Evaluation of Proximal Contact Tightness in Anterior Resin Composite Restorations: A Comparative In Vitro Study of Injectable, Flowable, and Universal Composites

ABSTRACT

Aim:

Proximal contact tightness (PCT) is critical for periodontal health, stability, and longevity of restorations. However, limited evidence exists comparing PCT in anterior restorations using injectable and flowable resin composites with transparent silicone indices versus universal composites with a putty index. This study compared the PCT of injectable resin composites (IJ), flowable resin composites (FL), and universal resin composites (U) in Class IV anterior restorations.

Methods:

Forty-five standardized Class IV restorations were performed on typodont models by an experienced restorative dentist, divided into three groups (n = 15/group): IJ, FL, and U. Modified Fédération Dentaire International (FDI) criteria were used for qualitative assessment of PCT scores, while a digital force gauge provided quantitative measurements. Statistical analyses were performed using the chi-square test for PCT scores and Kruskal–Wallis one-way analysis of variance for PCT forces, with significance set at P < 0.05.

Results:

The U group achieved the highest mean PCT force (5.05 ± 1.73 N, P < 0.01) and the highest percentage of ideal/strong PCT scores (80%–86.7%). In contrast, the IJ and FL groups demonstrated lower forces (2.07 ± 0.59 N and 2.09 ± 0.86 N, respectively) and a higher incidence of weak/inadequate contacts (20%–40% for FL; 33.33%–40% for IJ). No significant difference was found between IJ and FL groups (P > 0.05).

Conclusion:

Universal composites with a putty index provide superior PCT in Class IV restorations compared to injectable and flowable composites with transparent silicone indices. Clinicians using injectable or flowable composites should consider additional adjustments to optimize PCT. Future research should validate the effects of shrinkage and variability on PCT.

INTRODUCTION

Direct resin composite anterior restoration is a widely used clinical procedure due to its ability to effectively replicate natural teeth’ esthetics. A 10-year clinical study by Márcio et al.[1] concluded that direct resin composite is a reliable treatment with restorative effects, on aesthetic appearance and functional integrity. Anterior resin composite restorations can be completed in a single appointment and have minimal invasiveness compared to indirect restorations.[1,2] Consequently, patients may need direct composite restoration to achieve desired esthetic outcomes without higher laboratory costs associated with indirect restorations. The conventional method using the universal resin composite with a free-hand direct technique requires advanced artistic skills; therefore, results can be unpredictable.[2] To overcome this, a rigid silicone putty index creates palatal surfaces.[3] This index ensures precise alignment and fit by extending to cover adjacent unrestored teeth. Initially, a universal resin composite is applied to the palatal section of the silicone putty index to shape the palatal aspect, followed by manual completion of the restoration on the proximal and buccal surfaces.[4,5]

To achieve esthetic and functional restoration, some clinicians utilize highly filled low-viscosity resin composites for anterior restoration. Kouri et al.[6] reported that the injection technique is reliable for veneer restoration. Additionally, the injection technique is effective for single and multiple Class IV restorations.[7,8] This involves injecting and polymerizing the low-viscosity resin composite through a transparent silicone index. This index enables light penetration to polymerize the composite beneath it.[9,10,11]

The Genial™ universal injectable resin composite by GC Corp. has garnered significant attention for its enhanced durability, wear resistance, and reduced water absorption, attributed to the integration of high-strength, ultra-fine barium filler particles and full-coverage silane coating (FSC) technology. This injectable resin composite demonstrates high thixotropy, which enables its use in anterior restoration by injection into a transparent silicone index. Despite these advancements, no published research currently confirms the effectiveness of these materials and transparent silicone index in achieving optimal contact. Most existing studies focus on the esthetic outcomes rather than the functional aspects of proximal contact tightness (PCT). Proper PCT is important for maintaining periodontal health, preventing food impaction, and ensuring the stability of the teeth in the dental arch.[12,13,14]

Therefore, this study evaluated the performance of injectable and flowable composites with transparent silicone and universal composites with a putty index for anterior restoration, focusing on their ability to create PCT. The null hypothesis was that there was no difference in the PCT of anterior resin composite restorations among injectable and flowable resin composites with a transparent silicone index compared to those done with universal resin composites using a putty index.

MATERIALS AND METHODS

EXPERIMENTAL DESIGN AND SAMPLE SIZE

The investigation of the PCT score and force of three distinct materials and corresponding restoration techniques for class IV restoration is as follows [Figure 1]:

Figure 1.

Experimental design

• Group 1: Injectable resin composite with a transparent silicone index (IJ).

• Group 2: Flowable resin composite with a transparent silicone index (FL).

• Group 3: Universal resin composite with a putty index (U).

The sample size was calculated using G*Power software, with effect sizes of 2.07 and 2.83 estimated from previous studies by Kwon et al.[15] and Saber et al.[16] with 5% alpha error and 95% beta power, respectively. Although three samples per group were sufficient for significant effects, the sample size was increased to 15 specimens per group to account for potential variability in experimental design.

STANDARDIZED CLASS IV ANTERIOR RESTORATION MODELS

The 45 right upper anterior typodonts (Nissin Dental Product, Kyoto, Japan) were used to control variables such as cavity size and shape, preventing the influence of human subject variables like periodontal tissue, tooth size discrepancy, and contact alignment. After verifying proper PCT in the dentoform, the specimens were randomly assigned to three groups. Standardized Class IV cavities (2 mm mesiodistal depth and 7 mm axial height) were prepared by a single operator using a new flat-end cylinder diamond bur (Edenta AG, Zurich, Switzerland) for each group.

TRANSPARENT SILICONE INDEX AND PUTTY INDEX FABRICATION

For groups 1 and 2, transparent silicone indices were fabricated individually for each restoration to prevent distortion. Transparent silicone (Elite glass, Zhermack, Badia Polesine, Italy) was applied to a partial tray, positioned on a maxillary dentoform, and allowed to be set for 2 min and 30 s. After removal, two perforation holes were created: one at the incisal edge for composite insertion and another on the labial surface, 1 mm above the gingival wall, to allow the excess composite to escape [Figure 2].

Figure 2.

Transparent silicone index fabrication. (A) The auto-mixed transparent silicone is applied onto a partial tray and positioned on the anterior region. (B) The tray is precisely positioned, and the silicone is allowed to be set completely for 2 min and 30 s. (C) The transparent silicone index is removed from the tray. (D) Two perforation holes are created using a tip of injectable composite: one at the incisal edge and another on the labial surface, about 1 mm higher than the gingival wall

The putty index was created for group 3, each designed for a specific restoration to prevent silicone distortion. Putty silicone (I-Sil putty-Regular, Spident Co., Ltd., Seoul, South Korea) was used to replicate the desired palatal anatomy of restoration on a maxillary dentoform. The mixed putty was applied from canine to canine and allowed to set for 3 min.

CLASS IV RESIN COMPOSITE RESTORATION

A trained restorative dentist with 10 years of experience performed the restoration procedure on all three groups [Figure 3].

Figure 3.

Class IV restoration procedure

Group 1: Injectable resin composite with a transparent silicone index (IJ)

Teflon tape was placed on the adjacent left incisor typodont, and the index was checked for precise fit and stability. G-ænial Universal Injectable (GC Corp., Tokyo, Japan) was injected into the incisal hole until excess material emerged through the second hole, which was then removed. The composite resin was light-cured using an LED light curing unit (3M Elipar Deepcure-L, 3M ESPE, Minnesota, United States) for 20 s on labial and lingual surfaces through the clear silicone index. After removing the index, an additional 40 s of polymerization was applied to each surface, and the gingival margin of the restoration was refined using blade No. 12.

Group 2: Flowable resin composite with a transparent silicone index (FL)

The procedure followed the steps as Group 1, with the substitution of G-ænial Universal Injectable with flowable composite (Filtek Z350XT flowable, 3M ESPE, Minnesota, United States).

Group 3: Universal resin composite with a putty index (U)

Teflon tape was placed on the adjacent left incisor typodont, and the index was checked for precise fit and stability on the palatal surface. The first layer of universal resin composite material (Z350 XT, 3M ESPE, Minnesota, United States) for creating the palatal shelf was applied using a putty index as a guide and light-cured for 20 s. After removing the putty index, a subsequent composite layer was applied to create the proximal and labial composite regions manually. An additional 40 s of polymerization was applied to each surface, and the gingival margin of the restoration was refined using blade No. 12.

PROXIMAL CONTACT TIGHTNESS (PCT) SCORE AND FORCE EVALUATION

The PCT evaluation was performed blindly using two measurement methods: 1) the PCT score and 2) the PCT force.

The PCT score was assessed by three trained evaluators, each with over 10 years of clinical experience, including one prosthodontic and two restorative dentists. During the training period, each evaluator evaluated the same set of 10 standardized specimens twice to ensure reliability. The Kappa values for intra-operator reliability for Examiners 1, 2, and 3 were 1.0, 0.85, and 0.84, respectively, indicating high reliability. A qualitative 5-point rating system, based on dental floss combination with 25-, 50-, and 100-µm metal bands, was employed following modified FDI 2010 criteria[10,11] as follows.

• Score 1 (normal, ideal contact): A 25-μm metal band can pass through the contact area.

• Score 2 (slightly weak contact): A 50-μm metal band can pass through the contact area.

• Score 3 (strong contact): A 25-μm metal band cannot pass through the contact area.

• Score 4 (too weak contact; repair or adjustment is possible): A 100-μm metal band can pass through the contact area.

• Score 5 (loose contact, repair not possible): A 100-μm metal band easily passes through the contact area.

Additionally, a trained evaluator measured PCT force using a calibrated digital force gauge (SHAHE Model SF-10, Zhejiang, China) with a hook attachment on a 0.05-mm-thick metal strip. The gauge was stabilized, calibrated with a 0.5 kg weight, and zeroed to ensure accuracy before each material test. The metal strip was inserted into the proximal area and pulled buccally from the interproximal region of a specimen in the arch model [Figure 4]. The maximum frictional force recorded during strip removal was the PCT force. This measurement was repeated three times, and the mean of these three measurements was taken as the representative value for each contact area.

Figure 4.

Proximal contact tightness force measurement. (A) Insert a 0.05-mm metal strip between teeth 11 and 21 and attach a hook of the digital force gauge buccally to the strip. (B) Fix the typodont model to the surveyor and set the digital force gauge to 0 before evaluation. And (C) pull the strip buccally and record the maximum frictional force as the proximal contact tightness force

STATISTICAL ANALYSIS

The statistical analysis was conducted using SPSS 20.0 for Windows (IBM Corp., New York, United States) at P ≤ 0.05. The Kappa inter-evaluator reliability of the PCT score results among the three examiners was evaluated, and the chi-square test was employed to compare the frequency of the PCT scores among groups. For the PCT force, the Kruskal–Wallis one-way analysis of variance (ANOVA) and pairwise Mann–Whitney U tests were utilized to identify inter-group statistically significant differences.

RESULTS

The results of the PCT scoring by examiners 1, 2, and 3 are shown in Figure 5A and Table 1. The Kappa inter-examiner agreement for scoring PCT was 0.80 (between Examiners 1 and 2), 0.56 (between Examiners 1 and 3), and 0.57 (between Examiners 2 and 3). The chi-square test revealed statistically significant differences in PCT scores among resin composite types with their corresponding index (P = 0.01, <0.01, and <0.01 for examiners 1, 2, and 3, respectively).

Figure 5.

(A) Bar graph of restoration count numbers achieving each contact tightness score and percentage of restoration achieving each material assesses by all examiners. (B) The range of contact tightness force corresponds to each contact tightness score

Table 1.

The number of restorations achieved in each contact tightness score of injectable resin composite with a transparent silicone index (IJ), flowable resin composite with a transparent silicone index (FL), and universal resin composite with a putty index (U)

Examiner	Group	Number of restorations achieved each contact tightness score					P value
		1	2	3	4	5
		Normal contact	Slightly weak	Strong	Too weak	Loose contact
1	Injectable (IJ)	3a	6a	0a	6a	0a	P = 0.01
	Flowable (FL)	6a	6a	0a	3ab	0a
	Universal (U)	12b	0b	3b	0b	0a
2	Injectable (IJ)	3a	7a	0a	5a	0a	P < 0.01
	Flowable (FL)	5a	4a	0a	6a	0a
	Universal (U)	13b	0b	2a	0b	0a
3	Injectable (IJ)	3a	12a	0a	5a	0a	P < 0.01
	Flowable (FL)	3a	12a	0a	6a	0a
	Universal (U)	13b	0b	2a	0b	0a

The same subscript letter assigned by each examiner indicates that the count corresponding to contact tightness score does not differ significantly when compared among the experimental groups

Regarding PCT force, the results of Kruskal–Wallis one-way ANOVA indicated a significant difference between groups, as presented in Table 2. The range of PCT forces corresponding to each PCT score is presented in Table 3 and Figure 5B].

Table 2.

The contact tightness force (N) of injectable resin composite with a transparent silicone index (IJ), flowable resin composite with a transparent silicone index (FL), and universal resin composite with a putty index (U)

Groups	Contact force (N)				P value
	Mean	SD	Median	IQR
Injectable (IJ)	2.07	0.59	1.9	1.70–2.20
Flowable (FL)	2.09	0.86	2	1.65–2.60
Universal (U)	5.05	1.73	4.7	4.25–5.05

Table 3.

The range of contact tightness force (in N) corresponds to each contact tightness score

Examiner	Group	Range of contact tightness force (N)
		1	2	3	4	5
		Normal contact	Slightly weak	Strong	Too weak	Loose contact
1	Injectable (IJ)	2.5–3.8	1.9–2.2		1.4–1.8	0
	Flowable (FL)	2.4–3.3	1.6–2.3		0.1–0.5	0
	Universal (U)	2.7–5.7		5.2–9.6		0
2	Injectable (IJ)	2.5–3.8	1.8–2.2		1.4–1.7	0
	Flowable (FL)	2.6–3.3	2–2.4		0.1–1.7	0
	Universal (U)	2.7–5.7		8.2–9.6		0
3	Injectable (IJ)	2.5–3.8	1.4–2.2			0
	Flowable (FL)	3.2–3.3	2–2.6			0
	Universal (U)	2.7–5.7		8.2–9.6		0
Summary		2.5–5.7	1.4–2.6	5.2–9.6	0.1–1.8	0
Mean (SD)		3.83 (1.01)	1.82 (0.52)	7.67 (2.25)	1.46 (0.47)	0
Median (IQR1–IQR3)		3.95 (2.83–4.68)	1.85 (1.68–2.13)	8.20 (6.70–8.90)	1.65 (1.48–1.70)	0

DISCUSSION

Optimal PCT is essential for maintaining periodontal health, preventing food impaction, and ensuring the stability of teeth within the dental arch.[12,13,14] Weak PCT can lead to food impaction, discomfort, periodontal disease, caries development, and potential tooth movement and misalignment.[12,13,14] Recently, an advanced material, G-ænial Universal Injectable (GC Corp., Tokyo, Japan), a nano-filled high-strength injectable resin composite, was introduced to the market in 2018 for anterior and posterior direct restorative dentistry. This injectable composite is recommended for use with a transparent silicone index to assist in creating anatomical shapes, including contact areas. Many studies have investigated the PCT, primarily focusing on class II posterior restoration.[16,17,18,19,20,21] Thus, this study aimed to address this issue by comparing the effectiveness of creating PCT in class IV anterior restorations when using injectable and flowable resin composites with a transparent silicone index versus a universal resin composite with a putty index. A restorative dentist with over 10 years of experience used standardized material application and curing protocols to avoid operator bias or material handling variability.

This study employed two PCT assessment methods. In the first assessment, the clinically relevant modified FDI PCT score categorized the PCT into clinically acceptable (scores 1–3) and clinically unacceptable categories (scores 4–5).[12,13,22,23,24,25,26] Three experienced clinicians conducted the evaluations, revealing substantial agreement between examiners 1 and 2 (Kappa = 0.80) but only moderate agreement for examiner 3 with the others (Kappa = 0.56 and 0.57). This discrepancy may be attributed to the fact that examiner 3 was the operator who prepared all the experimental specimens, which is needed for careful interpretation of results, especially when clinicians assess their work. In the second assessment, PCT was quantitatively measured in Newton using a digital tooth gauge with a 50-micron metal blade, providing an objective evaluation.[27,28,29] To ensure consistency, all restorations were prepared by an experienced restorative dentist with over 10 years of expertise under controlled conditions.

The study demonstrated that the type of resin composite and its corresponding index significantly influenced PCT, as evidenced by consistent contact tightness scores and forces. The utilization of the universal resin composite with a putty index resulted in achieving clinical acceptability statistically tighter proximal contact forces (Mean = 5.05N, Median = 4.7N) compared to use of injectable (Mean = 2.07N, Median = 1.9N) and flowable resin composites (Mean = 2.09N, Median = 2N) with a transparent silicone index. The universal resin composite restorations predominantly (80%–86.7%) achieved ideal or normal PCT when assessed by all three examiners. A small percentage (13.3%–20%) had strong contact. In contrast, injectable and flowable resin composite restorations produce normal, slightly weak, and too weak PCT scores. Two examiners found that 20%–40% of restorations with flowable resin and 33.33%–40% with injectable resin had too weak PCT, requiring repair or adjustment for proper contact. This study was the first to combine PCT scores and force assessments, providing valuable data for future research on how each PCT score corresponds to a specific range of PCT force values. Unfortunately, direct comparisons with other studies were not feasible due to the absence of existing research specifically addressing PCT score and force in anterior composite restorations. Nonetheless, the proximal PCT force observed in this study aligns with those observed in Loomans et al.’s[29] study reported in posterior teeth, in which the variability ranged from 0.10 N to 12.43 N. The findings align with those of many previous studies on posterior Class II restorations, indicating that PCT in dental restorations is significantly influenced by factors such as the type of the resin composite, including variations in viscosity and shrinkage, and technique-related aspects like the use of matrix systems or contact-forming instruments and the incremental layering method during the restoration procedure.[18,25,29,30,31]

First, the type of resin composite significantly influences the creation of PCT. In this study, more viscous universal resin composites created more ideal contact and contact force than injectable and flowable composites. This finding aligns with that of the study by El-Shamy et al.[18] in Class II restorations, which found that the much more viscous bulk-fill (Tetric EvoCeram Bulk-fill) and nano-hybrid (Tetric EvoCeram) composites created significantly tighter contacts compared to flowable materials like G-ænial Universal Flo and SDR. Furthermore, Khoramian Tusi et al.[32] found that G-ænial Universal Injectable exhibited significantly higher polymerization shrinkage than conventional composites but similar shrinkage to flowable composites. The greater shrinkage volume of injectable and flowable resin composites toward the bonded surface of the restoration may potentially lead to reduced PCT of class II posterior restoration compared to universal resin composites. These studies support claims that higher viscosity and lower-shrinkage resin composites produce tighter proximal contacts, as evidenced by similar several studies conducted in posterior restorations.[21,31,33,34,35]

Second, this finding supports that technique-related aspects, such as the use of different indices like the putty index versus transparent silicone index, as well as the incremental layering of universal resin composites versus single bulk-fill of injectable and flowable composites, may affect the PCT in restorations. The putty index can significantly increase the number of restorations achieving ideal PCT. The layering restoration techniques can explain the achieving of tighter proximal contacts using a putty index, the palatal shelf, and the labial layer, which help minimize the polymerization shrinkage of the resin composite.[36,37] Furthermore, the application of pressure by using a hand instrument of the sculptable conventional resin composite labial layer offers better contact with the adjacent tooth. It relies on the clear visibility and skill of the operator rather than solely relying on the accuracy of the putty index.[38] Conversely, using a transparent silicone index with injectable and flowable resin composites often results in higher restorations with slightly weak or inadequate proximal contacts. Previous studies have demonstrated that using transparent silicone indices for injecting flowable resin composites can produce direct veneers comparable to diagnostic wax-ups without statistically significant differences.[6,39] That injectable flowable resin-based composite veneers can serve as successful long-term esthetic prototypes before ceramic veneers.[40] However, the findings of this study suggest that achieving proper proximal contacts has been compromised when compared to using a putty index with a universal composite. The lower PCT observed in injectable and flowable resin composite restorations can be explained by several factors. Using a transparent silicone index with injectable and flowable resin composites may be less effective in controlling polymerization shrinkage compared to the layering technique with a putty index.[41] This could lead to increased shrinkage of the restoration, potentially compromising PCT, as the composite material may pull away from the adjacent tooth during curing. Additionally, the flexible and distortable nature of transparent silicone indices may contribute to inaccuracies in replicating the contact area of the restoration.[42] Moreover, the observed challenge in determining whether there has been displacement of Teflon tape after inserting the transparent silicone index.

Based on the results, clinicians should prioritize universal resin composites with a putty index for class IV restorations, as this approach demonstrated superior performance in achieving optimal proximal contact. The study observed that 13.3%–20% of universal resin composite restorations initially exhibited strong tight contacts, which may relax to ideal contact over time in clinical use due to an “adaptation mechanism” involving periodontal tissue adjustments and/or proximal wear of adjacent tooth surfaces or restorative materials, consistent with findings by Looman et al.[30] However, in cases where injectable or flowable resin composites are preferred due to easier handling or anatomical guidance advantages, these materials may not achieve the same level of PCT as a universal composite with a putty index. Therefore, clinicians may need to consider adjustments or additional techniques to ensure satisfactory proximal contact. The study’s limitations include its in vitro design, focusing on the effect of resin composite and index on PCT. Future studies should assess the impacts of composite shrinkage and layering techniques on PCT, explore novel materials or techniques for improving PCT in anterior restorations, and propose in vivo validation to ensure clinical relevance. Additionally, research should investigate other critical factors for clinical success, such as the shape and dimensions of the contact area, marginal adaptation, and the occurrence of voids and gaps. The effects of the oral environment and functional use, such as PCT wear, indicate the need for further research.

CONCLUSION

Universal resin composites with a putty index provide superior PCT to injectable and flowable composites with transparent silicone indices in Class IV restorations. However, injectable and flowable composites offer handling and anatomical advantages, albeit with potential compromises in PCT. Clinicians must consider additional adjustments when using these materials. Future clinical studies must validate these findings and investigate factors like polymerization shrinkage and the effects of the oral environment on PCT.

CONFLICTS OF INTEREST

The authors declare no conflicts of interest related to this study.

AUTHORS CONTRIBUTIONS

N.K., S.L., C.P. and C.A.; methodology, N.K., S.L., P.S., W.B. and C.A.; formal analysis, N.K., P.S., W.B. and C.A.; investigation, N.K., P.S., W.B. and C.A.; data curation, N.K., S.L., P.S., W.B. and C.A.; writing—original draft preparation, N.K., C.P. and C.A.; writing—review and editing, N.K., C.P. and C.A.; project administration, N.K. and C.A. All authors have read and agreed to the published version of the manuscript.

ETHICAL POLICY AND INSTITUTIONAL REVIEW BOARD STATEMENT

This laboratory study did not involve human subjects or animals; therefore, ethical approval was not required.

PATIENT DECLARATION OF CONSENT

Not applicable as this was an in vitro laboratory study without patient involvement.

DATA AVAILABILITY STATEMENT

The data supporting this study’s findings are available from the corresponding author upon reasonable request.

Funding Statement

This research funding is supported by the Faculty of Dentistry, Thammasat University.