This controlled comparative in vitro study was approved by The Research Ethics Committee (REC), Faculty of Dentistry, X University (FP3-242,157) and was performed at the Department of Fixed Prosthodontics, Faculty of Dentistry, X University.

Twenty sound upper first premolars were collected from outpatient clinics at the Oral and Maxillofacial Surgery Department after obtaining the donors' signed informed consent. Teeth were brushed with non-fluoridated pumice, rinsed thoroughly with tap water and stored in 0.9% sodium chloride solution with 0.1% thymol (Normal saline, Cairo, Egypt) [11]. The teeth were then mounted in acrylic blocks using a surveyor (Surveyor B2, BioArt, São Carlos, Brazil), with the level set 3 mm apical to the cementoenamel junction.

Occlusal veneer preparation

After teeth had been mounted in acrylic blocks, standardized teeth preparations were accomplished using a linear precision saw (IsoMet 4000, Buehler; Lake Bluff, IL, USA), to obtain flat uniform surfaces. “All coronal tooth structure 4 mm occlusal to the cemento-enamel junction was removed, leaving exposed deep dentin centrally and enamel peripherally” (Fig. 1). In a pilot study, two designs were tested: one covering both enamel and dentin, and another leaving the peripheral enamel intact. The latter was selected to preserve occlusal surface integrity, ensure stable handling of the cemented veneers during sectioning, and reduce the number of specimens required.

Fig. 1The alternative text for this image may have been generated using AI.

Flat occlusal surface after cutting by Isomet

Specimen size calculation

Specimen size was calculated using G*Power software (version 3.1.9), targeting 80 specimens (resin–dentin sticks) from 20 teeth (experimental units). The experimental unit was considered to be the tooth rather than the individual sticks. Each tooth yielded four sticks, and their bond strength values were averaged to achieve a power of 96.8% with a significance level of 0.05 and a 95% confidence interval.

Specimen grouping:

All teeth (n = 20) were randomly allocated to the groups using a computer-generated randomization list, then assigned as follows:

Group I (control group): included 10 teeth (n = 10) that received vita enamic (Vita, Zanhnfabrik, Germany) occlusal veneers bonded with the original adhesive without the addition of NaF. The group was further divided into:

Sub-group I-A (n = 5): Was evaluated following a 24-h storage in artificial saliva at 37 °C.

Sub-group I-B (n = 5): Were tested subsequent to 6 months of storage under the same conditions.

Group II (test group): Included 10 teeth (n = 10) that received vita enamic occlusal veneers bonded with NaF containing adhesive (5000 ppm NaF). This group was further subdivided into:

Scanning and design

Peripheral enamel was marked, and the prepared surfaces were coated with scanning spray (Digiscan-Spray, Yeti Dental Produkte). The teeth were then scanned (Freedom, DOF Inc., Seoul, Korea), and the data were saved as STL files [12]. Occlusal veneers were designed in Exocad (Exocad GmbH, Eiterfeld, Germany) and customized to the tooth anatomy with standardized thicknesses (1.5 mm at the fossa, 2 mm at the cusp) and a 0.05 mm cement gap. The STL files were milled from Vita Enamic blocks using a 5-axis CAD/CAM machine (imes-icore, Germany) [13].

Adhesive preparation

The adhesive system used in this study is listed in (Table 1). For the NaF-containing formulation, 20 mg of NaF (5000 ppm) was weighed using an analytical balance (Citizen CY-204, India) and added to 4 mL of dentin adhesive in a clean beaker [10]. This concentration was selected based on its established use in experimental and clinical dentistry. It represents a relatively high yet controlled level that provides sufficient fluoride ions to produce measurable effects on material interactions while avoiding saturation or adverse structural changes. Thus, it offers a balance between efficacy and experimental sensitivity [9, 10].

Table 1 Manufacturer, commercial name, and composition of the adhesive system used in this study

Mixing was performed under a custom dark box made from thick cardboard and black paper. The mixture was stirred with a magnetic stirrer (IKA Werke GmbH & Co. KG, Germany) at 2000 rpm for 10 min to ensure homogeneity. The NaF-containing adhesive was then transferred to a labeled bottle.

For quality control, the unpolymerized material was dispensed into a laser-cut Teflon mold (4 mm × 4 mm). The mold was covered with a Mylar strip and a glass slide and then light-cured (Woodpecker, Guilin, China) for 20 s [14]. The specimens were polished (Sof-Lex, 3 M ESPE) and subsequently analyzed using a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDX) (JEOL–SEM IT200, Tokyo, Japan). This analysis assessed mixture homogeneity and fluoride distribution, and confirmed proper NaF incorporation.

Restorations luting procedures

Vita Enamic veneers were etched with 5% hydrofluoric acid for 60 s, rinsed, dried, and silanized using two coats of Bisco Porcelain Primer (Bisco, Inc., Schaumburg, IL, USA). Dentin was etched with 37% phosphoric acid for 15 s, rinsed, dried, and bonded with either the original or NaF-containing adhesive, followed by light curing for 10 s [15]. Dual-cure resin cement (Duo-Link, dual-cure resin cement, Bisco, Inc., Schaumburg, IL, USA) was then applied, and the veneers were seated under a constant load of 49 N (equivalent to 5 kg) for 5 min using a universal testing machine (Instron 3345 universal testing machine) to standardize pressure during cementation. Excess cement was removed, and the restorations were light-cured for 30 s [16].

Resin–dentin stick preparation

The specimens were cut longitudinally in both mesiodistal and buccolingual orientations to full depth across the adhesive interface using an Isomet saw (Fig. 2a) under copious irrigation to obtain beams. These beams were further trimmed to produce four resin–dentin sticks from each tooth: three from the center and one located 0.5 mm from the margin, each with an adhesive surface area of approximately 1 mm2 [17]. A digital caliper was used to measure the cross-sectional area of each stick. The resin–dentin sticks from each subgroup then underwent microtensile bond strength testing after storage in artificial saliva at 37 °C for either 24 h or six months. Artificial saliva was used as the storage medium to better simulate intraoral conditions and the oral environment [18]. It was replenished daily to maintain consistency [19].

Fig. 2The alternative text for this image may have been generated using AI.

Microtensile bond strength test. a Isomet cutting of the cemented occlusal veneer buccolingually and mesiodistally. b Application of microtensile force after attaching the beam by a cyanoacrylate adhesive

Microtensile bond strength test

The operator performing the microtensile bond strength testing and failure mode analysis was blinded to group allocation. Each stick was subsequently mounted on a jig for microtensile testing using a cyanoacrylate adhesive and subjected to tensile loading in a universal testing machine at a crosshead speed of 0.5 mm/min (Fig. 2b). Data were calculated and recorded using computer software.

Failure mode analysis

The fractured specimens were examined by a single evaluator under a stereomicroscope (× 40) and classified as adhesive (at the adhesive–dentin or adhesive–ceramic interface), cohesive (within dentin or ceramic), or mixed (involving both the interface and the substrate).

Statistical analysis

Data were analyzed using SPSS v26. The mean bond strength value per tooth was calculated and used as the statistical unit to avoid pseudo-replication. Numerical data were expressed as mean ± SD and range, while categorical data were expressed as frequency and percentage. The normality of data distribution was assessed using the Shapiro–Wilk test. The effect of adhesive type (between-subject factor) and storage duration (within-subject factor) on tensile bond strength was evaluated using repeated-measures ANOVA. In addition, pairwise comparisons between independent groups (Group I vs Group II) were performed using independent t test. All results are reported as mean ± standard deviation (SD), with P < 0.05 (*) considered significant and P < 0.001 (**) considered highly significant.