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NATIONAL BUREAU OF STANDARDS REPORT

3127

REPORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU OP STANDARDS

Progress Report

July 1 to December 31 * 1953

Dental Research Laboratory

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Aslin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards is suggested in the following listing of

the divisions and sections engaged in technical work. In general, each section is engaged in special*

ized research, development, and engineering in the field indicated by its title. A brief description

of the activities, and of the resultant reports and publications, appears on the inside of the back

cover of this report.

Electricity. Resistance Measurements. Inductance and Capacitance. Electrical Instruments. Magnetic Measurements. Applied Electricity. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic

Technology. Length. Gage.

Heat and Power. Temperature Measurements. Thermodynamics. Cryogenics. Engines and

Lubrication. Engine Fuels. Cryogenic Engineering.

Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State

Physics. Electron Physics. Atomic Physics. Neutron Measurements. Infrared Spectroscopy.

Nuclear Physics. Radioactivity. X-Rays. Betatron. Nucleonic Instrumentation. Radiological Equipment. Atomic Energy Commission Instruments Branch.

Chemistry. Organic Coatings. Surface Cheipistry. Organic Chemistry. Analytical Chemistry.

Inorganic Chemistry. Electrodeposition. Gas Chemistry. Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Aerodynamics. Engineering Mechanics. Hydraulics. Mass. Capacity, Density, and Fluid Meters. Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Polymer Structure. Organic Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

Mineral Products. Porcelain and Pottery. Glass. Refractories. Enameled Metals. Concreting Materials. Constitution and Microstructure. Chemistry of Mineral Products.

Building Technology. Structural Engineering. Fire Protection. Heating and Air Conditioning. Floor, Roof, and Wall Coverings. Codes and Specifications.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Machine

Development.

Electronics. Engineering Electronics. Electron Tubes. Electronic Computers. Electronic

Instrumentation.

Radio Propagation. Upper Atmosphere Research. Ionospheric Research. Regular Propagation Services. Frequency Utilization Research. Tropospheric Propagation Research. High

Frequency Standards. Microwave Standards.

Ordnance Development. These three divisions are engaged in a broad program of research

Electromechanical Ordnance, and development in advanced ordnance. Activities include

Ordnance Electronics. basic and applied research, engineering, pilot production, field testing, and evaluation of a wide variety of ordnance materiel. Special skills and facilities of other

NBS divisions also contribute to this program. The activity is sponsored by the Department of

Defense.

Missile Development. Missile research and development: engineering, dynamics, intelligence,

instrumentation, evaluation. Combustion in jet engines. These activities are sponsored by the

Department of Defense.

® Office of Basic Instrumentation • Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT NBS REPORT

0708-10-0707

0708-20-3824

0708-20-3857

January 31* 1954 3127

REPORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU OF STANDARDS

Progress Report

July 1 to December 31* 1953

Dental Research Laboratory

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 Army Dental Corps, the

Air Force Dental Service, the Navy Dental Corps, and the Veterans

Administration.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, r Approved for public release by the

unless permission Director of the National Institute of

25 ,d,c. Such pe Standards and Technology (NIST)

cally prepared If 0 n October 9, 2015

r In part, Is prohibited

tandards, Washington

eport has been speclfimport for Its own use,

REPORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU OF STANDARDS

1 . INTRODUCTION

Investigations of a wide variety of dental materials and

studies of natural tooth structures have been conducted in the

Dental Research Laboratory at the National Bureau of Standards

during the half year ending December 31, 1953 « Summaries of

results obtained on work now in progress are given below. In

addition, copies of reports issued on completed phases of the

investigations are included.

2 . REPORTS ISSUED

NBS No. 2738 Crazing of Acrylic Resins

J.A.D.A. 47 , 324 Hydraulic Turbine Contra -angle Handpiece

NBS No. 3085 Cutting Characteristics of Dental Burs as

Shown by High-Speed Microphotography.

3.

WORK IN PROGRESS

3.1 Structure of Human Tooth Enamel and Dentin

Investigation of the fluorescence of enamel and dentin when

irradiated with ultra-violet light has been continued. During

this period emphasis has been placed on the development of equipment for determining the characteristics of the fluorescence of

materials. A depolarization fluorometer, which will be used in

an attempt to obtain some idea of the molecular weight of the

fluorescent material in tooth structures, has been constructed

and is now being calibrated. Development of a decay fluorometer

is still in the planning stage and the relative advantages of

several methods of determining decay time of the order of 10 “9

seconds are being studied.

3.2 Low Temperature Alloys

Studies of low temperature alloys of the dental amalgam

type have included both investigation of experimental alloys containing various combinations of gallium, nickel, silver and indium and X-ray diffraction examinations of the phase changes which

occur in the amalgam alloys now in use.

Alloys of gallium and nickel were made and the effects of

particle size of the nickel powder of the alloy were determined.

Best results have been obtained with nickel powder having a fineness of minus 325 mesh. Powders having a fineness of minus 200

plus 325 mesh result in alloys of low physical properties. When

a flake nickel powder is used the alloy hardens too rapidly to

be used. The data indicate that particle size will be useful

in controlling the hardening time of the alloy.
’
2

The properties of the alloy improve with an increasing gallium content in the range of 20 to 40 atomic per cent gallium.

Lower gallium contents result in an alloy that is too dry to

handle and higher gallium-content alloys tend to harden as a result of the solidification of the excess gallium rather than

through reaction between the alloy components.

Alloys of silver and indium were made and powdered. This

in turn was mixed with gallium in several proportions. These

alloys did not harden in storage at 37°C for 30 days.

Use of X-ray diffraction methods at low temperatures

(-125°C) in the investigation of dental amalgam has provided

data on the diffusion of mercury through amalgam, the effect of

surface polishing and the effect of heating the amalgam. Surface polishing removed a tin-mercury phase revealing a surface

rich in the Ag2Hg 3 phase. Further polishing reduced this phase

and revealed the presence of unattacked alloy remnants. Aging of

dental amalgam at 65°C resulted in reduction in the SnyHg phase

and an increase in free mercury which disappeared on continued

heating apparently as a result of volatilization. Heating at

100 to 105 °C produced a decrease in both the Sn^Hg and Ag2 Hgo

phases and formation of the AgHg phase.

This X-ray diffraction technic has been shown to provide

a method for following the fundamental phase changes in dental

amalgams on which the physical properties depend.

3*3 Resins

The effects of various accelerators on the polymerization

of methyl methacrylate, the relationships between transition

temperature and water sorption, methods of determining monomer

content, and the properties of dental resin repair materials,

resin cements and resin posterior filling materials were investigated.

The study of the effect of amine accelerators in varying

concentrations on the polymerization kinetics of methyl methacrylate polymers has been continued. Wide variations in the

setting time were observed in studies of over 40 accelerators.

In the presence of a large number of the amines, no noticeable

change in the viscosity was observed even after a few days.

Only a very few accelerators showed short setting times. The

reactivity varied somewhat depending on the catalyst-accelerator

ratio. Small differences were observed at accelerator concentrations of between one and two percent. The polymerization

rate followed no simple overall kinetics. Most polymers formed

were highly colored. Further studies will be made using amine

mixtures and other catalysts in an effort to produce polymers

with short setting times and improved color stability.
.

'
- 3

-

The second order transition temperatures (Tg ) of methyl

methacrylate polymer samples previously used in sorption measurements were determined refractometrically . Values of Tg varied

from 55 to 93°C and were well within the temperature range in

which an abrupt increase in water sorption takes place. In general, Tg values increased and water sorption decreased with increasing molecular weight. Reversal from this behavior, i.e.

lower Ts. values at higher molecular weights was always accompanied by increase in water sorption. These discrepancies are probably caused by large variations in molecular weight distribution.

Several methods of determining the monomer content of cured

dental resins have been investigated. Infra-red examination of

the sublimate collected from frozen polymer -benzene solutions

indicates that methyl methacrylate monomer contents as low as

0.02$ can be detected by means of an absorption band at 5.75y*.

Mass spectrograph and ultra-violet absorption methods of detecting small amounts of monomer in benzene or other solvents were

not sufficiently sensitive.

Data obtained on materials used for repairing dentures

show that repairs made from self -curing materials had approximately 60$ of the strength of the original resin, while repairs

made from heat -curing resins had approximately 85$ of the original strength. Dimensional changes were smaller when the selfcuring materials were used.

Preliminary results in the investigation of the adhesive

properties of dental resin cements and resin posterior-filling

materials indicate that while the materials do adhere to dry extracted teeth, there is little or no adhesion to wet tooth structure .

3-4 Zinc Oxide -Eugenol Materials

The study of the mechanism of the zinc oxide -eugenol reaction was continued. Use of a zinc oxide of very small particle

size (125-250A) greatly intensified the reaction. X-ray diffraction indicated the formation of a hitherto unreported crystalline material on setting which has a different lattice structure than that of zinc oxide or previously reported "zinc hydroxides . "

Elementary analysis of purified samples of the zinc containing organic material obtained by chemical separation and checked

for purity by X-ray diffraction and electron micrographs gave

an empirical formula of C 59.8$, H 5»7$* Zn 17.1$. The molecular weight is presently being determined by boiling point elevation . Infra-red absorption spectra suggest the formation of a

chelate compound of the eugenolate type and substantiate chemical evidence that the propenyl group is unchanged in this setting

reaction

.
.
- 4 - 3.5

Chrome -Cobalt Alloys

A standard specimen 2 {- inches long and 0.09 inch in diameter with 12-24 threaded ends has been selected for tensile and

hardness tests on dental chrome alloys. Wax patterns of these

dimensions have been prepared and sent to Federal and manufacturers' laboratories for use in casting test specimens. Data

obtained on these specimens will be utilized in the preparation

of a specification for dental casting alloys.

3.6

Rotating Dental Cutting Instruments

High-speed motion pictures have been made of dental burs

cutting enamel, dentin, glass, brass and various other substances.

For the first time, it has been possible to show the actual cutting of individual bur blades and to demonstrate, for example,

how an eccentric bur behaves in various media.

A conference was held on December 21, 1953 > with research

workers' and manufacturers' representatives at which time a

discussion of bur test methods was carried on. As a result of

this conference standardization of bur testing will be attempted,

so that speeds of rotation, test materials, loads applied, etc.

will be the same in all laboratories. It is believed that data

obtained by the many workers in this field can be made more understandable and more valuable to all concerned, if more uniform

test methods are employed.

A survey of the various sizes and types of dental rotating

diamond abrasive instruments now in use has been made and a uniform nomenclature and decimal numbering system has been devised

for these instruments. The system, based on six basic classifications which are further subdivided to designate variations in

shape and position of abrasive, will do much to eliminate the

confusion which now exists because of the wide variations of nomenclature and numbering used by different manufacturers

.

3.7

X-ray Film Survey

To obtain information from the dental profession on the properties required in X-ray film, eighty-five questionnaires and

penetrometers have been sent out to cooperating dentists. Approximately 80$ of these have been completed and returned. All

films are developed immediately upon their return and the work

of measuring their density is now 65$ completed. The returns to

date indicate that there is a wide variation in the methods of

exposure and processing of X-ray film by the profession.
'

s
- 5

- 3.8

Panographic X-ray Investigation

The development of methods and equipment for making X-rays

of the entire dental arch on a single film was started. Literature describing previous panographic X-ray methods has been reviewed and the relative advantages of various approaches are being studied. A mechanical model is being constructed for use in

investigating the various motions and relationships inherent in

a device of the type under development. The data obtained will

be used in the development of electronic equipment for automatically producing the motion required.

A wax spatula was designed and developed to improve upon

the traditional spatula used in the construction of waxed prosthetic appliances. The new electric spatula, heated automatically rather than manually by a gas flame is a practical time

and labor saving device. Its components are: a 6 volt CT transformer, a rheostat, a switch, a heating element and interchangeable spatula blades.

Materials evaluated for the cooperating Federal dental

service and the American Dental Association by specification

or special test methods included amalgam alloys, silicate cements,

denture base materials, wrought gold wire alloys, zinc phosphate

cements and inlay casting golds.

3.9

Electric Wax Spatula

3.10

Evaluation of Material

For the Director

W. T. Sweeney, Chief^^^—-

Dental Research Laboratory
.

'
NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT MBS REPORT

0708-20-3824 August 14, 1953 2738

Progress Report

on

CRAZING OF ACRYLIC RESINS

by

W. T. Sweeney^

G. M. Brauer ^

I . C . Schoonover-^

1 Chief, Dental Research Section, Division of Organic and Fibrous

Materials

.

2 Chemist, Dental Research Section.

3 Chief, Polymer Structure Section, Division of Organic and Fibrous

Materials

.

This work is a part of the Dental Research Program conducted at the National

Bureau of Standards in cooperation with the American Dental Association, the

Army Dental Corps, the Air Force Dental Service, the Navy Dental Corps and

the Veterans Administration.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, reprinting, or reproduction of this report In any form, either In whole or In part, la prohibited

unleaepermlaiion la obtained In writing from theOfflce of theDIrector, National Bureau ofStandarda,Waahlngton

25, D. C. Such permlaalon la not needed, however, by a Government agency foi wh'ch a report haa been specifically prepared If that agency wishes to reproduce additional copies of that particular report for Its own use,
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CRAZING OF ACRYLIC RESINS

Abstract

Crazing is a common phenomenon in acrylic dentures cured

against tin-foil substitutes. In this investigation causes of

crazing in denture resins and plastic teeth were studied. The

results obtained show that the increased water sorption by

straight chain methyl methacrylate resins at elevated temperatures is an important factor in crazing. Polymer specimens exposed to water during the processing become susceptible to crazing. Materials protected from water during the heating -period

do not craze readily. Cbthe basis of the data obtained it is

believed that during storage of the polymer specimens at room

temperature, evaporation of excess water from the resin surfaces

sets up strains which ultimately are released by crazing of the

resin. .

1. INTRODUCTION

Appearance of small surface cracks is often observed in dentures and

plastic teeth. These cracks lower the esthetic qualities of the restorations

and greatly reduce their strength. Crazing is the result of local stresses

set up in the resin. When the stress exceeds that necessary to fracture the

resin, crazing occurs. The object of this investigation was to determine the

factors which produce crazing in dental acrylic resin and to explain the crazing mechanism.

Typical photographs of crazing in a denture and in plastic teeth are shown in Figures 1 and 2. It has been shown (references 1, 2 and others) that

crazing in polymethyl methacrylate is caused either by application of external mechanical stresses or by the action of chemical agents (solvent crazing).

The latter type of crazing may occur in the presence of residual monomer, solvents and oxygen or depolymerization products. These substances will produce

local strains greater than the material can withstand. Often the simultaneous

action of solvents and external stresses produces crazing. It is probable

that the crazing of dentures results mainly from solvent action in combination

with strains due to internal stresses in the resin.

2. METHODS AND RESULTS

Examination of dentures in service and of laboratory specimens showed

that there was a wide variation in the amount of crazing occurring in denture

bases and in plastic teeth made of the same brands of material. It was also

observed that plastic teeth which did not craze on the application of a solvent

before being placed on a denture were often craze susceptible after being subjected to the denture processing procedure. Since these observations indicated

that the craze susceptibility of the resins was dependent upon variations in

- 2 -

the processing procedure the effects of several of these variations were investigated. Included were the effect of the type of investment or mold material used

in processing the denture, the effect of water sorption during processing and the

effect of the molecular weight of the resin.

The "standard” craze test selected for determining the craze susceptibility

of the resins consisted of three cycles of alternately immersing the specimen in

methyl methacrylate monomer for 5 seconds and then drying it in air. Specimens

which were crazed by this standard procedure also crazed on the application of

ethyl alcohol and many other solvents.

To determine the effect on crazing of stresses produced due to the differential thermal expansion between resin teeth and investing media, teeth were invested in a number of materials varying widely in thermal expansion and in

strength. Teeth invested in dental stone, dental investment containing silica

gel, Boilsoft investment and alginate impression material were held in a water

bath at 100°C for one-half hour. After cooling, the teeth were subjected to the

standard craze test and crazing was observed in all cases regardless of the

type of investing material used. The uniformity of crazing observed after investment of the resins in materials varying so widely in physical properties

strongly suggested that differences between the thermal expansion of the investing media and the resin were not major factors in crazing.

The results of experiments in which teeth were maintained at 100°C by techniques which prevented their coming in contact with water, indicated that water

sorption was a major factor in the crazing of teeth. Teeth heated in air gave

no craze pattern. Teeth invested in Acrawax C (melting point 137°C) and then

suspended in a water bath at 100°C for 3 hours showed no craze marks on application of the craze test. Teeth heated in stone investment from which free

water had been removed by drying in vacuum and heating to 110°C did not craze. Acrylic dentures polymerized at 71.1°C and 100° C against tin-foil-lined plaster

molds did not craze, whereas those prepared against alginate tin-foil substitutes invariably blushed and showed extensive craze patterns. It has been reported [3,43 that water from the plaster or stone penetrates the resin when it

is polymerized against alginate separating media, which are water permeable.

Sweeney [3] showed that a loss in weight on drying of resin disks cured against

alginate base material was 2.7$ whereas tin-foil-lined specimens lost only 0.6$.

This loss of water was accompanied by volume shrinkage. The effect of water

sorption during processing was illustrated with sheets of resin 1.5 mm thick

polymerized against alginate on one side and against tin foil on the other side.

When these specimens were dried, the side polymerized against the alginate

shrank more than the side cured against tin foil due to the greater loss of

water, and the specimens warped.

Crazing was found to be more pronounced when specimens were allowed to dry

for a few hours before being subjected to the crazing test. Furthermore, if an increased amount of water was introduced into the specimen during processing (for example by processing the resin in an autoclave), more severe crazing resulted.

In order to determine the effect of water temperature on crazing, Plexiglas sheets 0.04 cm thick were immersed in water for a few days and weighed.

Positive craze test results were observed for samples stored at 100°C, 70°C,

50°C and 37°C. The results show that crazing occurs on exposure to water

above as well as below the second-order transition point, i.e. the temperature

in the 55° to 75°C range at which a relatively abrupt change takes place in the

mechanical, thermal, electrical and optical properties of the polymer. Speci- mens stored at 4°C did not craze. As can be observed from Figure 3 which shows

the water sorption-temperature relationship, lack of crazing at 4°C cannot be

attributed to difference in water sorption. Increase in craze resistance with

decrease in temperature has also been observed by Maxwell and Rahm for polystyrene [3 •

Measurements of the water sorption of polymethyl methacrylate disks

(0,4 thick) which were molded in a Carver press at 140°± 5°C and 10,000

lb/in. from duPont HG4 (fine) molding powder were made at various temperatures.

Samples were stored in water and weighed at various time intervals. Specimens

kept below the second-order transition temperature, T , reached sorption equilibrium (attained constant weight) within less than 10 days. Water sorption below

T„ was slightly temperature dependent and increased about 10% over the 4° to

70°C temperature range. (See Figure 3$ in this figure water sorption at 100°C

does not represent the equilibrium value. The disks still increased in weight

after 200 days of water exposure.) On storage of purified straight chain polymethyl methacrylate* at 100°C, which is well above Tg, a continuous weight in- crease was observed even after 100 days (Figure 4) . Depending on the viscosity

molecular weight, water sorption values of the order of 2 to 10 times those

found at room temperature were obtained.

Blushing (evidenced by the hazy appearance due to light scattering) occurred when a Plexiglas sheet which had been dipped into monomer was exposed

to water-saturated air. The bleached or milky white portion extended to a very

shallow depth as evidenced by the disappearance of blushing on removal of a thin

surface layer. Examination under the microscope showed no distinct cracks at a magnification of 500.

When a blushed and crazed sample was heated slowly to 150°C, blushing

and craze marks disappeared and did not return on cooling. Specimens annealed

in this manner showed no crazing. Efforts to reduce craze patterns by drying

samples at room temperature in a high vacuum were unsuccessful.

It was observed that crazing is affected by the molecular weight of the

specimen. Five samples of different molecular weights (45,000 to 1,615,000)

were given the test for crazing after being heated for one hour in water. The

higher molecular weight material was more craze resistant than specimens of

lower molecular weight.

A very slight increase in number average and viscosity average molecular

weight was observed for samples polymerized with tin-foil substitutes as com- pared with those processed against tin foil. This difference was too small,

however, to change crazing characteristics. These results show that crazing

of dentures packed with tin-foil substitutes is not due to a lower molecular

weight of the denture resin.

* The authors wish to thank W. F. Bartoe of the Rohm and Haas Co. for supplying the samples.

- 4 -

3 . DISCUSSION

It is evident that a complete description of the mechanism of crazing

is not possible from data available at this time. The observed effects, in

the opinion of the authors, may be explained as follows.

During processing the denture base material is heated above the transition temperature. After the polymerization is complete and while the polymer

is still above Tg the less rigid chains of the polymer remain uncoiled. The * chain network is expanded as compared to its normal arrangement below the transition temperature. This allows the water molecules to permeate more easily

between the polymer chains, and an increased quantity of water is absorbed.

Greater water sorption above Tg may also be caused by a possible increase in

hydrogen bonding due to the presence of small amounts of depolymerization and

hydrolysis products, especially if the material is a copolymer of polyacrylic

acid ester. On cooling below Tg, the resin becomes supersaturated with water.

On placing the specimen in air, water and residual monomer evaporate from the

surface. The shrinkage resulting from the evaporation of water and monomer

from the outer layer or skin causes the surface layer to be in tension. When

the strain is sufficiently large, fracture of the weak secondary forces between

the chains occurs and crazing cracks appear at right angles to the direction

of stress, Ofb*r^©layed fracture occurs because of the increase in magnitude

of stresses, wfaz.e marks appear after storing the resin for a few days, Cn

subjecting the sample to the rapid craze test, crazing probably occurs by a mechanism similar to that described above.

Whether the blushing of the material which takes place during the cooling

period is due to the precipitation of water as a distinct phase or is caused

by numerous submicroscopic openings which cause a large portion of the light

striking them to be reflected, cannot be decided from the present investigation.

Possibly clues regarding this mechanism can be obtained from X-ray diffraction

measurements and observations under the electron microscope. Samples of polymethyl methacrylate can be annealed in air at elevated temperatures. On heating above Tg, more and more segments of the molecule have sufficient energy to

execute vibrations and rotations. Strains are released and blushing and craze marks disappear. Furthermore, any water present in the polymer can diffuse

more readily to the surface where it evaporates.

Craze resistant teeth are now commercially available. These teeth con- tain cross-linked polymethyl methacrylate of high molecular weight. The

material is insoluble in most solvents, does not swell readily, and has a softening point above 130°C. In the preparation of the denture the teeth are probably not heated above the transition temperature. Also, reinforcement of

the chains through cross linking probably increases the amount of stress nec- essary to break chains. Hence, the craze resistance is increased through cross linking.

4. SUMMARY

Investigations of the crazing of acrylic dentures and plastic teeth demonstrated that when linear methyl methacrylate polymer comes in contact with

water during processing at elevated temperatures, the material is susceptible

to crazing. Material protected from water during the processing does not craze.

- 5 -

An explanation for the crazing process, based on the experimental evidence,

is advanced. Sorption of excess water at elevated temperature causes the surface of the material to be supersaturated with water when the specimen is cooled.

Evaporation of excess water sets up strains which eventually are released by the

formation of craze marks.
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Bibliography

1. Hsio, G. C. and, Sauer, J. A., J. Appl. Physics, 21, 1071 (1950).

2. Russell, E. W., Nature, 165 , 91 (1950).

3. Sweeney, W. T., Vernonite Work Bench, 6, (No. 6) June 1947.

4# Ferguson, G. W., Paffenbarger , G. C., and Schoonover, I. C.,

J. Am. Dent. Assn., J38, 573 (1949).

5. Maxwell, B. and Rahm, L. F., Ind. and Eng. Chem., ^1, 1989 (1949).
w/

; • r'i‘ s
FIGURE

T T

T T T

10

8

a*

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m

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2

TEMPERATURE VS WATER SORPTION

OF POLYMETHYL METHACRYLATE

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TEMPERATURE-*C FIGURE 3

0 20
MOLECULAR

WEIGHT

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WATER

SORPTION

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POLYMETHYL

METHACRYLATE

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FIGURE

4
Hydraulic turbine

contra-angle

handpiece

Robert J. Nelson, D.D.S.; Carl E. Pelander,

and John W. Kumpula, Washington, D. C.

Reprinted from Volume 47

Pages 324-329 September 1953

the Journal of the American Dental Association

Hydraulic turbine contra-angle handpiece

Robert J. Nelsen, D.D.S.* Carl E. Pelanderf and

John W. Kumpula,% Washington, D. C.

A high rotary speed enhances the effi- ciency of instruments used for cutting

teeth in operative procedures. The small

diameters of the dental cutting tools make it necessary to turn the instru- ments at high rates of speed in order to obtain effective linear speeds for the re- moval of enamel. For example, a Ys inch

(22.2 mm.) disk rotating at 1,000 rpm

has the same linear or surface speed as a 5/32 inch (3.9 mm.) disk dental tool rotating at 5,600 rpm. Some dentists have

been reluctant to use high speeds in many

operations in the mouth because of the

hazard to the patient. This danger is due

chiefly to the high inertia of the revolv- ing instrument.

This report describes a contra-angle

dental handpiece containing a small tur- bine which is propelled by a high speed

stream of fluid in a closed system. This

handpiece was developed at the National

Bureau of Standards in cooperation with

the American Dental Association. Because

of certain characteristics inherent in hydraulic systems, many of the objections

to the use of high rotary speeds in cavity

preparation can be eliminated with the

use of this new instrument.

THE UNIT

The unit is shown in Figure 1. It consists of a mobile cabinet which contains a fluid pumping mechanism, a reservoir, a pressure switch, a solenoid valve and

distribution lines. The contra-angle handpiece (Fig. 1, A) is connected to a flexi- ble coaxial double tubing (Fig. 1, B).

The Yq inch inside tube carries the propellent fluid (water) under pressure to the contra-angle handpiece while the Yq

inch outer tube carries the spent fluid back to the reservoir. The tubes join

(Fig. 1, C) with the line directly from the pump (Fig. 1, D) and the line to the

reservoir tank (Fig. 1, E). The flexible rubber hose (Fig. 1, F), which rests on

the floor, is connected to a relay system

by which the operator controls the fluid flow to the handpiece.

The unit is self-contained and needs

only to be connected to an electrical outlet for operation. The fluid is stored in a

reservoir tank and is used over and over; hence the unit requires no adjustment

during its normal operating use. The

pumping equipment is enclosed in a sound-insulated cabinet which minimizes

undesirable noise during operation.

THE PUMPING MECHANISM

The pumping mechanism consists of a constant-volume pump (Fig. 2, A), close- coupled to a one-half horsepower electric motor (Fig. 2, B). The output of the

*Research associate, American Dental Association, National Bureau of Standards.

{Senior instrument maker, National Bureau of Stan- dards. {Laboratory mechanic, National Bureau of Standards.

Reprinted from The Journal of the American Dental Association, Volume 47, pages 324-329, September 1953. All expressions of opinion and all statements of supposed facts are published on the authority of the writer over whose signature they appear and are not to be regarded as expressing the views of the American Dental Association unless such statements or opinions have been adopted by the Association.
NELSEN—PELANDER—KUMPULA . . . VOLUME 47, SEPTEMBER 1953 • 325

pump in this unit under no-load conditions is 1.6 gallons per minute. The fluid

is taken into the pump from the reservoir tank above through the connection (Fig.

2, C). The fluid is discharged from the pump into a line (Fig. 2, D) which leads

to the contra-angle handpiece. The fluid from the handpiece is returned to the

reservoir through a line (Fig. 2, E). To

stop the turbine, the operator steps on a tube (Fig. 2, F). This force closes the pressure switch (Fig. 2, G), completing a

circuit and opening the solenoid valve

(Fig. 2, H), which allows the fluid to bypass the turbine through a line (Fig. 2, I) and return to the reservoir. While the instrument is being used, the motor and pump operate continuously; the handpiece is energized as needed by the operator controlling the flow of fluid through the instrument as

just explained. The motor is controlled by a switch located on the side of the

unit.

THE HANDPIECE

The use of air or water as motive power

for dental rotary cutting instruments is not new. Iseman in a United States patent secured in 19401 and Norlen in a United States patent secured in 19522 both describe dental engines propelled

by compressed air. In United States patents secured in 1878, 1877 and 1879 re- spectively, Straub,3 Wilkerson4 and

Laurence5 describe different means of

driving a dental engine by the use of water as a propellent. Each of the engines was designed for the straight dental handpiece, and each required that a gear-type

contra-angle handpiece be attached.

The handpiece is driven by a small

turbine in the head of the contra-angle.

The over-all size and shape of the new

instrument are within the general proportions of the conventional gear-driven

contra-angle handpiece. Figure 3 shows

the comparative sizes of the hydraulic

handpieces, A and B, and a conventional

Fig. 1 • External view of assembly showing

handpiece, A; coaxial double tubing, B; and

junction, C, of line, D, from pump, and line, E, to reservoir tank

contra-angle handpiece, C, attached to a straight handpiece and wrist joint. The

instrument shown in Figure 3, A is of an

experimental design which permits vari- ous turbines and bearings to be inserted

for testing. Figure 3, B shows the design

of the instrument which would be more

suitable for clinical use than that in Figure 3, A. The cutting tool in these instruments is attached directly to the

shaft of the turbine.

1. Iseman, John. Dental tool. U. S. Patent No.

2,251,057, July 29, 1941.

2. Norlen, Per Ivar. Dental engine for compressed

air. U. S. Patent No. 2,618,064, Nov. 18, 1952.

3. Straub, Ambrose W. Improvement in instruments

for giving motion to tools. U. S. Patent No. 154,569, Sept. I. 1874.

4. Wilkerson, Basil M. Improvement in dental en- gines. U. S. Patent No. 189,409, April 10 1877.

5. Laurence, Henry. Improvement in dental engines.

U. S. Patent No. 213,909, April I, 1879.
326 • THE JOURNAL OF THE AMERICAN DENTAL ASSOCIATION

The comparative size of the turbine

and of the head of a gear-type contra- angle handpiece (S. S. White) is shown

in Figure 4. The turbine has six notched blades

fixed to the shaft and measures 7.5 mm.

in diameter and 4.8 mm. in height. In

the design shown in Figure 5, stainless

steel ball bearings are used. A thin metal

disk to direct the flow of water away

from the bearings is mounted on the shaft

at each end of the turbine (Fig. 5, A).

The radial load on the shaft is taken by

the two stainless steel ball bearings (Fig.

5, B) placed at each end of the turbine. The upper bearing fits into a collar (Fig.

5, C) while the lower bearing is a press

fit into the end plate (Fig. 5, D). The

axial thrust is taken on a sapphire bearing (Fig. 5, E) mounted in the cap screw

(Fig. 5, F )

.

Various combinations of plastic bearing materials have been used in place of the stainless steel ball bearings, which

proved rather noisy and harsh in this application. Figure 6 shows an expanded

view of a turbine assembly using plastic

bearings, A, and journals, B. Nylon, Teflon, Rulon and stainless steel have been

used in combinations. The three plastic

materials have a low coefficient of fric- tion and were not lubricated while in

use. Bearings of these materials are much

quieter in operation and give more sat- isfactory results than bearings of any

other material used in this application

to date. The turbine shaft (Fig. 6, C) fits into the hollow shaft of the cutting instru- ment. A spring key attachment (Fig. 6, D) fits into a keyway in the instrument

shaft and secures it to the turbine shaft. The centrifugal force developed by the

rotating shaft causes this spring to lock the cutting instrument in place. When

the shaft is still, however, it is relatively easy to attach and remove the cutting in- struments.

Fig. 2 • Pumping mechanism consisting of constant-volume pump, A, operated

by one-half horsepower motor, B. Fluid enters pump from reservoir through

line, C, and leaves pump through line, D, to handpiece. Line, E, returns the spent fluid to reservoir. Foot pressure on tube, F, closes switch, G, which opens

solenoid valve, H, so that fluid is permitted to by-pass handpiece through line , /
NELSEN—PELANDER—KUMPULA . . . VOLUME 47, SEPTEMBER 1953 • 327

Fig. 3 • Comparative size of hydraulic handpieces, A and B, and conventional

contra-angle handpiece attached to straight handpiece and wrist joint, C

OPERATIVE CHARACTERISTICS

Since the only rotating or moving ele- ment in this contra-angle handpiece is the turbine to which the cutting tool is directly attached, the vibrations due to the belt-and-gear driving mechanism of the conventional contra-angle are eliminated. The fluid passing over the turbine

and shaft acts as a coolant. In spite of the high rotational speed developed,

overheating of the instrument is pre- vented. Therefore, this instrument has a

distinct advantage over the gear-type

instruments which heat excessively at high speeds.

The rotational speed of the instrument

depends on the amount of fluid which

is forced past the turbine in a given

amount of time. Also, the amount of torque developed by the turbine is proportional to the pressure of fluid passing

through it. With the pumping mechanism previously described, 1.6 gallons per minute forced through the turbine pro- duced a speed of 61,000 revolutions per minute. At this speed and with a 2 ounce

radial load used against a Densco dia- mond instrument SC-6 (7 mm. in di- ameter and tapered from 1 mm. thick at the hub to a knife edge), an average of

4.2 mg. of tooth structure was removed

per second from fresh extracted teeth. During this test it was found that a load

of 3/2 ounces on this diamond tool stopped it from rotating. This, in effect, means that if the operator were to press too heavily on the cutting tool or if the

tool were to catch or bind in some manner that would ordinarily be hazardous,

this instrument automatically would stop

rotating. For instance, a belt-driven dia- mond-cutting instrument will entangle

in a rubber dam or will cut through it if

it makes contact with it while it is rotating. The same cutting instrument

mounted in the hydraulic handpiece will not engage the rubber dam to any extent because there is insufficient torque de- veloped to enmesh or tear the rubber. When disks of larger diameter are used, the effective torque is lessened. This is not a disadvantage, however, because

with the increased diameter of the disk, there is an increase in the peripheral

speed of the tool which compensates for the reduced torque. The actual cutting

rate remains approximately the same. With the larger disks an extremely light pressure on the tool is used. Also, the

disks will stop rotating when the cutting loads are light, and in this way the safety feature of the instrument is maintained.

Any of the cutting instruments used, in-
328 • THE JOURNAL OF THE AMERICAN DENTAL ASSOCIATION

eluding the % inch diamond disks, can

be stopped while rotating at top speed

(61,000 rpm) by placing a finger abruptly

against the edge of the disk. The disk will stop immediately and will not injure the

finger. As soon as the disk is released, the

instrument will start rotating again. If excess pressure is exerted on the cutting

tool and then released, no manipulation

or adjustment of the instrument is re- quired to start or stop the turbine. The use of city tap water (about 40

pounds per square inch) to operate the turbine does not produce the same amount of torque and rotational speed

that occur when the pumping unit is used. With tap water pressure one gallon

of water per minute is forced through

the instrument, and a rotational speed

of 35,000 rpm and proportionally less torque are produced. The cutting ability of the instrument is considered not adequate when city water pressure is used.

It is extremely important that all the

cutting tools used at these high speeds be

accurately balanced. Any eccentricity or unbalance is apparent, as it causes vi- bration of the instrument and immediately produces an uneven wear on the

cutting tool. The diamond instruments

which have been used with this contraFig. 4 • Head of conventional contra-angle

handpiece, A, compared with turbine as- sembly, B

Fig. 5 • Turbine assembly using stainless steel ball bearings. Thin disks, A, direct water away

from ball bearings, B, which are fitted into

collar, C, and end plate, D. Sapphire axial thrust bearing, E, is mounted in cap screw, F

Fig. 6 • Turbine assembly using plastic bearings, A, and plastic journals, B. Turbine shaft, C, contains spring key attachment, D, which

engages keyway on shaft of cutting tool

angle handpiece show little sign of wear

even though many old amalgam fillings

have been removed with each of them.

Silicon carbide mounted points and disks of medium grit used in this instrument were found to be highly efficient. The

fine grit points, however, did not cut

well. The silicon carbide points are less expensive than the average diamond in- struments. Although they wear faster than the diamond tools, they retain their cutting ability as they wear, whereas the

diamond instruments lose efficiency as soon as they begin to wear. With this handpiece the preparation of teeth in the mouth can be performed with

considerably less strain on the operator
NELSEN—PELANDER—KUMPULA . . . VOLUME 47, SEPTEMBER 1953 • 329

because of the extremely light pressures

used. The cutting tools do not climb or

roll out of the cavity as do those used in the slower-rotating instruments. The

greatest advantage of the instrument is in the making of extracoronal preparations. For intracoronal preparations, the

instrumentation advocated by Ingraham

and Tanner is most suitable.6 The ex- tremely rapid cutting rate of this instru- ment makes it uniquely suited for occlusal equilibration procedures.

PATIENT REACTION

The patient’s reaction to this type of in- strumentation has been varied. None have

remarked that this method of cutting

teeth was more unpleasant than the usual

method. The speed at which tooth struc- ture can be removed and the short time

that the instrument must be used are

factors which enhance its acceptance by

the patient. An insufficient number of patients have had cavities prepared with

the instrument to draw a definite conclusion regarding the patient reaction to the actual cutting sensations.

LIMITATIONS OF THE INSTRUMENT

Because of the extremely rapid rate of

cutting, this contra-angle handpiece does

not lend itself to every type of rotary

instrumentation. It is doubtful if the instrument can be used effectively with

diamond or silicon carbide cutting tools smaller than a no. 560 steel bur (1.8 mm.

in diameter) . The various types of car- bide burs did not cut tooth structure with any practical efficiency when used

in this instrument. It is believed that the design of the cutting blades of all burs

will have to be modified before they can

be used at the rotational speeds of this instrument.

SUMMARY

1

.

The advantages of using increased

rotary speeds in tooth-cutting procedures

have long been recognized.

2. The operational hazards attendant on the use of high rotational speeds with

conventional belt-and-gear-propelled cut- ting tools have been considerably re- duced by the development of a hydraulic

turbine contra-angle handpiece.

3. Rotational speed of 61,000 rpm

with considerably higher cutting rate and

lower cutting pressure than can be at- tained with conventional instrumentation is now possible.

4. The mechanical difficulties of excessive vibration and heating characteristic of the gear-type contra-angle

handpiece have been eliminated by positioning the turbine in the head of the new instrument. Therefore, no gears are

required.

5. Milling cutters such as steel and

carbide burs of current design do not function well in this hydraulic headpiece.

Grinding tools such as silicon carbide

and diamond points and disks used with

the handpiece cut with exceptional effi- ciency.

6. Even though some adjustment in the dentist’s operative procedures is necessary, the instrumentation is basically the same as his present methods. Therefore, the use of this equipment will re- quire no special training.

6. Ingraham, Rex, and Tanner, H. M. The adaptation

of modern instruments and increased operating speeds

to restorative procedures. J.A.D.A. 47:311 Sept. 1953.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT KBS REPORT

0708-20-3224 December 31, 1953 3085

CUTTING CHARACTERISTICS OF DENTAL BURS AS

SHOWN BY HIC-E-SPEED MICROPHOTOGRAPHY

Jack L. Hartley*

Donald C. Hudson**

W . T . Swee ney* *

W. P. Richardson**

* Guest Worker, Major U. S. Air Force Dental Service, Dental Research

Section, Division of Organic and Fibrous Materials.

** Guest Worker, Col. U. S. Air Force Dental Service, Dental Research

Section.

*** Chief, Dental Research Section, National Bureau of Standards.

**** Motion Picture Producer, Office of Scientific Publications,

National Bureau of Standards.

This work is a part of the Dental Research Program conducted at the National

Bureau of Standards in cooperation with the American Dental Association, the

Army Dental Corps, the Air Force Dental Service, the Navy Dental Corps and

the Veterans Administration.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, reprinting, or reproduction of this report In any form, either in whole or In part, Is prohibited

unless permission is obtained In writing from theOffice of theDlrector, National Bureau of Standards, Washington

25, D, C, Such permission Is not needed, however, by a Government agency for which a report has been specifically prepared If that agency wishes to reproduce additional copies of that particular report for Its own use,

GUTTING CHARACTERISTICS OF DENTAL BURS AS

SHOWN BY HIGH-SPEED MI GROPHOTOGRAPHY

Abstract

High-speed still and motion picture microphotography has

been used in the study of the mechanism of cutting of dental

burs. Two-microsecond flash still photographs were made of

dental burs cutting glass and Bakelite cast resin at 5 >000 rpm

with a load of 300 grams. Slow motion studies employing a highspeed motion picture camera at 3,000 frames per second were made

of steel and carbide burs at 2,500 rpm with a load of 300 to 500

grams, cutting human enamel, dentin and laboratory test materials.

The action occuring in one second was extended to approximately

three minutes for study by projection at 16 frames per second.

These methods permitted dynamic observations hitherto not recorded, of clogging, intermittent cutting, and eccentric rotation.

Selected frames from a cycle cf rotation were enlarged and printed

from negative l6mrn film for detailed studies. A slight superiority of carbide over steel burs was noted when cutting human enamel

and dentin. It is believed that this results from the extreme

hardness of the tungsten carbide instruments and superior workmanship. Data obtained by conventional laboratorj" test procedures based on weight loss were in accordance with the motion

picture studies.

1. INTRODUCTION

In the study of rotating dental instruments being carried on at the National Bureau of Standards, temperatures and cutting rates of revolving dental burs

have been measured (l) and work on the development of suitable tests for diamond

abrasive instruments is in progress. Considerable work has been accomplished

by other investigators in the study of the mechanism of milling and in the design

and production of industrial cutting tools (2). The characteristic forms of chips

have been classified. Charts are available for low and high-speed cutting, for

clearance and rake angles, and for correct speed and depth cf cut which give

optimum performance in a specific material. Little is known, however, of the

cutting action involved in the use of the dental bur for the cavity preparation

of a human tooth. A better understanding of this mechanism could logically

lead to improvement in the design and use of rotating dental cutting instruments.

To obtain basic information on the manner of performance of dental burs, a program of high-speed photography of the instruments as they rotate under load in

contact with human enamel, dentin, and other test materials was undertaken.

2. PROCEDURE AND RESULTS

Still photographs were made first, employing a General Radio nT lieroflashtf unit which emitted an intense light of 2-millionths of a second duration and

which effectively stopped the action of a dental bur rotating at 5,000 rpm„

This light was focused by means of a condensing lens system which increased

the effective illumination by concentrating the light on the bur head. Four

by five inch panchromatic film, ASA speed 64 Tungsten, was exposed by the openflash method in an American Optical Company photomicrographic camera equipped

with a Rausch and Lomb Micro-Tessar F 4.5 lens (Figure l). The film was developed four and one-half minutes in DK-19 at 68 degrees F.

Figures 2 and 3 are typical of the photographs obtained using this technique. Figure 2 shows a Number 559 tungsten-carbide bur cutting a glass microscope slide. The load was 300 grams and speed of rotation was 5,000 rpm. It can be seen that a pulverizing action rather than a chip formation was taking

place. The powdered-glass debris accumulated on the entrant side of the exea=

vation. This phenomenon demonstrates one reason for clogging of the bur, in

that the debris is not thrown out at the exit side but is carried around to the

entrant side, where it is occasionally picked up and carried back through the

excavation by the bur.

Figure 3 shows a Number 559 tungsten-carbide bur cutting a test material,

Bakelite cast resin BI-44-992, used in previous work (l), as a substitute for

dentin. Speed of rotation was 5,000 rpm with a load of 200 grams. The true

chip formation seen here, would be expected when a milling-type cutter is employed in its proper medium. The chips accumulated on the exit side of the

excavation, apparently because they were too large to be carried around by the

vortex of air created by the revolving bur.

The single exposure, high-speed flash method of photographing a rotating

dental bur, did not permit sequential studies of the cutting action of any

particular blade. It was decided, therefore, to employ a high-speed motion

picture camera, which would photograph the cutting action occurring in one second at a rate of 3,000 exposures per second. The film, when projected at

normal speed, would allow about three minutes for the study of this action.

These studies were made using the Eastman Kodak 16mm high-speed motion

picture camera. Adequate magnification was obtained through the use of extension tubes 8 inches long, in combination with an F 4.0, 6-inch telephoto lens

and a 2-plus Proxar auxiliary lens. This combination permitted a working distance between lens and bur of about six inches. The camera speed of 3,000

frames per second resulted in an exposure time of 1/15,000 second per frame.

This short exposure time, combined with the reduction in effective aperature

of the lens to F 9.3, due to the extension tubes ", established the requirement

for a high intensity light source. Eleven 750 watt focusing photoflood bulbs

were arranged on a ring above the subject (Figure 4 ) . An internally chromeplated ring about the bur chuck, aided in concentrating illumination on the

The formula EA -if applies; EA - effective aperture, K = lens-film

distance, f = marked lens aperture, and F ~ focal length of the lens. (
• . • •'

/

i . ,

'
subject. Reversal type 16mm motion picture film, ASA 64 Tungsten, was used when

photographs were made for projection study. Negative film of the same characteristics was employed when prints were desired.

Although the action of the burs is best seen by observation of the projected

motion picture, information can also be obtained from examination of selected

prints, prepared from individual frames of the l6ram motion picture film. Figure

is a print from a motion-picture film study of a Number 55° steel bur rotating

at 2,500 rpm, with a load of 300 grams against human tooth enamel. Very little

cutting is evident. The debris is so finely powdered that it is barely visible.

A complete rotation cycle of a bur turning at 2,500 rpm, photographed at

3,000 frames per second could be observed in approximately 74 frames, "'very

fourth frame from a rotation cycle was enlarged and printed. Key pictures chosen

from these studies are shown in Figure 6. Row A of Figure 6 illustrates the

sequence of events from left to right as a Number 559 eight-bladed tungstencarbide bur cuts human enamel. The force applied in all cases was 300 grams.

This bur was 0.003 inch eccentric. It can be seen that only four blades actually cut, and that in one phase of its rotation cycle, the bur was not in contact

with the tooth structure. Some of the debris is finely powdered and follows the

vortex of air about the bur, and is seen to be carried back into the excavation.

The bulk of the cut material, however, is being deposited at the exit side, where

it is seen to accumulate. Very little difference between the cutting effectiveness of steel and carbide burs is evident in these pictures (Figures 5 and 6-A),

taken during the first five minutes of use of each instrument. The effective

life of a carbide bur will, however, extend over a much longer period of time

since the extreme hardness of tungsten carbide permits burs of this material to

cut hard substances with less dulling of the blade edges. Superior workmanship

noted on microscopic examination of the carbide burs (Figures 7 and 8) may be a factor affecting the cutting ability of these instruments.

Row B of Figure 6 shows the key pictures in a rotation cycle of a Number 559

six-bladed steel bur cutting human dentin. This bur was 0.002 inch eccentric.

Note that there is definite chip formation, but that the chips are fragmented

and tend to follow the instrument as it rotates. Eccentricity is not evidenced

by lack of contact of the instrument with the dentin at any point in this cycle,

possibly due to flexure of the steel bur, however; a study of the debris reveals

that the cutting is intermittent.

Row C in Figure 6 illustrates the cutting cycle of a Number 559 tungstencarbide bur in human dentin. The chip formation is superior to that seen in

the case of a steel bur cutting the same material. These larger chips tend to

be thrown out of the operating field. The first photograph in the series shows

the bur in a phase of its cycle of cutting in which a great amount of dentin

has been removed and is leaving the excavation. The blades in cutting posit “on

at this stage are ineffective, since the instrument is 0.002 inch eccentri •„ The fourth photograph in this series shows the bur after rotation of 180° has

occurred; it can be seen that the blades now in the excavation are cutting

effectively. This intermittent cutting action was observed in all burs studied,

which were over 0.001 inch eccentric, but not observed in those which were less

than 0.001 inch eccentric. Increasing the load to 500 grams did not serve to

bring all blades into play, and only accentuated the intermittent cutting action.
'

.
- 4 -

Thif study would indicate that calculations of bur efficiency based on the amount

of material removed per blade per revolution would be of little value as all

blades of a bur do not remove equal amounts of material during a cutting cycle.

The relative cutting rates of steel and carbide burs in human enamel and

dentin are shown in Figure 9. Calculations were based on weight of material removed in one-minute periods of cutting at 2500 rpm and with a load o'" "00 grams.

These data showing slightly more effective cutting by the carbide burs are in

accordance with the action demonstrated by the motion picture studies.

3 . CONCLUSIONS

High-speed motion picture photography of dental burs under actual operating conditions permits detailed, slow-motion studjr of the action of each bur

blade throughout the cutting cycle. In combination with conventional laboratory test methods it provides a new approach to the study of rotating dental

instruments.

Under the conditions of this experiment, a slight superiority of carbide

burs over steel burs was noted when cutting human enamel and dentin. This

difference is possibly due to the greater hardness and to the higher quality

of X\

rorkmanship and finish of the carbide burs.

laboratory data obtained by conventional methods alone do not give a true

picture of the cutting efficiency of individual bur blades. Fhotographic evidence shows that, with very few exceptions, all burs used in these experiments

were intermittent cutters, due to eccentricity and irregularities in blade formation.

High-speed motion picture photography should prove of considerable value

in the study of experimental burs, since by this method, new designs can be

quickly evaluated dynamically, prior to complete investigation requiring many

hours of laboratorjr procedure.
2 -

- - .

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- 5 -

BIBLIOGRAPHY

1. Hudson, Donald C., and Sweeney, W. T., Temperatures

Developed in Rotating Dental Instruments, J. Am,

Dental Assn. (To be published February 1954).

2. Kent, William, Mechanical Engineers Handbook, 12th ed.,

Vol 2, Sec. 23, p. 46 (John Wiley & Sons, Inc., New York,

New York, 1952)

.

3. Lester, Henry M., Photo-Lab Index, 12th ed., Vol 2,

Sec. 17, p. 12. (Morgan & Lester, Publishers, New York,

New York, 1952)

.

COUNTER-CLOCKWISE

AS

VIEWED

BY

THE

READER.

AT

LEFT:

CARBIDE

BUR

CUTTING

GLASS,

5,000

RPM,

300

GRAM

LOAD;

RIGHT:

CARBIDE

BUR

CUTTING

A

DENTIN

SUBSTITUTE

(BAKELITE

CAST

RESIN),

5,000

RPM,

200

GRAM

LOAD.

FIG

p.

PRINT

FROM

A

SINGLE

FRAME

OF

l6MM

HIGH

SPEED

MOTION

PICTURE

FILM

OFA

STEEL

BOR

ROTATING

AT

2,500

RPM

CUTTING

HUMAN

ENAMEL,

300

(31AM

LOAD.

# 559 CARBIDE BUR CUTTING HUMAN ENAMEL

#559 STEEL BUR CUTTING HUMAN DENTIN

#559 CARBIDE BUR CUTTING HUMAN DENTIN

THE INSTRUMENTS ABOVE WERE ROTATING AT A SPEED OF 2,500 RPM AT A

LOAD OF 300 GRAMS.

FIG 6. KEY PRINTS FROM l6MM MOTION PICTURE FILM SLOW MG']1 ION

STUDIES. THESE WERE SELECTED TO SHOW A CYCLE OF ROTATION

OF EACH INSTRUMENT.

FIG

8.

NEW,

UNUSED

CARBIDE

BUR;

NOTE

THE

CLEAN-CUT

EDGES

16

54

12

50

8

6

4

2

0

MATERIAL REMOVED

BY STEEL AND CARBIDE BURS

2,500 R PM - 300 GM LOAD

FOR EACH OF FIVE

ONE MINUTE INTERVALS

1 ST MINUTE

2 ND MINUTE

3 RD MINUTE

4 TH MINUTE

] 5 TH MINUTE

NO. 558 GR0SS-CU7FISSURE BUR

NBS.

CARBIDE

IN DENTIN

STEEL

IN DENTIN

CARBIDE

IN ENAMEL

STEEL

IN ENAMEL

FIG 9* RELATIVE CUTTING RATES OF STEEL AND CARBIDE BURS IN HUMAN ENAMEL AND DENTIN DETERMINED BY CONVENTIONAL

LABORATORY METHODS.

THE NATIONAL BUREAU OF STANDARDS

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Information on the Bureau’s publications can be found in NBS Circular 460, Publications of

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found in NBS Circular 483, Testing by the National Bureau of Standards (25 cents). Both are

available from the Government Printing Office. Inquiries regarding the Bureau’s reports and

publications should be addressed to the Office of Scientific Publications, National Bureau of Standards, Washington 25, D. C.