The development of the dental drill–from its origins as a hand instrument to the foot-operated engine and then the electric engine–was a major advancement in the technology available for operative dental procedures. However, after the speed of the electric engine had increased to about 3,000 revolutions per minute by 1914, there was a period of relative stagnation, which lasted until the mid-1950s, when high-speed handpieces were introduced.1
A technological breakthrough came when Dr. John V. Borden, a former U.S. Navy dental officer associated with Dr. Robert J. Nelsen’s turbine project at the National Bureau of Standards, solved the engineering problems inherent in the air-driven turbine handpiece. In 1956, he demonstrated his high-speed drill at the District of Columbia Dental Society meeting. In 1957, the S.S. White Company introduced the Borden Airotor, the first successful air-driven handpiece, which ran at about 300,000 rpm. This instrument is regarded as the precursor to the present generation of ultra–high-speed handpieces.2
This literature review revealed studies with inconclusive results and further studies are indicated.
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RESEARCH ON TURBINE NOISE
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As early as 1959, Dr. Jerome S. Mittelman, a New York dentist, warned the profession about the health hazard caused by the whine of the turbine.3 His primary concern was that the vibrations per second could do "definite irreparable damage over a period of time."3 He recommended that regular audiograms be taken of dental personnel to check for hearing loss resulting from use of the air-turbine handpiece and ultrasonic scaler. He also suggested that cotton saturated in olive oil and squeezed of excess be used as ear plugs to minimize the hazard. Mittelman concluded that not enough information had been collected about the "effect of these instruments to succumb to complacency and neglect." He believed that it made good sense to take precautions to safeguard hearing.3
Air-turbine vs. belt-driven hand-piece.
In 1959, the U.S. Public Health Service supported a research project by Cantwell and colleagues4 on the noise effect from turbine handpieces. The instruments tested were the Ritter R-Borden Airotor air-turbine handpiece (Ritter Co., Rochester, N.Y.) and the Page-Chayes belt-driven handpiece (Chayes Dental Instrument Co., New York).
The air-turbine handpiece produced noise levels above 84 decibels in the range between 4,800 and 9,600 hertz, when operated free-running (that is, not on tooth structure). By contrast, the noise level of the belt-driven handpiece was well below levels that are harmful to hearing. Cantwell and colleagues4 concluded that neither the dentist (working eight hours per day for a lifetime) nor patients were at risk with either instrument. They said, "Temporary threshold shifts may occur but these cannot be considered as constituting a hazard to hearing."4
Need for audiometric checkups.
In the 1960s, Dr. Howard E. Kessler, a Cleveland dentist, seems to have been the main spokesperson for ear protection in the dental office.5 In 1960, he expressed his concern about the problem of possible hearing loss in the operating dentist. He too recommended regular audiometric checkups for the dentist performed by an audiologist. Kessler, who was also a dentofacial speech consultant, explained how an audiometer works and how the resulting audiogram can be used to graph the degree of hearing loss for several high and low frequencies. In this way, he explained, the overall hearing ability in each ear can be determined.5
Earplugs.
Kessler pointed out that "hearing and sight, the most important two of our five senses, deteriorate more quickly and more often than do our senses of smell, taste or touch."6 Although hearing loss does not preclude the practice of dentistry, it does cause "confusion, fear and loneliness, and the personal and personality problems" that arise in relationships with others. Kessler explained that dizziness, which accompanies some types of hearing loss, could cause a problem for the dentist.6 He recommended the use of an earplug for continuous attenuation of noise.7
In 1974, the ADA Council on Dental Materials and Devices recognized that extended exposure to the noise from ultra–high-speed cutting instruments could cause auditory damage.
In 1961, Kessler published an article about hearing as it related to dentistry and high-speed instrumentation.8 That same year, he warned about the possible deleterious effect from the popular audio-distraction analgesia equipment, which had been introduced into the dental office. He advised dentists not to construct their own sound machines. The proper "white sound" (masking sound) should be scientifically standardized; it should not be merely noise.9
In 1962, Hopp10 tested 64 second-year dental students at the University of California and found no acoustic trauma; however, he cautioned that the "noise levels and frequency spectrum of the instruments do indicate that they are capable of producing acoustic trauma." In 1963, Schubert and Glorig11 reported that "the inner ear structure can suffer permanent damage from prolonged exposure to sounds too weak to arouse the pain response." Unfortunately, their article did not include any clinical test results.
In 1965, Taylor and colleagues12 tested 70 dentists and 29 male teachers for hearing loss, and concluded that although the newer types of turbine drills were less noisy than the older ball-bearing types, a definite hearing loss occurred at high frequencies in the group exposed to drill noise. The dentists, who had been exposed to drill noise for three to seven years, demonstrated a significant noise-induced threshold shift in the 6-kilocycles/second and 4-kc/second frequency region that would be undetected by the dentist and that did not constitute a social handicap.12 However, continued exposure caused "gradual encroachment on the upper frequencies of the speech range."12
In 1968, von Krammer13 warned against chronic acoustic trauma from the turbine, which was irreversible, and recommended custom-made ear plugs or ear muffs. In 1969, Ward and Holm-berg14 tested 156 volunteer dentists in Minnesota and concluded that although "the danger to hearing from high-speed drills is small, we cannot claim it is completely negligible."
The Williams-Steiger Occupational Safety and Health Act, or OSHA, of 1970 was an attempt to provide standards to ensure that working environments are free of pollutants and other hazards.15 Occupational noise exposure was one of the areas covered by Part 1910 of Title 29 of the Code of Federal Regulations in 1974.16 The American Conference of Governmental Industrial Hygienists recommended slightly more stringent standards in 1975.17
In 1972, Weatherton and colleagues18 tested 30 dental students and faculty members from the University of Tennessee and found that the hearing levels of faculty members were markedly different from those of students. They attributed a degree of this loss to age differences and believed that the amount of noise exposure was probably limited in the dental school setting. However, they recommended further study of both the dental students and practitioners.
Council on Dental Materials and Devices.
In 1974, the American Dental Association’s Council on Dental Materials and Devices recognized that extended exposure to the noise from ultra–high-speed cutting instruments could cause auditory damage.19 However, the Council believed that the age and physical condition of the individual, frequency of exposure, intensity of loudness, length of exposure time and intervals between exposures were factors to consider. Although earlier-model turbines had recorded noise levels up to 8,000 Hz, the newer models were reduced to a range from 2,000 to 6,000 Hz. Anything higher than 1,000 Hz was considered dangerous. The Council recommended "optimum maintenance of rotary equipment, reduction of the ambient noise level in the operatory (soundproofing, acoustical ceilings, baffle drapes, resilient floors, rational location of the compressor and other noise-making equipment), and personal protection through the use of ear plugs."19
Park15 pointed out that the dentist did not use the turbine continuously during an eight-hour day, but was only exposed to the "siren-like sound" of the turbine for short bursts of 15 to 30 seconds, followed by periods of relative rest while pursuing other procedures. Sometimes, one ear may be more affected than the other, depending on the head position in relation to the turbine.20
By the 1980s, the medical profession recognized that exposure to loud sounds could cause temporary or permanent damage to the inner ear.
Years in dental practice.
Finally, in 1978, at the Long Island Jewish-Hillside Medical Center, New Hyde Park, N.Y., Forman-Franco and colleagues21 conducted a study of 72 dentists (56 percent of whom were general practitioners) with a single-channel audiometer to test for hearing loss. They found that when comparing the hearing levels, as adjusted for age, of the general population with the hearing levels of the dentists, no statistical differences were found. The authors reported that a correlation appears to exist between years in dental practice and progressive loss of hearing. However, this mimics the relationship of advancing age and loss of hearing in the normal population and suggests that when a loss of hearing occurred, it was primarily an effect of aging.21
In addition to the dental profession, industry had its problems with sound-induced hearing loss. Alberti and Riko22 called noise the most ubiquitous industrial pollutant. Compliance with OSHA regulations did not guarantee protection against work-related hearing loss. Medical statistics, which led to OSHA standards, projected in 1979 that 18 percent of workers exposed to the "legal limit of 90 dBA for eight hours would sustain a handicapping hearing loss."23 In some states, the age of the claimant was a factor in determining workers’ compensation (younger workers received more money). The hearing tests were to be conducted by a trained audiometric technician. If noise levels were lower than 85 decibels per ampere, OSHA did not require a hearing conservation program.23
Medical profession concerns.
By the 1980s, the medical profession recognized that "exposure to loud sounds" could cause "temporary or permanent damage to the inner ear."24 In 1980, Zubick and colleagues25 tested 217 people, including 111 dentists exposed to the air turbine, 26 dental specialists and 80 physicians. They found a "statistically significant difference in hearing sensitivity between both groups of dentists and the group of physicians."25
The authors were surprised to find high-frequency hearing loss in the dental specialists, because they thought these dentists would not have been subjected to the same noise levels as general practitioners. Apparently, they were exposed before their postgraduate specialty training or in their office environment. The results also demonstrated that right-handed dentists exhibited greater hearing loss in the left ear, whereas no such difference was seen among right-handed physicians. They concluded that the left ear is closer to the drill and therefore suffered more "toxic noise level" damage.25
In 1984, Sheldon and Sokol26 pointed out that noise-induced hearing loss generally went unnoticed because it was gradual, progressive and painless. In 1985, Coles and Hoare27 reported two cases of noise-induced hearing loss in dentists. However, they concluded that it was the "mistaken belief by some [ear, nose and throat specialists] and audiological consultants that noise exposure in the [dental office] is a likely cause of hearing difficulty and/or tinnitus in dentists." Wilson and colleagues28 concluded in their 1990 study that the "risk of significant hearing loss as a result of dental practice is small if modern handpieces in good condition are used."
In 1992, Merrell and Claggett29 concluded that if those who are susceptible to "noise-induced hearing loss" are exposed to daily harmful noise levels, they will "probably incur noise-induced hearing loss during their lifetime." They recommended that dentists receive regular hearing evaluations by an audiologist and wear ear protectors.29 In 1995, Fabry30 studied the question of whether dentists were at greater risk of experiencing hearing loss than was the remainder of the population, and found a cause-and-effect relationship between hearing loss and use of dental drills.
OSHA guidelines.
Some researchers had reported noise levels of 100 dB with air turbines. Daily cumulative drill noise of 12 to 45 minutes falls within the recommended OSHA guidelines, which allow eight hours of exposure to a 90-dB sound pressure level stimulus.30 However, the older drills produce 100 dB or more, and the allowable exposure durations are reduced to about two hours per day, according to the OSHA guidelines.30 Fabry30 recommended that dentists undergo periodic audiometry, with the use of modern equipment, and wear ear protection.
In 1999, Hinze and colleagues31 also recommended earplugs and provided the dentist with directions for constructing them in the dental office. Ahmed and colleagues32 studied 269 noise-exposed and 99 nonexposed factory worker subjects in 2001, and concluded that the noise-exposed workers were at "high risk of developing noise induced hearing loss due to excessive occupational exposure to noise."
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CONCLUSION
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Because this literature review revealed studies with inconclusive results, it seems appropriate to reevaluate the hearing loss potential among dental students, faculty members, practicing dentists and other dental staff members who work with air-turbine handpieces. Is there a correlation between the use of the air turbine and hearing loss? Should dentists and staff members wear ear protection? Unfortunately, there seems to have been a lack of research during the past 20 or so years. Further studies are indicated.
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ABSTRACT
RESEARCH ON TURBINE NOISE
