We are learning more and more about the multiple clinical advantages of composites, but also about the principles that govern their behaviour in space and time.
We are also well informed about their most intimate mechanisms, chemical as well as physical. However, except for the introduction of photopolymerisation and polymerisation in layers, little has been done to break out of the monotony that surrounds this marvellous tool.
Indeed it is a fact that for several decades it was common practice to use the polymerisation lamp during 40, or even 60 seconds (and maybe even 4 mn in certain cases ...). Nobody criticised this, nor did anybody question the foundations on which this choice was based, although everybody suffered the consequences every day.
Nevertheless, many things have changed over the last few years, and the new composites have an impressive degree of hardness, an increasingly reduced degree of retraction, and a better control of internal tension thanks to new molecules with exceptional properties.
It is only after the development of composites with a fibrous structure (Aristee) for CAD-CAM, and later with the perfection of Composipost, that we were astonished to see that clinicians still accept requirements which we now consider to have an adverse effect. How can we accept today that a lamp is applied for 60 seconds when we know now that polymerisation and retraction bear no relation to time, to the hardness obtained when the structural transformation is respected, and to the reduced internal tension if, amongst other things, the degree of contraction is less important. How can we accept that only dentistry claims that a good composite is a composite that was polymerised slowly?
We believe it is essential that clinical practice assumes precedence over the fundamental theory: reducing the time of exposure to less than 5 seconds gives the patient and the clinician so much comfort that every possible effort must be made in this respect. That is why we, in our laboratory, spent two years (from 1995 to 1997), developing the plasma lamp for rapid polymerisation, which we now call the Apollo 95E.
We will show you some scientific results that seem to prove that the mechanical properties of dental reconstitutions in which composites are used, are maintained and even improved by rapid polymerisation, and that it provides an exceptional clinical comfort instead of involving risks.
1. Materials and Method:
We performed 7 comparative physical analyses between two Halogen lamps and our Plasma lamp Apollo 95E. These lamps were used in 8 different composites available on the market in 3 different shades (Schein, Kulzer, Coltene, Espe, Kerr Herculine, 3M Z 100, Helio Progress and Cavex Clearfill).
The following tests were done by us, or on our authority, in independent and officially recognised laboratories of international repute, by lab assistants who had no relationship whatsoever with our research lab nor with DMD (Dental Medical Diagnostic Systems, Westlake Village California USA), the company that sponsored these analyses.
We performed the following analyses:
2. Scientific results and analyses:
2.1 - Spectrum Analysis:
Fig. 1: Comparison of the spectral curve of the halogen lamp (HL) and the plasma lamp (PL)
We know that the photosensitive molecules which are used as initiators of the polymerisation of dental composites are mainly situated between 460 and 480 nm, with a preference for 475 nm (family of the camphoroquinones). It is evident that any dental lamp within this band width will be more efficient.
Graph 1 clearly shows that the halogen lamp (HL) is more efficient in the red infrared light and is only slightly energetic in the zone in which we are interested. This is not the case with the plasma lamp (PL) which shows one of its most energetic peaks between 460 and 480 nm.
That is why we chose for this type of lamp, because, with the same power, this bulb is ten times more efficient and reacts to much more molecules in the same length of time.
Table 1: graph diagram of the rise in temperature
We thought it would be interesting to compare the anticipated rise in temperature in the pulp chamber, during the polymerisation time. We did this kind of research in various universities, so that we may conclude that:
It is common knowledge that red light produces more heat than violet light. The plasma lamp (PL) reaches an important peak between 460 and 480 nm (see fig. 1). Therefore there is no need to enlarge the spectrum width in order to supply more energy. The halogen lamp, on the other hand, which has a small energy value in the active wavelength (475 nm), must compensate this with a larger spectrum (between 400 and 510 nm in most cases). Therefore, it needs a higher thermal value to start the polymerisation process. Unfortunately, this implies that the temperature rises without improving the photopolymerisation significantly. This is why it is necessary to use the lamp for 40 to 60 seconds in order to get the same degree of efficiency.
2.3 - Analysis of the degree of polymerisation
Fig.2: Typical curve of differential calorimetry of the plasma lamp (PL) and the halogen lamp (HL)
The above-mentioned method of differential calorimetry by means of scanning is certainly one of the most sophisticated methods available to measure the real degree of polymerisation of a body, post-reaction. It is not based on random similarities between two physical properties, but it measures the residual material to be polymerised after a photopolymerisation of up to 100%. It measures directly the number of compounds that have not yet been polymerised.
Our research showed:
Even in those cases where a higher degree of polymerisation is not strictly required (e.g. to provide a better adhesion of two successive composite layers), we believe that a degree of 85% is acceptable, all the more so because the intensity produced by the plasma lamp during the curing of the second layer will complete the polymerisation of the first layer.
2.4 - Energetic analysis
Fig. 3: Energetic zones of the spectrums of halogen lamps (HL), plasma lamps (PL) and photo initiators (PI)
When analysing the polymerisation time, we find that, in the zone of spectral sensitivity that we are interested in, the plasma lamp is 6 to 10 times more efficient than the halogen lamp.
Still, a halogen lamp has a value of 0.700 mW/cm², whereas a plasma lamp has a value of 'only' 1.320 mW/cm². How can it be explained then that it is ten times more efficient?
An efficient energy level is not the same as the energy level of a lamp which spreads the energy over the entire band width used (fig. 3). Only the zone between 465 and 480 nm gives optimal efficiency and therefore, should be the only one recommended.
If we compare the performance of both lamps (HL) and (PL) within this narrow zone of high efficiency (integrating the surface) we see that the active surface of the plasma lamp is between 6 and 10 times higher than that of the halogen lamp.
2.5 - Analysis of the Knoop Hardness;
As you can see in the two histograms (fig. 5 and fig. 6) there are two types of composites:
Fig.5: Knoop hardness after 3 seconds with the Apollo 95E lamp and 40 seconds with the halogen lamp.
Fig. 6: Knoop Hardness at SC seconds for the Apollo 95E lamp and 40 seconds for the halogen lamp.
This difference in response to light results from the type of reaction in the polymerisation of the polymer itself rather than from the dispersed energy.
Anyway, we would recommend the 'devotees' of polymerisation in two steps to use the SC function, i.e. 1.5 seconds at half power followed by 3.8 seconds at full power, to allow them to put Professor Davidson's interesting theory into practice.
2.6 - Analysis of the contraction of the composite
Fig. 7: comparative dilatometric curve of the composites with a plasma lamp during 2s, 3s and 4s and a halogen lamp during 40 seconds.
We know that a composite contracts during polymerisation, not in proportion to the time of the reaction but in proportion to the number of double compounds transformed. Hence, there is no relation between the light source used and the variations of the volume observed.
All our studies prove that a rapid polymerisation gives less contraction, which, at first sight, seems contradictory to the theory expounded above. In reality, there is no contradiction at all. We believe that the quicker the adhesion, the quicker the body is cured. This reduces the contraction since it prevents the internal restructuring, which, with slow halogen polymerisation, stimulates the interpenetrations of molecules that are not completely polymerised in the other molecules. We prevent, so to speak, an easy redistribution of the molecular structure.
But then there is still the problem of internal tensions.
A recent theory suggests that it is better to polymerise slowly if you want to have a minimum of internal tensions. We think this is not correct since a rapid polymerisation results in a reduced contraction, i.e.: less overall tension. When looking at the results of our experiments, the tensions seem to be dispersed. We create a kind of dispersed tensions and there is not enough time for the resin in the composite material to create a global tension.
We use the composite's heterogeneity to create barriers for the internal tension trying to create a global tension (study of finished elements).
It goes without saying that we do not question the theory of a slow polymerisation. In case of a slow polymerisation, the composite behaves like a homogeneous instead of like a heterogeneous body, allowing a global distribution of the internal tensions. In that case, we absolutely must extend the polymerisation time as much as possible, up to 60 seconds and even up to (4 or 5 mn).
2.7 - Analysis of the interface between tooth and filling
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This analysis allowed us to validate our theory of rapid polymerisation and the dispersion of tensions. All the measurements we carried out showed that the interface, and consequently the contraction, is smaller with the plasma lamp than with the halogen lamp.
These two observations regarding the use of the plasma lamp show that:
Finally we also observed that the composite seemed to be attracted by the luminary source.
Since the power of the lamp enabled us to work with transillumination, we were able to reduce, in certain samples, the space between tooth and filling even more, by polymerising straight through the tooth (Dr. Bertrand)
3. Clinical results and analyses:
Over 500 Apollo 95E plasma bulbs are being used today in dental practices and some have been using this bulb for almost a year now. We have gathered some clinical information which, in our opinion, could be of interest for the every-day use in dentistry.
Indeed, you will be surprised by the power of the bulb from the first time you use it.
It is important always to be careful and never to direct the light rays towards the eyes of the patient, even though the risk is smaller than with LASER rays.
The most significant contribution of this power makes itself felt in four essential domains.
These are:
The second function, which, at first sight, seems to be the most spectacular, is the time reduction. It is obvious that:
Conclusion:
It is always difficult, even if it is not the first time, to present a new technology to the profession, especially when it concerns a common practice like the photopolymerisation of composites.
Even if we still have a long way to go before we will be able to apply CAD CAM, the concept of the plasma bulb has been surprising us for more than 15 years thanks to the potential it holds for the profession and the improvement it has brought in every-day use.
The optimisation of the development of the Plasma microtorch allowed us to master this technique gradually, and the striving for diversification made it possible to use it successfully in polymerisation.
After two years of work, we are surprised and impressed by the results obtained, although we are fully aware that they contravene certain established practices. Rapid polymerisation produces less heat, creates less contraction and makes the composites harder.
Indeed, we are sure this will lead to various experiments trying to refute this theory and to various scientific debates which we will follow with great interest. But, in our opinion, the comfort experienced by practitioner and patients thanks to this type of development will never be surmounted.
We and the people surrounding us are convinced that this development has created a revolution in every-day clinical practice, and that it has made a laborious task look like child's play, to say the least.
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