Optimization of the edge stability of lithium silicate

Lithium silicate and the influence of surface treatment, firing, and other parameters. A variety of modern materials are available for the fabrication of restorations to treat defects in tooth structure. In the field of fixed dentures, all-ceramic restorations have become established. This trend may be due to their high-quality aesthetics and solid material properties. 

From the class of dental ceramics, lithium silicate and its mechanical properties, depending on the processing protocol, will be examined below. Lithium silicate impresses due to its pronounced translucency and opalescent properties, which makes it particularly suitable for aesthetically demanding restorations in the anterior tooth area.

The influence of surface treatment on mechanical properties

Various post-processing methods are available to achieve optimal surface quality and ideal mechanical parameters. The strength of ceramic materials depends on their surface quality. Surface defects such as pores or microcracks act as crack initiators and weaken the material structure. Two primary processes are available for surface optimization:

1. Polishing: The mechanical smoothing of the surface leads to the elimination of superficial defects and thus to an improvement in the material strength.
2. Glaze firing: The application of a glaze followed by firing not only fills surface irregularities but also induces compressive residual stresses on the surface. This surface treatment effect contributes significantly to increased strength.

Definition: Edge Chipping Resistance (ECR) measures the edge stability of dentures. A high value indicates that the edges of a crown are robust and do not chip easily under chewing load.


material and methods

288 test specimens were manufactured from four CAD/CAM lithium silicate ceramics (Figures 1 and 2):

• Amber Mill (lithium disilicate); HASS Organic, Korea
• Amber Mill Direct (lithium disilicate); HASS Organic, Korea
• CEREC Tessera (lithium di-aluminosilicate); Dentsply Sirona, Germany
• IPS e.max CAD (lithium disilicate); Ivoclar AG, Liechtenstein

Experimental setup

The experimental setup was divided into the following clearly defined phases:

1. Test specimen production

The ceramic blocks were manufactured by cutting them into plates (Secotom-50; Struers, Ballerup, Denmark), which were then ground to the exact thickness (1,5 mm, 2 mm and 3 mm) in a polishing machine (Abramin; Struers).

2. Surface conditioning and fire control

After cleaning in an ultrasonic bath (L&R Transistor Ultrasonic T-14; L&R, New Jersey, USA), the surface treatment was carried out in the form of:

• Polish (Diapro R17DPmf, Diapro R17DP; EVE Ernst Vetter, Keltern, Germany),
• Glaze (IPS e.max CAD Crystall./Glaze Spray, Ivoclar, Schaan, Liechtenstein; Universal Spray Glaze Fluo, Dentsply Sirona, Konstanz, Germany) or
• no treatment.

The glazed or untreated test specimens were then subjected to their group-specific firing protocol (Austromat 654 press-i-dent, Dekema, Freilassing, Germany; Ivoclar Programat EP 5010; Ivoclar).

3. ECR measurement and artificial aging (longitudinal design)

The measurement of fatigue behavior under simulated in-vivo loading was carried out in three steps:

• Initial measurement: The first measurement of the Edge Chipping Resistance (ECR) was performed using a universal testing machine (ZHU 0,2; Zwick Roell).
• Thermal aging and second measurement: Aging in the thermal cycler (Thermocycler TCS-10; SD Mechatronik, Feldkirchen-Westerham, Germany; 10.000 cycles, 5° C/55° C, 20 sec., corresponding to one year of clinical wear time) followed by the second ECR measurement.
• Hydrothermal aging and third measurement: Finally, a hydrothermal aging cycle was performed in an autoclave (Euroklav 29-S; MELAG Medizintechnik, Berlin, Germany; 134°C, 2 h, 0,2 MPa; corresponding to an additional 6-8 years of clinical exposure), followed by the third ECR measurement.

4. Statistical evaluation

The collected data were statistically analyzed using IBM SPSS Statistics v29.0 using the Kruskal-Wallis, Mann-Whitney U, Friedmann, and Wilcoxon tests (α = 0,05).

Results

All materials tested demonstrated edge chipping resistance (ECR) that was convincing for clinical use (Figure 5). The key influencing factors were as follows:

• Influence of artificial aging: Artificial aging often led to a reduction in the ECR. However, even after completing the aging protocol, the value was considered sufficiently high for clinical use of the materials.
• Influence of firing and glaze: Of all materials and processing protocols tested, the glazed specimens fired according to the 'medium opacity' firing protocol showed the highest ECR values. This is due to increased cross-linking between the lithium disilicate crystals due to the higher firing temperature, resulting in greater strength while simultaneously reducing translucency.
• Comparison of surface treatments: Within a material group, the glazed test specimens showed a higher ECR than the polished or untreated test specimens.
• Influence of specimen thickness: It was demonstrated that variation of specimen thickness had largely no influence on the ECR of the specimens.
• Influence of the loading point: Varying the distance between the loading point and the edge (0,25 mm vs. 0,3 mm) did not result in any change in the ECR. This finding contradicts the general consensus in the literature, which may be due to the small difference in the selected distances.

Conclusion

The indication-specific selection of the restorative material and the subsequent processing protocol have decisive consequences for the long-term success and survival of the dental prosthesis in the mouth. The study presented here examined the influence of various factors relevant to the manufacturing process of a restoration and their impact on the ECR of the materials. To approximate real-world clinical stress, the test specimens underwent both thermal and hydrothermal aging cycles.

As a result of the results discussed here, the clinical user should be provided with the following practical takeaways:

1. Glaze as the surface treatment of choice: Glazing is considered the surface treatment of choice.
2. Balancing translucency levels and mechanical properties: Especially when producing restorations that lie outside the aesthetically relevant range, it is worthwhile to trade off a lower level of translucency in favor of higher mechanical properties.
3. Recommendation for thinner layer thicknesses: Thinner layer thicknesses of the restoration material and thus a preparation adapted to the defect to protect healthy tooth substance are recommended, without having to expect a loss of ECR of the restoration.