Clinical outcomes of zirconia-reinforced lithium silicate partial coverage crowns compared to lithium disilicate partial coverage crowns. A randomized controlled split-mouth clinical study

Introduction

High survival rates, fracture resistance and proper marginal integrity of CAD/CAM partial coverage restorations (PCRs) were reported in studies simulating 5-year clinical service^1–4.

However, clinical behavior of PCRs utilizing morphology driven preparation design was never assessed in randomized clinical trials^5–11.

Furthermore, long-term clinical studies have shown that bulk fracture and marginal deterioration of PCRs has a direct correlation to the use of brittle ceramic materials, such as feldspathic and leucite-based ceramics^12–15, which encouraged researchers to use higher strength lithium disilicate glass ceramic in such restorations^16–21. Although some clinical studies tested the performance of lithium disilicate PCRs, no randomized clinical trial tried to compare between lithium disilicate ceramic material and the newly introduced zirconia-reinforced lithium silicate ceramic material in posterior partial coverage^22–24.

The aim of this randomized controlled split-mouth clinical study was to evaluate the clinical outcomes of zirconia-reinforced lithium silicate (Vita Suprinity) and lithium disilicate (IPS e.max CAD) partial coverage restorations. The null hypothesis was that there would be no difference between the two ceramic materials over 12 months.

Methods

First visit (teeth preparation)

A new cavity preparation design; morphology driven preparation (MDP) design was selected for this study. In this design, preparations were guided by the anatomical and structural morphology of the teeth^25–27.

Interior walls were prepared with 6–10°divergence, well-defined margins and rounded inside angles. Inter-proximal box was prepared with 1–1.2mm butt-joint, all obtained with medium-grit 80μm diamond truncated conical bur (4137-856-025, Microdont, USA). (Figure 1)

Figure 1. Morphology driven preparation design in lower left first molar.

Occlusal reduction of 1.5–2mm was performed with egg-shaped football bur (3118-368-023, Microdont, USA), and verified with silicon index. The outer axial walls with inclined planes were prepared with hollow chamfer margin obtained with round bur (1014-801-014, Microdont, USA). (Figure 2)

Figure 2. Hollow chamfer margin on outer preparation surface.

All preparation was finished with fine finishing bur (4137F-856-025, Microdont,USA).

All undercuts were blocked with Herculite™ ultra-flow composite (Kerr-Germany, Catalogue no.: 2201-35392).

Full arch Vinylpolysiloxane (EliteHD+. Zermack-Germany, Catalogue no.: F121007 - 2016-05) impression and interocclusal records (Occlufast. Zermack-Germany, Catalogue no.: F121009 - 2016-05) were taken. Provisional restorations were fabricated with Structure-2 bis-composite (Voco-Germany, Catalogue no.: VC 84 001479 GB 0918 V) and cemented with temporary cement (Dentotemp-Itena. France, Catalogue no.: K03330 9).

Laboratory fabrication

Master models were poured with type IV dental stone (FujiRock-EP, GC-Belgium, Catalogue no.: 890366), then scanned with an extraoral scanner (Identica-blue. Medit, England). Final PCRs were designed using CAD/CAM software (Exocad-DentalCAD, Exocad GmbH-Slovenia) and milled with 5-axis machine (CAM5-S1impression.Vhf, Ammerbuch-Germany) of Suprinity (VITA-ZahnfabrikH. Rauter-GmbH-Germany, Catalogue no.: 2002E – 0114 (X.) S Version (02)) and e.maxCAD (Ivoclar-Vivadent, Schaan-Liechtenstein, Catalogue no.: 721198/e/2018-11) blocks. After staining, PCRs crystallization was done in ceramic furnace (Programat-P310) according to manufacturer instructions.

Second visit (PCRs try-in and bonding)

Definitive PCR try-in was performed to confirm the restoration proper seating, marginal integrity, shade matching and proper occlusal and proximal contacts.

All PCRs bonding steps were performed under rubber-dam isolation.

The internal surfaces of PCRs were etched for 20 second with 9.5% hydrofluoric acid (BISCO-USA, Catalogue no.: E-5702EP), rinsed, air dried, then Bis-silane (BISCO-USA, Catalogue no.: B-2221P) was applied, left for 60 seconds and air dried.

Enamel margins of the preparations were etched with 37% phosphoric acid (BISCO-USA) for 30 seconds, rinsed and air dried. All-bond universal adhesive was applied, air thinned, and cured for 20 seconds (Elipar™, 3MESPE, USA, Catalogue no.: 70-2013-0430-3-B). Duolink adhesive resin cement (BISCO-USA, Catalogue no: A-19010P), was applied to fitting surface; restoration was seated with gentle pressure, glycerin barrier was applied to margins (Deox.Ultradent-USA, Catalogue no: 238), then light curing was performed for 40 seconds (Elipar™, 3MESPE, USA, Catalogue no.: 70-2013-0430-3-B).

Residual cement was removed and occlusion was carefully checked.

Results

All 14 patients (8 females and 6 males) attended 6 and 12 month follow-up. A total of 46 PCRs were fabricated in this study. A patient flow diagram is available as part of the Reporting guidelines section.

Survival rates on Kaplan Meier survival curve are provided in (Table 1) and (Figure 3). After 12 months, all PCRs of both groups remained in situ, with a survival-rate of 100% (P=0.8254).

Figure 3. Kaplan Meier Survival Curve for two tested groups.

For criteria marginal adaptation, e-max CAD group showed a statistically non-significant decrease in Alpha ratings to 95.65% (p=0.1560), while Vita Suprinity group maintained 100% Alpha scores after 12 months (Table 2, Figure 4 and Underlying data³⁶).

Figure 4. Stacked column chart of marginal adaptation associated with restoration for both groups.

For marginal discoloration, e-max CAD group showed a non-significant decrease in Alpha ratings to 95.65 % (p = 0.1560) after 6 months and 87 % ( p=0.6078) after 12 months (Table 3 and Figure 5).

Figure 5. Stacked column chart of marginal discoloration associated with restoration for both groups.

Bravo ratings of 13% for discoloration and palpable marginal ditching were recorded in e-max CAD group after 12 month (Figures 6–Figure 8), while 4.35% Bravo rating were recorded in the Vita Suprinity group after 12 months.

Figure 6. Marginal discoloration of e.max partial coverage crowns (PCR)- 6 months follow-up.

Figure 7. Marginal discoloration and ditching (arrows) in e.max partial coverage crowns (PCR)- 12 months follow-up.

Figure 8. Marginal discoloration (arrow) of e.max partial coverage crowns (PCR). A: Baseline and B: 12 months follow-up.

Discussion

This study aimed to compare the clinical performance of Vita Suprinity and e-max CAD partial coverage restorations in a prospective double-blinded split-mouth design. Selection biases can be avoided in split-mouth studies as the patient acts as their own control, in this way direct comparison of two ceramic materials can be performed^37–39.

Compared to full coverage restorations, posterior partial coverage restoration utilizing more tooth structure conservation concept has the potential to reinforce and protect tooth structure, preserve enamel, and safeguard pulp vitality while achieving the desired aesthetic results^5,6. Overtime, various ceramic materials have been developed for restoring posterior teeth^7–9. The excellent combination of high mechanical strength and optical properties of lithium disilicate glass ceramic material made it the gold standard for comparison of new monolithic ceramic materials^9,11,12. In our study CAD/CAM lithium disilicate ceramic material (IPS e.max CAD) was selected as the control to compare the clinical outcomes of the newly introduced zirconia-reinforced lithium silicate ceramic material (ZLS) (VITA Suprinity). Reinforced with about 10% zirconium dioxide, ZLS belongs to a new generation of CAD/CAM ceramic that combines positive mechanical characteristics of zirconia with glass-ceramic aesthetic appearance²². Still all findings regarding this new material are either laboratory or initial clinical experience findings^22–24. Moreover, the indication for this material should be chosen with strict observation of the material-specific processing instructions regarding the necessary minimum wall thickness and required adhesive luting^22,23. All of these findings make it crucial to conduct randomized clinical studies to verify the clinical performance of this new material.

In this study, a novel tooth preparation design; MDPT was selected. According to Venezian M²⁵, this preparation design aims to minimize the loss of healthy tooth tissue and reduce the areas of dentin exposure. A hollow chamfer margin was created on the outer surface of the preparation to optimize the cutting of enamel prisms, thereby bonding and color blending at the transitional zone between tooth and restoration are enhanced^25–27(Figure 1), (Figure 2).

Regarding our results, Kaplan Meier analysis was used for survival assessment during the observational period of 12 months^32–35; both Suprinity and e.max-CAD had a survival rate of 100%. All restorations remained in situ and in good function.

Comparison of our results to similar clinical studies regarding Suprinity is limited due to the novelty of the material^22,23.

Clinical studies on e.max PCRs reported 98.99–100% survival rates over periods of 1–5 years^{15,17,40–43}. In a long-term evaluation study, Guess et al.¹⁶ reported 100% survival rates for e.max PCRs with only evidences of relative failures as small repairable chipping after 8 years, but none of PCRs were fractured or de-bonded.

In our study, none of PCRs were de-bonded after 12 months. Other studies reported de-bonding of PCRs as one of the common causes of failure^13,14. In those studies, de-bonding mainly was associated with endodontically treated teeth which were among the exclusion criteria in this study.

For marginal adaptation, all PCRs were rated Alpha at base-line and after 6 months. However; after 12 months, palpable margin ditching resulted in Bravo ratings for one of the e.max-CAD PCRs (4.35%). Marginal deterioration might be attributed to degradation of cement due to fatigue in the oral cavity^15,16.

Suprinity PCRs sustained Alpha rating after 12 months, which can be attributed to the higher marginal quality and fatigue resistance of zirconia lithium silicate over lithium disilicate as reported by Preis et al.^23,43.

Nevertheless, results by Elsaka and Elnaghy²⁴ were in disagreement with our results as they reported lower brittleness index of e.max CAD compared to Vita Suprinity, and consequently according to the parameters determined by Boccaccini⁴⁴ and Chaysuwan et al.⁴⁵; e.max CAD might show lower marginal chipping rates than Suprinity²⁴.

Marginal discoloration of e.max PCRs has been reported by Guess et al.^15,16 and Santos et al.¹⁷ as the most common clinical finding occurring in 37.5% of PCRs after 7 years.

For marginal discoloration; three of e.max CAD (13%) and one Vita Suprinity (4.35%) restorations showed yellowish marginal staining (Bravo) after 12 months. Still both materials showed clinically acceptable margins.

The null hypothesis for this study was accepted as there was no statistically significant difference in clinical outcomes of the two tested ceramic materials.

Conclusion

Both Vita-Suprinity and e.max CAD partial coverage restorations are considered reliable treatment options for restoring larger defects in posterior dentition.

Data availability

Consent

Written informed consent for publication of their clinical details was obtained from the patients.