Abstract:
A light curing device has a light source for curing light-polymerizable dental materials. A control circuit controls the light output of the light source according to a pre-determined output profile. The output profile has a starting time period with a decreased output relative to a maximum output, followed by a main curing time period. In the main curing time period, a pulsed output with alternating high output value and low output value is provided.

Description:
This application claims the benefit of No. 60/146,590, Jul. 30, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a light curing device and method for curing light-polymerizable dental materials. The device comprises a light source with which the light-polymerizable dental materials are cured. Apparatus of this type is known from U.S. Pat. Nos. 6,095,812 and 6,123,545 the subject matter of said patent applications being incorporated herein by reference thereto. A control circuit is provided for controlling light distribution according to a pre-determined output profile. In accordance with this invention the output profile is controlled to have a start-up period, followed by a main curing period. 
     A number of light curing devices are known which attempt by various means to achieve a curing effect that is as good as possible and attempt to avoid the creation of open margins. It has been known from the printed publication “K.-J. Reinhardt: Der Einfluss der Lichtquelle auf die Randständigkeit von Kompositfüllungen”, Carl Hanser Verlag, München, 1991, that light sources of a high light intensity have the propensity to create open margins. 
     In order to delay the Trommsdorff effect, it has further been suggested to provide a decreased light output value at the start of the entire lighting period in order to ensure that the viscosity of the dental material does not increase too fast. 
     A further problem of dental materials is their final curing grade (hardness). In order to achieve a satisfactory result, it has already been suggested to increase the light output step-wise over the length of the entire curing duration. 
     Despite these attempts, the shrinkage value has so far been unsatisfactory with known light curing devices. 
     Furthermore, light curing devices have been known for a long time with which the light output shall be provided by means of a Xenon flash lamp. With such light curing devices, the duration of voltage application of each light impulse lasts only 0.1 to 2 milliseconds, insofar reference is made, for example, to DE-OS 32 15 664. 
     Studies with such light curing devices have, however, led to the result that the curing effect is unsatisfactory so that such devices have not found acceptance in practice. Moreover, because of its size, the Xenon flash lamp which is of a usually complicated design is mostly suitable for stationary devices, with respect to which reference is also made to the aforementioned publication. 
     Therefore, it is an object of the present invention to provide a light curing device of the aforementioned kind with which the curing shrinkage and the temperature induced stress are reduced for curing light-polymerizable dental materials. By means of the low shrinkage value, the marginal integrity (marginal adaptation) shall be improved when curing composites. 
     SUMMARY OF THE INVENTION 
     This object is solved by the output profile showing a pulsed output with alternating high output value and low output value. 
     Accordingly, the inventive light-curing device, having a starting time period with an increase in the light output, is combined with a main curing time period during which the light output is alternated at an alternating high and low output value. Surprisingly, this combination results in a decreased curing shrinkage. Apparently, the time period of low light output allows the material to flow against the cavity edge. A gentle curing is achieved, and the propensity for the creation of open margins is reduced by the inventive light curing device. A factor that may also contribute to this result is that the temperature-induced stress is significantly reduced because of the pulsating action in comparison to conventional light curing devices. 
     While the earlier mentioned studies according to the publication of K. J. Reinhardt show the introduction of stresses due to high light intensity, studies based on the inventive device have surprisingly shown that the inventive short-term high light output is by no means detrimental, but that it favorably affects the hardness of the cured dental material. 
     A further favorable feature with respect to the invention is that the dental material becomes significantly less warm on curing in comparison to the continuous irradiation at a high light output. During a curing duration of 40 seconds, the temperature increase only amounts to 7° C. (when curing 25 mm 3  Tetric Ceram, a light-curing composite of the Vivadent company). 
     It is particularly advantageous according to the invention if the curing application is performed with a not too long alternating period between the high and the low output value. The alternating period can, for example, last for 2 seconds. With alternating periods in this range, the most favorable values result for the final hardness, on the one hand, and the curing shrinkage, on the other hand 
     An advantageous embodiment of the invention provides to use an incandescent lamp as a light source. An incandescent lamp has a certain inertia with respect to the light radiation. The delay ranges between 100 milliseconds and up to almost one second with larger lamps. Moreover, the internal resistance of a cold spiral filament is significantly higher than the internal resistance of a hot spiral filament, so that usually an electrical connection impulse has to be handled on alternating electrical connection/disconnection pulses, i.e., on changes between an electrically fully connected and an electrically fully disconnected incandescent lamp. This alternation of a connection/disconnection pulse also puts stress on the spiral filament, resulting in a decreased service life. 
     In order to avoid this result, it is particularly advantageous if a slanted connecting flank is used for the transition between the low light output and the high light output. The fact that the commercially available halogen lights have a light output which is, for example, reduced by 60% at a 30% reduced electrical power can be especially favorably taken advantage of for the present invention. Due to the distinctly lower temperature of the spiral filament at a reduced luminous power, the emission of light is more than proportionally lowered. This often undesired behavior of spiral filaments can be particularly favorably taken advantage of, according to the invention, for the recovery periods of the light-polymerizable dental materials. A reduction of the electrical power by 30%, for example, accordingly results in a reduction of the light output by 60% or 70% which is sufficient for providing the recovery period. 
     According to a further, particularly favorable aspect of the invention, it is provided to select the starting time as to be comparatively rather long, for example, up to 0 40% of the entire lighting duration, so that the main curing time is 60% of the entire lighting duration. An especially favorable pre-curing effect and a uniform curing effect result therefrom. According to the invention, the core curing can be especially favorably affected by the combination of the relatively long starting time with the pulsating main curing time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The object and advantages of the present invention will appear more clearly from the following specification in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows a diagram of an output profile for illustrating the emitted light output, wherein the inventive output profile is compared with two known output profiles; 
     FIG. 2 another diagram for illustrating the temperature increase during the lighting duration; 
     FIG. 3 a diagram for illustrating the shrinkage during the lighting duration; 
     FIG. 4 is a perspective view of a light curing device in which the present invention may be incorporated; 
     FIG. 5 is a sectional view of the hand held instrument shown in FIG. 4; and 
     FIG. 6 is a side view of a halogen lamp which may be used in the hand held device shown in FIG.  5 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Initially, with reference to FIG. 4, the present invention may be embodied in a light curing device indicated generally at  110 , which light curing device includes a supply station  112  and a pistol shaped hand-held instrument  114  connected to one another by an electrical cable  116 . The hand-held instrument includes housing  118 , a light guide  119 , a light source  120 , and a fan  122 . The supply source  112  is connected to electrical mains by power cord  128 , and the power to the supply station may be turned “on” and “off” by an on/off switch  130 . 
     A control circuit is provided which includes in addition to cable  116  a printed circuit board  132  in the hand-held device  114  and another printed circuit board  134  in the supply station, one of which may be provided with a PID controller, indicated at  136  in FIG.  5 . 
     The fan  122  may be on at all times when switch  130  is in its “on” position. Alternatively, it may be under the control of a further on/off switch  138  on the hand-held instrument, which switch may also initiate current flow to the lamp  120 . 
     In the embodiment illustrated the light source may be a halogen lamp, shown in detail in FIG. 6, which halogen lamp includes a bulb  140  carried by a base  142 , there being a spiral wound filament  144  in the bulb. The lamp  120  is mounted in a socket in a reflector assembly  146  which is in turn received by receiving socket  148  on the printed circuit board  132 . 
     The present invention will now be described in detail with the aid of several specific embodiments utilizing FIGS. 1 through 3. 
     The output profile  10  illustrated in FIG. 1 shows the measured light output of a light curing device with three programs, whereby one embodiment of the inventive light curing device shows the output profile designated as program  3 . 
     The output profile  10  is illustrated as a curve  12  of the light output. The applied electrical power which is not illustrated in FIG. 1 has to be distinguished therefrom. In the preferred embodiment, the applied electrical power results in the curve  12 , whereby it is preferred that this curve is provided by a control of the electrical power. 
     According to the invention, it is particularly advantageous to increase the light output continuously during a starting time period  14  and to vary the light output between a high output value  18  and a low output value  20  during the main curing time  16 . During the main curing time, the electrical power is, however, not substantially sinusoidal like the emitted light output, but shows at least steeper declining flanks, while the incandescent lamp  120  used as a light source cools down more slowly than would be proportional to the corresponding decline of the applied power. 
     It is understood that this dead time is inventively encompassed in the control of the control circuit  132 - 136 . 
     Apart from the inventive control according to the curve  12 , the inventive light curing device has two further output profiles according to the curves  22  and  24 . The curve  22  shows a steep incline of the light output up to the high output value  18 , the maximum value, and subsequently a flat decline and maintaining of that level essentially during the entire curing duration. In contrast thereto, the curve  24  shows a slightly less steep incline approximately up to the lower output value  20 . It is possible with an inventive light curing device to select at one&#39;s choice curve  22  by program  1 , curve  24  by program  2 , and curve  12  by program  3 . With programs  1  and  2  according to curves  22  and  24 , the supply voltage for the light source is maintained at a constant level so that a relatively high connecting current is present due to the internal resistance of the light source, the connecting current being slightly flatter, corresponding to the inclining sides of the curves  22  and  24 . 
     In contrast thereto, the incline angle of curve  12  with the inventive output profile  10  is significantly flatter. During a starting period which lasts 15 seconds of the entire lighting duration of 40 seconds, the emitted light output is first increased to 150 mW/cm 2  for approximately one second. Between the time of one second and 15 seconds, a controlled increase of the light output takes place from 150 mW/cm 2  to approximately 400 mW/cm 2 , thus, approximately to the lower output value. 
     The main curing time  16  follows during the time period of 15 seconds to 40 seconds. The light output is initially increased between 15 seconds and 17 seconds from approximately 400 mW/cm 2  to the high output value, namely, approximately 800 mW/cm 2  and subsequently declines to the low value  20  until 19 seconds have elapsed. The light output essentially follows a waveshaped curve, and, during the main curing time  16 , the curve  12  forms six wave-shaped curves or 12 alternations between the high and the low output values  18  and  20 . It is understood that the exact output profile can be adjusted to the requirements in wide ranges and can, in particular, be adjusted by program-control depending on the curing objective. 
     FIG. 2 shows that the inventive control of the light curing device with the output profile according to curve  12  causes a distinctly lower temperature increase than with program  1 . The curves of the programs  1 ,  2 , and  3  are designated by reference numerals  26 ,  28 , and  30 . The inventive output profile  10  results in the temperature curve  26 , thus, in a temperature increase by approximately 7° C. as a final value (when curing 25 mm 3  Tetric Ceram, a light curing composite of the Vivadent company), while program  1  causes a temperature increase by 12° C. It is apparent that the temperature curve  26  has a slightly wave-shaped inclining character during the main curing time  16 , while curve  30  shows a maximum value of almost 7° C. at an elapsed time of 9 seconds of the entire lighting duration and subsequently declines, and the temperature curve  28  reaches a temperature increasing value of slightly more than 10° C. after approximately 8 seconds and subsequently slowly increases to 12° C. 
     Thus, according to the invention, the temperature increase stressing the dental material and affecting the patient is distinctly lower than with the temperature curve  28  and approximately in the area of temperature curve  30 . 
     Furthermore, the core curing depth and the shrinkage measured as the Vickers pyramid hardness have been determined in studies of the inventive device. With all three programs, core curing depths resulted of approximately 5 mm, with program  1  having the deepest and program  2  the lowest core curing depth. However, the core curing depth was sufficient in all cases. Also the Vickers pyramid hardness did not show significant deviations between the individual programs. The measured curing values which showed a range between one and two percent, showed the following curing values according to the following chart: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Vickers pyramid hardness (HV 0.5130) 
               
               
                   
                 Program 
                 (NImm 2 ) 
               
               
                   
                   
               
             
             
               
                   
                 1 
                 584 
               
               
                   
                 2 
                 572 
               
               
                   
                 3 
                 574 
               
               
                   
                   
               
             
          
         
       
     
     The Vickers pyramid hardness measurements show that comparable curing results are achieved with all three programs. The results do not differ statistically. 
     Because of the sample variation, no significant statement can be made with reference to curing differences. 
     FIG. 3 illustrates the shrinkage for the individual programs. The shrinkage curve clearly shows the positive effect of program  3 . The slow-speed curing at the low light output value results in a slow-speed increase of the shrinkage which can have a positive effect on the marginal integrity of a filling. 
     A light source which employs a halogen lamp has been decribed above. However, it is possible that other light sources may be used in the practice of this invention. Thus it is possible to use either a laser diode, or an arrangement of LEDs.