ablation rate and morphological aspects of composite resins

Laser is now established as a suitable tool for the se-lective and precise removal of carious dental tissue,

as well as for many other applications.1-8 If correctlyused, laser ablation minimizes healthy tissue removaland increases patient comfort. The reduced noise andmechanical vibration result in less pain, making the pro-cedure tolerable for the majority of patients.9,10 Asidefrom removal of carious dental tissue, the removal ormodification of composite resin restorations is alsoquite a common procedure. The conventional toolsemployed in these cases are the low- and high-speedrotating instruments, and in some cases, an aluminum

oxide micro-etcher jet. As described previously,11-12 ad-vantages such as the possibility of selective resin re-moval and preserving healthy tissue are importantissues to be considered. The use of laser for this pur-pose requires knowledge of the basic features of com-posite resin laser ablation. Such information is basicallyabsent from the literature, making clinical applicationprogress difficult in this area. The knowledge of thebasic aspects of resin laser ablation can lead to the development of procedures that allow the selective removal of composite resin, preserving the dental tis-sue.12 Basically, all high-intensity laser systems available

Ablation Rate and Morphological Aspects ofComposite Resins Exposed to Er:YAG Laser

Rosane de Fátima Zanirato Lizarellia, Lílian Tan Moriyamab, José Eduardo Pelizon Pelinoc, Vanderlei Salvador Bagnatod

a Professor, Physics Institute of São Carlos (IFSC), University of São Paulo (USP), Brazil.b Physicist, Postgraduate Student, Physics Institute of São Carlos, University of São Paulo (USP), Brazil.c Professor, IPEN/FO, University of São Paulo (USP), Brazil.d Professor, Physics Institute of São Carlos, University of São Paulo (USP), Brazil.

Purpose: The aim of this work was to investigate the ablation rate and the morphological aspects of three com-posite resins using Er:YAG laser irradiation.

Materials and Methods: Three different types of composite resins were chosen (microfilled, hybrid, condens-able) regarding their chemical and structural composition. Composite tablets were irradiated with an Er:YAGlaser at different energy levels per pulse (100, 200, 300, and 400 mJ). The diameter and depth of each compos-ite resin tablet were measured, as was the microcavity produced by laser ablation, and from this the overall vol-ume removed was calculated. Observations of morphological aspects were performed using electron microscopy.The ANOVA and Tukey tests were used to statistically analyze the volume results.

Results: The only variable that influenced the ablation rate of the composite resins tested was the energy levelper pulse.

Conclusion: Examining the basic features of laser ablation of composite resin represents the first step to under-standing the mechanism of composite resin ablation by Er:YAG laser and the numerous differences due to resincomposition and structure.

Keywords: ablation rate, morphological aspects, Er:YAG laser, composite resin.

J Oral Laser Applications; 5: 151-160. Submitted for publication:10.08.04; accepted for publication:05.02.05.

Vol 5, No 3, 2005 151

SCIENCE

for dental clinical application can remove restorativematerials. Nevertheless, since Er:YAG laser is the sys-tem mainly used for dentistry, we have chosen it as thestarting point for a series of studies. We are planningto achieve a full understanding of laser ablation of com-posite resin, involving different types of laser radiation.

The purpose of this study was to investigate ablationrate and morphological effects of Er:YAG laser onthree types of composite resin: microfilled, hybrid, andcondensable.

Because Er:YAG laser interacts with tissues throughan explosive thermomechanical mechanism resultingfrom the interaction of laser with water droplets pre-sent in the ablation region, interaction with differentresin types may result in different morphological as-pects as well as different rates of material removal.

MATERIALS AND METHODS

Experiment I – Thermal Mapping

Thermal mapping during laser ablation was performed toconfirm that this laser is a safe tool for the proposed use.

Using a metallic mold, we made six 4-mm-thickcylindrical tablets with a diameter of 4 mm. An A2 hy-brid composite resin (batch OWN 2003-05, Z100,3M, St Paul, MN, USA) was used as a restorative mate-rial. The resin tablets were compressed and then curedunder a halogen lamp (KM200R, K&M, São Carlos,Brazil) for 80 s.

After this step, a cylindrical cavity of 2 mm depthand 1.5 mm diameter was prepared in the base of eachtablet using a #1091 diamond bur (SSWhite, Brazil).

This cavity was filled with a thermal conducting paste(Implastec, Votorantim, São Paulo, Brazil) and a ther-mocouple (Model 120-202 EAJ, Fenwal Electronic, Mil-ford, MA, USA) was introduced as shown in Fig 1.

The experimental setup is depicted in Fig 2. Thelaser system used was the Twin Light Er:YAG laser (Fo-tona Medical Lasers, Ljubljana, Slovenia), operating at2940 nm, peak energy up to 500 mJ, frequency up to15 Hz, and pulse duration of 200 to 450 ms. For thisexperiment, a 12.0-mm focal distance (focused laserbeam), frequency 10 Hz, exposition time of 10 s, andenergy levels of 80, 400 and 500 mJ per pulse wereset. Each parameter was repeated three times and anaverage value to plot a curve was calculated for them.

Experiment II – Ablation Rate and MorphologicalEvaluation

Using the same methodology, we made sixty 2-mm-thick cylindrical tablets with a diameter of 8 mm.11

Three different materials were chosen: a microfilledresin A2 (batch A30122, Durafil VS, Heraeus Kulzer,Hanau, Germany), an A2 hybrid composite resin(batch OWN 2003-05, Z100, 3M), and an A2 con-densable composite resin (batch 39722 2002-12,Alert, Jeneric Pentron, Wallingford, CT, USA).

The composite resin tablets were compressed andthen cured under conventional light (halogen lamp,KM200R, K&M) for 40 s for the hybrid compositeresin and 20 s for the others, following the manufac-turer’s recommendations.

The laser system used was the same as above. Thesmallest possible diameter of the laser beam is around

152 The Journal of Oral Laser Applications

SCIENCE

Fig 1 Introducing a thermocouple into the resin tablet with athermal conducting paste.

Fig 2 Experimental setup and focused laser tip.

700 µm. For the present experiment, we set a distanceof 12.0 mm, fixed the frequency at 10 Hz, an exposuretime of 10 s, and energy levels at 100, 200, 300 and400 mJ per pulse. Each sample was ablated at 12 spotswith three repetitions for each energy level.

The samples were immersed in epoxy resin for fur-ther processing and the micromorphological aspects ofthe ablated volume were evaluated under a scanningelectron microscope (SEM; DSM 960, Zeiss, Ober-kochen, Germany). The diameter and the depth of theablated regions were optically measured, which al-lowed the determination of the ablated material vol-ume. We f irst focused on the top surface of themicrocavity and then on the deepest part of the ab-lated cavity. A calibrated micrometer allowed the depthto be measured. The diameter was measured using acalibrated micrometer slide coupled with the mi-croscopy images.

RESULTS

Experiment I – Thermal Mapping

Temperature values were stored and plotted in a com-puter. As a result, plots show the temperature profilesas a function of time for each parameter used.

Figure 3 shows the obtained data. All curves de-monstrate the same performance: laser radiation startsto ablate the target at the 3rd second, visible on thecurves as a decrease in temperature. After that, tem-perature increases up to the 13th second, when thelaser irradiation ceases. Then, the curves drop, repre-senting the thermal relaxation of the composite resin.

The graphs allow the calculation of temperaturevariation during laser ablation for each energy perpulse.

Experiment II – Ablation Rate and MorphologicalEvaluation

Figure 4 shows the macroscopic morphology of the ab-lated region for all composite resins at 300 mJ. Charac-teristics such as ablation regularity and shape can beobtained from these observations.

At higher magnification, the microstructure of theablated region can be observed. Figure 5 shows the mi-crostructure at 1000X magnification of a microfilledcomposite resin ablated at four different energy levels.Equivalent results are presented in Figs 6 and 7 for hy-brid and condensable composite resins, respectively.

The observed ablation effect for different laser energylevels is clearly peculiar to each composite type andvery dependent on the energy delivered per pulse.

In all samples, the microcavities obtained using laserirradiation show shapes that resemble a conical cavity.The diameters and the depths of the ablated regionswere measured for each irradiated sample. In order tosimplify the comparison, Figs 8 and 9 present the mea-sured diameter and depth, respectively, for the ablatedsurfaces for the three composite resin types.

The removed volume was calculated for each pre-pared cavity based on the conical shape and the diame-ter and depth values (Fig 10). The results of theremoved volume were submitted to ANOVA andTukey’s test. Comparing all variance factors, a statisti-cally significant difference (p < 0.01) was observed(Table 1) when different types of composite resins andenergy per pulse were considered. It means that allthese factors may be statistically significant concerningthe influence on the volume removed. However, theTukey test (p < 0.05) showed that 300 mJ was able toablate the highest material volume.

Figures 5 to 7 show important aspects of laser-mate-rial interactions.

DISCUSSION

Thermal mapping during composite resin ablation withEr:YAG laser showed an average temperature variationfrom ca 0.7°C (80 mJ per pulse) to 0.9°C (400 and500 mJ per pulse) (Fig 3). These temperature varia-tions are not damaging to the pulp14 or the periodontaltissue.15

Vol 5, No 3, 2005 153

SCIENCE

Fig 3 Temperature variation as a function of the irradiation time,considering energies per pulse.

Although our experimental setup did not includeany additional cooling source, it is mandatory to con-sider that dental organs (teeth and supporting struc-tures) possess complex physiological cooling systems.Thus, it is probable that composite resin ablation withthe Er:YAG laser under clinically acceptable parametersdoes not result in damage to the pulp or periodontaltissues.

In contrast to natural dental tissue, observations ofthe ablated regions of the composite resins show avery regular ablation pattern. The edges visible in themicrographs of Fig 4 are well defined in all three com-posite resin types. This uniformity is probably a conse-quence of the homogeneity of the material itself. Thereare, however, small variations from one resin type tothe next, due to the differences in filler-polymer matrixbond energy in each composite type, resulting in differ-ences in the ablation resistance. Greater cohesion inthe material will result in greater resistance to laser ab-lation.

The overall aspect of the ablated area reveals resinbehavior itself as a soft material under laser irradiation.It seems that the hybrid composite resin can be moreeasily and efficiently removed by the Er:YAG laser thanthe microfilled and condensable ones. That may be dueto the microfiller microstructure, the resistance ofwhich relies entirely on the polymeric matrix, which,once ablated, releases the filler particles. The two com-posite resin types with cores consisting of polymer withdispersed fillers (microfiller and hybrid) show equiva-lent aspects. The condensable composite resin seems,at least macroscopically, to be more resistant to laserablation.

SEM analysis shows a different microstructure in theablated region for each composite resin. The surfacefeatures presented in Fig 5 show that for the micro-filled composite resin, laser ablation occurs mainlythrough the polymer matrix. The spaces previously oc-cupied by microparticles are left behind. An increase ofenergy in the laser pulse does not seem to change the

SCIENCE


Figs 4c Microcavity in condensable resin resulting from Er:YAGlaser irradiation using 10 Hz and 300 mJ for 10 s (80X magnifica-tion).

Fig 4a Microcavity in microfilled composite resin resulting fromEr:YAG laser irradiation using 10 Hz and 300 mJ for 10 s (80Xmagnification).

Fig 4b Microcavity in hybrid resin resulting from Er:YAG laser ir-radiation using 10 Hz and 300 mJ for 10 s (80X magnification).

overall aspects, but clearly produces greater modifica-tion of the polymer matrix.

For the hybrid composite (Fig 6), the microstructureof the irradiated surface does not show severe modifi-cation even when laser pulse energy varies. Neverthe-

less, larger energies seem to produce an ablated sur-face with a more regular aspect.

In both the microfilled and hybrid composite resinsdiscussed above, the low cohesion energy manifests asan apparent regularity in the ablated area. There seems

Vol 5, No 3, 2005 155

SCIENCE

Fig 5a Microfilled composite resin surface irradiated with Er:YAGlaser at 10 Hz for 10 s, energy level 100 mJ (1000X magnifica-tion).

Fig 5b Microfilled composite resin surface irradiated with Er:YAGlaser at 10 Hz for 10 s, energy level 200 mJ (1000X magnifica-tion).

Fig 5c Microfilled composite resin surface irradiated with Er:YAGlaser at 10 Hz for 10 s, energy level 3oo mJ (1000X magnifica-tion).

Fig 5d Microfilled composite resin surface irradiated with Er:YAGlaser at 10 Hz for 10 s, energy level 400 mJ (1000X magnifica-tion).

to be no evidence of resistance offered by the materialduring ablation.

On the other hand, the condensable compositeresin (Fig 7) behaves differently from the two previ-ously discussed cases. Here, the low energy per pulse

parameter seems to produce little effect on the mater-ial. At this level, the energy of the laser is insufficient toovercome the cohesion of the material and promotethe ablation. Increase of energy seems to overcomethis apparent barrier exposing the heterogeneity of the

SCIENCE


Fig 6a Hybrid composite resin surface irradiated with Er:YAGlaser at 10 Hz for 10 s, energy level 100 mJ (1000X magnifica-tion).

Fig 6b Hybrid composite resin surface irradiated with Er:YAGlaser at 10 Hz for 10 s, energy levels 200 mJ (1000X magnifica-tion).

Fig 6c Hybrid composite resin surface irradiated with Er:YAGlaser at 10 Hz for 10 s, energy levels 300 mJ (1000X magnifica-tion).

Fig 6d Hybrid composite resin surface irradiated with Er:YAGlaser at 10 Hz for 10 s, energy levels 400 mJ (1000X magnifica-tion).

structure. The presence of fiberglass became evident,and the general shape of the fibers seems preserved.Increasing the energy clearly causes a breakdown, eas-ily removing the inter-fiber polymeric matrix. At thehighest energy level per pulse, the micrographs clearly

show that the fiberglass arrangement is being disassem-bled.

Concerning the diameter of the ablated area, Fig 8shows that the behavior of all composite resins is simi-lar. Extrapolation of E (mJ) = 0 per pulse provides an

Vol 5, No 3, 2005 157

SCIENCE

Fig 7a Condensable composite resin surface irradiated with theEr:YAG laser at 10 Hz for 10 s, energy level 100 mJ (1000X mag-nification).

Fig 7b Condensable composite resin surface irradiated with theEr:YAG laser at 10 Hz for 10 s, energy level 200 mJ (1000X mag-nification).

Fig 7c Condensable composite resin surface irradiated with theEr:YAG laser at 10 Hz for 10 s, energy level 300 mJ (1000X mag-nification).

Fig 7d Condensable composite resin surface irradiated with theEr:YAG laser at 10 Hz for 10 s, energy level 400 mJ (1000X mag-nification).

irradiated diameter on the order of 0.75 to 0.80 mm,which is consistent with the nominal beam diameter ofabout 0.7 mm. Clearly, the ablated diameter increasesas the laser energy increases and tends to saturate atvalues of ca 1.2 mm. The increase of diameter with thepulse energy is a consequence of better use of the en-ergy tail present on the beam spot. Higher energy in

the tail makes it more similar to the hot part of thebeam, therefore causing ablation in a wider area. Thecurve showing progressive decrease in the rate of di-ameter increase with energy is probably a consequenceof the limited amount of water in the irradiated region,which is intrinsically related to the ablation mechanismfor the Er:YAG laser. Water is essential for ablation.Figure 8 shows that in almost all situations, the ablateddiameter is the smallest in the condensable compositeresin when compared to the others. The condensablecomposite resin, having the fiberglass component, pre-sents better resistance to material removal. On theother hand, the hybrid composite resin shows, for allconditions, the highest ablation rate concerning the di-ameter, which is also related to the structure. The vari-ety of particle sizes makes the material easier to bedisassembled.

Concerning the depth of penetration (Fig 9), the re-sults show the existence of a threshold energy perpulse before its penetration into the target tissue. Thisis more evident in the condensable composite resin,but it is present to some extent in microfilled and hy-brid ones as well. The overall observed behavior forpenetration is the following: from zero energy up, thespeed of penetration seems to be quite low until thethreshold value (Eth) is reached, whereupon the pene-tration rate rapidly increases. After this fast increase,the penetration depth seems to keep increasing at arate that decreases with increase of energy, again (simi-larly to the diameter phenomenon) with a possible sat-

SCIENCE


Fig 8 The observed ablated diameter for different resin types.

Fig 10 The removed volume of material as a function of energyper pulse for each type of composite resin.

Fig 9 The penetration depth of ablation is presented for eachcomposite resin type.

uration effect, even though less evident than in the di-ameter case. The leveling out effect in penetration maybe based on several factors. First, the amount of waterinteracting with the laser in the ablation mechanism candetermine the maximum potential ablation. Second,the increase of energy per pulse increases the ablationability, but the system’s ability to eject the ablated ma-terial is also limited, causing a saturation effect. Thethreshold value (Eth) can be estimated from Fig 9. Thiscan be done through the prolongation of the steepestpart of the curves. As indicated in Fig 9, Eth values forhybrid, microfilled and condensable composite resinsare approximately 55 mJ, 80 mJ, and 110 mJ, respec-tively. These threshold values indicate the conditions ofcohesion for the resin. In this study, the hybrid com-posite resin showed the lowest cohesion level com-pared to the others, while the condensable compositeresin presents the highest. This is in agreement withLuo et al,16 who evaluated the effect of filler porosityon abrasion resistance, and concluded that porous par-ticles prepared via sol-gel show some promise in termsof improving the wear resistance of photopolymerizedcomposite resins. The Z100 fillers are produced as aresult of sol-gel chemical reaction.

In this study, the condensable composite resin show-ed the highest resistance to ablation, followed by themicrofilled and finally the hybrid composite resins. Thisis in fact in agreement with the observed microstruc-tural morphology after laser irradiation under severalconditions, and is also supported by Berlin et al,17 whostate that regarding polymer composites with reinforc-ing fibers, the mechanism of stress transmission from

the matrix to the filler depends on the filler particleconfiguration.

Finally, Fig 10 shows the results concerning removedmaterial volume as a function of energy per pulse. Thegraph shows similar depth penetration curves. For theablated volume, there is Eth per pulse to start materialremoval. From zero energy up, the removed volumeseems to be quite low until the threshold energy valueis reached, after which the volume removed rapidly in-creases. Just after this fast increase, the volume in-creases at a rate that decreases with increase ofenergy. A possible saturation, similar to the penetra-tion depth is observed. The existence of Eth shows thatthere is a minimum energy able to overcome the en-ergy that holds the resin together.

This study presents a new finding: when Er:YAGlaser removes cured composite resin, the ablationmechanisms involved are explosive vaporization fol-lowed by a hydrodynamic ejection. Rapid melting cre-ates large expansion forces due to the volume changeof the material upon melting. Liquid expansion is op-posed by the liquid surface energy or surface tension.These counteracting forces combined with the compos-ite resin structure result in the formation of surfaceprotrusions which are accelerated away from the sur-face as droplets.

CONCLUSION

Our findings allow us to conclude that energy level perpulse is the only factor that influenced the ablation rate

Vol 5, No 3, 2005 159

SCIENCE

Table 1 ANOVA test results

ANOVA: original valuesVariance factors Square sum Degrees of freedom Square average F Prob. (H0)

Between tablets (T) 0.0051 2 0.0026 3.91 3.2993%Between energy (E) 0.0110 3 0.0037 5.59 0.4948%Interaction TXE 0.1866 6 0.0311 47.49 0.0000%Residue I 0.0157 24 0.0007Between composite resins (R) 0.8007 4 0.2002 264.62 0.0000%Interaction RXT 0.2707 8 0.0338 44.73 0.0000%Interaction RXE 2.4695 12 0.2058 272.06 0.0000%Inter RXEXT 0.4620 24 0.0193 25.45 0.0000%Residue II 0.0726 96 0.0008Total variation 4.2940 179

Calculated critical Tukey value: 0.18115

of the composite resins studied here. Micromorpholog-ical aspects resulting from Er:YAG laser irradiation de-pend on the chemical composition and structure of alltypes of composite resin. The existence of a thresholdenergy of ablation is consistent with the cohesion ofthe material. A different mechanism of composite resinablation was observed: the explosive vaporization fol-lowed by a hydrodynamic ejection. This is an interest-ing discovery in the search for a more convenient lasersystem to ablate composite resin using the real ablationmechanisms involved in that operation. The next stepwill be a comparative study of laser ablation of resinand natural material.

Acknowledgments

We greatly appreciate the financial support of FAPESP, and wethank the Master Student in physics, Denis Jacomassi, for helpingus in the laboratory.

REFERENCES

1. Hibst R, Keller U, Steiner R. Die Wirkung gepulster Er:YAG-Laserstrahlung auf Zahngewebe. Laser Med Surg 1988;4:163-165.

2. Paghdiwala AF, Vaidyanathan TK, Paghdiwala MF. Evaluation oferbium:YAG radiation of hard dental tissues analysis of tempera-ture changes, depth of cuts and structural effects. Scanning Mi-croscopy 1993;7:989-997.

3. Pelagalli J, Gimbel CB, Hansen RT, Swett A, Winn DW. Investiga-tion study of the use of Er:YAG laser versus dental drill for cariesremoval and cavity preparation – phase I. J Clin Laser Med Surg1997;15:109-116.

4. Dostálová T, Jelínková H, Krejsa O, Hamal K, Kubelka J,Procházka S, Himmlová L. Dentin and pulp response to er-bium:YAG laser ablation: a preliminary evaluation of humanteeth. J Clin Laser Med Surg 1997;15:117-122.

5. Tokonabe H, Kouji R, Watanabe H, Nakamura Y, Matsumoto K.Morphological changes of human teeth with Er:YAG laser irradia-tion. J Clin Laser Med Surg 1999;17: 7-12.

6. Hadley J, Young DA, Eversole LR, Gornbein JA. A laser-poweredhydrokinetic system for caries removal and cavity preparation. JAm Dent Assoc 2000;131:777-785.

7. Yamada Y, Hossain M, Nakamura Y, Suzuki N, Matsumoto K. Re-moval of carious dentin by mechanical, chemomechanical andEr:YAG laser in deciduous teeth. J Oral Laser Applic 2001;1:109-114.

8. Research Science and Therapy Commitee of the Annual Academyof Periodontology. J Periodont 2002;73:1231-1239

9. Kurachi C. Estudo comparativo do laser, do led azul e da lâmpadaconvencional no processo de polimerização da resina compostadental. Master Dissertation. São Carlos: IFSC-USP, 2000.

10. Fan PL, Edahl A, Leung RL, Stanford JW. Alternative interpreta-tion of water sorption values of composite resins. J Dent Res1985;64:74.

11. Lizarelli RFZ, Moriyama LT, Bagnato VS. Ultraconservative abla-tion in operative and esthetic dentistry. J Oral Laser Applic2003;3:73-78.

12. Lizarelli RFZ, Moriyama LT, Bagnato VS. Ablation of compositeresins using Er:YAG laser – comparison with enamel and dentin.Laser Surg Med 2003;33:132-139.

13. Craig RG. Restorative dental materials. St Louis: Mosby, 1993.14. Zach L, Cohen G. Pulp response to externally applied heat. Oral

Surg Oral Med Oral Pathol 1965;19: 515-530.15. Ericksson RA, Albrektsson T. Temperature threshold levels for

heat induced bone tissue injury. A vital microscopic study in rab-bit. J Prosth Dent 1983;50:101-107.

16. Luo J, Lanutti J J, Seghi RR. Effect of filler porosity on the abra-sion resistance nanoporous sílica gel/polymer composites. DentMater 1998;14:29-36.

17. Berlin AA, Volfson S A, Enikolopian NS, Negmatov SS. Principlesof polymer composites. Berlin: Springer-Verlag, 1986.

SCIENCE


Contact address: Rosane de Fátima Zanirato Lizarelli, Insti-tuto de Física de São Carlos – USP, Av. Trabalhador Sancar-lense, 400, São Carlos, SP, Brazil. Tel: +55-16-3371-2012,Fax: +55-16-3373-9811. e-mail: [email protected]

ablation rate and morphological aspects of composite resins

Documents