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ORIGINAL ARTICLE
Year : 2016  |  Volume : 2  |  Issue : 1  |  Page : 11-16

Quantitative assessment of surface microhardness of esthetic restorative materials after exposure to different immersion regimes in a cola drink: An in vitro study


1 Department of Pedodontics and Preventive Dentistry, Punjab Civil Medical Services Dental, Mohali, Punjab, India
2 Department of Pedodontics and Preventive Dentistry, Government Dental College and Hospital, Patiala, Punjab, India
3 Department of Pedodontics and Preventive Dentistry, Nagpur, Maharashtra, India

Date of Web Publication14-Mar-2017

Correspondence Address:
Navroop Kaur Bajwa
House No. 1662, Sector-70, Mohali, Punjab
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2454-3160.202122

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  Abstract 

Objective: To compare the effect of different immersion regimes in a cola drink on surface microhardness of esthetic restorative materials.
Subjects and Methods: Two hundred samples were grouped into four equal groups of fifty samples each: Group I - conventional glass ionomer, Group II – resin-modified glass ionomer, Group III - polyacid-modified resin composite, and Group IV - composite resin. Each group was further subdivided into five subgroups of ten samples each: Subgroup A - samples were kept immersed in artificial saliva. Subgroup B - samples were immersed in cola drink once a day. Subgroup C - samples were immersed in cola drink, three times a day. Subgroup D - Samples were immersed in cola drink five times a day. Subgroup E - samples were immersed in cola drink ten times a day. Each immersion lasted 5 min. The immersion protocol was repeated for 7 days.
Results: Maximum microhardness was seen in composite resin samples followed by conventional glass ionomer, polyacid-modified resin composite, and least microhardness was seen in resin-modified glass ionomer.
Conclusion: Resistance to change in surface microhardness was seen in the following sequence: Composite resin > polyacid-modified resin composite > resin-modified glass ionomer > conventional glass ionomer.

Keywords: Cola drink, immersion regimes, restorative materials, surface microhardness


How to cite this article:
Bajwa NK, Pathak A, Jingarwar MM. Quantitative assessment of surface microhardness of esthetic restorative materials after exposure to different immersion regimes in a cola drink: An in vitro study. Saint Int Dent J 2016;2:11-6

How to cite this URL:
Bajwa NK, Pathak A, Jingarwar MM. Quantitative assessment of surface microhardness of esthetic restorative materials after exposure to different immersion regimes in a cola drink: An in vitro study. Saint Int Dent J [serial online] 2016 [cited 2019 Jul 23];2:11-6. Available from: http://www.sidj.org/text.asp?2016/2/1/11/202122

Urbanization and industrialization have led to a change of lifestyle through the decades, and the total amount and frequency of consumption of acidic foods and drinks have increased.[1] Cola drink is a frequently consumed soft drink by children as well as the youth population. Tooth repair is increasingly being performed with tooth-colored restorative materials which may have varying degrees of longevity.

Restorative filling materials used in dentistry are required to have long-term durability in the oral cavity.[2] The aqueous environment of the oral cavity, the low pH due to acidic food or cariogenic microorganisms, and ionic composition of saliva are important parameters which may influence the physical and mechanical characteristics of restorative dental materials.

Under acidic conditions, restorative materials suffer degradation over time, which can be predicted by changes in the surface characteristics such as decrease in hardness. Therefore, this in vitro study was conducted to evaluate and compare the effect of cola drink (Coca Cola ®), on the surface microhardness of tooth-colored restorative materials after exposure to different immersion regimes.


  Subjects and Methods Top


A total of 200 samples were prepared using a ring-shaped brass mold of diameter 10 mm × 2 mm height. This ensured the standardization of shape and size of each sample. Samples were divided into four equal groups of fifty samples each on the basis of the restorative material used.

  • Group I - Conventional glass ionomer cement (GC Fuji II) (See [Table 1])
  • Group II - Resin-modified glass ionomer cement (GC Fuji II LC) (See [Table 1])
  • Group III - Polyacid-modified resin composite (Dyract Extra) and (See [Table 1])
  • Group IV - Composite resin (Filtek Z 350) (See [Table 1]).
Table 1: Description of the restorative materials used in the study

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Sufficient amount of material was added into the mold and pressed between mylar matrix strips supported by glass slides on either side [Table 1]. Only a polyester strip and a glass slab were used while making the samples with the intention of obtaining a smooth and flat surface. Before light polymerization or setting, the glass slab was replaced with a glass slide. Any form of additional polishing was avoided as it could lead to an increase in surface roughness.

Each group was further divided into five subgroups of ten samples each (See [Table 2]).
Table 2: The division of the samples into respective groups and further sub groups according to the immersion protocol

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  • Subgroup A (control subgroup) - Each sample was kept completely immersed in 25 ml of artificial saliva in an airtight container at room temperature for 7 days [Table 2]. Artificial saliva in each container was changed every day
  • Subgroup B - Samples were immersed in 25 ml of cola drink once a day
  • Subgroup C - Samples were immersed in 25 ml of cola drink, three times a day
  • Subgroup D - Samples were immersed in 25 ml of cola drink, five times a day
  • Subgroup E - Samples were immersed in 25 ml of cola drink, ten times a day.


In this study, the samples were completely immersed in cola drink (Coca Cola ®) for 5 min in contrast to in vitro studies that had employed extremely long time intervals of immersion in eroding solutions ranging from 15 to 180 min. On an average, a child can be assumed to consume soft drinks once or thrice during the time; he is awake in a day. The waking period in children is assumed to be approximately 12 h in a day. Hence, the immersions were evenly distributed over a 12 h period. Toward the higher end of the continuum, soft drink consumption may be five times or ten times per 12 h. Hence, this study used the above frequencies of consumption as the yardsticks for deciding the number of immersions per day in vitro. Before and after immersion in the cola drink (Coca Cola ®), samples in these subgroups were copiously rinsed in 0.1 M phosphate-buffered saline pH = 7.2.[3]

This was done for the following reasons:

  1. To buffer the effect of cola drink (Coca Cola ®) after the prescribed exposure time
  2. To return the pH to a neutral level once the exposure was over and
  3. To avoid prolonged insult to the materials while they were stored in the artificial saliva.


To ensure complete contact of the samples with the cola drink, the container was agitated continuously during the immersion.[4] This also simulated the intraoral conditions because at least some degree of agitation occurs intraorally as well while drinking. Cola drink used for immersion of samples was changed after each immersion. The samples in the experimental subgroups were kept immersed in artificial saliva in airtight containers when not subjected to the immersion regime. Artificial saliva was preferred over distilled water as the storage medium to simulate the oral environment and provide data closer to reality and reproduce clinical situations. Composition of artificial saliva as proposed by Klimek J et al., 1982][5] was used - 0.002 g ascorbic acid, 0.030 g glucose, 0.580 g NaCl, 0.170 g CaCl2, 0.160 g NH4 Cl, 1.270 g KCl, 0.160 g NaSCN, 0.330 g KH2 PO4, 0.200 g urea, 0.340 g Na2 HPO4, and 2.700 g mucin in 1000 ml distilled water. The solution was further titrated with a phosphate buffer of 26.4 ml 0.06 M Na2 HPO4.2H2O and 7.36 ml 0.06 M KH2 PO4.

As the greatest change in physical properties has been shown to occur within the first 7 days of exposure to solutions, the immersion protocol was carried on for 7 days.[6]

Vickers microhardness measurement was done for all the samples after the immersion protocol was completed. The microhardness tester was standardized before initiation of the testing procedure. The testing parameter of 100 g of force for 15 s initiated no cracks on the surface of the material, thereby, providing a size of indentation that allowed measurement of surface hardness of the samples.


  Results Top


The data obtained was compiled and put to statistical analysis. The mean and standard deviation were calculated for the comparison of microhardness of all subgroups of Groups I, II, III, and IV [Figure 1]. One-way ANOVA test was used as the study had more than two groups, and the data obtained was normally distributed. For intergroup comparison, multiple comparison post hoc test (Turkey honest significant difference) was employed.
Figure 1: Comparison of the effect of different cola drink immersion regimes on microhardness (Vickers hardness) of restorative materials tested (mean values), standard deviation included

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Maximum microhardness was seen in Group IV composite resin samples (Filtek Z 350) followed by Group I conventional glass ionomer cement (GC Fuji II), Group III polyacid-modified resin composite (Dyract Extra), and least microhardness was seen in Group II resin-modified glass ionomer cement (GC Fuji II LC).

Resistance to change in microhardness was seen in the following sequence: Group IV Composite resin (Filtek Z 350) > Group III polyacid-modified resin composite (Dyract Extra) > Group II resin-modified glass ionomer cement (GC Fuji II LC) > Group I conventional glass ionomer cement (GC Fuji II).

In Group I (conventional glass ionomer cement-GC Fuji II), highly significant decrease (P < 0.01) was observed in subgroups IC (3 immersions/day), ID (5 immersions/day), and IE (10 immersions/day) while subgroup IB showed insignificant decrease (P > 0.05) on comparison with subgroup IA (control). As the number of immersions increased, each consecutive subgroup showed highly significant decrease in microhardness [Table 3]. In Group II (resin-modified glass ionomer cement-GC Fuji II LC), subgroups IIC (3 immersions/day), IID (5 immersions/day) and IIE (10 immersions/day) showed significant decrease while subgroup IIB (1 immersion/day) showed insignificant decrease (P > 0.05) on comparison with subgroup IIA (control). As the number of immersions increased, each consecutive subgroup showed highly significant decrease (P < 0.01) in microhardness except subgroup IIB (1 immersion/day) and subgroup IID (5 immersions/day) which showed insignificant decrease (P > 0.05) on comparison with subgroup IIC (3 immersions/day) and subgroup IIE (10 immersions/day), respectively [Table 4]. In case of Group III (polyacid-modified resin composite-Dyract Extra), subgroups IIID (5 immersions/day) and IIIE (10 immersions/day) showed highly significant decrease (P < 0.01) while subgroup IIIB (1 immersion/day) and IIIC (3 immersions/day) showed insignificant decrease (P > 0.05) on comparison with subgroup IIIA (control). As the number of immersions increased, each consecutive subgroup showed significant decrease (P < 0.01) in microhardness except subgroup IIIB (1 immersion/day) and subgroup IIID (5 immersions/day) which showed insignificant decrease (P > 0.05) on comparison with subgroups IIIC (3 immersions/day) and IIIE (10 immersions/day), respectively [Table 5]. In case of Group IV composite resin (Filtek Z 350), subgroups IVB (1 immersion/day), IVC (3 immersions/day), IVD (5 immersions/day), and IVE (10 immersions/day) showed insignificant decrease (P > 0.05) on comparison with IVA (control). As the number of immersions increased, each consecutive subgroup showed insignificant decrease (P > 0.05) in microhardness [Table 6].
Table 3: Mean difference in microhardness (HV) among various sub groups of group I- conventional Ionomer cement (GC Fuji II)

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Table 4: Mean difference in microhardness (HV) among various sub groups of group II- resin modified glass ionomer cement (GC Fuji II LC)

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Table 5: Mean difference in microhardness (HV) among various sub groups of group III- polyacid modified resin composite (Dyract extra)

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Table 6: Mean difference in microhardness (HV) among various sub groups of group IV- composite resin (Filtek Z 350)

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  Discussion Top


Hardness is defined as the resistance to permanent indentation or penetration. Surface hardness correlates well to compressive strength, and abrasion resistance which ultimately determine the longevity of the restoration.

It has been established that the erosive potential of an acidic solution is related to its pH, titratable acidity, and buffer capacity. pH of Coca Cola ® is low, i.e., ~2.5. In addition, this soft drink has in its composition an inorganic and strong acid, phosphoric acid. Thus, the association of a low pH and the presence of a strong inorganic acid could have caused a more aggressive attack on the surface of restorative materials, hence leading to an increase in the surface roughness.

Maximum microhardness was seen in the control subgroup of composite resin samples followed by conventional glass ionomer cement; polyacid-modified resin composite and least microhardness were seen in resin-modified glass ionomer cement.

These findings are in congruence with those of Gladys S et al., 1997,[7] who conducted a study to determine several physical and mechanical properties of eight hybrid esthetic restorative materials in comparison with two conventional glass ionomers one micro-filled and one ultrafine compact-filled resin composite. Resin-modified glass ionomers revealed the lowest degree of hardness; polyacid-modified resin composite showed intermediate values and conventional glass ionomers, composite resin showed the maximum values.

The present study showed that greater number of immersions led to more accentuated decrease in microhardness of the restorative materials tested. These findings are in conjunction with those of Abd El-Latiff SB et al., 2007,[3] who conducted a study to assess the change in microhardness of enamel and three anterior restorative materials after exposure to eroding immersion in acidic beverages for 8 days. Results showed that increasing the frequency of immersion in the test beverages significantly increased the erosive effect.

These findings are further supported by the study conducted by Vanga V Narsimha, 2011,[8] who evaluated the change in microhardness of resin-modified glass ionomer cement and polyacid-modified resin composite after immersing them in a cola soft drink under low (one immersion), medium (five immersions), and high immersion (ten immersions) regimes for 7 days. Results revealed that surface microhardness was least affected in low immersion regime while medium and high immersion regimes showed a significant decrease in microhardness of restorative materials evaluated.

The results of this study showed a generalized reduction in surface microhardness of all the restorative materials tested. However, the greatest decrease in surface microhardness was seen in Group I - conventional glass ionomer group followed by Group II – resin-modified glass ionomer. Group III – polyacid-modified resin composite and Group IV - composite resin was the most resistant to the degradative effect of cola drink. These findings are in accordance with the findings of Francisconi LF et al., 2008,[9] who evaluated the effect of erosive pH cycling on the surface microhardness of different restorative materials and bovine enamel restored with these materials. Results showed that conventional glass ionomer and resin-modified glass ionomer cement showed the maximum degradative change in microhardness. Composite resin and amalgam showed negligible change in microhardness. This observation can be explained by dissolution of the glass ionomer matrix.[10]

These results were in accordance with those of Honorio HM et al., 2008,[11] who stated that erosive pH cycling in Coca Cola ® resulted in a significantly higher reduction in %SMHC for the glass-ionomer cements (both resin-modified glass ionomer and conventional glass ionomer) when compared with both the control medium (artificial saliva) and other restorative materials (composite resin and amalgam).

Silva et al., 2007[12] also stated that the immersion of resin-modified glass ionomer cements in aqueous solutions with low pH reduces their hardness, depending on several factors, including the time and means of immersion and the composition of the material.[12] Thus, the amount of resin matrix (HEMA) in the cement influences the water absorbed by these materials. The absorbed water may inhibit the secondary curing reaction at the superficial layer of the cement, reducing its hardness. These results were also confirmed by Abu-Bakr et al., 2000,[13] they showed that soft drinks affect the microhardness and surface texture of restorative materials. They found that Resin-modified glass-ionomer cement immersed in coca-cola soft drink exhibited significantly lower hardness values than when immersed in deionized water for 7 days.

Group II resin-modified glass ionomer (GC Fuji II LC) showed better resistance to degradation to decrease in microhardness when compared to Group I conventional glass ionomer cement (GC Fuji II).

De Gee et al., 1996[14] reported that in case of resin-modified glass ionomer cement, the interpenetrating matrices of cross-linked polyalkanoate and poly-HEMA are not chemically integrated but merely entangled with one another.[14]

However, the greater instability of resin-modified glass ionomer cement after immersion in acidic solutions when compared with polyacid-modified resin composites and composite resin could be explained by matrix dissolution in the periphery of the glass particles of glass ionomer.

On comparison of both the hybrid materials, after subjecting to various immersion regimes, the reduction in microhardness is greater in resin-modified glass ionomer as compared to polyacid-modified resin composite (Dyract Extra) probably due to difference in matrix formation. The higher deterioration of resin-modified glass ionomers can be explained as follows. The set cement of resin-modified glass ionomer materials has a polyalkenoate network loosely entangled with polymer chain of the HEMA monomers. The coherence of filler particles embedded in the interpenetrating matrices of polyalkenoate and polymers is inferior. This may be caused by the partial replacement of the rigid polyalkenoate network by the flexible polymer chains.[15]

According to Hse et al., 1999,[16] and Torstenson et al., 1988,[17] a probable reason for a lower difference in base line and final Vickers hardness number of polyacid-modified resin composite when compared to resin-modified glass ionomer could be the higher resin component of polyacid-modified resin composite than resin-modified glass ionomer.

However, significant reduction in microhardness was seen in subgroups IIID and IIIE in case of polyacid-modified resin composite (Dyract Extra). This can be explained by studies conducted by Watts et al., 1995,[18] who proved that these materials are surface-softened by an acidic environment. The acidic attack resulted in a loss of structural ions from the glass phase of polyacid-modified composites. Watts et al., 1995,[18] also reported that at pH 3–6, hardness of compomers decreased by a factor of 5. The pH of Cola soft drink is pH <3. Hence, more frequent exposure to this low pH could have caused generalized reduction of microhardness.

On comparison of polyacid-modified resin composite (Dyract Extra) and composite resin (Filtek Z 350) groups with glass ionomer based materials (GC Fuji II and GC Fuji II LC), both materials showed greater resistance to change in microhardness as compared to glass ionomer based restorative materials. According to Cattani-Lorente MA, 1999,[19] the properties of polyacid-modified composite resins resemble more with those of resin composites than those of glass-ionomer materials. However, greater decrease in microhardness was seen in polyacid-modified resin composite (Dyract Extra) as compared to composite resin (Filtek Z 350). This can be reasoned as follows.

According to Geurtsen W et al., 1998,[20] contrary to nano-filled composite resins, polyacid-modified composite resins generally contain more organic matrix and thus may be more susceptible to water absorption and a subsequent surface disintegration in a low pH aqueous environment. Vickers hardness is directly correlated with the filler content and hence, polyacid-modified composite resin having relatively lower filler content was more susceptible to decrease in microhardness.

In the present study, when comparing microhardness of Filtek Z 350 composite resin in all the subgroups, the storage in artificial saliva produced the highest surface hardness. This finding agrees with Martins de Oliveira et al., 2010,[21] who explained this result by the deposition of minerals on the surface of specimens, resulting in the formation of film probably composed of insoluble calcium phosphate compounds.

In Group IV composite resin (Filtek Z 350), all the experimental subgroups showed insignificant decrease on comparison with the control subgroup. This result agrees with Lim et al., 2002,[22] and Han et al., 2008,[23] who found that any portion of the organic matrix resin which is insufficiently polymerized can get dissolved by acidic solutions, and filler particles can thus easily fall out.

Furthermore, the period of exposure to acidic environment is more crucial than the volume of beverage consumed. Hence, greater decrease in microhardness of samples was observed when subjected to high-frequency immersion regime.


  Conclusion Top


The results of the present study tend to support the suggestion that the detrimental effect of cola drink (Coca Cola ®) on the surface properties of restorative materials is related to the frequency of consumption. In addition, in clinical decision-making, a material with less resistance to degradative effects of acidic beverages may not be suitable for patients who have the habit of frequent consumption of soft drinks. Hence, the restorative material of choice for the patients with frequent intra-oral acidic challenges should be either composite resin or polyacid-modified resin composite as these materials present minor alterations under high erosive challenges as well.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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Abu-Bakr N, Han L, Okamoto A, Iwaku M. Changes in the mechanical properties and surface texture of compomer immersed in various media. J Prosthet Dent 2000;84:444-52.  Back to cited text no. 13
    
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  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]



 

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