Physico-chemical features, optical and mechanical properties
Lithium disilicate (LS
2) is classified as a glass-ceramic, in the class of particle-filled glass materials. Introduced on the market in the 90s with the commercial formulation named “IPS Empress 2” (Ivoclar Vivadent, Schaan, Liechtenstein), it was composed of 65 vol% lithium disilicate, small needle-shaped crystals (3–6 μm × 0.8 μm) embedded in a glass matrix, with a 1 vol% porosity [
1‐
3], showing valuable mechanical characteristics (flexural strength: 350 MPa; fracture toughness (KIC): 3.3 MPa√m; heat extrusion temperature: 920 °C; thermal expansion coefficient (CTE): 10.6 + 0.25 ppm/°C). At first, this material was made commercially available as ingots, to be utilized according to the “heat-pressing” fabrication procedure, similar to the classic “lost wax” technique for metal-alloy casts, aimed at producing cores, hot pressed into a mold. In order to get an appealing reproduction of the optical characteristics of natural teeth, the cores are lately veneered with a very translucent fluorapatite ceramic, containing 19–23% of fluorapatite crystals (Ca
5(PO
4)
3F) embedded in a glassy matrix [
4].
Thanks to an optimization of the processing parameters, allowing the formation of smaller and more uniformly distributed crystals, in 2005 a new formulation of LS
2 was marketed as “IPS e.max Press” (Ivoclar Vivadent), exhibiting improved mechanical properties and optical features (flexural strength: 370–460 MPa; fracture toughness (KIC): 2.8–3.5 MPa√m), much higher than the older glass-ceramics. The high mechanical performance of this material is due, on one side, to a layered, tightly interlocked distribution of the elongated disilicate crystals, hindering crack propagation across the planes and, on the other side, to a mismatch between the thermal expansion coefficients of LS
2 crystals and the glassy matrix, so that the latter induces a tangential, compressive stress around the crystals [
2]. Besides the production of ceramic cores for bilayered crowns, the increase of strength and toughness of IPS e.max Press has allowed to extend its clinical indication to monolithic restorations, without veneering ceramic, anatomically shaped, colored by surface stains and characterized by a higher fatigue resistance than the bilayered ones.
Besides the heat-pressed technique, the widespread, increasing implementation of computer-aided design/computer-aided manufacturing (CAD-CAM) technologies has led to the introduction of ceramic blocks aimed at the production of restorations by milling devices (IPS e.max CAD), also suitable for chairside production of restorations. Partially, pre-crystallized blocks are manufactured in a “blue state”, containing 40% of metasilicates (Li
2SiO
3) in addition to lithium disilicate crystal nuclei (Li
2Si
2O
5). Such blocks are characterized by moderate flexural strength of ~ 130 MPa, resulting in higher cutting efficiency, easier and faster workability and lower wear of the milling tools [
2,
3,
5]. The milling procedure is performed in this pre-crystallized state and, after its completion, it is followed by a heating cycle (840°-850 °C for 10 min) that turns metasilicate crystals into lithium disilicate (~ 70%), increasing the flexural strength up to values of 262 ± 88 MPa, together with a fracture toughness of 2.5 MPa·m
1/2. The blocks are available in different colors, obtained by dispersing staining ions in the glassy matrix [
6] and in different degrees of translucency, on the basis of the size and distribution of the crystals in the glassy matrix [
4]. The variability of flexural strength of lithium disilicate among heat-pressed and CAD-CAM blocks with different translucency is still under debate [
7,
8]. Particularly, the flexural strength of IPS e.max Press and IPS e.max CAD was reported to be similar and the manufacturing process did not seem to affect the mechanical characteristics of lithium disilicate ceramics; moreover, the flexural strength was significantly influenced by translucency only for CAD-processed materials [
7].
In vitro fully anatomical e.max CAD crowns have been shown to exhibit fracture resistance that is suitable for posterior, monolithic restorations [
9] and to be more resistant to fatigue in cyclic loading than veneered zirconia, that is more prone to chipping [
10]. For the high interest generated by its clinical versatility, further developments are expected on this material, being it influenced by different production processes, like thermal gradients, times and rates, that affect its microstructure and mechanical properties. It has been shown, for instance, that extending temperature range (750–840 °C, compared to the standard 820–840 °C) or prolonging holding time (14 min vs 7 min at 840 °C) increase elastic modulus and hardness properties, without affecting flexural strength and fracture toughness [
11]. Moreover, new technologies, as spark plasma sintering, can induce a refinement and a densification of the nano-crystalline microstructure, increasing lithium disilicate and metasilicate phases and reducing lithium orthophosphate and cristobalite/quartz phases [
12,
13].
As regards mechanical resistance, it has been clearly demonstrated that, in vitro, veneered LS
2 crowns exhibit significantly lower fracture load values (1431.1 ± 404.3 N) compared to monolithic ones (2665.4 ± 759.2 N), the main failure mechanism being bulk fracture initiating from the occlusal surface [
14]. To date, there is strong evidence from in vitro studies that, differently from bilayered restorations, monolithic ones show fracture strength and fatigue resistance suitable for use in the posterior areas, both in tooth- and implant-supported single crowns (SC) and 3-unit fixed dental prostheses (FDPs) [
15‐
22].
Monolithic LS
2, as well as Zirconia reinforced-Lithium Silicate ceramics (ZLS), offers higher fracture resistance than bilayered, hand-veneered zirconia [
20], while a recent in vitro research has shown that load-to-fracture values of monolithic zirconia are higher than those of LS
2; the latter, in turn, are higher than those of ZLS [
23].
It has to be pointed out, however, that, particularly as regards LS
2, fatigue resistance is strongly influenced by many experimental variables, like amount of cyclic loading, abutment and antagonist design and material, thermocycling parameters and test environment; for this reason, the heterogeneity and lack of standardization in research designs, tested materials and experimental conditions make a comparison of data not easily feasible [
24].
Abrasiveness and wear
As to wear and abrasiveness, LS
2 shows quite favourable properties, that are highly depending on the surface characteristics of the restoration. When accurately polished at its surface, the material exhibits convenient tribological behaviour in vitro, in terms of friction and wear of restorations, being its abrasiveness quite close to enamel, although more aggressive when compared to type III gold [
25] or to polished monolithic zirconia in in vitro simulations [
26‐
28]. Such favourable wear behaviour and durability have been also confirmed by some in vivo evidence [
15].
On the other hand, it has been reported that grinding, glaze coating and fluorapatite ceramic veneering can increase wear, both of the antagonist teeth and of the restoration itself; at the same time, surface roughness can also be increased, besides a reduction of gloss, in the presence of basic pH environment and after toothbrushing with abrasive toothpaste [
29‐
33]. For these reasons, when it is not crucially needed for esthetic reasons, glazing of monolithic restorations should be avoided on the occlusal surfaces in posterior sites and only limited to the esthetically relevant zones; moreover, careful polishing procedures should always follow any occlusal grinding or esthetic refinement of disilicate restorations, although in vitro evidences at scanning electron microscope (SEM) have shown that LS
2 is one of the most critical materials to adjust intraorally, due to significant chip accumulation in the diamond burs, requiring higher machining forces and energy, with likely onset of intergranular and transgranular fractures, besides risks of thermal damage to tissues and restorations [
32].
Surface treatment and cementation
In addition to excellent biocompatibility and high mechanical properties, LS
2 exhibits very good esthetic features, especially as regards translucency, that is about 30% higher than conventional zirconia [
37]. Moreover, for the presence of silica, LS
2 is an acid-sensitive ceramics, so that high strength of adhesion to the substrate is expected, due to both micromechanical and chemical bonding mechanisms. Micromechanical interlocking between ceramics and resin cement at the intaglio surface is based on the creation of surface microirregularities, pits and roughness by means of acid etching and/or physical treatments like alumina particles sandblasting or diamond bur grinding. For the glass-ceramic class, to date hydrofluoric acid (HF) etching is the best-established procedure, to be performed according to validated protocols taking into account both acid concentration and etching time. For LS
2, 20 s HF etching (at 5% concentration) is suggested, that is a shorter time than requested for feldspathic and leucite-based ceramics (generally 60 s). Higher HF concentrations (9–10%) and longer etching times have been shown to be too aggressive and can introduce relevant damages, not only to the surface but also to the internal microstructure of the material, negatively influencing mechanical performance (reduction of flexure strength), adhesion potential and long-term success of ceramic restorations, particularly when thickness is low [
38‐
41]. Another system to create surface microirregularities is sandblasting LS
2 with aluminum oxide particles. Nevertheless, it has been shown that this procedure, as well as laser etching, can determine excessive loss of material, with surface modifications that are less uniformly distributed than after HF etching and that can significantly reduce flexural strength [
42,
43]. In addition to micromechanical interlocking, as for all silica-based materials, adhesive bonding of LS
2 is efficiently increased by silane, ensuring a chemical interaction between the resin-based agent and the ceramics, obtained forming strong siloxane linkages [
44‐
50].
Recently, it has been shown that the use of silane combined to a phosphate functional monomer, the 10-Methacryloyloxydecyl-Dihydrogen-Phosphate (10-MDP), creating an acidic environment further improves the bond strength of resin-based luting cement to lithium disilicate ceramics [
51].
Clinical indications and performances
As regards clinical indications of LS
2, it has to be pointed out that this is one of the most versatile metal-free materials for its high esthetic potential, good mechanical properties and favourable bonding strength to dental tissues, thanks to its silica content. Lithium disilicate ceramics can be utilized both for tooth- and implant-supported restorations, ranging from SCs to FDPs, from anterior veneers to posterior inlays, onlays and overlays [
4,
7].
To date, due to its relatively recent market introduction, there is still a lack of data about long-term outcomes of LS
2 restorations, particularly as regards CAD-CAM production. Prospective, medium-term studies reported good cumulative survival rates, both for tooth-supported crowns (94.8% after 8 years [
52]) and implant-supported crowns, made by CAD-CAM procedure following conventional impression (100% after 5 years [
53]). A recent prospective study on implant-supported, single-unit monolithic restorations made of LS
2 in a complete digital workflow has demonstrated survival rates of 100%, without any technical or biological complications, after 2 years of service [
54]. Similarly, retrospective studies have shown that LS
2 can yield satisfactory clinical performance with favourable survival rates and low incidence of mechanical failures, like debonding, fractures and chipping [
15,
55‐
58].
As regards chairside procedures, monolithic LS
2 crowns revealed a survival rate of 83.5% after 10 years of follow-up; the main complications were loss of retention, secondary caries and hypersensitivity [
59].
In the last decade, LS
2 has been proposed for producing full-contoured, monolithic SCs to be bonded to CAD-CAM zirconia full-arch frameworks supported by implants. In a mid-term study, such a restorative solution exhibited 100% survival rate, after 5 years of follow-up [
60]. Recently, an in vitro study has suggested that LS
2 crowns supported by ceramic-reinforced polyether ether ketone (PEEK) implant abutments may be an alternative to zirconia abutments with a titanium base for single-implant restorations in the anterior region [
61].
Thanks to the high reliability of resin bond to glass-ceramics, LS
2 clinical indications also include adhesively retained, tooth-supported restorations. In the anterior sites, in the authors’ and in other clinicians’ clinical experience, laminate veneers made of bilayered, hand-veneered LS
2 are a likely choice, particularly when clinical performance and high esthetic results are expected [
62]. Clinical and in vitro studies demonstrated that, in the presence of long teeth, margins positioned beyond the cemento-enamel junction (CEJ), large areas of exposed dentin or flexural tensile stresses due to high functional loads, laminate veneers are exposed to higher failure risks, being maximum enamel preservation and veneer mechanical resistance paramount success factors [
63,
64]. Due to its mechanical properties, lithium disilicate can be considered a viable option to fabricate ceramic veneers in the presence of unfavorable biomechanical conditions; in fact, it was reported that more rigid ceramic materials exert a kind of shield effect onto underlying tooth structures, strengthening the restorative complex [
65].
Since their introduction in 1991, all-ceramic, resin-bonded fixed dental prostheses (RBFDPs) have been increasingly utilized as minimally invasive restorations aimed at replacing one missing tooth in the anterior arch [
66]. Although recording a high rate of early (1-year), unilateral retainer fractures in conventional, two retainers all-ceramic adhesive bridges, the authors noticed that the fractured, unilaterally supported restorations stayed in situ for 5 to 10 years [
67‐
69]; for that reason, since 1997 cantilevered all-ceramic RBFDPs were proposed as a new conservative treatment modality for replacement of single anterior missing teeth, with minimal tooth preparation on the lingual side, just aimed at achieving a correct positioning during cementation [
70]. Different materials have been proposed over the years, mainly, for their high strength, glass-infiltrated alumina ceramics [
71] and densely sintered, bilayered zirconia, treated with a combination of moderate pressure air-abrasion and MDP, with promising medium-term outcomes [
72‐
75]. Thanks to its advantageous optical properties and to its HF etching/silane bonding option, LS
2 has also been proposed as an alternative material for such cantilevered restorations, showing comparably promising clinical results [
76‐
78]. In a systematic review, cantilevered RBFDPs showed a lower failure rate than conventional, two-retainer, “Maryland bridge-style” ones, in which higher biomechanical stress arises for the different directions of forces acting on the adjacent supporting teeth during anterior guidance in protrusive and lateral mandibular movements [
79]. In another recent review, an estimated 91.2% survival rate at 5 years was reported for all-ceramic RBFDPs, exhibiting higher debonding rate with zirconia resin-bonded restorations than with glass-ceramic ones; conversely a higher fracture rate was reported with glass-ceramics [
80], even though higher level of evidence will be necessary to draw final long-term evaluations of all-ceramic RBFDPs clinical performances. RBFDPs are a suitable prosthetic solution as an alternative to implant-supported SCs, in the presence of anatomical impairment requiring costly and invasive surgical procedures, financial problems, young age of patients with congenitally or post-traumatically missing incisors; in any case, to limit the risks of mechanical failure or debonding, after an extensive esthetic, occlusal and technical evaluation of the case, a very careful treatment planning has to be defined prior to proceed with the operative phases.
In the posterior sites, LS
2 can be successfully employed for resin-bonded single restorations, like inlays, onlays, non-retentive partial crowns and full coverage table-tops, in the monolithic form. The material offers undisputable advantages, like high fracture resistance, showed by high load-at-fracture values in table-tops/occlusal veneers, allowing reduced thickness of the restorations (1–1.5 mm), low wear and abrasive potential, adhesive bonding strength and high biocompatibility, properties that are very favourable when teeth are severely abraded or a heavy occlusal correction is needed (like in lateral post-orthodontic open bite) [
10,
81‐
85]. These restorative solutions have shown favourable clinical outcomes in the most recent literature, even though with limited follow-up [
86,
87]. A recent 3-years randomized, controlled prospective trial has shown that LS
2 partial crowns can be used as successful restorative solutions for endodontically treated posterior teeth, with no significant differences between premolar or molars and with or without the use of fiber posts [
88].
The utilization of LS
2 for FDPs is a controversial topic: literature data is quite scant and not homogeneous, with a high variability of reported survival and success rates, ranging from rather poor clinical results [
89‐
92] to acceptable long-term serviceability both in anterior and posterior sites, similar to metal-ceramics [
93]. In the opinion of the authors, from a strictly clinical point of view, taking into account the cost/benefit ratio in terms of esthetic needs and structural resistance, the material of choice for 3- or 4-unit FDPs is still zirconia, in all of its different typologies.
Marginal accuracy and internal fit
Several studies evaluated the adaptation of lithium disilicate restorations, fabricated in both conventional and digital workflow. According to the most recent literature, there is no significant difference in terms of marginal accuracy between conventional and full-digital procedures for the fabrication of monolithic lithium disilicate crowns [
94‐
96]. Moreover, some authors reported that hot-pressed LS
2 crowns made from conventional impressions with polyvinylsiloxanes exhibit better fit than CAD-CAM digitally produced ones [
97].
Furthermore, centralized milling production has been reported to result in better fit compared to chairside system; in the same study, occlusal internal adaptation was better in the conventionally manufactured crowns than in the digitally fabricated ones [
95]. Conversely, other studies reported that marginal and internal fit of LS
2 crowns were more accurate when using digital impression technique; in any case, whatever the workflow used, the adaptation was shown to be within clinical acceptability range [
98‐
101].
To date, drawing univocal conclusions about adaptation accuracy of lithium disilicate restorations is not easy, due to the high number of variables involved in the final prosthetic fit, like digital impression system and technique, used material and fabrication procedure, so there is still a noticeable amount of controversial debate [
3,
102]. As regards fabrication techniques, hot-pressed lithium disilicate is reported to offer better internal fit and mechanical performances compared to CAD-CAM pre-crystallized blocks, even if, also about this topic, further data will be necessary to definitely shed light on these aspects, due to the constant evolution and increasing quality of milling procedures and devices [
103‐
108].
Zirconia reinforced-Lithium silicate ceramics (ZLS)
In the last years, the continuous research and progress in prosthetic material field for dental CAD-CAM applications has led to the introduction on the market of promising materials, the ZLS, thanks to an alternative strategy to enhance translucency: a glassy matrix, containing a homogeneous crystalline structure made of lithium silicate crystals, is reinforced with tetragonal zirconia fillers (about 10% by weight) allowing higher strength values than LS
2 [
109]. The higher mean translucency, together with proper biaxial flexural strength values, make such material a proper choice for minimally invasive, single tooth esthetic restorations, like inlays, onlays, partial crowns, veneers, anterior and posterior crowns, both tooth- and implant-supported [
109,
110], also fulfilling the “no-prep, table-top” strategy [
85]. The restorations show higher translucency and ease of intraoral polishing than both feldspathic and disilicate blocks, but, at the same time, exhibit high brittleness [
110‐
112]. In case of a dark substrate, moreover, it has to be considered that the high translucency of the material requires adequate thickness (1.5–2.0 mm) in order to get a proper chromatic masking [
113].
To date, as regards mechanical properties and clinical performances of ZLS, data are still limited, often controversial and short-term; these highly promising ceramics need further studies, both in vitro and in vivo, in order to precisely define physical-mechanical properties, clinical indications, limits and long-term performance of such restorations [
114‐
117].