Influence of resin and matrix systems on the three-dimensional geometry of reproduced proximal surfaces in Class II resin restorations of primary molars

Abstract

Purpose: As the anatomy of proximal contact areas affects the risk of dental caries in primary molars, this study aimed to assess the impact of different resin and matrix systems on the geometry of reproduced proximal surfaces in primary molars.

Methods: Sixty mandibular primary first molar typodont teeth were prepared with standardized disto-occlusal cavities. Two experimental groups were created: Group I utilized GC UniFil Flow and Filtek Z250 XT; Group II utilized SDR Plus Bulk Fill Flowable Composite and Ceram.x Spectra ST. Each group was further subdivided into the Palodent V3 sectional matrix system (P) and the Tofflemire matrix system (T). Digital evaluations were then performed to calculate discrepancies.

Results: Significant discrepancies were found only between Group IT and Group IIP in the middle third (P < 0.05). Average root mean square values at proximal contact surfaces did not differ significantly among the groups. The proximal surface shape was also not significantly different.

Conclusion: Within the limits of this study, the reproduced proximal surface geometry was not significantly affected by the type of resin and matrix system.

Introduction

Proximal caries in primary teeth are highly prevalent among children and often restored with direct resin restorations [1]. However, accurately reproducing the anatomy of the proximal contact surface in harmony with the adjacent teeth using direct resin restorations can be challenging [2]. Establishing appropriate interproximal contact is essential for the stability of teeth and supporting periodontal tissues [3]. In addition, interproximal contact may influence the risk of food impaction [4]. It is noteworthy that a previous study showed a strong association between interproximal contact points and the risk of proximal caries in primary molars [5]. Recent studies have also found that the anatomy of proximal contact areas in primary molars is associated with risk of proximal caries [6,7]. Therefore, achieving appropriate proximal surface anatomy is crucial for successful direct resin restorations in primary teeth.

To achieve the appropriate proximal surface anatomy, it is recommended that the interproximal contact is shaped as an area rather than a single point [8]. A convex contact shape in permanent teeth is recommended to maintain oral hygiene, as it influences the interdental gingiva contour [9]. However, primary molars have distinct proximal surface anatomy compared to permanent molars, characterized by broader and flatter contact areas [10]. Moreover, recent studies found four unique types of proximal contact shapes in primary molars: open (O); point (X); straight (I); curved (S) contact shapes [6,7]. These studies further suggested that the distinct proximal surface in primary molars is responsible for the high susceptibility to dental caries [6,7].

To reproduce the anatomy of the proximal contact surface in direct resin restorations, techniques such as high-viscosity resins and matrix systems are indicated [8,11]. Typically, the incremental filling technique is preferred for proximal resin restorations, where flowable resin is used to fill the internal and proximal areas of the cavity, followed by the application of packable resin to cover the occlusal surface. This approach is favored because flowable resins adapt easily along the cavity walls, particularly at the margins [12]. However, concerns remain regarding the low mechanical properties, wear resistance, and high volumetric shrinkage of flowable resins [13]. As an alternative, flowable bulk-fill resins are now readily available, offering a fluid consistency that facilitates easier cavity adaptation and reduced volumetric shrinkage [14].

Establishing the proximal contact surface in direct resin restorations also depends on the type of matrix system [3]. Standard circumferential (Tofflemire) and sectional matrix systems are commonly used in pediatric dentistry. The Tofflemire system features a universal retainer and steel matrix band that wraps around the entire tooth. This system has been widely adopted due to its simplicity and well-established use. In contrast, sectional matrix systems offer pre-contoured bands and specialized rings designed to create anatomically appropriate proximal surfaces [15]. In spite of the fact that the selection between these systems may significantly influence the final proximal anatomy, it is still controversial. Studies have shown that the Tofflemire system is not favorable for ensuring appropriate proximal surface anatomy compared to sectional matrix systems due to its uniform thickness and lack of anatomical pre-contouring [3,11]. Another study demonstrated that sectional matrix systems are limited in primary molars, particularly in cases where securing the matrix is difficult due to the absence of adjacent teeth or very large cavities [11].

Given the shorter height of primary molars and their distinct proximal surface anatomy, it is necessary to evaluate the reproducibility of proximal surface created by resin restorations in primary molars. Therefore, the purpose of this in vitro study was to assess the influence of resin and matrix systems on the three-dimensional (3D) geometry of proximal surface in posterior resin restorations in primary molars. The null hypothesis of this study was that there is a difference in the 3D geometry of proximal surface depending on resin and matrix systems.

Materials and Methods

Sample size calculation

The study protocol was reviewed and approved by the Ethics Committee of Kyung Hee University Dental Hospital, Kyung Hee University, Seoul, Korea (KH-DT23047). This study followed the CRIS guidelines [16]. The sample size was calculated using the G*Power software (v.3.1.9.7; Heinrich Heine University, Düsseldorf, Germany) with a significance level of 0.05, a statistical power of 95% and an effect size of 0.57. The effect size was derived from the results of a previous study [17]. The required sample size was calculated to be at least 15 teeth per group.

Group allocation

The groups were allocated according to different resin and matrix systems. First, the groups were divided based on the resin systems: (i) Group I‒filling the gingival margin area with GC UniFil Flow (GC Corp., Tokyo, Japan) and completing the filling Filtek Z250 XT (3M ESPE, St. Paul, MN, USA) and (ii) Group II‒filling the gingival margin area with SDR Plus Bulk Fill Flowable Composite (Dentsply Sirona, Charlotte, NC, USA) and completing the filling with Ceram.x Spectra ST (Dentsply Sirona) (Table 1). The selection of resin composite combinations was based on their compatibility with the respective matrix systems employed in this study. GC UniFil Flow and Filtek Z250 XT were selected alongside the conventional Tofflemire matrix and wooden wedge system, as this represents a commonly used restorative technique in Class II restorations. The combination of SDR Plus Bulk Fill Flowable Composite and Ceram.x Spectra ST was selected as they are part of Dentsply’s integrated restorative system, specifically designed to work in conjunction with their sectional matrix system, ensuring optimal material performance and predictable results. Two subgroups were then divided according to the matrix systems: (i) Group P ‒ Palodent V3 sectional matrix system (Dentsply Sirona) and (ii) Group T ‒ Tofflemire matrix system (Hahnenkratt, Königsbach-Stein, Germany) (Table 2). The Palodent V3 sectional matrix system includes a 3.5 mm contoured matrix band, a universal ring, and small elastic plastic wedges (Palodent V3 Wedges; Dentsply Sirona) to facilitate optimal contouring of the proximal surfaces. The Tofflemire matrix system includes a circumferential matrix band stabilized with a wooden wedge (Hahnenkratt) to ensure proper gingival adaptation.

Table 1 Description of the resin systems used in this study

Resin system	Type	Composition
GC UniFil Flow (GC Corp., Tokyo, Japan)	Flowable composites	resin matrix: •Bis-GMA, UDMA, TEGDMA fillers: •strontium glass filler, silicon dioxide (silica) photoinitiators: •camphorquinone
SDR Plus Bulk Fill Flowable Composite (Dentsply Sirona, Charlotte, NC, USA)	Flowable composites	resin matrix: •modified UDMA, TEGDMA fillers: •barium aluminum fluorosilicate glass, silicon dioxide (silica) photoinitiators: •camphorquinone additives (not disclosed by manufacturer): •modifiers, stabilizers
Filtek Z250 XT (3M ESPE, St. Paul, MN, USA)	Sculptable composites	resin matrix: •Bis-GMA, UDMA, Bis-EMA, TEGDMA. fillers: •zirconia/silica fillers photoinitiators: •camphorquinone
Ceram.x Spectra ST (Dentsply Sirona)	Sculptable composites	resin matrix: •Bis-GMA, TEGDMA fillers: •SphereTEC fillers (granulated spherical fillers), barium aluminum borosilicate glass, silicon dioxide photoinitiators: •camphorquinone additives (not disclosed by manufacturer): •pigments

Table 2 Description of the matrix systems used in this study

Matrix system	Type	Manufacturer	Shape, height, thickness, characteristics	Wedge
Palodent V3 sectional matrix system (P)	sectional	Dentsply Sirona, Charlotte, NC, USA	contoured, 4.5 mm, 0.03 mm dead soft	elastic wedge (medium) (Dentsply Sirona)
Tofflemire matrix system (T)	circumferential	Hahnenkratt, Königsbach-Stein, Germany	Straight, 5.0 mm, 0.03 mm flexible	wooden wedge (Hahnenkratt)

Restorative procedure

The restorative procedure was performed on a total of 60 mandibular left primary first molar typodont teeth (Simple Root Tooth Model; Nissin Dental Products, Kameoka, Japan). Standardized disto-occlusal (DO) cavities were prepared with an isthmus width of 1 mm and a gingival floor width of 1.5 mm by an experienced pediatric dentist.

After cavity preparation, all teeth were acid-etched for 10 s using Scotchbond Universal Etchant (3M ESPE). This was followed by the application of Dentsply Prime & Bond Universal Adhesive (Dentsply Sirona), which was applied according to the manufacturer’s instructions and light-cured for 10 s before the placement of the restorative material.

Matrix system placement was standardized for all specimens. For the Palodent V3 sectional matrix system (Group P), the contoured matrix band was first inserted from the occlusal approach. The elastic wedge was then placed from the buccal side to ensure gingival adaptation, followed by the universal ring placement. For the Tofflemire matrix system (Group T), the universal retainer was mounted with the matrix band, positioned around the tooth, and tightened. The wooden wedge was then inserted from the buccal side to achieve gingival adaptation. All matrix systems were verified for proper adaptation before beginning the restoration procedure.

The cavities were restored incrementally, with each increment measuring 1 mm in thickness. Each increment was light-cured individually for 10 s using a light emitting diode (LED) curing light (Bluephase Style; Ivoclar Vivadent, Schaan, Liechtenstein; wavelength: 430-490 nm; output: 1,100 mW/cm²) to ensure complete polymerization. The final increment was carefully placed to extend above the contact points of adjacent teeth, ensuring proper proximal contact and anatomical occlusal relationships.

After curing the final increment, the matrix system and wedges were carefully removed to avoid damaging the restoration or proximal contacts. Finishing and polishing were performed using a slow-speed white stone finishing bur (Shofu Dental, Kyoto, Japan) to refine the anatomical contours and smooth the restored surfaces. This standardized finishing protocol ensured uniformity across all specimens while preserving the integrity of the proximal geometry.

Intraoral scanning procedure

After the resin-filled typodont teeth were detached from the dentiform model, intraoral scanning was performed using a TRIOS4 (3Shape, Copenhagen, Denmark). The intraoral scanning was performed under 24°C at room temperature with no dental chair light. As a standard, a natural typodont tooth with the same tooth number was also scanned. The scanned images were saved as STL files.

Assessment of 3D geometry of proximal surface anatomy

The stereolithography file (STL) format were transferred into the Gom Inspect 2018 software (GOM GmbH, Braunschweig, Germany). The surfaces of resin-filled typodont teeth were superimposed on the corresponding surfaces of the standard typodont tooth, using the root surfaces underneath the teeth as a reference point. Then, the discrepancies between the resin-filled and standard teeth were assessed on the designated points. The points were designed as 3-points which are intersections of horizontal and vertical lines on the reproduced proximal surface of the resin-filled teeth. The vertical line was drawn from the midline between mesiobuccal and mesiopalatal line angles and the horizontal lines were drawn at the gingival, middle, and occlusal 1/3.

For assessing the root mean square (RMS) values at proximal contact surfaces, a rectangular region of interest (ROI) with a width of 4 mm and a length of 3 mm was established from the midline of the mesiobuccal and mesiopalatal line angles on the superimposed proximal surfaces. Within the defined ROI, RMS values were automatically calculated using the ‘surface comparison’ command and visualized using color coded-mapping [18]. Assessments were performed by two independent examiners.

Assessment of proximal surface shape

The STL files were transferred into the Gom Inspect 2018 software (GOM GmbH) and aligned. The shapes were classified as concave, flat, and convex as previously described [19].

Statistical analysis

Data were analyzed using IBM SPSS Statistics 20 (IBM Corp., Armonk, NY, USA). Regarding the inter-examiner reliability, intraclass coefficient values were calculated and were 0.916 (P < 0.001) in the discrepancy evaluation, showing a highly acceptable level. For the pairwise comparisons, the Mann-Whitney U test was performed after assessing normality with the Shapiro-Wilk test. The comparison of RMS values was performed using the Kruskal-Wallis test. The data on the shape were analyzed using the Fisher’s exact test. The P-values <0.05 were considered to be significant.

Results

Regarding the discrepancies in the designated points, the discrepancy values at occlusal 1/3 were not significantly different (Table 3). At middle 1/3, significant differences in the discrepancy were observed between Group IT and IIP. At gingival 1/3, no significant differences were observed among the groups.

As shown in Fig. 1A, color-coded mapping showed no remarkable differences in discrepancies at proximal contact surfaces. Average RMS values at proximal contact surfaces were 0.05 ± 0.01 mm in the IP group, 0.06 ± 0.02 mm in the IT group, 0.06 ± 0.02 mm in the IIP group, 0.06 ± 0.02 mm in the IIT group with no statistical significance (Fig. 1B).

Regarding the proximal surface shape, concave shape was predominant when the Tofflemire system was used, whereas a flat surface was predominant when the Palodent system was used (Table 4). However, there were no significant differences among the groups (P = 0.239).

Table 3 Pairwise comparisons between the groups for the discrepancy

Groups	Median (mm)	IQR (Q1-Q3)	P-value
			IP	IT	IIP	IIT
Occlusal 1/3
IP	0.17	0.08 (0.13-0.21)		0.486	0.775	0.305
IT	0.16	0.05 (0.13-0.18)	0.486		0.838	0.061
IIP	0.16	0.11 (0.12-0.23)	0.775	0.838		0.116
IIT	0.22	0.13 (0.15-0.28)	0.305	0.061	0.116
Middle 1/3
IP	0.06	0.05 (0.02-0.07)		0.05	0.389	0.539
IT	0.02	0.02 (0.01-0.03)	0.05		0.026*	0.683
IIP	0.06	0.07 (0.02-0.09)	0.389	0.026*		0.250
IIT	0.03	0.06 (0.01-0.07)	0.539	0.683	0.250
Gingival 1/3
IP	0.02	0.04 (0.01-0.05)		0.567	0.775	0.461
IT	0.02	0.02 (0.01-0.03)	0.567		0.775	0.148
IIP	0.03	0.03 (0.01-0.04)	0.775	0.775		0.250
IIT	0.04	0.04 (0.02-0.06)	0.461	0.148	0.250

*P-values from the Mann-Whitney U test

 

Fig. 1 Three-dimensional (3D) discrepancy at proximal contact surfaces

(A) The representative images of the groups. The discrepancy is presented by color-coded mapping. (B) Bar graph of RMS values. Horizontal lines in the bars represents the median of RMS values. Dots in the bars represent the corresponding RMS values in each sample. Note that there were no significant differences among the groups (P = 0.539).

Table 4 Results of proximal surface shape according to the groups

Group	Proximal surface shape			P-value
	concave n (%)	flat n (%)	convex n (%)
IP	8 (53.3)	6 (40.0)	1 (6.7)	0.239
IT	4 (26.7)	11 (73.3)	0 (0.0)
IIP	9 (60.0)	6 (40.0)	0 (0.0)
IIT	6 (40.0)	9 (60.0)	0 (0.0)

Discussion

This study evaluated the effects of different resin and matrix systems on the reproducibility of proximal surface in posterior resin restorations in primary molars. The null hypothesis that there was a difference in the 3D geometry of proximal surface depending on resin and matrix systems was rejected.

An appropriate proximal surface anatomy is crucial for the long-term success of the Class II resin restorations. The influence of proximal surface anatomy in primary molars on the risk of dental caries is well-documented [10,20]. A previous systematic review on restorations in primary teeth found that Class II resin restorations failed more often than Class I resin restorations and secondary caries was the major reasons for the failure of the restorations [21]. A randomized controlled study showed an odds ratio of 5.1 for failure in Class II resin restorations compared to Class I resin restorations in primary molars [22].

The results of the 3D geometry of the proximal surface anatomy according to the resin and matrix system showed significant differences at the middle 1/3 between Groups IT and IIP. However, there were no significant differences in the RMS values among the groups. Furthermore, 3D evaluation results mostly differed by within 200 µm. In general, 200 µm is considered to be a clinically acceptable range for dental restorations [23]. While many studies have evaluated proximal contact tightness and contour [24,25], there are few studies evaluating proximal surface anatomy from a 3D perspective. Therefore, a deviation of less than 200 µm between standard and resin-filled typodont teeth is considered clinically acceptable when compared to dental restorations. This suggests that the choice of resin or matrix system may not be a critical factor in achieving accurate proximal surface reconstruction and that restorative success may depend more on technique than on material selection [26].

In this study, proximal surface morphology was more concave with the Tofflemire matrix system and had flatter surfaces with the Palodent sectional matrix system. However, there were no significant differences among the groups. These results are consistent with previous studies on proximal contact restorations of primary molars using three different matrix systems [27]. However, there are often conflicting results regarding proximal surface morphology depending on the matrix system [8,11,28]. Therefore, these results suggest that in determining the proximal contour of direct composite restorations, operator skill may have a greater influence than material selection [29].

This finding both supports and extends previous research in this field. While earlier studies have shown the importance of proper proximal contact in preventing secondary caries [21,22], the present study specifically addresses the technical aspects of achieving appropriate contacts in primary molars. The observation that operator technique may be more crucial than material selection aligns with previous findings in permanent teeth [26], but this study uniquely applies this to primary dentition.

However, this study has several limitations. First, this in vitro study may not fully replicate the clinical challenges encountered in pediatric patients, such as moisture control and limited access. The use of dentiform educational models, while advantageous for standardization of tooth morphology and cavity preparation, presents notable limitations. Most significantly, the absence of periodontal ligaments in typodont teeth may affect the behavior of matrix systems and the final proximal contour of restorations, as natural teeth allow for minor physiological movements during matrix placement and wedging. Second, the study evaluated only immediate post-restoration outcomes, while long-term clinical performance might differ. In the future, long-term clinical trials comparing different matrix systems in pediatric patients are needed to evaluate durability and clinical performance. Third, the use of different wedge types between groups (elastic wedges for Palodent V3 and wooden wedges for Tofflemire) might have influenced the final proximal contour outcomes. While elastic wedges were part of the Palodent V3 system, the variation in wedge type could affect the standardization of the study results. Finally, the standardized cavity preparations used in this study may not represent the variety of cavity configurations encountered in clinical practice.

In conclusion, the reproduced proximal surface geometry in Class II resin restorations was hardly influenced by resin and matrix systems. This finding suggests that creating the proximal surface in Class II resin restorations is less technique-sensitive and more dependent on pediatric dentists.

Abbreviations

Bis-EMA: bisphenol A ethoxylate dimethacrylate; Bis-GMA: bisphenol A-glycidyl methacrylate; IQR: interquartile range; LED: light emitting diode; P: Palodent V3 sectional matrix system; RMS: root mean square; ROI: region of interest; STL: stereolithography file; T: Tofflemire matrix system; TEGDMA: triethylene glycol dimethacrylate; UDMA: urethane dimethacrylate. 3D: three-dimensional

Ethical Statements

This study was reviewed and approved by the Ethics Committee of Kyung Hee University Dental Hospital, Seoul, Republic of Korea (KH-DT23047).

Conflicts of Interest

The authors declare that they have no conflict of interest.

Funding

This research did not receive any funding.

Author contributions

MJ and OHN: conceptualization and study design; FSA: data acquisition; JRY and YKC: methodology; FSA, JRY, and OHN: writing; MKB, YKC, HL, SCC, and OHN: review and editing. All authors gave their final approval and agreed to be accountable for all aspects of the work.

ORCID iD

¹⁾FSA: [e-mail], https://orcid.org/0009-0008-2202-4464

²⁾JRY: [e-mail], https://orcid.org/0000-0003-4278-5566

^3,4)MKB: [e-mail], https://orcid.org/0009-0008-6774-9258

⁵⁾MJ: [e-mail], https://orcid.org/0000-0001-9579-076X

^6,7)YKC: [e-mail], https://orcid.org/0000-0001-8059-9305

^6,7)HL: [e-mail], https://orcid.org/0000-0001-7287-5082

^6,7)SCC: [e-mail], https://orcid.org/0000-0001-7221-2000

^6,7)OHN*: [e-mail], https://orcid.org/0000-0002-6386-803X

Data Availability Statements

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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