d e nt n t a l m a te t e r ia i a l s 2 8 ( 2 0 1 2 ) 521–528
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Depth of cure of resin composites: Is the ISO 4049 method suit su itab able le fo forr bulk fill materials? Simo Si mon n Fl Flur ury y a , Stef Stefan anie ie Hayo Hayozz b , Anne Anne Peut Peutzf zfeld eldtt a , Jür Jürg Hüsl Hüsler er b , Adr Adrian ian Luss Lussii a ,∗
a b
Depart Departmen mentt of Preve Preventi ntive ve,, Restor Restorati ative ve and Pedia Pediatri tricc Dentis Dentistr try y, Schoo Schooll of Dental Dental Medici Medicine, ne, Univer Universit sity y of Bern, Bern, Switzer Switzerlan land d Instit Institute ute of Mathem Mathemati atical cal Statis Statistic ticss and Actuar Actuarial ial Scienc Science, e, Univer Universit sity y of Bern, Bern, Switzer Switzerlan land d
a r t i c l e
i n f o
Article history:
Receiv Received ed 6 Octobe Octoberr 201 20111 Receiv Received ed in revis revised ed form form 7 Feb February ruary 2012 Accept Accepted ed 7 Fe Febru bruary ary 201 20122
Keywords:
Increment Light-curi Light-curing ng time Restorative
1.
a b s t r a c t evaluate ate if depth depth of cure cure DISO determ determine ined d by the ISO 404 4049 9 method method is accura accuratel tely y Objectives. To evalu new reflect reflected ed with with bulk bulk fillmateria fillmaterials ls when when compar comparedto edto depth depth of cure cure D determinedby determinedby Vickers Vickers microhard microhardness ness profiles. profiles. Methods Methods.. DISO was was dete determ rmin ined ed acco accord rdin ing g to “ISO “ISO 4049 4049;; Dept Depth h of cure” cure” and and resi resin n comp compos osit ite e specim specimens ens (n =6 per per grou group) p) were were prep prepar ared ed of two two cont contro roll mate materi rial alss (Fil (Filte tek k Supr Suprem eme e Plus Plus,, Filtek Silo Silora rane ne)) and and four four bulk bulk fill fill mate materi rial alss (Sur (Surefi efill SDR, SDR, Venus enus Bulk Bulk Fill Fill,, Quix Quixfil fil,, Tetri etricc EvoEvonew Cera Ceram m Bulk Bulk Fill Fill)) and and ligh lightt-cu cure red d for for eith either er 10s or 20s. For D , a mold old was fill filled with with one of th of the e six six resincomp resincompos osit ites es and and ligh lightt-cu cure red d for for eith either er 10s or 20s ( n = 22 per per grou group) p).. The The mold mold was placed placed undera microhard microhardness ness indentatio indentation n device device and hardness hardness measureme measurements nts (Vickers (Vickers hardne hardness, ss, VHN) VHN) were were made made at defined defined distan distances ces,, begin beginnin ning g at the resin resin compos composite ite that that had been clos closes estt to the the ligh lightt-cu curi ring ng unit unit (i.e (i.e.. at the the “top “top”) ”) and and proc procee eedi ding ng towa toward rd the the uncu uncure red d resin resin compos composite ite (i.e. (i.e. toward toward the “botto “bottom”) m”).. On the basis basis of the VHN measur measureme ements nts,, Vicke Vickers rs hardne hardness ss profile profiless were were gener generate ated d for each each group group.. Results. Results. DISO variedbetw variedbetween1.76and een1.76and 6.49 6.49 mmwith thebulkfill materi materialsshow alsshowingthe ingthe highes highestt ISO new new ISO varie varied d betwee between n 0.2and 4.0mm. D was smaller smaller than D for all resincomposites resincomposites D .D except except Filtek Filtek Silorane. Silorane. Conclusions. For bulk bulk fill fill mate materi rial alss the the ISO ISO 4049 4049 meth method od over overes esti tima mate ted d dept depth h of cure cure comcompared pared to depth depth of cure cure determ determine ined d by Vicke Vickers rs hardne hardness ss profile profiles. s. © 201 2012 2 Academ Academy y of Dental Dental Materi Materials als.. Publis Published hed by Elsev Elsevier ier Ltd. Ltd. All rights rights reserv reserved. ed.
Introduction
Energ Energy y of the light light emitte emitted d from from a light light-cu -curin ringg unit unit decrea decreases ses dras drasti tica call lly y when when tran transm smit itte ted d thro throug ugh h resi resin n comp compos osit itee [1] [1],, lead leadin ingg to a grad gradua uall decr decrea ease se in degr degree ee of con convers versio ion n of the the resin resin compos composite ite materi material al at incre increasi asing ng distan distance ce from from the irrairradiated diated surface. surface. Decrease Decreasess in deg degree ree of convers conversion ion compromi compromise se physic physical al properti properties es and increase increase elution elution of monomer monomer [2–5] and thus hus may lea lead to prem emat atu ure failu ailurre of a rest estorat oratio ion n or may may negativ negatively ely affect affect the pulp tissue. tissue. When restoring restoring cavities cavities with
∗
light light-cu -curin ringg resin resin compos composite ites, s, it has there therefor foree bee been n regar regarde ded d as the the gold gold stand tanda ard to app apply and and cure the the resin comp omposit ositee in incr increm emen ents ts of limi limite ted d thic thickn knes ess. s. The The maxi maxima mall incr increm emen entt thic thickn knes esss has has been been ge gene nera rall lly y defin defined ed as 2 mm [6,7] [6,7].. However, restor restoring ing cavit cavities ies,, especi especiall ally y deep deep ones, ones, with with resin resin compos composite ite incre incremen ments ts of 2 mm thick thicknes nesss is time-c time-cons onsumi uming ng andimplies andimplies a risk risk of incorp incorpora oratin tingg air bub bubble bless or contam contamina inatio tions ns betwe between en the incre incremen ments ts.. Thus, Thus, variou variouss manufa manufact ctur urers ers have have recent recently ly introd introduc uced ed new new types types of resin resin compos composite ites, s, so-cal so-called led “bulk “bulk fill” fill” materi materials als,, which which are are claime claimed d to be cura curableto bleto a maxima maximall increincrement me nt thic thickn knes esss of 4 mm [8–11] [8–11]..
Corres Correspond ponding ing author author at: Freib Freibur urgst gstras rasse se 7, CH-301 CH-3010 0 Bern, Bern, Switze Switzerla rland. nd. Tel.: el.: +41 316 316322 322581 581;; fax: fax: +41 316 316329 329875 875..
E-mail E-mail address: address: simon.fl
[email protected] (S. Flury). Flury). 0109-5 010 9-5641 641/$ /$ – see front front matter matter © 201 20122 Academ Academy y of Dental Dental Materi Materials als.. Publis Published hed by Elsev Elsevier ier Ltd. Ltd. All rights rights reser reserved ved.. doi:10.1016/j.dental.2012.02.002 doi:10.1016/j.dental.2012.02.002
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A method for defining the maximal increment thickness of resin composites has been introduced by the International Organization for Standardization ISO in the second edition of ISO 4049 in the year 1988 [12]. The method is officially denominated as “ISO 4049; Depth of cure”, and according to the method the resin composite to be tested is filled in a tube-shaped mold, light-cured, pushed out of the mold, and uncured resin composite material is then removed (“scraped away”) with a spatula leaving a hard cylindrical specimen. Finally, the absolute length of this hard specimen is measured and divided by two. The resulting value is recorded as the depth of cure and defines the maximum increment thickness. The rationale for the division factor two is that not all the hardened specimen is actually optimally cured [2,13,14]. The ISO 4049 method was developed using a microfilled resin composite (Durafill, Kulzer & Co GmbH, Bad Homburg, West Germany) [15], one of the first visible light-curing resin composites. Ever since, the principle of the ISO 4049 method has basically remained the same [16]. Resin composites, however, have undergone continuous development through the years as regards their various components, e.g. the filler and the initiator. It seems likely that the new bulk fill materials have required certain changes or modifications in the composition, and it is therefore relevant to verify the accuracy of the ISO 4049 method and its division factor. Since hardnessmeasurement hasbeen shown to be a practical method to indirectly determine degree of conversion for a given resin composite [14,17–20], hardness profiles can be used to alternatively measure depth of cure. Consequently, the aim of this study was to evaluate if depth of cure determined bythe ISO4049method is accurately reflectedwithbulk fill materials when compared to depth of cure determined by Vickers hardness profiles. In order to arrive at this aim several subaims were set: (1) to determine the depth of cure by ISO 4049, (2) to measure Vickers hardness at increasing distances from the light-curing source, (3) to determine at which depth 80% of the maximum Vickers hardness was obtained, and (4) to determine which division factor should be used to arrive at this “80% of maximum Vickers hardness” depth. The overall hypothesis to be tested was that the ISO 4049 method accurately reflects the depth of cure determined by Vickers hardness estimations of the degree of conversion.
2.
Materials and methods
Six resin composites (Table 1) were used for investigating the accuracy of the ISO 4049 method: Two control materials (Filtek Supreme Plus and Filtek Silorane), two flowable bulk fill materials (Surefil SDR and Venus Bulk Fill), and two highviscosity bulk fill materials (Quixfil and Tetric EvoCeram Bulk Fill). All light-curing was performed with an LED light-curing unit (Demi, Kerr Corporation, Middleton, WI, USA) and light power density was verified to be at least 1000mW/cm2 at the beginning and end of each day of specimen preparation with a radiometer (Demetron L.E.D. Radiometer, Kerr Corporation).
2.1.
Depth of cure by ISO 4049
Depth of cure by ISO4049 wasperformed with re-usable stainless steel molds according to ISO 4049:2000 [16]. Pretests had found the absolute length of cylindrical specimens of the cured resin compositeto vary between 3.5and 13mm depending on the resin composite. The ISO 4049 method states that thestainless steelmoldsshall be at least 2 mm longer than the absolute length of the cylindrical specimens. Thus, stainless steel molds of6 mm, 9mm, or 15mm in lengthand an internal diameter of 4 mm were custom-made. Depending on the resin composite, the mold of either 6mm, 9mm, or 15mm in length was placed on a glass slide covered by a Mylar strip (Hawe Stopstrip Straight, KerrHawe, Bioggio, Switzerland). The mold was then filled in bulk with one of the six resin composites. The top side of the mold was covered with a secondMylarstripand theresin material made flush with the mold by use of a second glass slide. The mold was placed on white filter paper (Filter Paper Circles 589/1, Schleicher & Schuell MicroScience GmbH, Dassel, Germany). The second glass slide was removed and the resin composite was light-cured for either 10s or 20s keeping the light tip centered and in contact with the second Mylar strip. After light-curing, the cylindrical specimens were pushed out of the mold and the uncured resin composite material was removed with a plastic spatula. The absolute length of the cylindrical specimens of cured resin composite was thenmeasured witha digital caliper of ±0.01mm accuracy (Mitutoyo IP 65, Kawasaki, Japan). The absolute length ( AL) was divided by two and the latter value recorded as DISO . Sixspecimens were made in each of the 12 groups (i.e. six materials light-cured for either 10s or 20s). 2.2.
Depth of cure by Vickers hardness profiles
Depth of cure by Vickers hardness profiles was performed in a re-usable, block-shaped, and custom-made stainless steel mold with a semicircular notch of 15mm in length and 4 mm in diameter (Fig. 1A). The semicircular notch was entirelyfilled with one of the six resin composites. Then, the mold was covered with a Mylar strip (Hawe Stopstrip Straight, KerrHawe) and the resin composite was made flush with the mold by use of a glass slide. Excess resin material was removed and the mold was covered by a stainless steel shell (Fig. 1B). A second Mylar strip was placed on the semicircular opening (Fig. 1C) and the resin composite was light-cured through the semicircular opening (top surface) for 10s or 20s keeping the light tip centered and in contact with the second Mylar strip. After light-curing, the shell and both Mylar strips were removed (Fig. 1D) and the mold including the resin composite specimen was placed under a microhardness indentation device (Fischerscope HM2000, Helmut Fischer GmbH, Sindelfingen, Germany). Subsequently, hardness measurements (Vickers hardness, VHN) were made on the resin composite specimen at defined distances, beginning with the resin composite which had been closest to the light tip (i.e. from the “top”) and moving toward the uncured resin composite (i.e. toward the “bottom”) until VHN of the resin composite could not be measured anymore due to its softness. The defined distances ( ı) were: 0.1mm, 0.2 mm, 0.5 mm, 1.0mm,
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Table 1 – Resin composites used.
Resin composite
Type of resin composite (according to manufacturer)
Maximum increment thickness (mm) (according to manufacturer)
Shade
LOT-number
Filtek Supreme Plus 3M ESPE, St. Paul, MN, USA Filtek Silorane 3M ESPE, St. Paul, MN, USA Surefil SDR Dentsply Caulk, Milford, DE, USA Venus Bulk Fill Heraeus Kulzer, Hanau, Germany Quixfil Dentsply DeTrey, Constance, Germany Tetric EvoCeram Bulk Fill Ivoclar Vivadent, Schaan, Liechtenstein
Universal restorative
2
A3
N116619
Low shrink posterior restorative
2.5
A3
N138530
Posterior bulk fill flowable base
4
Universal
100128
Low stress flowable composite
4
Universal
010030
Posterior restorative
4
Universal
1007001127
Moldable posterior composite for bulk-filling technique
4
IVA (reddish universal shade)
IDS
1.5 mm, 2.0mm, 2.5mm, 3.0 mm, 3.5mm, 4.0mm, 4.5 mm, 5.0 mm, 6.0mm, 7.0mm, 8.0mm, 9.0mm, 10.0mm, 11.0mm, 12.0mm, and13.0mm. Programming of the hardness indentation device for defined distances and reproducible placement of the mold ensured that the VHN measurements were made along the same axis on each specimen. VHN measurements weremade ata loadof 3 g for 15s. For eachof the 12groups (i.e. six materials light-cured for either 10s or 20s), 22 specimens were prepared and thus 22 VHN measurements were made at each of the defined distances.
2.3.
Statistical analysis
Ineachofthe12groups,themaximum VHNmax oftheVHNvalues obtained at the defined distances ı = {0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0} was identified for each of the 22 specimens. For each group, max the median VHN of the 22 VHNmax values as well as the ISO D median ˜ of the six DISO values was derived. Then the VHN value VHNISO at the biggest depth that was equal to or smaller ISO was determined (Fig. 2). To assess which percentage D than ˜
Fig. 1 – Specimen preparation for depth of cure determination by Vickers hardness measurements. A = stainless steel mold with semicircular notch, B = stainless steel shell, C = semicircular opening for light-curing, and D = mold including the resin composite specimen.
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Fig. 3 – Light-curing for 10s: Vickers hardness profiles for each resin composite ( n = 22 per resin composite). Medians AL of ˜ ISO of depth of cure obtained by the ISO 4049 method, and new depth of cure Dnew for each resin absolute length, D composite.
max
at least 80% of VHN was attained varied between 0.2 and ˜ ISO for all resin com4mm, with D˜ new being smaller than D posites except Filtek Silorane (Figs. 3 and 4). The factor f new by which the absolute length should be divided in order to arrive at Dnew varied between 1.76 and 38.25, with f new being higher than the division factor two of the ISO 4049 method for all resin composites except Filtek Silorane. Nmiss varied between 0 and 9 withthe high-viscosity bulk fill material Tetric
EvoCeram Bulk Fill displaying the highest number of incalculable cases.
4.
Discussion
This study showed great variation between the six resin composites as regards depth of cure DISO determined by the ISO 4049 method with the bulk fill materials, true to their name,
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Fig. 4 – Light-curing for 20s: Vickers hardness profiles for each resin composite ( n = 22 per resin composite). Medians AL of ˜ ISO of depth of cure obtained by the ISO 4049 method, and new depth of cure Dnew for each resin absolute length, D composite.
yielding higher DISO than the two control materials. Possible explanations for the higher DISO of the bulk fill materials are more potent initiator systems and higher translucency. Nevertheless, even with a light-curing time as short as 10s all six resin composites met the requirement stipulated in the ISO
4049:2000 of a DISO not less than 1.5 mm. ISO 4049:2000 additionally requires that the depth of cure shall be no more than 0.5 mm below the value claimed by the manufacturer when using the recommended light-curing time. Only one resin composite, at only one of the two light-curing times, did not
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meet this requirement: The manufacturer indicates that Tetric EvoCeram Bulk Fill may be light-cured to a depth of 4mm using a light-curing time of 10s provided that the light power density is ≥1000mW/cm2 . Yielding a DISO of 3.32mm Tetric EvoCeram Bulk Fill did not quite obtain the minimum DISO required of 3.5 mm (i.e. 4.0 mm claimed by the manufacturer minus 0.5mm). As regards the influence of light-curing time, doubling the time from 10s to 20s led to an average increase in DISO of 17%. This increase is in agreement with the 16–23% previously found as a result of doubling the light-curing time [21,22]. The six resin composites also varied markedly regarding Vickers hardness profiles. The hardness profiles of the four bulk fill materials were wider than those of the control materials (as evidenced by the stretched-out hardness profiles) indicating that the point at which VHNcouldnot be measured anymore due to softness occurred at a much bigger depth for thebulk fill materials. It is noteworthy that this point at which VHN could not be measured occurred at smaller depth than the absolute length AL of the hardened resin composite specimens determined by the ISO 4049 method (Figs. 3 and 4) for all resin composites except Filtek Silorane. This discrepancy may be the result of a limited resolution of the microhardness indentation device at relatively low surface hardnesses. The hardness profiles of the two flowable bulk fill materials were flatter than those of the other materials indicating that max VHN of the two flowable bulk fill materials was markedly lower. The inferior hardness can be explained by a lower filler content necessary for obtaining the reduced viscosity. Furthermore, regardless of resin composite and light-curing time, most hardness profiles showed that VHNmax was not reached at the very first measuring depth of 0.1 m m but rather in subsurface areas at a depth of 0.2mm to 1.0mm. This phenomenon has been previously described [23]. max The percentage of VHN attained at the depth D˜ ISO was ISO assessed and expressed as p . Whereas pISO was above 80% for one of the control materials(Filtek Silorane), pISO was much lower particularly for the bulk fill materials, which showed in the median a pISO of only 40%, reflecting the different forms of the Vickers hardness profiles. Numerous studies have defined depth of cure based on hardness measurements performed on the top and bottom surface of a light-cured resin composite specimen. The hardness values obtained were used to calculate a bottom/top hardness ratio, and a ratio above 80% has often been used as a minimum acceptable threshold value [14,24]. Analogically, in the present study a threshold of 80% was used in order to determine a depth of cure based on the Vickers hardness measurements: Thus, Dnew was defined as the depth max at which at least 80% of VHN was obtained. Dnew varied between 1 and 4mm. There was, however, one exception in that Tetric EvoCeram Bulk Fill yielded a Dnew of only 0.2mm when light-cured for 20s. The reason for this very low value is mainly that the hardness of a majority of Tetric EvoCeram Bulk Fill specimens dropped drastically after the first measuring depth (0.1mm). Except for Filtek Silorane, the Dnew values were lower than the DISO values indicating that the ISO 4049 method overestimated the depth of cure, especially for the bulk fill materials as already evidenced by the pISO values. This finding is in corroboration with that of Moore
and coworkers, who concluded that the ISO 4049 method overestimated depth of cure compared with Knoop hardness profiles [24]. The factors f new by which the absolute length as determined by the ISO 4049 method should be divided in order to arrive at Dnew were, consequently, higher than the factor 2 stipulated in the ISO 4049 method except for Filtek Silorane. For the other resin composites, a division factor of 5 (in the case of 10s light-curing) or 4 (20 s light-curing) would be more accurate. Again, the drastic drop in hardness after the first measuring depth of 0.1 mm accounts for the exceptionally high f new calculated for Tetric EvoCeram Bulk Fill at 20s light-curing. In seven of the 12 groups it happened for certain specimens that none of the VHN values measured at the defined max distances were above 80% of the VHN of that group, and new new for these specimens D and f could not be calculated. The number of such incalculable cases Nmiss varied markedly and was especially high for Tetric EvoCeram Bulk Fill, which ledto a biased estimation of Dnew and f new. Onone handwhen light-cured for 10s, nine specimens of Tetric EvoCeram Bulk max Fill showed no VHN value above 80%of VHN and thus, Dnew new and f remained similar to those of the other resin composites. On the other hand when light-cured for 20s, for many max specimens only the first VHN value was above 80% of VHN , leading to very low Dnew and thus very high f new. It must be mentioned, however,that the TetricEvoCeram BulkFill usedin the present study wasa preliminary material not yet intended for in vivo use and not yet on the market at the time of the measurements. A recent study investigated depth of cure using numerous measurement techniques and the authors concluded that not only depth of cure measured similarly to the ISO 4049 method but also Vickers hardness profiles overestimated depth of cure [25]. This implies that in the present study hardness profiles and the resultant Dnew also overestimated depth of cure. However, it should be noted that depth of cure determined on the basis of hardnessprofiles was calculated differently in the two studies: In theprevious study, depth of cure wasdefined as the depth at which at least 80% of the hardness measured on the upper surface of the resin composite was obtained whereas in thepresentstudy, depthof cure Dnew wasdefined as the depth at which at least 80% of the maximum hardness( VHNmax ) was obtained. As VHNmax was most often obtained in subsurface areas and not at the upper surface, it is uncertain whether or to what extent Dnew overestimated depth of cure.
5.
Conclusion
The present study found that for bulk fill materials the ISO 4049 method overestimated depth of cure compared to the determination by Vickers hardness estimations of the degree of conversion.
Conflicts of interest The authors declare no conflicts of interest, real or perceived, financial or nonfinancial.
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