x 10” 6 /°C (25-45°C) and as

(3 4) previously stated for amalgam 25 x 10 6 /°C. ' These

differences in coefficients do cause different amounts

of expansion and contraction when the amalgam restoration

and the tooth is subjected to temperature fluctuations

that occur in the mouth. This range is fairly wide for

the anterior teeth, -9°C to 52°C^°^ when measured on

a relatively poor thermal conducting plastic filling

material. In amalgam restorations temperatures as high

(31)

as 80 °C may be reached. So in the mouth the amalgam

should expand and contract about twice as much as the

tooth during changes in temperature. The amount is

undoubtedly much greater as amalgam is a much better

thermal conductor than the tooth so the amalgam will

attain a much higher temperature and do it much more

rapidly than will the tooth. Consequently there should

be a very slight extrusion of the amalgam restoration

from the cavity as amalgam is plastically deformed at

temperatures occurring in the mouth when hot foodstuffs
-16-

are eaten. Limited laboratory tests confirmed this

when amalgam fillings in glass molds were cycled between

room temperature and 65 °C in water. Some extrusion of

(32)

the fillings occurred. After a few cycles water was

observed between the glass walls and the fillings. On

cooling small water droplets would collect at the margin,

y-p transition in Ag-Hg system. Under mouth temperature

conditions some of the Ag 2 Hg3 (Y

x ) phase of dental amalgam

changes over to the (3 silver-mercury phase with an accompanying volume shrinkage of 5% or an increase in porosity and

(33) with the liberation of free mercury. This free mercury

would combine with the unreacted alloy particles and the

( 10 , 20 ) matrix which should result in some expansion.

Suggestions

The dimensional changes in amalgam during hardening

are of a minor order compared to those occurring as a

result of the moisture-contamination of zinc- containing

amalgam and the expansion caused by reaction between the

mercury, released by corrosion of the y g phase, and the

other phases

.
-17-

Research, however, is needed to establish limits

of the amount of expansion and contraction that can be

tolerated without serious disarrangement of the amalgamcavity surface interface.

Research is also needed to determine to what extent

current zinc-containing amalgams are being contaminated

by moisture during normal technical procedures in filling

teeth and how much delayed expansion results . Such data

are necessary in deciding if zinc or how much zinc should

be permitted in alloys used for dental amalgam.

The development of amalgams without the Y a phase

the tin-mercury phase--is the only method that would

eliminate the expansion caused by the reaction of the

mercury, released on the corrosion of the tin-mercury

phase, and the set amalgam.

The thermal expansion of amalgam cannot be meaningfully

decreased so all patients with amalgam restorations should

be advised to refrain from "bathing" the teeth in extremely

hot and cold foodstuffs. This would also minimize the

Y to (3 (Ag-Hg) transition in dental amalgam.
-18-

References

1. Black, G. V . Operative dentistry Vol . II. Technical

procedure in filling teeth. Medico-Dental Publishing

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2. Souder, Wilmer. Possibilities of a specification for

dental amalgams. Bui. Chicago D. Soc. 5:4 July 24,

1925.

3. Souder, Wilmer. Physical properties of dental materials.

JADA 12:509 May 1925.

4. Souder, Wilmer. Measurement and application of certain

physical properties of dental amalgam. J. D. Res.

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5. Taylor, N. 0. Specification for Dental Amalgam Alloys,

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.
-19-

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- 20 -

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- 21-

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.

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- 22-

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