Characterization of Lithia‐Based Machinable Glass‐Ceramic Materials

ABSTRACT

Objective

Lithia‐based glass ceramics lead the indirect single‐unit restoration market, but the underlying evidence is dominated by a single material. New materials have been introduced. The purpose was to investigate the elemental composition, elemental oxide composition, as‐fabricated surface morphology, as‐fabricated flexural strength, color, contrast ratio, and absolute light transmission for a variety of lithia‐based glass ceramics.

Materials and Methods

Test and control materials included: Amber Direct, Amber Mill, Cerec Tessera, IPS Emax CAD, Enamic, IPS Empress CAD, Initial Lisi, Supriniy PC, and TriLuxe Forte. Inductively coupled plasma‐optical emission spectrometry, X‐ray fluorescence, scanning electron microscopy, mechanical testing (n = 10), visible light spectroscopy (n = 4), and transmission testing (n = 4) were used to evaluate the above parameters. Where appropriate, ANOVA and multiple comparisons testing were used to determine which of the materials differed from one another (α = 0.05).

Results

A range of lithia‐based glass ceramics exhibited substantial differences in the above parameters. Differences were of sufficient magnitude to have statistical significance (p < 0.05) and clinical importance. Milling partly crystallized blocks, followed by additional crystallization, almost doubled flexural strength values in comparison to milling fully sintered blocks. Differences in a wide range of color parameters, more than sufficient to be obvious to the eye, were measured even though the materials were all the same nominal shade.

Conclusions

A variety of lithia‐based glass‐ceramic materials differed substantially across a range of chemical and physical properties.

Clinical Significance

A range of dental lithia‐based glass‐ceramic materials exhibited substantial differences in chemical composition, strength, and optical properties of a magnitude expected to influence their clinical performance. The milling of partly crystallized blocks, followed by additional crystallization, almost doubled flexural strength values in comparison to the milling of fully sintered blocks or control materials. At high magnification, machining damage was evident for all materials except for one partially crystallized lithia‐based material, which had also exhibited the highest strength.

1. Introduction

Glass ceramics have become one of the most widely used restorative materials in dentistry [1, 2, 3, 4, 5]. Their combination of aesthetics and strength enables them to be used in a wide variety of clinical situations: crowns, onlays, and inlays [4, 6]. Initially, glass‐ceramic materials were heat‐pressed in a laboratory furnace. Currently, computer‐assisted design and computer‐assisted manufacturing (CAD/CAM) machining is the dominant fabrication method [7]. Unlike dense crystalline ceramics, for example, zirconia, glass‐ceramics are amenable to etching and adhesive bonding. The first commercially available glass‐ceramic material in restorative dentistry was heat‐pressed DICOR developed by Corning Glass works, derived from MACOR, a machinable industrial glass‐ceramic [8]. A later machinable version of DICOR was quickly taken off the market due to high fracture rates and failures [9, 10, 11]. In 1991, IPS Empress was introduced by Ivoclar Vivadent; this material had both heat‐pressed and machinable versions. After the clinical success of IPS Empress, the first lithia‐based glass‐ceramic material for dental usage, IPS Empress 2, was launched. In 2005, Ivoclar introduced pressed IPS e.max followed by a CAD version. IPS e.max press and CAD were found to be exemplary materials in terms of optical and mechanical properties and have shown excellent clinical performance ever since [4, 12, 13]. Many lithia‐based glass‐ceramic materials are now available as blocks for chairside in‐office milling.

Glass‐ceramics are the material of choice for anterior single‐unit crowns according to data from a national practice‐based research network [1]. Following the success of lithium disilicate glass‐ceramics and expiration of the original patents, several manufacturers developed their own materials, likely circumventing some patented compositions and processing methods. Although long‐term clinical data is lacking, a short‐term study on one such new material revealed a 26% 1‐year failure rate [14, 15]. Recent findings show the composition and microstructure of some of these newer glass‐ceramic materials differ from the well‐documented IPS emax Press and CAD [4, 16, 17, 18, 19, 20]. The newer materials, however, not only differ in composition but may be supplied as fully crystallized blocks which do not require additional heat treatment to facilitate crystallization after milling [15, 16, 21]. Some manufacturers of fully crystallized blocks recommend a glaze firing cycle or a post grinding heat treatment cycle, likely to attempt to heal their subsurface microcracks.

Machining generates subsurface defects in glass‐ceramics that are detrimental to their strength [17, 22, 23]. Defects on the outer cameo surfaces of restorations are removed during finishing and polishing. However, defects on the intaglio surfaces of restorations remain as sites of stress concentration and crack growth. Ceramic crowns generally fail from the application of tensile forces on fabrication defects [24, 25]. Much data on all‐ceramic materials derives from specimens with polished surfaces, not as‐fabricated milled surfaces. But polished surfaces are not representative of the real‐world milled intaglio surfaces of restorations [17, 22].

Hence, it is important that a variety of current lithium disilicate machinable materials are comprehensively evaluated so that their composition can be related to parameters that are relevant to clinical decision making.

The purpose of this study was to investigate the elemental composition, fabricated surface morphology, fabricated flexural strength, and a wide variety of optical properties for a variety of lithia‐based glass‐ceramic materials suitable for chairside milling.

2. Materials and Methods

2.1. Materials

Nine commercial lithia‐based glass‐ceramic chairside‐milling materials and controls (Table 1) were investigated and characterized with respect to elemental composition, elemental oxide composition, surface texture, flexural strength, color, contrast ratio, and light transmission. Some of the lithia‐based glass‐ceramic materials were supplied as fully crystallized blocks that required no additional crystallization after milling. Others were supplied as partly crystallized blocks that required further heat treatment after milling to reach their full level of crystallization. Per manufacturer's recommendation, an additional “glazing” heat treatment cycle was performed for Cerec Tessera. Control materials, a feldspathic porcelain, a leucite‐based glass‐ceramic, and a ceramic‐resin hybrid, were included. These control materials provide contextual information by allowing comparison to other materials used for the same clinical situations as lithia‐based glass‐ceramic chairside‐milling materials. The control materials physical properties were focused upon because physical properties are considered in clinical material selection. Vita shade A2 was used throughout.

TABLE 1.

Materials characterized.

Material	Manufacturer	Description	Manufactured block
Amber Direct	Hass, Gangwon, Korea	lithia‐based glass‐ceramic	Fully crystalized
Amber Mill	Hass, Gangwon, Korea	lithia‐based glass‐ceramic	Partly crystallized, crystallization after milling
Cerec Tessera	Dentsply Sirona, Charlotte, NC, USA	lithia‐based glass‐ceramic	Fully crystallized, additional heat treatment after milling
IPS Emax CAD	Ivoclar Vivadent, Liechtenstein	lithia‐based glass‐ceramic	Partly crystallized, crystallization after milling
Enamic	Vita Zahnfabrik, Bad Sackingen, Germany	ceramic‐resin hybrid	No additional treatment
IPS Empress CAD	Ivoclar Vivadent	leucite‐based glass‐ceramic	Fully crystallized
Initial LiSi	GC Corp, Tokyo, Japan	lithia‐based glass‐ceramic	Fully crystallized
Suprinity PC	Vita Zahnfabrik	lithia‐based glass‐ceramic	Partly crystalized, crystallization after milling
TriLuxe Forte	Vita Zahnfabrik	felspathic‐based glass‐ceramic	Fully crystallized

2.2. Fabrication

Specimens were milled from the centers of chairside blocks using a 5‐axis milling machine (Craft DOF, Seoul, Korea). To avoid overuse of burs, the life of milling diamond burs was always maintained at more than 50%. After milling, specimens were crystallized or heat‐treated per manufacturers' recommendations.

2.3. Elemental Composition

Elemental compositional fractions were quantified using inductively coupled plasma—optical emission spectrometry (ICP‐OES). An argon plasma torch generates high temperatures, exceeding 10,000°C, ionizing atoms in the specimen. As the excited atoms return to their ground state, they emit characteristic light wavelengths, providing information about the elements present and their concentrations. ICP‐OES covers a broad range of elements with high sensitivity and low detection limits, but it requires specimens to be in liquid form. In contrast to X‐ray fluorescence (XRF), ICP‐OES measures the constituents of the entire specimen, not just those on its surface. ICP‐OES is also more effective in quantifying lighter elements than XRF.

Specimens, powdered 100 mg of samples of each material, were placed in pre‐wetted 22 mL Savillex Teflon vials with 0.5 mL of 16 mol/L HNO₃ and 2 mL of 24 mol/L HF. Vials were capped and heated at 120°C for 12 h. After drying, the samples were treated with 4 mL of aqua regia and 1 mL of HF and heated for 2 h. Specimens were transferred to Teflon tubes for microwave digestion using the Milestone ETHOS UP system at 230°C for 40 min. After digestion, they were dried, treated with 8 mol/L HNO₃ and 12 m3ol/L HCl in sequence, and refluxed with HNO₃ to remove HF and HCl. Final solutions were diluted 1:1000 with Milli‐Q water for ICP‐OES analysis on an Optima 7000 DV ICP‐OES (PerkinElmer; Shelton, CT, USA) at MATFab, University of Iowa.

IV‐ICPMS‐71A and 71B multi‐element standards were used for calibration. The USGS standard BHVO‐2 was employed as a secondary standard to monitor the accuracy and precision of the analytical run, with accuracy RSD < 5% (N = 3) and precision within 10% of reference values.

Because the control materials were based on different chemistries, comparison of their elemental compositions was not considered to be meaningful. Because Si is dissolved away during the digestion process, its concentration cannot be directly estimated.

2.4. Elemental Oxide Composition

Elemental oxide compositional fractions were quantified using XRF analysis. XRF exposes atoms to high‐energy X‐ray radiation, ejecting inner‐shell electrons from the atoms, as they are replaced by outer‐shell electrons; XRF is emitted. The energy and intensity of the emitted X‐rays are used to identify and quantify the elements in the specimen. XRF has higher detection limits than ICP‐OES, so it is better suited to identification and quantitative analysis of trace elements, but is less effective with lighter elements. Sample preparation is much simpler than for ICP‐OES because XRF measurements can be made on bulk specimen surfaces. Chemical elements were traced from F (atomic number 9) to U (atomic number 92) for each material characterized. Li is the lightest metal element, with an atomic number of 3, so it was not expected to be detectable using XRF. Specimens 14 ± 2 × 14 × 1.0 ± 0.05 mm were analyzed using XRF, operated at 50 kV and 48 mA (ZSX Primus IV, Rigaku, Japan) [26, 27]. Because so many elemental oxides were present, analysis focused upon the elements identified as having higher concentrations by ICP‐OES. Because the control materials were based on different chemistries, comparison of their elemental oxide compositions was not considered to be meaningful.

2.5. Surface Texture as Fabricated

Secondary electron imaging (SE) by a scanning electron microscope (SEM, Hitachi S‐3400 N, Hitachi, Tokyo, Japan) was used for qualitative analysis of surface textures after specimen fabrication. After coating with a thin layer of gold (MNT‐JS1600, MicroNano Tools, British Columbia, Canada), images were collected in high vacuum mode (< 10⁻³ mbar) with an accelerating voltage of 25 kV, an aperture setting of 4 (30 μm), a probe current setting of 40, and a working distance of 5 mm. Images were collected at magnifications of ×100, ×500, and ×3000 to bracket the features related to machining damage. SEM investigations used a non‐field emission SEM, which required a high beam voltage, balanced with a low beam current to achieve a usable brightness and fine image resolution.

2.6. Flexural Strength as Fabricated

Bars (n = 10) of dimension 14 ± 0.2 mm length, 4 ± 0.2 mm width, and 1 ± 0.2 mm thickness were milled from the CAD/CAM blocks (N = 90). The sample size was previously found to be sufficient in other investigations and was as recommended by ISO specification 6872 [17, 22, 28]. No additional polishing was performed so as to replicate the effects of routine fabrication methods on the restorations' clinically important intaglio surfaces. Three‐point flexural strength was measured using a Universal testing machine at a cross‐head speed of 0.5 mm per minute (Instron 34SC‐5, Canton, MA, USA).

2.7. Color and Light Transmission

Square specimens (n = 4) of dimension 14 × 14 × 2 ± 0.1 mm were sectioned using a water‐cooled slow‐speed saw (Isomet, Buehler) and were hand‐polished up to 2000 grit of SiC abrasive paper. The sample size equaled or exceeded those previously used in other investigations [27, 29]. Light transmission was measured in a benchtop spectrophotometer (Ci‐7600, X‐Rite, Grand Rapids, MI, USA). CIE L*, a*, b*, C*, and h^o parameters, as well as whiteness index and contrast ratio were calculated for specimens 2 ± 0.1 mm thickness [27, 29, 30]. Established 50:50% thresholds would be used to establish clinical relevance [30, 31].

Absolute light transmission (n = 4) was measured using a spectrophotometer (Color‐i7; X‐Rite) for polished (2000 grit SiC) specimens of each layer/material (N = 36) [32, 33]. The sample size approximated, equaled, or exceeded those previously used in other investigations [27, 32, 33]. Absolute transmission was used as well as contrast ratio, because contrast ratio has been reported to be poorly suited to characterizing low‐translucency materials and it is an indirectly derived relative parameter [33]. In addition, light transmission was visualized for five representative materials by placing the bars (14 × 4 × 1 mm) on a daylight‐adjusted view box (True Colors; Dimond Tech). A camera (DSLR D850 and Macro 105 mm lens; Nikon Corp) was fixed on a horizontal mount, and the exposure was adjusted until the view box was imaged as being pure white. A clear acetate sheet with 2‐mm‐wide printed black lines was placed on the view box, and bars were placed over the black lines. Images were made at a 1:1 magnification.

2.8. Statistical Analyses

For appropriate outcome parameters, a priori, to guide dentists and technicians in clinical decision making, one‐way ANOVA and Tukey multiple pairwise comparisons testing were used to determine which of the materials differed from one another (α = 0.05).

3. Results

3.1. Elemental Composition

ICP‐OES provided estimates of elemental concentrations (Table 2). Materials differed substantially in their compositions. For example, the concentration of Li, a key constituent, varied almost twofold among the included materials. The concentrations of minor components often varied by several orders of magnitude. It should be noted that Si could not be detected because it was removed when the materials were digested in HF so that the components could be solubilized. The data reported for IPS emax CAD in its uncrystallized and crystallized forms differed slightly; this was likely attributable to differences in sample preparation and dilution factors resulting from differences in powdering the microstructurally and mechanically different forms of the material. Although Tb was detected at low levels in all materials tested, the machine's calibration reference standard lacked this element, so no attempt was made to quantify its concentration.

TABLE 2.

ICP‐OES estimates of elemental compositions, excluding Si. Units are μg/g.

	IPS Emax CAD uncrystallized	IPS Emax CAD crystalized	Suprinity PC	Amber Mill	Amber Direct	Initial LiSi	IPS Empress CAD	Cerec Tessera
Na	668	678	754	2580	766	8057	40,410	2076
Ca	258	243	228	1760	2725	340	3903	441
P	11,516	12,144	18,183	13,795	11,677	17,282	80	17,588
Li	323,861	348,789	441,295	322,213	286,142	235,981	823	327,974
Mg	281	998	524	302	1380	385	0	726
K	33,226	38,591	17,390	23,797	21,953	17,266	11,317	8388
Mn	0	244	0	0	0	0	0	0
Sr	6	< 1	2	5	2	3184	38	4
Zn	5508	3149	0	1026	5459	0	0	0
Al	6712	12,924	9181	14,841	32,721	24,508	13,941	11,580
Fe	0	0	0	231	0	0	146	0
Cr	0	0	0	0	0	0	12	0
Ho	0	0	0	86	2337	0	0	0
Er	268	176	0	342	1855	4046	0	76
B	31	48	16	164	31	11	1464	6201
Zr	23,635	21,021	247,246	437	1370	41,836	334	212,197
Ce	1347	2983	3386	825	7812	5997	1487	0
V	659	579	556	684	318	1254	0	1496
Ba	202	189	179	209	252	253	2707	335
Pr	3	236	314	0	1442	582	0	0
Tb	Present	Present	Present	Present	Present	Present	Present	Present
Hf	125	119	1071	0	0	225	0	1222
Ti	58	70	62	32	60	1217	0	80
Y	2683	2953	24,692	452	4368	5152	59	33,251
Ge	197	173	56	259	21	154	0	173

3.2. Elemental Oxide Composition

XRF provided estimates of elemental oxide compositions (Table 3). As with ICP‐OES, materials differed substantially in their compositions and like trends were evident. All materials were dominated by their SiO₂ content, but this varied from ~62% to ~81%, consistent with the high variation in Li concentrations discerned by ICP‐OES. One material contained ~18% Al₂O₃, whereas another contained just one tenth of that amount. Two materials contained ~15% ZrO₂, whereas most others contained minimal amounts.

TABLE 3.

XRF estimates of elemental oxide percentage compositions by weight, excluding Li₂O.

	IPS emax CAD	Suprinity PC	Amber Mill	Amber Direct	Initial LiSi	IPS Empress CAD	Cerec Tessera
Na2O	0.1	0.1	0.4	0.05	1.0	4.2	0.4
Al2O3	1.8	2.4	3.6	8.3	5.9	18.2	3.0
SiO2	80.9	68.9	83.2	79.5	79.3	61.7	68.3
P2O5	4.2	7.9	5.0	4.6	6.5	0.03	7.1
K2O	7.2	3.2	4.4	4.3	3.3	14.5	2.0
V2O5	0.2	0.2	0.3	0.1	0.4	0.1	0.7
Fe2O3	< 0.01	0	0.1	0.01	0.02	0.05	0.02
ZnO	1.1	< 0.01	0	1.0	0	0.06	< 0.01
Y2O3	< 0.01	0.03	< 0.01	< 0.01	< 0.01	0.08	0.6
ZrO2	1.5	14.9	0.03	< 0.01	2.4	0.07	14.6
BaO	0	0.1	0.1	0.02	0.03	0.5	0.02
CeO2	3.0	2.3	2.9	2.1	1.1	0.5	3.2

3.3. Surface Morphology as Fabricated

SEM images displayed pronounced machining grooves at low (100×) magnification on all materials (Figure 1). Machining grooves were also evident at moderate magnification (500×) on most materials except IPS emax CAD and Suprinity PC. At high magnification (3000×) machining damage, jagged chipping, was evident on all materials except IPS emax CAD. Suprinity PC exhibited less surface machining damage than the other materials, including the non‐lithia‐based control materials.

FIGURE 1.

SEM images of as‐fabricated materials at 100, 500, and 3000× magnifications.

3.4. Flexural Strength as Fabricated

ANOVA revealed significant differences in flexural strength, p < 0.0001, among the as‐fabricated materials (Figure 2). Mean strengths of lithia‐based materials varied from 340 MPa (IPS emax CAD) to 130 MPa (Cerec Tessera), a 2.6‐fold difference. The partly crystallized block materials were approximately twice as strong as the fully crystallized block materials. All three partly crystallized block materials (IPS Emax CAD, Suprinity PC, and Amber Mill) were significantly stronger than all the fully crystallized block materials (Amber Direct, Initial Lisi, Cerec Tessera). The non‐lithia‐based control materials were significantly weaker than the partly crystallized materials, approximately half their strength, and overlapped with the weakest fully crystallized material. Coefficients of variation were all relatively low, varying from 4% to 12%.

FIGURE 2.

Flexural strength of as‐fabricated beams, means (standard deviations) in MPa with percentage (coefficients of variation). Statistically similar groups are linked by horizontal lines.

3.5. Color

Differences among all materials were discerned for all CIE parameters studied, L*, a*, b*, C*, and h^o, even though all materials were of the same nominal Vita shade, A2 (Table 4). Differences in L*, lightness, and in the whiteness index, a perceptually relevant measure of whiteness, were the most pronounced. The differences in parameters among all materials were of sufficient magnitude to be considered noticeable to the human eye and relevant to clinical dentistry, per established thresholds. Moreover, differences in color and light transmission were large enough to be inescapable from the viewer's eye (Figure 3).

TABLE 4.

Optical properties: CIE L*, a*, b*, C*, h^o color coordinates, and whiteness index.

Material	L*	a*	b*	C*	ho	Whiteness index
IPS Emax CAD	66	−0.2	12	12	91	−36
Suprinity PC	72	1.1	16	16	86	−50
Amber Mill	61	2.3	13	13	80	−55
Amber Direct	66	−0.4	14	14	92	−51
Lisi	67	0.3	13	13	89	−40
Enamic	64	0.9	13	13	86	−45
IPS Empress	68	0.3	12	12	89	−36
Cerec Tessera	67	0.8	10	10	85	−22
Triluxe Forte	71	−0.3	15	15	91	−47

FIGURE 3.

Transmitted light photograph of bars of representative materials, all nominally Vita classic shade A2; from left to right, IPS emax CAD, Amber Direct, Suprinity PC, Cerec Tessera, and Initial Lisi. Differences cannot be escaped by the naked eye.

ANOVA revealed significant differences among materials in contrast ratio, p < 0.0001 (Figure 4). Amber Mill was the least opaque material at 63%, and Suprinity PC was the most opaque material at 93% (Figure 4).

FIGURE 4.

Contrast ratio by material, means (standard deviations). Statistically similar groups are linked by horizontal lines. Please see bar color key.

ANOVA revealed significant differences among materials in absolute light transmission, p < 0.0001 (Figure 5). IPS emax CAD was the most transparent glass‐ceramic at 35% light transmission, and Suprinity PC was the least transparent at just 17% light transmission, almost a two‐fold difference (Figure 5).

FIGURE 5.

Absolute light transmission by material, means (standard deviations). Statistically similar groups are linked by horizontal lines.

4. Discussion

A variety of lithia‐based glass‐ceramic materials differed substantially in their elemental compositions, as‐fabricated surface morphologies, as‐fabricated flexural strengths, and in a wide range of optical properties. Statistical analyses revealed highly significant differences among materials for many of the parameters tested (Figures 2, 4 and 5). However, visualization of the effect sizes may be more informative of expected clinical relevance (Tables 2, 3, 4, Figures 2, 4 and 5). In addition, qualitative insights can be gained from Figure 3 and Table 4.

Not only have manufacturers availed themselves of differing compositions (Table 2), but also of differing processing and fabrication methods (Table 1). Some materials are already fully crystallized, to be milled, finished, and delivered. One fully crystallized material requires additional heat treatment after milling. Other partly crystallized materials need to undergo additional crystallization after milling. The major commonalities among the compositions of these lithia‐based materials are that they contain relatively high amounts of Li. Concentrations of the minor constituents tend to have increased variability, and the trace elements exhibit the highest variability (Table 2).

Although Fe is sometimes used to attain dentin‐like shades in dental ceramics, it was detected using ICP‐OES in one material tested, but by XRT in others, all at very low levels. Low concentrations of Tb were discerned in all materials; Tb(III) oxide is used as a dopant in transparent glass ceramics and in fluorescent glazes, but is more commonly used in solid‐state or fluorescent devices. Low molar concentrations of Tb₄O₇ have previously been reported for a variety of lithia‐based materials in an XRF study [19]. The presence of substantial concentrations of Na, P, Mg, K, Al, Zr, and Y was unsurprising in these glass‐ceramic materials. Other metals such as Er, B, Ce, V, Pr, Ti, and Ge were identified. The compositional findings were consistent with previously published data insofar as the methodologies used and materials included allowed comparison [16, 34].

It must be remembered that these lithia‐based systems all have SiO₂ as their dominant chemical component (~ 50%–75%), followed by LiO₂ (~ 25%–35%) [16, 19]. Moreover, these materials comprise at least 6 major crystalline phases within glassy matrices. Thus, their complexity derives from both their elemental constituents and their microstructural organization, which is heavily influenced by processing and, for some materials, by post‐milling crystallization [16, 34]. Further insights into phase content may be gained through thermal analyses, differential scanning calorimetry, and X‐ray diffraction to quantify phase content [19, 34]. Efforts were made to quantify the metal oxide species present in these materials using XRF; however, in complex systems, the best statistical fits can produce nonsensical results [26]. Furthermore, detailed local microstructural examination will be necessary. Nonetheless, strength data (Figure 3) is consistent with the idea that partly crystallized blocks undergo both more crystallization and less surface damage (Figure 2) than fully crystallized blocks.

SEM is frequently used to examine microstructure; however, in this study, the focus was on machining damage on the intaglio or tensile surfaces of milled restorations from which mechanical failure typically occurs. At low magnification (100×), all materials exhibited surfaces reminiscent of neatly tilled fields; parallel rows of bur marks were evident (Figure 1). In this study, burs were discarded as soon as they reached 50% of their surface lives. Overuse of burs would likely decrease efficiency and further increase machining damage. At moderate magnification (500×), differences in surface texture among materials emerged. At high magnification (3000×), differences among materials were defined. Most materials exhibited jagged fracture surfaces. However, one material, IPS Emax CAD, differed from all others; it exhibited a smooth, gently wavy surface. Another material, Suprinity PC, exhibited a morphology intermediate between those of IPS Emax CAD and the other materials.

Surface morphology was broadly predictive of flexural strength (Figures 1 and 2). IPS Emax CAD and Suprinity PC were the strongest and second strongest materials, at 340 and 288 MPa, respectively (Figure 2). Whereas, most of the other lithia‐based materials slotted into a narrow strength range between 130 MPa and 153 MPa.

The third strongest material, Amber Mill, a partly crystallized block, was substantially stronger than its fully crystallized block sibling, Amber Direct (Figure 2). No obvious difference in surface morphology distinguished these stablemates. However, milling before full crystallization may reduce subsurface machining damage. Also, subsequent crystalline growth and the effect of heat on the glassy matrix may mitigate subsurface damage. Cerec Tessera underwent what its manufacturer described as a glazing heat treatment, but this did not produce discernible differences in surface morphology or strength in comparison to its peers.

Strength limitations have constrained the broader clinical application of lithia‐based glass ceramics. However, the materials evaluated in this study, which completed their crystallization after milling, IPS Emax, Suprinity PC, and Amber Mill, exhibited flexural strengths approximately twice those of the other lithia‐based materials (Figure 2). It is important that dentists appreciate the value of post‐milling crystallization block systems.

The strength data reported in this study were broadly consistent with, or substantially lower than that for, previously published data for the same materials; however, specimen surface preparation generally differed [17, 18, 21]. A few papers specifically demonstrated the importance of machining damage in limiting the strength of lithia‐based glass‐ceramic materials; reductions in strength from the polished state of 33%, 45%, and 55% have been reported [17, 22, 23].

Recently, several high‐translucency 5‐Y zirconia ceramic blocks, designed for chairside milling, have been introduced to the marketplace. These new zirconia products may provide a competitive alternative in terms of physical properties and cost to lithia‐based glass‐ceramic materials. Unlike the control materials used in this study, they have the potential to provide superior mechanical properties. Comprehensive evaluation and comparison are recommended.

Dentists certainly appreciate the importance of fidelity in shade selection and delivery. Data from a wide range of CIE‐related parameters indicated that the manufacturers had different interpretations of shade execution, even when the same nominal Vita classic shade, A2, had been used throughout (Table 4). Substantial differences were found among L* parameters, which represent the brightness or darkness of a shade on a 0–100 scale value, a critically important clinical factor. Hue‐related parameters demonstrated some substantial differences among materials. Differences in the red‐green axis, a*, and in the yellow‐blue axis, b*, were large enough to be visually obvious [31, 35]. Substantial differences were also discerned in C*, chroma, saturation, and in hue angle, h^o, a representation of wavelength. The CIE‐based whiteness index, designed to relate to people's perception of whiteness, the absence of color, likewise revealed substantial differences among materials [29, 31].

Translucency is critical to achieving a natural appearance of an artificial restoration. Contrast ratio is determined by separately measuring CIE color and luminance parameters of a single specimen over black and white backgrounds; thus, it is an indirect relative measure of opacity [30]. Contrast ratios varied substantially among materials (Figure 4). Likewise, absolute light transmission varied substantially among materials (Figure 5). Although contrast ratio and absolute light transmission are obtained through very different methodologies, their data demonstrated a fair level of complementarity (Figures 4 and 5). Differences in contrast ratio and in absolute light transmission were sufficient to have a substantial clinical impact. Notably, the strongest material, IPS Emax, exhibited the highest light transmission and the second‐lowest contrast ratio.

De vitrum‐crystallum, caveat emptor.

5. Conclusions

Within the limitations of this current study, it was concluded that a variety of commercially available high‐strength lithia‐based glass‐ceramics designed for chairside milling differed substantially in their elemental compositions, as revealed by ICP‐OES; in their elemental oxide compositions, as revealed by XRF; in their as‐fabricated surface morphologies, as revealed by SEM; in their flexural strengths, as revealed by mechanical testing; and in a range of optical properties, as revealed by spectroscopy and transmission testing, even though they were all nominally of the same shade and translucency. The milling of partly crystallized blocks, followed by additional crystallization, almost doubled flexural strength values in comparison to the milling of fully sintered blocks or of the non‐lithia‐based control materials.

Conflicts of Interest

The authors declare no conflicts of interest.

Maharishi A., McLaren E. A., and White S. N., “Characterization of Lithia‐Based Machinable Glass‐Ceramic Materials,” Journal of Esthetic and Restorative Dentistry 38, no. 3 (2026): 530–538, 10.1111/jerd.13489.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author.