Resin cement is a crucial component in the restoration of teeth and is regularly utilized in various dental specialties for a wide array of procedures such as cementing fixed dental prosthesis, acting as a root canal sealer, and bonding restorations [1], [2]. Particularly, the marginal seal holds a vital role in the clinical outcome. The failure of the marginal seal with the resin cement can lead to microleakage, recurrent caries, and ultimately jeopardize the longevity of the restoration or treatment [3], [4]. To overcome these shortcomings, modifications have been made to resin cement in the form of adding nanoparticles [5]. Nanoparticles are particles of matter that are smaller than 100 nm in dimension, and as a result are capable of unique properties that provide certain advantages over the larger-sized particles. They occupy a large surface area relative to their volume, which allows them to interact with dental materials at the nanoscale and consequently improve the physical, biological, mechanical, and physicochemical properties of the hybrid dental material. Furthermore, specific properties of the nanoparticles can be manipulated in order to achieve enhanced effects, such as biodegradability or antibacterial activity [6], [7].

By incorporating nanoparticles with resin cement, several benefits can be imparted onto the cement. Their nano-size allows them to be highly translucent, therefore maintaining the color stability [8]. Moreover, the nanoparticles are capable of strengthening the mechanical qualities of the cement by reinforcing the matrix, raising the elastic modulus of the adhesive layer, and enhancing the bond between the adhesive and the tooth by penetrating the tubules of the dentin [9], [10], [11]. They are also capable of reducing polymerization shrinkage due to an interaction in the molecular range in which the nanoparticles integrate with the polymer molecules, thereby reducing the polymerization stresses and volumetric shrinkage [12], [13], [14].

In particular, magnesium oxide nanoparticles (MgO NPs) have garnered consideration in integration with resin cement as it is especially biocompatible. However, MgO NPs tend to agglomerate and form clusters. Nanoparticles must be thoroughly and uniformly dispersed throughout the matrix in order to achieve the desired properties, and the agglomeration would obstruct this dispersion. The outcome would be the loss of the nano-attributes and disruption of the stability and properties of the cement [15], [16]. A recent solution was found in coating the MgO NPs with the zein polymer, an alcohol-soluble protein derived from corn and utilized in the pharmaceutical industry. The zein counteracts the nanoparticles hydrophobicity and stabilizes the particles, ultimately reducing the agglomeration without compromising on the MgO NPs’ other desirable characteristics [17]. Furthermore, the antimicrobial impact of zein-coated MgO nanoparticles (zMgO NPs) was found to be highly effective against the four bacteria S.mutans, S.aureus, E.faecalis, and C.albicans [18]. It was previously reported that resin cement modified with zMgO NPs produced a significant antimicrobial effect [19].

As secondary caries is still a leading cause of failure in restorations, the employment of antimicrobial resin cement can be a valuable asset in combating bacterial invasions beneath and at the restoration’s margin [20]. Moreover, the addition of zMgO NPs to resin composite aided in minimizing marginal leakage by improving the adaptation to the interface between the restoration and the tooth, thereby prolonging the longevity of the restoration [21]. An enhancement of the same nature to resin cement could achieve a similar outcome.

In order to evaluate the adhesive margin, morphological assessment by a microscope is often implemented [22]. In recent studies, cross-polarization optical coherence tomography (CP-OCT) has been utilized as a 2D and 3D visual biomedical technique to examine and assess both soft and hard tissues [23], [24]. Furthermore, it has been used to measure the interfacial gaps in composite restoration interfaces [21], [25], [26]. CP-OCT functions by applying an ultra-high-speed scan of an infrared light source to create tomographic images 1–3 mm below the surface of the analyzed specimens. The unique aspect of this laboratory test is that it can be done without the processing of the specimens or exposing them to ionized radiation. Additionally, several studies have reported that CP-OCT is capable of a remarkable degree of sensitivity and specificity for gap detection [27], [28], [29].

This study is part of a series of investigations regarding the dental implications of zMgO NPs on various dental materials [8], [17], [18], [19], [21], [30], [31], [32], [33]. To the authors’ knowledge, there is currently no data on the assessment of the internal adaptation of self-adhesive resin cement (SARC) modified with zMgO NPs using CP-OCT. The objective of this study was to analyze the effect of incorporating different concentrations of zMgO NPs to resin cement on the internal adaptation of the restoration utilizing CP-OCT and scanning electron microscopy (SEM). The null hypothesis states that there is no discrepancy in the adaptation of resin cement with or without zMgO NPs, while the alternative hypothesis states that incorporating zMgO NPs in resin cement will significantly increase its marginal adaptation.