An in vitro comparison of the fracture resistance of MOD inlay restorations using different lithium disilicates

Abstract

Objective:

The objective of this study was to evaluate and compare the fracture resistance of MOD inlay preparations restored with two different lithium disilicate glass ceramics.

Materials and Methods:

Standardized MOD inlay cavities were prepared on 64 human permanent premolar teeth and divided into two groups to be restored with lithium disilicate glass-ceramic materials, i.e. Group I with IPS e.max Press HT and Group II with Shofu Vintage LD Press T. The fracture resistance was assessed using a universal testing machine and recorded in Newtons (N). Statistical analysis was done using SPSS software (Version 17.0, IBM, USA). P <0.05 was set as a statistically significant level. Unpaired Student’s t-test was used to compare the fracture resistance values of both groups.

Results:

The mean fracture resistance value obtained for Group I (IPS e.max Press HT) was 565.04 N while that obtained for Group II was 732.75 N. Statistically significant difference was found in this intergroup comparison (P = 0.002).

Conclusion:

The fracture resistance of MOD inlay preparations restored with Vintage LD Press T (Shofu) was significantly higher than those restored with IPS e.max Press HT (Ivoclar Vivadent).

INTRODUCTION

Approximately 80% of people in developed countries have experienced dental caries, making it one of the most prevalent diseases. There is disagreement on the best restorative method when there are large carious lesions, inadequate restorations, and tooth fractures. An intracoronal cast restoration known as an inlay is made to restore the proximal and occlusal surfaces of posterior teeth without affecting the cusps. The overall benefits of utilizing inlay preparations are the preservation of the tooth’s anatomical shape, preservation of the pulp, enhancement of periodontal health, ease of cementation without hydrolytic behavior, and ease of visual margin management. Over the past 10 years, patients’ growing interest in esthetic dentistry has increased demand for metal-free restorations in both the anterior and posterior regions. Excellent color stability, remarkable resilience to the oral environment, remarkable esthetic qualities, and a high degree of biocompatibility with periodontal tissue are all displayed by all-ceramic inlays.[1,2]

Ceramics have grown in popularity in dentistry due to their natural appearance, translucency, biocompatibility, and optical qualities. Dental ceramics are fragile materials with poor tensile strength, a tendency toward catastrophic failure, and sensitivity to imperfections. These are the main drawbacks of these materials. Recently, glass ceramics reinforced with zirconia, lithium disilicate, and hybrid porcelain materials have been introduced. Among these systems are pressable materials, which acquire their strength from tiny crystals scattered within a glass matrix. Because of these materials’ superb visual qualities, patients have accepted them well. Various degrees of opaque pressable ceramic ingots can be used to create lithium disilicate glass ceramics. About 70% of the lithium disilicate that are needle-like (Li2Si2O5) crystals in their microstructures are embedded in a glassy matrix made up of oxides such as SiO2, K2O, MgO, Al₂O₃, P₂O₅, and others. Lithium disilicate glass ceramics offer greater translucency, increased mechanical strength of 400 MPa, and superior esthetics in a durable package.[3,4]

Ivoclar-Vivadent’s IPS e.max Press press ceramic material released in 2005 is a lithium disilicate pressed glass ceramic with the chemical base same as that of IPS Empress 2 (2SiO2-Li2 O), but a different firing method modifies its characteristics. It shows significantly better physical characteristics and more translucency than IPS Empress 2. Clinical studies have shown that IPS e.max Press has a 3-year survival rate of 96.6% for single crown restorations. A ceramic ingot made of lithium disilicate press glass is also produced by Vintage LD Press (Shofu Inc.); a system of interlocking crystals encased in a glass matrix makes up its microstructure with a mechanical strength of up to 400 MPa and an exceptionally strong break resistance.[5,6]

The critical stress intensity level at which a particular fault begins to grow is known as fracture toughness. Shear tests or three-or four-point bending tests are used to measure the fracture resistance of restorative materials. Because only small-dimensional specimens are needed and the crack growth parameter is comparable to what is predicted in clinical situations, the indentation fracture technique is particularly suitable for studying the behavior and characteristics of brittle materials.[7] Moreover, a complex interplay of variables, such as the geometry of the restoration, the mechanical characteristics of the restoration, the cementation material, and the damage brought on by occlusal forces, influences the clinical failure of ceramic restorations. Therefore, the best way to examine these restorations’ ability to withstand fractures is to look at them in the anatomical form that they are utilized in clinics, like inlays that break under strain.[8,9]

Thus, the purpose of this research is to assess and contrast the variations in the fracture resistance of MOD Inlay preparations that were restored using two distinct glass ceramics based on lithium disilicate on extracted human premolar teeth.

MATERIALS AND METHODS

Experimental in vitro research was carried out at the Department of Conservative Dentistry and Endodontics, to assess and compare the two distinct lithium disilicate glass ceramics – IPS e.max Press HT and Shofu Vintage LD Press T – in terms of their ability to withstand fracture in MOD Inlay preparations. The ethical clearance was obtained from the Scientific Advisory Committee and Institutional Ethics Committee (TDC/EC/18/2022).

G*Power 3.0.10 was the program used to calculate the sample size. Using the lottery technique, teeth samples were randomly assigned to two groups, each containing 32 samples (n = 32). Group I’s samples had numbers ranging from 1 to 32, whereas Group II’s samples had numbers ranging from 33 to 64 [Table 1].

Table 1:

Test groups

Groups	Sample size (n)	Test group
Group I	n=32	MOD inlay preparation + IPS e.max press HT (Ivoclar Vivadent)
Group II	n=32	MOD inlay preparation + Vintage LD press T (Shofu)

HT: High translucency

Inclusion criteria

Human permanent premolar teeth have normal morphology and are free of cavities and any tooth anomalies, such as dens evaginatus and dens invaginatus.

Exclusion criteria

Immature teeth and any other defects including cracks, teeth with previous restoration, fracture, attrition/abrasion, craze lines, caries and curved roots, endodontically treated teeth, and teeth with defective enamel and dentin formation.

Preparation of specimen

After giving the specimens a thorough water rinse, they were cleaned with an ultrasonic scaler and submerged in a 0.5% sodium hypochlorite solution (Prime Dental Products Ltd, India) to get rid of any organic material. Before being used, the teeth were kept in a regular saline solution. Every tooth was inserted beneath the CEJ into uniform methacrylate resin molds.

By using a calibrated William’s probe (GDC Fine Crafted Dental Pvt. Ltd., India) to measure the pulpal floor depth and occlusal isthmus width, the cavity preparations across all groups were brought to uniformity in terms of size. The pulpal floor depth was 2.5 mm from the occlusal surface, the occlusal isthmus was 2.5 mm wide, and the buccolingual widths of the mesial and distal boxes were 2.5 mm with the gingival floor depth of each box being 1.5 mm and axial wall height 2 mm [Table 2 and Figure 1].

Table 2:

Standardization of tooth preparation

Parameters	Measurements in mm
Pulpal floor depth	2.5 mm from the occlusal surface
Occlusal isthmus	2.5 mm
Buccolingual widths of the mesial and distal boxes	2.5 mm
Gingival floor depth of each box	1.5 mm
Axial wall height	2 mm

Figure 1.

Inlay MOD cavity

Using a close-ended metallic cylinder, impressions of the teeth with prepared inlay cavities were made using polyvinyl siloxane impression material (Zhermack, Italy). To create a die model, the impressions were poured using Type IV gypsum material (Ultrarock, Kalabhai). Every restoration was made in accordance with the guidelines provided by the manufacturer for the specific material. A sufficient quantity of ingots was prepared, and the inlay cavities on the isolated model die were waxed up. An investment ring of the proper size was chosen based on the weight of the wax pattern [Figure 2].

Figure 2.

Putty impression of MOD inlay cavity

Each restoration that had been waxed up had a wax sprue attached to it, along with the sprue to the ring base’s edge. The sprue had a maximum length of 5 mm and a diameter of 3.3 mm. The sprue was waxed or affixed to the ring base’s edge at an angle ranging from 45° to 60°, being sure to leave at least 10 mm between wax patterns and the outer wall.

IPS PressVEST or Shofu Ceravety Press investments were made with care to prevent air bubbles from forming when the investment material was inserted into the ring. To maintain the parallel top and bottom surfaces, the ring cap was positioned atop the ring.

After investing, the base and the muffle former were taken out, giving the investment muffle an additional 5 min to dry. The investment muffle was then put in an 850°C furnace that had been warmed. In the furnace, the plunger was also warmed. The 100-g ring was kept for 45 min and the 200-g ring for an hour after the furnace temperature was raised to 850°C.

To keep the investment ring from cooling down too much, it was taken out of the preheating furnace as soon as the preheating cycle was finished, keeping it at 850°C for an hour without stopping for longer than 30 s. After inserting the ingots and pressing them into the heated ring, the press program was initiated and the ring was positioned in the middle of the hot press furnace.

The ring was allowed to cool to ambient temperature before the restoration was removed. On the investment ring that had cooled, the plunger’s length was indicated. Using a separating disk, the investment was separated, and then it was roughly divested with Al₂O₃ (Korox110, Germany) at a pressure of around 0.2–0.3 MPa (2–3 bar). Al₂O₃ was used for fine divesting at a pressure of roughly 0.1–0.2 MPa (1–2 bar).

Using a fine diamond disk, the cutting surface was moistened and sliced approximately 2 mm above the accumulation point. Light pressure and low speed were used. Diamond-tipped tools were used to smooth the sprue attachment locations. Using the appropriate grinders, the final form and surface texture were contoured. Silicone polishers (CeraMaster Coarse/SoftCut PA) were used for prepolishing. Al₂O₃ was used to clean the repair at a pressure of roughly 0.1–0.2 MPa (1–2 bar), followed by an ultrasonic or steam cleaning. Staining and glazing the restoration came next. Following the try-in processes, the inlay cavities underwent surface treatment, and dual curing cement (Calibra, Dentsply Sirona) was used to luting the inlay restorations. Following the luting of inlay restorations, fracture testing was performed on the specimens [Figure 3].

Figure 3.

Inlay cavity immediately postcementation

Fracture testing

The apparatus utilized was a universal testing machine (Instron Corp., Massachusetts, USA) capable of handling loads up to 5000 N. Each specimen was set up in a mounting jig that allowed it to be loaded at a 0° angle to the tooth’s long axis. The sample was loaded and allowed to fracture on the premolar’s central fissure at a crosshead speed of 1.0 mm/min. For every specimen, the maximum load required to cause a fracture was noted in Newtons (N) [Figures 4 and 5].

Figure 4.

Universal testing machine

Figure 5.

Sample under load at an angle of 0° in the universal testing machine

Statistical analysis

Data were entered in Microsoft Excel Spreadsheet and subjected to statistical analysis using SPSS software (Version 17.0, IBM, USA). P <0.05 was set as a statistically significant level. Unpaired Student’s t-test was employed to compare the fracture resistance values of the two groups.

RESULTS

After calculating the mean fracture resistance values for each group, a comparison was performed. When IPS e.max Press HT (Ivoclar Vivadent) was compared to Vintage LD Press (Shofu) ceramic, descriptive statistics showed that the latter had greater fracture resistance values. The average fracture resistance value (IPS e.max Press HT) for Group I was 565.04 N, whereas Group II’s value was 732.75 N. In this intergroup comparison, a statistically significant difference was discovered (P = 0.002) [Table 3].

Table 3:

Mean fracture resistance values and standard deviation of Groups I and II

Group statistics
Groups	n	Mean	SD	SEM	P
Fracture resistance (n)
IPS e.max press HT	32	565.04	215.84	38.15	0.002
Vintage LD press	32	732.75	190.56	33.68

SD: Standard deviation, SEM: Standard error of mean, HT: High translucency

DISCUSSION

Ceramic inlays offer a competitive alternative to amalgam or cast gold restorations, both of which have a long history of clinical efficacy. They have become a potentially effective treatment for treating large cavities by replacing lost tooth structure. Heavy loading is still debatable because porcelain fracture is deemed as the primary reason for the failure of ceramic inlay.

Long-term clinical studies by Naik et al. and Hickel and Manhart with observation durations of up to 12 years revealed that ceramic fracture was the primary cause of failure and that the survival rates of ceramic inlays range between 74% and 100%.[10,11]

To sinter the ceramic body during the fabrication process of pressable ceramics, external pressure must be applied at high temperatures. Thus, these ceramics are also referred to as “hot-pressed” or “heat-pressed” ceramics depending on the processing technology used. With a concomitant increase in density and greater strength over typical ceramics, this fabrication approach eliminates porosity, excessive grain development, and secondary crystallization in ceramics. About 65% of the main crystalline components in second-generation heat-pressed ceramics such as IPS e.max Press HT and Shofu Vintage LD Press T contain lithium disilicate. Before the formation of lithium disilicate (Li₂Si₂O₅) crystals, it creates two distinct phases during the crystallization process, such as lithium metasilicate (Li₂SiO₃) and cristobalite (SiO₂). Highly interlocked lithium disilicate crystals and stacked crystals make up the final microstructure, which helps to strengthen.[1,12]

Therefore, the intend of the current study was to assess and contrast the fracture resistance of MOD Inlay preparations that were restored using two distinct lithium disilicate glass ceramics, namely, Shofu Vintage LD Press T and IPS e.max Press HT.

Human premolars extracted for periodontal purposes were chosen for this investigation because they more closely resemble the clinical setting in terms of bonding properties, heat conductivity, modulus of elasticity, and strength.[13]

The Instron universal testing machine was utilized to test the samples for fracture. According to published studies, it has a maximum load capacity of 5000 N and a linear force application pattern that most closely resembles the patterns of chewing force. Since the objective of the current study was to correlate with functional movements and achieve masticatory simulation, a loading angulation of 0o between the central fissure of the premolar and plunger orientation of the universal testing machine was established.[14]

The mean fracture resistance values in Groups I (IPS e.max Press) and II (Vintage LD Press) were 565.04 N and 732.75 N, respectively, according to the study’s results, indicating a statistically significant difference. Heat-pressed ceramic inlays typically have exceptional fracture resistance and great marginal adaptability. Lithium disilicate inlays have outstanding strength and visual appeal.

In contrast to IPS e.max Press, the hardness values obtained with Vintage LD Press porcelain at a thickness of 0.7 mm and cured at 1300 mW/cm² for 20 s were the highest in a study conducted by Naliani et al.[15]

IPS e.max Press used for all-ceramic partial coverage restorations of molar teeth were studied by Stappert et al., who showed that the restorations’ fracture resistance was on par with that of unprotected natural teeth. Based on clinical research, IPS e.max Press has a 96.6% survival rate for single crown restorations over a 3-year period.[1]

In an investigation of the fracture resistance of teeth restored with all-ceramic inlays and onlays, Saridag et al. found that the fracture strength values in the inlay group restored with lithium disilicate glass-ceramic, i.e. IPS e.max Press, were significantly higher than those in the onlay group reinforced with the same material and that the fracture resistance decreased as the amount of preparation (from inlay to onlay preparation) increased.[16]

In their study, RM Saleh et al. examined the fracture resistance and reason behind the failure of the three ceramic veneering materials that were fabricated using computer-aided design (CAD)/computer-aided manufacturing, i.e. Vita Enamic, Shofu HC, and IPS e.max CAD and concluded that Shofu HC has the highest fracture resistance. They ascribed this achievement to the makeup of the Shofu block’s densely packed nanofiller, which is contained in 61% zirconium silicate.[17]

In contrast to Vita Enamic and IPS e.max CAD, their lower modulus of elasticity (9.6 GPa) allowed them to tolerate deformation under applied force without breaking, demonstrating their capacity to absorb stress.

In contrast to the findings of the current study, Saad et al. stated a statistically significant difference (P = 0.005) between e.max and Shofu, the fracture resistance values of inlays made from lithium disilicate e.max CAD (1104.627) were higher than those of SHOFU (724.417).[18]

The development of an extraordinarily intertwined microstructure of lithium disilicate crystals was most likely the reason of the rise in fracture resistance seen following the crystallization of samples of IPS e.max. The fracture deflects because of the arbitrarily oriented tiny plate-like interlocked crystals with a particle size ranging between 0.5 and 4 µm.[19]

Ohashi et al. assessed the features of Vintage LD Press (LD; Shofu) and IPS e.max Press. The latter exhibited a considerably better flexural strength compared to Vintage LD. The observation of lithium disilicate crystals under a SEM could provide an explanation for this. The volume ratio of the crystal to the glassy matrix and the size of the e.max crystals vary. In LD, the crystals are sparser, indicating a larger percentage of the glassy matrix. Such a discrepancy could influence how far cracks extend.[19]

Lithium disilicate crystals may presumably provide resistance against crack propagation in the biaxial flexural strength test, which may be aided by the glassy matrix. Hence, e.max’s tightly packed lithium disilicate crystals have the potential to impede fracture formation more potently than LD’s, which could perhaps account for their greater flexural strengths.[18]

However, because different test parameters were used in the previous studies-such as different adhesive cement used for specimen cementation and different regions and directions of applied load during the load to fracture test-the results of those studies cannot be directly compared to those of the present study.

The surface of IPS e.max Press appears to be rougher, and there are some crystals visible embedded in the glassy matrix. This implies that the greater KIC of IPS e.max Press was caused by the crystals’ ability to obstruct the propagation of cracks.[20]

The current study reveals that there was a substantial difference between the two groups even though they used the same materials and techniques for bonding, inlay restoration fabrication, and tooth preparation. Despite the fact that lithium disilicate ingots were used in the production of both kinds of inlay restorations, there had to be some variations in the companies’ manufacturing procedures which translates into differing fracture strengths of the tested materials.

Limitations

Extracted human premolars were used which makes standardization of the bonded interface strenuous since these specimens vary in age and caliber. Factors pertaining to the periodontal ligament were not taken into consideration since the extracted teeth were embedded in acrylic resin which would present with a different biomechanical situation as compared to that in the oral cavity thereby not simulating the clinical condition. The possible loading angles that could create a difference in terms of fracture resistance values were not taken into consideration.

CONCLUSION

According to the study’s limitations, there were notable differences in the fracture resistance of MOD Inlay preparations restored using two distinct lithium disilicate glass ceramics, namely, Shofu Vintage LD Press T (Shofu) and IPS e.max Press HT (Ivoclar Vivadent) and that compared to those restored with IPS e.max Press HT (Ivoclar Vivadent), the fracture resistance of MOD Inlay preparations treated with Vintage LD Press T (Shofu) was noticeably higher. However, to evaluate the fracture resistance of inlay restorations cemented to teeth, additional clinical research is warranted.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.