1991_laird - ultrasound in dentistry part 1

8/11/2019 1991_Laird - Ultrasound in Dentistry Part 1

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14

J.

Dent. 1991; 19:

14 17

Review

Ultrasound in dentistry. Part l-

biophysical interactions

W R E Laird and A D Walmsley

Department of Restorative Dentistry Dental School Birmingham

UK

ABSTRACT

Ultrasound has many applications in the field of dentistry. H owev er, it is only recently that the applications

and effects of its physical p roperties have been rationalized and understood. Ultrasound may be generated by

either magnetostriction or piezoelectricity, although the former is more commonly used in dental applications.

Interactions of ultrasound with biological tissues may be caused by either thermal o r mechanical mechanism s.

The mechanical forces produ ced may be a result of cavitation, acoustic microstreaming and radiation pressure

forces. An understanding of these interactions alloys a more knowledgeable appreciation of the effectiveness,

safety limitations and rationale of dental ultrasonic instrumentation.

KEY WORDS: Ultrasound, Dentistry, Review

J. Dent. 1991; 9: 14-l 7 Received 26 July 1990; accepted 1 August 1990)

Correspondence should be addressed to: Professor W. FL E. Laird, Department of Restorative Dentistry, Dental

School, St Chads Queensway, Birmingham 84 6NN. UK.

INTRODUCTION

Ultrasound is sound which has a frequency of 16-20 kHz

and as such is unable to be detected by the human ear. Its

use in dentistry extends over three decades, although it

is only more recently that the application and effects of

its physical properties have been rationalized and

understood.

One of the earliest reported uses of ultrasound in

dentistry was in the form of an ultrasonic drill develop ed

for cavity preparation in human teeth Ca tuna, 1953 )

based on pioneering work by Balamuth 1963). The

ultrasonic drill opera ted at a frequency of 29 kHz and

required an abrasive slurry of aluminium oxide to assist

the proce ss of cutting or grinding enamel and dentine

Postle, 1958 ). Although it was possible to use this

instrument without recourse to local anaesthesia, it was

some wha t cumb ersome with a relatively slow action. In

addition it required very efficient suction ap paratus to

remov e the volumes of slurry used in the grinding proces s.

Neve rtheless it did receive favourable comm ent Oman

and Applebaum , 1954 ; Nielsen et aI., 1955 ), particularly in

respect of the low loads emp loyed during cutting and the

limitation of traumatic effects to the dental pulp Postle,

1958 ). In spite of this, ultrasonic cavity preparation never

became popular, being superceded by the much more

@

991 Butterworth-Heinemann Ltd.

0300-5712/91/010014-04

effective and efficient high speed rotary drills Street,

1959).

The potential use of ultrasound in dentistry how ever

was still recognized by Zinner 1955 ), who suggested that

the use of a modified ultrasonic instrument in conjunc-

tion with a water coolant might be effective in the removal

of plaque and calculus from human teeth. This instrument

used a probe design in the form of a modified scaling tip

based on that used with hand scaling instruments. The

scaling procedure using such an instrument was demon-

strated by Johnson and Wilson 1957 ) and the technique

has becom e established as a rapid and simple alternative

to hand instrumentation and now finds wide clinical

use.

The pattern of oscillation of the probe tip was also

recognized as having potential use in endodontic therapy

in the cleaning and preparation of root canals prior to

obturation Martin, 1976 ) and this has given rise to the

technique of endosonics. Other uses of ultrasound have

been the removal of debris from instruments prior to

sterilization, cleaning of dentures by immersion in an

ultrasonic bath, debonding of restorations Walm sley et

al., 1989 a, b), the treatment of disorders of the temporo-

mandibular joint and the detection of early caries Ng et

al., 1988).


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Laird and Walm sley: Ultrasound in dentistry

15

In order to appreciate the action of ultrasound in

dentistry however, it is necessary to have som e apprecia-

tion and understanding of its physical properties, together

with the possible biological effects on tissues.

ULTRASONIC WAVES

Ultrasonic waves are essentially a mechanical propagation

of energy through a suitable medium . The w aves occur

when particles of the medium are energized causing them

to

vibrate and transfer energy to adjacent particles, the

energy being transmitted in the form of a wave. In fluids

and solids, wave propagation may be either longitudinal

or transverse. In the former, vibration occurs in the

direction of the travelling wave whilst in the latter the

particle displacement is at right angles to the direction of

propagation. Transverse waves can travel efficiently only

in solids, where there are strong forces of attraction

between adjacent particles to ensure energy transfer. In

fluids however particles slide past each other with little

resistance and the energy is dissipated within the fluid. In

the hum an body hard tissues can transmit both longi-

tudinal and transverse waves, compared to soft tissues

which can only transmit longitudinal waves, with the

transverse waves being dissipated as heat.

Wh en an ultrasound wave encounters an interface

between different media, as will occur w ith the tissues of

the teeth for instance, part of it will be reflected back into

the original m edium and the remainder reflected into the

new medium at a velocity which is dependent upon the

transmissio n properties of the medium . The ratio of the

reflected to refracted waves is termed the acoustic

impedance, and there is greater energy transfer acro ss

bound aries where acoustic imped ances are similar. Large

impedance mism atches occur between solids to liquids to

gases and consequently little energy is transferred.

GENERATION OF ULTRASOUND

For clinical purposes ultrasound is generated by trans-

ducers which convert electrical energy into ultrasonic

waves. This is usually achieved by magnetostriction or

piezoelectricity, with the former being m ore commo n in

the generation of the low frequency ultrasound oscillations

used in dentistry.

Magn etostrictive devices undergo changes in their

physical dimension when a magnetic field is applied to

them. This is usually achieved by placing a ferromagnetic

stack within a solenoid through which is passed a direct

current. Th is produces stresses leading to a change in

shape of the material. W hen an alternating current is

passed throug h the solenoid the stack will then change its

shape at twice the frequency of the applied magnetic field.

Magn etostriction with a laminated ferromagnetic stack is

used commonly in the design of ultrasonic scaling

instrumen ts as it is a robust and easily manu factured

system.

The piezoelectric system is based on the fact that certain

crystalline structures such as quartz will be subject to a

shape change when placed within an electrical field. If an

alternating voltage at an ultrasonic frequency is applied

across a piezoelectric crystal, it will result in an oscillating

shape change of the crystal at the frequency applied. This

is then pas sed onto the working tip. Piezoelectric genera-

tors are more efficient at frequencies in the MH z rather

than the kHz range, although some have been developed

for use in dentistry. H owever, the crystalline structure has

poor shock resistance and such instrumen ts are more

fragile than their magnetostrictive counterparts.

Wh en the input energy to a system is in phase w ith the

natural frequency of oscillation of that system it is said to

be in resonance. This will maintain or increase the

amplitude of oscillation of a probe tip and allow it to work

at maxim um efficiency, if the tip and the generating stack

are cut to resonant length, w hich is usually one-half

wavelength or multiples thereof.

BIOLOGICAL EFFECTS OF ULTRASOUND

Wh en an ultrasonic wave passes through a biological

system changes may occur in that system. These may be

due to heat, cavitational activity, acoustic m icrostreaming

or radiation forces. All are important when co nsidering

the use of ultrasonic instrumentation in dentistry.

THERMAL EFFECTS

As a wave of ultrasound passes through tissues its energy

is reduced and is dissipated as heat, leading to an

elevation of tissue temperature. The effects of this on the

tissues are dependent upon the size of temperature rise,

the time over which it is maintained and the thermal

sensitivity of the tissue. In most tissues the normal

physiological response will be an alteration in the blood

flow in the region due to reflex relaxation of the arterioles.

The resultant increase in blood flow through the area will

tend to control heating effects within a limited increase in

temperature, with a temperature rise of less than 1 C

resulting only in a minor overall increase in local

metabolic rate. Excessively high temperatures however

will lead inevitably to tissue dam age.

CAVITATION

Cavitational activity in relation to ultrasound encomp as-

ses a continuous spectrum of bubble activity in a liquid

medium . It ranges from gentle linear p ulsation of gas-

tilled bodies in low amplitude sound fields stable

cavitation) to violent and destructive beh aviour of vapour-

filled cavities transient cavitation) in high amplitude

sound fields Flynn, 1964; Nyborg, 1977; Williams, 1983).

The energy generated within these bubbles may result in

shock waves or hydrodynamic shear fields which may

disrupt biological tissues, and it is the production of these


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J Dent 1991; 9: No 1

3

A

b

4

Fig. 7. Diagrammatic representation of possible bubble

collapse. a, A free bubble collapsing to smaller fragments

and radiating shock waves. b: 1, bubble on a solid surface; 2,

undergoing deformation; 3, producing a high velocity liquid

jet; 4, jet pierces bubble and damages solid surface.

large disruptive forces which are of use in the removal of

plaque and calculus during ultrasonic scaling Balamu th,

1963).

The occurrence of cavitation requires the presence of

gaseous bodies or bubb les in the medium , wh ich have

been termed cavitation nuclei William s, 19 83). In the

presence of an ultrasound field a bubble will grow and will

undergo breathing pulsation in response to the applied

pressure oscillations set up by the field Nyborg, 1977). As

the bubble pulsates transverse waves are set up on its

surface Lamb, 1945)which become distorted and unstable

as the ultrasonic amplitude increases. Microbubb les will

occur around the original bubble Nyborg and Rogers,

1967) and will act as new sites for cavitational activity

Nepp iras and Fill, 1969) . Formation of microbub bles is

associated with the onset of transient cavitation, w here the

bubbles show a collapse phenomenon Flynn, 1964) with

the temperature of the gas in the bubble reaching

thousan ds of degrees Celsius and several thousand

atmospheres of pressure Noltingk and Neppiras, 1956).

The demandin g effects of transient cavitation are due to

the shock waves radiated during the final stages of bubble

collapse Nyborg, 1977) or high velocity liquid jets Fig. I)

from non-linear motions of the bubble face Crum , 1982).

At low ultrasound frequencies in the order of 20-40 Wz

growth of micronuclei and subsequ ent transient cavita-

tion occur readily Esche, 1952).

Cavitation occurring in hum an blood can result in a

thrombogen ic effect Chater an d William s, 198 2) and

cause lysis of erythrocytes and platelets. This may explain

reduction in haemorrh age when using ultrasonic surgical

instruments and dental scalers Ewen, 1960; Goliamina,

1974).

ACOUSTIC MICROSTREAMING

The rapid cyclical volume pulsation of a gas bubble

results in the formation of a complex steady state

streaming pattern within the liquid close to the bubble

surface. This is termed acoustic microstreaming and can

be demon strated around an oscillating so lid cylinder

within a fluid or a stationary cylinder within an oscillating

fluid Fig. 2).

The dimensions of the patterns demo nstrate a rapid

rate of change of streaming velocity with distance

Nyborg, 1977) . Therefore although the velocities them-

selves are only of the order of a few centimetres per second

William s, 1983) the gradients due to the rate of change of

velocity will produce large hydrodynamic shear stresses

close to the oscillating object i.e. probe or gas bubble)

which may disrupt or damag e biological cells or tissues.

These most probably contribute to the efficiency of

endosonic instrumentation Lumley et al., 1990 ). Acoustic

microstreaming may also result in the disruption of blood

flow and cells such as hum an platelets exposed to probes

operating at 20 kHz the level used in dentistry). At higher

Fig. 2. A theoretical prediction of the acoustic microstreaming

field generated around a solid cylinder oscillating within a

stationary fluid modified from Holtzmark et al., 1954).


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Laird and Walmsley: Ultrasound in dentistry

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amp litudes gelatinous aggregates of platelets can form as

emboli resulting in possible blood vessel occlusion

Walmsley et al., 1987).

RADIATION FORCES

Anymedium or object in the path of an ultrasonic beam is

subjected to a radiation force which tends to push the

material in the direction of the propagation wave Wells,

1977). This force is small, b ut in a standing wave field may

be enhanced and act over a short distance, so that dense

particles in the medium are driven to regions of maxim um

acoustic pressure amplitude. In blood vessels this may

cause local aggregation of blood cells leading to stasis

Dyson et

a l .

1968 ). Radiation forces may also enhance

cavitational activity within a standing wave field Nyborg,

1977).

OCONCLUSION

The physical prop erties and biological effects of ultra-

sound are of importance in dentistry where low frequency

ultrasound instrumentation is used. From a theoretical

extrapolation this may be both beneficial and damagin g.

The understan ding of the basis of ultrasound and

methods of its clinical use allows us to consider more fully

the effectiveness, safety limitations, and rationale of

dental ultrasonic instrumentation.

References

Balamu th L. 1963 ) Ultrasonics and dentistry. Sound 2, 15-19.

Catuna M. C. 1953 ) Sonic energy. A possible dental

application. Preliminary report of an ultrasonic cutting

method. Ann. Dent. 12,256-260.

Chater B. V. and W illiams A R 1982) Absence of platelet

damage in vivo following the exposure of non-turbulent

blood to therapeutic ultrasound. Ultrasound Me d. Biol. 8,

85-87.

Crum L. A 1982) Acousti c Cavitat ion 1982 Ultrasonics

Symposium, IEEE, San Diego, USA

Dyson M., Pond J. B., Joseph J.

et al .

1968) The stimulation

of tissue regeneration by means of ultrasound. J. Clin. Sci.

35,273-285.

Esche R. 1952) Untersuchungen der schwingungs-Kavitation

in flussigkeiten. Acusifica 2, 208-2 18.

Ewen S. J. 1960) Ultrasound and periodontics. J. Periodonfol.

31, 101-106.

Flynn H. G. 1964 ) Physics of acoustic cavitation in liquids.

In: Mason W. P. ed.), Physical Acoustics vol. 1 B. New

York, Academic, pp. 57-172.

Goliamina I. P. 1974 ) Ultrasonic surgery. In:

Proceedings

of th e Eight Infema tional Congress on Acoustics. Guildford,

IPC Science and Technology Press, pp. 63-69.

Holtzm ark J., Johnson I., Sikkeland T. et al . 1954) Boundary

layer flow near a cylindrical obstacle in an oscillating

uncompressional fluid. J. Acou sf. Sot. Am. 26 , 26.

Johnson W. N. and W ilson J. R. 1957) Application of the

ultrasonic dental unit to scaling proced ures. .I Periodonfol.

28,264-271.

Lamb H. 1945) In: Hydrodynamics. New York Dover

Publications.

Lumley P. J., Walmsley A. D. and Laird W. R. E. 1990)

Streaming patterns produced around endosonic files. Znf.

Endodon f. J. in press).

Martin H. 1976 ) Ultrasonic disinfection of the root canal.

Oral Surg. Oral Med. Oral Pafhol. 42,92-99.

Nepp iras E. A and Fill E. E. 1969 ) A cyclic cavitation

process. J Acousf. Sot. Am. 46, 1264-1271.

Ng S. Y., Ferguson M. W. J., Payne P. A.

et al .

1988)

Ultrasonic studies of unblemished and artificially

demineralised enamel in extracted human teeth: a new

metho d for detecting early caries. J. Dent. 16, 201-209.

Nielsen A G., Richards J. R. and W olcott R B . 1 955)

Ultrasonic dental cutting instrument: I. J. Am. Dent. Assoc.

50, 392-399.

Noltingk B. E. and Neppiras E. A 1950) Cavitation produced

by ultrasonics. Proc. Phys. Sot. [Land.] B6 3, 674-6 85.

Nyborg W. L. 19 77) Physical Mechanisms

for

Biological

Eficfs of Ultrasound. 7 8-802 6, HEW Publications FDA].

Nyborg W. L. and R ogers A 1967) Motion of liquid inside

a closed vibrating vessel. Biofechnol. Bioeng. 9 , 235-2 41.

Oman C. R. and Applebaum E. 19 54) Ultrasonic cavity

preparation II. Progre ss rep ort. J. Am.

Dent . Assoc.

50,

414-417.

Postle H. H. 1958 ) Ultrasonic cavity preparation. J. Prosfhe f.

Dent 8, 153-160.

Street E. V. 1 959) Critical evaluation of ultrasonics in

dentistry. .I Prosfhef. Dent 9, 132-141.

Walmsley A D., Laird W. R. E. and Williams A R. 1987)

Intra-vascular thrombo sis associated with dental ultrasound.

J. Oral Pafhol. 16, 256-259.

Walmsley A. D., Jones P. A, H ullah W. et al . 1989a)

Ultrasonic debonding of comp osite retained restorations.

Br. Dent. J. 166,290-294.

Walmsley A. D., Lumley P. J. and Laird W . R. E. 1989b)

Ultrasonic instruments in dentistry III. Remov al of

restorations by ultrasonics.

Dent. Update

15, 401-404.

Wells P. N. T. 1977) Biomedical Ultrasonics. London,

Academic.

Williams A . R. 19 83) Ul tr asound: Biological Effecfs and

Potential Hazards. London, Academic.

Zinner D. D. 1955 ) Recent ultrasonic dental studies including

periodontia without the use of an abrasive. .I.Dent . Res.

34 748-749.

1991_laird - ultrasound in dentistry part 1

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