This laboratory study was done using two commercially available resin-modified glass ionomer (RIVA LC, SDI) and conventional glass ionomer cement (RIVA conventional glass ionomer cement, SDI) with its manufacturer recommended coat (RIVA LC coating material, SDI) and customized (vaseline) coat. The commercial name, manufacturer, composition, and application guidelines for the material are provided in (Table 1).

Table 1 Materials used in this studyTeeth selection

Seventy-two permanent human molars were used in this study. The collected molars were extracted for therapeutic reasons unrelated to the study, with prior informed consent from healthy individuals who were seeking dental care at The Oral and Maxillofacial Surgery Department Clinic, Faculty of Dentistry, Mansoura University, Faculty of Dentistry at Mansoura University. The patients were voluntary donating the extracted teeth to The Faculty for utilizing in research purpose. The study protocol was approved from Faculty of Dentistry at Mansoura University, ethical approval number (A15080622). Teeth were thoroughly cleaned of calculus and soft tissue deposits before being rinsed with distilled water and a low-speed rubber cup with prophy paste using a hand scaler. Following that, teeth with pre-existing flaws, cracks, or restorations were excluded using a stereomicroscope (SZ TP, Olympus, Tokyo, Japan). Teeth were kept in 0.5% chloramine T solution for 72 h before being placed in deionized water, which was changed every 2 weeks.

Specimen preparation

Molars were placed in polyvinyl chloride cylinders with 1.8-centimeter diameter and 2-centimeter height to fit the enormous roots of the molars. Auto-polymerizing acrylic resin was packed into these cylinders. Later, with the acrylic resin still in its doughy state, each tooth was vertically invested in the center of the cylinder with a metal ring. Two opposing screws were used to centralize the molar parallel to the long axis of an acrylic resin mold. The height of acrylic blocks were below the CEJ of the teeth.

A diamond saw (Isomet 4000, Buehler Ltd., Lake Bluff, IL, USA) was used to cut the specimens. Each tooth had its occlusal enamel and superficial dentin shaved away, revealing the dentin closer to its center. The dentin surface was ground using wet 600 grit silicon carbide sheets (SIA Brand, Switzerland) for 30 s to create uniform smear layers.

Study grouping

Seventy-two molars were randomly assigned into 2 groups according to type of restorative materials used; resin modified glass ionomer cement or conventional glass ionomer cement. Each group was subdivided into 3 subgroups according to the application of coat material; Sub-group1: without application of coat; Sub-group2: manufacturer recommended coat was applied and sub-group3: Vaseline® Brand (Unilever, UAE) coat was applied.

Half of the specimens of each group were tested immediately, while the remaining halves were tested after 6 months of storage in artificial saliva at (37 ± 1ºC) in an incubator (BTC, Model: BT1020, Egypt). The storage medium was changed with a new fresh one every week.

Sample size calculation was based on cross-sectional hardness between treated arm versus control group retrieved from previous research [9]. Using G power program version 3.1.9.7 to calculate sample size based on effect size of 1.17 (127 ± 1.03 versus 126 ± 0.62) ,using 2-tailed test, α error = 0.05 and power = 80%, the total calculated sample size will be 13 in each group.

Measurement of surface roughness using atomic force microscopy (AFM)

Three specimens were used in this test from each subgroup with a total number of thirty-six teeth (Fig. 1). The evaluation of surface roughness was carried out using AFM (Model. FleXAFM3). The scan area 10 × 10 Mm2 and number of data points: 256 × 256 at scan rate 1HZ. The AFM was operated in contact mode using nanoconductive silicon probe using Nanosurf C3000 (version 3.5.0.31) software. All measurements were performed at the middle one third of restoration surface for each tooth and the average was recorded. The results were obtained to compare restoration surface roughness before and after storage.

Fig. 1

Diagram showing study design for surface roughness test

Micrographs were taken for each specimens using a digital microscopy with a built in camera linked to a computer using 120 X magnification. The micrographs were documented with a resolution of 1280 × 1024 pixels per image. The images were cropped to 350 × 400 pixel using Microsoft picture manager to standardize roughness areas, and analyzed using WSxM software. All limits, sizes, frames and measured parameters were expressed in pixels within the WSxM software. So, the system calibration was made to convert the pixels into absolute real world units, this made by comparing an object of known size with a scale produced by software. Consequently, 3-D image of specimens profile surface was constructed, then collected for each specimen, in the central area and in the sides at area of 10 Mm x 10 Mm. WSxM software was used to analyze surface roughness average expressed in Mm, which can be assumed as a reliable indices of surface roughness.

Fluoride specific ion electrode test

Thirty test specimens of each GIC and RMGIC were formed into discs of 10-mm diameter and 2 mm thickness by using 3D dimension resin mold was specially designed with a central depression of the same dimension as the disc. All materials were handled according to the manufacturers’ instructions.

Each 30 discs subdivided in to 3 groups (n = 10), according to the type of coat placed for each one (no coat, SDI Riva coat or customized coat) (Fig. 2). For each coat group, coat was placed after complete setting of the material by using a dental brush (one layer for each disc). Each test specimen was immersed in artificial saliva in a sealed container stored in an incubator at 37 °C. The bottom of each container had a raised center that ensured that the sample disc was tilted to expose all of its surfaces to the artificial saliva. The fluoride levels in the solutions were measured in different time of storage (after 24 h, 7days, 14 days, 30 days, 45 days, 60 days ,75 days 90 days).

Fig. 2

Diagram showing study design for fluoride release test

The test was conducted for each sub-group by taking 4 mm from the artificial saliva media where each sub-group has been stored. Five reading for each sub groups were recorded from At each storage time period. Fluoride ion was measured in artificial saliva using The SPADNS colorimetric method (SM 4500 F standard method for examination of water and waste water 24th edition 2023). The method is based on the reaction between fluoride and a zirconium-dye lake. Fluoride reacts with the dye lake, dissociating a portion of it into a colorless complex anion (ZrF6 2- ); and the dye. As the amount of fluoride increases, the color produced becomes progressively lighter.

The SPADNS reagent is prepared by dissolving 958 mg (0.958 g) SPADNS (Sulfanilic acid azochromotrop, Sigma Aldrich, USA) in deionized water and dilute to 500 ml deionized water. Zirconyl-acid reagent is prepared by dissolving 133 mg (0.133 g) zirconyl chloride octahydrate, ZrOCl2–8H2O (Sigma Aldrich, USA) in about 25 mL deionized water then by adding 350 mL conc HCl and dilute to 500 ml with deionized water. Acid zirconyl-SPADNS reagent is prepared by mixing equal volumes of SPADNS solution and zirconyl-acid reagent.

A group of 6 standard solutions with fluoride concentration 0.0, 0.2, 0.0.4, 0.6 ,0.8,1.0 ppm were prepared from a certified reference material of fluoride ( Merck KGaA, Darmstadt, Germany) and the absorbance was measured at 570 nm for the standards using UV-visible double beam spectrophotometer ( LAMDA 365, PerkinElmer, USA ).Samples then measured against the prepared standards after been diluted by ratio 1 : 25.

Micromorphological analysis of restoration ̸ tooth interface using scanning electron microscope (SEM)

For this test, the same study design of surface roughness test was used (Fig. 1). The immediate subgroups (3 teeth from each sub groups) were sectioned and tested after 24 h of restorative procedure. The delayed subgroups (3 teeth from each sub groups) were sectioned and tested after 6month of aging. This test used to obtain and quantitative data of tooth structure, which explaining the complex between restoration-tooth interface for evaluation of hybrid layer ultra-morphology.

The specimens were sectioned mesio-distally into two equal halves along the long axis of the teeth in a direction perpendicular to the restoration-dentin interface using a water-cooled diamond disc under low speed (IsoMetTM 4000, Buehler Ltd; Lake Bluff, IL, USA). Each half was grounded with coarse (600 grit), medium (800 grit), fine (1000 grit), and extra fine (1200grit, 2000grit) silicon carbide papers (SIA Brand Switzerland) under water. Final polish was obtained with fine diamond pastes with particle size (4 μm, 2 μm, 1 μm) respectively (ENA polishing system, Micerium S.p.A.) with a polishing piece of velvet fabric. Then, the first half of each tooth was cleaned in ultrasonic bath for 30 s (XH-E412 ultrasonic cleaner, Xinghua, China). The second half of each tooth was subjected to an acid\base challenge for characterization of the hybrid-like layer. This process include application of 10% orthophosphoric acid solution for 5 s on the tooth/restoration interface to demineralize dentin collagen fibers. Then 5% sodium hypochlorite solution was applied for 5 min to remove organic components. The specimens were prepared for scanning electron microscope (SEM) by being gold sputtered twice (SPI Module Sputter Carbon/Gold Coater, EDEN Instruments, Tokyo, Japan), and then observed with secondary electron detection mode using SEM (JSM-6510LV, JEOL, Tokyo, Japan) at an accelerating voltage of 30 KV and a working distance of 9–12 mm at magnification of ( 2000 x).

Statistical analysis

All the data were collected, tabulated, and tested for the normality with shapiro-Wilk test. Based on the outcome of normality test, the significant effects of variables were assessed using appropriate statistical analysis testing methods. The threshold of significance was fixed at 5% level. The results were considered significant when p-value ≤ 0.05. Data analysis was performed by SPSS software, version 25 (SPSS Inc., PASW statistics for windows version 25. Chicago: SPSS Inc.).