The In Vitro Color Stabilityof Acrylic Resins for

Provisional Restorations

Roberto Scotti. MD, DDSProfessor and Chairman

Saverio Carlo Mascdlani, DDSVisiting Professor

Francesca Forniti, DDS

Lecturer

Department of ProsthodonticsUniversity of Ferrara Sihool of DentistryFerrara. Italy

This5tucly made an in vitro comparative evaluation of the color variation offour types of acrylic resin for provisional fixed prostheses, as determined usingcomputerized spectro photometry, before and after a 20- and 30-day cycle ofimmersion in four staining soiutions. The four acrylic resins used for provisionalfixed restorations were: Cold Pac, Trim, Protemp, and Mixacryl II. Thirty-twospecimens for each resin were divided into four suhgroups of eight elements,immersed in the four staining solutions (synthetic saliva, synthetic saliva andtea, synthetic saliva and coifee, and synthetic saliva and chlorhexidine in0.12% water solution), and then placed into four thermostatic baths at 37°C ±1 °C. Ail specimens were measured for each resin before immersion (haseline).After 20 and 30 days, the specimens were .inalyzed by computerizedspectro photometry and compared. All data were analyzed with analysis ofvariance for repeated measures IP < ,05), Only the Cold Pac resin was colorstable in all staining solutions, while the others showed color changes from thedifferent staining solutions. Int j Prosthodont 1997; 10: ¡64^168.

Provisional fixerJ prostheses (PFP) should helppromote periorJontal health and provide proper

occlusion while offering esthetic restoration of thedental abutment to avoid compromising the pa-tient's social relationships.''" This latter factor im-plies that a PFP is kept in the mouth for as short aperiod as possibie awaiting final restoration orlong-term interim fixed prostheses. In the authors'experience, the mean duration of a PPP is approxi-mately 20 to 30 days; however, clinical delay, dis-ease, and financial considerations may prolong theperiod in which a PFP is in use and may lead tosubstantial color alterations.

Color stabiiity of the PFP relates not only to thechemicophysical properties ofthe resin but also thepatient's habits. Tea, soy sauce, tannin, red wine,curry, licorice, cocoa, coffee, and chlorhexidine-based oral rinses all tend to stain natural teeth anddiscolor the PFP to an even greater extent, largelybecause of material porosity.-^''

There are many experimental in vitro studies^ "evaluating the discoloration of resin composite, rein-forced resins, and denture base resins'*" ^ soaked instaining solutions, but few studies on the discol-

Reprint requests: Professor Roberto Scotti, Department ofProsthodontics, University of Ferrara School of Dentistry, CorsoCiovecca. 203. 44100 Ferrara, llaiy.

oration of acrylic resins for PFP, Fluid pigments fromfood, beverages, drugs, and nicotine are deposited inthe interprismatic spaces of the teeth, on conserx'a-tive and prosthetic restorations, and especially on thePFP acrylic resins, which are more porous than resincomposite and reinforced resins.'- The fewer freeradicals a resin has, the more stable it will be.'"*

For chlorhexidine discoloration. Rolla et al^ re-ported tbat the mechanism for staining is the pre-cipitation of iron from foods and sulfide arisingfrom the mercaptan groups contained in denatu-rated proteins. However, Prayitno and Addy'""stated that tbe combination of the dietary chro-mogens contained mainly in tea and coffee andchlorhexidine can cause a surface precipitation re-action without the formation of metal sulfides.

Botb the concentration of the staining agents andthe period of exposure may affect tbe degree ofpigmentation of an acrylic resin for PFP.''^ Therole of finishing and polishing is crucial in reduc-ing pigmentation.^'"^

The purpose of this study was the in vitro com-parative evaluation of the color variation of fourdifferent types of acrylic resins used for PFP, as de-termined using computerized spectrophotometrybefore and after a 20- and 30-day cycle of immer-sion in different concentrated staining solutions.

The Iniemalional lournal of Prosthodoriii 164 Volume 10, Number 2, 1997

Color Stiiliiliiy ar Acrylic Resins

Fig 1 Basins with four staining solutions containing thespecimens in the thermostatically oontrolled bath at 37'C ±1 'C with magnetic mixers: the dark oover sheet is to simulatethe oral cavity.

Table 1 Materials Used for the Study

Com me re iaName

Trim

Protemp

Mixacryl II

Cold Pac

Composition

Vinyl-ethyl-poly (methyl meth aery late)

Di-aorylic

Bis-aery lie[methacrylic esters)

Methyl-poly (methyl meth aery late)

Manufacturer

Bosworth,Skokie, IL

ESPE, Seefeld,Germany

Sweden fi Martina,SarmeoladiRubano, Italy

Motloid,Chicago, IL

Materials and Methods

Four different types of acrylic resins for PFP (Table1) were evaluated after immersion in four stainingsolutions.

A total of 128 specimens were made, 32 foreach type of resin. The manufacturer's recom-mended powder-to-liquid ratio and mixing instruc-tions were followed for all materials. The speci-mens, 25 mm in diameter and 2 mm thick, wereautopolymerized at ruom temperature for 15 min-utes under constant pressure of 100 bars in a com-pression plate machine ("ON 57" F.lli Manfredi,-S, Secondo di Pinerolo, TO, Italy), After polymer-ization, the specimen surfaces were polished usingcalcium carbonate with a mean size of 70 to 80(jm (Blanc da Troie, EMD, Geneva, Switzerland)applied using a chamois leather brush (Rilat Hateo,Creiburg, Germany) on a lathe polisher at 2,800rpm,^'' All procedures were consecutively carriedout by one operator.

To evaluate color differences, the CIELAB colori-metric system was used,^''"^^ This system is basedon three parameters; "L," "a," and "b" for definingcolor, and the following equation to evaluate colordifference was used:

Fig 2 Maobeth spectrophotometer Color-Eye 700Qequipped with D6500 daylight and computerized system.

Before discoloration procedures, specimens weremeasured for L, a, b (baseline)'^ and then subdi-vided into four subgroups of eight specimens each.Each subgroup was immersed in one of the follow-ing solutions:

1, Synthetic saliva: 990 mL (KCL 1,47 g, NaHCOj1,25 g, KSNC 0,52 g, NaH,PO^ • H,0 0-19 g,distilled water up to 10 dm' buffered with lac-tic acid atpH 6.72),-°

2, Synthetic saliva: 660 mL; tea: 330 mL (4 g/500mL, Lipton, London, England),

3, Synthetic saliva: 660 mL; coffee: 330 mL (15g/100 mL, SAO Classic Erboris, EDA, Verona,Italy),

4, Synthetic saliva: 660 mL; chiorhexidine 330 mL(Plak Out, Byk Gulden Italy, Cormano, Milan,Italy).

The four staining solutions containing the specimenswere placed in four basins in a thermostatically con-trnlled bath at 37°C ± TC (Haake F3, Berlin,Germany) with 220 V magnetic mixers (Bicasamodel 34532, Bernareggio, Milan, Italy) at a speedof 600 revolutions per minute in a dark environmentto simulate conditions in oral cavity (Fig 1 ),

Color measurements ("L", "a", "b" coordinates,and AE) were made at baseline and at 20 and 30days following immersion in the staining solutions.

All specimens were analyzed using a spectro-photometer (Macbeth Color-Eye 7000, KollmorgenInstruments, New York, NY) using D6500 daylight(Fig 2), The unit is able to analyze 100 specimensper hour and is connected to a computer to recordthe spectrophotometric data.

_Volume10, Number 2, 165 The Inlernational lournal of Prosthodortics

Coior Stablilly ol Acryiic Resins

Table 2 Sheffe Test at 95% Probability: Analysis ot Vanance for L, a, b Values forAll Immersed Samples

Source of variation

L valuesSolutionsfi/ate rialsTimes

Interactions (L)Soiulions-mate riaisSoiutions-timesrelate rials-timesSoiutions-materiais-times

ResidualTotai (correoted)a values

Solutionsfi ateriaisTimes

Interactions (a)Solutions-materialsSolutions-timesMalenals-timesSolutions-materials-times

ResidualTotal (corrected)b values

Solutionsfvlate rialsTimes

Interactions (b)Soiutions-materiaisSo i ut ions-timesMatenals-timesSolution s-maierials-times

ResiduaiTotal (corrected)

Sum ofsquares

564 967183,36

2.56

475 8123 4736.3495.83

996.939379,26

89,51181,43

0,04

367,544,Q81,29

16,88127,94788,71

500,601321,10

0.29

961,207.246,63

52,74821.13

3670.94

Degrees offreedom

331

9339

224255

331

9339

224255

331

9339

224255

Meansquare

188,322394,45

2,56

52.877.82

12.1t10,654,45

29,8460.480.04

40.841.360.431.880.57

166.87440.37

0.29

106.802.412.215.863.67

F ratio

43.31538,01

0.58

11,881,762,722,39

52.24105,88

0,07

71,502,380,753,28

45.53120.13

0.08

29,140,660,60t.60

P

,0000,0000,4597

,0000.1561.0452.0132

,0000,0000,7887

.0000

.0703

.5227

.0009

0000.0000.7804

,0000,5786,6135,1167

All the data were submitted to ANOVA accordingto Scheffe test at 95% probability using Statgrapbics5,0 (Computer Statistical Graphics System, StatisticalGraphic, Rockville, MD) (Table 2).

Results

Table 3 reports the mean value and standard devia-tion of the measurements performed on the eightspecimens of eacb resin before immersion (baseline)and after immersion in the four staining solutions.Three colorimetric parameters, L, a, b, were evalu-ated after 20 and 30 days of immersion. All valuesare absolute numbers with the following meaning:(1) the L coordinate (range 0 to 100) measures thequantity of white-black; tbe greater tbe L value, theviibiter tbe sample (0 = black, 100 = white); (2) the acoordinate measures the color along the red-greenaxis: a positive value refers to the amount of red in

the sample, a negative value refers to the amount ofgreen; (3) the b coordinate measures the color alongthe yellow-blue axis: a positive b value is yellow, anegative b value is blue.

For example, the whitest material was the baselineof Trim and the blackest was the Mixacryl II after 30-day immersion in syntbetic saliva and coffee.

The differences between each mean value (L, a,b) after immersion in the four solutions and atbaseline was calculated (see Table 3), and thecolor variation AE is reported in Table 4, The meanAE for each product after 20 and 30 immersiondays is shown in the last line of Table 4, Tbesedata represent the general behavior of each mater-ial: the lower tbe value, the less tbe color variation.Initially the most color stable materials were GoldPac and Trim, and Trim was more stable after 30days. Tbe greatest color variation was found withMixacryl II immersed in syntbetic saliva and coffee

ThelnlerrationsUoufnalofProiltiodontrcs 1 6 6 Volume 10, Number 2, 7907

Coior Stabiiity of Acrylic Re

! , !n^ Q ^ 1 [ J f J ^ - ^- Ö) for Materials and Solutions Before (Baseline) and Atter Immersion ¡n the Fourning Soiutions, Means (Standard Deviations)

Synthetic SalivaTea and

synthetic salivaCoffee and

synthetic saiiiia20 d 30 d 20 d 30 d 30 d 30 d 20 d 30 d

Cold PacLab

TnmLab

PtotempL

Mixacryi ilL

76,10(4.06) 72 49(3.60)1,18(0.43) 105(0.81)

14.39(2.19) 13.93(3.15)

79,31 (0,73) 77,96 (0.98)1,21 (0,34) 1.50(0.18)

15,00(0.88) 17,48(0,54)

67.49(1.24) 65.85(2,31)1.06(0.70) 2.44(0,25)

13.32(0.93) 14,79(1,07)

68,04(3,65) 66,50(1,70)2,79(0.64) 3.19(0,29)

13,77(180) 14.19(1,42)

72.83(4.40) 72.80(409) 72.80(3.90) 71.04(1.80) 69,00(1.43) 71,10(0,91) 7110(129)1.37(1.06) 1.44(083) 1,47(0,92) 0.73(0.30) 0,68(0.35) 1.83(0,36) 1 8t (0 43)

1471(3.76) 14.49(247) 14,63(3,02) 12.97(1.08) 11,81(1.63) 12,18(0,50) 12.10(0.98)

77.42(0.88) 78.07(097) 77,83(1.14) 77.57(1,22) 77,65(1.18) 74,20(1,09) 74.30(010)2.32 (0,37) 1.83 (0,75) 1,83 (0,62) 1,49 (0,53) 1.44 (0,49) 7,35 (1,49) 6,60 (0,83)

17,79(0,93) 17,82(1.07) 17,80(0,67) 17,55(1,16) 17.39(0,93) 18,15(1,24) 19,80(1,02)

66,88(1.18) 66.16(139) 67,20(0,57) 61,70(2,60) 61.70(2,70) 64,93(145) 64 09(124)2,44 (0,31) 2.25 (0.50) 2,04 (0,39) 3.53 (0.92) 3,83 (0.95) 2.13 (0.74) 2.24(0,66)

14,97(1.13) 16.32(181) 14,57(1.11)20.36(3.68)21,39(2.59) 16.61(2,23) 17.50(2,22)

64,00(1.60) 66.49(4.45) 67,08(3,98) 60,41(3.65) 58,88(4.33) 65,40(0,95) 6595(010)3,76(0.46) 2.67(0.79) 2.65(0,84) 4,90(2.16) 4,15(1.65) 2.84(0,44) 3,48(0 30)

17,65(1.62) 17.07(1.57) 17.15(1,39) 25,93(3,86) 26,13(3,70) 17.74(1,55) 17,83(0.45)

Table 4 AE Color Difference Between Immersed Sample (Mean) and Baseline

Cold Pac Mixacryi II

Staining solution 20 d

3 643.315.275,504,43

30 d

3,283,327,575,534,92

20 d

2.843.143.108.514,40

30 d

3,553,332,928.844,63

20 d

2.603.499.444.304.96

30 d

2,241,61

10,315,064,80

20 d

1.643,6514,504,74

30 d

5,683,5215.444.62

Synthetic saliva (pH 6.72)Tea and synthetic saiiva (1:3)Coffee and synthetic saiiva (1:3)Chiorhexidine (0 12% water soiution)Mean

for 30 days; the least variation was found inProtemp in synthetic saliva and tea after 30 days.

As a result of the test performance, the largenumber of specimens, and the absence of outlyingdata, the authors supposed all the data to be nor-mally distributed. All data were submitted toANOVA according to the Scheffe test at 95% prob-ability using Statgraphics 5,0 (see Table 2), Table 2shows the results among solutions, materials, andtimes for L, According to the P value, there weresignificant differences among values for solutions.The same occurred with materials: no differenceswere found between the two periods of immersion(20 and 30 days).

Statistically, the HQ hypothesis was not verified forsolutions and materials, but it was verified for times(45% significance). Tbe interactions in which solu-tions, times, and materials are compared are alsoshown in Table 2. A significant difference (P = 0)

existed only for the materials and solutions. The val-ues and interactions for a and b are also reported inTable 2,

Table 2 indicates that for all times values therewas no significant difference, but there were signif-icant differences between solutions.

Discussion

Since chromatic instability increases over time, theuse of filled resins (compound resins and/or resincomposites) is preferred for long-term provisionalfixed prosthetic restorations. This study indicated thatit is better not to use Protemp and Mixacryi 11 resinswith patients who consume large quantities of coffee,or the Trim resin with patients using chiorhexidinemouthwashes. However, these experimental data re-quire confirmation by in vivo studies, ^

Voiump 10, Number 2, 1 167 Journal of Prosttiodontii

Culor Stability of Acryiic Resins

Conclusions

Four aufopolymerizing resins were immersed insynthefic saliva and solufions of fea, coffee, andchlorhexidine for periods of 20 and 30 days. Theresulting color changes were evaluated using aspectrophofomefer and compared to fhe baseline.All data were analyzed wifh ANOVA for repeafedmeasures. Within the limitations of this study, thefollowing conclusions may be made:

1. Solutions: Color instability was found fo be lessin saliva and saliva and tea than in fhe other twosolufions. Ali the solufions darkened all the ma-terials: L values decreased when compared withthe baseline. The solution of synfhefic saliva andcoffee produced the greatest darkening (lowesf Lvalues). All solutions except Cold Pac had a redpigmenfafion shift in all maferials compared wifhthe baseline. Changes in b component valueswere significanf for all materials after immersion.

2. Times: There were no differences in the valuesrecorded at 20 or 30 days.

3. Resins: Cold Pac was color stable in all stainingsolutions. Mixacryl II darkened more than allother materials in synthetic saliva and salivawith coffee. If compared with baseline, all ma-terials except Cold Pac increased in yellowness(higher b value). The most yellow material afterimmersion was Mixacryl II.

Acknowledgments

The authors wish to thank Dr P. Schiavina for his advice, andHimonl italia and Mr L. Buriani ior the use ofthe equipment.

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The Internationai lournal of Prosthodontii 168 VolumelO, Number?, 1997