Effect of different luting agents on vertical marginal fit of full contour CAD/CAM zirconia crown restorations: a comparative in vitro study

INTRODUCTION

The enlarging request for metal-free, aesthetically pleasant restorations has driven the broad use of all cera­mic systems, combined with computer-aided design and computer-aided manufacturing (CAD/CAM) techno- logy in dentistry [1, 2]. However, one concern of using ceramic systems is chipping or cracking of the interface between zirconia core and veneer facing. To overcome this drawback, restorations with no need for veneering were developed, such as monolithic zirconia restorations [2]. CAD/CAM technique is commonly used for the production of zirconia-based ceramic restorations. These CAD/CAM restorations are favored due to their reproducible outcomes, highly aesthetic valuation, reduced fabrication time, and minimal technical errors [3].
Holmes et al. [4] defined vertical marginal discrepancy as the perpendicular measurement from the marginal surface of casting to the axial wall of tooth preparation. They stated that the fit of casting to the tooth is determined by measuring specific points between them [4, 5].
Biological outcomes of ceramic zirconia restorations are crucial for their success and survival rates. Many stu­dies revealed that biological failures are mainly related to secondary caries, vitality loss, and periodontal problems, augmented by increasing marginal discrepancy of a restoration [1, 6, 7]. Marginal discrepancies encourage salivary infiltration and microleakage, resulting in dissolution of luting agent, consequently in-creasing vulnerability to biological failure [8]. Additionally, it can lead to severe dentinal sensitivity, promoting plaque and food debris accumulation around the exposed margins, which initiates periodontal breakdown in abutment teeth [9, 10].
Several studies reported that the acceptable limit of marginal discrepancy should be no more than 120 μm. At the same time, other authors assumed that by using CAD/CAM technology, the marginal discrepancy should be less than 100 μm [10-12]. Marginal fit, in general, may be affected by several variables, such as the type of used restorative material, fabrication techniques, preparation design, different types of finish lines, thickness of die spacers, and type of luting cement [1, 9, 13, 14].
Zirconia restorations can be cemented using conventional cements, including glass ionomer and zinc phosphate cement, and depending on whether the tooth preparation offers adequate retention. However, the development of luting agents and introduction of adhesive systems allowed operators to increase reten-tion in clini­cal conditions, such as tapered and short preparation, providing favorable aesthetic outcomes [3]. Additionally, resin-based cements and adhesive bonding systems exhibit superior physical and mechanical properties, low-er solubility, higher stability, superior aesthetics, and high adhesive strength to dental surfaces and restora-tive materials [3, 5]. Therefore, Ganapathy et al. [9] and many other researchers preferred adhesive resin cements in their ceramic restorations [15, 16].

OBJECTIVES

The purpose of the current research was to analyze the pre- and post-cementation vertical marginal fit of CAD/CAM zirconia crowns using three types of resin luting cement. The first null hypothesis was that no sig-nificant difference would be found between the pre- and post-cementation marginal gap for every type of cement. The second null hypothesis was that no significant difference would be detected in the post-cementation marginal gap among the three different luting agents investigated.

MATERIAL AND METHODS

SAMPLES COLLECTION AND PREPARATION

Twenty-four sound human maxillary first premolars of similar shape and size, extracted for orthodontic reasons were gathered and utilized in this in vitro study. Each tooth sample was then mounted in an acrylic resin block up to 2 mm below cemento-enamel junction (CEJ) to achieve a similar level of alveolar bone sup-port, and to facilitate tooth preparation [3, 8, 17, 18]. For standardization of samples, all crown’ preparations were performed by the same operator using a modified dental surveyor. Preparation was carried out with a high-speed handpiece that was fixed and adjusted to the modified dental surveyor, and utilized during the entire axial wall teeth sample preparations to attain a constant taper degree [19] (Figure 1).
Preparation of samples was done to obtain zirconia crowns, performed as recommended by KATANATM Zirconia: Chamfer finish line of 1.5 mm above CEJ, 0.8 mm circumferential axial reduction, total convergence angle of 6o, and standardized axial height of 4 mm for buccal and palatal aspects. These measurements were verified with a modified digital caliper [17] (Figure 2).

STUDY DESIGN AND SAMPLE GROUPING

The prepared tooth samples were classified into three major groups (n = 8), according to luting agent used. Each group was then subdivided into two subgroups (1 and 2), representing pre- and post-cementation conditions, as follows:
• Group A (A1, A2): zirconia crowns were cemented with RelyX™ Ultimate adhesive resin cement (3M ESPE; Germany),
• Group B (B1, B2): zirconia crowns were cemented with RelyX™ U200 self-adhesive resin cement (3M ESPE; Germany),
• Group C (C1, C2): zirconia crowns were cemented with RIVA Luting Plus resin modified glass ionomer (RMGI) cement (SDI; Australia).
The composition and key properties of the three luting cements used in this study, which may influence crown seating and marginal adaptation, are summarized in Table 1 [20-23].

CROWNS FABRICATION AND CEMENTATION PROCEDURE

In this study, KATANATM zirconia disks (Kuraray Noritake; Japan) were employed for crowns fabrication. Each sample was scanned with Medit I 700 intraoral scanner (Medit; Korea), and fabrication of the full con-tour zirconia crown restorations was performed by Sirona InLab MC X5 milling device, according to the manu­facturer’s instructions using Sirona InLab CAD 20.0 software. The design of all crown samples was fabricated with cement space thickness of zero μm around the marginal area [19]. Internal surfaces of completely fabricated crowns were sandblasted to enhance their bonding to luting agents. Zirconia crowns were sandblasted using aluminum oxide particles (≤ 50 μm) and a 2.5 bar with a space of 3 cm for 15 seconds (3M ESPE; Germany, 2017) using a Renfert sandblasting machine [17, 19].
A specially designed custom-made specimen holding device with a sensor load was utilized as a screw to se-cure the zirconia crown on the prepared tooth sample, preserving seating forces during marginal checking and cementation process. Each sample was seated within the specimen holding device to ensure complete crown seating over its respective tooth before cementation procedure [19]. The cementation procedure was done in three steps relative to each cement type: prepared tooth surface treatment, crown restoration surface treat-ment, and crown restoration cementation.
Prepared tooth surface treatment: For RelyX™ Ultimate adhesive resin cement, the surface of the prepared tooth sample was etched for 15 seconds with a phosphoric acid etchant (32% Uni-Etch; Bisco Inc., Schaumburg, IL, USA), then rinsed for 15 seconds, and dried before application of a light-cured single bond universal adhesive (3M ESPE; Germany), whereas the prepared tooth samples for both RelyX™ U200 self-adhesive resin cement and RIVA Luting Plus RMGI cement, received no surface treatment; they were just washed and dried to obtain glossy dentin.
Crown restoration surface treatment: The inner surface of crown restorations cemented with RelyX™ Ul-timate adhesive resin cement was coated with a thin layer of single bond universal adhesive. In crowns ce-mented with RelyX™ U200, their inner surfaces were coated with zirconia primer (Z-PRIME Plus; Bisco Inc., Schaumburg, IL, USA), then air-dried for 3-5 seconds prior cementation. However, the crown restorations cemented with RMGI cement received no surface treatment.
Cementation procedure: This was performed for all types of luting cements by filling half of the inner sur-face of each crown with a cement, and then seated it over its relevant tooth with rocking movement using full index finger pressure to ensure complete seating. To achieve standardization, every zirconia-crown assembly was placed inside the specimen holding cementation device under a load of 5 kg (5,000 g) for 6 minutes [24] (Figure 3). After the cementation procedure, the cemented crown-tooth assemblies were preserved in distilled water for 24 hours prior to post-cementation marginal gap measurements [17, 25].

VERTICAL MARGINAL GAP MEASUREMENTS

Vertical marginal gap (VMG) was assessed before and after cementation using a digital microscope (Dino- Lite; Taiwan) under a magnification power of 280× [17]. A total of 16 measurements were taken, with 8 rec-orded pre-cementation and 8 recorded post-cementation. Four reference points were estimated on each dis-tal and mesial aspects of the tooth; two were drawn at the midpoint of each tooth surface, while the other two points were positioned 1 mm apart [19]. Processing of the captured images was performed using Di-noCapture software, followed by analysis with ImageJ 1.50i (National Institutes of Health; Bethesda, USA) software program to investigate VMG reading in pixels and converting them into micrometers. All readings were taken by the same researcher and repeated three times to obtain accurate measu­rements [26] (Figure 4).

STATISTICAL ANALYSIS

Data analysis was performed using paired t-test to compare between pre- and post-cementation VMG re-sults for each cement type. Also, one-way (ANOVA) and Duncan’s test were applied to compare among groups before and after cementation (p ≤ 0.05).

RESULTS

The descriptive statistical results and paired t-test for the study groups are shown in Table 2. It was found that the highest mean value of VMG readings was seen in C2 subgroup (51.3238 µm), while the lowest was for B1 subgroup (22.1050 µm). Paired t-test revealed a signi­ficant difference between the pre- and post-cementation VMG measurements for each cement type. One-way ANOVA tests for the pre- and post-cementation meas-urements of the three subgroups and Duncan’s test for the post-cementation subgroups, are presented in Table 3. ANOVA test showed no statistical significant difference among the three pre-cementation subgroups (A1, B1, C1). However, a significant difference was observed among the three post-cementation subgroups (A2, B2, C2). Duncan’s test was employed to indicate which group mean values differ significantly from each other, and revealed that C2 subgroup differed significantly from both A2 and B2 subgroups.

DISCUSSION

The most widely used and clinically accepted method for assessing suitability of indirect restorations and determining their success rate is marginal fit, recognized as the golden standard for evaluating the adaptability of all ceramic fixed restorations [24]. An inadequate marginal fit can result in periodontal diseases, endodontic inflammation, and plaque buildup, which increase the risk of microleakage and carious lesions, and ultimately lead to restoration failure [27]. The VMG of all ceramic restorations can be measured via different methods. In the current investigation, a digital microscope was employed, since it is non-destructive equipment and can be applied without causing damage to the prepared samples. Additionally, utilizing a digital microscope to examine the marginal gap is an applicable technique in dental clinics [28]. Many studies illustrated a strong correlation between cement type and final restoration discrepancy [19, 24, 26]; depending on the type of luting agent used, several factors may promote the formation of marginal gaps after cementation, including luting agent film thickness, viscosity, flow ability, bonding strength, polymerization shrinkage, and water sorption [24, 27]. Therefore, this study investigated the pre- and post-cementation marginal fit of zirconia monolithic crowns luted with different cement types.
Our findings demonstrated that the pre- and post- cementation marginal gap values differed significantly for each cement type, rejecting the first null hypothe-sis. RelyX™ Ultimate adhesive resin cement showed lower VMG values post-cementation (from 24.3850 to 41.0163 µm) than both RelyX™ U200 self-adhesive resin cement (from 22.1050 to 41.9775 µm) and Riva Luting Plus RMGI cement (from 23.1175 to 51.3238 µm), as presented in Table 2, and in line with Hammood and Ibraheem [26]. This result can be interpreted by the low film thickness of RelyX™ Ultimate compared with RelyX™ U200 and Riva Luting Plus RMGI cement [17, 26]. Furthermore, RelyX™ Ultimate demonstrated excellent flow ability, low viscosity, and tiny particle size, which improved its flow, allowing for deeper penetration into micro retention gaps than the other two luting agents [17, 29, 30].
According to our results, ANOVA test identified no significant differences for the marginal gap among pre-cementation groups, which reflects samples’ standardization. However, it revealed that marginal dis-crepancy significantly increased after the cementation procedure in all groups, as illustrated in Table 3. Alt-hough this increase was still within the acceptable range (below 120 µm), and it rejected the second null hy-pothesis of the study. The increase in post-cementation marginal gap may be due to the action of hydraulic pressure that developed during the cementation procedure, possibly leading to pushing the crown to an upward direction [17]. Another plausible justification is that mar­ginal cement space thickness was estab-lished at zero μm. When the crown reached its final position, the cement had no room to escape through the cervical margins, which caused the luting agent to accumulate on the prepared tooth occlusally and increase its thickness layer within the crown; this interfered with appropriate seating of the crown restoration and raised marginal dis-crepancy [26]. Some researchers reported that increasing the thickness of resin cement might weaken its bond strength, as a thicker cement layer induces greater levels of polymerization shrinkage, which initiates heavier stress at the adhesive interface and yields gap for-mation [24, 26, 31]. In contrast, Ganapathy et al. [9] exhibited a reduction in marginal discrepancy following cementation. These conflicting results could be due to different experimental parameters, such as cement type, cementation protocol, seating force for the crown during the cementation process, type of zirconia res-toration, and finish line used.
Duncan’s test in Table 3 showed that the post- cementation VMG mean recorded higher value for Riva Luting Plus RMGI cement, and significantly differed from the other two resin cements, with no statistical difference between them observed. This may be related to the lower bond strength of Riva Luting Plus RMGI cement to both tooth structure and zirconia compared with the other two resin cements (RelyX™ Ultimate and RelyX™ U200) [31, 32]. This can be attributed to the fact that RMGI cements are self-curing and based on acid- base reaction for setting mechanism. Moreover, they demand no surface treatment of either the prepared tooth or zirconia monolithic crowns prior cementation procedure. In contrast, the higher bond strength of RelyX™ Ultimate adhesive luting agent succeeded in achieving lower VMG values due to the following rea-sons: etching the tooth structure using phosphoric acid as an initial step of cementation procedure improved the micromechanical interlocking between resin cement and tooth substrate and raised its binding capacity, resulting in extra bonding action [17, 33]. Furthermore, the single bond universal adhesive applied on tooth structure prior to RelyX™ Ultimate adhesive cement, improved chemical bonding, as it contains functional monomers, particularly acidic monomer 10-methacryloxydecyl dihydrogen phosphate (MDP). This can initiate chemical bonding with metal oxides of zirconia surfaces and generate secondary internal force at the resin-zirconia interface. Also, these functional monomers can demineralize the natu­ral tooth, chelate with calcium ions of the tooth structure (specifically dentin), and create indissoluble calcium salts at the tooth-resin interface [17, 34]. Concurrently, the polymerization of molecules of the acidic monomer cre-ates a polymer network. This polymerization reaction develops a micromechanical retention of the cement in the tooth structure, which promotes low rates of cement solubility and durability [35, 36]. Whereas the lower VMG value for RelyX™ U200 cement can be understood by its improved binding capacity to zirconia restoration surfaces due to the application of zirconia primer to the inner surface of zirconia crowns as a first step before starting cementation with RelyX™ U200; this primer contains MDP monomer in its chemical composition that enhances the bonding strength to the zirconia sur-face owing to chemical interaction between hydroxyl groups of MDP and cationic surface of zirconia [37]. Similarly, the chemical composition of RelyX™ U200 cement includes methacrylate monomer containing phosphoric acid groups. The phosphoric acid groups encourage adhesion to the ceramic surface through hy-drogen bonding, and also boost binding to the calcium of hydroxyapatite crystals of the tooth structure, de-veloping a reliable chemical bond to the tooth structure [35, 38].
A study by Jamel [39] observed that resin modified glass ionomer cements have higher rates of water sorption. However, resin cements show lower water sorption rates. An increased cement’s water sorption mediates hydrolytic reaction at the adhesive interface, hence, interfering with bonding to the tooth structure and decreasing marginal fitness, which consequently promotes marginal gap formation. With respect to the cement film thickness, resin modified glass ionomer cement (Riva) exhibited higher value (17 µm) com-pared with RelyX™ Ultimate adhesive resin cement and RelyX™ U200 self- adhesive resin cement (12 µm and 13 µm, respectively). Film thickness serves as an indirect indicator of cement viscosity and flow ability; a high level of cement film thickness correlates with high viscosity and poor flow ability, which restricts its ability to flow under seating pressure to fill small voids and gaps, resulting in larger marginal gaps. Conversely, a thinner cement film indicates low viscosity and good flow ability, facili-tating optimal adaptation of indirect restorations and minimizing marginal discrepancies for improved mar-ginal fit [17, 21-23].
Since this was an in vitro study, the results should not be generalized to clinical dental practice. Future re-searches ought to highlight the influence of luting agents on VMG after thermocycling, using different mar-ginal cement space thicknesses, different finish line designs, and different types of zirconia crowns to obtain more specific findings.

CONCLUSIONS

RelyX™ Ultimate adhesive resin cement, RelyX™ U200 self-adhesive resin cement, and Riva Luting Plus RMGI cement are effective luting agents for cementation of all ceramic zirconia monolithic CAD/CAM restora-tions, with each demonstrating minimal impact on VMG formation (below the maximum acceptable range of 120 µm). However, RelyX™ Ultimate ensures more optimal restoration outcomes and stands out as the reliable option due to its lowest values of marginal discrepancies, followed by RelyX™ U200 and Riva Lut-ing Plus cements.

DISCLOSURES

1. Institutional review board statement: This study was adhered to all applicable ethical guidelines and regu-lations. It was approved by the Research Ethics’ Committee of Mosul University/College of Dentistry, with reference number of UoM.Dent. 24/1024, issued on April 21, 2024.
2. Assistance with the article: None.
3. Financial support and sponsorship: None.
4. Conflicts of interest: The authors declare no potential conflicts of interest concerning the research, au-thorship, and/or publication of this article.

References