the artificial eye is outstanding ; although it 
 does not restore primordial function , that is sight , to the wearer , it satisfactorily 
 restores its facial esthetics .
furthermore , the artificial eye is responsible for the 
 individual 's reintegration into society , returning a condition of comfort to the user as 
 regards its social intercourse and acceptance of the deformity , thus improving patient 's 
 quality of life .
concern about esthetics and durability of the materials used for fabricating artificial 
 eyes has encouraged studies over the course of time .
one of the most worrying aspects in treatments with 
 artificial eyes is that they become disagreeable after a few months of use , and it 
 becomes necessary to have them redone due to color instability and decreased resistance 
 as a result of the effects of ultraviolet rays , deposition of microscopic residues on 
 the surface porosities , continual handling and cleaning of the prosthesis by 
 patients .
it is known that color stability is the property a material has of retaining color for a 
 period of time in a certain environment , and is an important physical property of many 
 dentistry materials .
shade determination in dentistry can be divided into two categories : visual and 
 instrumental .
this type of analysis is more exact than measurement by the naked 
 eye , because it is capable of assessing small difference in the coloring of 
 objects .
one aspect hardly mentioned by professionals in the area is the chromatic stability of 
 these resins when submitted to different polishing methods .
clinically , polishing of 
 artificial eyes is a factor of the utmost importance , as they must present a smooth 
 surface , otherwise the presence of roughness favors the accumulation of impurities and 
 bacteria , irritating the patient 's anophthalmic cavity , an in addition discomfort , also 
 causes infections .
conventional polishing of acrylic resins is performed in stages , in a bench lathe , and 
 any negligence in any one of the stages of the procedure , makes the subsequent polishing 
 stages more difficult . in relation 
 to chemical
polishing , this was described as an alternative technique to decrease water 
 absorption of resins . according to this technique
, the polymerized resin is immersed in 
 monomer heated to boiling point , for 1 minute , which results in well polished internal 
 and external surfaces . based on a literature review , and considering the degrading effects of environmental 
 agents on the physical properties of the acrylic resin , this study assessed the color 
 stability of acrylic resins specifically for fabricating sclera , submitted to chemical 
 and/or mechanical polishing , before and after accelerated aging .
to make the specimens , colorless autopolymerized acrylic resin disks were obtained by 
 using a metal matrix 15 mm in diameter and 2 mm thickness ( figure 1 ) , according to kiat - amnuay , et al .
metallic matrix with 15-mm diameter and 2-mm thickness these disks were included in the proper flasks for microwave polymerization ( stg , vipi , 
 pirassununga , sp , brazil ) , using type iii stone plaster ( gesso - rio , so paulo , sp , 
 brazil ) and extra hard packing silicone ( zetalabor , zhermack , badia polesine , rovigo , 
 italy ) for embedding the molds .
after final setting of the materials , silicone and stone plaster , the flasks were opened 
 and the disks removed ( figure 2 ) .
n2 and n3 for artificial eyes ( clssico , artigos 
 odontolgicos clssico ltda . , campo limpo paulista , so paulo , sp , brazil ) were mixed 
 according to the manufacturer 's instructions .
when the plastic stage was reached , the 
 resins were placed in the molds contained in the flasks and pressed in a hydraulic press 
 ( vh , midas dental products ltda . , araraquara , sp , brazil ) with a force of 1200 kgf , and 
 remained at rest for 30 min .
after this period , the flasks were taken for microwave 
 polymerization ( 840 watts ) using 60% of maximum power , for 3 min .
molds obtained after inclusion of colorless acrylic resin disks in silicone and 
 dental stone after allowing the flasks to cool naturally , the specimens were deflasked and submitted 
 to finishing for 1 min with abrasive papers no .
280 , 320 , 600 , 1200 ( norton , norton 
 abrasivos , ribeiro preto , sp , brazil ) and polishing with felt disk and 1/4 micron 
 diamond solution ( buehler , buehler ltd , lake bluff , il , usa ) in an automatic polisher 
 ( arotec , cotia , sp , brazil ) at 600 rpm .
of the 14 specimens obtained for each resin , 7 were submitted to mechanical polishing 
 and 7 to chemical polishing .
chemical polishing was performed in a chemical polishing machine termotron model pq-9000 
 ( termotron , piracicaba , so paulo , brazil ) with chemical polishing fluid ( poli - quim 
 artigos odontolgicos clssico ltda . , campo limpo paulista , sp , so paulo , brazil ) .
the 
 specimens were immersed in a receptacle containing the fluid at a temperature of 80c 
 for 10 s ( figure 3 ) .
after this period , the 
 specimens were removed from the receptacle and washed under running water according to 
 manufacturer 's recommendations after 15 s. the samples were storage in distilled water 
 for 24 h for reduction of residual monomer .
independent of the activation method , the literature shows the 
 presence of residual monomer .
the alterations in color and brightness were evaluated by reflection spectophotometry 
 ( shimadzu , shimadzu corporation , nakagyo - ku , kyoto , japan ) , as used by canadas , et 
 al .
the cielab system calculates in the distance between two readings of color through the 
 formula : e = [ ( l ) + ( a ) + ( b ) ] after initial chromatic analysis , the specimens were submitted to accelerated aging 
 according to astmg 53 standards ( american society for testing materials standard 
 53 ) in an accelerated 
 aging chamber for nonmetallic bodies - uvb/ condensation ( equilam , equilam ind .
the readings of chromatic stability were performed initially and following the periods 
 of 252 , 504 , and 1008 h of accelerated aging .
the results were submitted to anova followed by the tukey 's 
 test at 1% significance level .
table 1 displays the color change values ( e ) of 
 the acrylic resins after both polishing procedure and accelerated aging .
the n1 acrylic 
 resin presented the lowest e values that were clinically acceptable by visual analysis 
 of color change .
mean values of color derivatives ( e ) of resins regarding polishing methods e11.74 clinically unacceptable . according to tukey kramer test
, there was a statistical significant difference on color 
 stability among n1 , n2 and n3 resins ( p<0.01 ) regardless of accelerated aging and 
 polishing procedures .
the n1 acrylic resin exhibited the greatest color stability 
 ( e=10.74 ) , while n3 acrylic resin exhibited the worst color stability ( e=13.97 ) , with 
 statistically significant difference between them ( p<0.01 ) ( table 2 ) .
mean values of the color derivatives ( e ) of resins n1 , n2 and n3 means followed by the same capital letter do not differ statistically among 
 them ( p0.01 ) . considering the evaluation of color stability among the different periods of accelerated 
 aging
, there was significant difference only between 252 and 1008 h ( p<0.01 ) , 
 independent to the acrylic resin and polishing procedures .
the period of 504 h 
 ( e=12.26 ) did not differ in relation to the other periods ( table 3 ) .
there was no significant difference between the two types 
 of polishing procedures , regardless of acrylic resin and accelerated aging ( p>0.01 ) 
 ( table 4 ) .
mean values of the color derivatives ( e ) of different periods of accelerated 
 aging means followed by the same capital letter do not differ statistically among 
 them ( p<0.01 ) mean values of the color derivatives ( e ) of the two polishing techniques means followed by the same capital letter do not differ statistically between 
 them ( p0.01 ) .
when analyzing table 1 , it could be verified 
 that the derivative of shade ( e ) for all specimens during the first reading ( 252 h ) was 
 higher than zero , indicating color alteration by spectrophotometric analysis .
the intrinsic factors involve discoloration of the 
 material itself , with alteration in its matrix .
extrinsic factors , such as absorption and adsorption of stains 
 may also cause discoloration .
in addition to this , other factors are 
 responsible for color instability , such as : accumulation of stains ; water sorption ; 
 infiltration ; surface roughness ; chemical degradation by use ; oxidation during 
 double - carbon reactions , producing peroxide compounds ; and continuous formation of 
 pigments due to degradation of products .
there is a great deal of disagreement in the literature about establishing a clinically 
 unsatisfactory value for color derivation .
some authors established 3.5 as an unsatisfactory value while other 
 researches stated 3.7 . in present 
 study , we observed that only color derivatives greater than or equal to 11.74 presented 
 visual alteration ( table 1 ) .
, it appears that there is no way of 
 establishing a universal value clinically unacceptable for e , since each material has 
 particular physical and chemical properties , and behaves in a different manner , 
 depending on the medium in which it is found .
n3 , the 
 highest values of variation being for resin n3 ( table 
 2 ) . this difference in color derivation could be cause of intrinsic 
 pigmentation of each resin .
it is known that the coloring of these resins is achieved by 
 the addition of methylmethacrylate based pigments , which could have participated in 
 polymerization reaction , interfering in formation of polymeric chains , physically or 
 chemically altering the properties of the material . with altered polymeric chains ,
higher influence of accelerated 
 aging on these materials could have occurred , particularly in resin n3 ( table 1 ) , which presented the highest color 
 alteration values . in this study ,
accelerated artificial aging produced significant color alteration in 
 studied resins between 252 and 1008 h ( table 3 ) . 
 in results
it was verified that use of ultraviolet light , humidity and heat were 
 causative factors of polymer degradation .
this could occur because the majority of 
 polymers have functional groups in their molecular chains , which absorb ultraviolet 
 light ; among them are the carbonyl c = o group , aromatic rings , and c = c bonds .
when a 
 polymer molecule absorbs ultraviolet light through functional groups , this energy leads 
 to a more unstable state of the structure .
this excess of energy can be deactivated by various paths , such as 
 transferring the excitation to another molecule , and thus reestablishing itself .
these 
 groups can return to their original state by stages , reemitting the excess of energy in 
 a longer wavelength , such as visible light or heat .
if the excited molecule disperses 
 the excess of energy by some means , there would be rupture , that is , photochemical 
 degradation , and these factors contribute concomitantly to the appearance of 
 deterioration , such as loss of color or brightness , loss of opacity , appearance of 
 cracks and stiffness .
regarding to the polishing method , it was verified that the acrylic resin specimens 
 submitted to mechanical and chemical polishing presented no significant difference in 
 the chromatic stability values ( table 4 ) .
this 
 appears to indicate that the polishing method used had no influence on the color 
 stability of the test specimens , not even after accelerated aging .
therefore , chemical 
 polishing was performed without a detrimental effect to the physical and chemical 
 properties of the acrylic resins .
( 2001 ) verified 
 surface roughness in acrylic resins submitted to mechanical and chemical polishing 
 followed by mechanical brushing .
this result may contraindicate the chemical burnishing 
 for ocular prosthesis since roughness promotes the accumulation of microorganism . 

( 1998 ) , gonalves , et 
 al . ( 2008 ) and bollen , et 
 al . ( 1997 ) emphasized the 
 importance of this point , as the roughness of a denture fitting surface can help 
 determine its colonization by different microorganisms .
the importance of the 
 disinfection of ocular prostheses is to decrease the number of microorganisms before and 
 after using cleansing solutions .
( 2003 ) cited the iso 
 1567 , which established that 4.5% would be an acceptable limit for residual monomer for 
 this kind of resin .
( 2008 ) verified that the amount of residual monomer after 24 h of 
 storage reduced from 5.47% to 3.84% , which is a value considered as biocompatible to 
 tissues .
additional studies are necessary to confirm the long - term behavior of chromatic 
 stability after both polishing techniques .
it can be concluded that the n1 acrylic resin displayed the greatest color stability 
 both by visual analysis and by spectrophotometric analysis when compared to the other 
 resins .
the accelerated aging by ultraviolet radiation and water condensation influenced 
 significantly the color change of the ocular acrylic resins while the polishing 
 technique did not affect the color stability of these resins .
the authors would like to thank pibic / cnpq ( brazil ) for the financial support granted 
 to the project .
 esthetics and durability of materials used to fabricate artificial eyes has been an 
 important eissue since artificial eyes are essential to restore esthetics and 
 function , protect the remaining tissues and help with patients ' psychological 
 therapy . 
 however , these materials are submitted to degrading effects of environmental 
 agents on the physical properties of the acrylic resin.objectivethis study assessed the color stability of acrylic resins used to fabricate sclera 
 in three basic shades ( n1 , n2 and n3 ) when subjected to accelerated aging , 
 mechanical and chemical polishing . 
 material and methodsspecimens of each resin were fabricated and submitted to mechanical and chemical 
 polishing . 
 chromatic analysis was performed before and after accelerated aging 
 through ultraviolet reflection spectrophotometry . 
 resultsall specimens revealed color alteration following polishing and accelerated aging . 
 the resins presented statistically significant chromatic alteration ( p<0.01 ) 
 between the periods of 252 and 1008 h. conclusionsboth polishing methods presented no significant difference between the values of 
 color derivatives of resins . 
INTRODUCTION
MATERIAL AND METHODS
RESULTS
DISCUSSION
CONCLUSIONS
ACKNOWLEDGEMENTS