Indication of Glass-Ceramics Enriched with Lithium Disilicates: Systematic Review of the Literature

Introduction: Lithium disilicate is a popular restorative material, frequently used for aesthetic and functional rehabilitation due to its mechanical and aesthetic properties as well as its ease of implementation. However, this material also has certain limitations. The purpose of this systematic review is to submit the indications for lithium disilicate restorations to the judgment of the scientific evidence.

Materials and Methods: This systematic review adopted the guidelines of PRISMA (Preferred Reporting Items for Systematic Reviews and MetaAnalysis), followed the Guide of Cochrane Diagnostic Test Accuracy Protocol and reviews. An electronic search was performed in 3 databases (MEDLINE-PubMed, EMBASE-Science Direct, Cochrane Library) for studies published between 2011 and 2021 using the appropriate terms and keywords. The selected studies were filtered by title, abstract and full text.

Results: From 609 publications identified, 16 were deemed relevant, and met the eligibility criteria. Of a total of 33,295 fixed restorations placed in more than 7427 patients, this review investigated the indications for lithium disilicate according to the type of restoration, the mode of coating, and also examined the association between the indications for this glass-ceramic and location and method of assembly of prosthetic restorations.

Discussion: The results of this review will be compared with each other and with other publications. The methodology of the work, including the choice of inclusion criteria and the limits of this review.

Conclusion: Indications of lithium disilicate can be used anterior and posteriorly: Veneers, single crowns, Crowns on the implant, Onlays and Inlays. The limits of lithium disilicate will be bonded and cemented bridges, especially for posterior restorations. For coating mode, monolithic restorations seem to be better than bilayer ones. For single crowns, posteriorly, lithium disilicate must be used with caution if the occlusion is judged to be unfavourable. In the anterior, there is no worry. Posterior bridges should be avoided. Anterior bridges can be used but rigorously. Although this ceramic has proven itself, the appearance of new generations of lithium disilicate makes it possible to extend its field of application even further. It tends to make it a material capable of responding to the vast majority of clinical situations to which we are dealing

Introduction

All-ceramic restorations have become very popular because most clinicians and patients find metal-free restorations more attractive. Today’s aesthetic concerns have increased the demand for these materials not only when prosthetically restoring a tooth for the first time, but also when replacing existing ceramic-to-metal restorations [1].

Through the reflection of their metal framework and opaque coatings, traditional metal crowns have limited light exchange with the surrounding soft tissues and, in comparison to natural teeth, frequently have a decreased aesthetic appearance. In contrast, ceramic restorations can be considered aesthetically safe. Metal-free restorations offer reduced gingival inflammation and sufficient marginal adaptation. Nevertheless, the brittle nature of ceramics has led to high fracture rates, especially in the case of glass-ceramic systems inserted in the posterior region [2].

Several types of ceramic materials have been developed since their introduction in the early twentieth century [3]. Currently, the most popular ceramic restorative materials are lithium disilicate and zirconia [4].

Lithium disilicate glass-ceramic is a unique material that features an interlocking microstructure of needle-shaped lithium disilicate crystals, up to 70% by volume, in a glassy matrix. On the one hand, it offers a translucency similar to that of natural teeth and, on the other hand, a biaxial resistance of up to 400 MPa, almost three times more than conventional glass ceramics. In addition to this, as the chemical durability of lithium disilicate meets the standards of a veneer material, it can be used for monolithic restorations [2].

The first version of IPS Empress 2 lithium disilicate (Ivoclar Vivadent, Schaan, Liechtenstein) is still used as a veneering ceramic but was not designed to be used in its monolithic form [5], this version has been improved, in particular regarding strength, variety of translucency levels, and optional use as a monolithic restoration. The second generation of lithium disilicate was named IPS e.max (Ivoclar Vivadent) and was marketed in 2006 [2], it presents smaller and more homogeneous crystals and improved physical properties (flexural strength and toughness to about 10%) higher than its predecessor [5].

Given its growing popularity and the improvement of its mechanical and aesthetic properties as well as its ease of implementation, it is necessary to review and synthesize the current scientific data on the indication and survival of this glass-ceramic [6].

The purpose of this systematic review is to submit the indications for lithium disilicate restorations to the judgment of the scientific evidence.

Materiel and Methods

The PRISMA guidelines (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) [7] and the Guide of Cochrane Diagnostic Test Accuracy Protocol and Reviews [8] were followed in conducting this systematic review of the literature.

Three databases were searched electronically for documents: PubMed, EMBASE (Science Direct), and Cochrane Library. Lithium disilicate, survival, results, performance, veneers, failure, resistance, prosthodontics, clinical outcomes, and prostheses are the keywords that have been utilized. The selected “Boolean equations” (Table I). Published between 2011 and 2021, the inclusion criteria included human subjects, meta-analyses, systematic reviews, randomized controlled and non-randomized clinical trials, comparative clinical studies, cohort studies without language or country of implementation restrictions, and clinical examinations of the patients during the follow-up period.

Table I. Boolean Equations Chose

Databases	Boolean equations
PubMed	- ((lithium disilicate) AND (clinical outcomes) AND (prostheses));
	- ((lithium disilicate) AND (prosthodontics) AND (clinical outcomes) AND (failure));
	- ((lithium disilicate) AND (failure)) AND (resistance);
	- (lithium disilicate) AND (veneers).
EMBASE	- (lithium disilicate AND performance AND survival AND outcomes).
Cochrane library	(lithium disilicate) AND (clinical outcomes);
	(lithium disilicate) AND (survival).

An electronic documentary search was carried out in the three databases: PubMed, EMBASE (Science Direct) and Cochrane Library. The keywords that have been used are Lithium disilicate, Survival, Outcomes, Performance, Veneers, Failure, Resistance, Prosthodontics, Clinical outcomes and Prostheses. The “Boolean equations” were chosen (Table I). Inclusion criteria were published between 2011 and 2021, relating to living humans, meta-analyses and systematic reviews, randomized controlled clinical trials and non-randomized clinical trials, comparative clinical studies, Cohort studies without restriction of language or country of realization of the study, and patients were clinically examined during the follow-up period. Criteria for non-inclusion: articles that do not meet the objectives of the study, articles prior to 2011, articles with titles or abstracts that do not appear relevant to our literature review and articles deemed expert reports, letters, commentaries, and editorials.

In order to select only the relevant articles, and not to have to read several thousand articles, sorting is necessary and is carried out in 3 stages: the articles are first selected on their title, then on their summary and finally on their full text (Fig. 1).

Fig. 1. Flow diagram illustrating the different stages of selection of publications in the systematic literature review.

Results

A total of 16 studies were included in our review: 2 systematic reviews, 4 prospective studies, 5 retrospective studies, 3 cohort studies, 2 randomized clinical trials.These studies included more than 7427 patients, who received 33,295 restorations tooth-supported fixed prosthetics (6014 veneers, 23588 unit crowns, 11 endocrowns, 1915 bridges, 246 inlays, 1480 onlays and 41 crowns on implants). The studies selected are all in English, with the earliest publication date being 2012. All the characteristics of these studies have been grouped in Tables II and III.

Table II. Description of the Studies Included in the Review

Article no	Article title	Author	Date of publication/Country	Type of study
1	Clinical outcomes of different types of tooth-supported bilayer lithium disilicate all-ceramic restorations after functioning up to 5 years	Yang et al. [4]	2016-China	Retrospective study
2	Clinical outcomes of lithium disilicate single crowns and partial fixed dental prostheses	Pieger et al. [6]	2014-Germany	Systematic review
3	Clinical performance of pressable glass-ceramic veneers after 5, 10, 15, and 20 years	Aslan et al. [10]	2019-Türkiye	Retrospective study
4	Clinical results of lithium-disilicate crowns after up to 9 years of service	Gehrt et al. [11]	2012-Germany	Cohort study
5	Fifteen-year outcome of posterior all-ceramic inlay-retained fixed dental prostheses	Becker et al. [12]	2019-Germany	Prospective study
6	Fifteen-year outcome of three-unit fixed dental prostheses made from monolithic lithium disilicate ceramic	Garling et al. [13]	2019-Germany	Prospective study
7	Effect of cement type on the clinical performance and complications of zirconia and lithium disilicate tooth-supported crowns: A systematic reviewReport of the committee on research in fixed prosthodontics of the American Academy of fixed prosthodontics	Maroulakos et al. [14]	2018-USA	Systematic review
8	Survival rates of a lithium disilicate-based core ceramic for three-unit esthetic fixed partial dentures a 10-year prospective study	Sola Ruez et al. [15]	2013-Spain	Prospective study
9	The clinical performance of monolithic lithium disilicate posterior restorations after 5, 10, and 15 years: A retrospective case series	Van Den Breemer et al. [16]	2017-Germany	Retrospective study
10	Survival rate of lithium disilicate restorations at 4 years: A retrospective study	Sulaiman et al. [17]	2015-USA	Retrospective study
11	One-year clinical performance of lithium disilicate versus resin composite CAD/CAM onlays	Souza et al. [18]	2019-Spain	Randomized clinical trial
12	Monolithic lithium-disilicate single crowns supported by zirconia oral implants: three-year results of a prospective cohort study	Spies et al. [3]	2016-Germany	Cohort study
13	Ten-year outcome of three-unit fixed dental prostheses made from monolithic lithium disilicate ceramic	Kern et al. [5]	2012-Germany	Prospective study
14	Ten-year survival and complication rates of lithium-disilicate (Empress 2) tooth-supported crowns, implant-supported crowns, and fixed dental prostheses	Teichmann et al. [2]	2017-Germany	Cohort study
15	Clinical evaluation of 860 anterior and posterior lithium disilicate restorations : Retrospective study with a mean follow-up of 3 years and a maximum observational period of 6 years	Fabbri et al. [19]	2014-Italy	Retrospective Study
16	10.9-year survival of pressed acid etched monolithic e.max lithium disilicate glass-ceramic partial coverage restorations: Performance and outcomes as a function of tooth position, age, sex, and the type of partial coverage restoration (inlay or onlay)	Malament et al. [20]	2021-USA	Randomized clinical trial

Table III. Characteristics of the 16 Studies

Article	Number of patients	Average age (years)	Restoration number	Type of restoration	Localisation	Type of DL/Coating	Sealing	Follow up
1 [9]	4634	38.4	6486	Veneers 2295 Single crowns 4180 Bridges 111	Anterior Posterior	IPS emax Bilayer	Conventional	5 years
2 [10]	519	n.r	841	Single crowns 696 Bridges 145	Anterior Posterior	IPS emax /IPS emax CAD Monolithic/bilayer	Conventional	0.5–11 years
3 [1]	51	34.6	413	Veneers	Anterior Posterior	IPS empress II/IPS emax Monolithic	Adhesive	11.33 ± 4.85 years
4 [11]	37	34 ± 9.6	94	Single crowns	Anterior Posterior	PS emax Monolithic	Conventional/Adhesive	109.7 months
5 [12]	42	36.1	45	Bridge inlay 40 Hybrid Bridge 5	Posterior	PS emax Monolithic	Adhesive	100 months (min 4,max 234)
6 [13]	28	47.5	36	Bridges	Anterior Posterior	PS emax Monolithic	Conventional/Adhesive	167 months,(min 79–max 225)
7 [14]	1280	n.r	2120	Single crowns	Anterior Posterior	IPS emax/IPS emax CAD/IPS empress II Monolithic/bilayer	Conventional/Adhesive	4 years
8 [15]	19	49	21	Bridges	Anterior	IPS empress II Bilayer	Adhesive	10 years
9 [16]	13	n.r	87	Endocrowns 11 Single crowns76	Posterior	IPS empress II/ IPS emax Monolithic	Adhesive	12,8 years
10 [17]	n.r	n.r	21340	Veneers Single crowns Bridges Inlay/Onlay	N.r	IPS emax Monolithic/Bilayer	n.r	48 months
11 [18]	20	45	20	Onlays	Postérieur	IPS emax CAD Monolithic	Adhesive	1 year
12 [19]	24	42.7 ± 11.4	24	Implant supported crown	Anterior Posterior	IPS emax CAD Monolithic	Adhesive	3 years
13 [20]	28	47.5	36	Bridge	Anterior Posterior	IPS emax Monolithic	Conventional/ Adhesive	86 months
14 [2]	68	1) 32.9 2) 40.7 3) 40.7	131	1) Single Crowns 87 2) Bridges 27 3) Crowns on implant 17	Anterior Posterior	IPS empress IIMonolithic	Conventional/ Adhesive	1)11.4 ± 3.8 years 2) 8.9 ±5.4 years 3) 13.3 ± 2.3 years
15 [19]	360	45	860	Single crowns 480 Veneers 318 Onlay 62	Anterior Posterior	n.r Monolithic/ Bicouche	Adhesive	12–72 months
16 [20]	304	62	556	Inlay 246 Onlay 305	Anterior Posterior	IPS emax Monolithic	Adhesive	10,9 years

Assessment of the Risks of Bias of the Included Studies

Randomized Controlled Trials

The risk of bias in the articles was determined using the Guidelines from the Cochrane Handbook for Systematic Reviews of Interventions adapted by Higgins et al. [8] (Table IV).

Table IV. Risk of Bias Assessment of Included Randomized Controlled Trials

Authors	Selection bias	Attrition bias	Information bias	Notification bias	Detection bias	Risk of bias
Malament et al. [20]	Intermediate	Low	Low	Low	Low	Minimal
Souza et al. [18]	Low	Intermediate	Low	Low	Low	Uncertain

Cohort Studies

Critical Appraisal Skills Program (CASP) offers twelve questions supporting the analysis. (Table V).

Table V. Qualitative Assessment of the Risks of Bias of the Included Cohort Studies

Authors	Bias (score)
Gehrt et al. [11]	A
Spies et al. [3]	B
Teichmann et al. [2]	A

Survival Rate of Glass-Ceramic Enriched with Lithium Disilicate

Discussion

On the basis of the results and in comparison, with other studies in Tables VI and VII we can come out with the following synthesis:

Table VI. Failure and Survival Rates of Lithium Disilicate According to the Type of Prosthetic Restoration

Study	Year of publication	Average follow-up (years)	Sample size	Restoration number Veneers	Failure number	Failure rate %	Survival rate %
Yang et al. [4]	2016	5	4634	2295	57	2.5	97.2
Aslan et al. [10]	2019	11.33	51	413	15	3.63	98
Fabbri et al. [19]	2014	1–6	360	318	9	2.83	97,17
Sulaiman et al. [17]	2015	4	n.r	2998	42	1,4	98.5
Total	≈5,9	~5045		6024	123	2,04	97,95
Single Crown
Yang et al. [4]	2016	5	4634	4180	136	3.3	96.5
Pieger et al. [6]	2014	0,5–11	519	696	9	1,29	97.8
Gehrt et al. [11]	2012	2,8–9,14	37	94	4	4,25	97.4
Teichmann et al. [2]	2017	11.4	37	87	22	25.28	89,7
Fabbri et al. [19]	2014	1–6	360	480	16	3,33	96,67
Maroulakos et al. [14]	2018	4	1280	2210	104	4,91	95
Van Den Breemer et al. [16]	2017	12,8	13	76	13	17,1	92
Sulaiman et al. [17]	2015	4	n.r	15765	182	1,15	98,8
Total		≈ 6,55	>6880	23588	486	2,06	97,94
Endocrown
Van Den Breemer et al. [16]	2017	12,8	1280	11	0	0	100
Total		12,8	1280	11	0	0	100
Bridge
Yang et al. [4]	2016	5	4634	111	10	9.0	90.6
Pieger et al. [6]	2014	0,5–11	519	145	25	17,24	78.1
Becker et al. [12]	2019	8,33	42	45	33	73,33	57
Garling et al. [13]	2019	13,91	28	36	12	33,33	87,9
Kern et al. [5]	2017	7,16	28	36	4	11,1	90.8
Teichmann et al. [2]	2017	8.9 ± 5.4	19	27	15	55,5	63.0
Sola-Ruez et al. [15]	2013	10	19	21	8	n.r	71,4
Sulaiman et al. [17]	2015	4	n.r	1494	68	4,55	95
Total		≈ 7,88	~5289	1915	175	9,14	90,8
Crown on Implant
Spies et al. [3]	2016	3	23	24	0	0	100
Teichmann et al. [2]	2017	13.3 ± 2.3	12	17	1	3,7	100
Total		≈ 8,15	35	41	1	2,44	97,56
Onlay
Fabbri et al. [19]	2014	1-6	360	62	4	6,45	93,54
Malament et al. [20]	2021	10,9	304	305	3	0,98	95,6
Sulaiman et al. [17]	2015	4	n.r	1093	11	1,01	98,9
Souza et al. [18]	2019	1	20	20	0	0	100
Total		≈ 4,85	>684	1480	18	1,22	98,78
Inlay
Malament et al. [20]	2021	10,9	304	246	3	1,21	95,6
Total		10,9	304	246	3	1,21	95,6

Table VII. Failure and Survival Rates of Lithium Disilicate According to the Mode of Coating

Study	Restoration number	Failure number	Failure rate %	Survival rate %
Monolithic
Veneers
Aslan et al. [10]	413	15	3,63	98
Fabbri et al. [19]	265	n.r	n.r	100
Sulaiman et al. [17]	1612	21	1.3	n.r
Single crown:
Gehrt et al. [11]	94	4	4,25	97,4
Teichmann et al. [2]	87	22	25,28	89.7
Fabbri et al. [19]	199	n.r	n.r	96,1
Van Den Breemer et al. [16]	76	13	17,1	92
Sulaiman et al. [17]	11 603	106	0,91	n.r
Endocrown:
Van Den Breemer et al. [16]	11	0	0	100
Bridge:
Becker et al. [12]	45	33	73,33	57
Garling et al. [13]	36	12	33,33	87,9
Kern et al. [5]	36	12	11,1	90.8
Teichmann et al. [2]	27	15	55,5	63
Sulaiman et al. [17]	1494	68	4,55	n.r
Implant supported crown:
Spies et al. [3]	24	0	0	100
Teichmann et al. [2]	17	1	3,7	100
Onlay
Fabbri et al. [19]	46	n.r	n.r	97,83
Malament et al. [20]	305	3	0,98	95,6
Sulaiman et al. [17]	1093	11	1,01	n.r
Souza et al. [18]	20	0	0	100
Inlay
Malament et al. [20]	246	3	1,21	95,6
Bilayer
Veneers:
Yang et al. [4]	2295	57	2,5	97,2
Fabbri et al. [19]	53	n.r	n.r	97,91
Sulaiman et al. [17]	1376	21	1,53	n.r
Single Crown:
Yang et al. [4]	4180	136	3,3	96,5
Fabbri et al. [19]	281	n.r	n.r	96,92
Sulaiman et al. [17]	4162	76	1,83	n.r

Veneers

In Sulaiman et al. [17] review, the results of the studies were encouraging; lithium disilicate veneers were associated with very high survival rates: 98.5% over a 4-year follow-up period, 97.2% and 98% over a follow-up period of 5 years [4], [9], 97.91% over a follow-up period of 6 years [10] and 87% over a follow-up period of 20 years [9].

In Yang’s et al. study [4] the most common failures were restorative fractures and chipping. According to the author, possible reasons may be design and manufacturing flaws, including pores and irregularities. Isgro et al. [11] believed that the gaz could go inside the ceramic material and the porosities could increase and cause more fracture origins to form, which would affect the fracture toughness and resistance to fracture.

The prospective study by Malchiodi et al. [12] reported a survival rate of 98.7% with an average follow-up of 3 years. Moreover, Granell-Ruiz et al. [13] reported a survival rate of 84.7% once patients with parafunctional habits were included.

Studies [4] and [9] showed that the location of lithium disilicate veneers had no significant impact on the survival rate. With over 20 years, the long-term aesthetic and functional success of lithium disilicate veneers is excellent. The studies included in our review have shown that lithium disilicate veneers perform very well with respect to color match and marginal integrity, and are a reliable, effective and conservative treatment option for tooth restoration anterior and posterior. It can be concluded that veneers are part of the indications for glass-ceramics enriched with lithium disilicate.

Single Crowns

The survival rate of lithium disilicate crowns was satisfactory in all the included studies, except for study [2] where the survival rate was 89.7% and the failure rate 25.28%.

In Yang’s et al. study [4] the survival rate was 96.5% for a follow-up period of 5 years. As with veneers, the most common failures were restoration fractures and chipping, according to the author possible reasons may be design and manufacturing flaws, including pores and irregularities. In this study the crowns were located only at the anterior level, so it was not possible to make a connection between the restoration and the location.

In Pieger et al.’s review [6] the survival rate was 97.8% at 5 years and 96.7% at 10 years. The performance of porcelain fused to metal crowns was compared with that of IPS e.max Press single-layer crowns and IPS Empress 2 double-layer crowns and it was found that the survival rate was 100% for all crowns after 3 years. Nevertheless, lithium disilicate crowns exhibited more crown wear and surface roughness than porcelain fused to metal, with statistically significant differences in surface texture and crown wear at year 3. Regarding the reported fractures, most were located in the posterior region. Unlike other studies of all-ceramic crowns [14], [15], in Gehrt et al.’s study [16], the location of lithium disilicate crowns did not have a significant impact on the rate of survival (log rank test, p = 00.74).

Over a follow-up period of one to six years, the study by Fabbri et al. [19] found a survival percentage of 96.67%. Even though the restorations were thick enough to meet or exceed manufacturing criteria, all of the fractures and mechanical problems occurred in teeth where dentin or composite made up the majority of the support. However, when the bonding substrate was at least 50% enamel, no mechanical problems were seen with cement-retained restorations on those teeth. Sixty-three per cent of mechanical issues (chipping and fractures) on restorations with porcelain close by occurred in individuals with parafunctions. Patients without bruxism or grinding issues who had their original teeth as antagonists had the remaining 37% of mechanical difficulties.

Considering the high rate of parafunctional patients (30.3%) and the high survival rate in this study, lithium disilicate can be considered a valid option to treat these patients.

In the review by Maroulakos et al. [17], the failure rate of zirconia crowns was significantly high compared to lithium disilicate crowns with a value of 18.35% against 4.91%.

According to a systematic analysis by Sailer et al. [18], the failure rates of all-ceramic single crowns and the “gold standard” metal ceramics were considerably comparable. The greatest failure rates were observed with zirconia (9.04%) and feldspar ceramic (14.3%). The most frequent failures of all-ceramic crowns, technically, were ceramic and restoration fractures. Using poor ceramic materials was the particular cause of this. On the other hand, the most frequent failure for metal-ceramic crowns was ceramic fracture. In terms of biology, all-ceramic crowns performed better than the “gold standard,” which had greater rates of periodontal complications, secondary caries, and tooth fractures. These biological issues may affect the abutment teeth’s prognosis or may lead to their loss simultaneously with restorations. In contrast, all-ceramic crowns are rarely associated with these issues, which may encourage dentists to choose these kinds of materials.

Therefore, research has demonstrated that all-ceramic lithium disilicate crowns can be suggested as a substitute for metal-ceramic crowns in both the anterior and posterior sectors [18]. Nevertheless, feldspathic ceramic could only be recommended at the anterior level with lower occlusal forces.

Zirconia crowns demonstrated poor clinical effectiveness despite their mechanical reliability. Ceramic fractures were the most common failure for this type of ceramic in comparison to other types of all-ceramic or even metal-ceramic materials, not to mention the sintering issues frequently faced which demonstrated that zirconia crowns could not serve as the primary treatment option [18].

In Gehrt et al.’s study [16], it was demonstrated that the sealing method did not significantly influence the incidence of complications (log-rank test, 0.17%). The qualitative assessment by Maroulakos et al. [17] showed comparable survival and failure rates between bonded and cement-retained crowns.

The most common failures for bonded crowns were debonding as well as fracture of the restoration, and for cement-retained crowns, the most common failure was fracture of the restoration. All cement-retained or bonded crowns included in our review showed satisfactory clinical performance, and their use appears to be a reliable treatment option. In addition, attention should be paid to the location of the restoration, taking into account the patient’s occlusion.It can be concluded that crowns are part of the indications for glass-ceramics enriched with lithium disilicate.

Endocrowns

Only one study by Van Den Breemer et al. [19] treated endocrowns. No cases of failure have been reported.

There were no other studies reporting on endocrowns on humans to compare.

A study by Altier et al. [20] compared the performance of lithium disilicate and composite endocrowns on extracted human molars, and found that composite endocrowns failed more favorably than lithium disilicate endocrowns. Lithium disilicate glass-ceramic and composites have different fracture modes, which could be caused by variations in their modulus of elasticity. Composite materials tend to bend under load, distribute stress more uniformly, and absorb it because their modulus of elasticity is compatible with that of dentin. Lithium disilicate ceramics, on the other hand, are stiff materials that might break catastrophically due to stress concentrations in key places. Altier et al. [20] claims that the clinical success of endocrowns on molars was superior to that of endocrowns on premolars. This could be because premolars have a smaller pulp than molars, which means that their bonding surface is less.

Bridges

Studies [4] and [6] reported on cemented bridges, studies [21] and [22] on bonded bridges, and the three studies [5], [23] and [2] on bonded and cemented bridges.

Failure rates of lithium disilicate bridges were high compared to failure rates of other types of restorations, and ranged from 4.55% to 73.33%. The study by Yang et al. [4] reported a survival rate of 91.8% at 1 year and 90.6% at 5 years. In this study, restoration fracture was the most reported failure. These results suggest that bridges should be more vulnerable to ceramic damage. This could be due to the difference in mobility between the tooth and the different abutments, inducing higher tensile and shear stresses inside the restorations. The tensile and shear strength properties of the ceramic were much lower than the compressive strength properties. Therefore, lithium disilicate bridges should be used with caution. In this study the crowns were located only at the anterior level, so it was not possible to make a connection between the restoration and the location. In Pieger et al.’s review [6] the survival rate was discouraging with the value of 78.1% at 5 years and 70.9% at 10 years, and a failure rate of 17.24%. Examination of the fractured fragments under a scanning electron microscope showed that the connection zone was the main reason for the fractures. The authors also stated that catastrophic failures such as framework fracture occurred mostly in dentures replacing missing posterior teeth (68%).

In a clinical study with 30 bridge inlays over a ten-year observation period, the most common complications observed were chipping and debonding. The 10-year survival rate was 12% [24]. In laboratory studies, zirconia-based bridge inlays showed significantly higher fracture toughness than those made of lithium disilicate ceramic [25].

The study by Garling et al. [23] showed a dramatic drop in the survival rate of lithium disilicate bridges to only 48.6% after 15 years. Molar restorations were found to be more frequently affected by failures. This can be explained by the load of forces exerted in the posterior region. The high failure rate in the present study could also be related to the posterior position of most of the bridges evaluated. The study by Teichmann et al. [2] reported a relatively low survival rate of 63.0% at 5 years and 51.9% at 10 years. Restoration fracture was the most common failure. The comparison between the posterior and anterior prostheses revealed no significant difference (log-rank test, p = 0.98). In the study by Kern et al. [5] the survival rate was 100% at 5 years and 90.8% at 10 years, all fractures occurred in replacement bridges of molars.

Sealed Bridges

In a review by Pjetursson et al. [26], a comparison was made between the different types of cement-retained bridge materials. Cemented lithium disilicate bridges had a high failure rate (27.21%) compared to the “gold standard” ceramic-metallic and zirconia which showed failure rates of 17.9% and 17.15%, respectively. Another major problem with lithium disilicate bridges was restoration failure, especially in the posterior sector, which might be related to connection sizes less than 4 mm × 4 mm. Pjetursson et al. pointed out that zirconia has shown satisfactory performance in comparison to other types of all-ceramic and ceramic-metallic materials, especially when not used for long-span restorations, adhering to the connection diameter criteria, which is also applicable to other all-ceramic bridges, despite the challenges seen in zirconia bridges, whether related to biological or technical issues.

Bonded Bridges

Bonded lithium disilicate bridges had the greatest failure rate (90.91%) in research by Chen et al. [27]. It was considerably lower (36.84%) for ceramic-to-metal bridges than for lithium disilicate bridges and higher (31.03%) for zirconia bridges. The possibility that the mechanical characteristics of lithium disilicate would not be able to withstand masticatory stresses at the posterior level helped to explain this high rate.

In another study by Yushida et al. [28], ceramic-to-metal bridges had the highest survival rate, followed by zirconia bridges. Lithium disilicate bridges had the lowest survival rate. However, in the study by Thoma et al. [29], 0 failures were reported for zirconia bridges, while all-ceramic bridges had the highest failure rate 10.95%, followed by lithium disilicate 4.08%.

Because adherence to dentin is weaker than to enamel, which may affect retention, several studies have suggested bridges that are either prepared entirely or just to the enamel [30]. Most authors believe that less invasive preparations are adequate for anterior level bridges. In contrast to conservative techniques, it was proposed to prolong the preparations at this level in order to improve retention at the posterior sector [29]. Numerous studies have proposed expanding the bonding surface; Wyatt [31] showed that changing the shape of the preparation increased the strength and retention of bonded bridges, and wings with a 180° extension around the axial surface of the abutment are believed to improve retention [30] as well. Balasubramaniam [30], mentioning a number of studies, suggests supragingival boundaries to enable proper hygiene maintenance and ward against cavities and periodontal and gingival diseases. Furthermore, the recommendation of bonded bridges is significantly influenced by the endodontic and periodontal condition of the abutment teeth.

Implant-Supported Crowns

Two studies investigated lithium disilicate crowns on implants. In the study by Spies et al. [3], the fabrication technique of lithium disilicate crowns on zirconia implants was by CAD/CAM. In the present study, no restorations failed after an average observation period of 31 months. In addition, no spalling or fractures were observed. For this, the survival rate of single implant crowns was 100%. In the study by Teichmann et al. [2], the 5- and 10-year survival rates were 100%, and 93.8%, respectively. None of the crowns have been replaced. Although none of the implants were lost, an alumina-based abutment fractured. Due to the small number of cases, no comparisons were made between adhesive and conventional crowns or anterior and posterior crowns. Studies reporting on implant-supported lithium disilicate crowns were limited. Pozzi et al. [32] evaluated the outcome of full-contour lithium disilicate crowns bonded to full-arch zirconia CAD/CAM implant bridges (16 patients, 18 frameworks, 236 units) with a mean observation time of 49.3 month. Only one unit had a minor fracture that could be polished, giving a survival rate of 100%. Another study of lithium disilicate crowns supported by implants reported 100% survival of implants and abutments and no prosthetic complications in all 23 crowns after 5 years of clinical service [33]. A systematic study of implant-supported single crowns and their implants revealed a survival rate of 96.3% and 89.4% respectively. One of the limitations of this study is that there were only two studies of all-ceramic crowns versus 17 studies of ceramic-to-metal crowns, however, no significant differences were found between these materials. catering [34]. Zembic et al. [35] reviewed the outcomes of implant abutments and found no difference in the survival and failure rates of ceramic and metal abutments after 5 years (97.5% vs. 97.6%). Excellent results on all-ceramic crowns on implants have been reported by recent studies, in the medium and long term with a survival rate close to 100% [36].

Onlays/Inlays

The study by Fabbri et al. [10], on onlays, reported a survival rate of 93.54% and a failure rate of 6.45%. As in the case of crowns, all complications were observed on teeth where the majority of the support was dentine or composite. The study by Malament et al. [37] reported a survival rate of 95.6%, and a failure rate of 0.98% for onlays, and a survival rate of 95.6%, and a 1.21% failure for inlays. All failures occurred in the molars, which are subject to the highest occlusal loads. Nevertheless, the number of failures was low (n = 6). The failure production time suggests that the failures occurred after a random event rather than being related to cumulative fatigue. The high survival rate of lithium disilicate partial coverage restorations can be attributed to the mechanical properties of the latter with a flexural strength of 470 MPa and a breaking strength of 2.54 MPa.m1/2. Additionally, due to its vitreous phase, lithium disilicate is etchable, allowing a strong micromechanical bond with the dental substrate and greatly increasing its characteristic strength [37].

Direct-to-resin composite restorations can be provided as an alternative treatment method, with less cost and fewer patient visits. However, concerns have been raised about these large restorations in posterior teeth [37]. According to a practice-based research study, failure rates of posterior composite resin and dental amalgam restorations were similar, between 2% and 9% per year [38]. When compared to the findings from the study by Malament, the additional time and monetary expense of glass-ceramic inlay and onlay restorations is a reasonable trade-off. Moreover, composite resin restorations wear more than tooth structure [4] while lithium disilicate restorations have comparable wear rates to enamel, implying longer-lasting dental patterns due to bites [5].

The longevity of all-ceramic restorations can also be affected by the preparation geometry in addition to the restorative material itself. Specifically, the effect of enamel backing in minimally invasive preparations appears more favorable concerning the stability of the restoration than the dentinal backing [39]. A similar enamel-preserving technique was applied in the study by Edelhof et al. [40] where occlusal onlays were studied in patients with signs of heavy tooth wear, demonstrating excellent outcomes with a 100% survival rate. Except for minor discoloration in four restorations and one marginal fissure, there were no complications. Three of the discolorations that developed after five years were repolished, and one penetrated partially beneath the restoration where it was monitored. It was also examined after 10 years of clinical use for the fine marginal crack.

Souza et al. [41] in their study on onlays, specified that several patient-related factors influence the survival of the restoration, such as the risk of caries and occlusal loads, in addition, the risk of failure of partial restorations indirect in posterior teeth is higher for endodontically treated teeth compared to vital teeth.

According to Fabbri et al. [10], monolithic crowns and onlays occasionally did not produce the best shade results (9.1%). Since a number of factors influence the successful aesthetics of a restoration, including the color of the natural abutment, the thickness of the ceramic, the careful selection of stumps, and the color and value of neighboring teeth, the case selection is crucial to achieving a natural appearance with a monolithic approach.

In a study by Schultheis et al. [42], the authors analyzed the effect of fatigue on the survival rate and the fracture load of posterior three-unit, monolithic and two-layer CAD/CAM bridges, in comparison with metal-ceramic prostheses. Monolithic restorations have shown fractures at load levels to be comparable to porcelain fused to metal, and therefore appear to be as strong. Unlike monolithic restorations, the performance of ceramic-to-metal restorations was superior compared to those in bilayers, this is explained by the fact that ceramic-to-metal restorations have an intrinsic stress absorption mechanism in the metal infrastructure which limits the propagation of cracks.

Monolithic restorations seem to have good and better results compared to bilayer restorations, but the clinical success of lithium disilicate prostheses cannot be attributed to the material alone, as the manufacturing process and the clinical preparation of the teeth can play a role. major role in this success. Failure analysis methods for restorative material, thickness, and loading positions have been proposed to predict the success and longevity of restorations [43].

Association between Indications of Lithium Disilicate and Location

In Pieger et al.’s review [6], failures occurred mainly in prostheses teeth replacing missing posterior teeth (68%). In Becker et al.’s study [21], most failures due to fracture or detachment were related to prostheses replacing first molars. In the study by Garling et al. [23], molar restorations were found to be more frequently affected by failures. This can be explained by the load of forces exerted in the posterior region. In the study by Teichmann et al. [2], the comparison between posterior and anterior prostheses revealed no significant difference. In Kern et al. study [5], all fractures occurred in replacement bridges of molars.

Association between Indications of Lithium Disilicate and Method of Sealing

Studies by Kern et al. [5] and Garling et al. [23] showed the same survival and failure rates between bonded and cemented bridges. The survival of the restorations was not influenced by the cementation method, and no statistically significant difference was detected. The clinical results of bonded or cemented lithium disilicate bridges were not encouraging and not acceptable in comparison with metal-ceramic or zirconia restorations.

Due to the high risk of failure, it is suggested to avoid lithium disilicate bridges, especially posteriorly, and to replace them with standard treatments (metal-ceramic) or with other ceramic materials such as zirconia. It can be concluded that the indications of glass-ceramics enriched with lithium disilicate for bridges are limited.

Recommendations

Lithium disilicate is an essential material in our therapeutic arsenal despite its limitations for some types of restoration. Well known for its aesthetic characteristic, research today focuses on lithium disilicate prostheses whose mechanical properties are improved.

Recommendations for Each Type of Restoration

Veneers and Onlays/Inlays

These restorations can easily be used both anteriorly and posteriorly. Dental preparations for the realization of these restorations are governed by the principle of tissue saving. Indeed, the surface of the preparation must consist mainly of enamel in order to improve the bonding properties. Thus, it is essential to detect, from the clinical examination, excessive substance losses as well as major malpositions that would force the practitioner to make a preparation whose surface is largely composed of dentin.

Single Crowns

Although lithium disilicate crowns, regardless of their sealing methods, seem to be a reliable treatment, it is recommended to avoid placing them posteriorly in people with bruxism or who have strong muscles. These occlusal constraints can cause detachments or fractures and therefore affect the durability of the prosthesis.

Bridges

Due to the high risk of failure, it is recommended to avoid disilicate bridges lithium, especially posteriorly, and replacing them with standard treatments (metal-ceramic) or other ceramic materials such as zirconia. It is recommended to make only 3-unit bridges replacing one tooth up to the first premolar only. For bridges at the anterior site, less invasive preparations are sufficient for most authors. However, at the posterior site, increasing the bonding surface has been recommended to improve retention.

Recommendations for the Type of Coating

It is recommended to use monolithic restorations instead of restorations in bilayers, although the latter also offer satisfactory results but less than the monolithic ones.

Recommendations Regarding the Location of Lithium Disilicate Restorations

Avoid posterior bridges.The single crowns can be easily placed both anteriorly and posteriorly, but should be used with caution if the occlusion is judged to be disadvantageous.

Conclusion

In fixed prosthesis, for the success of a prosthetic restoration, the choice of materials must be the result of a well thought out study of the different materials available according to the clinical case.

From this work, we sought to demonstrate the indications of glass-ceramics enriched with lithium disilicate validated by scientific evidence.

We have therefore been able to observe that lithium disilicate is an essential material in our therapeutic arsenal despite its limitations for certain types of restoration. Well known for its aesthetic characteristics, research is now focusing on lithium disilicate prostheses with improved mechanical properties.

Despite the limitations of our review, we can conclude that indications of lithium disilicate will be: Veneers, single crowns, Crowns on implant, Onlays and Inlays.

The limits of lithium disilicate will be bonded and cemented bridges, especially for posterior restorations.

Lithium disilicate can be easily used anteriorly and posteriorly for: veneers, inlays, onlays and implant crowns. For single crowns: posteriorly, lithium disilicate must be used with caution if the occlusion is judged to be unfavourable. In anterior, there is no worry. Posterior bridges should be avoided. Anterior bridges can be used but rigorously.

Although this ceramic has proven itself, the appearance of new generations of lithium disilicate makes it possible to extend its field of application ever further and tends to make it a material capable of responding to the vast majority of clinical situations to which we are dealing with.