NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT NBS REPORT

31 1.05-1 1-3 1 10260 January 31, 1968 9842

31 1.05-1 1-3 1 10560

31 1.05- 1 1-31 1056 1

REPORT ON DENTAL RESEARCH

AT THE

NATIONAL BUREAU OF STANDARDS

Progress Report

July 1 to December 31, 1967

The dental research program at the National Bureau of Standards is carried on in cooperation with the Council

on Dental Research of the American Dental Association;

the National Institute for Dental Research; the Army

Dental Corps; the Dental Sciences Division of the School

of Aerospace Medicine, USAF; and the Veterans

Administration.

IMPORTANT NOTICE NATIONAL BUREAU OF S'

for use within the Government, and review. For this reason, tl whole or in part, is not authc Bureau of Standards, Washing

the Report has been specificall;

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

ess accounting documents intended

> subjected to additional evaluation

e listing of this Report, either in

le Office of the Director, National

by the Government agency for which

copies for its own use. <NBS>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

TABLE OF CONTENTS

Page

1

.

Introduction 1 2. Reports Issued 1 Papers Published 1

4. Work in Progress 2

4.1. Crystallography of Calcium Carbonates

and Calcium Phosphates 2

4.2. Solubilities of Calcium Phosphates 4

4.3. Adsorption Studies 10

4.4. Reactivity of the Components of Tooth

Structure 11

4.5. Characterization of Components of Tooth

Structure and Dental Materials by

Differential Thermal Analysis 12

4.6. Mechanical Behavior of Skin 13

4.7. Mechanical Properties of Materials 14

4.3. Stress Analysis 15

4.9. Metallurgy 16

4.10. Development of an Adhesive Dental Restorative Material 17

4.11. Zinc Oxide -Eugenol Materials 10

4.12. Rapidly Fabricated Surgical Splint 20

Appendix

NBS Report 0631 Preparation and Solubility of Hydroxyapatite.

NB5 Report 0650 Isothermal Diffusion in the Dilute Range

of the System Ca(0H) p-H-,P0h-Hp0.

I - Theory. p

NBS Report ^654 Calculations of Diffusion Coefficients in

Ternary Systems from Diaphragm Cell

Experiments

.

NBS Report 9304 Aluminum Oxide as a Reinforcing Agent for

Zinc Oxide-Eugenol-EBA Cements.

NBS Report Q820 The Reactivity of the Components of Tooth

Structure

.

REPORT ON DENTAL RESEARCH

at the

NATIONAL BUREAU OF STANDARDS 1.

INTRODUCTION

This report covers dental research conducted at the

National Bureau of Standards during the period July 1,

IO67 through December 31, 1067 under the cooperative program sponsored by the American Dental Association; the

National Institute of Dental Research; the U. S. Army

Dental Corps; the Dental Sciences Division of the School

of Aerospace Medicine, USAF; and the Veterans Administration. In the summaries below of work In progress, sponsors

of various projects are identified. However, since most

of the projects are closely related both technically and

administratively, all results are reported for the Information of all sponsors. Detailed reports issued during

the period on several phases of the work are listed below

and are appended to this report.

2.

REPORTS ISSUED

NBS Report 0631

NBS Report 06RO

NBS Report 065A

NBS Report o3o4

NBS Report 0320

Preparation and Solubility of Hydroxyapatite . Isothermal Diffusion In the Dilute Range

of the System Caf 0H)o-H^P0h-Hp0.

I - Theory.

Calculations of Diffusion Coefficients in

Ternary Systems from Diaphragm Cell

Experiments

.

Aluminum Oxide as a Reinforcing Agent for

Zinc Oxide- Eugenol -EBA Cements.

The Reactivity of the Components of Tooth

Structure

.

3.

PAPERS PUBLISHED

Pertinent Data on Some Fhysical Properties of Different Investments Used in the Casting of Gold Alloys. M. Ohashi,

J. W. Stanford, and G. C. Paff enbarger . J. Nihon University School of Dentistry, 0:l2l, Sept. 1967.

1
Composition and Structure of Bone Mineral. W. E. Brown.

Proceedings, First Conference on Biology of Hard Tissue,

Princeton, 1965. N. Y. Acad. Sci. 1067, p. 46.

Ionization Constants of Substituted Benzoic Acids in

Ethanol-Water. G. M. Brauer, George Durany, and Harold

Argentar. J. Res. MBS (Phys. and Chem. ) 71 A: 379, Sept.-

Oct. 1967.

Analysis of Gold and Platinum Group Alloys by X-ray Emission

with Corrections for Interelement Effects. J D Eick, H. J.

Caul, D. L. Smith, and S. D, Rasberry. Applied Spectroscopy,

21:324, Sept. -Oct. 196?.

Diminishing Discoloration in Methacrylate Accelerator Systems.

R. L. Bowen and H. Argentar. JADA 75: Ql3, Oct. 1967.

Elastic Constants of Dental Amalgam. George Dickson and P. L.

Oglesby. J. Dent. Res., 46:1475, Nov. -Dec. 1967.

4. WORK IN PROGRESS

4.1. Crystallography of Calcium

Carbonates and Calcium Phosphates

Sponsor: NIDR, .ADA

Crystal Structures : Work on the structures of various

calcium carbonates and phosphates and related compounds is

underway to provide background for understanding chemical

reactions such as dissolution and crystallization and the incorporation of impurities in hard tissue. The objectives are

described in greater detail In the preceding report.

CaC0^'6H20: The structural analysis on this compound

was suc^s'sfuT“and is complete except for cleanup of errors

in the observed data, location of hydrogen atoms, and interpretation of the structure. An outstanding feature in the

structure was the presence of Ca-CO-^ Ion pairs surrounded

by envelopes of water molecules. This is probably the first

reported instance of occurrence of this ion pair in a crystalline structure, and it gives evidence of the structure of

the ion pair in aqueous solutions.

NagCafCO-^g* 5H20 f gaylussite ) : Our interest in this

calcium carbonalleH^ lates T6‘"THeTMpresence of water molecules

fin analogy with CaC0^*6Hp0) and because of the sodium ions

2
which are components, also, of serum.

The structure of this compound has also yielded to

analysis. It has been refined to completion and the hydrogen atoms have been located. The structure determining

forces seem to be the formation of CO^-Ca-CO^ ion triplets

as the primary force with subsequent coordination of one

oxygen of 4 different CO^ groups to the Na ions as the

secondary force. The rest of the cation environments is

made up of water molecules.

2Na2S0^ *Na2C0^ (burkeite): It appears that carbonate

substitutes for suifate'Tri t~b<? lattice of this crystal,

making it an interesting analog to the proposed substitution

of carbonate for phosphate in apatites. Single crystals of

burkeite were grown from aqueous solution. Most of these

did not extinguish under crossed nichols, but one which did

was examined by x-ray dlffration. The superlattice reflections, reported by others, were smeared, indicating that the

proposed substitution does occur, but that there was considerable disorder in the sulfate and carbonate sites. This

disorder would make difficult precise location of the carbonate groups, and for this reason, new attempts will be made

to grow ordered crystals.

Ca^OfPO^^ (tetracalcium phosphate): An earlier optical

and single -crystal' x-ray“study on This salt revealed that

it had a probable structural relationship to hydroxyapatite.

In the same way that the relationship between octacalcium

phosphate and hydroxyapatite explains much of the anamolous

chemistry of apatites with Ca/P ratios less than 5 O , tetracalcium phosphate may have bearing on those with ratios

greater than 5

/3. The crystals were found to be monoclinic

instead of orthorhombic, as had been reported previously.

X-ray intensity data for use in a structure determination

has been collected. Work on the structure is being

continued.

3
H^PO^'l 72 I^O: The structure of this compound was

determTh¥^~T1T“cooperation with Tennessee Valley Authority.

A major interest in this structure relates to the positions

of the hydrogen atoms. Most of these were located, and

evidence was obtained that two of the hydrogens may be delocalized. A manuscript describing this work was completed

during the project period.

Apatite Analogs : The unit cells and spacegroups of the

following apatite analogs have been determined:

a c Na2Lag fS104 ) 6 F

2 9.72 7.16

Na2Pr

3 (Si02,) 6 F

2 9.53 7.05

Na2Nd8 (Si04 ) 6 F2 9.51 7.02

The spacegroup determined from the X-ray data is either

Pb-j or Pb^/m. The criterion of 1 formula weight per unit

cell suggests that the true spacegroup is Pb^, where 2Na and

2fa in the 2b position form a column which alternates Na and

La to replace the column of Ca*s formed in hydroxyapatite by

5+ the 4f position in spacegroup Pb-/m, The fact that the La . 3 +

2

La electrostatic interaction is 3 times as destabilizing

3+ +

as the La . . . .Na interaction suggests that the Na/La

columns are ordered.

CaRe2

* A fairly regularly shaped crystal for which the

necessary"^ -ray absorption corrections can be made has been

selected and aligned in a manner suitable for data collection

The unit cell is approximately a - 9.24, c = 4.81A. We are

awaiting the completion of the X-ray absorption program for

the Unlvac 1108 computer before starting data collection.

The objective of this study is to establish the degree of

ordering in this compound.

4.2. Solubilities of Calcium Phosphates

Sponsor: ADA, NIDR

Solubility of CaHPO^* 2h

20 W.o.rk at 5°C on this compound

4
was completed. The solubility Isotherms In the ternary

system, Ca (OH^-H^POjj-f^O are now known at four temperatures, 5°, 15°, 25®, and 37.5°C. This salt was selected

for Intensive study Initially because previous work indicated that it was the most amenable to accurate solubility

measurements. In this way It was possible to study with

greater accuracy some of the interlonic effects that occur

in the solution phase. As a result it was possible, for

the first time, to establish fairly accurate stability constants for the ion pairs [CaHPO^l and fCa.^PO^]’4’ at the four

temperatures listed above along with the solubility product

constants. By collecting data at four temperatures, it was

possible to evaluate enthalpies of dissolution and ion-pair

formation. These constants are listed in Table 1:

Table 1

Thermodynamic functions for CaHPO^ • and formation

constants for the ion pairs CaHPO^® and CafHgPO^)^

Temp.

®C

K

sp

xl °

7 AF° a

Kcal /mole

AH® a

Kcal /mole

AS ° a

e.u.

(CaH0P|, °

) Ca(H2PCV) +

4

5.00 2.33 8 . 4

3 1.3 -26 393 9.0

15.00 2.53 3 . 7

0 0.5 -23 200 4.7

25.00 2.54 q .°0 -0.3 -31 244 3.2

37.50 2. 4l Q 4• 1

• =t 1 r-l -35 306 3.0

a Standard values for dissolution of CaHPO^^^O

The techniques employed in the study were described in a paper, "Solubility of CaHOP^'^H^O and Formation of Ion Fairs

in the System CafOH^-H^PO^-^O at 37.5°C", and a paper on

the work at 5, 15 and 25 °C is under preparation. As an Important part of this work, we have developed a least-squares

method for computer evaluation of the constants from experimental data. This method takes into account the experimental

observables (Ca, P, and pH) with appropriate weighing of

errors, and it calculates the constants or parameters subject

5
to minimization of three condition functions:

1. Electroneutrality

2. Variation in the solubility product

3. Variation from the theoretical value in the

stoichiometric ration, Ca./P, of dissolution

By treating the data rigorously in this way, and by designing the experiments in the way described elsewhere in this

report, we feel that one achieves a maximum in the reliability of the derived results.

Solubility of Ga^OH(PO^)^.: The data on solubility of

hydroxyap'at i re ^ Tn the ternary system was reprocessed by the computer techniques described above. Its

solubility product constant was found to vary depending on

the history of the sample; this helps to explain some of the

disagreement on this subject in literature reports. We believe that the solubility constant for the steam-heated

cQ

sample, = 3.3 x 10“^ , represents that of the most stable

form of hydroxyapatite, and constitutes a thermodynamic frame

of reference for this material.

A manuscript, "Preparation and Solubility of Hydroxyapatite", summarizing the work on hydroxyapatite is appended.

Solubility of CaHPO^t The determination of the solubility isotherms for uahiFO^ in the ternary system at the

temperatures 5, 15 * 25 and 37®C was completed. The solubility products and ion-pair formation constants are listed

in Table 2;

Table 2

Thermodynamic constants for CaHPO^ and formation

constants for the ion pairs CaHP0^° and Caf^PC^) 4

’

Temp,

®c

K_ xlOY

sp

AF®

Kcal /mole

AH®

Kcal /mole

AS®

e.u.

(CaHPO^ °

)

C a ( HpP Oj,

)

+

y

5.00 2.03 3 - 3

6 -4.1 -17 222 5.3

15.00 1.63 3.9, -4.1 -17 142 10.2

25.00 1.29 9 . 40 -4.1 -18 291 9.9

37.00 0.97 9.% -4.1 -19 212 4.1

6
The ion-pair constants are in reasonably good agreement

with those obtained with CaHP0^*2H20 given in Table 1.

Solubility of B-Ca-^ (PO^ ) 2 (whit! ocki te ) : In our solubility st udTes , we"'“i^uriTieTy “caTculate the Ton- activity

products of all the calcium phosphates of each solution for

which we have analytical data. These calculations reveal

that in many instances, whitlockite may be more insoluble

than has been recognized. Since It is a component of calculus, it has obvious importance. As a result, we have

estimated its solubility-product constant using cryogenic

and thermal data, and we have Initiated an experimental program to measure its solubility. Our preliminary results show

, , p that the solubility product for this compound, (Ca ) (POT,

-

) , -2Q 4

is approximately 10

Stoichiometry of Whitl oc kite : The suggestion has been

made that whitlockite has the formula Ca^ qH(P 0^ rather

than the one usually given, Ca-2j(P0^) 2

, The suggestion raises

serious uncertainties concerning the thermodynamic properties and the nature of the solubility product for this material. Experiments are being carried out to test the Ca T

ratio of whitlockite by measuring the amount of pyrophosphate

that is produced when acid calcium phosphates are heated.

The Initial results to date Indicated that the composition

of whitlockite Is variable, at least in the temperature range

of the experiments, 700 to Q00°C, but more recent data do

not confirm the initial results. Work on the stoichiometry

of whitlockite is continuing.

Solubility of Ca^H2 (P0^)^ - 5H2

0 : In our studies we have

placelTTmporfarice on this saltbecause of its relationship

to hydroxyapatite and because it may participate in the formation of tooth and bone mineral. Our knowledge of the solubility of this salt is based on one point In the ternary

system at 25°C. In view of Its importance, this is an inadequate basis to predict Its chemical behavior. We have

7
initiated a program to measure its solubility over a range

of solution compositions and at other temperatures. Materials for the study are being prepared; the first series of

solubility measurements proved to have an apparent unbalance

in the electroneutrality of the solutions, and are to be

repeated.

Diffusion in the ternary system CaCOH^-H^PO^-HgO: It

is not generally realized by workers in this field that diffusion in a ternary system is very complex, particularly when

weak electrolytes such as H-^PO^ are present. As a result,

it is common practice to oversimplify treatment of experimental data by the use of Pick's law. Our work shows that

this may lead to gross errors in the Interpretation of kinetic

measurements

The cells previously used in our experimental studies

introduced an extra complication in that surface diffusion

occurred in the sintered glass diaphragm. A new cell using

a millipore membrane was constructed which would obviate

this particular problem. Experimental data are being collected using this cell.

Another result of this work has been the development of

a novel method for the calculation of diffusion coefficients

from experimental data from diaphragm cells. In the past,

it has been necessary to carry out two experiments when threecomponent systems were being studied. The present method,

which is an application of a generalized least-squares procedure, allows this to be done from a single experiment and

without any restrictions on the initial conditions.

Two reports covering this work, "Calculation of Diffusion Coefficients in Ternary Systems from Diaphragm Cell Experiments" and "isothermal Diffusion in the System

Ca(0H)2~ I. Theory", are appended.

3
Theoretical Consideration of the Solubility of Apatites:

Solubility studies on hydroxyapatite have yielded discordant

results which frequently have led the investigators to propose unusual explanations, some of which are inconsistent

with thermodynamic theory, and others have no known parallels

in other chemical systems. Therefore, the solubility problem was examined, starting from fundamentals. The following

are some of the conclusions:

1. It is possible to Introduce the ’’surface" as a phase

without violating the basic principles that lead to Gibbs

phase rule. The surface, as a phase, does not change the

number of degrees of freedom normally associated with the

system. This comes about because more Information Is needed

to describe the composition of a surface than for any other

phase

.

2. The surface cannot restrict the composition to an

isotherm nor will it lead to a valid solubility product constant, per se. It is only when the lattice phase is in

equilibrium with the solution that solubility product principles can be applied. This is true even though the surface

causes the stoichiometry of the overall dissolution reaction

to be different from the stoichiometry of the solid.

3. The system, Ca (OH^-H-^PO^-f^O, is normally considered to have three components. Actually it contains four

constituents, and limiting It to three components is valid

under a stoichiometric restriction which may not be applicable

to nonstoichiometric hydroxyapatite.

It is shown through use of the Gibbs-Duhem equation

that small variations In stoichiometry of the solid do not

invalidate application of solubility product principles.

5. The charge on the surface of finely divided hydroxyapatite particles can be treated as a fourth component in

the system, CafOH^

Q
6. Factors such as particle size, surface energy,

nonst oichiometry , surface equilibrium, and surface phases

make it mandatory to define carefully the mode] for the

equilibrium and to use a thermodynamic treatment which is

consistent with that model. Much of this difficulty arises

from the sparingly soluble character of hydroxyapatite.

A manuscript describing this work is in preparation.

Several of these points were verified in experimental studies

described elsewhere in this report.

4,3. Adsorption Studies

Sponsor; NIDR

Surface Studies of HA : Earlier, reference was made to

a theoretical treatment which distinguishes between "surface"

and "lattice" reactions. Finely divided hydroxyapatite

(surface area 25 m /8) was found to have a high capacity for

calcium and phosphate ions through adsorption experiments.

For example, on equilibrating a finely divided HA preparation with dilute H,PO^ solution, the final PO^ concentration

was less than the initial even though part of the solid had

gone into solution as evidenced by an Increase in calcium

concentration. In spite of the adsorption of phosphate, It

was found that the final equilibrium behavior could be explained on the basis of the solubility product for the lattice reaction, as anticipated from theory.

The results of surface exchange studies (described in

previous reports), using Ca and F" in hydroxyapatitesaturated solutions in the ternary system were used in a "compartmental" analysis. The results are consistent with

the idea that three processes were occurring, one very rapid

(and presumably reversible), a reversible reaction with intermediate rate, and a slow irreversible reaction. The isotopic dilution technique proved very useful for measuring

10
the second of these components. Evidence was found that

the composition of the surface changes as the pH of the

saturating solution is varied.

It was discovered that the earlier approach to the

two-process problem of adsorption from solution was only an

approximation whose degree of exactness was greatest when

the degree of competition (of the two adsorption processes

for the adsorbable species) was least appreciable. An exact

solution for the problem was ultimately found which resorted

to a numerical integration when it was learned that the

system of differential equations could not be Integrated in

closed form. Application of the isotopic exchange measurements to the adsorption theory is the basis of a paper now

in preparation. Determination of the six parameters involves

a non-linear least square optimization procedure which utilizes

two iterative processes alternately. The relative magnitude

of certain of these parameters in the isotopic exchange of

calcium and of phosphate sheds light upon the stoichiometry

of the deposited crystals.

4.-4. Reactivity of the Components of Tooth Structure

Sponsor: NIDR, USA, USAF

A restorative material capable of adhering to tooth

structure Is greatly needed. Bonding is complicated by the

complex nature of tooth structure as well as the mechanical,

toxicological and bacteriological conditions which occur in

the oral cavity. Apparent heats of wetting (AH ) measurements

of dentin, enamel, and anorganic whole tooth tissue of known

specific surface have been used to study the reactivity and

degree of surface modification produced by various ions and

functional groups In an aqueous environment employing the

technique previously developed.

A decrease In. the AH, , value of dentin in water from 4l00

11
2 2

ergs /cm to 1590 ergs /cm In 2% to 40% aqueous solutions of

organic solvents suggests the strengthening of electrostatic

bonds between polar groups of the collagenous matrix of

dentin. In absolute ethanol and hexane, dentin absorbs heat

indicating the weakening of hydrophobic bonds. AH values' W Q

for anorganic whole tooth in organic solvents (—34 ergs /crn)

are not appreciably different from those obtained in water

2 - (109 ergs 'em ). The presence of N0^, NOg, CNS^ SO^ and I",

at pH 5 depressed the AH of dentin to 2,000 ergs /cm

2

. Other w

anions (F“ and Cl") had no effect. At pH 3, however, anions

did not alter the AH of dentin. The addition of aldehyde

groups to the liquid adsorbate at pH 3, however, resulted in

the lowering of the apparent AH of dentin. Significant

changes in AH values for anorganic whole tooth were not obw

served in the presence of anions or aldehyde groups. Thus,

the surface of dentin may be modified by pre-treatment with

non-polar solvents. Ionic bonding to exposed polar groups

can be achieved at the optimal pH. A progress report on

this work is appended.

4.5. Characterization of Components of Tooth Structure

and Dental Materials by Differential Thermal Analysis

Sponsor: NIDR

Differential thermal analysis has been extended up to

1150°C. The melting ranges of dental golds of Types I, II,

III and IV were determined in an argon atmosphere. In all

cases, well-defined endothermic peaks were observed for the

gold component. On reheating, the peak temperature changed

for some of the alloys indicating oxidation of the less noble

alloy constituents. Although differences were observed between the peak temperatures obtained in the DTA measurements

and the melting ranges determined from cooling curves, the

DTA thermograms are very useful for characterizing dental

gold alloys. The thermograms of components of tooth structure.

12
and hydroxyapatite, fluorapatite, octacalclum phosphate

and trl calcium phosphate have been obtained. Presence of

certain impurities such as carbonate in 3-tricalcium

phosphate can be detected from the thermograms.

^.6. Mechanical Behavior of Skin

Sponsor: MIDR

Using the model system of rat skin to gain insight

into the mechnical behavior of the mucous membrane of the

oral cavity, a study is being carried out on the thermodynamic and viscoelastic properties of rat skin under various

mechanical conditions and at various temperatures. It was

noted in the previous report that the mechanical behavior

of rat skin in the 50°C ?ange and above became thermodynamically controlled due to first order phase transition phenomena

all or in part related to the collagen gelatin transition.

For example, as previously reported, the transition was followed as a function of the force of contraction which began

around 56°C and reached a maximum between 60 and 70°C, which

I 3 in general agreement with the shrinkage temperature for rat

skin reported In the literature of 65°C due to the collagen - gelatin transition.

During this period, a study has been made on the first

order phase transition phenomena occurring in skin above

50°C using a differential calorimeter. A calorimeter has

been developed which will determine the onset temperature,

as well as measure quantitatively the magnitude of the first

order phase changes occurring in skin from 50°C to 100°C.

It has been shown that the onset of the first order phase

transition in samples of back skin from a ^98 gm rat occurred

around 54°C Irrespective of sample size as followed calorimetrically. However, if the sample was soaked In water,

this onset appeared to be shifted to a lower temperature.

13
which is in agreement with the dilution effect on the

collagen transition reported in the literature. This skin

fluid content effect on the transition- phenomena needs

further investigation.

It was further noted, using the back skin from a 498

gm rat, that when the total calories released between 54°C

and 98 °C due to first order transition phenomena were plotted

against sample size, a linear plot was obtained. Thus, the

total calories released per grain of back skin could be calculated and were found to be approximately 4 . 2 cal /gm as

compared to that reported in the literature for pure collagen

at 12.5 c&l 'gm. The value for back skin of a 493 gm rat

may not only be due to the collagen “* gelatin transition,

but due as well to other protein transition since a plateau

was observed with every sample size at around 63°C.

4.7. Mechanical Properties of Materials

Sponsor: NIDR, USAF

A torsion pendulum system has been developed for determination of elastic properties and internal friction of

materials. The specimen serves as the torsion bar in the

system. From the frequency of the system, the shear modulus

of the specimen material can be determined and from the

damping the internal friction can be measured. Shear modulus

is determined using the equation G = where G is the

r

shear modulus, 1 and r are the length and radius of the

cylindrical specimen and M and f are the moment of inertia

and frequency of the system. Since the maximum length to

radius ratio that can be obtained with many dental materials

is small, specimen end effects have a relatively large effect on the frequency of the system. Therefore, an effective length must be determined by making measurements with

various specimen lengths. A value of 3 x 1CT psi has been

14
obtained for the shear modulus of dental amalgam. This is

in agreement with values obtained by ultrasonic methods.

The torsion apparatus is applicable to many restorative

materials and to natural tooth structure. Studies of the

effects of temperature variations on the mechanical properties of tooth structure will be made with this apparatus

*1.9. Stress Analysis

Sponsor: NX DR

In order that a more scientific approach can be implemented in the design of restorations, bridges and dentures

a study is being made on the way stresses are distributed in

teeth under various loading conditions. The problem is extremely complex due to the fact that the structures are made

up of several different materials, several of which are not

homogeneous or isotropic.

To solve some of these complex problems, the assumption

of plane stress has been utilized by several authors who

constructed and analyzed thin photoelastic models. Besides

the obvious discrepancies In this approach, such as the use

of homogeneous, isotropic materials and the use of only one

material to represent a structure which is composed of more

than one material, the use of the assumption of plane stress

is also questionable. To test Its validity, a computer program which solves stress analysis problems for either a plane stress or an axis yminetric case was obtained. It was

theorized that a cuspid could perhaps be more validly

represented as an axisymmetric rather than plane stress

structure. An idealized cuspid was then studied under an

axial loading by the plane stress and axisymmetric structures

approach. The results from the two studies were considerably different, so it was concluded that a simplifying assuinp

tion (such as plane stress or axisymmetric structure) was

15
not valid, and that a more exact three-dimensional approach

should be used.

Consequently, work is currently under way to write a computer program which will determine the stresses in an

arbitrarily shaped body subjected to an arbitrary loading.

It is felt that this program, which will require almost a year to complete, will have general applications in stress

analysis, and will be of particular help in the determination of stress distributions in periodontal structures,

4.Q. Metallurgy

Sponsor: ADA, USM? VA

Gallium Alloys . Our work on gallium alloys has now

been completed and will be summarized in a published paper.

During the past six months we have established that a delayed

expansion does not occur in gal Hum- palladium- tin alloys

which had been contaminated with a saline solution. However,

an excessive initial setting expansion appears to result

from such contamination. The same precautions must therefore be exercised to prevent contamination by moisture as

when using zl nc-containlng amalgam alloys.

The elastic constants of several gall ium-palladiumtin alloys were measured using ultrasonic methods. A Young‘s

modulus value of 12.0 ± 0,6 x 10^ psi was obtained which

may be compared to a value of 91 ± 0.1 x 10^ psi for dental

amalgam. It is therefore concluded that the stiffness or

rigidity of the gal llum-pal ladlum-tin alloys under rapid

loading is significantly greater than the stiffness of dental

amalgam

.

A result of even greater significance for clinical use,

however, Is the recent finding that actual restorations of

the gallium-palladlum-tln alloy resist leakage at their

margins to a significantly greater extent than Is the case

16
with similar restorations of dental amalgam. In these

studies, we used the method of Pickard and Gayford^ 1

^ in

which air pressure is applied beneath the hardened restoration through a channel in the root canal of extracted teeth

and the amount of pressure needed to produce leakage under

water is measured. The gallium alloy restorations consistently resisted pressures up to 100 psi while amalgam restorations generally leaked at 20 to 60 psi.

Noble Metals. Studies of phase diagrams for noblemetal alloys are continuing. Alloys of unusually uniform

composition and homogeneity have been prepared and analyzed

quantitatively using an electron beam microprobe. Calibration of the microprobe has been completed for chromiumplatinum alloys and subsequent work now in progress should

reveal the location of phase boundaries directly. The nature

of the order-disorder reactions in this alloy system are now

being studied with a view toward possible hardening of these

alloys by using a suitable heat-treatment

.

M. Pickard and J. J. Gayford, Brit. Dent. J. , 119 , 6° (1965)

.

4.10. Development of an Adhesive

Dental Restorative Material

Sponsor: ADA, USAF, VA

The material under investigation can be divided into

three parts: (l) a polymerizable liquid that is mixed with

(2) a reinforcing filler, this mixture making up the bulk

of the restorative material, and (5) adhesion-promoting

primers that are applied to the tooth surface in the form

of a very thin film.

With regard to the monomer system (the ternary eutectic

system HEMA-isophthalate, HEMA-terephthalate and HEMAphthalate that we discovered previously - these materials

17
are now commercially available) we have purchased a quantity sufficient for our continuing research. These monomers

have been formulated with suitable stabilizers, accelerators and other additives under investigation, and a resultant formulation has been evaluated for systemic toxicity

and local irritation. It was found to be non-toxic and nonirritating. One of the necessary additives to this monomer

system is a tertiary aromatic amine that functions to accelerate the polymerization to give the desired three-minute

hardening time. The materials previously available have

contributed to discoloration of the filling and are in themselves somewhat toxic. New tertiary aromatic amine compounds

were synthesized in an effort to reduce discoloration (by

sterlc hindrance) and to decrease toxicity (by increasing

the molecular weight of the compounds into the neighborhood of

300-500). Accordingly, the following amine accelerators

were synthesized:

N - methyl , N- 1 auryl - p- 1 olui di ne

N- methyl , N- 1 auryl

sym-xyli dine

N,N dimethyl -_p- laurylaniline

The first two compounds were synthesized by reacting lauryl

bromide with the corresponding ring-substituted N,N-dimethyl~

aniline. The last compound was prepared by methylating

p- lauryl aniline. Generally, the compounds were comparable

In their ability to accelerate polymerisation and in the

color stabilities of the resulting composite materials. More

definitive comparisons are currently being made. With regard

to the reinforcing fillers for the restorative material,

special glasses were developed. They contained barium to

give X-ray opacity so that the fillings will contrast with

decalcified (carious) dentin in roentgenograms. Thus, the

dentist will be able to detect recurrent decay. Attention

was given to minimizing the thermal expansion coefficient

18
and to matching the refractive index with that of the matrix

polymer

,

With regard to the adhesion- promoting compounds, numerous candidate materials were synthesized. Only one of these

mediated a significant improvement in the bond between the

filling material and dentin, and this improvement was not

as great as that produced by NPG-GMA (the adduct of N-phenylglycine and glvcidyl methacrylate). Numerous formulations

based on NPG-GMA were prepared and tested for their relative

adhesion-promoting ability. Adhesion in the range of 700-

800 pounds per square inch was obtained with the best of

these. However, a number of these formulations were not

stable, and they polymerized in their containers. Consequently, work has commenced with the objective of synthesizing improved stabilizers for the adhesion-promoting formulations. Independently, investigators in Tokyo reconfirmed

the ability of NPG-GMA to promote adhesion to hard tooth

tissues; it gave the highest adhesion values of a number of

compounds which they have synthesized and tested.

Clinical Studies : Clinical investigations of these

composite materials and other experimental materials are

continuing. Since the toxicity tests were favorable and

approval was given by the Committee of Institutional Associates of the American Dental Association, a formulation

utilizing the ternary eutectic monomer systems has been used

in clinical restorations. X-rays have been taken of these

restorations, and It has been established that the barium

concentration in the barium-glass reinforcing filler is at a suitable level for diagnostic purposes.

4.11. Zinc Oxide-Eugenol Materials

Sponsor: USA

Cements: Recent work in this laboratory has led to

19
greatly Improved aluminum oxide reinforced zinc oxideo-ethoxybenzoic acid (EBA )- eugenol crown and bridge cements.

These products have become commercially available and have

been well accepted by the dental profession. A report on

studies to improve these cements is appended.

Restorative Materials; The objective of the present

research is the development of a longer lasting temporary

zinc oxide- EBA- eugenol restorative material that will yield

sedation and protection of the tooth from irritants and decay. Formulations containing 64$ ZnO, 30$ Al o0^ and 6$

hydrogenated rosin and 62.5$ o ethoxybenzoic acid (EBA) and

37 . 5$ eugenol have excellent physical properties. Limited

clinical studies have shown that the restoration does not

dissolve in oral fluids and can be used for one or two

surface restorations in non- functional areas. These cements

do not hold up under extreme masticatory loads. To increase

the resiliency of the hardened cement, resinous substances

such as polyfmethyl methacrylate) have recently been incorporated into the powder component. Further improvements

have been obtained by employing a carbonate-containing zinc

oxide and a bentonite coated non-caking rosin. With these

components diametral tensile strength values as high as 1000

psi, as compared to 500 psi for other commercially available "improved11 zinc oxide- eugenol restorative materials,

have been obtained. It is believed that these products will

last one to two years, resulting in great savings in the

clinician’s and patient’s time.

4.12. Rapidly Fabricated Surgical Splint

Sponsor: USA

About 25$ of all Injuries incurred in combat are manifested as damage to facial structures. Among these, fractures of the mandible commonly occur. The presently employed

20
technics of splinting fractures of the mandible using archbars are not only traumatic to the oral tissues, but are

also time-consuming and require highly trained personnel

and specialized laboratory equipment and facilities.

A polymeric material suitable for the rapid fabrication of splint-type appliances for the fixation and stabilization of fractures of the mandible has been developed.

This material consists of a very small particle size polymer

powder ( 70$ poly (methyl methacrylate) resin with 30$ CaCO^

and. 0.2$ benzoyl peroxide) and a monomer liquid (methyl

methacrylate with 0.2$ dimethyl paratoluidine ) . The doughlike product formed on the mixing of the powder and liquid

can be readily adapted to the cervical third of the intact

teeth. Auto- polymerization occurs in the mouth within 4-7

minutes at peak temperatures not exceeding 50®C . The polymerized product exhibits sufficient strength, rigidity, and

dimensional stability to function as a splint. Preformed,

spring-like lingually inserted clasps prevent the buccal

displacement of the polymerized splint and serve also as

anchor points for interarch elastics. The use of the "rapid

splint" precludes the need for cast arch bars and complicated

wiring procedures. Limited clinical tests that will supply

a feedback for further modification, if necessary, will be

conducted at Walter Reed Medical Center. The same polymeric

materials may find application as a hard denture reliner and

as orthopedic splints.

21

USCOMM-NBS— DC