J. Dent. 1991;
14
19: 14-17
Review
Ultrasound in dentistry. biophysical interactions
Part l-
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. However, it is only recently that the applications and effects of its physical properties 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 or mechanical mechanisms. The mechanical forces produced 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: J. Dent. 1991;
Ultrasound, Dentistry, Review 19: 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 Chad’s 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 developed for cavity preparation in human teeth (Catuna, 1953) based on pioneering work by Balamuth (1963). The ultrasonic drill operated at a frequency of 29 kHz and required an abrasive slurry of aluminium oxide to assist the process of cutting or grinding enamel and dentine (Postle, 1958). Although it was possible to use this instrument without recourse to local anaesthesia, it was somewhat cumbersome with a relatively slow action. In addition it required very efficient suction apparatus to remove the volumes of slurry used in the grinding process. Nevertheless it did receive favourable comment (Oman and Applebaum, 1954; Nielsen et aI., 1955), particularly in respect of the low loads employed 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 In spite of this, ultrasonic cavity preparation never became popular, being superceded by the much more @1991 Butterworth-Heinemann 0300-5712/91/010014-04
Ltd.
effective and efficient high speed rotary drills (Street, 1959). The potential use of ultrasound in dentistry however was still recognized by Zinner (1955), who suggested that the use of a modified ultrasonic instrument in conjunction 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 demonstrated by Johnson and Wilson (1957) and the technique has become 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 (Walmsley et al., 1989a, b), the treatment of disorders of the temporomandibular joint and the detection of early caries (Ng et al., 1988).
Laird and Walmsley: Ultrasound in dentistry
In order to appreciate the action of ultrasound in dentistry however, it is necessary to have some appreciation 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 waves 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 human body hard tissues can transmit both longitudinal and transverse waves, compared to soft tissues which can only transmit longitudinal waves, with the transverse waves being dissipated as heat. When an ultrasound wave encounters an interface between different media, as will occur with the tissues of the teeth for instance, part of it will be reflected back into the original medium and the remainder reflected into the new medium at a velocity which is dependent upon the transmission properties of the medium. The ratio of the reflected to refracted waves is termed the acoustic impedance, and there is greater energy transfer across boundaries where acoustic impedances are similar. Large impedance mismatches occur between solids to liquids to gases and consequently little energy is transferred.
GENERATION OF ULTRASOUND For clinical purposes ultrasound is generated by transducers which convert electrical energy into ultrasonic waves. This is usually achieved by magnetostriction or piezoelectricity, with the former being more common in the generation of the low frequency ultrasound oscillations used in dentistry. Magnetostrictive 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. This produces stresses leading to a change in shape of the material. When an alternating current is passed through the solenoid the stack will then change its shape at twice the frequency of the applied magnetic field. Magnetostriction with a laminated ferromagnetic stack is used commonly in the design of ultrasonic scaling instruments as it is a robust and easily manufactured system.
15
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 passed onto the working tip. Piezoelectric generators are more efficient at frequencies in the MHz rather than the kHz range, although some have been developed for use in dentistry. However, the crystalline structure has poor shock resistance and such instruments are more fragile than their magnetostrictive counterparts. When the input energy to a system is in phase with 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 maximum efficiency, if the tip and the generating stack are cut to resonant length, which is usually one-half wavelength or multiples thereof.
BIOLOGICAL EFFECTS OF ULTRASOUND When an ultrasonic wave passes through a biological system changes may occur in that system. These may be due to heat, cavitational activity, acoustic microstreaming or radiation forces. All are important when considering 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 damage.
CAVITATION Cavitational activity in relation to ultrasound encompasses a continuous spectrum of bubble activity in a liquid medium. It ranges from gentle linear pulsation of gastilled bodies in low amplitude sound fields (stable cavitation) to violent and destructive behaviour of vapourfilled 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
16
J.Dent. 1991; 19: No.1
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 subsequent transient cavitation occur readily (Esche, 1952). Cavitation occurring in human blood can result in a thrombogenic effect (Chater and Williams, 1982) and cause lysis of erythrocytes and platelets. This may explain reduction in haemorrhage when using ultrasonic surgical instruments and dental scalers (Ewen, 1960; Goliamina, 1974).
ACOUSTIC MICROSTREAMING
A
The rapid cyclical volume pulsation of a gas bubble
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.
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 demonstrated around an oscillating solid cylinder within a fluid or a stationary cylinder within an oscillating fluid (Fig. 2). The dimensions of the patterns demonstrate a rapid rate of change of streaming velocity with distance (Nyborg, 1977). Therefore although the velocities themselves are only of the order of a few centimetres per second (Williams, 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 damage 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 human platelets exposed to probes operating at 20 kHz (the level used in dentistry). At higher
large disruptive forces which are of use in the removal of plaque and calculus during ultrasonic scaling (Balamuth, 1963). The occurrence of cavitation requires the presence of gaseous bodies or bubbles in the medium, which have been termed cavitation nuclei (Williams, 1983). 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. Microbubbles will occur around the original bubble (Nyborg and Rogers, 1967) and will act as new sites for cavitational activity (Neppiras and Fill, 1969). Formation of microbubbles is associated with the onset of transient cavitation, where the bubbles show a ‘collapse’ phenomenon (Flynn, 1964)with the temperature of the gas in the bubble reaching thousands of degrees Celsius and several thousand atmospheres of pressure (Noltingk and Neppiras, 1956). The demanding 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)
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).
1
2
A 3
b
4
Laird and Walmsley:
amplitudes 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, but 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 maximum acoustic pressure amplitude. In blood vessels this may cause local aggregation of blood cells leading to stasis (Dyson et al., 1968). Radiation forces may also enhance cavitational activity within a standing wave field (Nyborg, 1977).
OCONCLUSION The physical properties and biological effects of ultrasound are of importance in dentistry where low frequency ultrasound instrumentation is used. From a theoretical extrapolation this may be both beneficial and damaging. The understanding 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 Balamuth 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 Williams A R (1982) Absence of platelet damage in vivo following the exposure of non-turbulent blood to therapeutic ultrasound. Ultrasound Med. Biol. 8, 85-87. Crum L. A (1982) Acoustic Cavitation, 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, 208-218. Ewen S. J. (1960) Ultrasound and periodontics. J. Periodonfol. 31, 101-106.
Ultrasound
in dentistry
17
Flynn H. G. (1964) Physics of acoustic cavitation in liquids. In: Mason W. P. (ed.), Physical Acoustics, vol. 1B. New York, Academic, pp. 57-172. Goliamina I. P. (1974) Ultrasonic surgery. In: Proceedings of the Eight Infema tional Congress on Acoustics. Guildford, IPC Science and Technology Press, pp. 63-69. Holtzmark J., Johnson I., Sikkeland T. et al. (1954) Boundary layer flow near a cylindrical obstacle in an oscillating uncompressional fluid. J. Acousf. Sot. Am. 26, 26. Johnson W. N. and Wilson J. R. (1957) Application of the ultrasonic dental unit to scaling procedures. .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. Endodonf. J. (in press). Martin H. (1976) Ultrasonic disinfection of the root canal. Oral Surg. Oral Med. Oral Pafhol. 42,92-99. Neppiras 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 method for detecting early caries. J. Dent. 16, 201-209. Nielsen A G., Richards J. R. and Wolcott R B. (1955) 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.] B63, 674-685. Nyborg W. L. (1977) Physical Mechanisms for Biological Eficfs of Ultrasound. 78-8026, HEW Publications (FDA]. Nyborg W. L. and Rogers A (1967) Motion of liquid inside a closed vibrating vessel. Biofechnol. Bioeng. 9, 235-241. Oman C. R. and Applebaum E. (1954) Ultrasonic cavity preparation II. Progress report. J. Am. Dent. Assoc. 50, 414-417. Postle H. H. (1958) Ultrasonic cavity preparation. J. Prosfhef. Dent 8, 153-160. Street E. V. (1959) 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 thrombosis associated with dental ultrasound. J. Oral Pafhol. 16, 256-259. Walmsley A. D., Jones P. A, Hullah W. et al. (1989a) Ultrasonic debonding of composite 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. Removal of restorations by ultrasonics. Dent. Update 15, 401-404. Wells P. N. T. (1977) Biomedical Ultrasonics. London, Academic. Williams A. R. (1983) Ultrasound: 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.
14
19: 14-17
Review
Ultrasound in dentistry. biophysical interactions
Part l-
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. However, it is only recently that the applications and effects of its physical properties 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 or mechanical mechanisms. The mechanical forces produced 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: J. Dent. 1991;
Ultrasound, Dentistry, Review 19: 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 Chad’s 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 developed for cavity preparation in human teeth (Catuna, 1953) based on pioneering work by Balamuth (1963). The ultrasonic drill operated at a frequency of 29 kHz and required an abrasive slurry of aluminium oxide to assist the process of cutting or grinding enamel and dentine (Postle, 1958). Although it was possible to use this instrument without recourse to local anaesthesia, it was somewhat cumbersome with a relatively slow action. In addition it required very efficient suction apparatus to remove the volumes of slurry used in the grinding process. Nevertheless it did receive favourable comment (Oman and Applebaum, 1954; Nielsen et aI., 1955), particularly in respect of the low loads employed 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 @1991 Butterworth-Heinemann 0300-5712/91/010014-04
Ltd.
effective and efficient high speed rotary drills (Street, 1959). The potential use of ultrasound in dentistry however was still recognized by Zinner (1955), who suggested that the use of a modified ultrasonic instrument in conjunction 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 demonstrated by Johnson and Wilson (1957) and the technique has become 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 (Walmsley et al., 1989a, b), the treatment of disorders of the temporomandibular joint and the detection of early caries (Ng et al., 1988).
Laird and Walmsley: Ultrasound in dentistry
In order to appreciate the action of ultrasound in dentistry however, it is necessary to have some appreciation 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 waves 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 human body hard tissues can transmit both longitudinal and transverse waves, compared to soft tissues which can only transmit longitudinal waves, with the transverse waves being dissipated as heat. When an ultrasound wave encounters an interface between different media, as will occur with the tissues of the teeth for instance, part of it will be reflected back into the original medium and the remainder reflected into the new medium at a velocity which is dependent upon the transmission properties of the medium. The ratio of the reflected to refracted waves is termed the acoustic impedance, and there is greater energy transfer across boundaries where acoustic impedances are similar. Large impedance mismatches occur between solids to liquids to gases and consequently little energy is transferred.
GENERATION OF ULTRASOUND For clinical purposes ultrasound is generated by transducers which convert electrical energy into ultrasonic waves. This is usually achieved by magnetostriction or piezoelectricity, with the former being more common in the generation of the low frequency ultrasound oscillations used in dentistry. Magnetostrictive 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. This produces stresses leading to a change in shape of the material. When an alternating current is passed through the solenoid the stack will then change its shape at twice the frequency of the applied magnetic field. Magnetostriction with a laminated ferromagnetic stack is used commonly in the design of ultrasonic scaling instruments as it is a robust and easily manufactured system.
15
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 passed onto the working tip. Piezoelectric generators are more efficient at frequencies in the MHz rather than the kHz range, although some have been developed for use in dentistry. However, the crystalline structure has poor shock resistance and such instruments are more fragile than their magnetostrictive counterparts. When the input energy to a system is in phase with 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 maximum efficiency, if the tip and the generating stack are cut to resonant length, which is usually one-half wavelength or multiples thereof.
BIOLOGICAL EFFECTS OF ULTRASOUND When an ultrasonic wave passes through a biological system changes may occur in that system. These may be due to heat, cavitational activity, acoustic microstreaming or radiation forces. All are important when considering 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 damage.
CAVITATION Cavitational activity in relation to ultrasound encompasses a continuous spectrum of bubble activity in a liquid medium. It ranges from gentle linear pulsation of gastilled bodies in low amplitude sound fields (stable cavitation) to violent and destructive behaviour of vapourfilled 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
16
J.Dent. 1991; 19: No.1
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 subsequent transient cavitation occur readily (Esche, 1952). Cavitation occurring in human blood can result in a thrombogenic effect (Chater and Williams, 1982) and cause lysis of erythrocytes and platelets. This may explain reduction in haemorrhage when using ultrasonic surgical instruments and dental scalers (Ewen, 1960; Goliamina, 1974).
ACOUSTIC MICROSTREAMING
A
The rapid cyclical volume pulsation of a gas bubble
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.
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 demonstrated around an oscillating solid cylinder within a fluid or a stationary cylinder within an oscillating fluid (Fig. 2). The dimensions of the patterns demonstrate a rapid rate of change of streaming velocity with distance (Nyborg, 1977). Therefore although the velocities themselves are only of the order of a few centimetres per second (Williams, 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 damage 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 human platelets exposed to probes operating at 20 kHz (the level used in dentistry). At higher
large disruptive forces which are of use in the removal of plaque and calculus during ultrasonic scaling (Balamuth, 1963). The occurrence of cavitation requires the presence of gaseous bodies or bubbles in the medium, which have been termed cavitation nuclei (Williams, 1983). 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. Microbubbles will occur around the original bubble (Nyborg and Rogers, 1967) and will act as new sites for cavitational activity (Neppiras and Fill, 1969). Formation of microbubbles is associated with the onset of transient cavitation, where the bubbles show a ‘collapse’ phenomenon (Flynn, 1964)with the temperature of the gas in the bubble reaching thousands of degrees Celsius and several thousand atmospheres of pressure (Noltingk and Neppiras, 1956). The demanding 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)
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).
1
2
A 3
b
4
Laird and Walmsley:
amplitudes 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, but 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 maximum acoustic pressure amplitude. In blood vessels this may cause local aggregation of blood cells leading to stasis (Dyson et al., 1968). Radiation forces may also enhance cavitational activity within a standing wave field (Nyborg, 1977).
OCONCLUSION The physical properties and biological effects of ultrasound are of importance in dentistry where low frequency ultrasound instrumentation is used. From a theoretical extrapolation this may be both beneficial and damaging. The understanding 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 Balamuth 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 Williams A R (1982) Absence of platelet damage in vivo following the exposure of non-turbulent blood to therapeutic ultrasound. Ultrasound Med. Biol. 8, 85-87. Crum L. A (1982) Acoustic Cavitation, 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, 208-218. Ewen S. J. (1960) Ultrasound and periodontics. J. Periodonfol. 31, 101-106.
Ultrasound
in dentistry
17
Flynn H. G. (1964) Physics of acoustic cavitation in liquids. In: Mason W. P. (ed.), Physical Acoustics, vol. 1B. New York, Academic, pp. 57-172. Goliamina I. P. (1974) Ultrasonic surgery. In: Proceedings of the Eight Infema tional Congress on Acoustics. Guildford, IPC Science and Technology Press, pp. 63-69. Holtzmark J., Johnson I., Sikkeland T. et al. (1954) Boundary layer flow near a cylindrical obstacle in an oscillating uncompressional fluid. J. Acousf. Sot. Am. 26, 26. Johnson W. N. and Wilson J. R. (1957) Application of the ultrasonic dental unit to scaling procedures. .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. Endodonf. J. (in press). Martin H. (1976) Ultrasonic disinfection of the root canal. Oral Surg. Oral Med. Oral Pafhol. 42,92-99. Neppiras 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 method for detecting early caries. J. Dent. 16, 201-209. Nielsen A G., Richards J. R. and Wolcott R B. (1955) 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.] B63, 674-685. Nyborg W. L. (1977) Physical Mechanisms for Biological Eficfs of Ultrasound. 78-8026, HEW Publications (FDA]. Nyborg W. L. and Rogers A (1967) Motion of liquid inside a closed vibrating vessel. Biofechnol. Bioeng. 9, 235-241. Oman C. R. and Applebaum E. (1954) Ultrasonic cavity preparation II. Progress report. J. Am. Dent. Assoc. 50, 414-417. Postle H. H. (1958) Ultrasonic cavity preparation. J. Prosfhef. Dent 8, 153-160. Street E. V. (1959) 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 thrombosis associated with dental ultrasound. J. Oral Pafhol. 16, 256-259. Walmsley A. D., Jones P. A, Hullah W. et al. (1989a) Ultrasonic debonding of composite 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. Removal of restorations by ultrasonics. Dent. Update 15, 401-404. Wells P. N. T. (1977) Biomedical Ultrasonics. London, Academic. Williams A. R. (1983) Ultrasound: 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.
