Effe

mec

pro

Xingxue Hu, DM

This study was supported by The Oh

aAdjunct Assistant Professor, DivisioDentistry; Advanced Standing PrograbBiostatistical Scientist, Center for BcProfessor, Division of Restorative, P

Hu et al

cts of pigments on dynamic

hanical properties of a maxillofacial

sthetic elastomer

D, PhD,a Xueliang Pan, PhD,b andWilliam M. Johnston, PhDc

College of Dentistry, The Ohio State University, Columbus, Ohio;Henry Goldman School of Dental Medicine, Boston University,Boston, Mass; Center for Biostatistics, The Ohio State UniversityCollege of Medicine, Columbus, Ohio

Statement of problem. Pigmentation and coloration play a key role in fabricating a maxillofacial prosthesis. The addition ofpigments and dyes to the maxillofacial material may alter the dynamic mechanical behaviors of the prosthesis, possiblyinfluencing the success of the prosthesis.

Purpose. The purpose of this study was to investigate the effects of the type and concentration of intrinsic pigments on thedynamicmechanical properties of a commercially availablemaxillofacial silicone elastomer over a small range of low frequencies.

Material and methods. Ten pigmented mixtures (6 specimens per mixture) were made by using a base silicone elastomermixed with each intrinsic silicone pigment (Black, Red, Tan, or Yellow) or all the pigments (MixAll) in a designated high orlow concentration. The base elastomer without pigment (Unpigment) was prepared as a control. Dynamic mechanicalanalysis was performed over 5 low frequencies (0.5, 1.0, 1.5, 2.0, and 2.5 Hz) at room temperature. The storage modulus(E0), loss modulus (E00), and loss tangent (tand) in compression were determined. Mixed models for repeated measures wereused for the comparisons of E0, E00, and tand among mixtures (a¼.05).

Results. The means of E0, E00, and tand of all the pigmented specimens were lower than those of Unpigment. MixAll withhigh concentration had the lowest values in E0 and E00. The means of E0 and E00 of Red and Yellow in high concentrationwere lower than those in low concentration, whereas the means of E0 and E00 of Black and Tan in low concentration weresignificantly lower than those in high concentration; the means of tand for all the mixtures in high concentration weresignificantly lower than those in low concentration. The means of E0, E00, and tand of all the specimens tested increasedas frequency increased from 0.5 to 2.5 Hz (P<.05).

Conclusions. Within the limitations of this study, it was concluded that the addition of intrinsic silicone pigments into abase maxillofacial elastomer significantly influenced dynamic mechanical properties of the maxillofacial silicone elastomertested over the low frequencies from 0.5 to 2.5 Hz at room temperature. This effect, which was a quick elastic return toits original shape after deformation during pigmentation or coloration, seems desirable to a certain extent in clinicalapplication. The type and concentration of pigment may influence the elastic and viscous portion of the properties of themaxillofacial elastomeric materials tested. Low frequencies (0.5 to 2.5 Hz) affect the dynamic viscoelastic properties of thematerials. (J Prosthet Dent 2014;-:---)

Clinical Implications

Pigments and dyes are often used in the pigmentation and colorationof a maxillofacial prosthesis. Understanding the effects of pigmentadditives on the dynamic viscoelastic behaviors of maxillofacialprosthetic materials may lead to improvements in clinical applicationand to more scientific material developments.

io State U

n of Restom, Bostoiostatisticrosthetic a

niversity College of Dentistry.

rative, Prosthetic and Primary Care Dentistry, The Ohio State University College ofn University Henry M. Goldman School of Dental Medicine.s, Ohio State University College of Medicine.nd Primary Care Dentistry, The Ohio State University College of Dentistry.

Table I. Composition for type and concentration of pigments tested

MixtureBlack(wt%)

Red(wt%)

Tan(wt%)

Yellow(wt%)

Unpigment (control) 0 0 0 0

Black High 0.2 0 0 0

Black Low 0.03 0 0 0

Red High 0 1.2 0 0

Red Low 0 0.1 0 0

Tan High 0 0 1 0

Tan Low 0 0 0.1 0

Yellow High 0 0 0 1

Yellow Low 0 0 0 0.08

MixAll High 0.2 1.2 1 1

MixAll Low 0.03 0.1 0.1 0.08

wt%, percentage by weight.

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A maxillofacial prosthesis is consid-ered a reliable treatment option forrestoring maxillofacial defects andimproving quality of life.1 An idealmaxillofacial prosthetic material shouldhave physical andmechanical propertiescomparable with those of the humantissue being replaced and should be easyto process and insensitive to processingvariables.2 Silicone elastomer is currentlythe material of choice for maxillofacialprostheses because of its clinical inert-ness, biocompatibility, mechanical pro-perties, and ease of manipulation.1,3

Pigmentation and coloration playa key role in fabricating a successfulmaxillofacial prosthesis. Both intrinsicand extrinsic colorings are often usedto match the prosthesis to human tis-sue in clinical practice. Compared withextrinsic coloring, intrinsic coloring thatsets the basic color and translucency isless vulnerable to environmental con-ditions and handling but more likelyto affect the structure and properties ofthe mixture.4,5 Recent studies havefound that silicone intrinsic pigment orsilicone extrinsic paste (Factor II), rayonflocking fibers, and dry earth pigmentsare frequently used to fabricate extrao-ral maxillofacial prostheses.6-8 Dryearth pigments and kaolin have alsobeen reported to decrease the tensilestrength and increase the hardness ofthe materials tested, and rayon flockingto increase the hardness; the addition ofnano-oxides (for instance, ZnO, TiO2,and CeO2) as opacifiers improved theoverall mechanical properties of maxil-lofacial silicone elastomers.9,10

Dynamicmechanical analysis (DMA)has proved effective in evaluating theviscoelastic properties of maxillofacialprosthetic materials.11,12 Because theoral tissues are constantly moving andsubjected to a multitude of forces (inmany directions, in both tension andcompression) and because maxillofacialprosthetic materials should simulatethe oral tissues, the dynamic mechan-ical properties of silicone elastomerscan greatly affect the clinical success ofmaxillofacial applications.13,14 There-fore, to understand the effects ofpigment additives on the properties of

The Journal of Prosthetic Dentis

maxillofacial prosthetic materials, thelaboratory evaluation of elastomersfor maxillofacial applications shouldinclude not only the static propertiesbut also the dynamic properties ofthe materials. Maxillofacial siliconematerial showed elastic behavior withalmost no viscous component, and thestorage modulus (E0), loss modulus(E00), and loss tangent (tand) of sili-cone elastomers were insensitive tochanges in temperature.13 Adding ZnOin different concentrations to a maxil-lofacial silicone material did notsignificantly affect E0 and E00 but didaffect tand. Low frequencies (0.1 to15 Hz) did not seem to play a signif-icant role in the change of the dy-namic mechanical properties of thesilicone materials tested.14-16 However,very few studies of the effects of pig-ments and dyes (especially organicintrinsic colorants) on the dynamicmechanical properties of silicone elas-tomers have been reported.

The purpose of this study was toinvestigate the effects of the type andconcentration of intrinsic pigments onthe dynamic mechanical properties of amaxillofacial silicone elastomer over asmall range of low frequencies. The nullhypothesis was that the addition of thepigments would not affect the dynamicmechanical properties of the siliconemixture tested.

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MATERIAL AND METHODS

A representative platinum siliconeelastomer (A-2000; Factor II Inc) and4 types of commercially available pig-ments (Black, Red, Tan, and YellowFunctional Intrinsic Skin Color; FactorII Inc) were used. The pigments con-taining one or more of the FederalFood, Drug, and Cosmetic Act (FD&C)organic oxides are proprietarily dis-persed in a polydimethylsiloxane fluid.17

Ten types of silicone elastomericmixture were made from a base elas-tomer mixed with each pigment or acombination of all the pigments withhigh or low concentration. The baseelastomer without pigment was pre-pared as the control. The mixing proce-dure was performed according to themanufacturer’s instructions and thefabrication procedures of the materialsin previous studies.7,18 The determina-tion of high and low concentrations wasbased on those used in previous studiesfor elastomer specimens that yieldedcolors within the range of human skincolor.19,20 The compositions of the typesand the concentrations of the pigmentsused are listed in Table I.

Six cylindrical specimens (approxi-mately 15 mm in diameter and 4 mm inthickness) per mixture were prepared.Each specimen was first subjected to aninitial 15% compressive strain before

Hu et al

- 2014 3

being cycled and then to an additional�10% strain at a frequency of 0.5, 1.0,1.5, 2.0, and 2.5 Hz at room temper-ature (25�C). The force and deflectionvalues were recorded and interpreted bythe ElectroForce 3200 loading devicewith a 450-N load cell loading device(ElectroForce Systems Group; BoseCorp) and the Win Test DMA software(Bose Corp). The storage modulus (E0),loss modulus (E00), and loss factor(tand¼E00/E0) in compression werecalculated according to the ASTM In-ternational D5992-96 standard (reap-proved in 2006).21

Mixed models for repeated mea-sures were used to compare E0, E00, andtand at different loading frequenciesamong all mixtures (SAS 9.3, SASInstitute Inc). The differences amongall 11 mixtures were tested by using theleast square mean estimation based onthe mixed models. Sensitivity analyseswere conducted by using the ANOVAmodel to compare the difference ofE0, E00, or tand among the differentmixtures at each frequency level toconfirm that the conclusions weresimilar. The significance level was .05.The sample size of 6 specimens per

0 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 2

BlackHigh BlackLow MixHigh MixLow5 0 5 0 5. . . . . . . . . . . . . . . . . . . .

5 0 5 0 5 5 0 5 0 5 5 0 5 0 5

5E'

4

3

2

1

0

1 Mean and standard deviation of sto

Hu et al

mixture provided at least 80% power todetect an effect size of 1.8 standarddeviation difference between any 2mixtures at a significance level of .05.Assuming a coefficient of variance (CV)of 5% for either E0, E00, or tand, the 1.8standard deviation difference wasabout a 9% change of the measure,which was considered a significantchange for this study.

RESULTS

The mean and standard deviation ofthe storage modulus (E0), loss modulus(E00), and loss tangent (tand) for the 11mixtures (n¼6) over the frequenciesfrom 0.5 to 2.5 Hz are summarized inFigures 1-3. Table II shows that thestorage modulus, loss modulus, andloss tangent of all the specimens testedwere significantly affected by mixture,frequency, and the combination ofmixture and frequency (P<.001).

Table III summarizes the leastsquare means estimations based on themixed models and the statistical com-parisons of all the mixtures in terms ofstorage modulus, loss modulus, andloss tangent. All of the mean values of

00 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 2

RedHigh RedLow TanHigh TanLow Un

. . . . . . . . . . . . . . . . . . . . .5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5

rage modulus (E0) for elastomer mixtures

E0, E00, and tand of all of the pigmentedmixtures were significantly lower thanthose of Unpigment (P<.05) exceptthat no significant differences wereobserved between YellowLow (4.39MPa) and Unpigment (4.48 MPa) instorage modulus, between RedLow(0.151) and Unpigment (0.154) in losstangent, or between MixAll Low (0.150)and Unpigment (0.154) in loss tangent(P>.05). For MixAll High mixturespecimens, the means of E0 and E00 werethe lowest values among those of allthe mixtures, and the mean of tandwas significantly lower than thoseof Unpigment, BlackHigh, BlackLow,RedLow, and MixAll Low (P<.05) butsignificantly higher than those ofTanHigh and YellowHigh or not signif-icantly different from those of Yellow-Low, RedHigh, or TanLow. In addition,the means of E0 and E00 of Red, Yellow,and MixAll in high concentration weresignificantly lower than those in lowconcentration, whereas the means of E0

and E00 of Black and Tan in low con-centration were significantly lower thanthose in high concentration; the meansof tand for all the mixtures in highconcentration were lower than those in

0 1 1 2 20 1 1 2 21 1 2 2

pigment YellowHigh

YellowLow

Mixture

Frequency(Hz) . . . . . . . . . . . . . .

0 5 0 5 5 0 5 0 5 5 0 5 0 5

with frequency 0.5 to 2.5 Hz (n¼6).

0 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 2

BlackHigh BlackLow Mixhigh MixLow RedHigh RedLow TanHigh TanLow Unpigment YellowHigh

YellowLow

Mixture

Frequency(Hz)

5 0 5 0 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5

0.9E"

0.7

0.6

0.8

0.5

0.4

0.3

0.2

0.1

0.0

2 Mean and standard deviation of loss modulus (E00) for elastomer mixtures with frequency 0.5 to 2.5 Hz (n¼6).

0 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 20 1 1 2 2

BlackHigh BlackLow MixHigh MixLow RedHigh RedLow TanHigh TanLow Unpigment YellowHigh

YellowLow

Mixture

Frequency(Hz)

5 0 5 0 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5 5 0 5 0 5

0.18

tan δ

0.170.160.150.140.130.120.110.100.090.080.070.060.050.040.030.020.010.00

3 Mean and standard deviation of loss tangent (tand) for elastomer mixtures with frequency 0.5 to 2.5 Hz (n¼6).

4 Volume - Issue -

low concentration, and all of the mix-tures (except for Black) were statisticallysignificant (P<.05).

As shown in Figures 1-3, the meansof E0, E00, and tand for all of the 66

The Journal of Prosthetic Dentis

specimens tested increased as the fre-quency increased from 0.5 and 2.5 Hz.Although the interactions of mixtureand the frequency were statisticallysignificant in the mixed models for all 3

try

measures, the change patterns overfrequencies of different mixturesappeared similar from these figures. Toensure the robustness of the compari-son among different mixtures using the

Hu et al

Table II. Mixed models for repeated measures outputs for storage modulus (E0),loss modulus (E00), and loss factor (tand)

df

Variable Effect numerator denominator F Pr >F

E0 Mixture 10 55 20.47 <.001

Frequency 4 220 1603.93 <.001

Mixture�Frequency 40 220 3.90 <.001

E00 Mixture 10 55 32.24 <.001

Frequency 4 220 8655.08 <.001

Mixture�Frequency 40 220 19.39 <.001

Tand Mixture 10 55 17.14 <.001

Frequency 4 220 5663.40 <.001

Mixture�Frequency 40 220 9.06 <.001

Table III. Least square means estimations and statistical comparison resultsof mixtures in terms of storage modulus (E0), loss modulus (E00), and losstangent (tand) across different frequencies

MixtureMean E0

(MPa)Mean E00

(MPa) Mean tand

Unpigment 4.48a 0.691g 0.154m

Black High 4.07d 0.605i 0.148n

Black Low 3.71e,f 0.557j,k 0.150n

Red High 3.84e 0.553j,k 0.144p,q

Red Low 4.04d 0.610i 0.151mn

Tan High 4.16c,d 0.577j 0.139r

Tan Low 3.72e,f 0.539k 0.144p

Yellow High 4.18c,d 0.577j 0.138r

Yellow Low 4.39a,b 0.615i 0.140q,r

MixAll High 3.55f 0.505l 0.142p,q

MixAll Low 4.28b,c 0.645h 0.150m,n

Means with same letter were not significantly different (P<.05, without multiplecomparison adjustment).

- 2014 5

least square means estimations fromthe mixed models, sensitivity analyseswere conducted with 1-way ANOVA ateach frequency level and yielded similarconclusions.

DISCUSSION

This study evaluated the dynamicviscoelastic properties of 11 mixtures(6 replicates per mixture) of a repre-sentative platinum silicone elastomer,A-2000, with or without intrinsicpigments. According to the manufac-turer’s instructions, A-2000 was the first

Hu et al

generation of a 1:1 room temperaturevulcanized platinum silicone elastomerproduced by Factor II, and in a survey6

it was 1 of the top 3 silicone elastomersused for maxillofacial prostheses. Fourtypes of intrinsic silicone pigment(black, red, tan, and yellow) used tomake skin-colored specimens maycontain one or more organic oxides ofprimary colors and other inorganicsubstances, and both the intrinsic sili-cone pigment systems are a blend ofFD&C cosmetic pigments crushed intoa silicone cross-linking fluid to createa viscous but liquid or paste silicone

pigment.17 The addition of intrinsic sil-icone pigments may influence the staticand dynamic mechanical properties ofsilicone elastomers. Therefore, the nullhypothesis (that the addition of thepigments would not affect the dynamicmechanical properties) was rejected.

Understanding the effects of pig-ment additives on the dynamic me-chanical properties of maxillofacialprosthetic materials may lead to im-provements in clinical applicationsand to more scientific development ofmaxillofacial silicone elastomer mate-rials. Compared with static testingmethods, dynamic mechanical analysiscan be used to evaluate time-dependentviscoelastic behaviors of a material byvarying factors such as temperatureor frequency to simulate the environ-ments of the face or the oral cavity.13

The storage modulus represents theelastic portion of the elastomer andreflects the material’s stiffness; the lossmodulus represents the viscous portionof the material and reflects the energythat can be converted to heat; the losstangent represents the capacity for en-ergy absorption. Figures 1-3 show theranges of the means of E0 (3.55 to 4.48MPa), E00 (0.505 to 0.691 MPa), andtand (0.138 to 0.154) for the A-2000silicone specimens tested at frequenciesfrom 0.5 to 2.5 Hz. These ranges indi-cate that the elastic behavior of thesilicone elastomer tested dominates itsviscous behavior, which is consistentwith the findings in previous studieswith regard to the dynamic mechanicalproperties of commonly used maxillo-facial silicone elastomers at a frequencyof 1 Hz.13-15

Table III shows that the means of E0,E00, and tand of the elastomeric speci-mens without pigments (Unpigment)were higher than those of all the mix-tures mixed with the pigments, indi-cating that adding pigments coulddecrease the stiffness, viscous property,and capacity for energy absorption ofthe mixtures. This suggests a quickelastic return of the material testedto its original shape in response todeforming forces after pigmentation orcoloration, which is a desirable change

6 Volume - Issue -

in clinical application. The specimenscombining all the pigments in highconcentration demonstrated the leaststiff and viscous material. Within therange of pigment concentration usedfor skin-colored specimens in thisstudy, the red and yellow pigmentsin higher concentration cause moredecreases in the storage modulus andloss modulus than those in lower con-centration. The loss tangent values ofthe respective mixture in high concen-tration show more decreases than thosein low concentration, suggesting thatincreasing the concentration of thepigment may reduce the energy ab-sorption capacity of the material.

Frequencies from 0.5 to 2.5 Hz wereused in this study, because the facialtissues are often subjected to a lowconstant movement whose frequencyis usually 0.5 to 1 Hz in normal con-ditions.14 Figures 1-3 show that themeans of E0, E00, and tand present asmall increase as frequency increasesfrom 0.5 to 2.5 Hz; the P values for thefrequency effects in E0, E00, and tand areall less than .001. This suggests thatfrequencies from 0.5 to 2.5 Hz influ-ence the dynamic viscoelastic propertiesof the silicone specimens tested. Thisfinding is consistent with the previousstudies.13,15 In addition, because it wasreported that the silicone elastomerwould not vary substantially in E0, E00,and tand as a function of temperature,this study was carried out isothermallyat room temperature.12

Further study is warranted to inves-tigate the effects of a wider range oftype and concentration of the siliconepigments and of the condition of agingon the dynamic viscoelastic proper-ties of the maxillofacial elastomer.Further exploration is also warrantedof the relationship between changesin dynamic mechanical properties andchanges in microstructure and compo-sition during pigmentation and colora-tion of maxillofacial silicone mixtures.

CONCLUSIONS

Within the limitations of this study,it is concluded that adding intrinsic

The Journal of Prosthetic Dentis

silicone pigments into a base maxillo-facial elastomer significantly influencesthe dynamic mechanical properties ofthe maxillofacial silicone elastomertested over the low frequencies from 0.5to 2.5 Hz at room temperature. Thiseffect, which is a quick elastic return toits original shape after deformationduring pigmentation or coloration,seems desirable to a certain extent inclinical application. In addition, thetype and concentration of pigment mayinfluence the elastic and viscousportion of the properties of the maxil-lofacial elastomeric materials tested.Increasing the concentration of pig-ment decreases the energy absorptioncapacity of the materials tested. Lastly,frequencies ranging from 0.5 to 2.5 Hzinfluence the dynamic viscoelastic pro-perties of the maxillofacial elastomericmaterials tested.

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1. Ariani N, Visser A, van Oort RP, Kusdhany L,Rahardjo TB, Krom BP, et al. Current state ofcraniofacial prosthetic rehabilitation. Int JProsthodont 2013;26:57-67.

2. Craig RG, Koran A, Yu R. Elastomers formaxillofacial applications. Biomaterials1980;1:112-7.

3. Aziz T, Waters M, Jagger R. Analysis ofthe properties of silicone rubbermaxillofacial prosthetic materials. J Dent2003;31:67-74.

4. Seelaus R, Troppmann RJ. Chapter 17:Facial prosthesis fabrication: technicalaspects. In: Taylor TD, editor. Clinicalmaxillofacial prosthetics. Chicago:Quintessence Publishing; 2000. p. 245-64.

5. Han Y, Zhao Y, Xie C, Powers JM,Kiat-amnuay S. Color stability of pigmentedmaxillofacial silicone elastomer: effects ofnano-oxides as opacifiers. J Dent2010;38(suppl 2):e100-5.

6. Montgomery PC, Kiat-Amnuay S. Survey ofcurrently used materials for fabrication ofextraoral maxillofacial prostheses in NorthAmerica, Europe, Asia, and Australia.J Prosthodont 2010;19:482-90.

7. Hu X, Gilbert AB, Johnston WM. Interfacialcorrections of maxillofacial elastomers forKubelka-Munk theory using non-contactmeasurements. Dent Mater 2009;25:1163-8.

8. Hungerford E, Beatty MW, Marx DB,Simetich B, Wee AG. Coverage error of com-mercial skin pigments as compared to humanfacial skin tones. J Dent 2013;41:986-91.

9. Haug SP, Andres CJ, Moore BK. Colorstability and colorant effect on maxillofacialelastomers, part I: colorant effect onphysical properties. J Prosthet Dent 1999;81:418-22.

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10. Han Y, Kiat-amnuay S, Powers JM, Zhao Y.Effect of nano-oxide concentration on themechanical properties of a maxillofacialsilicone elastomer. J Prosthet Dent 2008;100:465-73.

11. Koran A, Craig RG. Dynamic mechanicalproperties of maxillofacial materials.J Dent Res 1975;54:1216-21.

12. Wunderlich B. Thermal analysis of polymericmaterials. 3rd ed. Knoxville: Springer; 2005.p. 408-22.

13. Murata H, Hong G, Hamada T, Polyzois GL.Dynamic mechanical properties of siliconemaxillofacial prosthetic materials and the in-fluence of frequency and temperature on theirproperties. Int J Prosthodont 2003;16:369-74.

14. Waters M, Jagger R, Polyzois G, Williams K.Dynamic mechanical thermal analysis ofmaxillofacial elastomers. J Prosthet Dent1997;78:501-5.

15. MouzakisDE, Papadopoulos TD, Polyzois GL,Griniari PG. Dynamic mechanical propertiesof a maxillofacial silicone elastomer incorpo-rating a ZnO additive: the effect of artificialaging. J Craniofac Surg 2010;21:1867-71.

16. Gupta A, Jain D, Tripathi K. Dynamicvisco-elastic analysis of silicon maxillofacialprosthetic material using custom-madedynamic visco-elastomer and LASERmeasuring device. J Ind Prosthodont Soc2009;9:127-35.

17. Factor II Inc. Functional intrinsic skin pig-ments. Material Safety Data Sheet. June 3,2008. Available at: http://www.factor2.com/Articles.asp?ID¼131.

18. Hu X, Johnston WM, Seghi RR. Measuringthe color of maxillofacial prosthetic material.J Dent Res 2010;89:1522-7.

19. Hu X, Johnston WM. Concentration addi-tivity of coefficients for maxillofacial elas-tomer pigmented to skin colors. Dent Mater2009;25:1468-73.

20. Wee AG, Beatty MW, Gozalo-Diaz D,Kim-Pusateri S, Marx DB. Proposed shadeguide for human facial skin and lipea pilotstudy. J Prosthet Dent 2013;110:82-9.

21. ASTM International. D5992-96. Standardpractice for dynamic testing of vulcanizedrubber and rubber-like materials usingvibratory methods. Reapproved 2006. Avail-able at: http://www.astm.org/Standards/D5992.htm.

Corresponding author:Dr Xingxue HuDivision of Restorative, Prosthetic andPrimary Care DentistryThe Ohio State University College of Dentistry305 W 12th Ave, Rm 3001AColumbus, OH 43210E-mail: hu.148@osu.edu

AcknowledgmentsThe authors thank Dr Do-Gyoon Kim for hisvaluable collaboration through his knowledgeand the loan of the test instrument used in thisstudy. Support from Drs Robert R. Seghi andWilliam A. Brantley is also gratefullyacknowledged.

Copyright ª 2014 by the Editorial Council forThe Journal of Prosthetic Dentistry.

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