- Email: [email protected]
Potential hazards of the dental ultrasonic descaler
Ultrasound in Med. & Biol. Vol. 14, No. 1, pp. 15-20, 1988 Printed in the U.S.A.
0301-5629/88 $3.00 + .DO © 1987 Pergamon Joul'nal$ Lid.
OOriginal Contribution POTENTIAL
HAZARDS
OF THE
DENTAL
ULTRASONIC
DESCALER
A. D. WALMSLEY Department of Dental Prosthetics, The Dental School, St. Chad's Queensway, Birmingham, B4 6NN (Received l I February 1987; in revised~rm 29 May 1987) Abstract--The use of a dental ultrasonic descaler may be associated with biological hazards to structures within the oral cavity. Thermal hazards may result from either frictional contact between the oscillating probe and tooth or from absorption of acoustic energy within the tooth. Transmission of ultrasound along the tooth may result in thrombogenic damage to nearby blood vessels. The vibrating probe tip may produce scratching of the tooth surface, and incorporation of oral bacteria within the aerosol generated by the instrument which may result in transmission of infected material. Damage to the ear may arise from the coupling of ultrasound to the bones of the skull via the tooth. Furthermore the electro-magnetic field produced by these devices may interfere with cardiac pacemakers. It is the patient receiving treatment who is mainly exposed to these potential hazards. However, the clinician and his supporting staff may also be at risk.
Key Words: Ultrasound, Dentistry, Ultrasonic descaler, Biological hazard.
identifies potential hazards that still require further investigation.
INTRODUCTION The increased and unlimited use of the dental ultrasonic descaler and its modifications has resulted in many benefits to both the dentist and the patient (Walmsley, 1988). However, there is a lack of appreciation of the possible biological hazards to structures contained within and around the oral cavity of the patient. Such hazards may be produced by thermal, vibrational, or cavitational effects on those dental structures with which the instrument comes into contact during clinical use. Ultrasound transmission into the tooth may result in potential damage to structures such as blood vessels both within and around the teeth. Such transmission may also result in damage to distant structures such as the ear, which are not immediately in contact with the instrument. Finally the electromagnetic field associated with certain devices may interfere with the discharge rate of cardiac pacemakers. It is the patient who is receiving treatment with the ultrasonic descaler who is predominantly at risk from any potential hazards associated with the instrument. However both the dentist and his supporting staffare also in close proximity to the instrument and may be exposed to the ultrasonic vibrations. This paper identifies those hazards which have been reported following use of the instrument and
Interaction of ultrasound with dental tissues For specular reflections of ultrasound at plane surfaces it is possible to estimate the amount of ultrasound energy that could enter the tooth from acoustic impedance equations (Wells, 1977). If the ultrasonic descaler is perfectly coupled at the probe/enamel interface then about 37% of ultrasonic energy would leave the metal and enter the tooth (Walmsley et al., 1986a). However, this will not occur in practice because (1) the dimensions of the probe tip (approx. 10 mm 2) are much smaller than the wavelength of sound at these kHz frequencies (approximately 23 cm at 25 kHz); (2) there is usually a thin layer of water imposed between the probe and tooth; (3) there will be a difference in transmission if the longitudinally oscillating tip is applied perpendicular or parallel to the tooth. If heavy contact pressures are used, however, then the coupling will be improved increasing the amount of ultrasound entering the tooth. One of the major sources of damage to the tooth is the result of frictional heating between the probe and enamel especially if there is inadequate or no water cooling. Usually the frictional heating is largely removed by the flow of a cooling water. However the presence of the water may act as a matching layer 15
16
Ultrasound in Medicine and Biology
allowing ultrasound energy to enter the tooth. This energy will be uniformly absorbed resulting in heating of the tooth, although any heat produced would be expected to be removed largely by blood flow together with the transmission of ultrasound into the surrounding bone. The tooth may also act as a potential waveguide and transmit the ultrasonic vibrations along its length to the apical portion. It is in this region that the delicate nutrient vessels enter the dental pulp. In summary, biological damage is most likely to occur to the surface of the teeth if the instrument is run dry due to frictional heating between probe and tooth. However, when a water coolant is used, energy may be transferred into and along the tooth where it may produce damage to structures not immediately adjacent to the descaling process. Finally cavitational activity has been shown to occur in the associated cooling water as it flows over and away from the oscillating ultrasonic descaling probe (Walmsley et al., 1984). The destructive effects of the cavitation assist in the removal of tooth deposits. However, the occurrence of cavitational activity may also be a further source of damage to the associated tissues. THERMAL HAZARDS Dental pulp
Frictional heating from the ultrasonic descaler has been shown to produce thermal damage to the dental pulp (Frost, 1977), although, this has not been fully evaluated and the information available is relatively sparse. In contrast, a considerable amount of investigations have been undertaken on the potential effects on the tooth pulp from the use of ultrasonic vibratory drills, operating in the same frequency range as the ultrasonic descaler. Many workers found that these drills were no more harmful to the pulp than conventional rotary methods of cavity preparation in either animal teeth (Zach et al., 1957; Butt et al., 1957; Mallerne, 1958) or in the human dentition (Zach and Brown, 1956; Healey et al., 1956; Mitchell and Jensen, 1957). Other workers however demonstrated that the ultrasonic drill could produce damaging effects such as pulpal necrosis and tooth exfoliation, presumably due to excessive heating (Hansen and Nielsen, 1956; Sausen and Jensen, 1957; Wakely, 1959). As the ultrasonic drill was superseded by rotary instruments for cavity preparation, further investigations into these pulpal effects ceased, despite the conflicting evidence in the literature. The above investigations attributed the biologi-
Volume 14, Number 1, 1988
cal changes to the frictional heating effect of the instrument as it contacted the tooth. Work by Holden (1962) demonstrated that a high speed dental rotary drill was capable of inducing acoustic microstreaming within the dental pulp of extracted human teeth during cavity preparation. He postulated that this was the result of mechanical vibrations from the drill being transmitted through the calcified tooth substance to the pulp. Recent work with an ultrasonic descaler (Walmsley et al., 1986a) has shown that despite the small area of contact and the large acoustic mismatch between the steel descaling tip and the tooth, some vibrational energy is transmitted into the tooth. Absorption of this acoustic energy alone can result in an elevation in tooth temperature in vitro of up to 2°C. Further research is required however to determine whether sufficient vibratory transmission also occurs in vivo to a level which will produce a deleterious biological effect on the dental pulp. Periodontal tissues
The effect of the ultrasonic descaler on the supporting periodontal tissues has also been studied and in general has indicated that uneventful healing of traumatised tissues takes place after routine instrumentation (Goldman, 1960; Ewen, 1961 ; Schaffer et al., 1964; Sanderson, 1966). Histological examination of tissues immediately after ultrasonic descaling has suggested that superficial *,issue coagulation occurs (Goldman, 1960; Schaffer et al., 1964) with the probe tip in contact or even in close proximity (i.e. with water contact only) to the gingival tissues (Ewen, 1961). These observations were attributed to frictional heating of the tissues due to inadequate water cooling of the oscillating probe tip. The biological effects of any cavitational activity occurring within the cooling water however did not appear to have been considered. In general medicine, therapeutic ultrasound has been used for many years to stimulate wound healing (Summer and Patrick, 1964) and skin repair (Dyson et al., 1976), and in this respect it has been suggested that the contraction part of the repair process is enhanced by a cavitation mechanism (Webster et al., 1978). Therapeutic ultrasound is used at MHz frequencies, whereas in dentistry, ultrasound of a lower frequency (25-42 kHz) is used and the threshold for cavitation is decreased (Esche, 1952). It has been noted that following ultrasonic instrumentation the periodontal tissues heal more quickly than those treated solely by hand instruments (Goldman, 1961; Sanderson, 1966; Bhasker et al., 1972). More recent studies suggest however that at the frequencies and
Dental ultrasonic descaler • A. D. WALMSLEY
displacement amplitudes of ultrasound employed by the ultrasonic descaler, there is probably no real stimulatory effect of the ultrasound (Walmsley, 1985). Any beneficial effect of the ultrasonic descaler is likely to be inherent in its superior cleaning efficiency. CAVITATIONAL HAZARDS Within the blood vascular system, human platelets have been shown to be extremely susceptible to physical damage from the hydrodynamic shear forces produced by acoustic microstreaming occurring around a wire oscillating at frequencies of 20-85 kHz both in vitro (Williams, 1974), and in vivo (Williams, 1977). The ultrasonic descaler operating at similar frequencies has been shown also to damage platelets in vitro (Williams and Chater, 1980). Furthermore, a sickle probe of an ultrasonic descaler operating at displacement amplitudes of 9-33 um may induce platelet damage and thrombus formation within mammalian blood vessels in vivo (Walmsley et al., in press). These observations together with the potential for the transmission of ultrasound through the tissues of the tooth (Walmsley et al., 1986a) suggest that during ultrasonic descaling of the crown of a tooth, sound energy may be conducted towards the root apex, setting it into oscillation. This may result in platelet damage in nearby blood vessels. It is possible also that thrombus formation may occur within the afferent vessels of the pulp with possible loss of tooth vitality. In situ studies have shown that the attachments holding the tooth in its bony socket effectively reduce the ability of the tooth to act as a waveguide (Walmsley et al., in press). For example, a probe tip operating at a displacement amplitude of 44 um directed perpendicular to the tooth surface under heavy contact pressure only resulted in a displacement amplitude at the root tip of 1-2 um. Changing the direction of the oscillating tip so that it was parallel to the tooth reduced this vibration at the root apex to a barely detectable level. Therefore, the possibility of a thrombogenic hazard may result from incorrect use of the descaling instrument when the longitudinal oscillations are directed perpendicular to the tooth surface. Ultrasonic cavitation occurring within the cooling water flowing away from the probe may also produce small petechial lesions within the blood vessels of the immediate gingivae (Williams, 1983), and small particles of dislodged calculus and associated bacteria may be driven into these tissues by the shock waves of cavitation, resulting in areas of necrosis
17
which may then become infected (Williams, 1983). It is not known, however, whether this will result in an increased incidence of gingival infection. VIBRATIONAL HAZARDS A tooth surface which is completely smooth is the desired end result following descaling procedures, as any roughness or damage will become an area for further deposition of dental plaque. Initial reports have suggested that surface roughness of the tooth is common following ultrasonic descaling (Johnson and Wilson, 1957; Belting and Spjut, 1964), although later studies have been contradictory with both surface roughness (Kerry, 1967; Wilkinson and Maybury, 1973; D'Silva el al., 1979) and unaltered surface structure (Jones et al., 1972; Pameijer et al., 1972) being reported. Damage to amalgam restorations near the gingival margin has also been demonstrated (Rajstein and Yal, 1984). Most of these studies lack standardisation and are difficult to compare as the power setting of the ultrasonic unit was either adjusted to the operator's preference or set at either an arbitrary "medium" or "high" level. It has been shown previously (Walmsley et al., 1986b; and 1986c) that standardisation is important so that results from different workers may be compared in a meaningful manner. This lack of instrument calibration therefore may explain some of the contradictory results obtained. The displacement amplitude of the oscillating probe tip is a convenient measure of the acoustic power output from the instrument (Walmsley et al., 1986c). Such measurements for the displacement amplitude related to individual tips at differing power settings should be displayed in manufacturers literature. This is necessary in order to regularize and improve clinical performance which in turn would allow more meaningful comparison of results obtained from different clinical investigators (Walmsley, 1988). This will enable the clinician to use the instrument with maximum efficiency coupled with the minimum potential biological damage to the structures with which the probe is in contact with during operation. Where an attempt has been made to define the various parameters involved, a smoother tooth surface was achieved with low instrument loadings (Bj6rn and Lindhe, 1962), adequate use of cooling water (Allen and Rhoades, 1963) and low instrument power settings (Clark et aL, 1968). However, such findings would have been more conclusive if adequate standardisation of their instruments had been made prior to use.
18
Ultrasound in Medicineand Biology
Previous observations on the load applied during ultrasonic descaling by clinicians may differ. Light loads were measured as 6 grams, mediumloads were 14 grams and heavy loads were 37 grams (Walmsley et aL, 1984). Therefore it may prove useful if the manufacturers could employ a sensing device which would indicate to the clinician how much loading is being employed so that surface damage is reduced. For example a cut-out mechanism within the generator would prevent excessive loading being applied to the tooth. AEROSOL PRODUCTION As the flow of cooling water passes over the oscillating probe tip, surface waves will be formed along the air/water interface. When the displacement amplitude of the probe is sufficiently high, this water will be ejected into the air as droplets to form an aerosol. The diameter (din) of these droplets may be found from the equation (Lang, 1962) dm = 0.34
3•2a pf 2
where a is the interfacial tension between water and air, p is the density of the liquid andfis the frequency of the acoustic wave. Aerosol formation will be increased as the displacement amplitude of the driving frequency is increased. An ultrasonic descaler operating at 25 kHz will produce a fine mist of droplets each approximately 30 #m in diameter. During ultrasonic descaling, oral bacteria may be incorporated in the aerosol produced (Lorato et aL, 1967), and will result in an increase in the number of microorganisms liberated into the surrounding air with their likely inhalation by patient or operator. The risk of a respiratory tract infection is therefore increased, and such aerosol production may constitute a significant health hazard (Lorato et al., 1967; Holbrook et al., 1978). The risk to the patient and operator from the larger amounts of particulate matter which may be produced by calculus removal may be reduced by the use of high suction aspiration techniques and operator protection such as spectacles and face masks, or by asking the patient to rinse his mouth with 0.2% chlorhexidine solution two minutes prior to descaling (Muir et al., 1978). THE AUDITORY SYSTEM During ultrasonic descaling, the inner ear of a patient may be subject inadvertently to ultrasound
Volume14, Number 1, 1988 (Williams, 1983). However due to the mismatch in acoustic impedances between the metal probe and air, it is unlikely that excessive high displacement amplitude airborne sound waves are produced by the working instrument. Furthermore within the ear the sensory elements of the cochlea and vestibule will be protected from high displacement amplitude airborne sounds of frequencies in the order of tens of kHz by the relatively inefficient mechanical conduction system of the eardrum and auditory ossicles. However ultrasonic descaling may circumvent such protective mechanisms by coupling the energy directly to the bones of the skull via the tooth or teeth. Despite the divergent nature of the acoustic beam (the probe tip is acting as a point source of ultrasound), the displacement amplitude of the sound waves transmitted to the inner ear by bone conduction may be high enough to damage the sensory structures. This is most likely to occur when descaling the upper molar and premolar teeth due to their proximity to the auditory system (Williams, 1983). Any potential biological effects may result from mechanical vibration of the delicate ?,tructures within the ear or by thermal mechanisms. Qualitative measurements of the possible toothborne acoustic transmission have been made for the audible range of 0-40 kHz (Walmsley et aL, 1987). It has been shown by utilising an in vitro tooth model that acoustic/vibrational energy is transmitted through a tooth during ultrasonic descaling and the frequency range of such tooth-borne emission was similar to that of airborne emission. Therefore, appreciable amounts of both ultrasonic and audible sound may potentially enter the inner ear of the patient during a typical descaling procedure. To date this hazard has not been adequately investigated with previous results being conflicting. Mrller et al. (1976) reported that both tinnitus and temporary threshold hearing shifts (a commonly accepted index of early damage to hearing) occurred in 10 out of 20 patients following a routine ultrasonic descaling. Walmsley et al. (1987) repeated Mrller's experiment and found no deterioration in the hearing thresholds of 10 volunteers following a routine 5 minute ultrasonic descaling as against a matched control group. Any variation found was inherent in the testing procedure, such as headphone placement and test familiarisation. Furthermore, Coles and Hoare (1985) found in a survey of dentists exposed to high frequency instruments such as the air rotor and ultrasonic descaler, that there was no statistical evidence of hearing disorders/ thresholds when compared to the normal population.
Dental ultrasonic descaler • A. D. WALMSLEY
CARDIAC PACEMAKERS The cardiac pacemaker is a tissue implanted electrical transmitter designed to regulate the rhythm of the heart. Two types are used, competitive (fixed rate type) and noncompetitive (demand type), the former discharging at a fixed rate while the latter only discharges if the rate becomes irregular. The noncompetitive/demand pacemaker is used exclusively at present (Adams et al., 1982). The electromagnetic field produced by magnetostrictive ultrasonic descalers during operation may interfere with the pacemaker discharge rate, resulting in a serious life threatening hazard to the patient (Griffiths, 1978). Reports have been conflicting however with both interference (Adams et al., 1982) and noninterference (Simon et al., 1975; Luker, 1982) being reported. It has been suggested that any effects which have been observed may be the result of a noncompetitive type of pacemaker switching over to a fixed mode during the period of interference (Mokrzycki, 1982). No reports of interference caused by piezoelectric descalers have been reported (Adams et al., 1982). There is clearly a conflict between the results obtained by different workers however, and caution is recommended when treating the cardiac pacemaker patient with a magnetostrictive ultrasonic descaler. SUMMARY Although dental ultrasonic descaling instruments have been found to be clinically beneficial, the possibility of related unwanted biological effects must be recognised. Some of these are well recognised and have received considerable attention in the literature, including frictional damage to tooth surfaces and the effect of the bacterial contaminated aerosol. Other areas such as the hazards posed by the generation of cavitational activity and acoustic microstreaming together with the possibility of energy transmission into the tooth require further clarification. In this respect, adequate standardisaton of instruments and procedures is essential in order that the ultrasonic descaler can be used both effectively and safely (Walmsley, 1988). Such standardisation should include: 1. Adequate training of the clinician in the use of the ultrasonic descaler. 2. Avoidance of heavy contact loads. 3. An indication of the displacement amplitude of the descaling probe tip and the direction of its oscillation.
19
This is important in order that reports on either the efficiency or potential hazards/damage associated with the instrument may be evaluated in a meaningful manner. Acknowledgements--The author would like to thank Professor W. R. E. Laird and Dr. A. R. Williams for their assistance in the preparation of this paper.
REFERENCES Adams D., Fulford N., Beechy J., MacCarthy J. and Stephens M. (1982). The cardiac pacemaker and ultrasonic scalers. Br. Dent. J. 152, 171-173. Allen E. F. and Rhoades R. H. (1963) Effects of high speed periodontal instruments on tooth surfaces. J. Periodontol. 34, 352-356. Belting C. M. and Spjut P. J. (1964) Effects of high speed instruments on the root surface during subgingival calculus removal. J. Am. Dent. Assoc. 69, 578-584. Bhasker S. N., Grower M. F. and Cutright D. E. (1972) Gingival healing after hand and ultrasonic scaling--biochemical and histological analysis. J. Periodontol. 43, 31-34. Bj6rn H. and Lindhe J. (1962) The influence of periodontal instruments on the tooth surface. Odontologisk. Revy. 13, 355-369. Butt B. G., Harris N. O., Shannon I. and Zander H. A. (1957). Ultrasonic removal of tooth structure: I. A histopathological evaluation of pulpal response in monkeys after ultrasonic cavity preparation. J. Am. Dent. Assoc. 55, 32-36. Clark S. M., Grupe H. E. and Mahler D. B. (1968) The effect of ultrasonic instrumentation on root surfaces. J. PeriodontoL 39, 135-137. Coles R. R. A. and Hoare N. W. (1985) Noise induced hearing loss and the dentist. Br. Dent. J. 159, 209-218. D'Silva I. V., Nayak R. P., Cherian K. M. and Mulky M. J. (1979) An evaluation of the root topography following periodontal instrumentation--a scanning electron microscope study. J. Periodontol. 50, 283-290. Dyson M., Franks C. and Suckling J. (1976) Stimulation of healing varicose ulcers by ultrasound. Ultrasonics 14, 232-236. Esche R. (1952) Untersuchungen der schwingungskavitation in flussigkeiten. Acusitica 2, 208-218. Ewen S. J. (1961) The ultrasound wound--some microscopic observations. J. Periodontol. 32, 315-321. Frost H. M. (1977) Heating under ultrasonic dental scaling conditions. In Symposium on Biological Effects and Characteristics of Ultrasound Sources, pp. 64-76. U.S. Dept. HEW Publications (FDA) 78-8048, Washington DC. Goldman H. M. (1960). Curettage by ultrasonic instrument. Oral Surg. 13, 43-53. Goldman H. M. ( 1961 ) Histologic assay of healing following ultrasonic curettage versus hand instrument curettage. Oral Surg. 14, 925-928. Gritfiths P. V. (1978) The management of the pacemaker wearer during dental hygiene treatment. Dent. Hyg. 52, 573-576. Hansen L. S. and Nielsen A. G. (1956). Comparison of tissue response to rotary and ultrasonic dental cutting procedures. J. Am. Dent. Assoc. 52, 131-137. Healey H. J., Patterson S. S. and Van Huysen G. (1956). Pulp reaction to ultrasonic cavity preparation. U.S. Armed Forces M.J.. 7, 685-692. Holbrook W. P., Muir K. F., MacPhee 1. T. and Ross P. W. (1978) Bacterial investigation of the aerosol from ultrasonic scalers. Br. Dent. Z 144, 245-247. Holden G. G. P. (1962) Some observations on the vibratory phenomena associated with high speed air turbines and their transmission to living tissue. Br. Dent. J. 113, 265-275. Johnson W. N. and Wilson J. R. (1957) Application of the ultra-
20
Ultrasound in Medicine and Biology
sonic dental unit to scaling procedures. J. Periodontol. 28, 264-271. Jones S. J., Lozdan J. and Boyde A. (1972) Tooth surfaces treated in situ with periodontal instruments--scanning electron microscope studies. Br. Dent. J. 132, 57-64. Kerry G. J. (1967) Roughness of root surfaces after use of ultrasonic instruments and hand curettes. J. Periodontol. 38, 340-346. Lang R. J. (1962) Ultrasonic atomization of liquids. J. Acoustic Soc. Am. 34, 6-8. Lorato D. C., Ruskin P. F. and Martin A. (1967) Effect of an ultrasonic scaler on bacterial counts in air. J. Periodontol. 38, 550-554. Luker J. (1982) The pacemaker in the dental surgery. J. Dent. 10, 326-332. Mallerne R. E. (1958) The effects of ultrasonic energy on the periodontal membrane, alveolar bone, and gingivae. J. Prosthet. Dent. 8, 147-152. Mitchell D. F. and Jensen J. R. (1957) Preliminary report on the reaction of the dental pulp to cavity preparation using an ultrasonic device. J. Am. Dent. Assoc. 55, 57-62. Mokrzycki J. (1982). The cardiac pacemakers and ultrasonic scalers (letter). Br. Dent. J. 153, 250. Mrller P., Grevstad A. O. and Kristofferson T. (1976) Ultrasonic scaling of maxillary teeth causing tinnitus and temporary hearing shifts. J. Clin. Periodontol. 3, 123-127. Muir K. F., Ross P. W., MacPhee I. T., Holbrook W. P. and Kowolick M. J. (1978) Reduction of microbial contamination from ultrasonic scalers. Br. Dent. J. 145, 76-78. Pameijer C. H., Stallard R. E. and Hiep N. (1972) Surface characteristics of teeth following periodontal i n s t r u m e n t a t i o n - - a scanning electron microscope study. J. Periodontol. 43, 628-633. Rajstein J. and Tal M. (1984) The effect of ultrasonic scaling on the surface of class V amalgam restorations--a scanning electron microscope study. J. Oral Rehab. 11,299-305. Sanderson A. D. (1966) Gingival curettage by hand and ultrasonic i n s t r u m e n t s - - a histologic comparison. J. Periodontol. 37, 279-290. Sausen R. E. and Jensen J. R. (1957) Ultrasonic applications with heavy pressure--a histologic report. North West Dent. J. 36, 310-315. Schaffer E. M., Stende G. and King D. (1964) Healing of periodontal pocket tissues following ultrasonic scaling and hand planing. J. PeriodontoL 35, 140-148. Simon A. B., Lindhe B., Bonnette G. H. and Schlentz R. J. (1975) The individual with a pacemaker in the dental environment. Z Am. Dent. Assoc. 91, 1224-1229. Summer W. and Patrick M. K. (1964) Ultrasonic Therapy--A Textbook fi~r Physiotherapists. Elsevier, London.
Volume 14, Number 1, 1988 Wakely J. W. (1959) Effects of ultrasonic applications on deciduous and developing monkey teeth. J. Dent. Res. 38, 739. Walmsley A. D. (1985) Some biological effects of ultrasound. Ph.D. Thesis, University of Manchester. Walmsley A. D. (1988) Applications of ultrasound in dentistry. Ultrasound in Med. & Biol. 14, 7-14. Walmsley A. D., Laird W. R. E. and Williams A. R. (1984). A model system to demonstrate the role of cavitational activity in ultrasonic scaling. J. Dent. Res. 63, 1162-1165. Walmsley A. D., Laird W. R. E. and Williams A. R. (1986a) Acoustic absorption wihtin human teeth resulting in an increased pulpal temperature rise. J. Dent. 14, 2-6. Walmsley A. D., Laird W. R. E. and Williams A. R. (1986b) Inherent variability of the performance of the ultrasonic descaler. J. Dent. 14, 121-125. Walmsley A. D., Laird W. R. E. and Williams A. R. (1986c) Displacement amplitude as a measure of the acoustic output of ultrasonic scalers. Dent. Mater, 2, 97-100. Walmsley A. D., Laird W. R. E. and Williams A. R. (in press) Intra-vascular thrombosis associated with dental ultrasound. J. Oral. Pathol. Walmsley A. D., Hickson F. H., Laird W. R. E. and Williams A. R. (1987) Investigation into patients' hearing following ultrasonic scaling. Br. Dent. J. 162, 221-224. Webster D. F., Pond J. B., Dyson M. and Harvey W. (1978) The role of cavitation on the in vitro stimulation of protein synthesis in human fibroblasts by ultrasound. Ultrasound in Med. & Biol. 4, 343-351. Wells P. N. T. (1977) Biomedical Ultrasonics pp. 15-18. London, Academic Press. Wilkinson R. F. and Maybury J. E. (1973) Scanning electron microscopy of the root surface following instrumentation. J. Periodontol. 44, 559-563. Williams A. R. (1974) Release of serotonin from human platelets by acoustic microstreaming. J. Accoust. Soc. A m . / 56, 1640-1643. Williams A. R. (1977) Intravascular mural thrombi produced by acoustic microstreaming. Ultrasound in Med. & Biol. 3, 191-203. Williams A. R. (1983) Ultrasound Biological Effects and Potential Hazards. Academic Press, London. Williams A. R. and Chater B. V. (1980) Mammalian platelet damage in vitro by an ultrasonic therapeutic device. Arch. Oral Biol. 25, 175-179. Zach L. and Brown G. N. (1956) Pulpal effect of ultrasonic cavity preparation: preliminary report. N. Y. Dent. J. 22, 9-17. Zach L., Morrison A. H. and Cohen G. (1957) Effect of ultrasonic operative procedures on the developing dentition. J. Dent. Res. 35, 71-72.
0301-5629/88 $3.00 + .DO © 1987 Pergamon Joul'nal$ Lid.
OOriginal Contribution POTENTIAL
HAZARDS
OF THE
DENTAL
ULTRASONIC
DESCALER
A. D. WALMSLEY Department of Dental Prosthetics, The Dental School, St. Chad's Queensway, Birmingham, B4 6NN (Received l I February 1987; in revised~rm 29 May 1987) Abstract--The use of a dental ultrasonic descaler may be associated with biological hazards to structures within the oral cavity. Thermal hazards may result from either frictional contact between the oscillating probe and tooth or from absorption of acoustic energy within the tooth. Transmission of ultrasound along the tooth may result in thrombogenic damage to nearby blood vessels. The vibrating probe tip may produce scratching of the tooth surface, and incorporation of oral bacteria within the aerosol generated by the instrument which may result in transmission of infected material. Damage to the ear may arise from the coupling of ultrasound to the bones of the skull via the tooth. Furthermore the electro-magnetic field produced by these devices may interfere with cardiac pacemakers. It is the patient receiving treatment who is mainly exposed to these potential hazards. However, the clinician and his supporting staff may also be at risk.
Key Words: Ultrasound, Dentistry, Ultrasonic descaler, Biological hazard.
identifies potential hazards that still require further investigation.
INTRODUCTION The increased and unlimited use of the dental ultrasonic descaler and its modifications has resulted in many benefits to both the dentist and the patient (Walmsley, 1988). However, there is a lack of appreciation of the possible biological hazards to structures contained within and around the oral cavity of the patient. Such hazards may be produced by thermal, vibrational, or cavitational effects on those dental structures with which the instrument comes into contact during clinical use. Ultrasound transmission into the tooth may result in potential damage to structures such as blood vessels both within and around the teeth. Such transmission may also result in damage to distant structures such as the ear, which are not immediately in contact with the instrument. Finally the electromagnetic field associated with certain devices may interfere with the discharge rate of cardiac pacemakers. It is the patient who is receiving treatment with the ultrasonic descaler who is predominantly at risk from any potential hazards associated with the instrument. However both the dentist and his supporting staffare also in close proximity to the instrument and may be exposed to the ultrasonic vibrations. This paper identifies those hazards which have been reported following use of the instrument and
Interaction of ultrasound with dental tissues For specular reflections of ultrasound at plane surfaces it is possible to estimate the amount of ultrasound energy that could enter the tooth from acoustic impedance equations (Wells, 1977). If the ultrasonic descaler is perfectly coupled at the probe/enamel interface then about 37% of ultrasonic energy would leave the metal and enter the tooth (Walmsley et al., 1986a). However, this will not occur in practice because (1) the dimensions of the probe tip (approx. 10 mm 2) are much smaller than the wavelength of sound at these kHz frequencies (approximately 23 cm at 25 kHz); (2) there is usually a thin layer of water imposed between the probe and tooth; (3) there will be a difference in transmission if the longitudinally oscillating tip is applied perpendicular or parallel to the tooth. If heavy contact pressures are used, however, then the coupling will be improved increasing the amount of ultrasound entering the tooth. One of the major sources of damage to the tooth is the result of frictional heating between the probe and enamel especially if there is inadequate or no water cooling. Usually the frictional heating is largely removed by the flow of a cooling water. However the presence of the water may act as a matching layer 15
16
Ultrasound in Medicine and Biology
allowing ultrasound energy to enter the tooth. This energy will be uniformly absorbed resulting in heating of the tooth, although any heat produced would be expected to be removed largely by blood flow together with the transmission of ultrasound into the surrounding bone. The tooth may also act as a potential waveguide and transmit the ultrasonic vibrations along its length to the apical portion. It is in this region that the delicate nutrient vessels enter the dental pulp. In summary, biological damage is most likely to occur to the surface of the teeth if the instrument is run dry due to frictional heating between probe and tooth. However, when a water coolant is used, energy may be transferred into and along the tooth where it may produce damage to structures not immediately adjacent to the descaling process. Finally cavitational activity has been shown to occur in the associated cooling water as it flows over and away from the oscillating ultrasonic descaling probe (Walmsley et al., 1984). The destructive effects of the cavitation assist in the removal of tooth deposits. However, the occurrence of cavitational activity may also be a further source of damage to the associated tissues. THERMAL HAZARDS Dental pulp
Frictional heating from the ultrasonic descaler has been shown to produce thermal damage to the dental pulp (Frost, 1977), although, this has not been fully evaluated and the information available is relatively sparse. In contrast, a considerable amount of investigations have been undertaken on the potential effects on the tooth pulp from the use of ultrasonic vibratory drills, operating in the same frequency range as the ultrasonic descaler. Many workers found that these drills were no more harmful to the pulp than conventional rotary methods of cavity preparation in either animal teeth (Zach et al., 1957; Butt et al., 1957; Mallerne, 1958) or in the human dentition (Zach and Brown, 1956; Healey et al., 1956; Mitchell and Jensen, 1957). Other workers however demonstrated that the ultrasonic drill could produce damaging effects such as pulpal necrosis and tooth exfoliation, presumably due to excessive heating (Hansen and Nielsen, 1956; Sausen and Jensen, 1957; Wakely, 1959). As the ultrasonic drill was superseded by rotary instruments for cavity preparation, further investigations into these pulpal effects ceased, despite the conflicting evidence in the literature. The above investigations attributed the biologi-
Volume 14, Number 1, 1988
cal changes to the frictional heating effect of the instrument as it contacted the tooth. Work by Holden (1962) demonstrated that a high speed dental rotary drill was capable of inducing acoustic microstreaming within the dental pulp of extracted human teeth during cavity preparation. He postulated that this was the result of mechanical vibrations from the drill being transmitted through the calcified tooth substance to the pulp. Recent work with an ultrasonic descaler (Walmsley et al., 1986a) has shown that despite the small area of contact and the large acoustic mismatch between the steel descaling tip and the tooth, some vibrational energy is transmitted into the tooth. Absorption of this acoustic energy alone can result in an elevation in tooth temperature in vitro of up to 2°C. Further research is required however to determine whether sufficient vibratory transmission also occurs in vivo to a level which will produce a deleterious biological effect on the dental pulp. Periodontal tissues
The effect of the ultrasonic descaler on the supporting periodontal tissues has also been studied and in general has indicated that uneventful healing of traumatised tissues takes place after routine instrumentation (Goldman, 1960; Ewen, 1961 ; Schaffer et al., 1964; Sanderson, 1966). Histological examination of tissues immediately after ultrasonic descaling has suggested that superficial *,issue coagulation occurs (Goldman, 1960; Schaffer et al., 1964) with the probe tip in contact or even in close proximity (i.e. with water contact only) to the gingival tissues (Ewen, 1961). These observations were attributed to frictional heating of the tissues due to inadequate water cooling of the oscillating probe tip. The biological effects of any cavitational activity occurring within the cooling water however did not appear to have been considered. In general medicine, therapeutic ultrasound has been used for many years to stimulate wound healing (Summer and Patrick, 1964) and skin repair (Dyson et al., 1976), and in this respect it has been suggested that the contraction part of the repair process is enhanced by a cavitation mechanism (Webster et al., 1978). Therapeutic ultrasound is used at MHz frequencies, whereas in dentistry, ultrasound of a lower frequency (25-42 kHz) is used and the threshold for cavitation is decreased (Esche, 1952). It has been noted that following ultrasonic instrumentation the periodontal tissues heal more quickly than those treated solely by hand instruments (Goldman, 1961; Sanderson, 1966; Bhasker et al., 1972). More recent studies suggest however that at the frequencies and
Dental ultrasonic descaler • A. D. WALMSLEY
displacement amplitudes of ultrasound employed by the ultrasonic descaler, there is probably no real stimulatory effect of the ultrasound (Walmsley, 1985). Any beneficial effect of the ultrasonic descaler is likely to be inherent in its superior cleaning efficiency. CAVITATIONAL HAZARDS Within the blood vascular system, human platelets have been shown to be extremely susceptible to physical damage from the hydrodynamic shear forces produced by acoustic microstreaming occurring around a wire oscillating at frequencies of 20-85 kHz both in vitro (Williams, 1974), and in vivo (Williams, 1977). The ultrasonic descaler operating at similar frequencies has been shown also to damage platelets in vitro (Williams and Chater, 1980). Furthermore, a sickle probe of an ultrasonic descaler operating at displacement amplitudes of 9-33 um may induce platelet damage and thrombus formation within mammalian blood vessels in vivo (Walmsley et al., in press). These observations together with the potential for the transmission of ultrasound through the tissues of the tooth (Walmsley et al., 1986a) suggest that during ultrasonic descaling of the crown of a tooth, sound energy may be conducted towards the root apex, setting it into oscillation. This may result in platelet damage in nearby blood vessels. It is possible also that thrombus formation may occur within the afferent vessels of the pulp with possible loss of tooth vitality. In situ studies have shown that the attachments holding the tooth in its bony socket effectively reduce the ability of the tooth to act as a waveguide (Walmsley et al., in press). For example, a probe tip operating at a displacement amplitude of 44 um directed perpendicular to the tooth surface under heavy contact pressure only resulted in a displacement amplitude at the root tip of 1-2 um. Changing the direction of the oscillating tip so that it was parallel to the tooth reduced this vibration at the root apex to a barely detectable level. Therefore, the possibility of a thrombogenic hazard may result from incorrect use of the descaling instrument when the longitudinal oscillations are directed perpendicular to the tooth surface. Ultrasonic cavitation occurring within the cooling water flowing away from the probe may also produce small petechial lesions within the blood vessels of the immediate gingivae (Williams, 1983), and small particles of dislodged calculus and associated bacteria may be driven into these tissues by the shock waves of cavitation, resulting in areas of necrosis
17
which may then become infected (Williams, 1983). It is not known, however, whether this will result in an increased incidence of gingival infection. VIBRATIONAL HAZARDS A tooth surface which is completely smooth is the desired end result following descaling procedures, as any roughness or damage will become an area for further deposition of dental plaque. Initial reports have suggested that surface roughness of the tooth is common following ultrasonic descaling (Johnson and Wilson, 1957; Belting and Spjut, 1964), although later studies have been contradictory with both surface roughness (Kerry, 1967; Wilkinson and Maybury, 1973; D'Silva el al., 1979) and unaltered surface structure (Jones et al., 1972; Pameijer et al., 1972) being reported. Damage to amalgam restorations near the gingival margin has also been demonstrated (Rajstein and Yal, 1984). Most of these studies lack standardisation and are difficult to compare as the power setting of the ultrasonic unit was either adjusted to the operator's preference or set at either an arbitrary "medium" or "high" level. It has been shown previously (Walmsley et al., 1986b; and 1986c) that standardisation is important so that results from different workers may be compared in a meaningful manner. This lack of instrument calibration therefore may explain some of the contradictory results obtained. The displacement amplitude of the oscillating probe tip is a convenient measure of the acoustic power output from the instrument (Walmsley et al., 1986c). Such measurements for the displacement amplitude related to individual tips at differing power settings should be displayed in manufacturers literature. This is necessary in order to regularize and improve clinical performance which in turn would allow more meaningful comparison of results obtained from different clinical investigators (Walmsley, 1988). This will enable the clinician to use the instrument with maximum efficiency coupled with the minimum potential biological damage to the structures with which the probe is in contact with during operation. Where an attempt has been made to define the various parameters involved, a smoother tooth surface was achieved with low instrument loadings (Bj6rn and Lindhe, 1962), adequate use of cooling water (Allen and Rhoades, 1963) and low instrument power settings (Clark et aL, 1968). However, such findings would have been more conclusive if adequate standardisation of their instruments had been made prior to use.
18
Ultrasound in Medicineand Biology
Previous observations on the load applied during ultrasonic descaling by clinicians may differ. Light loads were measured as 6 grams, mediumloads were 14 grams and heavy loads were 37 grams (Walmsley et aL, 1984). Therefore it may prove useful if the manufacturers could employ a sensing device which would indicate to the clinician how much loading is being employed so that surface damage is reduced. For example a cut-out mechanism within the generator would prevent excessive loading being applied to the tooth. AEROSOL PRODUCTION As the flow of cooling water passes over the oscillating probe tip, surface waves will be formed along the air/water interface. When the displacement amplitude of the probe is sufficiently high, this water will be ejected into the air as droplets to form an aerosol. The diameter (din) of these droplets may be found from the equation (Lang, 1962) dm = 0.34
3•2a pf 2
where a is the interfacial tension between water and air, p is the density of the liquid andfis the frequency of the acoustic wave. Aerosol formation will be increased as the displacement amplitude of the driving frequency is increased. An ultrasonic descaler operating at 25 kHz will produce a fine mist of droplets each approximately 30 #m in diameter. During ultrasonic descaling, oral bacteria may be incorporated in the aerosol produced (Lorato et aL, 1967), and will result in an increase in the number of microorganisms liberated into the surrounding air with their likely inhalation by patient or operator. The risk of a respiratory tract infection is therefore increased, and such aerosol production may constitute a significant health hazard (Lorato et al., 1967; Holbrook et al., 1978). The risk to the patient and operator from the larger amounts of particulate matter which may be produced by calculus removal may be reduced by the use of high suction aspiration techniques and operator protection such as spectacles and face masks, or by asking the patient to rinse his mouth with 0.2% chlorhexidine solution two minutes prior to descaling (Muir et al., 1978). THE AUDITORY SYSTEM During ultrasonic descaling, the inner ear of a patient may be subject inadvertently to ultrasound
Volume14, Number 1, 1988 (Williams, 1983). However due to the mismatch in acoustic impedances between the metal probe and air, it is unlikely that excessive high displacement amplitude airborne sound waves are produced by the working instrument. Furthermore within the ear the sensory elements of the cochlea and vestibule will be protected from high displacement amplitude airborne sounds of frequencies in the order of tens of kHz by the relatively inefficient mechanical conduction system of the eardrum and auditory ossicles. However ultrasonic descaling may circumvent such protective mechanisms by coupling the energy directly to the bones of the skull via the tooth or teeth. Despite the divergent nature of the acoustic beam (the probe tip is acting as a point source of ultrasound), the displacement amplitude of the sound waves transmitted to the inner ear by bone conduction may be high enough to damage the sensory structures. This is most likely to occur when descaling the upper molar and premolar teeth due to their proximity to the auditory system (Williams, 1983). Any potential biological effects may result from mechanical vibration of the delicate ?,tructures within the ear or by thermal mechanisms. Qualitative measurements of the possible toothborne acoustic transmission have been made for the audible range of 0-40 kHz (Walmsley et aL, 1987). It has been shown by utilising an in vitro tooth model that acoustic/vibrational energy is transmitted through a tooth during ultrasonic descaling and the frequency range of such tooth-borne emission was similar to that of airborne emission. Therefore, appreciable amounts of both ultrasonic and audible sound may potentially enter the inner ear of the patient during a typical descaling procedure. To date this hazard has not been adequately investigated with previous results being conflicting. Mrller et al. (1976) reported that both tinnitus and temporary threshold hearing shifts (a commonly accepted index of early damage to hearing) occurred in 10 out of 20 patients following a routine ultrasonic descaling. Walmsley et al. (1987) repeated Mrller's experiment and found no deterioration in the hearing thresholds of 10 volunteers following a routine 5 minute ultrasonic descaling as against a matched control group. Any variation found was inherent in the testing procedure, such as headphone placement and test familiarisation. Furthermore, Coles and Hoare (1985) found in a survey of dentists exposed to high frequency instruments such as the air rotor and ultrasonic descaler, that there was no statistical evidence of hearing disorders/ thresholds when compared to the normal population.
Dental ultrasonic descaler • A. D. WALMSLEY
CARDIAC PACEMAKERS The cardiac pacemaker is a tissue implanted electrical transmitter designed to regulate the rhythm of the heart. Two types are used, competitive (fixed rate type) and noncompetitive (demand type), the former discharging at a fixed rate while the latter only discharges if the rate becomes irregular. The noncompetitive/demand pacemaker is used exclusively at present (Adams et al., 1982). The electromagnetic field produced by magnetostrictive ultrasonic descalers during operation may interfere with the pacemaker discharge rate, resulting in a serious life threatening hazard to the patient (Griffiths, 1978). Reports have been conflicting however with both interference (Adams et al., 1982) and noninterference (Simon et al., 1975; Luker, 1982) being reported. It has been suggested that any effects which have been observed may be the result of a noncompetitive type of pacemaker switching over to a fixed mode during the period of interference (Mokrzycki, 1982). No reports of interference caused by piezoelectric descalers have been reported (Adams et al., 1982). There is clearly a conflict between the results obtained by different workers however, and caution is recommended when treating the cardiac pacemaker patient with a magnetostrictive ultrasonic descaler. SUMMARY Although dental ultrasonic descaling instruments have been found to be clinically beneficial, the possibility of related unwanted biological effects must be recognised. Some of these are well recognised and have received considerable attention in the literature, including frictional damage to tooth surfaces and the effect of the bacterial contaminated aerosol. Other areas such as the hazards posed by the generation of cavitational activity and acoustic microstreaming together with the possibility of energy transmission into the tooth require further clarification. In this respect, adequate standardisaton of instruments and procedures is essential in order that the ultrasonic descaler can be used both effectively and safely (Walmsley, 1988). Such standardisation should include: 1. Adequate training of the clinician in the use of the ultrasonic descaler. 2. Avoidance of heavy contact loads. 3. An indication of the displacement amplitude of the descaling probe tip and the direction of its oscillation.
19
This is important in order that reports on either the efficiency or potential hazards/damage associated with the instrument may be evaluated in a meaningful manner. Acknowledgements--The author would like to thank Professor W. R. E. Laird and Dr. A. R. Williams for their assistance in the preparation of this paper.
REFERENCES Adams D., Fulford N., Beechy J., MacCarthy J. and Stephens M. (1982). The cardiac pacemaker and ultrasonic scalers. Br. Dent. J. 152, 171-173. Allen E. F. and Rhoades R. H. (1963) Effects of high speed periodontal instruments on tooth surfaces. J. Periodontol. 34, 352-356. Belting C. M. and Spjut P. J. (1964) Effects of high speed instruments on the root surface during subgingival calculus removal. J. Am. Dent. Assoc. 69, 578-584. Bhasker S. N., Grower M. F. and Cutright D. E. (1972) Gingival healing after hand and ultrasonic scaling--biochemical and histological analysis. J. Periodontol. 43, 31-34. Bj6rn H. and Lindhe J. (1962) The influence of periodontal instruments on the tooth surface. Odontologisk. Revy. 13, 355-369. Butt B. G., Harris N. O., Shannon I. and Zander H. A. (1957). Ultrasonic removal of tooth structure: I. A histopathological evaluation of pulpal response in monkeys after ultrasonic cavity preparation. J. Am. Dent. Assoc. 55, 32-36. Clark S. M., Grupe H. E. and Mahler D. B. (1968) The effect of ultrasonic instrumentation on root surfaces. J. PeriodontoL 39, 135-137. Coles R. R. A. and Hoare N. W. (1985) Noise induced hearing loss and the dentist. Br. Dent. J. 159, 209-218. D'Silva I. V., Nayak R. P., Cherian K. M. and Mulky M. J. (1979) An evaluation of the root topography following periodontal instrumentation--a scanning electron microscope study. J. Periodontol. 50, 283-290. Dyson M., Franks C. and Suckling J. (1976) Stimulation of healing varicose ulcers by ultrasound. Ultrasonics 14, 232-236. Esche R. (1952) Untersuchungen der schwingungskavitation in flussigkeiten. Acusitica 2, 208-218. Ewen S. J. (1961) The ultrasound wound--some microscopic observations. J. Periodontol. 32, 315-321. Frost H. M. (1977) Heating under ultrasonic dental scaling conditions. In Symposium on Biological Effects and Characteristics of Ultrasound Sources, pp. 64-76. U.S. Dept. HEW Publications (FDA) 78-8048, Washington DC. Goldman H. M. (1960). Curettage by ultrasonic instrument. Oral Surg. 13, 43-53. Goldman H. M. ( 1961 ) Histologic assay of healing following ultrasonic curettage versus hand instrument curettage. Oral Surg. 14, 925-928. Gritfiths P. V. (1978) The management of the pacemaker wearer during dental hygiene treatment. Dent. Hyg. 52, 573-576. Hansen L. S. and Nielsen A. G. (1956). Comparison of tissue response to rotary and ultrasonic dental cutting procedures. J. Am. Dent. Assoc. 52, 131-137. Healey H. J., Patterson S. S. and Van Huysen G. (1956). Pulp reaction to ultrasonic cavity preparation. U.S. Armed Forces M.J.. 7, 685-692. Holbrook W. P., Muir K. F., MacPhee 1. T. and Ross P. W. (1978) Bacterial investigation of the aerosol from ultrasonic scalers. Br. Dent. Z 144, 245-247. Holden G. G. P. (1962) Some observations on the vibratory phenomena associated with high speed air turbines and their transmission to living tissue. Br. Dent. J. 113, 265-275. Johnson W. N. and Wilson J. R. (1957) Application of the ultra-
20
Ultrasound in Medicine and Biology
sonic dental unit to scaling procedures. J. Periodontol. 28, 264-271. Jones S. J., Lozdan J. and Boyde A. (1972) Tooth surfaces treated in situ with periodontal instruments--scanning electron microscope studies. Br. Dent. J. 132, 57-64. Kerry G. J. (1967) Roughness of root surfaces after use of ultrasonic instruments and hand curettes. J. Periodontol. 38, 340-346. Lang R. J. (1962) Ultrasonic atomization of liquids. J. Acoustic Soc. Am. 34, 6-8. Lorato D. C., Ruskin P. F. and Martin A. (1967) Effect of an ultrasonic scaler on bacterial counts in air. J. Periodontol. 38, 550-554. Luker J. (1982) The pacemaker in the dental surgery. J. Dent. 10, 326-332. Mallerne R. E. (1958) The effects of ultrasonic energy on the periodontal membrane, alveolar bone, and gingivae. J. Prosthet. Dent. 8, 147-152. Mitchell D. F. and Jensen J. R. (1957) Preliminary report on the reaction of the dental pulp to cavity preparation using an ultrasonic device. J. Am. Dent. Assoc. 55, 57-62. Mokrzycki J. (1982). The cardiac pacemakers and ultrasonic scalers (letter). Br. Dent. J. 153, 250. Mrller P., Grevstad A. O. and Kristofferson T. (1976) Ultrasonic scaling of maxillary teeth causing tinnitus and temporary hearing shifts. J. Clin. Periodontol. 3, 123-127. Muir K. F., Ross P. W., MacPhee I. T., Holbrook W. P. and Kowolick M. J. (1978) Reduction of microbial contamination from ultrasonic scalers. Br. Dent. J. 145, 76-78. Pameijer C. H., Stallard R. E. and Hiep N. (1972) Surface characteristics of teeth following periodontal i n s t r u m e n t a t i o n - - a scanning electron microscope study. J. Periodontol. 43, 628-633. Rajstein J. and Tal M. (1984) The effect of ultrasonic scaling on the surface of class V amalgam restorations--a scanning electron microscope study. J. Oral Rehab. 11,299-305. Sanderson A. D. (1966) Gingival curettage by hand and ultrasonic i n s t r u m e n t s - - a histologic comparison. J. Periodontol. 37, 279-290. Sausen R. E. and Jensen J. R. (1957) Ultrasonic applications with heavy pressure--a histologic report. North West Dent. J. 36, 310-315. Schaffer E. M., Stende G. and King D. (1964) Healing of periodontal pocket tissues following ultrasonic scaling and hand planing. J. PeriodontoL 35, 140-148. Simon A. B., Lindhe B., Bonnette G. H. and Schlentz R. J. (1975) The individual with a pacemaker in the dental environment. Z Am. Dent. Assoc. 91, 1224-1229. Summer W. and Patrick M. K. (1964) Ultrasonic Therapy--A Textbook fi~r Physiotherapists. Elsevier, London.
Volume 14, Number 1, 1988 Wakely J. W. (1959) Effects of ultrasonic applications on deciduous and developing monkey teeth. J. Dent. Res. 38, 739. Walmsley A. D. (1985) Some biological effects of ultrasound. Ph.D. Thesis, University of Manchester. Walmsley A. D. (1988) Applications of ultrasound in dentistry. Ultrasound in Med. & Biol. 14, 7-14. Walmsley A. D., Laird W. R. E. and Williams A. R. (1984). A model system to demonstrate the role of cavitational activity in ultrasonic scaling. J. Dent. Res. 63, 1162-1165. Walmsley A. D., Laird W. R. E. and Williams A. R. (1986a) Acoustic absorption wihtin human teeth resulting in an increased pulpal temperature rise. J. Dent. 14, 2-6. Walmsley A. D., Laird W. R. E. and Williams A. R. (1986b) Inherent variability of the performance of the ultrasonic descaler. J. Dent. 14, 121-125. Walmsley A. D., Laird W. R. E. and Williams A. R. (1986c) Displacement amplitude as a measure of the acoustic output of ultrasonic scalers. Dent. Mater, 2, 97-100. Walmsley A. D., Laird W. R. E. and Williams A. R. (in press) Intra-vascular thrombosis associated with dental ultrasound. J. Oral. Pathol. Walmsley A. D., Hickson F. H., Laird W. R. E. and Williams A. R. (1987) Investigation into patients' hearing following ultrasonic scaling. Br. Dent. J. 162, 221-224. Webster D. F., Pond J. B., Dyson M. and Harvey W. (1978) The role of cavitation on the in vitro stimulation of protein synthesis in human fibroblasts by ultrasound. Ultrasound in Med. & Biol. 4, 343-351. Wells P. N. T. (1977) Biomedical Ultrasonics pp. 15-18. London, Academic Press. Wilkinson R. F. and Maybury J. E. (1973) Scanning electron microscopy of the root surface following instrumentation. J. Periodontol. 44, 559-563. Williams A. R. (1974) Release of serotonin from human platelets by acoustic microstreaming. J. Accoust. Soc. A m . / 56, 1640-1643. Williams A. R. (1977) Intravascular mural thrombi produced by acoustic microstreaming. Ultrasound in Med. & Biol. 3, 191-203. Williams A. R. (1983) Ultrasound Biological Effects and Potential Hazards. Academic Press, London. Williams A. R. and Chater B. V. (1980) Mammalian platelet damage in vitro by an ultrasonic therapeutic device. Arch. Oral Biol. 25, 175-179. Zach L. and Brown G. N. (1956) Pulpal effect of ultrasonic cavity preparation: preliminary report. N. Y. Dent. J. 22, 9-17. Zach L., Morrison A. H. and Cohen G. (1957) Effect of ultrasonic operative procedures on the developing dentition. J. Dent. Res. 35, 71-72.