Journal of Dental Implants
   About JDI | Search | Ahead of print | Current Issue | Archives | Instructions | SubscribeLogin 
Users Online: 537  Wide layoutNarrow layoutFull screen layout Home Print this page  Email this page Small font size Default font size Increase font size


 
 
Table of Contents
ORIGINAL ARTICLE
Year : 2015  |  Volume : 5  |  Issue : 1  |  Page : 25-30

Removal force of cast copings to abutments with three luting agents


Department of Oral Implantology, School of Dentistry, Cayetano Heredia University, Lima, Peru

Date of Web Publication2-Apr-2015

Correspondence Address:
Frank R Mayta-Tovalino
Department of Oral Implantology, School of Dentistry, Cayetano Heredia University, Lima
Peru
Login to access the Email id

Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0974-6781.154426

Rights and Permissions
   Abstract 

Purpose: The purpose of this study was to evaluate the removal force of cast copings over abutments cemented with 3 dental cements.
Methods: The experiment used 36 abutments divided into six groups (n = 6) Cast copings with CeraOne cemented with zinc oxide-free eugenol; with glass ionomer; with zinc phosphate and solid 5-mm-high abutments cemented with the same cements wax loops (3 mm in diameter) were made and added to the occlusal surface to allow for tensile testing, and the copings were cast in cobalt-chromium alloy using the lost wax method. The copings were sandblasted with 50-μm aluminum oxide particles to prevent irregularities and cleaned with acetone. The copings were cemented with a 49-N load for 10 min. Subsequently, they were separated with a universal testing machine (Tensometer; Monsanto) at a rate of 5 mm/min until the detachment. The effects of cement and abutment type were evaluated using the means and standard deviations of the loads and were analyzed with ANOVA and the Tukey test (α =0.05).
Results: The abutments with a height of 5.5 mm had a statistically significant (P < 0.001) higher tensile strength with a mean of 84.60 N (±8.82) than that of the CeraOne abutments with an average of 36.09 N (±11.66) when cemented with zinc oxide.
Conclusions: We conclude that the type of cement and abutment can significantly influence the retention of implant-supported crowns. The results do not suggest that any one type of cement is better than another, but they rank cements in order of retention capacity. Zinc phosphate provided the greatest retention while the eugenol-free zinc oxide provided the least (P < 0.001).

Keywords: Dental implant, luting agent, tensile strength


How to cite this article:
Mayta-Tovalino FR, Ccahuana-Vasquez VZ, Rosas-Díaz JC. Removal force of cast copings to abutments with three luting agents. J Dent Implant 2015;5:25-30

How to cite this URL:
Mayta-Tovalino FR, Ccahuana-Vasquez VZ, Rosas-Díaz JC. Removal force of cast copings to abutments with three luting agents. J Dent Implant [serial online] 2015 [cited 2019 Mar 22];5:25-30. Available from: http://www.jdionline.org/text.asp?2015/5/1/25/154426


   Introduction Top


Treatment with dental implants to replace missing teeth is well documented. The prosthetic restorations are connected to the implant abutment screws or are cemented. The use of cemented implant restorations has gained in popularity over retaining screws because of their improved esthetics, [1],[2] passive fit, [3] reduced cost, and ease of fabrication with a simple laboratory technique.

Reversibility is an essential factor in these restorations. In the case of screw loosening, fracture, or perimplantitis, removing single or multiple crowns can mean the loss of the implant abutments. The abutments can be prefabricated, or custom made in stainless alloy, titanium, or zirconia ceramic. According to Hebel and Gajjar, [4] the form of the abutment, the material of which the abutment is made, and the type of luting agent can affect the longevity of these restorations.

Although screwed crowns are easier to remove if necessary, [5] the use of interim cements can add to the reversibility of cemented prostheses, according to Henry et al. [6] Levine et al. [7] and Santosa et al. [8] few published data are currently available regarding their long-term success. In particular, the choice of a suitable cement for a specific clinical situation is still based more on the experience of the clinician than on scientific data. Ideally, the cement must be strong enough to hold the implant prosthesis in place indefinitely but sufficiently weak to allow the clinician to remove it if necessary. Unfortunately, no evidence is available as to which cement is indicated or about the likely behavior of each cement. [2] In addition, implant abutments come in a wide variety of shapes and heights in order to improve aesthetics, preserve the health of the soft tissues, and provide adequate retention for the prostheses, all of which are fundamental to successful treatment.

Recommendations as to whether to use interim or definitive cements for implant-supported prostheses vary. [9] Water-based cements produce a variety of results and often do not behave predictably. Some studies suggest the use of interim cement for multiple implant prostheses and definitive cement for unit prostheses. The use of interim cements is considered suitable for a restoration likely to be removed in the future. In some situations where reversibility is not required, a permanent dental resin-based cement could be used. [10] For that reason, the purpose of this study was to evaluate the removal force of cast copings over abutments cemented with three dental cements.


   Materials and methods Top


Specimen preparation

Thirty-six abutments were distributed into six groups (n = 6). Cast copings with CeraOne abutments cemented with eugenol-free zinc oxide; with glass ionomer; and with zinc phosphate and solid 5-mm-high abutments cemented with the same luting agents. All specimens were embedded in acrylic resin-based on previous studies. [11],[12],[13]

Abutment casting

CeraOne abutment analogs were used (NACONIH code fix Titanium Implant System SP, Brazil) and 5.5-mm-high solid abutments (CMAAC 5.5 code, Titanium Fix Implant System SP, Brazil) to which a loop was added to the occlusal surface of each coping with a wax sprue (no. 10) to allow for tensile testing (Breeding et al., 1992). The copings were cast in a metal ceramic Co-Cr alloy (Metalloy CC Germany). The castings were sprued with a 5-mm diameter wax wire (Wax wire for placement of sprues, Dentaurum, Germany) and invested in phosphate-bonded investment (Hi -Temp, Whip Mix Co. Louisville, KY, USA) with an unrestricted expansion technique in a 9-cm diameter flexible ring (Dentaurum - Germany) [Figure 1]. They were cast with the lost wax method.
Figure 1: The cast copings were sprued with 5-mm wax sprues

Click here to view


Sandblasting of cast copings

The castings were separated from the sprue with a carborundum disk, sandblasted, and their external surface refined slightly to produce intimate contact with the fixtures of the universal testing machine. The intaglio of the copings was sandblasted with 50-μm aluminum oxide particles (Ivoclar Vivadent). Before cementation, the abutments and copings were cleaned of impurities with distilled water for 15 min in an ultrasonic bath. The intaglio was examined for irregularities with a stereomicroscope (Leica Microsystems LAS EZ version 2.0.0 Switzerland) at ×80 magnification.

One interim cement (eugenol-free zinc oxide) and two definitive cements (glass ionomer and zinc phosphate) were used [Table 1]. Each cement was used as directed by the manufacturer. All cast copings were cemented by a calibrated operator. The copings for all specimens were cemented with a standardized force of 49 N applied along the axis of the abutment pair within 10 min, according to American National Standards Institute/American Dental Association. Specification No. 96. [14] Finally, all specimens were stored for 24 h in distilled water at 37°C.
Table 1: Luting agents used in the study

Click here to view


Removal force test

The specimens were separated with a universal testing machine (Tensometer; Monsanto UK) in the Materials Laboratory of Pontificia Universidad Catolica del Peru. All specimens were subjected to a uniaxial load rate of 5 mm/min to cause the separation of the copings on the replicas [Figure 2] and [Figure 3]. The force values were recorded and expressed in newtons (N).
Figure 2: All specimens were subjected to a uniaxial load rate of 5 mm/min to remove the copings from the abutments. Removal force was expressed in newtons.

Click here to view
Figure 3: Titanium abutment after tensile test

Click here to view


Statistical analysis

For the univariate analysis, we obtained the mean and standard deviation, which were recorded in a frequency table. In addition, the sample was subjected to the Shapiro-Wilk test and found to have a normal distribution. For the bivariate analysis, the Tukey test and ANOVA test were used, establishing a significance level of P < 0.05. Statistical analysis was performed with Stata V° 12 (Data analysis and Statistical software).


   Results Top


We found that the 5.5-mm-high solid cemented abutments cemented with zinc phosphate required a mean force of 400.49 N ± 9.80 N to detach the coping from the cemented casting. In addition, a maximum value of 499.50 N and a minimum value of 186.20 N were obtained. The specimens had normal distribution [Table 2]. A comparison of in vitro resistance with tensile strength found that the removal force was higher for the 5.5-mm-high solid abutment group and was statistically significant (P < 0.001). In addition, the CeraOne abutments cemented with glass ionomer showed a mean force of 48.51 N as compared with the 5.5-mm-high solid abutments cemented with glass ionomer at 56.02 N; the difference was not statistically significant (P = 0.279). The CeraOne abutments cemented with zinc phosphate required a mean force of 459.62 N as compared with the 5.5-mm-high solid abutments at 400.49 N to displace the coping cemented on the abutment; the difference was not statistically significant (P = 0.355) [Table 2].

In assessing the in vitro removal force of cast cylinders and CeraOne abutments cemented with eugenol-free zinc oxide, glass ionomer, and zinc phosphate, it was found that CeraOne abutments had an average of 36.09, 48.51 and 459.62 N, respectively. The difference in the force required to achieve detachment among these groups was statistically significant (P < 0.001). Moreover, in assessing the resistance to tensile strength of cast cylinders of 5-mm-high solid abutments cemented with eugenol-free zinc oxide, glass ionomer, and zinc phosphate, it was found that 5.5-mm-high solid abutments had a mean of 84.60, 56.02 and 400.49 N, respectively. The difference in the removal force among these groups was statistically significant (P < 0.001) [Table 1].
Table 2: Mean tensile retention strength values of the three luting agents

Click here to view



   Discussion Top


The success of oral rehabilitation with dental implants depends not only on osseointegration, but also on maintaining the integrity of the connection of the prosthetic restoration to the implant. [15] Implant restorations can be screw-retained, cemented, or a combination of both. Numerous advantages and disadvantages have been described for each type of retention. [16],[17] Dental cements can be used for interim or definitive bonding. Currently, resin cements are used for definitive cementation but have the disadvantage of being more expensive; zinc phosphate cements are more affordable. Cements with weaker physical properties, such as eugenol-free zinc oxide, are generally used as interim luting agents. [18],[19],[20]

In contrast, the retention of cemented restorations can be influenced by different factors such as the size of the abutment (height and width), the texture, the convergence angle between the abutment walls, and the cement used. [21] For an implant-supported prostheses, the cement must provide sufficient retention to prevent loosening during normal operation but should also allow the restoration to be removed without damaging the abutment. [22] Thus, the selection of cement is of importance and should be made with the specific patient and/or site in mind. The geometry of the abutment can also influence the function of cementation. Although a threshold force for clinical success has not been established, most failures have been reported in cements with a lower tensile strength.

This study showed that the highest removal force was provided by the zinc phosphate cement, followed by glass ionomer and eugenol-free zinc oxide in the CeraOne abutment group (P < 0.001). In addition, the zinc phosphate exceeded the other cementing agents in the solid abutment group (P < 0.001). This finding is expected because of the affinity of zinc phosphate cement to metallic structures, providing good retention between the Co-Cr casting coping and the titanium abutment, regardless of the height and shape of the abutment. These results were similar to those described by Al Hamad et al. [13] Akça et al. [18] but different from those described by Ergün et al. [10] who found that the resin cement Panavia F2.0 showed even greater retention than zinc phosphate, zinc polycarboxylate, or zinc oxide eugenol (P < 0.05). Mansour et al. also disagreed with the results of this study. They reported that zinc phosphate showed lower values than the protec cem resin cement and even the zinc polycarboxylate Durelon, which with most testing exhibits a lower removal force than that of zinc phosphate. [18],[23],[24] Glass ionomer and zinc polycarboxylate have adhesive properties and can adhere to the tooth structure by calcium-ion chelation and chelating to the titanium ions in the metal surface.

The retention strength of resin cements is attributed to the adhesion system. [22] Some authors found that the resin and glass ionomer cements were more retentive, [15],[23],[25] but the results obtained by this study are consistent with other authors, who also found that zinc phosphate had the greatest removal force, [26],[27] which tests the hypothesis that was raised at the beginning of the investigation. However, Akça et al. [18] Mansour et al. [9] Sheets et al. [19] and Wolfart et al. [28] claim that zinc polycarboxylate was more retentive than the glass ionomer, zinc phosphate, and zinc oxide cement.

In contrast with the present investigation, Squier et al. [20] compared the retentive forces of different luting agents used to cement crowns to implant abutments. They found similar removal forces for the cementing agents as in this study.

Meanwhile, Keith et al. [29] mentioned that one of the limitations of zinc phosphate is that when it is used to cement crowns on implants, it generates a higher marginal discrepancy compared with glass ionomer (P < 0.001); this may be a factor in bacterial filtration and accumulation of food particles. Gilson [30] and Myers [31] reported that the interim cementing agents provide adequate retention for cemented prostheses with multiple splinted abutments; similar results were found by this study. However, Sheets et al. [19] and Bernal et al. [25] found that the retention values for interim cementing agents were very low. Many authors have concluded that zinc oxide is more suitable for cementing implant prostheses when the reversibility of the restoration is important.

When cementing agents are not comparable among themselves, the values of the adhesive strength will probably be different and the tensile strength will be influenced by several variables such as angulated abutment, type of luting agent, abutment height, and sandblasting. [33],[34],[35],[36],[37],[38],[39],[40] The present study showed that 5.5-mm-high solid abutments had greater tensile strength than the CeraOne abutments in the three groups (eugenol-free zinc oxide, glass ionomer, and zinc phosphate). Another point to consider is that most studies related to the retention of implant prostheses use abutments of different types, heights, and diameters. In addition, the surface roughness of the abutment and the use of thermocycling can cause variation in the cementation of crowns on implants. For example, a survey done in the United States showed that dental schools teach the use of resin cements, followed by zinc oxide, glass ionomer, zinc phosphate, and zinc polycarboxylate as the protocol for cementing prosthesis implants. [27] For this reason, cementing techniques and the other variables described earlier should be standardized.

A limitation of this study was the use of a pull-off test. In a clinical situation, it is likely that other forces can contribute to the loosening of the implant crown. Masticatory forces create a combination of tensile and compressive stresses on the prosthesis. However, the pull-off test was used since it has been adopted in other studies and allowed the comparison of our results with previous investigations. [8],[12],[13],[32] Thus, these studies and the current research have determined that zinc phosphate cement provides the greatest retention required regardless of the type of abutment used. Therefore, the present in vitro study provides a basis for expanding knowledge about the retention of cemented implant restorations.


   Conclusions Top


  • Within the limitations of this in vitro study, it can be concluded that the influence of the type of abutment in the performance of cementing agents depends on the particular type of cement. Zinc phosphate cement showed the greatest increase in retention, followed by glass ionomer, while eugenol-free zinc oxide showed the lowest tensile strength (P < 0.001)
  • When the tensile strength of the CeraOne abutment was compared with that of the 5.5-mm-high solid titanium abutment cemented with eugenol-free zinc oxide, the latter was found to need greater traction and the difference was statistically significant at P < 0.001
  • When definitive cementing is needed, zinc phosphate is appropriate. However, if the prosthesis may require removal, cementation with a eugenol-free zinc oxide is appropriate. The geometry and dimensions of the abutment will play a role in adequate retention.



   Acknowledgment Top


We wish to thank Titaniumfix for supplying implant parts used in this study.

 
   References Top

1.
Chee W, Felton DA, Johnson PF, Sullivan DY. Cemented versus screw-retained implant prostheses: Which is better? Int J Oral Maxillofac Implants 1999;14:137-41.  Back to cited text no. 1
[PUBMED]    
2.
Taylor TD, Agar JR, Vogiatzi T. Implant prosthodontics: Current perspective and future directions. Int J Oral Maxillofac Implants 2000;15:66-75.  Back to cited text no. 2
    
3.
Chiche GJ, Pinault A. Considerations for fabrication of implant-supported posterior restorations. Int J Prosthodont 1991;4:37-44.  Back to cited text no. 3
    
4.
Hebel KS, Gajjar RC. Cement-retained versus screw-retained implant restorations: Achieving optimal occlusion and esthetics in implant dentistry. J Prosthet Dent 1997;77:28-35.  Back to cited text no. 4
    
5.
Jivraj S, Chee W. Treatment planning of implants in posterior quadrants. Br Dent J 2006;201:13-23.  Back to cited text no. 5
    
6.
Henry PJ, Laney WR, Jemt T, Harris D, Krogh PH, Polizzi G, et al. Osseointegrated implants for single-tooth replacement: A prospective 5-year multicenter study. Int J Oral Maxillofac Implants 1996;11:450-5.  Back to cited text no. 6
    
7.
Levine RA, Clem DS 3 rd , Wilson TG Jr, Higginbottom F, Solnit G. Multicenter retrospective analysis of the ITI implant system used for single-tooth replacements: Results of loading for 2 or more years. Int J Oral Maxillofac Implants 1999;14:516-20.  Back to cited text no. 7
    
8.
Santosa RE, Martin W, Morton D. Effects of a cementing technique in addition to luting agent on the uniaxial retention force of a single-tooth implant-supported restoration: An in vitro study. Int J Oral Maxillofac Implants 2010;25:1145-52.  Back to cited text no. 8
    
9.
Mansour A, Ercoli C, Graser G, Tallents R, Moss M. Comparative evaluation of casting retention using the ITI solid abutment with six cements. Clin Oral Implants Res 2002;13:343-8.  Back to cited text no. 9
    
10.
Ergün G, Ceylan G, Yilmaz N, Esin B. Luting agent effectiveness on implant crown retention. Int J Oral Implant Clin Res 2011;2:7-11.  Back to cited text no. 10
    
11.
Di Felice R, Rappelli G, Camaioni E, Cattani M, Meyer JM, Belser UC. Cementable implant crowns composed of cast superstructure frameworks luted to electroformed primary copings: An in vitro retention study. Clin Oral Implants Res 2007;18:108-13.  Back to cited text no. 11
    
12.
Cano-Batalla J, Soliva-Garriga J, Campillo-Funollet M, Munoz-Viveros CA, Giner-Tarrida L. Influence of abutment height and surface roughness on in vitro retention of three luting agents. Int J Oral Maxillofac Implants 2012;27:36-41.  Back to cited text no. 12
    
13.
Al Hamad KQ, Al Rashdan BA, Abu-Sitta EH. The effects of height and surface roughness of abutments and the type of cement on bond strength of cement-retained implant restorations. Clin Oral Implants Res 2011;22:638-44.  Back to cited text no. 13
    
14.
American National Standards Institute/American Dental Association. Specification No. 96 - Dental Water-Based Cements: 2000 [reaffirmed 2005]. Available from: http://www.ada.org/830.aspx#96. [Last accessed on 2012 Aug 22].  Back to cited text no. 14
    
15.
Breeding LC, Dixon DL, Bogacki MT, Tietge JD. Use of luting agents with an implant system: Part I. J Prosthet Dent 1992;68:737-41.  Back to cited text no. 15
    
16.
Michalakis KX, Hirayama H, Garefis PD. Cement-retained versus screw-retained implant restorations: A critical review. Int J Oral Maxillofac Implants 2003;18:719-28.  Back to cited text no. 16
    
17.
Karl M, Graef F, Taylor TD, Heckmann SM. In vitro effect of load cycling on metal-ceramic cement- and screw-retained implant restorations. J Prosthet Dent 2007;97:137-40.  Back to cited text no. 17
    
18.
Akça K, Iplikçioglu H, Cehreli MC. Comparison of uniaxial resistance forces of cements used with implant-supported crowns. Int J Oral Maxillofac Implants 2002;17:536-42.  Back to cited text no. 18
    
19.
Sheets JL, Wilcox C, Wilwerding T. Cement selection for cement-retained crown technique with dental implants. J Prosthodont 2008;17:92-6.  Back to cited text no. 19
    
20.
Squier RS, Agar JR, Duncan JP, Taylor TD. Retentiveness of dental cements used with metallic implant components. Int J Oral Maxillofac Implants 2001;16:793-8.  Back to cited text no. 20
    
21.
Browning WD, Nelson SK, Cibirka R, Myers ML. Comparison of luting cements for minimally retentive crown preparations. Quintessence Int 2002;33:95-100.  Back to cited text no. 21
    
22.
Pan YH, Ramp LC, Lin CK, Liu PR. Retention and leakage of implant-supported restorations luted with provisional cement: A pilot study. J Oral Rehabil 2007;34:206-12.  Back to cited text no. 22
    
23.
Pan YH, Ramp LC, Lin CK, Liu PR. Comparison of 7 luting protocols and their effect on the retention and marginal leakage of a cement-retained dental implant restoration. Int J Oral Maxillofac Implants 2006;21:587-92.  Back to cited text no. 23
    
24.
Bresciano M, Schierano G, Manzella C, Screti A, Bignardi C, Preti G. Retention of luting agents on implant abutments of different height and taper. Clin Oral Implants Res 2005;16:594-8.  Back to cited text no. 24
    
25.
Bernal G, Okamura M, Muñoz CA. The effects of abutment taper, length and cement type on resistance to dislodgement of cement-retained, implant-supported restorations. J Prosthodont 2003;12:111-5.  Back to cited text no. 25
    
26.
GaRey DJ, Tjan AH, James RA, Caputo AA. Effects of thermocycling, load-cycling, and blood contamination on cemented implant abutments. J Prosthet Dent 1994;71:124-32.  Back to cited text no. 26
    
27.
Clayton GH, Driscoll CF, Hondrum SO. The effect of luting agents on the retention and marginal adaptation of the CeraOne implant system. Int J Oral Maxillofac Implants 1997;12:660-5.  Back to cited text no. 27
    
28.
Wolfart M, Wolfart S, Kern M. Retention forces and seating discrepancies of implant-retained castings after cementation. Int J Oral Maxillofac Implants 2006;21:519-25.  Back to cited text no. 28
    
29.
Keith SE, Miller BH, Woody RD, Higginbottom FL. Marginal discrepancy of screw-retained and cemented metal-ceramic crowns on implants abutments. Int J Oral Maxillofac Implants 1999;14:369-78.  Back to cited text no. 29
    
30.
Gilson TD, Myers GE. Clinical studies of dental cements. I. Five zinc oxide-eugenol cements. J Dent Res 1968;47:737-41.  Back to cited text no. 30
    
31.
Gilson TD, Myers GE. Clinical studies of dental cements. II. Further investigation of two zinc oxide - Eugenol cements for temporary restorations. J Dent Res 1969;48:366-7.  Back to cited text no. 31
    
32.
Kanno T, Milleding P, Wennerberg A. Topography, microhardness, and precision of fit on ready-made zirconia abutment before/after sintering process. Clin Implant Dent Relat Res 2007;9:156-65.  Back to cited text no. 32
    
33.
Ady AB, Fairhurst CW. Bond strength of two types of cement to gold casting alloy. J Prosthet Dent 1973;29:217-20.  Back to cited text no. 33
[PUBMED]    
34.
Anusavice K. Materiais Dentários. 11 th ed. Rio de Janeiro: Elsevier; 2005. p. 763.  Back to cited text no. 34
    
35.
Covey DA, Kent DK, St Germain HA Jr, Koka S. Effects of abutment size and luting cement type on the uniaxial retention force of implant-supported crowns. J Prosthet Dent 2000;83:344-8.  Back to cited text no. 35
    
36.
Di Francescantonio M, de Oliveira MT, Garcia RN, Romanini JC, da Silva NR, Giannini M. Bond strength of resin cements to Co-Cr and Ni-Cr metal alloys using adhesive primers. J Prosthodont 2010;19:125-9.  Back to cited text no. 36
    
37.
Emms M, Tredwin CJ, Setchell DJ, Moles DR. The effects of abutment wall height, platform size, and screw access channel filling method on resistance to dislodgement of cement-retained, implant-supported restorations. J Prosthodont 2007;16:3-9.  Back to cited text no. 37
    
38.
Macchi R. Dental Materials. 3 rd ed. Colombia: Panamericana; 2000.  Back to cited text no. 38
    
39.
Tarica DY, Alvarado VM, Truong ST. Survey of United States dental schools on cementation protocols for implant crown restorations. J Prosthet Dent 2010;103:68-79.  Back to cited text no. 39
    
40.
Wahl C, França FM, Brito RB Jr, Basting RT, Smanio H. Assessment of the tensile strength of hexagonal abutments using different cementing agents. Braz Oral Res 2008;22:299-304.  Back to cited text no. 40
    


    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2]



 

Top
 
  Search
 
    Similar in PUBMED
   Search Pubmed for
   Search in Google Scholar for
 Related articles
    Access Statistics
    Email Alert *
    Add to My List *
* Registration required (free)  

 
  In this article
    Abstract
   Introduction
    Materials and me...
   Results
   Discussion
   Conclusions
   Acknowledgment
    References
    Article Figures
    Article Tables

 Article Access Statistics
    Viewed3521    
    Printed55    
    Emailed0    
    PDF Downloaded310    
    Comments [Add]    

Recommend this journal