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 Table of Contents  
ORIGINAL ARTICLE
Year : 2022  |  Volume : 19  |  Issue : 3  |  Page : 132-139

Trueness and tissue surface adaptation assessment of digital versus conventional mandibular implant-assisted complete overdenture


Department of Prosthodontic, Faculty of Dentistry, Tanta University, Tanta, Egypt

Date of Submission04-Jun-2022
Date of Decision15-Jul-2022
Date of Acceptance22-Jul-2022
Date of Web Publication14-Sep-2022

Correspondence Address:
Nour E Ibrahim Abouelazm
BDS, Department of Prosthodontic, Faculty of Dentistry, Tanta University, Tanta
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/tdj.tdj_19_22

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  Abstract 


Purpose
Evaluation of trueness and tissue surface adaptation of digital versus conventional mandibular implant-assisted complete overdenture (IACO).
Materials and methods
Two implants were installed in the inter foraminal region of an epoxy resin model, which was scanned and saved as a Standard Tessellation Language (STL) file. The reference model with the implants was duplicated into 20-stone models. All models were scanned and the STL files were saved for digitally designed 20 IACOs. Half of them were fabricated by three-dimensional (3D) printing technology and considered as group I (the digital group). The other half were fabricated by the conventional pack and press technique and considered as group II (the conventional group). To evaluate the trueness, all IACOs were scanned and superimposed to the STL files of the original design by digital software. By using the same software, the gap between the intaglio surface of the scanned IACOs and the reference model was measured to evaluate the tissue surface adaptation.
Results
The statistical analysis by using t test revealed a highly significant difference between the two groups with less mean deviation value of the digital group with a P value less than 0.001. There is a significant difference between the two groups with P value of 0.035 at the implant region in the digital group. The overall adaptation between the two groups showed a significant difference with less mean deviation value of the digital group with a P value of 0.041.
Conclusion
The digital group has better trueness and adaptation than the conventional group. Despite the gap in the implant area in the digital group, the overall fit of the digital group is better.
Clinical implications
Three-dimensional printing technology can achieve an acceptable fit and good retention. Digital technology and implant dentistry provide great hope for complete edentulism, especially in those with a completely edentulous mandibular arch. Digital implant-assisted complete overdentures showed better fit than conventional ones.

Keywords: trueness, accuracy of fit, 3d printing, digital overdenture, tissue surface adaptation


How to cite this article:
Ibrahim Abouelazm NE, Aboutalb FA, El-Moneim El-Segai AA. Trueness and tissue surface adaptation assessment of digital versus conventional mandibular implant-assisted complete overdenture. Tanta Dent J 2022;19:132-9

How to cite this URL:
Ibrahim Abouelazm NE, Aboutalb FA, El-Moneim El-Segai AA. Trueness and tissue surface adaptation assessment of digital versus conventional mandibular implant-assisted complete overdenture. Tanta Dent J [serial online] 2022 [cited 2023 Jan 31];19:132-9. Available from: http://www.tmj.eg.net/text.asp?2022/19/3/132/356082




  Introduction Top


Edentulism is considered a poor health outcome and may adversely affect an individual's quality of life. For many years, the most common clinical protocol for edentulous patients has been conventional complete overdenture treatment. Many of these patients suffer from denture problems. These include decreased masticatory function and insufficient stability and retention, especially in the mandibular dentures. This can be attributed to the condition of the mandibular ridge. This has led researchers to focus more on the lower jaw [1].

With implant dentistry, the edentulous patient has great hope of having a denture that is sufficiently retentive, stable, and more comfortable. McGill consensus has suggested that the first choice of treatment for mandibular edentulous arches is the two implant-assisted overdentures, which is better for oral health and general quality of life [2].

A successful complete overdenture must provide proper function, esthetics, phonetics, and healthy denture-bearing tissues. Well-fitted and adapted dentures promote chewing efficiency and patient comfort. A denture base that is well-adapted to the denture-bearing tissues is essential for adequate retention and stability, especially because complete overdentures depend almost entirely on physical means of retention [3]. Research is ongoing to determine the best fabrication technique for successful dentures and to overcome the defects of the previous method. Every new emerging technique promises fewer dimensional changes during processing, and better adaptation and retention [4].

Using computer-aided designing/computer-aided manufacturing (CAD/CAM) technology was introduced to the dental field in 1971 and gained both popularity and confidence among dentists and patients alike. Complete digitalization leads to promising clinical outcomes, enhanced retention, fewer patient visits, improved material properties, standardization of both clinical and research results, reduced time for teeth setting, simplified data storage, and denture duplication. The application of CAD/CAM technology was launched into the field of complete overdenture prosthesis over 20 years ago [5],[6].

CAD/CAM technology has two fabrication approaches. With the subtractive method, or milling technology, the prosthesis can be milled from pre-polymerized discs using a special milling machine. The other approach is an additive manufacturing, three-dimensional (3D) printing, or free-forming technology [7]. This approach includes the addition of a printable material layer by layer by special computer-slicing software to create a 3D object from a virtual design. The idea behind this innovative technology is that the 3D object is sliced into many multiple layers. The 3D printer uses these geometric data to build each layer successively from liquid or powder. These layers are polymerized by ultraviolet light, laser, or lamplight until the final finishing of the manufacturing process [8].

Additive manufacturing is a promising technology because of its multiple advantages, including producing objects with delicate surface details and complex geometries from liquid or powder printable materials. Thus, it is more time-saving and reduces raw material waste. It also can overcome well-known problems of the subtractive method technique, such as fit problems, which depend on the size of the milling tool, milling tool abrasion, and microcracks in the produced object [9].

Multiple methods with different degrees of complexity have been used to evaluate the extent and position of processing dimensional changes of the denture base. Using laser and scanning software has become popular for assessing denture deformation during processing. Using these new techniques, scanned Standard Tessellation Language (STL) files can be superimposed and evaluated using innovative computer software. Research has shown that these evaluation methods are valid and effective [4].

To date, studies have reported the accuracy of milled denture bases in comparison with conventional fabricated maxillary ones [10]. However, no published research has evaluated the accuracy of the implant-assisted complete overdenture (IACO) manufactured by 3D-printing technology. Moreover, the accuracy and fit have been evaluated primarily in the maxillary complete dentures, and little research has focused on mandibular edentulous arches. The International Organization for Standardization stated that accuracy includes Trueness and Precision of the methods of measurement and results, Trueness means the closeness of agreement between the expectation of a measurement result and a true value [10].

The purpose of this in vitro study was to compare the trueness and tissue surface adaptation of the 3D-printing and pack-and-press (PAP) techniques of IACO. The first null hypothesis was that there would be no difference in the trueness between the 3D-printed and PAP overdentures. The second null hypothesis was that there would be no difference in the tissue surface adaptation between the 3D printing and conventional PAP techniques.


  Materials and methods Top


For this study, a ready-made mandibular epoxy resin model was selected (Ramses Co., Alexandria, Egypt) which the residual alveolar ridge was covered by a resilient silicon layer to simulate the residual ridge mucosa [11]. The residual alveolar ridge form was class I type A, and it was selected according to the American College of Prosthodontists classification, with no undercut areas [Figure 1] [10].
Figure 1: Mandibular completely edentulous epoxy resin model.

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To create a virtual model, the reference cast was scanned using a blue-light scanner (SHERA Eco-Scan 7, Ledford, Germany), and the STL file was saved. The STL file of the virtual model was loaded to the software (BlueSky Plan Co., Libertyville, Illinois, USA) for digital drilling guide fabrication. The fully limited, digital drilling guide template was fixed to the model through which all drilling procedures were performed [Figure 2]a. Two implant analogs (T6 implant analogs; Nucleoss Co., Izmir, Turkey) were installed in the intraforaminal region and two ball attachments (ball abutment; Nucleoss Co.) were tightened on the implant analogs then the female housings were inserted [Figure 2]b. As a reference point for superimposition, two dimples were prepared anterior to the retromolar pad on both sides [4],[11]. The reference cast with the ball attachments was sprayed with a scanning spray (Renfert Scanning Spray, Hilzingen, Germany) to scan the model using a light scanner (Egsolutions DS Mizar 3D Scanner, Bol Ogna, Italy) and was saved as an STL file for further stages of evaluation. The reference cast was duplicated using a silicone material (REPLISIL 9 N, Dent-E-Con, Lonsee, Germany) [Figure 3]a, and 20 identical master casts were poured in a type IV dental stone (Dental Elite Stone, Zhermack Co., Badia Polesine, Italy) [Figure 3]b.
Figure 2: (a) Drilling procedures of the epoxy resin model for implant installation. (b) Installed implant analogs with the ball attachments and female housing.

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Figure 3: (a) Position of the implant analogs in the silicon duplicating mold. (b) Final master cast with the ball abutments and female housings.

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After 24 h of dry storage, all duplicated master casts with the ball attachments and female housings were digitally scanned to get a virtual master cast, in which the undercut area below the female housing was blocked by modeling wax (Standard Modeling Wax, Bredent GmbH and Co. KG., Senden, Germany). The ball abutments and their housings were used as an extraoral scan body to transfer the correct implant position. The reference CAD mandibular IACO was virtually designed using the CAD software (Exocad Dental CAD Software, Darmstadt, Germany) according to the master cast STL file [Figure 4]a, [Figure 4]b [12]. All virtual IACOs were designed by a highly qualified dental technician. The STL files were loaded to the 3D printer (Phrozen 3D Printer, Sonic XL 4 K, Hsinchu City, Taiwan) software (Chitubox 3D Slicer Software V1.4.0, Guangdong, China) [Figure 5]a.
Figure 4: (a) Final virtual designing of the denture base. (b) Completed design of the IACO. IACO, implant-assisted complete overdenture.

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Figure 5: (a) Position of virtual denture bases on the 3D-printer platform. (b) Final denture bases after ending of 3D printing process. 3D, three dimensional.

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According to CAD reference overdenture, 10 3D-printed IACOs were fabricated with a 3D-printable material (Denture Bases, Nextdent Co., Soesterberg, the Netherlands) using a digital light processing (DLP) 3D printer, with a light source is (light emitted diode) and a 405 nm wave length. The supporting structures were located on the retromolar area of the denture base with 100° build angle [10] [Figure 5]b. After printing, the denture bases were cleaned with alcohol in an ultrasonic cleaner for 10 min and were additionally post-light polymerized in a light-curing unit (Bre. lux Power Unit 2, Bredent Medical GmbH and Co. KG) for 15 min, in accordance with the manufacturer's instructions. A space of 0.5 mm was created at the area of the attachments in the denture base on each side and a drop of resin cement (DTK, Kleber, Bredent Medical GmbH and Co. KG) was applied in this space, and the female housings were picked up on the ball attachment on the model for ensuring accurate positioning.

For the PAP technique, ten 3D-printed trial denture bases were fabricated with a 3D-printable material (NextDent Try-In, Nextdent Co.) according to the reference CAD mandibular IACO, using a DLP-based printer as mentioned for the digital group (3D-printed group). The female housings were cemented and picked up on the stone master castes. The 10 identical DLP trial dentures were flasked to produce 10 mandibular IACOs, which were fabricated from heat-activated polymethyl methacrylate (PMMA; Vertex Regular, Vertex-Dental Company News, Osaka, Japan) using a conventional dental flask (Hanau Varsity Flask, Pearson Dental Supply, USA). These dentures represented group II (PAP group).

To ensure the same position and angle for all scanned IACOs, the fitting surfaces of the 20 IACOs were sprayed using a scanning spray (Renfert Scanning Spray, Hilzingen, Germany), and the IACOs were attached to a silicone index (Zeta Plus Putty; Zhermack Co., Badia Polesine, Italy) and digitally scanned [10]. Twenty 3D images were obtained for the scanned IACOs and were saved as STL files. The evaluation methods were as follows: each STL file of the scanned IACOs was superimposed on the STL file of the reference CAD mandibular IACOs with surface-matching software (Geomagic Control X 2018, 3D System, Tokyo, Japan) 3D compare and best-fit alignment to assess the trueness of both techniques. There were 10 matched files for each technique.

To evaluate the tissue surface adaptation of the IACOs to the reference cast, the STL files of all IACOs were superimposed to the STL file of the reference cast separately using the two reference points posteriorly, the attachments, and the female housings anteriorly by using the same surface-matching software. The measurements were achieved at a total of 60 points for each of the 20 overdentures, which represents all measurement areas [4]. The points were distributed as follows: 12 points on the buccal slope areas, six points for each side; 12 points on the lingual slope area, six points for each side; 10 points on the crest of the residual ridge; 10 points on the retromolar pad, five points for each side; and 16 points around the female housing of the ball attachments, eight points on each side [13]. All measurements were recorded by the same investigator. The nominal deviation for the superimposition analysis was set at ±50 μm and the critical deviation at ±300 μm [10],[14]. To compare the trueness of the 3D-printed and conventional techniques, the means, SDs, and P value of all measured surface deviations [root mean square error (RMSE), average positive deviation, and average negative deviation] were calculated. To compare the degree of tissue surface adaptation, the average of all points representing the specific areas measured were calculated. Data were collected and tabulated. Statistical analysis were performed using the Statistical Package for Social Sciences (SPSS, version 26). Numerical variables are expressed for descriptive statistics as mean, SD, and range. To compare groups in each parameter, an independent t test was used. P value less than 0.05 was considered to indicate a significant difference, and P value less than 0.001 was considered to be a highly significant difference. The t test was selected because the comparison was between two different groups.


  Results Top


The color surface map of 3D-printed IACOs showed more homogenous and fewer deviation errors with only positive deviation values at the retromolar pad and the implant area (yellow to red areas; [Figure 6]a), but PAP-processed IACOs showed the highest deviation and color change errors [Figure 6]b. Conventional IACOs showed negative deviation (light to dark blue areas) at the crest of the ridge area and positive deviation at the buccal and lingual flange areas. Thus, from the visual analysis, the digital IACOs are more adapted than the conventional IACOs. As shown in [Table 1], the mean ± SD of the RMSE of the digital group was 0.174 ± 0.008, and the mean ± SD of the RMSE of the conventional group was 0.323 ± 0.078 (P = 0.000). Statistical analysis revealed a highly significant difference between the two groups. The mean ± SD of the positive deviation of the digital group was 0.097 ± 0.003, and the conventional group was 0.209 ± 0.057 (P = 0.000). The mean ± SD of the negative deviation of the digital group was −0.074 ± 0.009, the conventional group was −0.132 ± 0.019 (P = 0.000).
Figure 6: (a) Color map of the fitting surface of the 3D-printed group. (b) Color map of the fitting surface of the conventional group. 3D, three dimensional.

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Table 1: Comparison of the overall trueness of implant-assisted complete overdenture between the three-dimensional printed and conventional groups

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As shown in [Table 2], the mean ± SD of the gap distance of the digital group was 0.008 ± 0.02, and the conventional group was 0.026 ± 0.01 (P = 0.030). This showed that the positive deviation values were greater in the digital group. The mean ± SD of the gap distance at the lingual slope of the digital group was 0.026 ± 0.02, and the conventional group was 0.13 ± 0.08 (P = 0.002; [Table 2]). This points to a greater gap space between the model and tissue surface of the IACO at both the buccal and the lingual slopes in the conventional group.
Table 2: Comparison of the gap distance of the tissue surface of implant-assisted complete overdenture to the model between both groups at selected areas

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[Table 2] shows the mean ± SD of gap distance at the crest of the ridge area between the two groups. The mean ± SD of the gap distance of the digital group was 0.046 ± 0.02, and the conventional group was −0.11 ± 0.06 (P = 0.001). The statistical analysis revealed a significant difference between the two groups. This shows that the negative deviation values were greater in the conventional group. The mean ± SD of gap distance at the retromolar pad of the digital group was 0.099 ± 0.05, and the conventional group was 0.047 ± 0.02 (P = 0.005). The statistical analysis revealed a significant difference between the two groups. This showed that the positive values were greater in the digital group.

The mean ± SD of the gap distance at the implant area of the digital group was 0.123 ± 0.018, and the conventional group was 0.097 ± 0.031 (P = 0.035). Statistical analysis revealed a significant difference between the two groups. The positive deviation values were greater in the digital group.

The mean ± SD and P value of the previously selected 60 points were calculated to measure the overall (gap distance) tissue surface adaptation between the digital group versus the conventional group to the reference model. [Table 3] shows the mean ± SD of the gap distance over all tissue surface of the denture base to the model between the two groups. The mean ± SD of the gap distance of the digital group was 0.051 ± 0.011, and the conventional group was 0.071 ± 0.032 (P = 0.041) Statistical analysis revealed a significant difference between both groups in the tissue surface adaptation, showing that the overall gap distance was less in the digital group.
Table 3: Comparison of the overall fit of the implant-assisted complete overdenture to the model between both groups

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  Discussion Top


The evaluation of the trueness of the digital IACO and conventional IACO showed that the trueness of the digital group was highly significantly better than that of the conventional group.

The first null hypothesis was rejected, indicating that there is no difference between the trueness of digital and conventional fabrication methods of IACO. This finding agrees that of Yoon et al. [10] and Tasaka et al. [15], who compared the trueness of complete overdentures fabricated by digital technology and PAP, they found a better trueness in the digitally fabricated group. In addition, in 2019, researchers studied the accuracy of the 3D-printed metallic dentures versus conventional processing and reported better accuracy of the digital dentures [16]. This might be attributed to the series of manual laboratory steps, the use of multiple materials with different mechanical properties, and the conventional processing technique itself.

Denture dimensional changes after PAP processing are mainly because of the trail packing method and induction of internal stresses, and their later release after deflasking [17]. It can also be attributed to traditional pressure techniques, in which it is difficult to maintain metal-to-metal contact before the flask is placed in the traditional spring clamp. This causes a premature release of the residual stresses from the PMMA dough before closure of the final flask [18]. Kawara denture bases during resin polymerization are inevitable and have been well documented. These problems associated with the PAP technique compromise the denture fit and increase the gap between the denture base and the underlying mucosa.

PMMA tends to absorb water and expand slowly. Water absorption may release the internal stresses that occur during processing, and it can compromise the denture fit [19]. This contrasts with digital fabrication techniques in which the trail packing, internal stress release, and water sorption are absent, so it is more accurate. PMMA has a relatively high coefficient of thermal expansion in comparison with dental stone. Consequently, this difference between the coefficient of thermal expansions of dental stone and PMMA can result in thermal linear shrinkage in the heat-curing PMMA during the flask cooling to room temperature [20]. The features of thermal expansion, contraction, and polymerization shrinkage of PMMA during processing induce internal stress that is released during the deflasking step. Digital fabrication techniques have been invented to overcome these drawbacks of conventional processing techniques [21].

In this study, the trueness results were disagreed with the results of Hsu et al. [22], who studied the effect of different fabrication techniques, including digital and conventional compression molding, on the accuracy of the upper and lower denture base. Those authors found that the 3D-printing technique had the least trueness and that both milling and compression molding were better. They explained that 3D printing is a new technique that requires further investigation.

Regarding to IACO adaptation at the buccal and lingual slopes, the denture borders lifted from the model in the present study. It was stated that heat-cured PMMA denture bases tend to undergo marginal lifting and central pressure into the master cast [23]. It has been reported that thermal shrinkage and internal stress release can arise after processing with heat-curing resin during deflasking [24].

In relation to the results of tissue surface adaptation at the retromolar pad, a greater gap space was found between the IACO and the model in the digital group, which agrees with the findings of Prashant et al. [25], who reported that during the DLP process, many factors, including the printable material, printer resolution, buildup conditions and also type of 3D-printer machine, may affect the extent of the surface deviation. This might also be related to changes in the location of the support bars based on the build angle change [26]. The upward movement of the 3D printer platform and the sagging effect of the printable material may also play important roles in the surface deviation of the 3D printed prothesis [27].

In the present study, build angle of 100° was used [10], and the direction of the supporting bars was toward the retromolar pads. Because the tissue surface adaptation was the research point of this study, these parameters were selected based on the size of the plate form of the printer to accommodate two printed denture bases at the same time and to be away from the fitting surface. These factors were suspected to cause the gap distance in the retromolar pads on both sides toward the supporting bars.

In relation to the result of tissue surface adaptation at the crest of the ridge, there is a significant difference between the two groups and pressure on the ridge crest in the conventional group with a 0.11 mm value. This is due to the heat-cure behavior, which tends to contract and press on the center. This result is in agreement with the findings reported by Yoon et al. [13] However, Hsu et al. [22] reported higher pressure at the crest of the ridge in the 3D-printed group, which disagrees with the results of this study. This may be due to changes in the position of the supporting structures or build angle and type of 3D printer; in addition, Hsu and colleagues used the silicon index method to evaluate dimensional changes, which might be another reason for the difference in results.

Ranganath et al. [14] reported that the oral mucosa can be adapted with a misfit of ±1 mm, and because of the biological and mechanical characteristics of the oral tissues, they can adapt with a relatively limited deviation from the desired dimensions of the processed denture base. Thus, the pressure values in this study are clinically acceptable [11],[13].

The results of tissue surface adaptation in the implant area showed better fit and less space in the PAP group than in the digital group. This can be explained by the cementation of the female housing in the DLP group, in which a cement space was created and the application of resin cement. However, in the conventional group, the indirect pickup technique was used, in which the female housing was packed with the heat cure during processing and adhered by mechanical means, which is more accurate than the direct technique [28].

In the light of the present study, the second null hypothesis is that there is no difference between tissue surface adaptation of DLP and PAP fabrication method of complete implant-assisted overdenture was also rejected. The results of the average overall adaptation of IACO, in which there is better adaptation of the DLP group can be confirmed by the results of overall trueness as the DLP technique also showed better trueness values. This result can be attributed to the fact that the digital fabrication techniques decrease the manual work in processing and eliminate the multiple laboratory steps, thus decreasing the resulting errors.


  Conclusions Top


With the limitations of this in-vitro study and based on the results, the following conclusions were drawn:

  1. The 3D-printed IACO showed better trueness than conventionally fabricated IACO by the PAP method.
  2. The 3D-printed IACO exhibited gap space at the female housing area and retromolar pad on both sides.
  3. The 3D-printed IACO showed the best overall fit within the two experimental groups.


Recommendations

  1. Further investigation of trueness of the digital IACO versus the other conventional fabrication techniques.
  2. More studies of the tissue surface adaptation of the 3D printed overdenture bases at the implant regions.
  3. Future improvement of the design software to have cement space option in the attachment area during designing of removable overdenture.
  4. The deviation and tissue surface adaptation of the digitally fabricated complete overdenture need to be studied and evaluated clinically and for more follow up periods.
  5. The cost of CAD/CAM techniques for designing and fabrication is very high so, it needs further modifications to be readily available with reasonable cost.
  6. Studies to compare between 3D printed IACO and milled IACO are needed.
  7. Further research is needed about the perfect build angle and building direction of the AM technology in the field of removable prosthodontics.


Limitations

  1. High cost of the CAD/CAM technology including material, software designing and machines and no constant price.
  2. Availability of all CAD/CAM systems.
  3. Presence of highly qualified and well-trained technicians who can deal with the digital workflow of the dental prothesis.
  4. Time consuming procedures.


Acknowledgements

The authors thank Next Dent Co. and Bredent Medical GmbH for their materials, Nucleoss Co. for their support in terms of implant components, SKY milling center Dental Lab for their CAD/CAM supports Eng. Mohammed Yehia for his assistance and Mr Ahmed Tayl for his effort in preparing this manuscript.

Financial support and sponsorship

Nil.

Conflicts of interest

None declared.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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