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Oliveira et al investigated the surface roughness

Oliveira et al. (2008) investigated the surface roughness of HC and AC resins after submitting them to mechanical polishing (MP) and chemical polishing (CP). They concluded that MP produced lower surface roughness mean values than CP and is not influenced by the acrylic resin type. In contrast, Berger et al. (2006) reported that AC acrylic resin performed less favorably in terms of surface roughness than MP. Rahal et al. (2004) evaluated the influence of MP and CP on surface roughness of HC and AC resins. Their results lead them to conclude the following: MP produced smoother surface than CP and surface roughness was not influenced by acrylic resin type. However, CP results were dependant on acrylic resin type. Radford et al. (1999) reported that acrylic resin has been less frequently investigated for its surface roughness and effect of polishing than other dental materials.

Materials and methods

Results
The mean, standard deviation, minimum and maximum values of surface roughness between MP and CP for HC and AC materials are presented in Table 1. The mean roughness values for the combination of polishing technique and material in decreasing order are: CP-HC=1.4132μm; CP-AC=1.3494μm; MP-AC=0.7364μm and MP-HC=0.6333μm.
Two way ANOVA showed that surface roughness was influenced by the polishing procedures significantly (P<.0001) and not by the acrylic resin materials (see Table 2). There was no significant difference between MP-AC and MP-HC as well as between CP-AC and CP-HC. However, a significant difference in surface roughness was found between all other combinations (P<.0001) as shown in Table 3.
Discussion
Polished surface of acrylic resin denture material is important, as it affects the oral health of serine protease inhibitor that are in direct contact. Rough surfaces of oral appliances promote colonization of bacteria and plaque accumulation (Quirynen et al., 1990; Verran and Maryan, 1997; Bollen et al., 1997). The threshold surface roughness for bacterial attachment was reported to be 0.2μm (Quirynen et al., 1990). Surface roughness values more than 0.2μm may promote plaque formation. Bollen et al. (1997) and Radford et al. (1999) reported that high concentration of bacterial colonization occurs if the surface roughness value is greater than 2.0μm. The authors considered that characterization of smooth acrylic resin surface may vary between 0.03μm and 0.75μm depending upon the technique used for finishing and polishing (Bollen et al., 1997; Radford et al., 1999). The results of this study showed that the mean surface roughness values for MP of HC acrylic resin was 0.6333μm and 0.7364μm for CP of AC acrylic resin. The mean values found in this study agree with the range reported by Busscher et al. (1984), Oliveira et al. (2008) and Radford et al. (1999).
Furthermore, the results of this study demonstrated that the surface roughness of MP-HC acrylic resin was not significantly different from MP-AC acrylic resin although superior surface characteristics of HC acrylic resin may be expected due to higher degree of conversion of monomer compared to AC acrylic resin which is concurring with that of Oliveira et al. (2008). Also, MP produced lower surface roughness values than CP for both types of acrylic resins. This finding is in agreement with the results of other studies (Ulusoy et al., 1986; Rahal et al., 2004; Oliveira et al., 2008; Berger et al., 2006). These results were expected because the abrasive mechanical action progressively removes surface notches during polishing.
The polishability of the surface with chemical polishing method may be explained by the penetration of the polishing liquid, which contains methyl-methacrylate monomer molecules, through the superficial polymeric chain of acrylic resin breaking the secondary bonds that join them, promoting a final plasticizing effect of the acrylic resin surface. This superficial layer has no effect on the under lying irregularities caused by finishing procedures (Rahal et al., 2004). It is interesting to note that there was no significant difference in surface roughness values between HC and AC acrylic resin specimens subjected to mechanical polishing, although the surface of AC specimens being more porous than HC resin where a higher surface roughness should be expected.