Microhardness andSurface Topography of a

Composite Resin CementAfter Water Storage

Percy Miileding, LDS'Fredrik Ahlgren, LDS''Ann Wennerberg, LDS, PhD'^Ulf Örtengren, LDS''Stig Karlsson, LDS, Odont Dr

Purpose: The aims of this study were to assess changes in the microhardness and surface roughnessof a dual-cured composite resin cement after water storage for different periods of time.Materials and Methods: Sixty specimens were divided into four groups comprising high- and low-viscosity cement specimens stored either dry or in water for 1 to 60 days. Microhardness andsurface roughness measurements were made after 1, 7, and 60 days of storage. Results: It was foundthat that although interactions complicated the interpretation of the results, the water-stored sampleshad a significantly lower microhardness compared to the dry-stored specimens for every timeinterval. In addition, the high-viscosity specimens had a significantly higher microhardnesscompared to the low-viscosity specimens. An increased microhardness was found for ali groups,except for the low-viscosity, water-stored specimens after 60 days of water storage, which wasattributed to an effect of the chemical postturing process. Although difficuit to evaiuate from aclinical point of view, the laser profilometry analysis revealed that a significantly increased surfaceroughness was found after water storage and increased storage time that was possibly caused by adeterioration of the resin-matrix surface. Conclusion: For the permanency of the interfacial lutingmaterial, a high microhardness value seems to be important. Int I Prosthodont 1998:1:21-26.

In recent years, much interest has been focusedon composite resin luting materials because of

their potential for adhesive bonding to tooth sub-stance and restorative materials. Clinically, thetooth-composite resin cement restoration interfaceis subjected to degradation and wear that results insubmargination,' generally described as "marginalditching."1'2 The outcome of this may be marginal

^Associate Professor. Department of Prostt^etic Dentistry. Faculty

Odontoiogy, Göteborg University, Göteborg. Sweden."Research Assistant, Department of Prosthetic Dentistry, Facuity ofOdontology, Göteborg University, Göteborg, Sweden.

''Associate Professor. Department of Biomateriais/HandicapResearch. Göteborg University, Göteborg. Sweden.

Mssistint Professor, Department of Prosthetic Dentistry, FacuityOdontoiogy, Göteborg University, Göteborg, Sweden.

'Professor and Chairman, Department of Prosthetic Dentistry,Faculty Odontology, Göteborg University, Göteborg, Sweden.

Reprint requests: Dr Percy Miileding, Department of ProstheticDentistry, Facuity of Odontoiogy, Göteborg University,Medidnaregatan 12. 413 90 Göteborg. Sweden,e-maii: miiieding@odontoiogi.gu. se

fracture, discoloration, and secondary caries, all ofwhich may impair the longevity of the restoration.^When exposed to water or saliva, hydrolytic degra-dation is likely to occur and could be one of thereasons for submargination. It has earlier beenstated that water enhances the surface deteriora-tion of polymer resin materials because of water-matrix interactions and/or filler debonding." It hasalso been suggested that oral fluids will mediate anincrease in surface roughness on composite resinsurfaces.^ Among factors influencing the resistanceto abrasion, the surface hardness value is one ofthe most important,'" although environmental con-ditions, direction, and magnitude of acting forcesand time will also strongly influence the perma-nency of exposed surfaces. As tbe prognosis forbonded prosthetic restorations is largely a functionof the maintenance of both tbe luting material andtbe adhesive bond/ and as moisture and water in-teract with both, it seemed relevant to study the ef-fects of water on the surface properties of a com-posite resin cement.

- - I t , Number 1,199a 21 The International Journal uf Prosthodontics

Micnjhiirdne54 and Surlüce Topoj;r,iphv After Water Slo

Table 1 Materials, Storage Conditions, and Time Intervals for the Microhardness andSurface Roughness Analyses

and storageconditions Symbol

and observationtime

Variolink oatalysthigh-viscosity(batch no. 616853)

Variolink catalystlow-viscosity(batch no. 614243)

30 (60)

30 (60)

15 (30) Dry

15 (30) Water

15 (30) Dry

15 (30) Water

DH 5 (10) 1 day5 (10) 7 days5 (10) 60 days5 (10) 1 day5 (10) 7 days5 (10) 60 days5 (10) 1 day5 (10) 7 days5 (10) 60 days5(10) 1 day5 (10) 7 days5 (10) 60 days

Variolink (Viuadent) base paste (batch no. 614053) was used lor all spacimans. Thetigures in parentheses referióIhe surface roughness test

The objectives of tbis study were to analyze theinfluence water has on the microhardness of acomposite resin luting cement of two different vis-cosities and to examine the effects of water on sur-face roughness.

Materials and Methods

Microhardness

The material used was a dual-cured compositeresin cement of high and low viscosity (Table 1).The fabrication of the test specimens as well as themicrohardness testing were performed according tomethods described by Darr and Jacobsen.^ The ma-terial was mixed according to ihe manufacturer'srecommendation and placed on a glass slide. Metalspacers with a thickness of 200 [irr\ were placed oneach side of fhe material, and a second glass slidewas positioned on top of the composite resin ce-ment. The glass slides were then pressed togetherwith the hand of the operator until contact with themetal spacers was established, after which the poly-mer resin cement was light-cured for 60 secondswith a hand-held light-curing unit (Model 5520BH,serial no- 103265, Visilux 2, 3M). After light curing,the top slide was removed, and the bottom one waskept as a support for the sample, immediately afterspecimen preparation, 15 specimens of each hif̂ h(H) and Iow (L) viscosity material were transferredto an oven kept at a temperature of 37°C ± 1°C fordry (D) storage (Table 1 ). An equal number (n = 15)of each high and low viscosity samples were placedin a covered container containing 10.0 mL distilledwater at a temperature 37°C ± 1°C for water (W)storage. Before testing, water-stored specimenswere rinsed in distilled water, carefully wiped dry.

and finally air dried for 15 seconds. The microhard-ness was tested with a microindentator (Type 60-366.002 no. 747972, Vickers pyramid-diamond,Durimet Leitz Microindentator, Ernst Leitz) with aload of 300 g and a dwell time of 10 seconds.Readings were made by measuring the size of thediagonals of the indentation (in pm) made by the tipof the microindentator. At every test occasion, afterintervals of 1, 7, and 60 days, five random areas oneach of five specimens from each group (DH, WH,DL, and WL) were examined.

Surface Roughness Characterization

The specimens for the surface roughness analysiswere produced using material from the same batchas for the microhardness test and using the samespecimen preparation technique (Tahle 1).Altogether, 120 specimens were prepared, 10 speci-mens for each test group. After specimen prepara-tion, 60 specimens were transferred to containerswith 10.0 mL distilled water, and 60 specimenswere left in air. All the samples were stored at a tem-perature of 37°C ± r C for 1, 7, or 60 days. Afterthis time they were subjected to surface topographicanalysis. A confocal laser scanning profilometer(TopScan 3D, Heidelberg Instruments) was used,and measurements were performed according to amethod described by Wennerberg et al."' The instru-ment used a He-Ne laser beam as an optical stylus.Because of the confocal arrangement of the optics, ahigh vertical resolution was achieved. After 1 7and 60 days, 10 specimens from each of the fourgroups, DH, WH, DL, and WL, were analyzed. Theassessment area was 245 x 245 pm. Two com-monly used parameters—a vertical (Sa) and a hori-zontal (Sex)—were chosen to numerical ly

Tde Internalional Journal of Pioslhodrjndi 22 Volumen,Ni imber l ,1998

Milledingetal MicrohardneBí and Surlace Topography After Waler Storage

Table 2 Microhardness in Vickers Hardness Number (VHN) of Composite ResinCement (kg/mm^) after 1, 7, and 60 days (n = 5)

Time (d)

1760

DH

Mean

62 667.271.3

SD

2.94.13.6

WH

Mean

56.059.060.7

SD

2.61.02.6

DL

Mean

52.156.456.6

SD

2.91.13.6

WL

Mean

48.451.747.2

SD

2.02.62.5

SD = Standard devialion; DH = flry-stored, high-vis cosily; WH = waler-stored, tiigfi-viscosity; DL = flry-slored, low-viscosity: WL = water-stored, iow-viscosity.

characterize the surface topography. The Sa is thearithmetic mean deviation of the surface, ie, meanheight deviation, and Sex is the mean spacing be-tween the surface peaks in the x direction, ie, a spa-tial parameter. A mathematical description of thethree parameters'^ and a detailed description of themethod have previously been reported.''

To detect significant differences among the ex-perimental groups, analysis of variance (ANOVA)for a three-factorial design was applied to the re-sults, followed by multiple comparisons accordingtoScheffe.'i

Results

The results showed an overall increase in surfacehardness within all four experimental groups afterwater storage for 60 days, except for tbe low-vis-cosity/water-stored samples, for which a decline insurface hardness was found compared to registra-tions at 1 and 7 days (Table 2).

A significant reduction in microhardness (P <0.001) was registered for water-stored compositeresin cement specimens after 1, 7, and 60 days{Table 3¡ in comparison to dry storage. In addition,the higb-viscosity composite resin cement demon-strated significantly higher microbardness valuesthan the low-viscosity cement at all test intervals(Tables 3 and 4). Differences in microhardnesscaused by either water or viscosity were botb si-multaneously influenced by tbe time factor.

Under tbe infiuence of water, the specimens dis-played an increase in surface roughness, evi-denced by the height descriptive parameter, Sa(Table 5) . Altbough numerically small, the in-crease ¡n surface roughness was found to be statis-tically significant (Table 3]. In addition to tbe type

Table 3 Statisticai Anaiyses of Differences inMicrohardness and Surface Roughness (P)

Faclors

WateiViscosityTime

Ivlicrohardness

< 0.001< 0.001< G.001

Surtace roughness

Sa

< 0.0010.836

< 0.001

Sex

< 0.0010.013

< 0.001

Sa ^ Mean iieigrit deiíialion; Se* :̂ mean spacing betwpeaks.

Table 4 Statistical Analyses of Differences inMicrohardness for Dry- and Water-stcred Specimensand for Higfi- and Low-viscosity. Water-storedSpecimens at Different Time Intervals

Storage conditions

Dry-waterDry-waterDry-walerWaterWaterWater

Time

1 day7 days60 days1 day7 days60 days

Viscosity

High-LowHigh-LowHigh-Low

95% 00 nf ¡den celevel

5.2 + 4.2 kg/mm^6.4 ± 4.2 kg/mm=

1 0 . 0 - 4 . 2 kg/mm^9.0 + 4.2kg/mm^9.1 ±4 .2kg /mm2

14.1 ±4 .2kg /mm2

For the statistical Gualuatinn an anaiysis oí variance for a tliree-facloriaidesign was performed, followed by muitiple comparisons according toScheffe.

of Storage media, time was found to have a signifi-cant effect on the surface roughness, and increasedsurface roughness resulted after increased water-storage times. No statistically significant influenceof the different viscosities on the Sa was found(Tables 3 and 6¡ . According to the values in Table5, which represent the mean spacing between thesurface peaks in tbe x direction (Sex), waterseemed to reduce the spacing in both low- andhigh-viscosity cements.

n,Njmber1,1S 23 Journal of Prostliodontics

Table 5

Time

1 day7 days60 days

Microh rdiicss and Surlart Tnpog iphv After Wate. Sic r̂ ee M Hedingot

Surface Roughness of Composite Resin Cement (pm) after 1,

DH

Mean

0.060.080.07

SD

0.010.010.01

Mean height deviation -

WH

Mean

0.080.080,10

SD

0.010.020,01

DL

Mean

0.060.080.09

Sa (|jm)

SD

0.010,010.03

WL

Mean

0.080.08011

SD

0.010.020.02

DH

Mean

14.417.214.5

al

7, and 60 days (n =

SD

1.71.72.0

Mean

Wl-

Mean

12.213.011.4

spacing

SD

1.23.01,7

10)

- Sex (|im|

DL

yean

13.614.413.3

SD

1.61.62.0

WL

Mean

12.312.711.1

SU

2.21.8

SD = Standard deviation; DH = dry-storad, high-viscosity, WH = water-stored, high-visoosity; DL = dry-stored, lom-viscosity; WL = water-stoied, low-uiscosily.

Table 6 Statistical Anaiysis of Factors Influencing theluiean Surface Roughness, Sa, and the Mean SpacingBetween the Surface Peaks in the X Direction, Sex

95% Confidence level

Viscosity

Storage conditionsWater-dry High and low 0.019 + 0.006Dry-waterDry wafer

HighLow

3.18 ±1.361.75 ± 1 36

For the statistical évaluation an analysis of variance for a three-factor-iai design was performed, followed by multipie comparisons accordingto Scheffe

Discussion

The results of the present study indicate that waterhas an influence on the surface properties of thestudied composite resin cements and that the de-gree of surface degradation displayed by the ce-ments is low.

Water significantly reduced the microhardness ofthe studied composite resin cements at every testoccasion as compared to dry storage conditions(Tables 3 and 4), although interactions betweenstorage conditions, viscosity, and time complicatedthe interpretation of the results. As expected, be-cause of the supposed higher filler load, the highviscosity cement demonstrated significantly higherVickers hardness number (VHN) values comparedto the low-viscosity cement at all test intervalsITabie 4). The overall increase in microhardnessthat was registered after 7 days (see Table 2) proba-bly depended on a continuing chemical-curing ef-fect,'^ which is usually delayed compared to ihemore direct effect of light curing. With increasedstorage time in air as well as in water the micro-hardness values leveled out, possibly because of aconsumption of the catalyst and/or steric reasons.Despite the postcuring effects, tbe VHN for low-vis-cosity specimens after 60 days storage in water waseven lower than the baseline values, indicating thatwater had a more pronounced influence on the sur-

face of low-viscosity cement than on high-viscositycement. Composite resin materials absorb a smallpercentage of water," which is reported to affectthe surface hardness^* both by the amount of waterSorption and the length of storage time and initiatea hygroscopic swelling of the matrix in the surfaceIayer,i5 The surface of the material is affected be-fore the bulk,^^ which means that the change in mi-crohardness of the surface could be expected to belarger than the changes in the inherent mechanicalstrength. When evaluating the water absorption ofthe same two types of cements and for the sametime intervals as in the present study, no differencein water absorption was found between the two vis-cosities.''' Therefore, not only the amount of waterabsorbed and the time factor, but also the effects ofwater resulting from the composition and mi-crostructure of the composite resin material will in-fluence the microhardness. According to availableinformation from the manufacturer, the differencein filler ioading between the high- and low-viscos-ity materials (mixed base and catalyst 1:1) is at most10%, which perhaps is too small to allow a dis-crimination between the two viscosity types.However, besides differences in filler content, thelow-consistency catalyst contains a lower weightfract/on of high molecular-weight resins and ahigher weight fraction of triethylene glycoldimethacrylate (TEG-DMA), which is a lower mole-cular weight resin. Bisphenol glycidyl methacrylate(bis-CMA)/rEG-DMA-polymer specimens demon-strated a lower surface microhardness after incuba-tion in human saliva than untreated samples, whichwas attributed to the hydrolysis of methacrylateester bonds.'^ The different performance of thehigh- and low-viscosity types after water storagemay result from differences in the properties of thecomposite resin matrix^^ rather than from mintjr dif-ferences in filler content.

Another factor that probably will influence sur-face microhardness, at least in the long run isstress corrosion of the filler particles,^'' which hasbeen shown to take place in water-stored compos-

24 VolLiinen,Niimljeil,1998

Mitteding et E Microhardreis and Surface Topography After Water Slorage

ite restorative materials.''^-^"•^i However, the fillerdegradation process is a rather slow-proceeding re-action and is not likely to have had any major in-fluence on the resulti of the present study. In addi-tion, the surface of the composite resin cement wasprotected from the influence of oxygen duringpolymerization, which is known to enhance thedegree of polymerization of composite resin mate-rials.-- In this respect the test specimens repre-sented a higher quality than is usually reached inclinical practice.

The surfaces analyzed with confocal laser pro-filometry displayed very small changes in surfaceroughness, even after a water-storage period of 60day5. However, the tests performed with the opti-cal profilometer revealed that the mean height de-viation value (Sa) was significantly higher afterstorage in water for 60 days than at baseline (seeTables 3 and 6). The numerical values were verysmall, and no significant difference in the Sa valuecould be demonstrated between high- and low-vis-cosity composite resin cements; either the differ-ences between the two viscosity types were toosmall, the time factor was too short, or the testingconditions were not discriminating enough.Distilled water used as the corrosive medium inthe present study has been reported to have alower deteriorating effect on composite materialsthan, for example, lactate buffer of pH 4.0 or otherlow-pH solutions.^'' It should also he kept in mindthat the composite resin cements were not influ-enced by any abrasive action, which is reported asone of the most important factors in the submar-gination process.

Contact surfaces are reported to display a signifi-cantly larger percentage of submargination com-pared to noncontacting surfaces.' The mean spacingbetween the surface peaks was significantly reducedby water in both the high- and low-viscosity cement(see Tables 3 and 6), which may be attributed to theeffects of water-induced hygroscopic expansion ofthe resin matrix at the surface. As with the Sa values,the numeric change in Sex after water storage for 60days was very small. The surfaces of the specimenswere probably subjected to dynamic changes inwhich volumetric changes, because of a continuingsetting reaction, the effects of water sorption, andthe surface deterioration, may have resulted in dy-namic changes of the surface topograhy. However,the exact nature of the surface reactions of compos-ite resin cements at water contact is not known.

Although statistically significant, the differencesin microhardness, and specifically in surface rough-ness values after water storage, were numericallyvery small and the clinical implications difficult to

foresee, at least from a practical clinical point ofview. However, at a nanometer scale, even the verylow surface roughness values registered in the pre-sent study may influence, for example, the proteinabsorption to exposed surfaces. Under the in vivoconditions, several additional factors will influencethe surface deterioration, as will differences in com-position and microstructure of different commercialproducts. As microhardness is an important surfacefactor that strongly influences the wear pattern onexposed surfaces, a high microhardness valueseems to be important for the permanency of the in-terfacial luting material.-^ To decrease the problemof submargination, the polymerization of the com-posite resin cement must be optimal and the inter-facial area must be protected and not polished im-mediately following placement.

Acknowledgments

The materials used in ihis study were supplied by Vivaderit. Theauthors would also like to thank Miss C. t-tallgren for her assis-tance with the optical profilonnetry readings.

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The aim of this study was to evaluate the survival and prognosis ot oral implants inserted in

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The Internationai lournal of Pioslliodontic! 26 Voiume I I , Niiml)erl,199B