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Year : 2013  |  Volume : 17  |  Issue : 4  |  Page : 423-428  

Mechanized scaling with ultrasonics: Perils and proactive measures

Department of Periodontics, Krishnadevaraya College of Dental Sciences, Bangalore, Karantaka, India

Date of Submission25-Jun-2012
Date of Acceptance13-Jun-2013
Date of Web Publication17-Sep-2013

Correspondence Address:
Rashmi Paramashivaiah
No. 307, 6th C Cross, 3rd Main Road, OMBR Layout, Banaswadi, Bangalore - 560 043, Karantaka
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0972-124X.118310

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Mechanized scaling for plaque removal is a routine procedure in the practice of periodontics. Though it appears innocuous by itself, there are retinues of hazards associated with it on various organ systems in the body. Some of these unwanted effects and measures to avoid or ameliorate the same are elaborated here. Exposure to ultrasonic scaling is inevitable before any other treatment procedure. Aerosol contamination, vibrational hazards, thermal effects on the dental pulp, altered vascular dynamics, disruption in electromagnetic device, diminished hearing and dental unit waterline contamination are some of the probable off-shoots a patient has to bear. Uses of barrier devices, proper attention to usage of equipment, protection for ear and water treatment are few of solutions for the same. Though documented evidence for the existence of all effects is lacking, it is never the less significant for the overall safety of the patient. A conscientious clinician should therefore inculcate the available steps to overcome the hazards of ultrasonic scaling.

Keywords: Aerosols, auditory perception, dental plaque, dental pulp, scaling, ultrasonics

How to cite this article:
Paramashivaiah R, Prabhuji MV. Mechanized scaling with ultrasonics: Perils and proactive measures. J Indian Soc Periodontol 2013;17:423-8

How to cite this URL:
Paramashivaiah R, Prabhuji MV. Mechanized scaling with ultrasonics: Perils and proactive measures. J Indian Soc Periodontol [serial online] 2013 [cited 2022 May 28];17:423-8. Available from:

   Introduction Top

The periodic removal of supra and sub-gingival plaque is essential for controlling inflammatory periodontal diseases. [1] Mechanized instruments use the water-cooled instrument tip vibrating at high frequency for eradication of bacterial deposits from teeth and periodontal pockets. The two categories of mechanized instruments are ultrasonic and sonic handpieces. [2] Though they are indispensable tools of periodontics, they are not without their set of perils. Thus, some of the inherent problems associated with the use of ultrasonic scalers and the practical solutions for the same are elaborated here.

   Indoor Air Pollution Top

Health problems related to poor indoor air quality are often related to contaminants, which include bioaerosols, non-viable particulates and volatile organic chemicals. [3] Many routine dental procedures produce aerosol and splatter composed of varying combinations of water, tissue and tooth dust, blood and saliva. At a minimum distance of 50 cm from patient undergoing ultrasonic scaling about 33% of blood borne aerosols were isolated in a recent study. [4]

Micik et al. defined dental aerosols as particles smaller than 50 μm with any particles larger than 50 μm being described as splatter. [5] Aerosol particles of 50 μm or smaller remain airborne for a longer period and enter the nasal passages with penetration deep into the respiratory tree creating health concern. All aerosol particles may contain blood elements with attached viral particles, such as human immunodeficiency virus and hepatitis B virus. [6] Other diseases known to be spread through airborne route are pneumonic plague, tuberculosis, influenza, legionnaire's disease and severe acute respiratory distress syndrome. [7]

   Reduction of Indoor Air Pollution Top

Recommendations by American Dental Association (ADA) and Prevention and Occupational Safety and Health Administration support the use of a "spray-wipe-spray" technique. Use of a surface disinfectant with documented evidence of bactericidal, tuberculocidal and virocidal activity is recommended. All areas within six feet radius of the operation should be cleaned. [7]

The dental team should not rely on a single precautionary strategy.

In the reduction of dental aerosols, the first layer of defense is personal protection barriers such as masks, gloves and safety glasses. For masks to be protective, they must filter effectively aerosol particles of 50 μm or less with minimal marginal leakage. [8]

The second layer is antiseptic pre-procedural rinse with a mouthwash such as chlorhexidine.

The third layer of protection is the routine use of a high volume evacuator (HVE).

An additional fourth layer would be the use of a device to reduce aerosol contamination that escapes the operating area, such as a high efficiency particulate air filter (HEPA). Extra-oral HVE covered with a towel can filter out blood-borne aerosols. [4]

According to the ADA and centers for disease control a room ceiling equipped with HEPA will be 99.97% efficient in removing all particles 0.3 μm or larger in diameter. [3]

The advantages and disadvantages of commonly used protective measures are given in [Table 1]. [7]
Table 1: Methods of reducing airborne contamination

Click here to view

   Vibration Hazards Top

It is well-recognized that the large amplitudes produced by pneumatic drills will cause "white finger" or "acrocyanosis." This is a disruption in the blood flow to the fingers, caused by the vibration that is passed from the drill through to the hand. [9]

A study was undertaken on 120 subjects comprising of 60 dentists and hygienists exposed to vibrations, from high-speed hand pieces and ultrasonic scalers and a control group of 60 dental assistants and medical nurses. [10] They were assessed for manual performance, tactility, strength, etc., In this study, it was found that the vibrations could produce a reduction in strength and tactile sensitivity and performance. It was stated that vibrating tools are used for an average of 75 min a day highlighting the need for the reduction of exposure time.

   Reduction of Hazards From Vibration Top

Prolonged operation of an ultrasonic scaler may cause alteration of blood and nerve supplies to the operator finger. In this respect, adequate standardization of instruments and procedures is essential including:

  1. Adequate training of the clinician in the use of the ultrasonic scaler
  2. Avoidance of heavy contact loads
  3. An indication of the displacement amplitude of the scaling probe tip and the direction of its oscillation. [11]
Research is needed to aid in the development, design and production of an ultrasonic hand piece that will eliminate any vibration hazards to the operator. [9]

   Thermal Hazards Top

If the ultrasonic scaler is perfectly coupled at the probe/enamel interface then about 37% of ultrasonic energy would leave the metal and enter the tooth. However, this does not occur because:

  1. The dimensions of the probe tip are much smaller than the wavelength of sound at these frequencies
  2. There is usually a thin layer of water imposed between the probe and the tooth.
If heavy contact pressures are used the coupling will then 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 the enamel, especially if there is inadequate or no water cooling. However, the presence of water may act as a matching layer allowing uniform ultrasound energy to enter the tooth. [11]

A study has shown that temperature rise in the tooth caused by heating can cause damage to the pulp and dentine. An increase above 11°C was shown to invariably destroy the pulp and a 17°C increase produced pulp death. [12] However, a recent work suggests that an increase of 11.2°C produces no damage on the pulp tissues. [13]

   Down-Regulation of Thermal Hazards Top

Consistent use of the unit on the highest power setting will increase the potential for thermal damage without necessarily increasing the efficiency. [14] Temperature sensitivity can be controlled by slightly reducing the water flow or in the case of units with self-contained lavage warm water can be added to the container. When soft-tissue sensitivity is perceived as a problem and local anesthetics are not viable treatment choice, long acting topical anesthetic solutions have been used successfully for sub-gingival debridement. [2]

A study was designed to compare the temperature rise in dentin during ultrasonic scaling using either ultrasonic handpiece irrigation, intermittent bulb irrigation or no irrigation. A single clinician applied light forces, ultrasonic device was set to a fixed power setting and water flow was adjusted to 15 ml/min to reduce the number of study variables. Ultrasonic coolant temperature increases with duration of instrumentation and reaches a plateau, which is affected by power setting and irrigation volume. Bulb syringe irrigation and ultrasonic device water spray significantly reduced heat generation from ultrasonic scaling to physiologically tolerable levels. [15]

Thus, most studies conclude that powered scaling should not be considered without irrigation and the flow rate in the region of at least 20-30 ml/min. [9]

   Thrombogenic Hazards Top

During dental treatment, the oscillating tip of ultrasonic scaler will be in contact with the tooth. It may be possible that the tooth acts as a waveguide conducting the vibrational energy from the scaler toward the apex of the root. If sufficient energy reaches the root, then it could pose a thrombogenic hazard to the blood vessels passing through the apical foramen into the pulp. This may lead to a potential loss of tooth vitality. [16]

A study conducted in vivo on mice to assess the effect of a wire vibrating at a frequency of 20 kHz placed against an intact blood vessel resulted in the production of platelet emboli and the formation of thrombi. [17] This work was repeated with a commercially available ultrasonic scaler, which demonstrated similar findings. [9] Thus, ultrasound transmissions into the tooth may result in potential damage to the structures such as blood vessels both within and around the teeth.

   Alleviation of Thrombogenic Hazards Top

A probe tip operating at displacement amplitude of 44 μm directed perpendicular to the tooth surface under heavy contact pressure only resulted in displacement amplitude at the root tip of 1-2 μm. Thus, thrombogenic hazard may result when the longitudinal oscillations are directed perpendicular to the tooth surface. Thus, changing the direction of the oscillating tip so that it is parallel to the tooth reduces the vibration at the root apex to a barely detectable levels. [11]

   Interruption of Electronic Devices Top

The electromagnetic device of vital importance is the cardiac pacemaker. 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 non-competitive (demand type), the former discharging at a fixed rate while the latter only discharges if the rate becomes irregular. [18]

The handpiece of the ultrasonic scaler produces an electromagnetic field and the severity of interference is dependent on the strength of this field. [9] The electromagnetic field produced by magnetostrictive ultrasonic scalers may interfere with the pacemaker discharge rate, resulting in a serious life-threatening hazard to the patient. [19] It has been suggested that any effects, which have been observed may be the result of a non-competitive type of pacemaker switching over to a fixed mode during the period of interference. [18] Work investigating the magnetostrictive scaler stated that interference can be caused if the pacemaker lead comes within 37.5 cm of the scaler. [20] No reports of interference caused by piezoelectric scaler have been reported. [19]

   Avoidance of Interference with Electromagnetic Devices Top

An elaborate medical history for every patient is prerequisite to rule out the presence of cardiac pacemakers. Consultation with the patient's physician and cardiologist is a must in patients with pacemakers. A safe distance of 50 cm or more is a must in these patients. [21] Magnetostrictive ultrasonic scalers should not be used on patients with a pacemaker. [22]

   Auditory Hazards Top

Lesions of hearing apparatus can be chronic or acute. Acute acoustic trauma is caused by a high intensity noise stimulus such as an explosion or gunfire. The onset is painful and may be reversible or irreversible. Chronic acoustic trauma is caused by prolonged exposure to lower intensity sound irritant and is painless. The damage is irreversible because cochlear hair cells cannot regenerate. Although a certain amount of gradual hearing loss is considered normal as a part of ageing (presbycusis), prolonged exposure to excessive noise can greatly add to hearing loss.

The cognitive sound frequency for human ear is 20-20,000 Hz. Sounds below this range are infrasonic and those above are ultrasonic. The important speech frequency falls between the ranges of 250 Hz and 4000 Hz. A high-intensity, high-frequency noise can damage the high frequency receptors. [22]

Kilpatrick proposed a number of sounds in the dental office that may be hazardous to dentists hearing:

  1. High-speed turbine
  2. High-volume aspirator
  3. Ultrasonic scaler
  4. Mixing devices for stone, amalgam, etc.
  5. Music playing loudly and continuously. [23]

Damage to operator hearing is possible through airborne sub-harmonics of the ultrasonic scaler. For the patient, damage can occur through the transmission of ultrasound through tooth contact to the inner ear through the bones of the skull. [9]

Degree of risk to the individual dentists depends on several factors.

  1. Intensity of noise
  2. Frequency spectrum of noise
  3. Duration of exposure each day
  4. Distance from the source
  5. Individuals age, physical condition and susceptibility
  6. Type of procedure
  7. The intensity of noise emitted from hand pieces differs from manufacturer to manufacturer
  8. Previous exposure to damaging noise resulting in permanent injury to hearing
  9. Working environment: Materials in the dental operatory, i.e., smooth cement walls and floors reflect noise almost completely, whereas draperies absorb noise considerably. [24]
Hearing loss is expressed in terms of hearing threshold shifts in decibels (dB) as determined by audiometry. Permanent threshold shifts due to moderate noise levels, such as those encountered in dentistry would be detectable after years of exposure. [25]

In United Kingdom, the noise at work regulations state that a maximum exposure of 85 dB is permissible daily during an 8-h working period. This was ascertained using a formula given in the regulations. [26]

A study was undertaken to measure the noise levels made by different dental handpieces and equipment in dental practices and laboratories. Noise levels were measured in four dental practices and three dental laboratories. In the dental clinics, almost all noise produced by the dental instruments did not exceed the maximum permissible level of 85 dB. The only instrument that seemed to emit noise that was higher than 85 dB was the ultrasonic scaler in one of the dental clinics. [24]

Möller et al. reported temporary threshold shifts (TTS) in hearing in eight out of 20 subjects following a 5-min ultrasonic scaling procedure. [27] Unilateral changes of 10-20 dB in the frequency range of 3-10 kHz were demonstrated in these patients, three of whom had bilateral tinnitus. Both tinnitus and TTS are commonly accepted as early predictors of noise-induced hearing loss (NIHL). The liberal use of dental ultrasonic instruments may therefore pose a potential hazard to the hearing of the patient. [28]

A longitudinal study of 10 years duration on general practitioners and general dental practitioners concluded that dentists are at higher risk of hearing impairment although not generalized. [29]

Thus, ultrasonic scaler has been shown to cause no permanent harm to hearing through airborne noise. Transmission of ultrasound through the bone may potentially damage the inner ear; although, this has not been demonstrated. Work is needed to identify if hearing loss is a problem for patients who receive regular ultrasonic scaling. [9]

Attenuation of auditory hazards

As NIHL is not a treatable condition it can only be partly relieved by rehabilitative means, prevention therefore assumes a great importance. [24]

Theoretically there are five methods of prevention of NIHL:

  1. Exclusion from noise exposure of those who are most susceptible to NIHL
  2. To reduce the exposure time
  3. Personal evaluation: Technicians should have baseline otologic and audiometric examination to determine the current hearing status. Annual retests should be carried out to detect any degree of change taking place
  4. Reduction in noise level may be achieved by regular maintenance of equipment and early repair or replacement of defective items. The working room should be made more acoustically satisfactory by minimizing the hard surfaces
  5. Personal protection with the use of ear muffs or plugs. Ear plugs are inserted into the ear canal, whereas ear muffs cover the entire external ear by means of a sealed cup. In general, plugs provide less noise attenuation than muffs and are useful in the noise levels from 100 dB to 105 dB. Muffs subdue noise levels from 110 dB to 115 dB. [30]

   Root Surface Removal by Ultrasonics Top

Evidence shows that ultrasonic and sonic scalers are effective in plaque and calculus removal, surface alterations including scratches, gouges and nicks increase exponentially as the ultrasonic power is increased from medium to high. Studies also reveal that as instrument contact time, tip to tooth angle and instrument pressure is increased, the likelihood of root surface damage is also increased. [31],[32] A growing body of evidence suggests that forceful instrumentation of the root surface to achieve cementum removal may be unnecessary and harmful. Cemental thickness can be assessed using fluorescence microscopy, contact microradiography and toluidine blue staining after the injection of labeling agents. [33] In fact, overzealous root planning removes important protein components such as bone morphogenetic proteins and slows down critical fibrous attachment from bone to root. [34]

Increasing the power setting from low to high may actually increase the loss of root substance from 11.37 ± 3.64 mm to 23.37 ± 15.76 mm. [35]

   Diminution of Root Substance Removal Top

Defect volume and defect depth created by magnetostrictive and piezoelectric scalers are influenced by the combination of working parameters such as lateral force and tip angulations. Instrument power setting has to be low or medium to minimize tooth substance removal. 30B severe root damage at 40 s instrumentation occurred under most combination of lateral force tip angulation and power settings. Therefore, minimal forces, correct angulations and optimum power setting are mandatory to prevent the root surface damage. [2] Though, it is reported that thinner tips cause more recession the data are not conclusive to preclude the usage of these tips. [36] Rather than creating a glassy smooth root surface, Thompson believes that velvety smooth, clean surfaces with a resulting decrease in root sensitivity is most conducive to tissue healing. [34]

   Dental Unit Waterline Contamination Top

Water recovered from dental units connected to municipal water supplies may contain millions of bacterial colony forming units per millimeter. Biofilms have been shown to be a primary source of contaminated water delivered by dental units. [37] The medical risk from the microbial contamination of water is the most significant to immunosuppressed individuals. Patients and clinicians temporarily compromised by infection and stress may also be at risk of infection. [2] This water can spread infection by the dispersed contaminated aerosols. [38]

A survey was undertaken by Williams et al. wherein DUWL samples were collected from 116 three-way syringe lines, 54 high-speed hand-pieces and 12 scaler lines from about 150 operatories at 54 sites in Washington, Oregon and California. Samples from 12 scalers showed a similar pattern of severe microbial contamination (mean 19,800 CFU/ml, SD = 37,300). Sections of functioning DUWL showed complete biofilm layers lining the inner surface. [39]

   Prevention of Duwl Contamination Top

The ADA has called for dental researchers and dental manufacturers to design equipment and measures to reduce waterline contamination. The announced goal was to match the standards for kidney dialysate, or 200 CFU/ml. [40]

The various approaches to risk reduction of DUWL contamination are: [41]

Anti-retraction valves and retrograde aspiration of oral fluids

Anti-retraction valves will limit re-aspiration and are most efficacious when fitted immediately distal to the handpiece. Autoclaving the hand-pieces after use and flushing of units for 30 s at the end of the day will augment the action of the anti-retraction valve and should help to eliminate any aspirated liquid. Back flow from dental units to the mains water supply may occur and it is necessary to install check valves to prevent this from occurring.


Using filters on the dental waterline to eliminate bacteria from the water entering the handpiece, especially to reduce the planktonic bacteria. Therefore, disposable filters are recommended, which must be changed daily. Filters should be inserted just distal to the point of entry of water into the hand-piece for maximum efficiency. A pore size of 0.2 microns is recommended.


ADA recommends that waterlines should be flushed through for several minutes at the start of each clinic day. Flushing for 20 min, which would be impractical in most dental surgeries, will reduce the bacterial count to zero. However, the persistent nature of contamination is demonstrated when 30 min later, shedding of bacteria from the biofilm returns the total colony counts to within pre-flush range.

Biocides and chemical disinfectants

Biocides have been used in an attempt to remove the biofilm and eliminate the planktonic bacterial count. These include chlorhexidine gluconate, povidone-iodine, ethanol, hypochlorite, peroxide and glutaraldehyde. The intrinsic resistance of the biofilm ecosystem has hampered their value.


Chlorine, as sodium hypochlorite, is the most commonly employed biocide in water treatment plants and has proven efficacy in cold water hospital systems. Independent reservoir clean water systems can be used to deliver chlorine flushes to the dental waterline. The system can be "shocked" or hyper-chlorinated intermittently with high doses of 50 ppm chlorine every 6 months.

Peroxide, ozone and ultraviolet light

Hydrogen peroxide and ozone are compounds that can be introduced continuously into waterlines. UV treatment of water has been used alone or in conjunction with ozone and other biocides for the control legionellae and reduction of endotoxins in water. A major advantage of these systems is that they avoid introducing chemical disinfectants into the effluent water system with the potential for pollution and destructive effects on wildlife.

Independent clean water systems

A separate pressurized clean water reservoir system filled with sterile water plumbed to the mains connections to the municipal water provides sterile water. The total volume of water consumed per day as an irrigant is in the range of 1-2 L/dental unit; thus, the use of small reservoir is sufficient. Reservoirs should be used preferably with sterile water or boiled water that is allowed to cool in a sterile sealable container. Letting the system to drain down to dry and purging with air or ethanol will help to prevent the biofilm proliferation due to desiccation.

Autoclavable systems

A fully autoclavable assembly of water reservoirs, silicon multi-lumen DUWL tubing and fittings to be sterilized between patients could be used for surgical procedures.

   Conclusion Top

Considering the ethical issues of treating a patient under absolute safety it is the responsibility of every practitioner to pay heed to the above adverse effects and apply stringent measures to counteract the same.

   References Top

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2.Nield-Gehrig JG. Basic concepts of ultrasonic instrumentation. Fundamentals of Periodontal Instrumentation. 4 th ed. Baltimore: Lippincott Williams and Wilkins; 2009.  Back to cited text no. 2
3.Webber TC. Indoor air quality and the dental profession. Invironment 1999;12:49-56.  Back to cited text no. 3
4.Yamada H, Ishihama K, Yasuda K, Hasumi-Nakayama Y, Shimoji S, Furusawa K. Aerial dispersal of blood-contaminated aerosols during dental procedures. Quintessence Int 2011;42:399-405.  Back to cited text no. 4
5.Micik RE, Miller RL, Mazzarella MA, Ryge G. Studies on dental aerobiology. I. Bacterial aerosols generated during dental procedures. J Dent Res 1969;48:49-56.  Back to cited text no. 5
6.Harrel SK, Barnes JB, Rivera-Hidalgo F. Aerosol and splatter contamination from the operative site during ultrasonic scaling. J Am Dent Assoc 1998;129:1241-9.  Back to cited text no. 6
7.Harrel SK, Molinari J. Aerosols and splatter in dentistry: A brief review of the literature and infection control implications. J Am Dent Assoc 2004;135:429-37.  Back to cited text no. 7
8.Micik RE, Miller RL, Leong AC. Studies on dental aerobiology. 3. Efficacy of surgical masks in protecting dental personnel from airborne bacterial particles. J Dent Res 1971;50:626-30.  Back to cited text no. 8
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10.Akesson I, Lundborg G, Horstmann V, Skerfving S. Neuropathy in female dental personnel exposed to high frequency vibrations. Occup Environ Med 1995;52:116-23.  Back to cited text no. 10
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12.Zach L, Cohen G. Pulp response to externally applied heat. Oral Surg Oral Med Oral Pathol 1965;19:515-30.  Back to cited text no. 12
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14.Wilkins EM. Ultrasonic and Sonic Scaling Clinical Practice of the Dental Hygienist. 8 th ed. Philadelphia, London: Lippincott Williams and Wilkins; 1999.  Back to cited text no. 14
15.Nicoll BK, Peters RJ. Heat generation during ultrasonic instrumentation of dentin as affected by different irrigation methods. J Periodontol 1998;69:884-8.  Back to cited text no. 15
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17.Williams AR. Intravascular mural thrombi produced by acoustic microstreaming. Ultrasound Med Biol 1977;3:191-203.  Back to cited text no. 17
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22.Miller CS, Leonelli FM, Latham E. Selective interference with pacemaker activity by electrical dental devices. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1998;85:33-6.  Back to cited text no. 22
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27.Möller P, Grevstad AO, Kristoffersen T. Ultrasonic scaling of maxillary teeth causing tinnitus and temporary hearing shifts. J Clin Periodontol 1976;3:123-7.  Back to cited text no. 27
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31.Lie T, Leknes KN. Evaluation of the effect on root surfaces of air turbine scalers and ultrasonic instrumentation. J Periodontol 1985;56:522-31.  Back to cited text no. 31
32.Ritz L, Hefti AF, Rateitschak KH. An in vitro investigation on the loss of root substance in scaling with various instruments. J Clin Periodontol 1991;18:643-7.  Back to cited text no. 32
33.Ogura N, Mera T, Sato F, Ishikawa I. Longitudinal observation of cementum regeneration through multiple fluorescent labeling. J Periodontol 1991;62:284-91.  Back to cited text no. 33
34.Thompson D. Faster, easier, better root planing with ultrasonics. Dent Econ 1993;83:80-1.  Back to cited text no. 34
35.Lampe Bless K, Sener B, Dual J, Attin T, Schmidlin PR. Cleaning ability and induced dentin loss of a magnetostrictive ultrasonic instrument at different power settings. Clin Oral Investig 2011;15:241-8.  Back to cited text no. 35
36.Casarin RC, Bittencourt S, Ribeiro Edel P, Nociti FH Jr, Sallum AW, Sallum EA, et al. Influence of immediate attachment loss during instrumentation employing thin ultrasonic tips on clinical response to nonsurgical periodontal therapy. Quintessence Int 2010;41:249-56.  Back to cited text no. 36
37.Surgical, nonsurgical, occlusal and furcation therapies. Periodontol 2000 1995;9:1-185.  Back to cited text no. 37
38.Kumar S, Atray D, Paiwal D, Balasubramanyam G, Duraiswamy P, Kulkarni S. Dental unit waterlines: Source of contamination and cross-infection. J Hosp Infect 2010;74:99-111.  Back to cited text no. 38
39.Williams JF, Johnston AM, Johnson B, Huntington MK, Mackenzie CD. Microbial contamination of dental unit waterlines: Prevalence, intensity and microbiological characteristics. J Am Dent Assoc 1993;124:59-65.  Back to cited text no. 39
40.ADA statement on dental unit waterlines. Adopted by the ADA Board of Trustees, December 13, 1995 and the ADA Council on Scientific Affairs, September 28, 1995. Northwest Dent 1996;75:25-6.  Back to cited text no. 40
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   Indoor Air Pollution
    Reduction of Ind...
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    Down-Regulation ...
   Thrombogenic Hazards
    Alleviation of T...
    Interruption of ...
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