Poor Mercury Hygiene from Ultrasonic Amalgam Condensation

Poor Mercury Hygiene from Ultrasonic Amalgam Condensation

ORIGINAL ARTICLES The article “Poor Mercury Hygiene from U ltra­ sonic Amalgam Condensation,” while still in proof-form, was shown to several manufact...

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ORIGINAL ARTICLES The article “Poor Mercury Hygiene from U ltra­ sonic Amalgam Condensation,” while still in proof-form, was shown to several manufacturers of ultrasonic dental prophylaxis equipment. A t least one manufacturer has announced already that it is cooperating with the Association to the extent that its sale of amalgam-condensing tips for use with such ultrasonic equipment will be suspended until there is more information on the relative roles of office carpeting, ventilation, condensing devices, and other factors. The de­ velopment of standards for ventilation and hy­ gienic principles in the use of mercury may be indicated after that information is gathered. Council on Dental Materials and Devices American Dental Association

Poor mercury hygiene from ultrasonic amalgam condensation

H. H. Chandler, ODS N. W. Rupp, DDS G. C. Paffenbarger, DDS, Washington, DC

A cloud of mercury droplets and alloy particles was emitted from soft amalgam at the working tip of the ultrasonic instrument tested. Mercury vapor levels were 20% of the allowable threshold limit value and probably are not hazardous. However, continued use of the ultrasonic instrument would result in deposition of numerous mercury droplets throughout the dental operatory and could thereby cause higher mercury vapor levels, especially in poorly ventilated places. Inhalation of the emitted material by the patient and dental health person­ nel is not good hygiene. Therefore, the use of this instrument for amalgam condensation is contra­ indicated until the safety of the instrument for this purpose is firmly established.

The potential hazard of mercury poisoning of den­ tal health personnel and dental patients has been studied by many investigators.1-8 Souder and

Sweeney1 found that the amount of mercury vapor emitted from dental restorations o f silver am al­ gams was undetectable and that there was little danger from the ingestion of mercury in solution from these restorations. Nixon and Smith2 found that the amount of mercury in the fingernails and hair of dental workers was often many times that of a control group. Mercury vapor levels in dental offices are in­ creased during periods of m anipulation of m er­ cury and the mixed amalgam. Airaksinen3 ana­ lyzed the air in dental offices and concluded there were safe levels of mercury vapor in all instances. Highest levels were found during mixing, but he considered the danger of poisoning to be slight. Nossek and Seidel4 determined the mercury va­ por content in the air after ultrasonic amalgam condensation and observed no increase in m er­ cury vapor over that produced by hand condensa­ tion, in spite of macroscopically visible spraying of plastic amalgam. Meyer5 determ ined that the threshold limit value6 of 0.1 mg mercury/m3 of air was exceeded by two to four times during p ro ­ cedures such as amalgam carving and removal of old amalgams but that these concentrations were of short duration. Most investigators, including Grossm an and Dannenberg7 and Frykholm ,8 agree that if recJADA, Vol. 82, March 1971 ■ 553

Fig 1 ■ Cloud of m aterial (arrows) being em itted from am al­ gam at tip of u ltra so n ic amalgam condensing instrum ent. Particles (also shown in Fig 3) se ttle on instrum ent handle (A) and may be responsible fo r some of haze surrounding handle.

ommended precautions are followed, the mercury vapor concentration is maintained well below the threshold limit.6 But it is also agreed that the ef­ fects of long periods of exposure to even minimal amounts of mercury vapor are not well known and therefore, such exposure should be kept at a minimum. A preliminary investigation in this laboratory designed to study the effects of ultrasonic conden­ sation on the properties of dental amalgam, re­ vealed that when the ultrasonic instrument was being used, significant amounts of material were emitted from the working area in the form of a cloud (Fig 1). The nature of this cloud and the amount of mercury vapor in the surrounding area while dental amalgam was being condensed with the ultrasonic device were determined.

sured and found to be 24,850 ± 150 Hz and 0.9 ± 0 .1 thousandths of an inch, respectively.) An amalgam condensing insert was used with maxi­ mum water cooling and a power setting of 2 (me­ dium). The temperature of the plashy amalgam after removal from the mechanical mixer and dur­ ing compacting with the ultrasonic plugger was about 35 C as recorded with a 36-gauge (B and S) Type T thermocouple. During compaction occa­ sional transient peaks of 55 to 60 C were recorded. Other inserts and power settings, and in one in­ stance an ultrasonic instrument of the same brand in a different operatory, produced essentially the same type of cloud. Amalgam mixes were prepared with commer­ cially available alloys at 1:1 mercury-to-alloy ratio. To collect a sample of material from the aero­ sol cloud, we held a suction device containing an in-line filter (maximum opening 0.2/u.m) 7 cm above the operating area. This distance from the working tip was used for the collection of particles whereas with the vapor detector the intake hose opening was held 30 cm from the working tip. The material gathered on the filter was examined and photographed with a metallographic micro­ scope. Mercury vapor levels were determined with an Instantaneous Vapor Detector (General Electric Catalog no. 9790339G1). The instrument func­ tions on a principle of ultraviolet light absorption as it passes through an atmosphere containing mercury vapor. Vapor levels between 0.01 and 3.0 mg/m3 of air are detectable.9 The intake hose of the detector was placed 30 cm from the tip of the condenser to simulate operator-to-tooth working distance. The position of the intake hose was adjusted so that it was approx­ imately in line with the air currents in the im­ mediate working area. For comparison purposes, identical tests were run with use of hand conden­ sation and two mechanical condensers.* The amalgam mixes for vapor detection were condensed for three minutes in a steel die con­ taining a 4 X 8 mm cylindrical mold. The amal­ gam cylinder was left undisturbed for three min­ utes before it was removed to another room; ex­ cess was not removed from its top surface.

Materialsandmethods The ultrasonic device used in this study is reported to develop 25,000 mechanical strokes per second at the working tip with a stroke length of 0.001 inch. (The frequency and amplitude were mea554 ■ JADA, Vol. 82, March 1971

Resultsanddiscussion ■ Aerosol dispersion: Figure 1 shows the aero­ sol dispersed from the area of the tip of the ultra­

sonic condensor. In some instances the cloud rose as high as 60 to 90 cm. This cloud is carried by air currents, the way cigarette smoke is carried, and the particles settle in a 60 to 90 cm radius from the working area. The operator, assistant, and patient could inhale, ingest, or inhale and in­ gest the emitted material during amalgam con­ densation. Higher power settings and mixes con­ taining more mercury resulted in the emission of more material than low power settings and mixes containing less mercury. However, the same phe­ nomena occurred even at the lowest power setting and in mixes squeezed with pliers (as recommend­ ed by the manufacturer of the condenser) contain­ ing as low as 45.5% mercury. The greatest cloud formation occurred when the tip of the instrument touched the wall of the cavity prepared in an ex­ tracted or porcelain tooth or the side of the steel die. ■ Composition o f dispersed material: Figure 2 shows the material collected on the filter as viewed through a metallographic microscope. The m ateri­ al was composed of ~1 to 100/am size spheres of mercury and numerous alloy particles. The per­ centage of particles penetrating the pulmonary air spaces rose from essentially zero at 10//,m to a maximum at and below l^m , where it equaled the fraction of tidal air that reaches the lungs.10 It is assumed that the spheres of mercury contained some dissolved alloy and that the alloy particles were reacted partially with mercury. Souder1 ana­ lyzed the liquid squeezed from an amalgam mix before crystallization had caused it to harden. He reported 1.07% tin and 0.13% silver; therefore, presumably these mercury droplets would be 9 8 % + mercury. ■ Mercury vapor levels: Tests for mercury vapor revealed little vapor in the air drawn into the in­ take hose of the detector when the opening was 30 cm away from the working area. The highest reading obtained at 30 cm was 0.02 mg Hg/m3 of air during ultrasonic condensation. This repre­ sents 20% of the threshold limit value of 0.1 mg Hg/m3 of air as established by the American C on­ ference of Governmental Industrial Hygienists.8 The vapor levels were about zero when hand and mechanical methods of condensation, other than ultrasonic, were used. It was necessary to move the intake hose of the detector to within 7 cm of the working area to obtain vapor levels above the threshold limit value. When condensation was stopped, the vapor

Fig 2 a M aterial collected by suction and filte r device held 7 cm above ultrasonic condensing in stru m e n t d u ring am al­ gam condensation. M ercury droplets and alloy p a rticle s can be observed throughout.

levels returned to near zero within approximately three to four minutes, even when the condensed alloy and excess plashy amalgam were allowed to remain in place. Shepherd and others11 determined that the lev­ el of mercury vapor present in scientific labora­ tories was primarily dependent on the amount of ventilation in the rooms. Other important factors were the number and types of sources and the de­ gree to which these sources were disturbed. The low levels of vapor found in the present study may have been due to good air ventilation in the operatory (10 m3 air/minute in a room con­ taining approximately 50 m3), but reducing the air circulation by blocking the intake and exhaust vents did not produce significantly higher levels. ■ Operating field: Figure 3 shows a porcelain tooth and the surrounding operating field after condensation of three amalgam mixes into the tooth. It is evident, from viewing the mercury droplets deposited in the field, that there is a con­ siderable surface area for the evaporation of rela­ tively large amounts of mercury vapor. Since these droplets contain dissolved tin and a slight


Fig 3 ■ Porcelain tooth and surrounding operating fie ld a fte r condensation of three amalgam mixes into tooth. Background is plain black paper. Notice great number of m ercury and alloy particles th a t have been deposited around operating site. Dark area behind tooth is shadow caused by oblique lig h tin g used in m aking photograph.

amount of silver, the evaporation of mercury may be reduced. Extended use of the ultrasonic device would ultimately result in the deposition of nu­ merous fine droplets of mercury throughout a den­ tal operatory; this in turn could cause a high level of vapor, especially if the mercury droplets were disturbed. Whether toxic levels could be achieved is not known, but certainly awareness of the po­ tential hazard is one of the primary factors in prevention. No quantitative value of the amount of m er­ cury and alloy particles that are dispersed is given because the amount dispersed for any one opera­ tion is subject to the following variables and probably to others: the mercury-alloy proportions, the size and shape of the instrument tips, the size and shape of the restoration, the power adjust­ ments on the instrument, the length of time the instrument tip is in contact with the amalgam mix, and the length of time the tip of the instrument is in contact with the walls of the cavity. In any event, the dispersion of mercury drop­ lets and fine partially amalgamated alloy particles in the area of operation when amalgam is con­ densed by ultrasonic means is undesirable. Some of the debris will be swallowed and some inhaled by the patient and the operating team. 556 ■ JADA, Vol. 82, March 1971

Summaryandconclusions Amalgam condensation with an ultrasonic device resulted in the emission of a cloud of material from the area of the working tip. This aerosol was composed of mercury droplets and alloy particles. Mercury vapor levels of 0.02 mg Hg/m:i of air were found 30 cm from the condensing point. This value represents 20% of the current (July 1970) threshold limit value for mercury vapor and, in itself, probably does not represent a poison hazard. However, the additive effect of introduc­ ing this technique into an office having safe levels of mercury vapor may be sufficient to develop hazardous concentrations. The inhaling and swal­ lowing of alloy particles and mercury droplets cannot be considered beneficial and may repre­ sent a health hazard even though excessive mercury vapors are apparently not being produced. The continued use of an ultrasonic condensing instrument for placement of amalgam restorations would certainly result in the deposition of nu­ merous small mercury droplets throughout a den­ tal operatory. It would appear, therefore, that the use of ultrasonic amalgam condensers would be contraindicated until the safety of the instruments

has been well established, especially after long periods of use in places with poor air ventilation.

Certain com m ercial m ate ria ls and equipm ent are id e n ti­ fied in th is paper to specify adequately the experim ental procedure. In no instance does such id e n tifica tio n im ply recom m endation or endorsem ent by the National Bureau of Standards or th a t the m aterial or equipm ent id e n tifie d is necessarily th e best available fo r th e purpose. This investigation was supported in pa rt by Research Grant DE02742-02 to th e Am erican Dental Association from th e National In s titu te o f Dental Research and is part o f the dental research program conducted by th e National Bureau o f Standards in cooperation w ith th e Council on Dental Re­ search o f th e American Dental Association; the Dental Re­ search D ivision of th e U nited States Arm y Medical Research and Developm ent Command; th e Dental Sciences Division o f th e School o f Aerospace M edicine, USAF; th e National In s titu te o f Dental Research; and th e Veterans A d m in istra ­ tion. Doctor C handler is a research associate of the American Dental A ssociation Research Program at the National Bu­ reau o f Standards, W ashington, DC 20234 on a two-year leave of absence from th e Ohio State U niversity College of Dentistry, Colum bus, 43210; Doctor Rupp is a research as­ sociate and D octor Paffenbarger is a senior research asso­ cia te of the Am erican Dental Association Research Program a t th e N ational Bureau o f Standards.

* H ollenback condenser, Clev-Dent, Cleveland. Vibrapak, Superba Dental Products, San Diego.

1. Souder, W., and Sweeney, W.T. Is m ercury poisonous in dental amalgam restorations? Dent Cosmos 73:1145 Dec 1931. 2. Nixon, G.S., and S m ith, H. M ercury hazards in dental surgeries. Abstracted. J Dent Res 43:968(Suppl) Sept-Oct 1964. 3. Airaksinen, S. Y lioppilaiden ham m oshoitola. Hammashoito he n kilo kiin n a n elohopeam yrktysvaarasta. Suom Hammaslääk Toim 57:27 March 1961. 4. Von Nossek, V.H., and Seidel, W. Der Q uecksilber­ dam pfgehalt in der L u ft zahnärztlicher Praxisräum e unter besonderer B e rücksichtigung der U ltraschallkondensation von Amalgam. Deutsch S tom at 19:787 Oct 1969. 5. Meyer, A. M ercury poisoning: A p o te n tia l hazard to dental personnel. Dental Prog 2:190 A pril 1962. 6. Sax, N.I. Dangerous properties of in d u stria l m aterials, ed 3. New York, Van N ostrand-Reinhold Books 1968, p 902. 7. Grossman, L.I., and Dannenberg, J.R. A m o u n t o f m er­ cury vapor in th e a ir of dental o ffice s and laboratories. J Dent Res 28:435 Oct 1949. 8. Frykholm , K.O. On m ercury from dental amalgam. Its toxic and alle rgic effects and some com m ents on occu­ pational hygiene. Acta Odont Scand 15:7(Suppl 22) 19579. D irections for use of Instantaneous Vapor D etector (GEI-37634) Catalog no. 9790339G1 and G2 Feb 1952. Gen­ eral E lectric Co., Schenectady, NY. 10. Hatch, T.F., and Gross, P. Pulm onary deposition and retention o f inhaled aerosols. New York, Academ ic Press, 1964. 11. Shepherd, M., and others. Hazard o f m ercury vapor in sc ie n tific laboratories. J Res Nat Bur Stands 26:357 JanJune 1941, Research Paper RP1383.