NATIONAL BUREAU OF STANDARDS REPORT

ms PROJECT NBS REPORT

31 1.05-1 1-31 10260 June 30, 1969 10 073

31 1.05-1 1-3110560

311.05-11-311056 1

REPORT ON DENTAL RESEARCH

AT THE

NATIONAL BUREAU OF STANDARDS

Progress Report

January 1 to June 30, 1969

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 Dental

Research Division of the U. S. Army Medical Research

and Development Command; the Dental Sciences Division of

the School of Aerospace Medicine, USAF; and the Veterans

Administration.

IMPORTANT NOTICE NATIONAL BUREAU OF

for use within the Governme

and review. For this reason whole or in part, is not au Bureau of Standards, Wash

the Report has been specific

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

jgress accounting documents intended

it is subjected to additional evaluation

ture listing of this Report, either in the Office of the Director, National

er, by the Government agency for which

lal copies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

TABLE OF CONTENTS

Page

1. Introduction 1

2. Reports Issued 1

3. Papers Published 2

4. Work in Progress 2

4.1 Determination of Crystal Structures 2

4.2 Physicochemical Studies of Apatite and Related Inorganic Substances 5

4.3 Study of Dentinal and Enamel Surfaces from

Sorption Isotherms and Heats of Wetting 8

4.4 Mechanical Properties of Oral Structures and

Restorative Materials 10

4.4.1 Dimensional Changes In Teeth Components 10

4.4.2 Mechanical Behavior of Soft Tissues 11

4.5 Dental Metallurgical Studies 13

4.6 A Petrographic Study of Dental Investments l4

4.7 Reinforced EBA Cements 16

4.8 Improved Restoration Adhesion by Specific

Tooth Surface Treatments 17

4.9 Grafting to Hard and Soft Tissues 19

4.10 Composite Direct Filling Materials 21

4.11 Composite Resin Material for Temporary

Posterior Restorations 23

4.12 Clinical Research 25

4.13 Specifications and Advisory 28

Appendix:

NBS Report 10059

NBS Report 10060

NBS Report 10062

NBS Report 10063

NBS Report 10064

NBS Report 10065

NBS Report 10068

NBS Report 10072

Simple Extension of Model Making Technique

of Wood* Geith, Pan Liang and Doc krill.

Crystalline Dimethacrylate Monomers.

The Crystallography of Other Biologically

Significant Inorganic Compounds.

Clinical Investigation of a Radiopaque

Composite Restorative Material.

Plastic Reinforced EBA Cements as Temporary

Restorative Materials.

Characterization of a Composite Resin Material

Developed for Temporary Posterior Restorations,

Determination of Shear Modulus of Dental Materials by Means of the Torsion Pendulum.

The Solubility of CaHPOh at 5, 15, 25 and

37°C.

REPORT ON DENTAL RESEARCH

at the

NATIONAL BUREAU OF STANDARDS

1. INTRODUCTION

Dental research at the National Bureau of Standards

during the half year ending June 30, 1969 included investigations of a wide range of materials used in restorative dentistry as well as studies of mineralized tissues

and related compounds. The research program Is supported

by the Council on Dental Research of the American Dental

Association, the Collaborative Research Office of the

National Institute of Dental Research, the Dental Research

Division of the U. S. Army Medical Research and Development Command, the Dental Sciences Division of the U. S.

Air Force School of Aerospace Medicine, and the Veterans

Administration. Support from the National Institutes of

Dental Research is also provided in the form of grants to

Research Associates sponsored by the American Dental

Association. This support is designated NIDR (Grant) where

reference to it Is made. 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, reports on all

projects are presented for the information of all sponsors.

Detailed reports Issued during the period on some phases

of the research are listed below and are appended to this

report

.

2. REPORTS ISSUED

NBS Report 10059 Simple Extension of Model Making Technique

of Wood, Geith, Pan Liang and Dockrill.

NBS Report 10060 Crystalline Dimethacrylate Monomers.

1
NBS Report 10062

NBS Report 10063

NBS Report 10064

NBS Report 1006

The Crystallography of Other Biologically

Significant Inorganic Compounds.

Clinical Investigation of a Radiopaque

Composite Restorative Material.

Plastic Reinforced EBA Cements as Temporary Restorative Materials.

Characterization of a Composite Resin

Material Developed for Temporary Posterior

Restorations

NBS Report 10068 Determination of Shear Modulus of Dental

Materials by Means of the Torsion Pendulum.

NBS Report 10072 The Solubility of CaHFO,, at 5, 15, 25 and

37°C. 4

3. PAPERS PUBLISHED

Bowen, R. L and Cleek, G. W. X-ray Opaque Reinforcing

Fillers for Composite Materials. J. Dent. Res. 48:79-32,

January-February 1969

Patel, P. R . ; Moreno, E. C.j and Gregory, T. M. Calculation of Diffusion Coefficients in Ternary Systems from

Diaphragm Cell Experiments. J. Res. NBS, A. (Physics and

Chemistry), 73:43-49, January-February 1969.

Bowen, R. L. and Mullineaux, A. L. Adhesive Restorative

Materials. Dent. Abstr. 14:80-82, February 1969.

Waterstrat, R. M. Evaluation of a Gallium-Palladium-Tin

Alloy for Restorative Dentistry. JADA 78:536-541, March

1969.

Mighell, A.j Smith, J. P.; and Brown, W. E. The Crystal

Structure of Phosphoric Acid Heml hydrate, H,P0|, *l/2HpO.

Acta Cryst. 25:776, 1969. P

Hein, R. A.; Cox, J. E.j Blaugher, R. D.; and Waterstrat,

R. M. Superconducting Behavior of A- 15 Compounds. Solid

State Communl cat ions 7:381-384, 1969.

Waterstrat, R. M. Crystal Structure Transformations in

VPt Produced by Plastic Deformation at Room Temperature.

Transactions of the Metallurgical Society of AIME, 245:

1360-1361, June 1969.

4. WORK IN PROGRESS

4.1. Determination of Crystal Structures

Sponsor: NXBR, NXBR (Grant), ADA

Slllco-camotite : Ca^ (P0^) 2Si0^

The structure of siiico-camotite was determined to

2
learn the extent to which it is related to the structures

of hydroxyapatite and glaserite, and has been refined to

R = 0.05. Thus it is well determined. The structure of

w

Ca^ (P0^

)

oSi0^ is intermediate to the structures of the two

above compounds, with a relationship much closer to that

of K^Na(S0^) 2 (glaserite) than that of Ca^(P0^)^0H

(hydroxyapatite). For simplicity, the structure of silicocaraotite is discussed in terms of the ideal formula,

Ca^(P0^) 2Si0^, although the crystal used (obtained from a

slag from a steel furnace) contains 3$ V as V0^“.

The crystal structure of K^Na^SO^^ contains K/SO^

columns and K/Na columns. The K/Na columns are analogous

to two different Ca zigzags in Ca^(P0^) 2S10^, and the K/SO^

columns are analogous to the Ca/PO^ columns and the SiO^

columns. In the SiO^ columns every cation site may be

considered to be vacant. The relationship between the

K^Na(S0^) 2 structure and the Ca^(P0^)^0H structure has

been pointed out by Wondratschek. This relationship is

that the (OH) column in Ca^(P0^]^0H has been replaced in

K^Na(S0^) 2 by the movement of the anions toward the central

column and the movement of the cations away from the central

column. The Cas (PO^) 2SiO^ structure is related to the

hydroxyapatite structure in that the central OH column in

hydroxyapatite is vacant in Ca^(P0^) 2Si0^, though in the

latter, one-third of the Ca ions have moved from the environment to partially occupy these columns.

Ca^(P0^) 2Si0^ is also related to Ca^Mg^PO^)^, the

structure of which we have recently determined (June - December 1968). The Ca^ (PO^

)

2SiO^ structure may be considered to have one cation vacancy per formula weight in

its relationship to glaserite. Ca^Mg^ (PO^)g is also related to glaserite but has three cation vacancies per

formula weight. Such vacancies may be important in the

incorporation of structural defects in crystal structures.

3
There are some unexplained anomalies in the oond

distances in our determination of the Ca^fPO^^SiO^

structure. The average P-0 and Si-0 distances found here

9 —

are 1.595 and 1.55^ A, respectively, where the converse Is

expected since typical P-0 and Si-0 bond lengths for P0?~

2j_ ~ ^ and SiO ” ions are 1.54 S and 1.6l K respectively. p and

Si were not distinguished in our x-ray refinements, but

electron microprobe analyses Indicate that P:Si =2:1. The

resolution of these anomalies may not be possible without

further experimental work. We plan to collect data from a crystal of Ca^CPO^^SiO^ prepared in the laboratory known

to be free from impurities to obtain independent values for

the P-0 and Si-0 distances.

Computer Programs

(i) The latest and most convenient version of the

ORTEP structure-drawing program has been made compatible

with the Univac 1108 computer and other crystal structure

programs in use at the National Bureau of Standards. Considerable experience in its use has recently been gained.

(il) Programs have been adapted and new ones have been

written to provide a currently available series of methods

of calculating x-ray powder patterns fairly routinely.

These programs have been used extensively in the dental

metallurgical research of this section on VPt and Nb^Al-^Ge.

(iii) A program to read and process the output paper

tape from the recently acquired Picker single crystal x-ray

diffractometer so that the quantities obtained are in a suitable stage and form for use in presently available crystallographic programs has been written and tested

successfully.

(iv) The program (from ORNL) to calculate lattice

energies has been tested with calculations on NagCO^'HgO

and has failed to refine the water orientation to a removable

value. The problem is presumably not linear with respect

4
to the parameters involved in specifying the water environment. This program has now been altered so that a logical sequence of changes in the orientations or translation (or other parameters) of a group in the crystal

structure can be forced on it, and the optimum parameters

thus chosen by searching for the minimum energy in the

range covered.

Automatic diffractometer ; The Picker automatic single-crystal x-ray diffractometer equipped with a highly oriented graphite crystal

monochromator was delivered in early May 1969. Intermittent, but significant, progress in aligning the diffractometer and judging its performance has been made; interruptions by malfunctioning equipment and unclear instructions have been frequent. The diffractometer is now nearly

ready for the routine collection of x-ray intensity data

used in determinations of crystal structure.

k. 2 . Physicochemical Studies of Apatite

and Related Inorganic Substances

Sponsor: NIDR (Grant), ADA

Solubility of Calcium Phosphates

The generalized least squares procedure that we use

for the treatment of solubility data has been mentioned

in previous reports. During the present report period, a new and very useful feature has been Incorporated into

this procedure; the adjustment of data is now done considering a functional dependence of the equilibrium constants

(solubility or/and stability) on temperature. The new approach was used successfully with the solubility data on

CaHPO^^HgO. The procedure can be used to treat simultaneously all the solubility data we have gathered on the

various calcium phosphates in order to derive more reliable

thermodynamic constants. As an example, solutions saturated

5
with respect to Ca^OH(PO^ give information on the

stability constant of the ion pair (CaHPOjj), but their

low concentrations and relatively high pH ranges do not

permit a reliable calculation of the stability constant

of the ion pair (CaH2P0j)

e

. However, with the new procedure, it is possible to combine the solubility data

for CaHPO^ and CaHP0^*2H2G with those for more basic

phosphates to obtain more reliable values for the stability

constants than by the use of only one set of solubility

data on a given compound.

Measurements on the solubility of B-Ca^PO^

)

2

* Whitlockite, were completed at 5, 15, and 37°C. These

data, together with the data at 25®C mentioned in the

previous report, have been processed according to the

above procedure. The values for the solubility product

constant of Whitlockite, K_x ICT^, are 0.97 ± 0.090, s p

1.7 ± .12, 1.1 ± 0.08, and 0.22 ± 0.027 at 5, 15, 25, and

37°C. It appears that the solubility product constant of

Whitlockite, like that of CaHPO^ •

h

2

0, has a maximum in

the temperature range investigated. In the case of

CaHP0^*2H20 we could explain this feature on the basis of

the known thermodynamic properties of the salt. Calculations are being made to ascertain whether this is also the

case with Whitlockite.

A manuscript describing the results from a study on

the solubility of CaHPO^ at 5, 15, 25, and 37°C was

completed and was submitted to the Journal of Physical

Chemistry for publication.

Hydroxyapatite Studies

Previous studies made by workers in our laboratory

and also data presented by other Investigators have shown

the solubility product constant for hydroxyapatite,

Ca^(PO^)-jOH, to vary depending on the history of the sample.

We are presently making solubility measurements on

6
particularly dentin. Changes in the relative humidity

of the air surrounding dentin are rapidly reflected in

dimensional changes of the dentin.

One question to be answered by future work is whether

or not structural changes in the collagen component, such

as the linear shrinkage observed In soft tissue collagen

above 60°C, are responsible for some of the dimensional

changes of dentin. In this regard, it is recognized

that the magnitude of the dimensional changes in collagen

are far larger than those in dentin.

Future studies will Involve measurement on anorganic

materials as well as whole tooth materials. Compacted

specimens made from ground tooth substance may offer some

advantages for this work since the organic material can

be readily removed from dentin in small particle form.

Attention will also be given to the effect of changes in

water content with change in temperature of tooth structure immersed in water.

4.4.2. Mechanical Behavior of Soft Tissues

Sponsor: NIDR

Employing rat skin as a model system in the study

of the mechanical behavior of the mucous membrane of the

oral cavity, physical properties and their variations,

such as specific volume, density, change in density with

temperature, enthalpy changes, stress-relaxation, stressstrain and creep, have been previously measured in this

laboratory. A complicating factor has been the demonstration of non-linear elastic behavior. The question

arises as to what structural change in the structure of

the skin accompanies the non-linear elastic behavior.

From a physical point of view, it would be highly

desirable to construct a physical analog model relating

the mechanical components of skin and their physical

11
Interaction in various phases of the elongation of the

skin. This is one of the ultimate objectives of the

project

.

From a chemical point of view, the mechanical behavior of skin must be relatable to its structural

components. This can be approached by varying the quantities of the respective structural components, e.g.,

water, or by introducing specific structural changes in

specific components, e.g., changing the cross-linking

status of the collagen component. A preliminary investigation of both methods of modifying the mechanical system

has been initiated. In the former procedure, the role

of the water component Is being examined, while in the

latter, treatment of skin specimens with highly specific

enzymes or reagents has been initiated. The enzymes used

were collagenase (bacterial), elastase (pancreatic) and

hyaluronidase (bovine testes). D-penici 11 amine, reputed

to induce solubilization of collagen fibrils, was the

reagent employed. Wet skin specimens (130-200 mg) were

suspended for 48 hours at 37°C in 50 ml of saline solution

containing approximately 5 mg of enzyme or 0.8 g of Dpenicillamine . Controls were run by exposure of specimens to the saline solution only.

Exposure of the skin to collagenase greatly reduced

the tangent modulus (stress-strain measurement), but also

induced an unstretched length twice that of the control

specimens. This latter effect was not observed in specimens treated with the other enzymes or with D-penicillamine

.

The tensile strength of the collagenase treated specimens

was reduced by a factor of 20. This loss in tensile

strength and modulus on reaction with collagenase supports

the conclusion that collagen is a major contributing

component to the mechanical properties of skin.

Hyaluronidase treatment caused a moderate reduction

12
In the stress-strain modulus of the skin, indicating

much less dependence of the mechanical properties of the

skin on its polysaccharide components. Hyaluronidase

(bovine testes) is believed primarily to attack the polysaccharide hyaluronate yielding a tetrasaccharide

.

Elastase treatment induced no change in the stressstrain behavior of the skin.

Saline solution containing approximately a 0.1 molar

concentration of D-penicillamine affected the stressstrain behavior of the skin to approximately the same degree as hyaluronidase treatment. The D-penlclllamine is

presumed to cleave Intermolecular cross-linkages of collagen by formation of hemimereaptals with the aldehydiclike linkages.

The results reported here have been derived from

very preliminary experiments and are only of an exploratory nature. It is anticipated that dynamic testing

(equipment is being procured for this purpose) will play

a dominant role in the next phase of this research.

4.5. Dental Metallurgical Studies

Sponsor: ADA, NIDR (Grant), USAF, VA

During the past six months the accumulation of data

leading to the construction of phase diagrams for binary

alloys containing noble metals has continued. The work

on the systems Cr-Pt, V-Pt, and Cr-Ir has progressed to

the point where final examinations of alloys that have

already been prepared are under way. These alloys will

be subjected to thermal analysis, high-temperature x-ray

dif fractometry and dllatometry. The experiments are essential if the diagrams are to be completely usable.

In all three systems, one observes order-disorder

reactions through which certain types of atoms occupy

preferred positions within the atomic lattice structure.
These order- disorder reactions are known to play an

important role in determining the mechanical properties

in certain alloys such as the Au-Cu dental alloys. It

is, therefore, of considerable value to learn more about

the various factors which determine the rate of such reactions or the ordering mechanism Itself.

We have completed a study of the effects of alloy

composition on the degree of ordering In noble metal

phases having the A15 type structure. This structure is

a simple one which is unusually amenable to such studies.

Composition changes which can be related to the position

of the constituent elements in the periodic table were

observed

.

Also, it was observed that elements such as osmium

tend to be less highly ordered than elements such as

platinum or gold. This information may lead to means

for controlling the ordering reactions to obtain the

optimum properties for dental applications. This work

will be published shortly in the proceedings of an international conference on ordered alloys to be held in

September 1969 at Lake George, New York.

4.6. A Petrographic Study of Dental Investments

Sponsor: ADA, NIDR (Grant)

During this period, work has centered on the petrographic study of dental investments used in the preparation of partial dentures. Gypsum, ethyl silicate and

phosphate-bonded investments were examined after casting

in both polished and thin sections. In all examined investments, cast alloy was found to have reacted with the

investment or its refractory coat. Magnetic concentrates

were prepared of these reaction products and were examined

in transmitted light as crushed fragments and in reflected

light as polished sections.

14
Examinations reveal that the sand-sized quartz filler in gypsum-bonded Investment is not greatly mobilized

in the high fire prior to casting. However, it appears

that a weak sinter bond between calcium sulfate particles

is the main bonding mechanism in these investments. In

high-fired silicate-bonded investments, sand-sized quartz

is corranonly greatly mobilized and the bond appears to be

primarily a sinter bond. Phosphate-bonded investments

exhibit extensive sinter bonding and the sand-sized quartz

filler has been highly mobilized. The exact determination of investment bonding reactions is now being undert aken

.

The gypsum and phosphate-bonded investments and siltsized zircon-and-quartz filler refractory coats applied

to silicate Investments have been vitrified at the cast

surface. Chromium spinels and other chromium- rich phases

appear to be the major cast-reaction products in all investments. Detailed study of these reactions under different oxygen pressures is now being undertaken.

Preliminary examinations reveal that cast surface

roughness and investment gas permeability are influenced

by the degree of cast-investment reaction and the extent

of liquefaction in the formation of sinter bonds. The

character of the sinter bonding affects investment-particle

configuration. Extended liquefaction in sinter bonding

may promote coarseness of investment-particle configuration

which, in turn, promotes cast roughness. Liquefaction

or vitrifaction in the cast-investment reactions probably

inhibits gas permeability by the filling of interface

pores

.

Crucibles used for high fusing and low fusing alloys

exhibit vitrefaction and corrosion. In Ticonium crucibles, the formation of a chromium- rich reaction layer

promotes thermal-shock spalling.

15
4.7. Reinforced EBA Cements

Sponsor: U.S.A.

Previous investigations have resulted in improved

zinc oxlde-eugenol-o-ethoxybenzoic acid (EBA) cements

useful as luting media and as insulating bases. Because

of their brittleness, these cements are not contraindicated for temporary restoration of multiple surface

carious lesions In areas subject to heavy masticatory

forces. In the study completed during this period, incorporation of powdered polymers of relatively low

elastic moduli properties lead to products with improved

stress-bearing properties. Special emphasis was placed

in determining the physical, mechanical and solubility

characteristics of selected formulations. Among the

polymeric reinforcing agents investigated, acrylic and

vinyl copolymers appeared to be the most suitable additives.

The resultant products exhibited significant Increases

in tensile strength (maximum tensile strength 1700 psi

as compared to 700 psi for unreinforced EBA cements and

170 psi for ZOE cements). Tissue tolerance studies conducted by Walter Reed Army Institute of Research indicate

that these materials are tolerated as well as zinc oxideeugenol formulations.

These materials present good manipulative qualities.

Limited clinical evaluation indicates that certain polymer-reinforced materials, on the basis of demonstrated

mechanical and palliative properties, may be highly useful as long-duration temporary restoratives.

Details of this study, including the physical properties and solubility characteristics of the cements

investigated, are described In the report which is appended (NBS Report 10064).

16
4.3. Improved Restoration Adhesion by

Specific Tooth Surface Treatments

Sponsor: U.S.A.

One of the unsolved problems in restorative dentistry

is the lack of adhesion between tooth tissues and permanent restorations under conditions existing in the oral

cavity. Microleakage between cavity and restoration can

result in the formation of secondary caries.

Development of materials that adhere to the tooth

structure by Interfacial forces and thus prevent fluid

flow at the tooth-filling Interface would greatly prolong

the effective service life of the restoration.

During this report period, the effect of specific

surface treatment of dentin and enamel with respect to

improvement of bonding to acrylic restorative materials

was studied.

The tensile adhesion test of Lee, Swartz and Culp

was used to determine bond strength. This procedure is

rapid and, therefore, quite suitable as a screening test.

To decrease the number of teeth needed, all polished

enamel or dentin surfaces were reused In subsequent experiments . Later, experiments actually showed that much

higher tensile adhesion was obtained with reused teeth.

Thus, the actual values should be considered as relative

as compared to the respective control. However, the

relative order of adhesion obtained on varying the surface

treatment should not be altered appreciably by using

this protocol.

Tooth treatment was conducted approximating conditions that would be clinically feasible. The surface was

cleaned with a chelating agent such as tetrahydrofurane

tetracarboxylic acid and primed with a solution of sodium

hydroxide. A dilute aqueous solution of a heavy metal or

transition metal was then applied. A steel cylinder with

17
a cavity filled with dental resin was pressed on the

tooth surface and the samples were stored either at 100$

relative humidity or in water for 20 to 24 hours. Treatment of enamel surfaces with certain halides (FeCl^,

AlCl^'SHgO, CrCl^^HgO, PbClg, HgCl2

, SnF^, etc.) gave a

marked adhesion after 24-hour storage in water. It is

not certain if the adhesion resulted from an exchange of

+2 Ca ion for metal ions. Compounds like FeCl^, Sr^,

PbCl 2 or AgNO^, probably formed a thin layer of a coherent precipitate after addition of the base which may

have improved adhesion to the filling resin. Pretreatment with the chelate cleansing solution is necessary for

effective bonding. It is likely that the cleanser removed

+2 Ca ions from the surface layer which were replaced by

+3 A1 ions. Priming with sodium hydroxide is essential

when FeCl-* or A1C1-, solutions are used. For enamel surfaces, treatment with silver nitrate, ceric ammonium

nitrate or sodium molybdate Is effective even without

the NaOH primer. Only the silver nitrate was effective

to dentinal surfaces, but because of the poor tissue

tolerance of this material. Its application in clinical

dentistry appears to be limited. On the other hand,

ceric ammonium nitrate has been used as a catalyst for

graft polymerization to collagen and further studies of

its usefulness and that of other transition metal salts

appear in order.

A more complete study of the bond strength on extended exposure to water showed that the adhesion is lowered

rapidly on prolonged storage in water, so that the tensile strength after 30 days Is insignificant. Storage in

an acidic environment also lowers the tensile strength.

Based on the results of this study, it Is hoped that

the use of potential adhesives in combination with treatment of the surfaces with salts, especially those of

18
transition metals, may lead to improved bonding at the

tooth surface-acrylic interface.

4.9. Grafting to Hard and Soft Tissues

Sponsor: U.S.A.

In the last few years, significant progress has been

made in the chemical modification of proteinaceous materials with the aim of improving the physicochemical

properties of the resulting products . Grafting of monomers polymerizable to high molecular weight polymers on

natural products offers an attractive technique of changing the overall properties and especially the surface

behavior of proteinaceous fibers such as wool, corium

collagen, dentin or bone. In such a modified product,

a second polymer is attached at intervals along the chain

of the original backbone polymer. By judicious choice

of the type of monomer unit to be "graftecf1 on to the

original base polymer, tailor-made products with new and

desirable properties may be obtained. The initial aim

of this investigation is to delineate the optimum conditions (such as kind and concentration of initiators, pH,

rate of diffusion of solvent and monomer Into the "backbone") of grafting monomers to collagenous surfaces.

Similar experimental parameters should give satisfactory

results for other soft tissues.

Whenever possible, conditions and reagents were used

that may be employed clinically. In the grafting experiments, purified collagen or powdered dentin was reacted

with methyl methacrylate or acrylonitrile monomer in

water or in 50$ aqueous dimethyl formamide solutions in

the presence of ceric ammonium nitrate. Under carefully

controlled conditions, treatment with monomer resulted in

an increase in weight of collagen ranging from 6$ to 114$.

This increase in weight, which is caused by formation of

19
homopolymer or graft polymer, was obtained if the collagen was soaked for one hour In water prior to the addition of monomer and catalyst. This soaking period appears

to be necessary to swell the collagen fibers and thus increase the number of active sites accessible for grafting.

Swelling also will increase the rate of diffusion of monomer into the collagen.

In a typical experiment, 1 gm of purified collagen

was stirred in 30 ml of water for one hour and 1 ml of 0.05

M ceric ammonium nitrate in 1 N nitric acid was added. The

mixture was deaerated with nitrogen for 15 min. Then 2.25

ml of freshly distilled methyl methacrylate was added and

the mixture stirred for 3 hours at approximately 37°C.

During this time the mixture became viscous and frothy.

The product was filtered, dried and weighed. The appearance of the product differed considerably from the original collagen. Infrared transmission spectra using the

potassium bromide pellet technique clearly indicated the

presence of collagen and poly (methyl methacrylate).

Twenty-four hour extraction of the product with ethylene

dichloride, which should remove any homopolymer, did not

indicate the presence of any soluble material (homopolymer)

and produced only a negligible decrease in weight. Thus,

the product appears to be collagen onto which methyl

methacrylate side chains have been grafted.

Reducing the reaction period to one hour or omitting

the removal of oxygen from the solution lowered the amount

of polymer grafted into the collagen. Largest weight Increases were obtained on addition of 0.2 percent of a wetting agent to the reaction mixture. Preliminary investigations have been made to graft methyl methacrylate monomer onto the dentin surfaces. No appreciable increase in

weight was obtained on adaptation of the procedure used

successfully to graft monomer onto collagen. However, on

20
employing an anionic wetting agent, products showing a

10$ increase in weight were obtained indicating possible

grafting to the dentin. Definite characterization of this

modified dentin is being conducted.

To ascertain that grafting between collagen and the

vinyl monomer has taken place at the collagen surface

(as opposed to an intimate mixture where no chemical bonding between collagen and the vinyl homopolymer has occurred),

a study was initiated to utilize the infrared spectra obtained by multiple internal reflection or ATR (attenuated

total reflection) spectroscopy techniques for the possible

elucidation of chemical modifications of collagen, enamel

or dentin surfaces. A single 25 reflection ATR module

with KRS-5 prism and a variable beam attenuator for balancing the reference beam was procured. No sharp spectra

were obtained for purified collagen strands or collagenpoly (methyl methacrylate) mixtures. Dentinal powders

yielded reasonably distinctive spectra. However, further

experimentation will be necessary to use routinely this

rapid non-destructive technique for the accurate study of

the chemical composition of odontic powders within a few

microns of their surface.

4.10. Composite Direct Filling Materials

Sponsor: ADA, NXDR (Grant)

The development of an adhesive restorative material

for anterior teeth continues. A manuscript, "Crystalline

Dimethacrylate Monomers", has been prepared which describes

the syntheses and properties of three crystalline dimethacrylate monomers that appear to have advantages over the

monomers previously used in composite materials.

Methacrylate monomers require various additives such

as stabilizers and polymerization accelerators. Compounds

were synthetized having characteristics especially suited

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for use with these monomers. Special emphasis was placed

on the synthesis of tertiary aromatic amines having unususally high molecular weights so as to minimize their solubility in tissue fluids and thus their freedom to diffuse

into the pulp or other tissues. Knowledge previously

gained regarding the requirements for color stability was

utilized, resulting in experimental composite formulations

having greater color stability than previously obtainable.

A manuscript describing these compounds is nearly ready

for publication.

A close match between the x-ray opacity of the tooth

and the restorative material greatly facilitates the detection of secondary caries. Our previous work on this

subject is described in a paper, ”X-ray Opaque Reinforcing

Fillers for Composite Materials” , that was published during the report period. Sixteen additional glass formulations have been prepared, most of which contain reduced

concentrations of barium oxide or barium fluoride. The

objectives are to obtain a reinforcing filler glass that

(1) can be spheroidized without adversely affecting the

refractive index, (2) has a low coefficient of thermal expansion, and ( 3 ) obviates the necessity of mixing two

parts of fused silica with the barium glass powder in the

formulation of the reinforcing filler. This would simplify the manufacture of such materials; dental companies

have shown considerable interest in x-ray opaque reinforcing fillers

.

A review article entitled "Adhesive Restorative

Materials” was prepared and was published. As a continuation of work in this area, adhesion tests were made to

evaluate various synthetic coupling agents that differed

slightly from the coupling agent NPG-GMA (the reaction

product of N-phenylglycine and glycidyl methacrylate, described in our previous reports) which we have found to be

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most effective. Pour such compounds were prepared and

each significantly improved bonding between a composite

material and dentin surfaces relative to the application

of the same solvent containing no coupling agent. However* none gave greater bond improvement than did NPG-GMA.

Means were discovered for prolonging the storage stability

of NPG-GMA solutions.

4.11. Composite Resin Material for

Temporary Posterior Restorations

Sponsor: U.S.A.

Laboratory evaluation of certain experimental composite materials as potential temporary restoratives in

posterior teeth has continued. These composites are based

on formulations previously developed for use in restorations for anterior teeth. These formulations originated

in the course of research at MBS sponsored jointly by the

American Dental Association and the National Institute of

Dental Research. The materials are based on a monomer

formulation of a ternary eutectic system (isomeric pthalate

esters of 2-hydroxyethyl methacrylates) and a resin filler

consisting of vitreous silica and x-ray opaque glass.

The latter component is included to enable the dentist to

distinguish between the restoration and caries.

The previous report delineated physical properties of

a restorative material prepared by employing a ratio of

1.55 g of powder per 0.4 ml of liquid. In the current

report period* two additional mixing consistencies have

been laboratory evaluated. These additional ratios* 1.10 g

and 1.45 g of powder per 0.4 ml liquid* were included to

give a range of mixing consistencies within which one might

optimize certain desirable physical properties and handling

characteristics. The following properties were evaluated:

setting time* temperature rise on setting, setting shrinkage.

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compressive strength, tensile strength, elastic moduli

(Young's and Shear), thermal expansion, indentation resistance, hardness, water sorption, solubility, color

stability and optical opacity. Prom the point of view

of ease of manipulation for the tests involved, the 1.35

g to 0.4 ml ratio has exhibited optimum working consistency of those tested.

A setting time of 5 min., which includes a mixing

period of 1 min, and 15 sec., indicates these formulations allow sufficient working time for clinical use irrespective of powder-liquid ratio. An increase in temperature above room temperature of 5°-7° C was measured 4 - 5.5 min. after beginning the mix. It is during this period

that a large part of the polymerization shrinkage will

have occurred, although such shrinkage is observed to

continue on a much reduced scale for up to one week. Considering that the size of the mix used for the measurement

was much larger than would be placed in a cavity preparation, this temperature rise would not be sufficient to

cause injury to the pulp.

Compressive strength data (measured at 1.5 hr., 6

hr., 24 hr., 1 week and 4 weeks) show that the powder-liquid

ratio did not significantly affect this property. One-day

strengths were on the order of 26,000 psi and increased

to about 28,000 psi after one week. These data indicate

strengths which are one-half that of amalgam, comparable

with silicates, and two times that of unfilled resins.

Tensile strengths were determined at the same time

intervals as indicated above for compressive strengths.

After 6 hrs . , tensile strength of the 1.10 g per 0.4 ml

ratio measured 3700 psi as compared with 4400 psi for

each of the other mixes. Tensile strengths of about 5900

psi were attained with each ratio in one week.

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Varying the powder-liquid ratio did not have a significant effect on modulus data. Shear modulus was

determined by two dynamic methods giving about 0.7 -

fZ (Z 0.3 x 10° psl (torsion pendulum) and 1.0 x 10° psi

r (ultrasonic). A Young's modulus of 2.5 x 10° psi was

determined ultrasonically . The Shear and Young's modulus values for the composites are much lower than for

amalgam.

Interferometer measurements of thermal expansion

over the temperature range 24-58°C indicated a value approximately one-half that for unfilled acrylic resins and

about 10 ppm greater than that of amalgam. Values of

this extremely important physical property were 59, 30

and 54 ppm per °C for the 1.10 g, 1.55 g, and 1.45 g to

0.4 ml ratios, respectively.

Water sorption values were measured after 24 hr. and

2 2

1 week at 57°C to be 0.2 - 0.5 mg/cm and 0.4 mg/cm , respectively. These data indicate considerably less

sorption than the maximum American Dental Association

Specification limit for denture base resin. Water solubility, 0.15$, was low compared to silicate cements.

From this data, it may be concluded that precise

proportioning does not appear to be a critical factor

for producing optimum properties of these composite cements.

A new silica powder has been made. This contains the

same proportions of silica glass and BaFg glass as used

previously. The materials are also from the same batch as

those of the previous work. The difference is that the

silica was "spheroidized" by being put through a plasma

gun. The same percentages of silane and peroxides were

used as before. This powder is now ready for testing.

4.12. Clinical Research

Sponsor: ADA, NIDR (Grant)

Activities in the clinical testing of dental materials

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were increased, with particular emphasis placed on

materials recently developed in these laboratories.

A. Development of criteria and guides for clinical

evaluation of operative restorative materials: This

project is preliminary to the evaluation of newly developed restorative materials. A series of guides or

references was developed, including a shade guide for

estimating lightness or darkness, a step block for

estimating flushness at the margins, a gauge for estimating the crevice size at the margins, and a series of

glass blocks of varying roughness to estimate the surface

condition of the restorations. Other criteria were developed, but without reference guides.

B. Clinical evaluation of a newly developed composite

restorative material: Fifty pairs of restorations were

placed clinically. A newly developed radiopaque composite

filling material for anterior teeth was placed in one of

each pair; in the other was placed a commercially available

radiolucent material. The pairs of restorations will be

compared over an extended period using the reference guides

in (A) above.

C. Clinical evaluation of an adhesion-promoting agent for

resin restorations: Fifty-five pairs of restorations

were placed in 26 patients using the newly developed radiopaque material. In one of each pair, an adhesion-promoting

substance (NPG-GMA) was applied to the preparation surface.

The other preparation in each pair was restored without

the use of the NPG-GMA and will serve as a control. These

restorations will be observed for an extended period.

D. Evaluation of radiolucencies found at the tooth restoration interface during placement of the restorations

in (B) and (C) above: During the placement of the above

restorations, it was noted in post-operative radiographs

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that there was an unexpected Incidence of radiolucent

areas. An attempt was made to determine the causes of

these radiolucencies by serial grinding of extracted

teeth containing similar restorations. It was concluded

that the radiolucencies were caused by the operator’s

Inability to introduce the first Increments of the filling material into restricted areas due to lack of access

and/or air entrapment.

E. Development of a radiopaque denture base resin: The

feasibility of making a resin-glass composite material

for denture bases is being investigated. The glass would

make the material radiopaque so that It could be located

by present medical radiographic technics if aspirated or

swallowed. Specimens for screening purposes Indicate

that such a material can be developed. However, physical

properties and handling characteristics have not yet been

determined.

F. Resilient liners: In a continuation of the study of

resilient liners for dentures, 13 lined with Silastic 6l6

and nine with Coe’s Super Soft were re-examined after being

in service for 22 months. Three of the cold-curing silicone

(Silastic 6l6) and one of the Coe Super Soft resilient

liners have retained their original resiliency, adhesion

and surface Integrity. The others had varying degrees of

deterioration sufficient to warrant replacement. These

observations Indicate that resilient liners, particularly

those designed for chair-side curing, should be Inspected

routinely and replaced as needed.

G. Other programs: Two new programs have been Initiated.

One concerns cavity-margin sealing by dental amalgams to

determine the relevance of national and international

specifications on dimensional changes that occur during

hardening. The other concerns a method for the bonding

of self-curing denture base acrylic to acrylic teeth. On

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both of these projects, work has been limited to laboratory investigations preliminary to clinical trials.

4.1^. Specifications and Advisory

( Sponsor: ADA, NIDR, NXDR (Grant), VA

I Active participation in the specification work of

the Dental Materials Group, IADR, continues. This includes guidance and aid in organization of the new USASI

Standards Committee for Dental Materials and Devices.

Dr. Paffenbarger, as chairman of USASI Sectional Committee

PH6 for Dental X-ray Machines, Radiographic Films and Accessories, has made major contributions to progress in

establishing standards for designation of timer dials for

x-ray machines, specifications for dental radiographic

films, specifications for dental x-ray machines, and revision of standards for dental radiographic film-speed

classification and size.

I A notable development was the revision and approval

by the IADR subcommittee of the first specification on

dental materials -- American Dental Association Specification No. 1 for Alloys for Dental Amalgam. The specification has been updated by using mechanical amalgamation

for setting changes at 3 minutes instead of 15, and by

adding an early tensile strength requirement.

The evaluation of stainless steel orthodontic wires

by developing necessary physical test methods has been

initiated.

The tensile adhesion test developed under NIDR contract

(Lee, Swartz and Culp, J. Dent. Res. 48:211 ( 1969 )) was

critically evaluated for the Biomaterials Research Advisory Committee, NIDR. Only when the test conditions were

rigidly controlled could fairly reproducible results be

obtained. Length of application of cleansing and primer

liquids must be specified. Changes in the viscosity (or

28

{
consistency) of the adhesive resin when applied to the

surface affect the bond strength. Bond strength depends

largely on the length of storage as well as the environmental conditions employed (atmospheric, 100$ relative

humidity or water). Potential adhesives for a roundrobin evaluation test were screened under conditions considered to be clinically feasible and approximating those

encountered in the oral cavity. After storage in water

for 24 hours none of the systems investigated gave a

high enough adhesive tensile strength (bond strength of

more than 300 psi) to highly polished enamel surface to

be useful for an interlaboratory evaluation test. Considerably higher values were obtained with previously

used or roughened enamel surfaces. It is hoped that a

test evaluation protocol can be developed in the near

future using carefully prepared enamel or dentin surfaces.

The availability of a scanning electron microscope

(scheduled for delivery to NBS during fiscal 1970) should

greatly facilitate the standardization of the surfaces of

enamel and dentin to be used in these evaluation tests.

To determine solely which operating parameters affect the results of the test procedure, flat, polished

aluminum or stainless steel disks were substituted for

the usually curved enamel specimens. The recess of the

cleaned cylinder was filled with a commercial resin. The

bond strength was determined after 24 hours of storage

under atmospheric conditions. This procedure appears

promising for interlaboratory comparison of the reproducibility of the test method since manipulative variables

used by the adhesive are kept at a minimum.

USCOMM-NBS-DC

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