Abstract:
A radiometer and method for providing an indication of the amount of time (exposure duration) needed to cause a light curable dental resin composite material to maximally polymerize in response to the application of light from any light-curing source during the preparation of a dental restoration. It describes a radiometer comprising a sample holder of a size designed to hold a sample equivalent of said dental light curable dental material such that the thickness of the sample of light curable material in the sample holder will correspond to the depth of the composite material in the dental restoration; a solid state light sensor; a microprocessor programmed to respond to the output voltage or change in electrical resistance of the light sensor based upon an algorithm defining a mathematical model representative of the change in light transmission through the light curable dental material as a function of the degree to which said light curable dental material is being polymerized. It includes a time display that is responsive to the output signal generated from the microprocessor for displaying the amount of time (exposure duration) needed to maximally polymerize the light curable dental material in the dental restoration.

Description:
FIELD OF INVENTION 
     This invention relates to the field of radiometers and more particularly to a dental radiometer for providing the exposure time required to polymerize a light curable composite independent of light source. 
     BACKGROUND ART 
     Dentistry has used light curable composite resins for over 20 years with great success for preparing restorations, cementation of restorations, and a number of other dental restorative procedures such that light curing is now a standard procedure in dentistry. 
     Initial curing lights consisted of halogen devices, first with light sources removed from the point of application and thereafter with light transmitted to the point of application through long fibers. Following that, light curing guns were introduced. These devices typically used halogen light sources with short fused fiber optic light guides close to the lamp to apply high intensity light at the point of application. Along the way, radiometers were introduced into the dental profession for the purpose of measuring light output as a means of assessing the curing light&#39;s ability to properly polymerize the dental restorative materials. 
     Halogen curing lights suffer from a wide variety of mechanisms that cause degradation of intensity. These mechanisms include loss of light output from the halogen lamp, filter degradation, buildup of resin on light guides, degradation of light guides due to sterilization and faulty voltage control circuitry. The radiometer, therefore, has become widely accepted as a means of assessing light output of these devices and indirectly determining whether or not a material or restoration will be properly polymerized. 
     The popular radiometers in dentistry use either silicon or selenium detector cells with filters that block energy outside of the 400–500 nanometer range. Initially, radiometers were developed specifically for use with halogen light sources with their filters matched fairly closely to the wavelength distribution of the curing lights themselves. In recent years, other types of light sources have been introduced, namely plasma arc or gas pressure lamp devices, using xenon lamps to produce high intensity light in the 400–500 nanometer range. More recently, light emitting diodes (LED&#39;s) have been used to produce light specifically peaking at 470, 450 or 420 nanometers that match the absorption characteristics of photoinitiators currently used in dentistry to polymerize these restorative materials. However, when one uses a different light source on the same radiometer designed for halogen usage, erroneous readings result. Accordingly, to properly use a radiometer, the radiometer must be calibrated for use relative to a given light source and no standard of comparison exists to permit comparing the results between radiometers calibrated for different light sources. 
     The National Institute of Standards and Technology (NIST) presently requires every radiometer to be designed specifically for the light source it&#39;s being used with. Moreover, even if one were to use a separate radiometer designed specifically for each of the three types of light sources currently used in dentistry, the problem would still remain as to how long to expose the material under a given set of conditions i.e. depth, shade, and type of material. 
     Researchers in the dental field typically use a sensitive analytical laboratory tool employing a technique called Fourier Transform Infrared Spectroscopy (FTIR) to determine when a light curable material is maximally polymerized by measuring the ratio of aliphatic carbon-to-carbon double bonds pre- and post-exposure. Such laboratory equipment costs thousands of dollars and is clearly beyond the practical needs of the clinical dentist It would therefore be desirable to have a simple radiometer device that can assess the degree of polymerization and not be affected by which type of light source is used. It would further be desirable for the dentist to be able to determine the exposure time necessary to effect maximal polymerization of the restorative material selected for use in the preparation of a given restoration independent of the light source used to cure the material. 
     SUMMARY OF INVENTION 
     The present invention provides the dentist with a simple and effective method and radiometer device for determining the exposure time that provides maximal polymerization of a light curable composite material independent of which light source is used to polymerize the material. The radiometer of the present invention operates by exposing a test sample of light curable material to light regardless of which light source is used and calculates the exposure time necessary to achieve maximal polymerization with that light source. The selected test sample of material is placed in a sample holder in the radiometer which is designed in accordance with the present invention. The exposure time is automatically calculated to achieve maximal polymerization for the test sample of material and will correspond to the time necessary to achieve polymerization of the actual restorative material selected for use in the preparation of a given restoration when the actual restorative material is of identical composition to the test sample and the light source is the same as used to expose the test sample. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       FIG.  1 —is a graph of the light detector response (output voltage measured in millivolts) of a conventional radiometer when exposed to light with respect to time over two independent 40-second exposures spaced 30 seconds apart, with no interposing photocurable composite sample between the light tip end and the radiometer detector; 
       FIG.  2 —shows the real-time polymerization of a sample of light curable resin material being exposed to a conventional light-curing source for 40 seconds with respect to time; 
       FIG.  3 —is a graph of real-time change in radiometer detector response when a 2 mm-thick increment of uncured composite paste is interposed between the light tip and the light detector while giving two sequential 40-second exposures, spaced apart by a 30 second non-exposure time interval 
       FIG.  4 —is a graph of  FIGS. 2 and 3  superimposed upon one another using the same time scale to demonstrate that the same trend In light detector response while photo-curing composite ( FIG. 2 ) correlates well with changes seen in measuring the real-time polymerization ( FIG. 3 ); 
       FIG.  5 —shows the values of composite cure and light detector response occurring at similar time points as seen in  FIG. 4 . The data presented are the same as in  FIG. 4 , but with each “x” marking the correlation of detector response with real-time cure at similar time points (seconds) into giving the 2 sequential 40 second exposures spaced 30 seconds apart. 
         FIG. 6(   a )—is a view of the housing assembly of the radiometer of the present invention shown with a sample holder for holding a test sample of light curable material separated from the radiometer adjacent a light guide for a standard light source; 
         FIG. 6(   b )—is another view of the sample holder shown in  FIG. 6(   a ) for holding a test sample of light curable material; 
         FIG. 6(   c )—is yet another view of the sample holder of  FIG. 6(   b ); and 
       FIG.  7 —is a block diagram of the radiometer of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The subject invention results from experimental evidence proving that there Is a direct correlation between the percent composite cure of a light curable resin material and the degree of light transmission through the material as measured by the output of the light detector cell. To substantiate this correlation, the output voltage of a light detector cell in a conventional radiometer was measured based solely upon exposing the detector cell to light.  FIG. 1  is a graph showing the relationship between the output of a standard light detector cell with time over two sequential 40 second exposures spaced 30 seconds apart. As can be seen from this graph, whenever the detector cell is subjected to the light source under the same conditions by e.g. turning the light source on and off, a voltage (in millivolts) is generated by the detector cell with the output over each consecutive time period being essentially identical to one another within very dose tolerances. In the tests performed, the differences between the first and second light exposures was only about 0.75 millivolts, and for purposes of the present invention, may be deemed insignificant. 
       FIG. 2  shows the real-time curing profile of a sample of light curable material exposed to a light source (obtained using FTIR spectroscopy) indicating the change in composite cure of the material with respect to time. As noted previously, 2 successive 40-second exposures were given, 30 seconds apart as was the case in  FIG. 1  as well. Note the plateau for maximal exposure of this sample of material is reached in approximately 125 seconds, and that no additional curing occurs from the second exposure, but cure increases slowly as a result from the previous exposure (a finding well established in the literature). 
       FIG. 3  shows the change in light detector response as measured by millivoltage response when a sample of uncured composite (of similar thickness to that as the specimen used for  FIG. 2 ) as it is exposed to 2 sequential 40 second exposures spaced 30 seconds apart. The same light source was used for both test conditions. Note that during the first exposure, light transmission increases with exposure time resulting in a plateau. Upon second exposure, no obvious trend is seen to relate the second exposure with a change in detector output other than that which would be a slow, continuous increase resulting from the first exposure. Upon overlapping  FIG. 2  and  FIG. 3  on the same time scale ( FIG. 4 ), it becomes evident that extent of cure of the light curable material and the change in light transmission, noted as an increase in detector millivoltage response, appear to be related and follow one another very closely with respect to time. Although a slight deviation from one wave to the other (0.75 mv) was noted during the first two exposures in  FIG. 1 , the graph of  FIG. 4  shows that light transmission and conversion are essentially identical at the end of the second exposure. Accordingly, this deviation can be ignored. 
     Eliminating time as a separate axis and correlating composite cure with voltage output at similar time points is shown in  FIG. 5 , based upon measured data points taken at fixed intervals in time e.g. once per second. As millivoltage generation increases, the percentage of composite cure also increases and they tended to accumulate in the same portion of the graph, which indicates the existence of a point of diminishing return for either parameter. Stated otherwise, the data points tend to accumulate on the right side of the graph corresponding to where composite cure and millivoltage are maximal. This finding indicates that the effect of further light exposure would be insignificant. A curve can be mathematically derived that simulates this relationship based on measured data from which a mathematical model can be predicated with millivoltage generation predicting the extent of composite cure. These results show that an algorithm can be written with an accuracy of up to 99.5%, showing the change of light transmission can be used to accurately predict when the composite cure value will reach a plateau with respect to exposure duration. In this way, the time it takes for any specific light curing composite to approach maximum monomer conversion can be accurately determined. The relationship shown for this specific example in  FIG. 5  shows the shape of the algorithm, Y=−15.368x 2 +127.273X−213.217 having a coefficient of correlation r 2  of 0.995, where Y=percent composite cure and X=detector millivoltage output. The coefficient of correlation, r 2 , is a number between 0 to 1, with 0 indicating absolutely no correlation between factors, and 1 representing complete correlation. Thus, the observed correlation of 0.995 shows a great predictability of millivoltage change being an indicator of the level of composite cure. The value actually indicates that of 100% of the variability seen in the data, the predicted model can explain 99.5%, leaving only 0.5% attributed to unexplained error. 
     The present relationship uses a second order polynomial to describe the correlation between change in optical density (represented by change in detector millivoltage response) and change in composite cure (extent of polymerization). Thus, a change in light detector output can accurately predict a level of composite cure. Other mathematical algorithms may be applied as well, such as higher degree polynomial, logarithmic, exponential, power, or a combination of these functions as is well known to those skilled in the art. 
     DESCRIPTION OF OPERATION OF A PREFERRED EMBODIMENT OF THE INVENTION 
     The radiometer  10  of the present invention is shown in both  FIGS. 6 and 7  with  FIG. 7  representing a block diagram of the internal electronic components of the radiometer  10 . Accordingly, the radiometer  10  comprises a conventional detector cell  11  which may represent any conventional light sensor such as a silicon or selenium detector cell for providing either an output voltage or a change in electrical resistance in direct response to the degree of light exposure. In addition the radiometer  10  further comprises a micro-controller (microprocessor)  12 , battery  13 , serial input/output port  14 , LCD display  1 , an on/off function switch  2 , a scroll switch  3 , and a mode switch. The function switch  3  permits the radiometer to be scrolled to perform either an “Optical Conversion” function mode, “Power” function mode, “Energy Function” mode, or a “Calibration” function mode. When scrolled to Optical Conversion function mode, the LCD display  1  provides a time display output in seconds that will indicate the shortest exposure time to provide maximal composite cure for a test sample of uncured composite using any type light source as explained hereafter. 
     Optical Conversion Mode: Any conventional light curing source (not shown) having e.g., a standard light guide  9  may be used to cure a sample of an uncured dental composite  6 . The thickness of the sample of uncured composite  6  is adjusted by use of different thickness sample holders  7  with each sample holder  7  having a thickness corresponding to a typical depth of a dental restoration. The sample of composite material  6  is placed in a sample holder  7  of appropriate thickness for a given restoration. It is held by grip detail  8  as shown in  FIG. 6   c  and inserted along a groove or track  5  ( FIG. 6   a ) so that the sample sits directly over the detector window  4  of the light sensor  11 , which is shown in  FIG. 7 . The light guide  9  is placed over the sample in line with the detector window  4  so that light may be shined through the uncured dental composite sample  6 . The Function switch  3  is then scrolled to “Optical Conversion” mode of operation. The display  1  will then display time in seconds needed to maximally cure the composite, i.e., will stop when the display shows a time corresponding to the exposure duration needed to achieve the composite cure for the sample composite that represents a time when the sample is cured in accordance with the algorithm used in programming the micro-controller  12 . In accordance with the present invention, the micro-controller  12  is programmed using an algorithm such as the one explained earlier. The degree of composite cure measured can be determined to be anywhere between 80% and 99.5% of maximum. It should be understood that, for most composite resin materials, no matter how long the material is exposed to light, the extent of composite cure will plateau at between 45% to 65% of the maximum cure value (100%) for that material, and generally at about 50% as evident from  FIG. 2 . Thus, for example, using the 2% preferred change as the basis upon which the micro-controller  12  is programmed, when the display times out, the sample has cured to 98% of its maximum achievable value. The micro-controller  12  sampling rate is 0.1 seconds or less to insure accuracy. 
     Power Mode: When the Function switch  3  is scrolled to “Power Mode” the radiometer  10  will measure the curing light output intensity in watt/cm 2  or milliwatt/cm 2  and the display  1  for this mode of operation is programmed to update for as long as the push button is held. When the push button is released, the radiometer will continue to measure the curing light output intensity but the display will correspond only to peak measurements. 
     Energy Mode: When the Function switch ( 3 ) is scrolled to “Energy Mode”, a momentary push of the Function button will set the energy measured in joules or millijoules to zero (start) and begin to accumulate values once the intensity is above a preset level. The term “energy” is the mathematical product of the power density (measured in W/cm 2  or mW/cm 2 ) times the exposure duration (seconds). Thus, as a light exposure continues over time, the accumulated energy delivered to the target also increases and is thus measured by the instrument. 
     The On button ( 2 ) turns the radiometer unit on and it will remain on for two minutes if not used and then will automatically power down to conserve battery life. 
     Calibration Mode: The radiometer is calibrated at the factory by using a standard lamp and a plastic filter with the same optical transmission characteristics as that of well polymerized dental composite. The user can then compare the exposure time displayed using the calibration filter and the light unit being tested. Comparing the standard reading and the actual value will indicate the offset to which the unit is out of calibration. An auto ranging feature of the micro controller will adjust this offset to zero by holding down the On switch ( 2 ) (in the optical conversion mode) and the Function switch ( 3 ) simultaneously for two seconds). 
     LCD Display: This panel will display real-time light intensity (power density), accumulated light energy delivered, or recommended exposure time depending on the mode of operation. 
     The Light sensor is a solid-state photo detector with 400 to 500 nm sensitivity, but other ranges such as 300–400 nm are possible to measure the intensity of the light coming through the dental composite. 
     Mode Switch: This switch will allow scrolling through the functions of optical conversion, power and energy. 
     Function/Calibration Switch: This switch is used to calibrate the radiometer using a standard plastic filter as described previously. 
     Serial I/O Port: This port is configured as RS232C and will allow two-way communication between the radiometer and a computer or remote display. A “Blue Tooth” or USB Port can also be used. Battery: Two alkaline, lithium or rechargeable batteries power the radiometer. Either button can be pushed to turn the radiometer on, it will remain on for two minutes after the last button is pushed, and then, for battery life conservation the radiometer will go into a “sleep” mode. Low battery indication is evidenced by flashing the display. 
     Micro controller: The radiometer programs are controlled by a microprocessor. Inputs include measurement of light, reading mode and function switches. Outputs include RS232C or USB communication and display drivers. 
     The plastic filter is designed to simulate the light transmission characteristics of a well-cured dental composite restoration and can be used for calibration. It may come in four different depths (i.e. 2, 3, 4 &amp; 6 mm) or any depth that is desired. The plastic is selected from a group of plastic materials that have optical transmission characteristics identical to that of a well cured dental restorative material of a given thickness.