The effect of thermocycling on fracture resistance of zirconia crowns cemented with polylysine modified resin cements (Comparative in vitro study)

Abstract

Background

resin cement type and intraoral temperature fluctuations may affect the fracture performance of successful zirconia restorations. To fill this gap, the purpose of this study is to evaluate and compare the influence of thermocycling on fracture resistance and mode of failure of monolithic zirconia crowns luted with Rely X™ U200 and Breeze^TMself-adhesive resin cements as well as imply the effect of adding 2 % of polylysine (PLS) to these cements.

Materials

64 maxillary premolars were milled out of zirconia blocks using CAD/CAM milling system. They were divided into four groups (n = 16) according to the cement type. Four different resin cements were used (RelyX^TMU200, Breeze™, RelyX™ U200 with 2 % PLS and Breeze™ with 2%PLS). Each group was further subdivided into experimental and control groups (n = 8). The experimental specimens were exposed to thermocycling protocol of 10,000 cycles in water bath at 5 °C and 55 °C.Each specimen was subjected to axial load until fracture using universal testing machine. Fracture modes were analyzed using digital microscope. Data were statistically analyzed using paired t-test at a level of significance of 0.05.

Results

there was a statistical significant difference in fracture load among groups (p < 0.05) with the highest mean in Rely X cement. Although the fracture loads statistically decreased after thermocycling (p < 0.05) there was no significant effect on the addition of 2 % PLS (p > 0.05). Microscopical analysis demonstrated a majority of catastrophic mode of fracture.

Conclusion

both cement type and thermocycling exert significant effects on fracture resistance of premolars crowns restored with monolithic zirconia, while the addition of 2 % PLS exerted negligible effect.

Graphical abstract

1. Introduction

Zirconia ceramics have gained widespread acceptance in restorative dentistry due to their excellent aesthetics, biocompatibility, minimal tooth reduction requirements, and superior mechanical properties.^1,2 These advantages have made zirconia the material of choice for both clinicians and patients. However, despite its high flexural strength and fracture toughness, the clinical success of zirconia restorations is not determined solely by their mechanical properties. A critical factor influencing their long-term performance is the quality and durability of the bond between the zirconia and the tooth structure.³

Traditionally, zirconia crowns have been cemented using conventional luting agents such as glass ionomer or zinc phosphate. While these cements offer ease of use, they often fall short in providing adequate retention and marginal sealing. Resin-based cements, particularly adhesive resin cements, offer improved mechanical properties, superior marginal adaptation, and better retention. However, their bonding protocol typically requires multiple steps, including surface pre-treatment of both the tooth and the restoration, which can introduce variability and technique sensitivity.⁴

To overcome these limitations, self-adhesive resin cements were introduced. These materials simplify the cementation process by incorporating acidic functional monomers capable of simultaneously demineralizing and infiltrating the tooth substrate, eliminating the need for separate etching and bonding steps. This results in both micromechanical retention and chemical adhesion. Furthermore, in cases involving thick or opaque ceramic restorations, dual-cure self-adhesive resin cements are often recommended to ensure complete polymerization where light transmission is compromised.⁵

Despite these advancements, the longevity of resin–zirconia bonds remain a concern due to the challenging oral environment, which is characterized by fluctuating temperatures, moisture, pH changes, and mechanical stresses. To simulate such conditions, thermocycling is widely employed as an in vitro aging method. As outlined in ISO 10477 (1996)⁶ thermocycling mimics the thermal stress caused by daily intake of hot and cold substances. It is commonly used to assess the durability of resin cements and adhesive interfaces. While several studies have reported a decline in bond strength and fracture resistance after.^7,8 However, Fouda et al., 2024 suggested that thermocycling, under the conditions of their study, exert negligible effects on both fracture toughness and hardness. These authors attributed this finding, to the concentrated crystalline content found within glass ceramics enhances their hardness and elastic modulus values, rendering them more resistant to thermal changes compared to hybrid materials.⁹

In addition to thermocycling, fracture resistance remains a crucial determinant of the clinical viability of zirconia restorations. Failure due to fracture is one of the most common complications associated with all-ceramic restorations.¹⁰ Factors such as elastic modulus, surface flaws, fabrication technique, and the cementation process significantly influence fracture resistance. Notably, zirconia's transformation toughening mechanism where stress induces a phase transformation from tetragonal to monoclinic structure enhances its resistance to crack propagation. However, zirconia may undergo low-temperature degradation (LTD) in the presence of moisture over time, leading to reduced mechanical performance.¹¹ Consequently, to estimate the mechanical performance of zirconia crowns, it is not sufficient to test them without aging by thermocycling. Another important consideration is the type of resin cement used. A study by Jourado et al. (2025) demonstrated that fracture resistance varied significantly when the same crown type was cemented using different adhesive resin cements, emphasizing the role of cement composition on mechanical outcomes.¹²

Recently, attention has turned to enhancing the antibacterial properties of resin cements to help prevent secondary caries a leading cause of restoration failure. Polylysine (PLS), a cationic homopeptide composed of L-lysine units, has been incorporated into resin cements such as RelyX U200 and Breeze at a 2 % concentration. PLS is known for its broad-spectrum antimicrobial activity, low cytotoxicity, and high thermal stability.¹³ Its inclusion is intended to allow continuous release at the tooth-restoration interface, thereby inhibiting bacterial colonization and caries initiation.¹⁴ However, the mechanical implications of PLS incorporation, particularly under thermocycling conditions, have not been thoroughly investigated. While initial studies indicate promising antimicrobial benefits, the potential trade-off with mechanical performance especially fracture resistance remains unclear. Although several studies have evaluated the fracture resistance of zirconia restorations using different cements and aging protocols, limited evidence exists on the combined effect of thermocycling and PLS incorporation into self-adhesive resin cements. As PLS is being considered for routine inclusion in dental materials due to its antibacterial benefits, understanding its effect on mechanical properties specifically fracture resistance is vital for ensuring long-term clinical success. Therefore, this laboratory study aimed to evaluate the effect of thermocycling and 2 % polylysine (PLS) addition on the fracture resistance of zirconia crowns cemented using two self-adhesive resin cements: RelyX U200 and Breeze.The first null hypothesis postulated that thermocycling does not significantly affect the fracture resistance of zirconia crowns cemented with different types of self-adhesive resin cements, while the second null hypothesis stated that the addition of 2 % Polylysine to these resin cements does not influence the fracture resistance of zirconia crowns.

2. Materials and methods

2.1. Sample collection and preparation

This study was approved by the academic research ethics committee at the College of Dentistry/University of Baghdad, Iraq, (no.1029525). A total sample size was calculated to be 64 based on G*power statistical analysis program version (3.1.9.7) (16 per group, subdivided into 8 per subgroup, n = 8) was considered to be sufficient where α = 0.05 and 85.0 % power. 64 sound human second premolar teeth were extracted for orthodontic purposes from patients aged 18–22 years. Teeth were carefully cleaned from debris then polished with a rubber cup and pumice, all teeth should be free of caries, enamel flaws, or cracks. To assure tooth size uniformity, the recorded dimensions of all dentition samples deviated from the calculated mean by no more than 10 % ¹⁵.

2.2. Teeth mounting and preparation

Each tooth was embedded individually 2-mm apical to cementoenamel junction (CEJ) in a specially designed custom-made cuboid rubber mold that was filled with a freshly mixed cold cure acrylic (Duracryl plus, Kerr, USA). A dental surveyor (Paraline, Dentaurum, Germany) was used for the alignment of each tooth to be vertical to the horizontal plane of the mold ¹⁶. Modified dental surveyor was used to assure a standard axial taper and consistent convergence angle during tooth preparation. The finished preparation had a 1 mm chamfer finishing line, 4 mm occlusogingival height 6 % axial wall convergence and 1.5 mm occlusal clearance ¹⁷ (Fig. 1). and (Fig. 2).

Fig. 1.

(A) Planar occlusal reduction with Barreled-shaped trapezoid bur, (B): Axial preparation, (C): final preparation.

Fig. 2.

A modified dental surveyor for teeth preparation.

2.3. Crowns fabrication

Crowns were fabricated according to the manufacturer ‘s instructions of e.max ZirCAD with CAD/CAM milling system. The process included model scanning, designing, milling, and sintering. A CEREC Connect digital intra-oral scanner (CEREC-Omnicam, Dentsply-Sirona, Bensheim, Germany) was used to capture a 3D image of each prepared tooth. Crown restorations were designed using Sirona InLab 15.1 software (Dentsply-Sirona) and then milled with the InLab MC X5 milling machine (Dentsply-Sirona) using e.max ZirCAD material.The milling/design parameters used were: Radial spacer: 100 μm,Occlusal spacer: 100 μm, Occlusal Milling Offset: 0 μm, Proximal contacts strength: 25 μm, Occlusal contacts strength: 25 μm,Dynamic contacts strength:25 μm,Radial minimal thickness: 500 μm, Occlusal minimal thickness: 700 μm,Margin thickness: 50 μm. The “Biogeneric Reference” design mode was used to standardize crown designs across all samples.An unprepared maxillary first premolar (right or left) served as a reference tooth to calculate the restoration suggestion. In a separate step, the reference tooth was scanned using the same procedure as for the prepared tooth by selecting the “BioRef Upper" image field. The resulting 3D virtual images were saved in STL file format and exported to Sirona InLab CAD 18.0, where the subsequent phases of crown fabrication were carried out. Dry milling was performed by using In-Lab MC X5 milling apparatus. Each crown's milling needs Twelve minutes. Using a handpiece fitted with a fissure bur to separate the crown from attachment with the block. Zirconia crowns were milled 20–25 % larger to compensate for shrinkage and then sintered using the In Fire HTC Speed Sintering Furnace following the manufacturer's instructions to achieve the final size, strength, and color (Sirona, 2020) ^c.The intaglio surface of each crown was sandblasted with 50 μm aluminum oxide at 1 bar from a 10 mm distance for 15 s (Ivoclar Vivadent, 2017) to enhance mechanical interlocking with the luting cement. After sandblasting, each crown was seated on its corresponding tooth to verify proper fit (Fig. 3).

Fig. 3.

Crown fabrication steps.

2.4. Sample grouping

Zirconia specimens (n = 64) were randomly divided into four groups based on the resin type bonded to the specimen (n = 16). Four different types of self-adhesive resin cements were used (Table 1); namely: RelyX U200 (3 M ESPE, Germany), RelyX U200 with 2 % PLS, Breeze (Pentron Clinical, Connecticut, USA) and Breeze with 2 % PLS.

Table 1.

Description of materials used in the study.

Materials	Description	Composition	Manufacture
RelyX™U200Automix	Self-adhesive Dual-cure	-Methacrylate monomers containing phosphoric acid groups Methacrylate monomers -Monomers Alkaline (basic) fillers -Silanated fillers -Initiator components Stabilisers-Rheological Additives Pigments -Mixture of BISGMA, UDMA, TEGDMA, HEMA, & 4-MET resins, silane-treated	(3M ESPE, Germany)3M Deutschland
Breeze™	Self-adhesive Dual-cure	-Bariumborosilicate glasses -Silica with initiators, stabilisers and UV absorbers, -Organic and/or inorganic pigments, opacifiers	Pentron Clinical Technologies, LLC68 North Plains Industrial Road Wallingford, CT USSA 06492
Polylysine		(C6H12N2O)n	ZHENGZHOUBAINAFO Bainofa Bioengineering LTD-China
Zirconia	IPS e.max ZirCAD LT	Zirconium oxide (ZrO2) 88.0–95.5, yttrium oxide (Y2O3) > 4.5 % – ≤ 6.0 %, Hafnium oxide (HfO2) 5.0 %, aluminium oxide (Al2O3) ≤ 1.0 %, Other oxides ≤1.0 %	Vivadent, Schaan, Liechtenstein

2.5. Cementation with the resin cement

Before cementation, the crowns were thoroughly cleaned with distilled water in an ultrasonic cleaner for 5 min to remove any residue from sandblasting according to the manufacturer's instructions. Each crown was cemented on its respective tooth by using the corresponding self-adhesive resin cement. The cement was applied to the intaglio surface of each crown using a disposable mixing tip, covering half of the crown for standardization.

The crown was initially seated by finger pressure, then a vertical load of 5 kg was applied for 6 min by using a custom-made cementation device to simulate clinical biting forces, with a rubber piece placed at the end of the vertical arm of the holding device to ensure equal load applied on the occlusal surface of the crown restoration and to simulate the cushion effect applied by the cotton roll during the cementation clinically ¹⁸ (Fig. 4) and (Fig. 5). Excess cement was removed by probe after 1-s spot curing using a light cure unit, each cement was cured according to their manufacturer's instructions (40 s for Breeze cement and Breeze PLS; 20 s for RelyX cement and RelyX PLS). All cemented samples were kept for 1 h to bench set, all samples were then stored in distilled water at 37 °C for 24 h¹⁹

Fig. 4.

The custom-made Cementation device.

Fig. 5.

A: cement application, B: Samples after cementation.

2.6. Incorporation of PLS into resin cements

The incorporation of (2 %) PLS powder (Zhengzhou Bainafo bioengineering company, CHINA) in resin cement involved accurate weighing of the powder using digital balance with accuracy of (0.0001 gm) (Sartorius, Germany), base and catalyst components were dispensed separately. The PLS powder was divided into two equal portions; one was introduced to the base while the other was introduced to the catalyst, each mix was mixed separately and thoroughly using a mixing machine run (Zhermack, Italy) at 3600 rpm for 1 min to achieve the intended 2 % concentration²⁰. The resulting paste was then transported into 3 ml disposable syringe which was placed in an upright position on a vibrator to get rid of any trapped air bubbles; then each paste was retransferred to its original barrel by pushing the material through a specialized customized connector. Each barrel was left for 5 min to allow the paste to settle. The two barrels were then joined together to be ready for use. All steps of mixing were performed in a dark room to avoid any possible unwanted cement polymerization (Fig. 6) ²¹

Fig. 6.

Steps of the preparation of modified resin cement.

2.7. Thermocycling and fracture resistance test

The specimens of the four groups (according to cement type) were further subdivided into experimental and control groups (n = 8). The experimental specimens underwent 10,000 cycles in a water bath with temperature ranging from 5 °C to 55 °C, with a dwell time of 30 s at each temperature, and a transfer time of 5 s²². The thermocycled specimens were left to dry and stored for 24 h in a non humid environment before testing, while the control; specimens remained incubated at 37 °C until testing.

All crowns were loaded until fracture using an electronic universal testing apparatus (Laryee, China). A round-end stainless steel indenter at a crosshead speed of 0.5 mm/min was used to apply a vertical load on each zirconia crown. A piece of 1 mm thin rubber was placed between the indenter and the crown to avoid distortion of the crowns., and the fracture load was recorded automatically in Newton (N).

A digital microscope (Dino-Lite capture 2.0, version 1.3.6., Taiwan) at 70× magnification was used to evaluate the modes of fracture for all samples according to Burke's classification²³ (Table 2).

Table 2.

Burke's classification for the mode of fracture.

Mode of fracture	Description
Code I	Minimal fracture or crack in the crown
Code II	Less than half of the crown lost
Code III	Crown fracture through midline (half of the crown displaced or lost)
Code IV	More than half of the crown lost
Code V	Severe fracture of the crown and/or tooth

2.8. Statistical analysis

Data were analysed using SPSS (Statistical Package for Social Science) version 26.0 software at a significance level of 0.05. The normality of the data was evaluated using the Shapiro-Wilk test (p > 0.05). Descriptive statistics (mean and standard deviation) were used to describe the quantitative outcome variable (fracture strength). Multiple comparison by paired t-test was used to investigate the effect of three factors, “cement type”, “PLS addition” and “thermocycling”, and their interaction on the fracture resistance of zirconia crowns.

3. Results

Descriptive statistics for fracture resistance values are presented in Fig. 7It showed that the mean fracture load gradually decreased in the following order: RelyX, RelyX PLS, Breeze and Breeze PLS cement group with significant difference (p<0.05). All these values are decreased statistically after being subjected to thermocycling (p<0.05) while there was no significant effect on the addition of PLS on the same type of cement (P>0.05) as shown in Table 3, Table 4, Table 5.

Fig. 7.

Bar chart showing the fracture resistance mean values in Newton.

Table 3.

Comparison of fracture resistance of the tested cements before and after thermocycling.

Type of cement	before thermocycling				after thermocycling
RelyX	Type of cement	mean	SD	Type of cement	mean	SD	Paired t- test	P value
	RelyX without PLS	3006.88	173.124	RelyX without PLS	2881.56	76.195	2.560	.022∗
	RelyX with PLS	2951.31	168.801	RelyX with PLS	2826.25	131.734	2.210	.043∗
	t-test	.919		t-test	1.454
	P value	.365		P value	.159
Breeze	Breeze without PLS	2714.69	236.387	Breeze without PLS	2554.75	191.529	2.216	.043∗
	Breeze with PLS	2677.13	231.917	Breeze with PLS	2513.19	213.593	2.195	.044∗
	t-test	.454		t-test	.579
	P value	.653		P value	.567

∗ significant difference at p ≤ 0.05.

PLS = poly-lysin.

SD=Standard Deviation.

P value = probability value.

Table 4.

Comparison of fracture resistance of the tested cements before thermocycling.

before thermocycling	Type of cement	Mean	SD	Type of cement	mean	SD	t-test	P value
	RelyX without PLS	3006.88	173.124	Breeze without PLS	2714.69	236.387	3.989	.000∗
	RelyX with PLS	2951.31	168.801	Breeze with PLS	2826.25	231.917	3.824	.001∗

∗Significant at p ≤ 0.05.

PLS = poly-lysin.

SD=Standard Deviation.

P value = probability value.

Table 5.

Comparison of fracture resistance of the tested cements after thermocycling.

After thermocycling	Type of cement	mean	SD	Type of cement	mean	SD	t-test	P value
	RelyX without PLS	2881.56	76.195	Breeze without PLS	2554.75	191.529	6.342	.000∗
	RelyX with PLS	2826.25	131.734	Breeze with PLS	2513.19	213.593	4.999	.000∗

∗Significant at p ≤ 0.05.

PLS = poly-lysin.

SD=Standard Deviation.

P value = probability value.

Regarding the mode of fracture before thermocycling, 50 % of crowns cemented with Breeze PLS showed a catastrophic fracture of teeth and/or restoration (Code V), However, after thermocycling, 62.5 % of crowns cemented with Rely X PLS showed Code V fractures as shown in (Table 6), (Fig. 8) and (Fig. 9).

Table 6.

Percentage of the different fracture modes for all groups.

	Type of cement	Code I	Code II	Code III	Code IV	Code V	Total
Before thermocycling	RelyX without PLS		2 (25 %)	1 (12.5 %)	3 (37.5 %)	2 (25 %)	8 (100 %)
	RelyX with PLS		0 (0 %)	2 (25 %)	3 (37.5 %)	3 (37.5 %)	8 (100 %)
	Breeze with PLS		1 (12.5 %)	2 (25 %)	1 (12.5 %)	4 (50 %)	8 (100 %)
	Breeze without PLS		1 (12.5 %)	2 (25 %)	2 (25 %)	3 (37.5 %)	8 (100 %)
After thermocycling	RelyX without PLS		0 (0 %)	2 (25 %)	2 (25 %)	4 (50 %)	8 (100 %)
	RelyX with PLS		0 (0 %)	1 (12.5 %)	2 (25 %)	5 (62.5 %)	8 (100 %)
	Breeze without PLS		0 (0 %)	3 (37.5 %)	2 (25 %)	3 (37.5 %)	8 (100 %)
	Breeze with PLS		1 (12.5 %)	1 (12.5 %)	2 (25 %)	4 (50 %)	8 (100 %)

PLS = poly-lysin.

Fig. 8.

Different modes of fracture: (A) Code II, (B) Code III, (C) Code IV, (D) Code V.

Fig. 9.

Images showing Code V fracture mode Under digital microscope.

4. Discussion

Cementation is an essential step in indirect restoration since it has an influential effect on the adhesion between the tooth structure and the internal surface of indirect restoration which is primarily responsible for long-term clinical success. Because residual bacteria beneath restorations and plaque formation around restoration margins are considered key factors for indirect restoration failure; luting cement with antibacterial properties is required. Unfortunately, commercially available luting cements do not have any antibacterial activity. However, attempts to provide antibacterial properties to luting cement were very few and limited, based on the incorporation of antibacterial agents into resin cements.24, 25, 26 The clinical success of indirect ceramic restoration strongly relies on its durable adhesion to the dentin. Hence, the proper selection of the luting agent and the application of the accurate cementation protocol are of paramount importance.¹² Polylysine (PLS), a biodegradable, water-soluble, and non-toxic cationic polypeptide with known antimicrobial properties, has previously been applied in dental composite restorations. However, published literature contains no information regarding the relationship between modified cement and fracture resistance after aging. To our knowledge, this is the first study to incorporate PLS into two types of resin cements, providing novel insights into its effect on the mechanical properties of these materials. RelyX U200 and Breeze resin cements were selected in this study as representatives of self-adhesive resin cements, which have gained widespread clinical adoption due to their simplified application and reduced technique sensitivity. Despite belonging to the same class, their compositions differ significantly. Breeze contains 2-hydroxyethyl methacrylate (HEMA), a hydrophilic monomer that enhances wettability and facilitates diffusion into demineralized collagen fibrils. In contrast, RelyX U200 is HEMA-free, which minimizes water sorption and reduces susceptibility to hydrolytic degradation over time.²⁷

This comparative in vitro study investigated the effect of thermocycling on fracture resistance of four self-adhesive resin cements luted to commonly used zirconia dental ceramic crown and determine whether the addition of 2 % of PLS to Rely X and Breeze cements affected their fracture resistance. The results have demonstrated that the artificial aging reduced the fracture resistance of zirconia crowns of all the cement materials used. Therefore, the first null hypothesis was rejected. Moreover, the result revealed that the addition of 2 % PLS to the two types of cements has no statistical significant effect on the fracture resistance of zirconia crowns, therefore, the second null hypothesis was accepted.

The artificial aging method used in the current study is accepted and commonly used in in vitro studies^28,29 as it provide essential information on life time predictions and durability of ceramic restorations. There is no agreement among researchers about the number of thermal cycles, however, it is suggested that10,000 cycle is equivalent to approximately one year of clinical service,³⁰ in which the samples are embedded in alternating cold and hot water baths to simulate the temperature fluctuations occurring in the oral cavity between 5 and 55 °C aiming to create thermal strains at the bonding interface. The repetition of temperature change and water absorption by resin could change the material properties, because the absorbed water has a plasticizer effect which result in unsupported regions beneath the crown and raises the possibility of fracture under stress³¹ additionally, the water sorption process takes into consideration the hygroscopic expansion caused by prolonged hydrophilicity. It has a close relationship with bonded crowns longevity.³² Cracks in ceramic crowns were brought on by hygroscopic expansion stresses produced by restorative and luting materials. Therefore, in this study, the fracture resistance of zirconia crowns was reduced after thermocycling; however, it was higher than the maximum biting force which is ranged from 490 to 520 N in normal patients and may reach up to 790 N in patients suffering from bruxism³³

Indirect restorations that can withstand mastication loads of approximately 500N in the premolar area could be considered favorable material for posterior indications.³⁴ The average fracture loads of the crowns tested in the current study ranged from 2640 to 3288N without aging and 2305–2960 N after aging; this indicates that all crowns cemented with different cement materials could maintain their mechanical fracture load to withstand the clinical masticatory forces. However, a significant drop in fracture strength was noticed in all cement types used after thermocycling, which could be attributed to the hydrophilicity of resin cements due to the high content of acidic phosphate functional monomers, that induce higher water sorption and, consequently be susceptible to hydrolytic degradation at the cement-zirconia interface. Further hydrolytic degradation at the bonding interface could be created by the difference in coefficients of thermal expansion between zirconia and adhesive resin cement after aging.³⁵ This finding was in line with Abdulkader et al., 2021 and Alrabeah et al., 2023 who stated that thermocycling significantly reduced the bond strength between zirconia and self-adhesive resin cement.^1,36

The results of the present study revealed inferior fracture resistance of zirconia crowns cemented with Breeze resin cement, with superior values in crowns cemented with Rely X, this could be attributed to that resin cement's flexural strength dramatically reduced by water absorption. A previous study by Mansi and Al-Shamma 2023 revealed that the water sorption of breeze was higher than that for RelyX U200 because the former contains Bis-GMA and TEGDMA monomers, However, Breeze contains additional resin monomers (including HEMA and UDMA).²⁰Uncured HEMA absorbs water, which can cause monomer dilution. After polymerization, poly-HEMA also attracts water and forms hydrogels, thereby reducing the polymer's mechanical strength. The same study found that, PLS addition had no discernible effect on either cement's compressive strength, because PLS fillers were added without any salination process. Therefore, enhancement of mechanical properties is not expected since such enhancement necessitates the presence of a chemical bond between the resin matrix and filler via a coupling agent,³⁷ these findings may explain the non-significance of the mean fracture load between cements with and without 2 % addition of PLS. Nevertheless, incorporating 2%PLS antibacterial particles into both types of cement could be a promising clinical step, through their release at indirect restoration-tooth margin may prevent secondary caries initiation, this could be the key to extending the survivability of fixed prostheses.

More than half of the samples of all groups showed a catastrophic mode of fracture with sever fracture of both tooth and crown, this may be attributed to the high strength of low translucent zirconia used in this study that is classified as (3 Y-TZP) with no cubic phase. Moreover, the sharp inclination of the buccal and palatal cusps in the upper premolars makes them more vulnerable to a mesiodistal split vertical fracture under occlusal load whereby the slope of the cusp and the position of the loaded applicator during loading play an important role in determining the fracture behavior³⁸ as reported in previous studies39, 40, 41

Even though the study presents the limitations that only axial static load was applied that didn't mimic dynamic clinical loading situation. Although thermocycling served as a method of simulating artificial aging, it is noteworthy that other environmental factors such as chemical and mechanical stimuli were not taken into considerations. Further researches with long-term clinical investigations are required to evaluate the clinical outcomes of zirconia crowns bonded to different self-adhesive resin cements.

5. Conclusion

Zirconia crowns tested can withstand forces higher than the maximum biting forces in the premolar area with the highest fracture resistance in those cemented with Rely X U200. Also the addition of 2 % PLS to both cements had no impact on the fracture resistance of zirconia crown while, thermocycling significantly reduce their values. So it is recommended to add 2 % PLS to resin cements due to its antibacterial property.

Patient consent

This in vitro study, so there is no patient consent.

Ethical clearance

The study was conducted in accordance with the Declaration of Helsinki, the research ethics committee of the College of Dentistry, University of Baghdad, reviewed and approved the research project (protocol code 1029525, ref. number 1029 on 18/2/2025).

Source of funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of competing interest

None.