Short-term clinical and micromorphological comparison of injectable and paste composites in occlusal cavities

Abstract

Background

The aim of this clinical trial was to evaluate and compare the 12-month performance of a contemporary injectable and a conventional paste-type resin composite in Class I cavities using the modified USPHS criteria, supported by SEM analysis.

Methods

A total of 72 teeth from 34 volunteers were included. Each participant received at least two restorations (one per material); in cases where more eligible cavities were present, additional restorations were performed accordingly. All restorations were performed under rubber dam isolation following conservative Class I cavity preparation, using selective enamel etching and a universal adhesive. All procedures were carried out by the same clinician using a paste-type resin composite (Filtek Z250 Universal Restorative) and an injectable resin composite (G-ænial Universal Injectable). Two calibrated investigators evaluated restorations at baseline, 6, and 12 months using modified USPHS criteria. SEM analysis was conducted on a total of 66 epoxy replicas obtained at baseline, 6, and 12 months from 11 randomly selected participants. Statistical analysis was performed using SPSS (Version 23.0, IBM, NY) with Pearson’s chi-square and Cochran’s Q tests.

Results

Both materials demonstrated comparable and clinically acceptable performance over the 12-month evaluation period according to the modified USPHS criteria, with no statistically significant differences observed at any time point across all criteria, although minor changes were observed in marginal adaptation and color match in a few restorations (p > 0.05). All participants attended the follow-up visits, resulting in a 100% recall and retention rate.

Conclusion

The injectable (G-ænial Universal Injectable) and paste-type (Filtek Z250) resin-composites demonstrated comparable and clinically acceptable short-term performance in Class I occlusal restorations, although longer-term follow-up is needed to confirm these findings.

Trial registration

As trial registration took place after the first participant was enrolled, the study was retrospectively registered at ClinicalTrials.gov on December 21, 2023, with ID: NCT06192667. https://register.clinicaltrials.gov/prs/beta/studies/S000DTX600000072/recordSummary/.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-026-08472-7.

Introduction

Resin composites have become the preferred material for direct restorations in dentistry due to their aesthetic appeal and improved mechanical properties. Despite these advantages, composite restorations can still fail over time, often due to secondary caries or fractures, and their success is influenced by technique-sensitive factors [1]. To accommodate different clinical needs, resin composites are generally classified by viscosity into conventional paste-type and flowable resin composites.

Flowable resin composites were first introduced in the mid-1990s as less viscous versions of composites designed for improved cavity adaptation [2]. The primary indication for first-generation flowable composites was their use as a base or liner beneath resin restorations [3]. However, a recent systematic review has reported that the use of flowable composites as cavity liners does not have a significant effect on the long-term clinical performance of direct composite restorations. Moreover, this approach has been described as neither offering a distinct clinical advantage nor disadvantage, but rather representing a viable option in routine clinical practice [4].

Flowable composites are characterized by their low viscosity, which facilitates ease of application and improved adaptation to cavity walls. However, their lower filler content may adversely affect their mechanical and optical properties, thereby limiting their use mainly to Class V restorations, liner applications, pit and fissure sealants, and marginal repair procedures [5–7]. Furthermore, the relatively high volumetric polymerization shrinkage of approximately 5% has also been reported as a significant drawback of conventional flowable composites [8]. Consequently, due to their lower filler content, early-generation flowable composites exhibited reduced mechanical strength and wear resistance, limiting their use in high-stress areas [7].

Recently, highly filled flowable composites with refined nano-filler technology have been developed to overcome these limitations. These newer materials possess enhanced mechanical properties and wear resistance, enabling their use even in posterior restorations that bear significant occlusal loads [9]. In fact, changes in filler composition and coupling have improved resistance to crack propagation in these composites, potentially giving them better long-term wear characteristics than earlier versions.

This latest generation of flowable composites is often referred to as “injectable composites” because they can be delivered directly into the cavity via a syringe, while still setting to a high-strength restorative material. These materials are marketed as universal composites indicated for Class I–V cavities [10, 11]. The injectable placement technique has been reported to offer practical advantages such as more efficient cavity filling, improved adaptation to cavity walls, and reduced procedure time compared to the traditional application of paste-type composites. These handling improvements could translate to fewer voids at the tooth–restoration interface and a more streamlined restorative workflow [12].

An in-vitro study [13] has reported that low-viscosity resin composites can match or even exceed the wear resistance and flexural strength of conventional hybrid composite. Moreover, a 36-month clinical trial that investigated the application of earlier generations of flowable composites in posterior teeth reported no significant differences in clinical effectiveness with a conventional paste composite [14]. The authors also noted that flowable material provided easier handling and demonstrated comparable success rates, with no failures observed over the three-year evaluation period. These findings indicate that properly formulated contemporary injectable composites can be considered safe for routine clinical application.

Beyond clinical evaluations, assessing restorative materials through micromorphological analyses can provide a more comprehensive understanding of their performance [15]. Scanning electron microscopy (SEM) allows high-resolution visualization of surface morphology and is widely used to examine the restoration–tooth interface, surface defects, and microleakage [16]. SEM analyses of silicone replicas have frequently been employed in clinical research to provide micromorphological insights that complement traditional clinical evaluations [17].

Furthermore, the literature on these next-generation composites is relatively limited, and inconsistent terminology (e.g. “high-filled flowable” vs. “injectable” vs. “bulk-fill”) can make it challenging to generalize findings. Although injectable resin-composites have been investigated in broader restorative contexts, no split-mouth clinical trial with complementary SEM assessment has specifically compared their performance with conventional paste-type materials in strictly confined Class I occlusal cavities over multiple evaluation periods. This clinical scenario—minimally invasive occlusal restorations—remains insufficiently documented for contemporary high-filled injectable formulations. Therefore, the present clinical trial was designed to evaluate and compare the 12-month performance of a contemporary injectable and a conventional paste-type resin composite in Class I cavities, complemented by SEM analysis of the restorations. The null hypothesis was that there would be no difference in the clinical performance between the injectable and the paste-type resin composite over 12 months.

Materials and methods

Study design

This randomized controlled clinical trial was based on the guidelines outlined in the Consolidated Standards of Reporting Trials (CONSORT) 2010 statement.

Ethical approval

Ethical approval for the protocol was granted by the Institutional Research Ethics Committee for Clinical Investigations (Date: 21.03.2023, Protocol No: KA-22112 ). The trial was retrospectively registered at ClinicalTrials.gov, as registration occurred after enrollment of the first participant. All participants were informed about the study protocol and signed written informed consent forms.

Sample size calculation

The sample size calculation was performed using G*Power (version 3.1.9.2), with the participant defined as the unit of analysis in accordance with the split-mouth design. Using an effect size of 0.5, α = 0.05, and 90% power, the minimum required sample was 25 participants, providing 50 paired restorations. An effect size of 0.5 was assumed as a medium effect size according to Cohen’s criteria, due to the lack of sufficient prior clinical data for this specific comparison. To compensate for possible drop-outs, the number of participants was increased to 34, resulting in 72 restorations.

Patient selection

A total of 34 volunteers (23 female, 11 male), aged between 18 and 35 years (mean age: 26.5), were recruited for the study. Baseline characteristics and distribution of selected teeth by restorative material are presented in Table 2. The included teeth consisted predominantly of posterior molars, with a limited number of premolars presenting with occlusal Class I carious lesions. Participants were selected from patients receiving care at the Department of Restorative Dentistry, Faculty of Dentistry, Hacettepe University.

Table 2.

Baseline characteristics and distribution of selected teeth by restorative material

Number of Restorations	G-ænial Universal Injectable n (%)	Filtek Z250 Universal Restorative n (%)
Tooth type
Premolar	2 (5.6)	2 (5.6)
Molar	34 (94.4)	34 (94.4)
Arch distribution
Maxillary	9 (25)	13 (36.1)
Mandibular	27 (75)	23 (63.9)

Inclusion and exclusion criteria

Participants included in the study were healthy volunteers who had no systemic disease that would contraindicate restorative treatment, demonstrated good oral hygiene habits, had completed initial periodontal therapy, possessed at least 20 teeth in occlusion, had an opposing antagonist tooth in the opposing arch, and presented with at least two occlusal carious lesions.

Individuals were excluded from the study if they had advanced periodontal disease, any systemic condition that could interfere with restorative procedures, a history of bruxism, malocclusion, known allergies, refused to attend follow-up appointments, or were pregnant or breastfeeding at the time of recruitment.

Teeth included in this study were selected according to specific inclusion criteria to ensure sample standardization. Only teeth presenting carious lesions strictly limited to the occlusal surface, with no involvement of other surfaces, were considered eligible. Additionally, teeth that required endodontic intervention or pulp capping were excluded. Prior restorative treatments or application of fissure sealants also constituted exclusion criteria to eliminate any confounding variables that could affect the study outcomes.

Randomization and group allocation

Participants were consecutively recruited for the study, which employed a split-mouth design to enable intra-individual comparisons. Each participant received two standardized Class I occlusal restorations: one using G-ænial Universal Injectable and the other using Filtek Z250 Universal. Only individuals presenting with two, or more suitable teeth were included, and in cases where additional eligible cavities were present, more restorations were performed accordingly.

Randomization was performed using a computer-generated random number table created via the website www.randomizer.org. The allocation of materials to specific teeth was prepared by an independent statistician who was not involved in any other phase of the study. Allocation details, including composite material and tooth sequence, were placed in sequentially numbered, opaque, sealed envelopes opened before treatment. The tooth with the highest number received the first protocol, followed by the next tooth in sequence. Although operator blinding was not possible, both participants and evaluators were blinded to the composite material used, making the study double-blind.

Restorative procedure

Clinical evaluations were conducted using visual inspection with air–water spray, a mouth mirror, reflector light, and, when necessary, a blunt-ended periodontal probe (WHO probe). Occlusal caries were assessed according to the International Caries Detection and Assessment System II (ICDAS-II), and only teeth scored as Code 4 were included. Radiographic evaluation was also performed, and teeth classified as D1 (outer one-third of dentin) were selected. All tooth surfaces were cleaned using a pumice-water mixture and a silicone rubber cup prior to treatment. Local anesthesia was administered when needed. All restorations were performed using a rubber dam (OptraDam, Ivoclar Vivadent, USA) and a saliva ejector.

Class I cavities were prepared in a conservative manner, ensuring the lesions remained within the occlusal surface. Preparations were performed under water cooling using round (ISO 801 − 014) and fissure (ISO 847 − 016) diamond burs (G&Z Instrumente, Austria), followed by caries excavation using a slow-speed round steel bur (ISO 008, G&Z Instrumente, Austria). Cavity preparation was guided by the extent of caries and completed when the dentin demonstrated sufficient hardness upon tactile examination; only lesions with a depth of 2 mm measured from the cavosurface margin, confirmed using a periodontal probe, were considered eligible for inclusion.

Selective enamel etching was performed with 37% phosphoric acid (Super-Etch, SDI, Australia) for 15 s. A universal adhesive (G2 Bond Universal, GC, Tokyo, Japan) was applied with a two-step protocol and light-cured for 10 s (Radii Plus, SDI, Australia; 1000 mW/cm²). The same adhesive protocol was employed for both restorative groups.

G-ænial Universal Injectable (GC, Tokyo, Japan) was applied in 2-mm increments and light-cured for 20 s. Filtek Z250 Universal (3 M ESPE, USA) was placed using a layering technique with the same increment thickness and curing time. A total of 72 restorations were completed over a three-month period. The details of the restorative materials are summarized in Table 1.

Table 1.

Materials used in the study

Material/Batch	Type	Manufacturer	Composition
G-ænial Universal Injectable	High-Filler Flowable Composite / Nanohybrid	GC Corporation, Tokyo, Japan (Lot: 2201121, Lot: 2204061)	UDMA, Bis-MEPP, TEGDMA, pigments, photopolymerization initiators; fillers: silicon dioxide and strontium glass (10–200 nm); filler content: 69 wt%, 50 vol%; incorporates Full Silane Coverage (FSC) technology.
Filtek Z250 Universal Restorative	Paste-Type Resin Composite / Microhybrid	3 M ESPE, St. Paul, MN, USA (Lot: 9251732)	Bis-GMA, UDMA, Bis-EMA, TEGDMA; inorganic fillers: zirconia/silica particles (0.01–3.5 μm, mean 0.6 μm); filler content: ~82 wt%, 60 vol%.
G2 Bond Universal	Adhesive System	GC Corporation, Tokyo, Japan (Lot: 2208021)	Two-bottle universal adhesive system; contains functional monomers (4-META, MDP), UDMA, Bis-MEPP, solvents (acetone, water), photoinitiators, and silica fillers. Designed for both self-etch and total-etch modes with dual-cure compatibility.
Super Etch (Etching Gel)	Etching gel	SDI Limited, Bayswater, Victoria, Australia (Lot: 1210613)	37% phosphoric acid gel with a thixotropic consistency, water-soluble formulation, blue-tinted for visibility; used for enamel and dentin etching prior to adhesive application.

Finishing was performed under water cooling using fine-grit diamond burs (Diatech; Coltene/Whaledent, Altstatten, Switzerland), and polishing was completed using a two-step spiral rubber system (Diacomp Plus Twist, EVE, Germany) operated at 10,000 rpm for 15 s per step. Occlusion was verified with articulating paper. All procedures were carried out by a single experienced operator.

Clinical evaluation

The clinical assessments were conducted by two calibrated and blinded examiners who were unaware of the restoration assignments. Evaluations were performed at baseline (one week postoperatively) and subsequently at 6, 12 months, following the modified United States Public Health Service (USPHS) criteria [18]. Examiner calibration ensured a minimum Kappa coefficient of 0.90 for both inter- and intra-examiner reliability for each evaluation parameter. In instances of discordance, consensus was achieved chairside through joint discussion. To assess postoperative sensitivity, a gentle air stream was directed at a standardized distance of 2 cm from the restoration, while adjacent teeth were shielded with cotton rolls; tactile evaluation using a dental explorer was also performed. Standardized digital photographs and bitewing radiographs were obtained at each follow-up interval to support the clinical findings.

SEM analysis

Among the 34 participants, 11 were randomly selected using a computer-generated random number table for SEM evaluation. Impressions were obtained at three time points (baseline, 6 months, and 12 months), resulting in a total of 66 replicas (11 patients × 2 restorations × 3 evaluation periods).

At each evaluation session, impressions were taken using custom trays and a type C silicone material (Oxasil, Kulzer, Germany) after isolation and drying of the teeth, and subsequently disinfected with 0.5% sodium hypochlorite. All impressions were poured within 10 min using an epoxy resin (Resinin, Turkey). The resulting replicas were carefully trimmed and mounted on aluminum stubs. A thin layer of gold (approximately 10 nm) was sputter-coated under vacuum conditions (MED 010, Balzers Union, Liechtenstein) to render the surface electrically conductive.

Surface morphology was examined using a scanning electron microscope (GAIA3 Triglav, Tescan, Czech Republic) under ×17 and ×100 magnification, which was deemed sufficient to evaluate marginal adaptation and anatomical form. A total of 66 replicas were assessed by two calibrated examiners, blinded to the group allocations. SEM observations were then compared with clinical USPHS findings to validate the consistency between clinical and microscopic criteria.

Statistical analysis

All statistical analyses were performed using SPSS for Windows (version 23.0; IBM Corp., Armonk, NY, USA). The association between composite type and the clinical performance of Class I restorations was evaluated using the Pearson Chi-square test. To assess the temporal changes in performance based on the modified USPHS criteria, the Cochran Q test was applied. A p-value less than 0.05 was considered statistically significant.

Results

Recall rates and retention

The flow chart of the study is displayed in Fig. 1. In total, 72 restorations were placed in 34 patients, as some participants presented with more than two eligible cavities.

Fig. 1.

CONSORT Flow chart: enrollment, allocation, and follow-up representing the relevant appointments and information. Np: Number of patients, Nr: Number of restorations

A total of 34 participants (23 females and 11 males; mean age: 26.5 years) were included in the study. A total of 72 restorations were performed. The majority of the restored teeth were molars (n = 68), while a limited number were premolars (n = 4). Both materials were applied within the same participants due to the split-mouth design, and no baseline differences were observed between the groups in terms of demographic or clinical characteristics. The baseline demographic and clinical characteristics of the study population are presented in Table 2.

All participants attended the baseline, 6-, and 12- month recalls. Thus, the follow-up rate for the evaluated restorations was 100% at all time points. The retention rate of the restorations was 100% at all evaluation periods. Table 3 presents the results of the clinical evaluation of the restorations.

Table 3.

Distribution of the restorations evaluated in the study according to the Modified USPHS criteria

Evaluation Criteria	Score	Baseline n (%)		6-month n (%)		12-month n (%)
		G-ænial Universal Injectable (n = 36)	Filtek Z250 (n = 36)	G-ænial Universal Injectable (n = 34)	Filtek Z250 (n = 34)	G-ænial Universal Injectable (n = 34)	Filtek Z250 (n = 34)
Retention	Alpha	36 (100)	36 (100)	36 (100)	36 (100)	36 (100)	36 (100)
	Bravo	0 (0)	0 (0)	0 (0)	0 (0)	0 (0)	0 (0)
Color Match	Alpha	36 (100)	36 (100)	35 (97.2)	36 (100)	35 (97.2)	36 (100)
	Bravo	0 (0)	0 (0)	1 (2.8)	0 (0)	1 (2.8)	0 (0)
Marginal Discoloration	Alpha	36 (100)	36 (100)	36 (100)	36 (100)	36 (100)	36 (100)
	Bravo	0 (0)	0 (0)	0 (0)	0 (0)	0 (0)	0 (0)
Marginal Adaptation	Alpha	36 (100)	36 (100)	35 (97.2)	35 (97.2)	34 (94.4)	34 (94.4)
	Bravo	0 (0)	0 (0)	1 (2.8)	1 (2.8)	2 (5.6)	2 (5.6)
Secondary Caries	Alpha	36 (100)	36 (100)	36 (100)	36 (100)	36 (100)	36 (100)
	Bravo	0 (0)	0 (0)	0 (0)	0 (0)	0 (0)	0 (0)
Surface Texture	Alpha	36 (100)	36 (100)	36 (100)	36 (100)	36 (100)	36 (100)
	Bravo	0 (0)	0 (0)	0 (0)	0 (0)	0 (0)	0 (0)
Anatomic Form	Alpha	36 (100)	36 (100)	36 (100)	36 (100)	36 (100)	36 (100)
	Bravo	0 (0)	0 (0)	0 (0)	0 (0)	0 (0)	0 (0)
Postoperative Sensitivity	Alpha	36 (100)	36 (100)	36 (100)	36 (100)	36 (100)	36 (100)
	Bravo	0 (0)	0 (0)	0 (0)	0 (0)	0 (0)	0 (0)

Representative images of G-ænial Universal Injectable and Filtek Z250 Universal restorations scored as alpha for all evaluation criteria from the same patient at baseline, 6- and 12-month recalls are presented in Figs. 2 and 3.

Fig. 2.

Representatives case of a G-aenial universal injectable restoration that received alpha scores for all modified USPHS criteria at every recall. A Pre-operative clinical photograph and radiograph of toot #36. B Baseline postoperative photograph, radiograph, and scanning electron micrograph (SEM). C Six-month follow-up photograph, and SEM micrograph. D Twelve-month follow-up photograph, radiograph, and SEM micrograph

Fig. 3.

Representative case of a Filtek Z250 Universal restoration that received Alpha scores for all modified USPHS criteria at every recall. A Pre-operative clinical photograph, radiograph of tooth #26. B Baseline postoperative photograph, radiograph, and scanning electron micrograph (SEM). C Six-month follow-up photograph, radiograph, and SEM micrograph. D Twelve-month follow-up photograph, radiograph, and SEM micrograph

Color match

At baseline, all restorations exhibited Alpha scores. During the 6- and 12-month follow-up, all Filtek Z250 restorations maintained Alpha ratings (100%), while in the G-ænial Universal Injectable group, one restoration (2.8%) was downgraded to Bravo. No statistically significant differences were detected between materials or across time (p = 0.368).

Marginal discoloration and adaptation

At 12 months, no marginal discoloration was observed in any restorations. At baseline, all restorations showed perfect marginal adaptation. After 6 months, one restoration (2.8%) in each group was rated Bravo, whereas the others were scored Alpha. By 12 months, two restorations (5.6%) per group were rated Bravo. No statistically significant differences were observed between materials or over time (p = 0.223).

Among the 11 epoxy replicas, only one—belonging to the G-ænial Universal Injectable group—corresponded to a restoration with a Bravo score. The marginal discrepancy detected clinically in tooth 46 was consistent with the defect observed under SEM (Fig. 4). All other replicas corresponded to restorations with Alpha scores, showing full agreement between clinical and SEM evaluations.

Fig. 4.

Representative G-aenial Universal Injectable restoration on tooth #46 that received a Bravo score for marginal adaptation at the 6-month recall: (A) intraoral photograph: (B) scanning electron micrograph (SEM) at x17: (C) higher-magnification SEM (x100) highlighting the localized marginal discrepancy

Anatomic form and surface texture

At the 12-month follow-up, none of the restorations in the G-ænial Universal Injectable or Filtek Z250 groups received Bravo or Charlie scores for the anatomic form criterion; all maintained Alpha ratings, indicating preservation of anatomical contour. Surface texture was also evaluated, and all restorations maintained Alpha scores throughout the follow-up period. SEM evaluations of epoxy replicas from 11 patients were fully consistent with the clinical findings. Representative SEM photomicrographs at 6 and 12 months for restorations scored Alpha are shown in Figs. 2 and 3.

Secondary caries and postoperative sensitivity

None of the restorations exhibited secondary caries during the follow-up period. No post-operative sensitivity was reported by the patients throughout the evaluation period.

Discussion

Ongoing innovations are being made in restorative materials to enhance application convenience for practitioners. In the present study, two resin composites with different properties were compared, and no statistically significant differences were observed between them for any of the modified USPHS criteria over 12-months. Therefore, the null hypothesis of the study was accepted. Resin composites are widely preferred in dentistry due to their esthetic qualities and adhesive bonding to dental tissues. To assess the clinical performance of newly developed materials, a reference group is required; therefore, Filtek Z250, a well-established conventional microhybrid composite, was selected as the control in the present study [19–21]. Its Bis-GMA, UDMA, and Bis-EMA-based organic matrix provides both mechanical durability and low polymerization shrinkage, while its 82 wt% silanized zirconia/silica filler content contributes to long-term success in posterior restorations [22]. In a study by Ozduman et al. [23], the microhybrid resin composite Filtek Z250 was reported to exhibit higher microhardness, flexural strength, and elastic modulus compared to other composites, findings that were consistent with those of Yancey et al. [24] In the present study, restorations with Filtek Z250 showed clinical success after 12 months according to modified USPHS criteria, consistent with reports that microhybrid composites remain reliable and clinically acceptable for long-term use [25, 26].

G-aenial Universal Injectable contains a UDMA-based composition and utilizes FSC technology, in which all filler particles are silanized, features that are intended to improve handling and physical properties [27]. In vitro findings have been heterogeneous, with some studies reporting enhanced flexural strength and wear resistance for highly filled injectable composites [11, 28], whereas others demonstrated comparatively lower performance of G-ænial Universal Injectable compared with certain commercial alternatives [29]. According to a meta-analysis, flowable composites are considered a suitable option for the restoration of minimally invasive occlusal cavities [30]. A two-year clinical study on Class II restorations revealed that G-ænial Universal Injectable exhibited better surface gloss and more favorable marginal adaptation than the conventional resin composite [31]. Although previous reports have mainly highlighted the use of G-ænial Universal Injectable in anterior smile design with the injection molding technique [32, 33], evidence regarding its performance in occlusal cavities—particularly supported by SEM analysis—has been lacking. This study demonstrated that the material achieved clinically favorable results during the 12-month evaluation period.

A consistent observation in the literature is that Filtek Z250 achieves very high retention rates, often approaching 100% even at 30 months [34, 35]. In the present study, all restorations with both materials were successfully retained after 12 months, with no failures observed. Even in posterior teeth, where esthetic demands are sometimes underestimated, proper shade matching remains an important factor for clinical success. In a clinical study, Hashir et al. reported that Filtek Z250 restorations achieved a clinically acceptable color match with the surrounding tooth structure [36]. Consistent with anterior esthetic rehabilitation cases reporting satisfactory shade matching for G-ænial Universal Injectable [32, 33], our findings showed that all restorations received an Alpha score for color match—except for a single G-ænial Universal Injectable restoration (Bravo)—thus showing general agreement with these results.

Marginal discoloration is a frequent clinical finding and a surrogate marker of interfacial degradation driven by polymerization-shrinkage stresses, marginal gap formation, and subsequent uptake of dietary chromogens. Prior clinical studies have reported no significant differences in this parameter between high-filled flowable composites and paste-type microhybrid composites [14, 37]. In the present study, no marginal discoloration was detected at any recall, a result that likely reflects rigorous isolation, standardized finishing/polishing protocols, and the favorable polishability of both materials. Similar to discoloration, marginal adaptation is pivotal for long-term survival, as its deterioration ultimately necessitates replacement. While some long-term cohorts describe progressive loss of marginal integrity for Filtek Z250 [38], in the present study, only minor, non-significant discrepancies were occasionally observed for both groups. SEM evaluation corroborated the clinical scores: the single Bravo finding in the G-aenial Universal Injectable group corresponded to a discrete marginal defect, whereas restorations scored Alpha exhibited intact margins, indicating strong agreement between clinical and microscopic assessments. The isolated Bravo score observed in the G-ænial Universal Injectable group may be associated with polymerization shrinkage or technique sensitivity during material placement. In addition, patient-related factors may also have contributed to this finding. However, as this was an isolated case, it should not be interpreted as a general material-related limitation.

In this study, enamel was selectively etched with 37% phosphoric acid, and bonding for both restorative materials was performed using a HEMA-free, two-step universal adhesive (G2-Bond Universal). Evaluations based on the modified USPHS criteria—particularly retention, marginal discoloration, and marginal adaptation—indicate that G2-Bond Universal provides an effective and durable bond when used with both Filtek Z250 and G-ænial Universal Injectable. These findings are consistent with prior in-vitro reports demonstrating favorable dentin bonding performance for G2-Bond Universal [39, 40]. It should be noted that HEMA-containing adhesives are more susceptible to moisture, which may lead to long-term degradation within the hybrid layer and potentially compromise bonding stability and increase microleakage [39].In this regard, the use of a HEMA-free adhesive may offer potential advantages in terms of long-term performance [41, 42]. However, as the study was not designed to compare adhesive systems, a single adhesive was used, and the short follow-up period limits conclusions on long-term durability. Surface quality is another factor influencing clinical outcomes. Laboratory observations suggest microhybrids may exhibit more filler loss than nanofilled composites, leading to increased roughness [43]. In contrast, all restorations in the present study maintained smooth surfaces across evaluations, suggesting effective polishing, consistent with the literature linking finishing and polishing to improved esthetics and longevity [44]. Wear resistance, as reflected in anatomic form, depends strongly on filler size and distribution [45, 46]. While in-vitro evidence indicates superior wear resistance in nanofiller-reinforced flowables [11, 47], a 2-year clinical study evaluating Class II restorations placed with a resin composite of two different viscosities found that both materials exhibited acceptable clinical behaviour [48]. In the present study, no wear was clinically observed, and SEM replicas also confirmed the absence of detectable wear over 12 months. These findings are limited by the short follow-up and qualitative nature of SEM evaluation, highlighting the need for longer and more detailed analyses.

Secondary caries remains the primary cause for restoration failure in long-term studies, but such outcomes typically emerge only after several years [49, 50]. Consistent with this, no secondary caries were observed in the present 12-month evaluation, echoing earlier short-term reports. This outcome may be associated with the clinician’s careful placement technique, the inclusion of patients with good oral hygiene, and the advances in adhesive systems and restorative materials. In the present study, none of the patients from either group suffered from postoperative sensitivity. The absence of sensitivity may be related to proper technique and the relatively shallow cavity depths included.

Several limitations should be considered when interpreting these findings. The 12-month follow-up captures early clinical performance but is insufficient to determine medium- or long-term durability; extended observation is warranted, while, on the other hand, as SEM assessment primarily focuses on detecting distortions in marginal edges and anatomical form, the quantitative level of interface evaluation remains limited, and future studies should combine SEM with complementary imaging methods (e.g., three-dimensional surface scanning) to better assess edge congruence and anatomical form. Finally, the cohort comprised carefully selected participants with good oral hygiene and regular attendance, which may introduce selection bias and limit the generalizability of these findings to broader patient populations.

Conclusion

Both the injectable and paste-type resin-composites showed acceptable short-term performance in Class I occlusal restorations, with no statistically significant differences between the materials. SEM observations were consistent with the clinical findings, although they were qualitative and therefore limited in interpretive strength. Within the constraints of this 12-month evaluation, both restorative approaches appear clinically suitable; however, longer-term studies are needed to support more definitive conclusions.

Abbreviations

SEM

Scanning Electron Microscopy

USPHS

United States Public Health Service

CONSORT

Consolidated Standards of Reporting Trials

ICDAS

International Caries Detection and Assessment System

Authors’ contributions

A.C.K. was responsible for the experimental work, data collection, laboratory procedures, and manuscript writing. C.A. provided conceptual input, contributed to the study design, and critical editing of the final text. E.E. contributed to manuscript editing. All authors reviewed and approved the final version of the manuscript.

Funding

This research received no external funding.

Data availability

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

Declarations

Ethics approval and consent to participate

The study has ethics committee approval. Ethical approval for the study was granted by the Clinical Research Ethics Committee of Hacettepe University (Date: 21.03.2023, Protocol No: KA-22112). The authors declare that the study was approved by the ethics committee and that all participants took part in the study by signing voluntary informed consent documents. Written informed consent was obtained from all individuals who agreed to participate in the study. The study protocol was conducted in accordance with the ethical principles of the World Medical Association (WMA) Declaration of Helsinki for medical research involving human participants.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.