|Year : 2014 | Volume
| Issue : 3 | Page : 136-143
Placement Technique and Microleakage in Posterior Composite Restorations
Peter Kuyaya Welime
Department of Conservative Dentistry and Prosthetics, School of Dentistry, Moi University, Eldoret, Kenya
|Date of Web Publication||29-Oct-2014|
Peter Kuyaya Welime
Department of Conservative Dentistry and Prosthetics, School of Dentistry, Moi University, P.O Box × 4606 - 30100, Eldoret
Source of Support: Self and University of the Western Cape, South
Africa, Conflict of Interest: None
Context : Advances in composite restorative materials have positioned them as credible alternatives to dental amalgam. However, polymerization shrinkage and the resultant microleakage remain major obstacles in the successful use of these materials. Aims: The aim of this study was to investigate the effects of placement technique on microleakage of Quixfil® composite restorations bonded with Xeno III® bonding agent Settings and Design : In vitro study. Materials and Methods : Thirty-four cylindrical cavities measuring 5 mm in diameter and 3 mm in depth were prepared on the buccal aspect of extracted human teeth. The apices of all the teeth were sealed and the teeth randomly assigned into the bulk and the incremental groups for restoration. Restorations were polished and two layers of nail varnish applied on all tooth surfaces except a rim of 1 mm around the restorations. The teeth were thermo-cycled and stained with 2% methylene blue dye. The restorations were sectioned and examined under light microscopy. Dye penetration on the tooth-restoration interface was scored on an ordinal scale of 0 to 4. Statistical Analysis Used : Pair-wise comparison of the two placement groups was done using the Wilcoxon Signed Ranks Test. Results : No statistically significant difference was observed in the microleakage of the two study groups (P value > 0.05). Conclusions : There is no statistically significant difference in the microleakage of the bulk and the incremental Quixfil restorations. It is feasible to place Quixfil restorations in layers of up to 3 mm in depth.
Keywords: Microleakage, posterior composite restorations, polymerization shrinkage, placement technique
|How to cite this article:|
Welime PK. Placement Technique and Microleakage in Posterior Composite Restorations
. J Res Dent 2014;2:136-43
| Introduction|| |
Recently, composite restorative materials have been launched commercially that manufacturers claim could be placed in a single layer of up to 4 mm without any significant polymerization shrinkage stress development. One such material is Quixfil® (Dentsply DeTrey, Germany), a light-cured posterior composite restorative material. The purpose of this study was to determine whether Quixfil® restorative material (Dentsply DeTrey, Germany) could be placed in layers of up to 3 mm without causing excessive stress build-up at the tooth-restoration interface. Stress build-up at the tooth restoration interface is a major cause of failure in posterior composite restorations. ,,
| Materials and methods|| |
The aim of this study was to determine the effect of placement technique on microleakage of Quixfil composite restorations bonded with Xeno III bonding agent. The null hypothesis was that there is no significant difference in the microleakage of Quixfil restorations placed either in bulk or in increments with Xeno III as the bonding agent.
An in vitro experimental study design was used. [Figure 1] is an illustration of the study design. Thirty-four extracted human molar and premolar teeth were used in this study. All the teeth were collected from the Oral Health Centres of the Faculty of Dentistry, University of the Western Cape. The teeth were stored at room temperature in normal saline, with crystals of thymol added, for a maximum of 2 months before the study.  The inclusion criteria were permanent molar and premolar teeth that had no visible caries and no previous restorations. The exclusion criteria were carious teeth, restored teeth, teeth with fractured crowns, primary molars, and teeth with any developmental defects where the enamel would have required a different etching duration compared to the selected teeth. 
All the teeth were carefully cleaned with flour of pumice and screened under a dissecting microscope (Wild M5, Switzerland) at a 10 times magnification. Only undamaged teeth were used. [Figure 2] depicts the dissecting microscope used for screening the teeth.
The roots of all the teeth were sectioned 2 mm from the apex. Small cavities were prepared in the apical ends of the root canals and sealed with a resin modified glass-ionomer restorative material (Vitremer® , 3M ESPE, Germany). Using a correction pen commercially available as Steadtler® Correction Pen (Steadtler, Nuremberg, Germany), all the teeth were labelled with a number between 1 and 34 on the occlusal surface.
A random sampling and assignment package  was used to assign the 34 teeth into two experimental groups designated group 1 and 2. [Table 1] shows the two experimental groups. Without covering the crowns, the teeth were mounted in blocks made from freshly mixed polyvinyl siloxane impression material putty (President, Coltene Whaledent).
Cylindrical cavities measuring 5 mm in diameter and 3 mm in depth were prepared on the buccal aspect of each tooth. Although it was desirable to test the manufacturer's claim using 4-mm cavities, specimen anatomy could not allow the preparation of such cavities at the cervical areas of the teeth without perforation into the pulp. Such perforation could in turn adversely affect the standardization of cavities. The cavities were prepared in such a manner that one margin of the cavity was placed 3-mm above and the other margin 1-mm below the cemento-enamel junction. A carbide fissure bur (number FG-330; SS White, USA) mounted in a high-speed hand-piece (Midwest, USA) under copious water coolant was used to prepare the cavities. A new bur was used after every five cavities  since etching effectiveness is influenced by the surface texture of the cut enamel. 
After cavity preparation, the cavities were thoroughly rinsed with an air/water spray. The cavities were blot-dried with a cotton pellet but not desiccated. Each group of 17 cavities was then restored according to the manufacturer's instructions as summarized in [Table 2].
To ensure a constant output of energy, calibration of the light-curing unit was performed using a light meter (CURE RITE® , Caulk, USA: Serial number 5918). The intensity of light from the curing-light unit was verified with the light meter before exposing the bonding agent and the restorative material to the light. This calibration is important since variations in the energy output may affect the development of polymerization shrinkage stress in resin composite restorations making it difficult to determine the role played by the placement technique. 
Group 1 (Bulk Quixfil + Xeno III): Xeno® III bonding agent (DentsplyDeTrey, Germany. Lot 0702002760, Expiry date 2009-01) was applied according to the manufacturer's instructions  (appendix II). Xeno III is a bonding agent recommended for use with Quixfil restorative material. Twenty seconds after application, the layer of Xeno III was light-cured for 10 seconds with Smartlite™ PS (Dentsply DeTrey, Germany). Smartlite is a High-Power LED curing-light unit and is recommended for use with the Xeno® III bonding agent. After light-curing the Xeno III bonding agent, Quixfil®restorative material (DentsplyDeTrey, Germany: Lot 0703000720, was placed in a single 3 mm-thick layer according to the manufacturer's instructions  (appendix III). The restorative material was light-cured for 20 seconds with Smartlite, whichis recommended for use with Quixfil restorative material.
Group 2 (Incremental Quixfil + Xeno III):Xeno® III bonding agent was applied according to the manufacturer's instructions. Twenty seconds after placement, the layer of Xeno III was light-cured for 10 seconds with Smarlite. After curing the bonding agent, the occluso-gingival (horizontal) layering technique was used to place Quixfil restorative material in two layers of 1.5-mm each.  Each layer of the restorative material was cured for 10 seconds using a Smartlite. A total of 20 second cure-time was used for the incremental Quixfil restorations so that the total energy delivered per restoration was identical to that delivered to the bulk Quixfil restorations.
All the restored teeth were kept in normal saline at room temperature in the dark for 24 hours to allow for delayed polymerization shrinkage.  The same finishing procedures were carried out on all the restorations with Sof-lex polishing discs (3M Dental products, St. Paul, MN, USA) under water cooling. Contouring was done with Sof-lex polishing discs with a grit size of 400. Final polishing procedures were accomplished with Sof-lex polishing discs with an increasing grit size (550, 625, and 1250). 
All the teeth were removed from the mounting blocks. All tooth surfaces except a rim of 1 mm around the restorations were covered with two layers of a commercially available nail varnish (Revlon, France). [Figure 3] depicts a tooth with surfaces covered with nail varnish.
The teeth were placed in two separate porous bags according to the experimental groups [Figure 4]. The porous bags were placed in 2% methylene blue dye in a thermal cycling machine [Figure 1]0] and thermal cycled for 500 cycles between 5 o and 55 o C with a 30-second dwell time , (ISO TR 11450 standard, 1994). The thermal cycling was done to simulate thermal changes in vivo, since there is a direct relationship between microleakage and thermal changes in composite restorations. ,
After thermal cycling, the teeth were cleaned under running tap water to remove any excess dye material and acetone was used to remove the nail varnish. The teeth were embedded in a clear casting resin (Fibroglas®, South Africa) that was allowed to harden. The restorations were sectioned longitudinally in a bucco-lingual direction into sections of 400 microns with a water-cooled diamond disk cutter (Minitom, Struers, Denmark) at slow speed. , [Figure 5] shows the diamond cutter. Four slices were obtained from each restoration. The slices were cleaned under running tap water and placed on labelled light-microscopy glass slides.
Dye penetration was measured using a light microscope (Wild, Switzerland) [Figure 6] at 100 = times magnification. An ordinal scale ranging from 0 to 4 was used to score dye penetration. Data collection was performed by one previously calibrated examiner. The calibration process involved randomly selecting slides for re-examination until the scores were reproducible. Twenty slides were randomly selected for re-examination from the total of 136 slides using a random sampling and assignment package.  Scoring of dye penetration was done according to the method illustrated in [Figure 7]. The scoring criteria are summarized in [Table 3] and [Table 4].  Each restoration had four slices that were scored on both sides giving a total of eight readings per restoration margin. Only the median score from the eight readings was assigned to each restoration margin to be used later in data analysis.
Dye penetration scores from the enamel and the dentin restoration margins were recorded in an excel spreadsheet. Raw data was submitted to a statistician for analysis. The Wilcoxon Signed Ranks Test was used to determine whether there were statistically significant differences between the two groups at a P ≤ 0.05.
| Results|| |
The raw data for dye penetration scores is presented in appendix IV. Summed dye penetration scores at the enamel margins are presented in [Table 5]. From these results, it is clear that no specimen had an enamel margin dye penetration score of zero. [Figure 8] depicts summed enamel margin dye penetration scores.
The bulk Quixfil + Xeno III group (group 1) had the highest number of restorations with the least enamel margin dye penetration score. No specimen in any of the two groups had an enamel margin dye penetration score of 4. Groups 1 and 2 had similar median enamel margin dye penetration scores [Table 6]. The enamel margin dye penetration scores were compared using the Wilcoxon Signed Ranks Test to determine whether there were any differences between the two groups. No statistically significant differences were found between the two groups (P-value = 1.00). This implies that any differences in the enamel margin dye penetration scores for the two groups could only have been by chance. [Table 7] is a summary of the Wilcoxon Signed Ranks Test comparing the enamel margin dye penetrations scores for the two groups.
|Table 6: Summary of medians, median absolute deviations, and the minimum and maximum dye penetration scores at the enamel margins|
Click here to view
Summed dye penetration scores at the dentin margins are presented in [Table 8]. From these results, it is clear that there was no specimen with a dentin margin dye penetration score of zero. This implies that all specimens in the study exhibited some level of dye penetration at the dentin margins. [Figure 9] depicts summed dye penetration scores at the dentin margins. Group 2 had the highest number of restorations with dye penetration limited to the outer one-third of dentin (score 1) and the lowest number of restorations with dye penetration extending onto the pulpal floor (score 4).
[Table 9] shows a summary of the medians, the median absolute deviations, and the minimum and maximum dentin margin dye penetration scores. The dentin margin dye penetration scores for the two experimental groups were compared using the Wilcoxon Signed Ranks Test. No statistically significant differences were found between the two groups (P-value = 1.00). This implies that any differences in dentin margin dye penetration scores between the two groups could only have been by chance. [Table 10] is a summary of the Wilcoxon Signed Ranks Test comparing dentin margin dye penetration scores of groups 1 and 2.
|Table 9: Summary of medians, median absolute deviations, and the minimum and maximum dye penetration scores at the dentin margins|
Click here to view
| Discussion|| |
Based on the results of this study the null hypothesis that there is no significant difference in the microleakage of Quixfil restorations placed either in bulk or in increments was accepted. However, these observations as regards placement technique are contrary to the views of several authors in the literature. ,,,
The incremental placement techniques (occluso-gingival, oblique, facio-lingual, or U-oblique) have been recommended because they may reduce polymerization shrinkage stress due to; (1) the small volume of material that is polymerized at one time, (2) the reduction in the cavity configuration factor, and (3) the minimal contact of the restorative material with the opposing cavity wall during the polymerization process.  No statistically significant differences were observed among the incremental placement techniques in providing shrinkage stress relief at the tooth-restoration interface.  In the same study, the occluso-gingival (horizontal) incremental technique was ranked as the easiest to use clinically amongst the incremental placement techniques. Therefore, this technique was used in the present study.
Although it is generally accepted in the literature that the incremental placement techniques are desirable, the role of these techniques in reducing interfacial stress build-up in composite restorations has been questioned previously.  In a study using finite element analysis, it was observed that all the incremental techniques for the placement of resin composite restorations (occlusal-gingival, oblique, U-oblique, and facio-lingual) caused more inward deformation of the cavity walls compared to the bulk placement technique.  The incremental placement techniques resulted in cavities that were volumetrically filled with less composite resin compared to their original volume. They argued that the increased cavity wall deformation in an incrementally-placed resin composite restoration resulted in a more stressed tooth-restoration complex compared to the bulk-placed restoration. Therefore, a possible explanation for the observations in the present study is that any benefits of placing resin composite restorations in increments may have been cancelled by the progressive deformation of the cavity wall. Although the incremental placement of resin composite has obvious benefits including thoroughness of light polymerization, allowing bond maturation, and ease of adaptation; its role in the total stress relief has not been demonstrated conclusively.  Furthermore, it has been reported that not all polymerization shrinkage occurs immediately after light activation.  Seventy to 85% of the shrinkage could occur immediately after polymerization while up to 95% only occurs after 5 minutes. From this it becomes evident that as polymerization shrinkage of the last increment occurs, considerable strain could still be under way from the first layer. Therefore, the combined simultaneous shrinkage of the different layers may result in much more shrinkage stress compared to the shrinkage stress from a single bulk cured layer.  The idea that a subsequent increment could compensate for the polymerization shrinkage could only be valid if the increment is placed in all regions where volume reduction had occurred.  This seems to be possible only for the free surface but not the interface where gap formation may have occurred.
In the present study, neither the bulk nor the incremental placement technique was superior to the other as regards microleakage of restorations. Furthermore, neither the bulk nor the incremental placement technique could completely eliminate microleakage in this study. This observation is in agreement with previous studies on microleakage. , The integrity of the tooth-restorative interface is affected by the polymerization shrinkage stress that is associated with all composite restorations. A direct correlation has been described between microleakage and polymerization shrinkage stress in resin composite restorations.  Since none of the placement techniques evaluated in the present study could totally eliminate microleakage, additional measures that could reduce microleakage such as application of a surface sealer or a layer of dentin bonding agent over the restoration margins would be beneficial. ,
Poorly polymerized bulk restorations could result in low polymerization shrinkage stress and low microleakage scores due to a low degree of conversion.  On the other hand, optimal polymerization of the incremental restorations could result in a better degree of conversion compared to the bulk restorations. 
| Conclusions|| |
From the results of this study, it seems reasonable to place Quixfil resin composite restorations in single layers of up to 3 mm. The similarity in microleakage of the bulk Quixfil + Xeno III group and the incremental Quixfil + Xeno III group implies that neither of the two techniques was superior to the other as regards to shrinkage stress relief.
- Many factors affect the integrity of the tooth-restoration interface and they have a complex inter-relationship that makes it difficult to isolate the role played by any one factor in interfacial stress development and microleakage
- In vitro studies are only a prediction of what may actually happen in vivo. Simulation of oral conditions in the clinical environment such as thermal changes using thermocycling may be misleading since such clinical conditions are individual specific
- Incomplete polymerization of the bulk restorations could be a source of error in this study since such a scenario may have masked the effects of polymerization shrinkage stress on microleakage in the bulk restorations.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8], [Table 9], [Table 10]