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J Am Dent Assoc, Vol 136, No 6, 766-773.
© 2005 American Dental Association |  |
CLINICAL PRACTICE |
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ABSTRACT |
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 TOP ABSTRACT METHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Methods. Research assistants visited 100 dental offices and assessed 214 QTH light units. They recorded each unit’s model, age, service history, light intensity and heat/glare emissions.
Results. Mean light intensity was 526 milliwatts per square centimeter (120–1,000 mW/cm2), with 26 units having intensity less than 300 mW/cm2. The mean light unit age was 5.6 years. Light units older than three years had significantly lower output intensities than those that were one, two or three years old. The authors found a wide range of heat/glare measurements (3–300 mW/cm2), with 4.6 percent of the units having values greater than 50 mW/cm2, including three with values of more than 200 mW/cm2. The mean light intensity of units serviced in the preceding year was 539 mW/cm2; it was 418 mW/cm2 for units serviced from one to six years previously.
Conclusions. Light intensity and heat/glare values varied among the 214 units; some units had values well outside the recommended levels. Each unit’s age and service history significantly affected its intensity. An awareness campaign is needed to promote testing, repair or replacement of light polymerization units. Periodic testing of light polymerization units should be considered by regulatory bodies to ensure optimum quality of composite restorations.
Clinical Implications. Light polymerization units in some private dental offices in Toronto had intensities that may result in composites restorations with inferior properties. Dentists need to regularly monitor the intensity of the light polymerization units and maintain the units to ensure quality composite restorations.
Key Words: Intensity; quartz-tungsten-halogen light units; resin-based composite
Photopolymerization of resin-based composites has been used in dentistry for more than 25 years.1 The majority of photopolymerization units based on conventional quartz-tungsten-halogen (QTH) lamps emit light within the wavelength range of 400 to 500 nanometers.2 During polymerization, light passes through an increment of the material in which it is absorbed and scattered; consequently, its intensity decreases.3,4 This may cause deeper layers of an increment not to polymerize sufficiently, resulting in inferior physical and mechanical properties of the restoration.4
Factors that may affect polymerization of resin-based composite restorations include light intensity,5–7 exposure time,5,8–10 wavelength as related to the type of photoinitiator incorporated in the resin-based composite material,4 thickness of the resin-based composite increment,10,11 distance between light guide tip and surface of the resin-based composite,12 shade of the resin-based composite13 and composition of the resin-based composite material.14,15 Of all these factors, light intensity seems to be the most important.
Although no acceptable single minimum value of intensity has been validated scientifically, light units with intensities of less than 300 milliwatts per square centimeter are described in the literature as inadequate, unusable or unsuitable.16,17 Three international studies16–18 reported that 27 to 33 percent of the lights in dental offices had intensities of less than 200 mW/cm2, while two reported that 45 to 55 percent of the lights had intensities of less than 300 mW/cm2.16,17 A fourth survey indicated that 14 percent and 25 percent of the lights had intensities of less than 200 and 300 mW/cm2, respectively.19
These four surveys reported on the intensity of light units used in private dental offices in Texas; Tokyo; Tel Aviv, Israel; and Australia from 1994 to 1999.16–19 The authors found a wide range of light intensities and concluded that light intensities used in some private offices were lower than what is needed for optimum polymerization of resin-based composite restorations. In addition, one of these surveys reported on heat/glare measurements from 130 light units used in 107 dental offices and found a number of them to have heat/glare emissions higher than the manufacturers’ recommendations.17 Excessive heat/glare emissions are secondary indicators of light unit inefficiency.
In spite of these published studies that urged dental practitioners to conduct regular checks on their light-curing units, it is suspected that many dental practitioners did not heed the recommendations. Therefore, we decided to conduct a study to determine the current situation in dental offices in a North American city. If the results of our study were similar to those found previously, this would indicate that dentists need to be alerted again about potential problems associated with less-than-ideal light intensity of curing units. In today’s practices in which posterior resin-based composite restorations are being placed extensively by dentists, inadequate light intensity can be an important quality issue, as it may result in insufficient polymerization with subsequent postoperative sensitivity.20,21 It also may lead to cytotoxicity if incomplete polymerization of the resin-based composite material occurs.22
The aim of our study was to determine the light intensities and heat/glare measurements of light polymerization units used in a select group of private dental offices in Toronto and to compare these results with those found internationally.
We used the 2002 listing of dentists published by the Royal College of Dental Surgeons of Ontario to identify dental offices in the greater Toronto area. For each letter of the alphabet, we telephoned the first seven offices that fell within the targeted population area and had a dentist who was a general practitioner to find out if the office also met the other two criteria. After we identified an office that met all four selection criteria, we telephoned the dentist, briefly explained the study’s purpose and methods and asked him or her to take part in the study. We repeated this process until dentists from 100 offices agreed to participate in the study. We initially contacted 416 offices.
Once the dentist agreed to participate, we made an appointment to visit the office. Two trained teams, each consisting of two research assistants with identical equipment, made the office visits. On arrival at each office, the team members presented a detailed letter explaining the purpose and methods of the study and a thank-you message to the dentist for taking part in it. The team members then recorded the number of QTH light units in the office and information about each unit, including make, model, date of purchase, last date of service and whether a light meter was available in the office.
The team members used an analog radiometer with a range of 0 to 1,000 mW/cm2 (Optilux, model 100, Kerr, Orange, Calif.) to measure light intensity. This radiometer measures light in the wavelength range of 400 to 500 nm and was used by Pilo and colleagues.17 Each day before the dental office visits, the team members checked the light radiometers against another light radiometer at the Restorative Dentistry Laboratory at the University of Toronto Faculty of Dentistry to ensure consistency of readings. At each office, after a short warm-up period, the team members recorded three measurements of light intensity for each light unit in the office. For heat/glare measurements, the team members used a heat/glare radiometer (Optilux, model 200, Kerr), which measures optical power density from 520 to 1,100 nm, as indicated by the manufacturer. They also checked this heat/glare radiometer daily at the Restorative Dentistry Laboratory, and they obtained three measurements from each light unit in the office after a short warm-up period. Since both radiometers were analog, team members recorded measurements as estimated values according to the nearest scale markings as accurately as possible.
At the end of the visit, the team members provided each dental office with a written report about its intensity and heat/glare test results. They also provided written instructions on maintenance of light polymerization units.
A team member entered the collected data into a database and, after editing to correct data entry errors, averaged the three readings of light intensity and of heat/glare for each light unit. The team member then exported these mean values and the other data collected on each light unit to a statistical analysis program (SPSS for Windows, Release 6.1.2, SPSS, Chicago) file for analysis. Using the statistical analysis program, the team members performed a variety of data summary (means, frequencies), significance testing (
The mean light intensity of the 214 light units was 526 mW/cm2, but the distribution was variable with a wide range, from a low of 120 mW/cm2 in one unit to a high of 1,000 mW/cm2 (the maximum possible value on the meter) in 11 other units (Figure 1Regular use of light meters by dentists is essential to ensure optimum quality of resin-based composite restorations.
It is suspected that many dental practitioners did not heed the recommendations to conduct regular checks on their light-curing units.
METHODS AND MATERIALS
TOP
ABSTRACT
METHODS AND MATERIALS
RESULTS
DISCUSSION
CONCLUSIONS
REFERENCES
The study was approved for scientific merit by the University of Toronto Faculty of Dentistry’s Research Committee. Our four selection criteria to guide private dental office participation in this study included that the targeted population be represented by offices in the greater Toronto area that were serviced by the public transportation system to enable easy access for office visits by research team members, that the dentist operating the office be a general practitioner and not a specialist, that the light polymerization units in the office use QTH lamps and that the dentist routinely use resin-based composites for restoration of posterior teeth. 
2, t test, analysis of variance [ANOVA], linear regression) and graphical procedures.
RESULTS
TOP
ABSTRACT
METHODS AND MATERIALS
RESULTS
DISCUSSION
CONCLUSIONS
REFERENCES
We analyzed 214 lights from the 100 offices, including two light units from each of 55 of these offices. In the remaining 45 offices, about one-half (22) had one light unit, while the other one-half (23) had three or more; one had seven light units. The units were of 19 different makes, though two makes (Dentsply, York, Pa., and Kerr) represented 31.3 percent and 48.1 percent, of the 214 lights, respectively. We found 52 different light unit models. 
). The modal category containing the most lights (47) was 400 to 499 mW/cm2. Twenty-six (12.1 percent) of the 214 light unit intensities were less than 300 mW/cm2, and nine (4.2 percent) were less than 200 mW/cm2. At the opposite end of the distribution, 28 (13.1 percent) were 800 mW/cm2 or higher.
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The mean light intensity decreased as light age increased (Figure 2). One-way ANOVA revealed that the mean differences in light intensity among the light age groups were highly significant (P < .0001). Furthermore, using the Scheffé test, we found that the intensity means of the one-, two- and three-year groups were each significantly higher (P < .05) than the means of each of the five-year, six-to-10-year and 11-to-20-year light age groups.
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The mean heat/glare values measured for the 214 lights was 36 mW/cm2; however, the distribution of heat/glare values was skewed at a highly positive rate, since the heat/glare values from nearly 81.3 percent (174) of the lights was lower than this overall mean (Figure 3). The lowest heat/glare value recorded was 3 mW/cm2 for two light units, and the highest was 300 mW/cm2 for two light units. Thirty (14.0 percent) of the 214 lights had heat/glare values greater than 50 mW/cm2, including three light units with emissions greater than 200 mW/cm2.
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2, P > .05). The mean light intensity of the 77 light units that were reported to have been serviced within the last 12 months was 539 mW/cm2, whereas the mean light intensity of 418 mW/cm2 for the 24 lights serviced from 12 to 72 months previously was significantly lower (t test, P < .01).
Dentists reported having either built-in or free-standing light meters for monitoring light intensity for 113 (52.8 percent) of the 214 light units. Seventy-three (64.6 percent) of these meters were built into the polymerization units, with significantly more (2, P < .0001) being built into lights less than four years old (66.7 percent) than into the oldest lights, which were 10 to 20 years old (2.6 percent). Only 18 of the lights had turbo light guides.
The relationship between light intensity and heat/glare emissions for each of the 214 light units is shown in Figure 4. Because of concerns about the suitability of light units with an intensity of less than 200 mW/cm2 and heat/glare emissions greater than 50 mW/cm2, we placed reference lines at these two points on the plot.17 These reference lines divide the scatterplot into four quadrants. One hundred seventy-nine of the 214 light units (83.6 percent) in the lower right quadrant met the "acceptable" output criteria: intensity was equal to or greater than 200 mW/cm2 and heat/glare value was equal to or less than 50 mW/cm2. In the upper right and lower left quadrants, 31 of the remaining 35 light units met one of the two criteria. Only the four light units completely in the upper left quadrant failed to meet both of the criteria. It is noteworthy that of the 30 light units completely in the upper left and upper right quadrants that exceeded the greater than 50 mW/cm2 heat/glare criterion, about two-thirds had light intensities of 300 to 650 mW/cm2.
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DISCUSSION |
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TOPABSTRACTMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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). Since the use of resin-based composites in private dental offices has been on the rise over the last five to 10 years, this likely is the reason for the higher mean number of lights and the finding that most dentist offices in Toronto (78 percent) had more than one light unit.
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).16–19 Since light intensity is associated directly with the polymerization of resin-based composite restorations, low intensity values are of concern.5 Inadequate light polymerization of resin-based composite restorations might cause a number of problems. Postoperative sensitivity due to partial dissolution of unpolymerized material at the tooth/restoration interface would occur first, and recurrent caries along the interface may follow.23 Inferior mechanical properties of restoration may lead to excessive wear or possibly bulk fracture.24 Cytotoxicity due to ingestion of leached monomer or other unbound chemicals can take place with relatively thicker increments that are not well-polymerized.22,25,26 In the four international studies, the highest light intensity recorded (1,368 mW/cm2) was in Tokyo19 (Table). While 11 units in our study had readings of 1,000 mW/cm2, their actual intensities might have been higher; the upper range of the light meter was 1,000 mW/cm2, and readings higher 1,000 mW/cm2 could not be recorded. The same issue occurred with the heat/glare measurements, as the radiometer had an upper range of 300 mW/cm2. Two of the units tested had readings of 300 mW/cm2. Relatively fewer light units in our study had light intensities lower than 200 mW/cm2 (4.2 percent) and lower than 300 mW/cm2 (12.1 percent) compared with the lights in the international studies with percentages of 14 to 33 percent and 25 to 55 percent, respectively.16–18 Similarly, 16.4 percent of the light units in our study had intensities of less than 200 mW/cm2 or heat/glare emissions greater than 50 mW/cm2, compared with 46 percent in the Tel Aviv study.17
According to Rueggeberg and colleagues,5 light sources with intensity values of less than 233 mW/cm2 should not be used because of their "poor cure characteristics." They also stated that "incremental layer thickness of resin-based composite should not exceed 2 mm, with 1 mm being ideal, and exposure time of 60 seconds using a light source with an intensity of at least 400 mW/cm2 is recommended."5 Tate and colleagues27 confirmed these recommendations. In our study, 65 units had intensity values lower than 400 mW/cm2, while 14 units had intensity values less than 233 mW/cm2. This suggests that 30.3 percent of all tested light units emitted light with less-than-recommended intensity, and 21.5 percent of those 65 units emitted light with an intensity that would result in "poor cure characteristics." In comparison, about 52 percent of the lights tested in Australia18 had light intensities lower than 400 mW/cm2, with 27 percent at or lower than 200 mW/cm2; in Tel Aviv,17 these same percentages were 75 percent and 33 percent, respectively.
Our study clearly shows that light intensity decreases as the square root of light age increases and that this association is significant and linear. However, since only 26 percent of the variation in light intensity was explained by light age, it is evident that other factors account for the observed variability in light intensity. The frequency of service may be one of these factors. Using the limited available data on service history, however, our finding that light units serviced within the last 12 months had significantly higher light intensity than light units serviced more than 12 months previously is dampened somewhat because, as mentioned earlier, more frequently serviced light units tended to be newer.
Light polymerization units for resin-based composites are based on QTH, plasma arc, laser or, more recently, light-emitting diodes.28 QTH technology is considered the original1 and is the most widely used by dentists worldwide. Perhaps the main reason for this is the lower cost of QTH light polymerization units compared with plasma arc and laser units. In addition, some of these other light units have limited applications, unlike QTH units that have virtually unlimited ability to polymerize different brands of resin-based composites. Therefore, we limited our study to only QTH units to ensure a larger sample size and help control variability in study design.
A number of factors can lead to deterioration of QTH light unit intensity. They include age of lamp, condition of filter and condition of light guide.22 Miyazaki and colleagues19 have shown that replacement of lamps in two light polymerization units that had light intensities less than 300 mW/cm2 resulted in increases in light intensity of 21 percent and 36 percent, while changing the filters resulted in increases of 88.8 percent and 157.7 percent. Changing the fiber-optic light guide of the same light units resulted in increases in light intensity of 36 percent and 46.2 percent. When all three parts (lamp, filters and fiber-optic light guides) were replaced at the same time, however, light intensity increased by 208 percent and 322.7 percent.19 It has been suggested that light polymerization units’ lamps be replaced once every six months to maintain the appropriate level of intensity.29 However, it seems important that other components (filters and fiber-optic light guides) also be replaced routinely. Perhaps decisions about lamp replacement should be made based on intensity readings.
Temperature increase during polymerization of resin-based composites is related directly to the condition of filters in the light units.30 Defective filters may allow for emission of light with wavelengths greater than 500 nm, which increases the potential for heat buildup during a polymerization cycle. Measurements of the heat/glare values with the radiometer used in this survey, which measured optical power density from 520 to 1,100 nm, may give an indirect indication about the appropriateness of the wavelength of the light emitted. However, the 50 mW/cm2 upper limit that the manufacturer of the radiometer recommended does not seem to have strong scientific basis.17
Filters used in light polymerization units serve to limit light to the blue region of the spectrum (400–500 nm) that is necessary for activation of the photoinitiators in the resin-based composites.31 A cracked, blistered or defective filter may not function properly, and, as a result, the wavelength of the produced light might not be in the required range. In addition, a defective filter will affect light intensity negatively.16 Therefore, it is possible that though light intensity of a light unit is adequate, an incorrect wavelength due to a defective filter may cause the light unit to function ineffectively.
Dentists are unable to determine the adequacy of light intensity by looking at the light emitted from a light polymerization unit. Measuring light intensity with a light meter is the only reliable way to determine light intensity. In addition, assessing the hardness of the surface of a polymerized resin-based composite increment with an explorer is not a reliable method for testing the effectiveness of the light source. A light unit with inferior light intensity was shown to be able to harden the surface of a resin-based composite increment just as well as a light unit with superior light intensity.32,33 However, the subsurface layer of a resin-based composite increment is affected most by inferior light intensity.
Dentists need to adopt policies to ensure that monitoring the efficiency of all light polymerization units in their offices is performed routinely. A free-standing light meter will enable dentists to test all light units in their offices. Lamps, light guides and filters should be checked routinely and be replaced with new components when there is evidence of deterioration. Manufacturers of light polymerization units should consider organizing an awareness campaign to test and offer repair or replacement services for defective units used by dentists. In addition, regulatory bodies should consider the need for obligatory testing of light polymerization units used by dentists in private dental offices. This could be done in a manner similar to that used by Health Canada (Ottawa) with radiograph machines used in dental offices that are subjected to annual testing by trained technicians.
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FOOTNOTES |
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