To those who would like more information about the polymerisation of composite…. and Why are Lysta’s curing lights different from all others?

Articles by: Flemming Brandt, DDS and Professor Erik Asmussen and Associate Professor Anne Peutzfeldt

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This article is written by Flemming Brandt, DDS.

Polymerisation of Composites It ought to be so simple and yet we see many different solutions to the problem of polymerisation. Intensity x Time = total dissipated energy 600 mw/cm2 x 20 sec = 12,000 mwE/ cm2 It is easy to see from the above formula that, if we wish to change the recommended amount of energy (600mwE/ cm2 ), we only need to change the time and/or the intensity of the lamp. It is a simple mathematical fact. This formula is applied by manufacturers of lamps and composites to explain how their product is able to polymerise in 10 seconds or less. However, simple mathematical formulas cannot replace real clinical issues. It is not that simple. If we increase the input, polymerisation will of course be quicker but we will also increase the stress on the composite considerably. This is true of all plastic composites. The gel point (g point) is reached much more quickly and, as is evident from the simple diagram below, it only increases the stress inside the composite. The quality of the polymerisation is no better than polymerisation with lower input over a longer period of time. Studies show that, on the contrary, there are many more cohesive cracks in the composite and poor adhesion to the tooth. Energy. mwE/cm2 G point stress G point stress Time/polymerisation

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Stress is an undesired effect. It does not only put strain on the composite itself, it also creates stress between the tooth and the composite and this puts a stress on the teeth. The diagram at the end of the article shows that quite a lot of stress is built up in the various different composites. The diagram shows the correlation between the contraction of the polymerised material and the build up of stress in the various different composites. This can be quite considerable. (Can the established bond between tooth and composite cope with this?) On the other hand, if we reduce the intensity and increase the time, we would achieve the same degree of polymerisation without inappropriate stress. The composite would have time to flow from the unpolymerised area.

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See the explanation in the following diagram

Tests which I have performed several times show that there is a minimal difference: A variation in polymerisation depth of approx. 10%. The test was performed with LYSTA’s LCD88 HI-POWER. This lamp meets the requirements for step polymerisation. If the same test is carried out regularly, it will indicate the efficiency of the lamp and warn against ineffective polymerisation. The test will also prove whether the clinic’s composites are suitable for the curing light(s) used. Over the years, many different methods of polymerising composite have appeared: Ramp, pulse, interval and step polymerisation. It has been an accepted fact for a long time now that the way in which a dentist uses his lamp is of little significance. This is incorrect.

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Step polymerisation, starting at a relatively low output and increasing the effect after a period of time, has been proven to reduce the formation of marginal gaps by 30%. energy

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With a simple test, which everyone can try, I have proven that this is clinically true. Fill an empty glass ampoule with composite to a depth of approx. 10 mm and polymerise it at maximum output for only 20 seconds. You will be surprised to see the intensity of the contraction. It is also evident that there is a great difference between the ways in which the different types of composite react. I can actually determine the composite’s gel point with the naked eye and simultaneously measure the time from the start of polymerisation to the time at which the composite begins to contract.

Shocking.

An ampoule with dye penetration

An ampoule after step polymerisation

Immediately after polymerisation, the ampoules were placed in a very thixotropic fluid. I used SEE-it® from Zacho·Rønvig. After a couple of minutes the fluid penetrated the first ampoule with maximum effect. There is a clear dyecoloured gap between the composite and the glass tube. It is quite different in the case of the other test in which I used the step method (30 + 20 secs). There is hardly any dye between the composite and the glass tube. This shows that we can apply this method in clinics to great advantage and we will achieve restorations with fewer gaps. The results apply both to the clinic and the laboratory. 5

I strongly advise dentists to use the above-mentioned method when polymerising composite which has contact with the tooth. It is particularly recommendable in deep proximal boxes and in the case of sclerosed, discoloured dentin where the bond between the plastic components and the tooth is already reduced. The use of selfcorroding adhesives does not provide better conditions. In the case of the new bonding materials, the bond with the enamel is often a problem. Contact between composite and composite is better because the bond between composite and composite is better than the bond between composite and tooth. I recommend consistent construction of layers using a sloping layer technique. This keeps the contraction factor to a low and acceptable level. Controlled polymerisation together with the above-mentioned technique provides predictable results and ensures a long lifetime for the restoration.

T ime is an essential factor:

DDS. Flemming Brandt: Member of: Founder and owner of:

Former teacher, Lecturer (both nationally and internationally), author of several clinical papers, Prosthodontist and Oral Surgeon. DSOI. and SPBT, in Denmark EAO, ITI member and ESCD. “Center for Odontological development”: A company dedicated to post-graduate education in a wide range of subjects.

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This article was previously published in Danish in the Danish Dental Journal (no. 8, 2004). Professor Erik Asmussen, associate professor Anne Peutzfeldt (Copenhagen Dental School) and the Danish Dental Journal have given Lysta permission to translate the article into English.

Soft-start polymerization of composite resin: Does it work?

Erik Asmussen and Anne Peutzfeldt It is characteristic of resin composites that they shrink during polymerization and that this may lead to gaps at the margins of composite restorations. It has been suggested that the size of marginal gaps can be reduced by the application of so-called soft-start photo-curing techniques. The soft-start polymerization method consists of two steps: the first step is carried out at reduced intensity and the second step at high intensity. It was the purpose of the present study to investigate the effect of two-step polymerization on the formation of marginal gaps at composite restorations. The restorations were constructed in cylindrical cavities prepared in dentin after the surfaces of the extracted teeth had been polished. An LED curing unit was used for polymerization. The first step (precuring) was carried out at intensities of 70, 140 or 200 mW/cm 2 for 15, 30, 60 or 120 seconds. The final polymerization as well as the polymerization in the control group was carried out for 20 seconds at 600 mW/cm2. After polymerization, the gaps were measured under a microscope. It was found that two-step polymerization significantly reduced the width of the marginal gaps. The greatest reduction was observed when the duration of precuring was 30 seconds. There was no difference between the three groups precured at 70, 140 or 200 mW/cm2. It was concluded that certain methods of two-step curing may have a gap-reducing effect. When compared with the results of other studies, these results indicated that, if two-step polymerization is to be effective, the first step must include a relatively long exposure time at a relatively low intensity. All composite resins shrink during polymerization. As a result, the resin has a tendency to recede from the walls of the cavity, and this leads to the formation of marginal gaps. These gaps are undesirable as they allow bacteria and dye to penetrate, and this leads to a risk of secondary caries, pulp damage and discolouration of the restoration margin. Over the years, many methods have been suggested for the reduction of the occurrence of marginal gaps at composite restorations, e.g. the application of adhesive techniques (1, 2), the construction and polymerization of restorations in oblique layers (3), and so-called soft-start polymerization (4). During soft-start polymerization the composite resin is exposed to relatively low light intensities during the first step of the polymerization (precuring) and is then cured at full power during the final polymerization phase. The intention of the “soft” start is to induce slow polymerization so that the composite resin has time to “flow” in from unbonded surfaces before the resin becomes too hard. This reduces the build up of stress as well as the tendency of the composite resin to shrink away from the walls of the cavity (5). There are two possible soft-start methods: two-step polymerization and pulse-delay polymerization. In the case of two-step polymerization, curing begins at a low intensity, which is either sustained for a number of seconds (typically 10 seconds), or increases slowly to the final intensity. This is known as “ramped” polymerization. In the case of pulse-delay polymerization, the composite resin is irradiated at high intensity – typically for a couple of seconds and, after a delay of perhaps a minute, the final polymerization is then carried out (6). Whereas several independent studies have proved the pulse-delay method to be effective (6,7), the results of the two-step method have been mixed: Some studies revealed a positive effect (4,8,9), while others have not been able to discover any advantages to the method (10-12). The reasons for the failure of the two-step method in some studies can only be conjecture. It is possible that the soft start was not soft enough, or, on the contrary, that it was too hard and the composite resin was not able to

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flow as intended. Another possibility is that prepolymerization was so short that there was not enough time for the resin to flow. The purpose of this study was to investigate these possibilities by varying the intensity and duration of prepolymerisation and by measuring the effects on the marginal gap. Materials and method The resin composite used in the study was Filtek Supreme, A2 Dentin Shade (3M ESPE). Polymerization was carried out using the DioCure LED curing unit (CMS Dental). The version of the unit supplied by the manufacturer enabled prepolymerization at 70, 140 and 200 mW/cm 2 and final polymerization at approx. 600 mW/cm 2. (It should be noted that a different lightmeter, from the same manufacturer, measured intensities of 100, 200 and 300 respectively as well as over 1000 mW/cm2.) The mesial or distal surfaces of extracted human teeth were ground flat until a sufficiently large area of dentin was exposed. Cavities with a diameter of 3 mm and a depth of 1.5 mm were prepared using a cylindrical drill. The cavities were filled with a small amount of composite resin, covered by a transparent matrix band and precured at the three intensities mentioned above for 15, 30, 60 or 120 seconds before final polymerization was carried out for 20 seconds. Fillings in the control group were not precured, but were only subjected to final polymerization. After polymerization, the filled teeth were immersed in lukewarm water (37ºC) for 15 minutes before the excess composite resin was removed using carborundum paper (1000 grit) and the fillings were polished with corundum powder (grain size 0.3 µm). Marginal gaps were then measured under the microscope at their widest part and wall-to-wall polymerization contraction was calculated as the gap width as a percentage of the diameter of the cavity. Six fillings were tested for each set of experimental conditions. The results were treated statistically using analysis of variance and a level of significance α = 0.05. Results The results of the study are presented in fig. 1. The control group, which was not subjected to prepolymerization, had significantly larger marginal gaps than the groups in which fillings were precured. Statistically, there was no significant difference between the groups which were precured at 70, 140 or 200 mW/cm2. A prepolymerization time of only 15 seconds resulted in larger marginal gaps than prepolymerization for 30, 60 and 120 seconds, whereas there was no statistically significant difference between the latter three groups. Discussion The present study found that certain methods of two-step polymerization result in a significant reduction of the width of the gaps at the margins of composite restorations. As mentioned in the introduction, this was also found in a number of previous studies – but not in all. It is very difficult to say why soft-start works in some cases but not in others. One explanation could be that the duration of prepolymerization by curing lamps with integral soft-start programmes is too short (e.g. only 10 seconds). This explanation is supported by the results of this study, as fig. 1 shows by interpolation that the effect of prepolymerization for only 10 seconds would be too modest to be statistically significant, and that the maximum effect would only be achieved if the duration of prepolymerization was 30 seconds. In addition, the intensity of the prepolymerization could also be of significance. If the intensity is too low the composite resin will be undercured; if the intensity is too high it will be overcured. In the study described here, there was no difference between the groups precured at 70, 140 or 200 mW/cm 2. Therefore, it is impossible to tell whether a lower intensity over a period of time longer than 30 seconds would have led to a further reduction in the width of the marginal gaps. In the case of the pulse-delay

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method investigated in a previous study (7), which was found to be effective based upon a method similar to the one applied in this study, the energy dose (the product of light intensity and exposure time) produced by a pulse of 2 seconds, an intensity of 425 mW/cm 2 and a delay of 1 minute before the final polymerization was 950 mJ/cm 2. In the study described here, an exposure time of 30 seconds at 70 mW/cm2 results in an energy dose as high as approximately 2100 mJ/cm 2 before final polymerization. This indicates that even lower intensities, possibly coupled with longer exposure times, may be effective. With regard to the fact that two-step polymerization was not found to have a positive effect on the formation of marginal gaps in all studies, it has also been found that there can be a difference between the composite resins investigated: the method works for some makes of composite but not for others (8). Thus, it is possible to conclude that the sensitivity of the composite resin’s initiator system is of some significance. If the selected combination of time and intensity of prepolymerization is to provide the composite resin with both the time and opportunity to flow as intended, the sensitivity of the initiator system is crucial. A two-step polymerization technique takes longer than conventional polymerization, as does the pulsedelay method. Therefore, the question is whether it is necessary to apply this method to each individual layer of the composite restoration being constructed. It must be said that, theoretically, it would be an advantage from the point of view of the marginal gap. However, it is the assessment of the inventors of the pulse-delay method (6) that it would be sufficient to apply the method to the final layer of composite resin in an occlusal cavity; their argument being that as long as the restoration is sealed to the oral cavity it will be impossible for either bacteria or dyes to penetrate. This study showed a reduction in gap width of approx. 30%. This reduction may appear to be modest in comparison with the time required to achieve the reduction. However, it must be considered that it is not only the width of the gap that is reduced but also the extent of the gap along the margin of the restoration (13). In addition, there are other procedures, such as the use of dentin and enamel bonding agents and the construction of the restoration in oblique layers, which help to reduce contraction stress. Together, these methods could perhaps prevent the formation of marginal gaps altogether.

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Fig.1: Gap width (mean values) in percentage of cavity diameter. The pooled standard deviation was 0.04%. The pre-cure was carried out at 70, 140 or 200 2 mW/cm .

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References: 1. van Meerbeek B, Perdigão J, Lambrechts P, Vanherle G. The clinical performance of adhesives. J Dent 1998; 26: 1-20. 2. Hansen EK, Asmussen E. Comparative study of dentin adhesives. Scand J Dent Res 1985; 93: 280-7. 3. Hansen EK. Effec t of cavity depth and application technique on marginal adaptation of resins in dentin cavities. J Dent Res 1986; 65: 1319-21. 4. Uno S, Asmussen E. Marginal adaptation of a restorative resin polymerized at reduced rate. Scand J Dent Res 1991; 99: 440 -4. 5. Davidson CL, de Gee AJ. Relaxation of polymerization shrinkage stresses by fl ow in dental composites. J Dent Res 1984; 63: 146 -8. 6. Kanca J III, Suh BI. Pulse activation: Reducing resin -based stresses at the enamel cavosurface margins. Am J Dent 1999; 12: 107-12. 7. SahafiA, Peutzfeldt A, Asmussen E. Effect of pulse delay curing on in vitro wall-to-wall contraction of composite in dentin cavity preparations. Am J Dent 2001; 14:295 -6. 8. Ernst CP, Brand N, Frommator U, Rippin G, Willershausen B. Reduction of polymerization shrinkage stress and marginal microleakage using soft -start polymerization. J Esthet Restor Dent 2003; 15: 93-103. 9. Ernst CP, Kurschner R, Rippin G, Willershausen B. Stress reduction in resin -based composites cured with a two-step light-cu- ring unit. Am J Dent 2000; 13: 69 -72. 10. SahafiA, Peutzfeldt A, Asmussen E. Soft-start polymerization and marginal gap formation in vitro. Am J Dent 2002; 14: 145 -7. 11. Brackett WW, Covey DA, St Germain HA jr. One -year clinical performance of a self -etching adhesive in class V resin composi - tes cured by two methods. Oper Dent 2002; 27: 218 -22. 12. Amaral CM, de Castro AK, Pimenta LA, Ambrosano GM. In fl uence of resin composite polymerization techniques on microleakage and microhardness. Quintessence Int 2002; 33: 685 9. 13. Hansen EK, Asmussen E. Effect of postponed polishing on mar - ginal adaptation of resin used with dentin-bonding agent. Scand J Dent Res 1988, 96: 260 -4. Authors: Professor Erik Asmussen and Associate Professor Anne Peutzfeldt Department of Dental Materials, Institute of Odo ntology, Faculty of Health Sciences, Copenhagen University.

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Together with Dr. odont. Erik Keith Hansen, Lysta has designed a simple device for the testing of cured composite. The Lysta hardness testing device is a 10mm high Teflon block with a conical hole for the plastic filling. 1.

Place the Lysta hardness testing device on a sheet of white paper with the small opening facing down.

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Fill the cavity with the composite material you wish to test.

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Position the tip of the light guide as close to the plastic as possible without touching it and illuminate it for a period of 40 seconds.

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Push the composite material out of the mould after five minutes, and scrape off the unpolymerised plastic with a plastic spatula. Only remove the soft plastic. Due to the fact that polymerisation continues for quite a while after the curing lamp is removed, the test piece should not be measured directly after polymerisation. If you scrape off the unpolymerised plastic too soon, the final result of the polymerisation depth will be too small.

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Measure the polymerisation depth using a slide gauge. Research shows that the test sample is almost always polymerised deeper at one side than at the other side. Therefore, you should measure the depth at both sides, and calculate the average.

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Subtract 1.5 mm from the average. The result is the depth to which the lamp can polymerise the plastic in a human tooth.

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The final result will include plastic which is both well polymerised and poorly polymerised. Therefore, we can assume that plastic which is sufficiently polymerised only corresponds to 60% of the computed polymerisation depth.

Example The test piece is 6.5 mm at one side and 4.1 mm at the other. The average is 5.3 mm. Subtract 1.5 mm from this average. The result is a polymerisation depth of 3,8 mm in a human tooth. 60 % of this will be plastic which is sufficiently polymerised, which corresponds to 2,3 mm.

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