Clinical Evaluation of an Alkasite-Based Resin Composite in Class I and II Cavities Conditioned with Self-Curing Primer: Preliminary Results up to One Year Open Access

Simplified restorative procedures are an ongoing demand for posterior applications. Beyond improved esthetic and mechanical properties, contemporary restorative materials are expected to have additional features such as adequate polymerization in large increments, bioactivity, and self-adhesiveness.

The increasing acceptance of resin composite (RC) is due to better esthetics and a more minimally invasive treatment approach compared to dental amalgam.¹  Nevertheless, the longevity of RCs is frequently reported to be lower than amalgam restorations, especially in load-bearing areas.^2,3  Secondary caries, fractures, marginal deficiencies, wear, and postoperative sensitivity are the main reasons for failure of RCs.^3,4  Thus, there is ongoing research to develop innovative materials with properties such as bulk application, low polymerization shrinkage, cariostatic effect, and self-adhesiveness to eliminate the shortcomings of RCs.

Bulk-fill resin composites (BF-RCs) are one of the options for a simplified restoration concept enabling up to four- or five-millimeter (mm) thick increments to be cured in one step.⁵  BF-RCs have the potential to be less technique-sensitive due to the reduced risk of incorporating air bubbles or contaminants between the increments. Still, the reasons for failure, such as fracture and secondary caries, are not different from conventional RCs.^6–8  The anti-cariogenic properties of glass ionomer cements are well known.⁹  However, they have some disadvantages, such as rough surface texture, high porosity, and inferior mechanical properties resulting in brittleness, surface wear, and decreased fracture toughness over time.¹⁰  Giomers were introduced to be bioactive BF-RCs, combining the fluoride release properties of glass-ionomers with the esthetic and mechanical properties of RCs.¹¹  However, the degree of conversion, which has a major impact on the physical properties and biocompatibility of the restorative materials, seems not promising for giomers.^12–15 

Recently, Cention N (Ivoclar Vivadent, Schaan, Liechtenstein) has been introduced as a new category of material defined as an alkasite-based resin composite (AB-RC) with bulk application and optional photo-polymerization properties. With the implementation of alkaline fillers in a methylmethacrylate resin matrix, AB-RC is capable of releasing ions such as fluoride, calcium, and hydroxide. Some previous clinical trials demonstrated that Cention N (CN) performed as well as conventional nanofilled RCs within a one-year observation period.^16,17  However, CN might be advantageous over RCs to prevent secondary caries formation.^18–20 

Most recently, to improve the functional properties of AB-RCs, in terms of retention, fracture resistance, and marginal adaptation, the capsulated automix formulation (Cention Forte, Ivoclar Vivadent, Schaan), and self-curing primer (Cention Primer, Ivoclar Vivadent, Schaan) have been introduced. However, Cention Forte (CF) and Cention Primer (CP) have not been tested in vivo. Therefore, this study aimed to evaluate the clinical performance of CF in conjunction with CP. The primary outcome of the study was to assess esthetic, functional, and biological properties of CF restorations in Class I and II cavities according to FDI criteria at baseline and after one year in function. The null hypothesis was that there would be no change in FDI scores regarding color, staining, marginal adaptation, secondary caries, fracture, and pulpal vitality after one year of observation period compared to baseline.

The Consolidated Standards of Reporting Trials (CONSORT) statement was followed to design the study.

This study was a prospective single-arm clinical trial registered at ClinicalTrials.gov (registration No. NCT04796974) and approved by the National Ethical Committee of Medicines and Medical Devices Agency (Vote number of the local Ethical Committee: E-68869993-511.06-1464523, Date: 10.03.2021). All participants were provided written informed consent explaining the scope, duration, alternative treatment options, and risks and benefits of the study.

The sample size was calculated using G* Power statistical software (ver. 3.1.9.4) (Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany). Considering a small standardized effect size (Cohen's d=0.3) with 80% power and an alpha error of 5%, the sample size was defined as 82 restorations. Considering the possibility of drop-out during the observation period, 90 teeth in need of restoration were included in the study.

Potential patients referred to the Restorative Dentistry Department of the University Clinic for restorative needs between March 2021 and June 2022 were invited to participate in the study. The inclusion and exclusion criteria were as follows.

• At least 18 years of age

• Vital posterior teeth with Class I and II caries lesions or defective restorations in need of operative treatment

• Good or moderate oral hygiene (plaque score of less than 30% in the anterior region before treatment)

• Subjects classified ASA III or IV

• Subjects breastfeeding

• Subjects with allergic reactions to any component of the restorative materials

• Pulpal exposure during treatment

• Untreated additional caries lesion(s) or periodontal disease

• Tooth whitening completed within the past three weeks

• Tooth hypersensitivity

• No opposing dentition

• No tooth adjacent to Class II caries lesion to be restored

• Parafunctional habits

• Ongoing orthodontic therapy

Clinical treatments were carried out by three operators (B.B.A, N.Ü., F.Ö.B) who have experience in restorative dentistry for at least eight years since graduation. The operators were trained in all clinical procedures before the beginning of the study. Preoperative radiographs and occlusal photographs were made for each subject. Initial periodontal therapy, if required, was performed two weeks before the cavity preparation. Age, gender, caries risk status, Simplified Oral Hygiene Index (OHI-S), type of tooth, and reason for restoration were recorded before the operative procedure.

Operative procedures were performed under local anesthesia. A round diamond bur operated at high speed under water cooling was used to access caries or remove defective previous restorations. Carious tissue was selectively removed to firm dentin using round tungsten carbide burs operated at low speed. The cavity walls were finished with a fine-grit diamond bur to achieve smooth surfaces. Principles of minimally invasive dentistry were followed; over-extension and beveling were avoided. A hand excavator was used to remove caries near the pulp. No liner was applied after caries removal.

Cavity dimensions were measured using a periodontal probe. Occlusal photographs of the cavity preparations were taken. A circumferential matrix band (Adapt Super Cap Matrix, KerrHave SA, Bioggio, Switzerland) was placed and fixed with wooden wedges before the restoration of Class II cavities. A dry working field was obtained using cotton rolls, suction, and an air syringe before the placement of restorations. No dental dams, retraction cords, or hemostatic agents were used.

The materials used in the study are listed in Table 1. The primer (Cention Primer, Ivoclar Vivadent, Schaan, Liechtenstein) was dispensed into a dish and protected from light. A single-use applicator was soaked in the liquid for five seconds, applied to the internal surfaces of the cavity for at least 10 seconds, and then air-dried until a shiny, thin, immobile film had formed. According to the manufacturer's instructions, the primer self-cures when it is overlayed with AB-RC material. The capsule of AB-RC material (Cention Forte, Ivoclar Vivadent, Schaan, Liechtenstein) was mixed in a universal amalgamator (Silamat S6, Ivoclar Vivadent AG) for 15 seconds (room temperature at 22-26°C). Immediately after, the capsule was inserted in the applicator (Capsule Applicator, Ivoclar Vivadent AG) and the resin was placed into the cavity. The matrix band was removed after a setting time of 6.5 to 9 minutes.

Necessary adjustments were made using fine-grit diamond burs and discs (Opti Disc, KerrHave SA, Bioggio, Switzerland). The proximal contacts were checked using dental floss. Rubber points were used for polishing. Thereafter, postoperative radiographs and occlusal photographs were made. The patients were asked not to eat or drink and rinse their mouths for at least two hours.

Two calibrated evaluators who have experience in restorative dentistry for more than 15 years since graduation independently evaluated the restorations at baseline (three to four weeks after treatment) and after one year according to FDI World Dental Federation criteria (Table 2). Restorations were evaluated for each of 10 FDI criteria and scored as (1) clinically excellent, (2) clinically good, (3) clinically sufficient, (4) clinically not sufficient but repairable, or (5) clinically unacceptable. Additionally, the Semi-Quantitative Clinical Evaluation (SQUACE) method was carried out to assess the proportion of the total length of the restoration margin scored as not being clinically excellent (2-5).²¹  Sketches were made on digital occlusal photographs and the proportions of marginal staining and marginal gap or irregularities were recorded. Disagreements in evaluations were discussed between the evaluators and a consensus was reached.

SPSS software (IBM SPSS Statistics for Windows, Version 23.0, IBM Corp., Armonk, NY) was used for statistical analyses. Wilcoxon signed-rank test was used to evaluate the differences between FDI scores at baseline and after one year to test the null hypothesis. The Kappa (K) value of each criterion was calculated using the Fleiss multi-rater kappa statistic to measure inter-examiner reliability. The significance level was set at 5%, and the unit of analysis was the tooth.

Among the 1118 teeth assessed for eligibility to participate in the study, 1028 were excluded (Figure 1). In total, 90 restorations in 49 patients (32 females, 17 males; mean age: 26.3; range: 18-45 years old) were included in the study. Forty-nine patients at baseline and 47 patients at one year were evaluated between March 2022 and June 2023. Two patients did not return for their one-year recall; one was out of the city at the designated follow-up period and the other was not willing to attend the recalls.

The mean observation time was 12.8 ± 1.4 months. The trial has been planned for a longer observation period of three years with annual recalls. Herein, the data regarding clinical outcomes of one year have been analyzed.

All baseline characteristics are listed in Table 3. The caries risk of the patient population was moderate to high. Most of the teeth received Class II restorations due to the presence of superficial primary caries restricted to the outer half of dentin. The clinical outcomes for 90 restorations at baseline and 86 restorations after one year evaluated according to FDI criteria are presented in Table 4. Intense hypersensitivity observed in one restoration disappeared without any treatment three weeks following the intervention. One restoration was clinically not sufficient but repairable (FDI 5.4.1) due to a small material chip affecting marginal quality at one year. All restorations were clinically acceptable in terms of esthetic properties at baseline and after one year. Thus, the success rate of the restorations was found to be 98.8% after one year in function.

Based on the primary outcome of the study, the differences in color change (p=0.001), surface luster (p=0.002), and surface staining (p=0.007) were statistically significant, whereas no change was found for marginal staining (p=0.157) after one year compared to baseline. Forty-two restorations were scored as clinically excellent for surface luster (FDI 1.1) at baseline; 33 of these restorations showed a score of 2 and one of them scored 3 after one year. Eight of 86 restorations with no surface staining at baseline (FDI 2a.1) showed minor (FDI 2a.2; n=7) or moderate (FDI 2a.3; n=1) staining after one year. Six of the 84 restorations with no marginal staining (FDI 2b.1) at baseline received a score of 2, indicating minor staining, with a SQUACE value of 9.8 at one-year recall. Approximately 35% of restorations were found to be more opaque after one year compared to baseline. Representative photographs of restorations with excellent FDI scores and minor changes regarding esthetic properties are shown in Figures 2-6.

No statistically significant differences were found for scores regarding fracture (p=0.317), marginal deficiency (p=0.593), occlusal wear (p=1.00) and patient's view (p=0.655). Six of 76 restorations scored as 1 at baseline showed slight ditching (FDI 6.2) with a mean SQUACE value of 21.6 after one year in function. No changes were observed in terms of tooth integrity (FDI 13) and secondary caries development (FDI 12) after one year compared to baseline (p=1.00). None of the teeth needed endodontic treatment due to persistent hypersensitivity or pain.

All the scores given by two independent observers were the same for FDI 2a, FDI 5, FD 7, FDI 10, FDI 11, FDI 12, and FDI 13 at baseline and one-year follow-up. Inter-examiner agreement was statistically significant for FDI 1, FDI 2b, FDI 3, and FDI 6 parameters both at baseline and one-year follow-up (p<0.001). The kappa values were excellent for FDI 1, FDI 2b, and FDI 3 criteria (range: 0.85-0.98), while substantial for FDI 6 criterion (0.64 at baseline and 0.72 at one-year follow-up).

The clinical performance of newly developed bioactive restorative material CF in conjunction with CP in posterior teeth was tested in this study. After one year in function, the marginal quality of the restorations has maintained in terms of marginal adaptation, marginal staining, and secondary caries formation.

None of the teeth received endodontic treatment due to hypersensitivity or loss of vitality during the observation period. No retention loss or tooth fracture was observed. However, the color of the restorations slightly changed, and minor surface roughness and staining increased over time. Thus, the null hypothesis was rejected.

Previous studies showed a high survival rate and good clinical performance for CN over follow-up periods of one to three years.^{16,17,22–24}  Notably, failure seemed to be less likely in Class I and Class II cavities conditioned with selective etching and multimode dental adhesives,^17,23  compared to studies not using any primer or adhesive system^16,22  before CN placement. Loss of retention was reported to be the main reason for the failure of posterior cavities restored with CN in previous clinical trials not using an adhesive system.^16,22  Moreover, a previous in vitro study showed higher bond strength for CF in conjunction with CP compared to resin-modified glass ionomer, glass ionomer, and RC with a universal adhesive system.²⁵  The results of the present study showed good clinical success of CF applied with CP, corroborating the findings of this in vitro study. Taking the results of the present study into account together with those cited above, it seems that conditioning of the tooth substrate is essential to improve the retention of the AB-RCs.

Apart from adhesion quality and retention, adequate mechanical resistance is equally important for the durability of restorations. In the present study, only one small material chipping was evident during the observation period of one year. This result is consistent with the results of a previous clinical trial.¹⁷  CN was introduced with the intent of replacing amalgam.²⁶  Poor esthetics, loss of sound tooth structure (extension for prevention), and mercury release are potential concerns with amalgam restorations.²⁷  Some previous studies demonstrated superior or similar fracture strength for CN compared to RC and amalgam.^23,29,30  In the present study, a 98.8% success rate provided further evidence for the applicability of CF in load-bearing areas. Considering these results, CN and CF seem to perform as well as amalgam and RCs in Class I and II load-bearing areas.

In the present study, CF was applied in self-curing mode, which was considered advantageous due to reduced polymerization contraction stress arising from slower monomer conversion.³¹  No change in marginal adaptation scores in the present study might be attributed to reduced polymerization shrinkage provided by the slow chemical curing process and isofillers acting as shrinkage stress relievers.³¹  However, the marginal adaptation (34.5%) of self-cured CN restorations scored as 2 was higher in the study by Albelasy and others²³  compared to self-cured CF restorations in the present study (9.3%) over the same observation period. Using self-curing CP primer in the present study instead of an etch-and-rinse adhesive system might have provided better marginal quality.

The degree of conversion and flexural strength for CN in self-cure mode were lower compared to dual-cure mode irrespective of storage time of 24 hours or 12 months in water.³²  Contrarily, Ilie²⁸  proposed no difference in the degree of conversion or flexural strength between self-cured (11 minutes) or dual-cured CN after storage time of three months in both acidic and neutral solutions. In the present study, most of the cavities were deeper than 2 mm (mean cavity depth: 3.12 ± 1.07 mm; range: 1-5 mm; data not shown) and CF in self-curing mode showed good performance irrespective of the cavity depth. Similarly, Albelasy and others²³  showed no significant difference in FDI scores of shallow and deep cavities restored with self-cured CN after two years in function. These findings agree with the findings of İlie,²⁸  demonstrating similar polymerization kinetics at 2 mm and 4 mm depth in vitro for the self-cure mode. Considering the benefits of self-curing and the results in terms of functional properties of the CF restorations in the self-cure mode in the present study, not using a polymerization device might be preferable. However, longer-term evidence is required to make a conclusive statement.

CF and CN have the same composition except for the difference in the powder-to-liquid proportion. To improve handling properties, the manufacturer optimized this ratio in favor of a higher amount of powder in the self-mixing CF capsules. Additionally, CF might be advantageous over CN considering the possibility of forming air voids during hand mixing, which can jeopardize the fracture toughness.³³  While the filler content of inorganic fillers (58-59 vol%) is similar to RCs and BF-RCs, CN and CF have larger particle sizes of 0.15-1.7 μm compared to nano hybrid composites.³²  An increase in filler size may lead to an increase in the fracture toughness of RC materials, whereas decreased wear resistance is an expected outcome for this formulation.^33,34 

The slightly increased surface roughness in the present study might be related not only to large filler size but also degradation of calcium fluorosilicate glass upon acidic challenge.³⁵  The proportion of CF restorations scoring a 2 or 3 for surface luster after one year in the present study was higher (76.7%) compared to the proportion of CN restorations scoring a 2 (8.1-10.7%) in previous studies.^16,17  This difference might arise from the use of light polymerization in the previous studies,^16,17  which might have provided a more tightly bound resistant surface against degradation. Notably, the surface roughness of light-cured CN was shown to be lower or comparable to RC (Z350 XT, St Paul, MN, USA) in vitro and in vivo, respectively.^17,34  In contrast, Shekhar and others found the surface roughness was higher for light-cured and polished surfaces of CN than Z350 XT.³⁶  However, both above-mentioned in vitro studies stated that the mean value of surface roughness of CN was below the minimum value of 0.2 μm to promote plaque retention and the minimum value of 1 μm to be esthetically unacceptable.^34,36  The results of the present study showing acceptable clinical scores for esthetic properties corroborate the findings of these previous in vitro studies. Nevertheless, further investigations are required to clarify the effect of light polymerization on the surface texture of the CN and CF.

The alkaline fillers incorporated into the tested material to prevent secondary caries formation are deemed responsible for the opaque color and surface staining. While the decrease in the translucency of the material might arise from the high refractive index of alkaline fillers, the strong affinity of these fillers for water is probably the reason for staining over time.^28,37,38  Restorative materials with hydrophilic resin matrices and a high degree of water sorption were proposed to be more susceptible to staining.³⁹  However, previous studies revealed comparable water sorption properties for both light- and self-cured CN compared to conventional RCs, which can be attributed to the UDMA-based resin matrix of CN with a low degree of water sorption.^32,38  Thus, it might be suggested that staining of the tested material might be related to water solubility rather than water absorption, as previously shown for giomers.⁴⁰ 

Previous studies presented conflicting evidence regarding the esthetic properties of CN restorations. Derchi and others²⁴  observed worsening esthetic properties for CN restorations in primary teeth, whereas Öz and others¹⁶  reported alpha scores for color match in 91.9% of the light-cured CN restorations. In the present study, 38.7% of CF restorations showed acceptable deviations as being darker and more opaque. However, the marginal staining of CF restorations scored as 2 (6.9%) was lower in the present study compared to the one-year results of a study by Albesasy and others.²³  This finding is important to show that CP might have the potential to provide not only better marginal adaptation but also less marginal staining compared to etch-and-rinse adhesives.

In addition to good sealing ability, ion release to prevent demineralization at restoration margins is one of the desired features for the longevity of restorations. Recently, the prevalence of secondary caries for resin composites was reported to be higher compared to amalgam, irrespective of the caries risk of the patient.⁴¹  Notably, no incidence of secondary caries was reported in either the present study or previous studies evaluating AB-RCs within the observation period of 1-2 years.^16,17,23  The caries-preventive efficacy of AB-RCs also has been shown in previous in vitro studies.^18–20  Additionally, self-curing CN was suggested to have higher fluoride ion release and alkalizing potential than light-cured CN.⁴²  Considering the results of these studies, AB-RCs seem to be promising for the prevention of secondary caries. However, longer-term clinical outcomes are required to make a conclusive statement for caries preventive efficacy of self- or light-cured AB-RCs.

Based on our review of the literature, we believe this study is the first clinical trial evaluating the clinical outcome for CF applied with the specific primer CP introduced to be used with this material. The strictly followed protocol with three operators and two blinded evaluators, well-defined variables regarding caries risk and oral hygiene of the patients, description of cavity depth, and low (4.4%) drop-out rate were the strengths of the study. The use of the SQUACE method provided data for the severity of deviations in marginal quality.

One of the limitations of the present study was the short follow-up period of one year. However, this study has been designed to reevaluate the restorations for up to three years. Herein, preliminary results were given, since a newly developed material has been tested and the presentation of the preliminary results might be valuable for future clinical trials. This study has revealed important clinical implications in terms of mechanical and optical properties of CF restorations applied with CP and without photo-polymerization after one year in function. Further clinical trials are advisable to compare CF with CP and other simplified restorative procedures with bulk application and ion-releasing properties.

Based on the results of the present study, the clinical performance of CF applied with CP after one year of function appears promising. Minor staining and slight changes in color and surface luster over time might be disregarded, considering the benefits in terms of ease of application, mechanical resistance, and marginal quality, especially in high caries-risk patients.

The present work was sponsored by Ivoclar Vivadent AG, Schaan, Liechtenstein. The authors extend their gratitude to Dr. L. Enggist for valuable discussions.