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Background:
Systematic Review

Thermal Influence on the Mechanical Properties of CAD/CAM Ceramics: A Systematic Review

by
Andrea Ordoñez Balladares
1,2,3,
Cristian Abad-Coronel
4,5,*,
Patrícia Pauletto
6 and
Benjamín José Martín Biedma
3
1
Faculty of Dentistry, Universidad Bolivariana del Ecuador, Durán 092406, Ecuador
2
Faculty of Dentistry, Universidad de Guayaquil, Guayaquil 090514, Ecuador
3
Faculty of Dentistry, University of Santiago de Compostela, 15782 Galicia, Spain
4
Faculty of Dentistry, Universidad de Cuenca, Cuenca 010107, Ecuador
5
Faculty of Dentistry, Universidad San Francisco de Quito, Quito 170901, Ecuador
6
Faculty of Dentistry, Universidad de las Americas (UDLA), Quito 170517, Ecuador
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(12), 5188; https://doi.org/10.3390/app14125188
Submission received: 11 April 2024 / Revised: 28 May 2024 / Accepted: 29 May 2024 / Published: 14 June 2024
(This article belongs to the Special Issue CAD & CAM Dentistry)
Browse Figure
Versions Notes

Abstract

:
Aim: The influence of different heat treatments on the mechanical properties of CAD/CAM ceramics was assessed. Methodology: A systematic search in five databases and gray literature was performed. In vitro studies providing data about the influence of various thermal treatments on the mechanical properties of CAD/CAM ceramics were included. Results: Out of 6500 articles found, 15 were included for results analysis. In the disilicate group, e.max CAD showed the best results in flexural strength (FS) and Vita Suprinity in microhardness (MH) and fracture toughness (FT). In the zirconium oxide group, Incoris exhibited better performance in FS while Razor Zirconia demonstrated superior MH and FT. Eleven studies had low and four had a moderate risk of bias (RoB). Conclusions: Both reinforced vitreous ceramics and zirconia ceramics, when subjected to high temperatures for short periods of time, significantly improve their mechanical properties. Favoring the biomechanical behavior of restorations present in the oral cavity, they are always subjected to constant changes in temperature, forces of different nature, intensity, or direction, changes in acidity, presence of moisture, etc., which make this a difficult environment for their clinical survival.

1. Introduction

In recent years, metal-free dental ceramics with high strength have gained popularity due to the fact that current restorative dentistry focuses on treatments that are conservative, functional, and long-lasting, always seeking to achieve optimal prosthetic–periodontal integration [1,2,3]. This has led to a constant search for restorative materials that are biocompatible with soft tissue and possess high aesthetic value [4,5]. Two major categories of such materials are feldspathic ceramics with crystalline reinforcement [6] and zirconium oxide materials [7].
Chemically, silicate-reinforced crystalline ceramics consist of a crystalline phase (lithium disilicate and lithium orthophosphate). The mechanical strength of lithium silicate ceramics is increased due to the homogeneous dispersion of round and submicrometer elongated metasilicate grains. Usually, tetragonal zirconia fillers are added to them [8], with the aim to increase the strength values. This structural typology has been developed in order to combine favorable optical properties with improved mechanical characteristics [9,10].
This set of ceramics possesses exceptional and highly beneficial mechanical properties, making them primarily ideal materials for plural fixed prostheses (PFPs) in both natural teeth and implants [11].
Zirconium dioxide, a polycrystalline ceramic endowed with attributes of polymorphism and allotropy, exhibits an unstable nature as it, under thermal influence, undergoes three distinct phases. Up to 1170 °C, it is in the monoclinic phase; between 1170 °C and 2370 °C, it adopts the tetragonal phase; and above 2370 °C up to its melting point, it transforms into the cubic phase [12].
There are different generations of zirconium according to the amount of additives or dopants incorporated [13]. The most common variant is that partially stabilized with yttrium oxide (Y2O3), characterized by low porosity and high density [14]. Restorations fabricated with zirconium dioxide are distinguished by their remarkable mechanical properties, which, depending on the generation (e.g., 1 and 2), significantly outperform silica-based ceramics [15]. The flexural strength of conventional yttria-stabilized polycrystalline tetragonal zirconia (3Y-TZP) ranges from 1000 to 1500 MPa [16].
Early versions of zirconia were opaque, intended for use as copings and frameworks that required feldspathic ceramic veneering to enhance translucency. However, these experienced low success rates due to ceramic detachment, a situation also observed in metal–ceramic restorations. Hence, current trends favor monolithic restorations [17,18], which enhance translucency through slight adjustments in the Y2O3 content (between 4 and 5 mol%) instead of the standard 3 mol%, resulting in a higher presence of cubic-phase particles [19]. Zirconia with a more cubic phase offers greater light transmission but exhibits lower bending strength values than conventional zirconia, ranging from 550 to 800 MPa, limiting its widespread application for PFPs [14,20,21].
Both silicates and zirconium oxides can undergo additional heat treatment (AHT), a decisive and crucial procedure that not only determines their final geometry but also enhances their mechanical and optical characteristics. The lack of control over sintering or crystallization parameters can compromise the microstructure and inherent properties of the material. Achieving a fine-grained and homogeneous geometry, along with high density, ensures optimal biomechanical performance of the material [22,23]. Additionally, AHT plays a key role in repairing microcracks originating during milling processes and facilitates the transformation of a pre-crystallized metasilicate, which is intrinsically weak, into a strong and tough lithium disilicate [24,25].
Due to the importance of AHT in CAD/CAM ceramic materials, companies have developed a variety of furnaces with specific protocols adapted to different thermal units. Their purpose is to optimize time and save energy, both in the clinical (“chairside”) and laboratory (“labside”) environments, aligning with the CAD/CAM philosophy of offering solutions that are easy, fast, and accurate [26,27].
Therefore, the objective of this systematic review is to address the research question: “How do heat treatments influence the mechanical properties of CAD/CAM ceramics?”

2. Materials and Methods

A systematic review was conducted following the guidelines outlined in “Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols” (PRISMA-P) [28]. The review protocol was registered in the Prospective International Register of Systematic Reviews (PROSPERO; Centre for Reviews and Dissemination, University of York), with the identification CRD42023474444. The report of this article adheres to the PRISMA checklist [29].

2.1. Eligibility Criteria

The central question was formulated following the participant; intervention; outcome; and studies (PICOS) structure as follows: Participant: CAD/CAM ceramics; intervention: Heat treatment; control: Not Applicable; outcome: Mechanical properties of CAD/CAM ceramic materials (flexural strength, fracture toughness, strength, and hardness); study design: In vitro.
The studies were meticulously selected for this review using specific inclusion and exclusion criteria. In vitro studies that provided quantitative data about the process underlying the influence of heat treatments (sintering and crystallization) on the mechanical properties of CAD/CAM ceramics were included.
The exclusion criteria were as follows: (1) Studies lacking separate quantitative data on the thermal influence on the mechanical properties of CAD/CAM ceramics; (2) studies omitting mention/specification of the types of heat treatments applied to CAD/CAM ceramic materials; (3) studies exclusively involving CAD/CAM resin materials; (4) studies encompassing cemented ceramic restorations; (5) studies concentrating on any other conditions, such as cavity preparation designs, surface analysis with tribochemical treatments, and physical property analysis; (6) studies that did not incorporate monolithic materials; (7) studies with duplicate data from another included study; (8) reviews, letters, books, conference proceedings, case–controls, case reports, case series, opinion articles, technical articles, posters, and guidelines; and studies with duplicate data from another included study; and (9) full text not available, even after attempting to contact the corresponding authors (three attempts within a 3-week period).

2.2. Information Sources and Search Strategy

An electronic bibliographic search strategy was designed and implemented on 7 October 2023. This search covered five databases: Embase, Scopus, LILACS (in Spanish: Literatura Latinoamericana y del Caribe en Ciencias de la Salud), Web of Science, and PubMed. Additionally, an exploration of the gray literature was carried out using Google Scholar and ProQuest Dissertations & Theses Global.
Zotero, a reference management software, version. 6.0.37 (developed by the Corporation for Digital Scholarship and the Roy Rosenzweig Center for History and New Media, George Mason University, North Virginia, Washington DC, USA), was employed to organize references and eliminate duplicates.

2.3. Selection Process

Three independent reviewers conducted the selection in a two-phase process. In both phases, the three reviewers (CAC; AOB; PP) assessed the references based on eligibility criteria. They initially reviewed titles and abstracts (Phase 1) and later examined the full-text studies (Phase 2). Any discrepancies were resolved through consensual discussion among the three reviewers. To streamline the selection process, the Rayyan software from the Qatar Computing Research Institute, Data Analytics, Doha, Qatar, was utilized.

2.4. Data Collection Process and Data Items

Data were collected by the first (AOB) and reviewed by the second and third (CAC; PP) authors. The information from the included studies was classified as follows: Authors, year of publication, total samples, CAD/CAM ceramics (type, trade name, and manufacturer), heat treatment (furnace used, manufacturer, time, and units); mechanical properties (flexural strength (FS), compressive strength (CS), microhardness (MH), fracture toughness (FT), and test type), conclusions, funding, and conflicts of interest.

2.5. Study Risk of Bias Assessment

The risk of bias of the in vitro studies was assessed using the QUIN tool [30]. Three authors (CAC; BMB; PP) reviewed the included studies and assessed the following criteria established by the tool: Clearly stated aims/objectives; detailed explanation of sample size calculation; detailed explanation of sampling technique; details of comparison group; detailed explanation of methodology; details of the operator; randomization; method of outcome measurement; details of outcome assessor; blinding; statistical analysis; and presentation of results. As shown in Table 1, each criterion was rated with the following possibilities: Adequately specified (score 2), inadequately specified (score 1), not specified (score 0), and not applicable. The studies were classified as high, medium, or low risk of bias (>70% = low risk of bias, 50% to 70% = medium risk of bias, and <50% = high risk of bias). Final score = (total score × 100)/(2 × number of criteria applicable).
The quality of the in vitro studies was assessed using the QUIN tool [30], consisting of 12 items with scoring and rating options, allows investigators to assess the quality of in-vitro studies. Three authors (CAC; BMB; PP) reviewed the included studies and assessed the following criteria set by the tool. Of the included studies, only 11 had a low RoB [31,32,35,36,37,38,41,42,43,44,45], while 4 had a moderate RoB [33,34,39,40].

2.6. Effect Measurements and Synthesis Methods

The extracted data were synthesized descriptively.

3. Results

3.1. Selection of Studies

From a total of 6500 references identified in the five databases, 3221 were considered. After eliminating duplicate records in phase 1 of study selection, 58 articles went on to phase 2 of full-text reading (Figure 1).

3.2. Characteristics of the Studies

The 15 studies included in this analysis were in vitro in nature and were published in the period from 2016 to 2023. In classifying the studies, two main groupings of ceramic materials were made: The group of ceramics with silicate crystalline reinforcement and that of zirconium dioxide. Then, a detailed organization was carried out, identifying the results of the mechanical properties analyzed under different time protocols in their crystallization or sintering. Each property was broken down by the authors of the studies, ceramic/trade name, furnace/trade name, temperature, time, the results obtained, and the standard deviation associated with each mechanical property analyzed. It is important to note that the sample sizes of the ceramics with crystalline reinforcement with silicate and zirconium oxide varied in a range of 10 to 50 units in the studies considered.

3.3. Evaluation of Results

3.3.1. Crystalline Silicate-Reinforced Ceramics

The FS data for the time interval from 2–5 min are presented in Table 2. Specifically, it is highlighted that at a temperature of 820 °C for 4 min and 30 s, the Celtra Duo ceramic showed outstanding FS performance [32].
IPS e.max CAD demonstrates superior FS performance when using a temperature of 840 °C for a period of 7 min [34], as shown in Table 3.
Analyzing Table 4 reveals that increasing both time and heat units results in slightly lower mechanical performance for e.max CAD [33] than the one presented in Table 3.
On examining the MH results presented in Table 5, it is observed that Vita Suprinity exhibited higher hardness. This performance was achieved using an Ivoclar Vivandent furnace and applying a heat treatment of 840 °C for 8 min [39].
From Table 6, it is evident that the Vita Suprinity ceramic demonstrated higher FT when the manufacturer’s recommended furnace was utilized, with a heat treatment of 840 °C applied for 8 min, in comparison to Celtra Duo [39].
It is important to note that only one study focused on evaluating the MH and FT of the Celtra Duo material. This study utilized a treatment time of 1.5 min at a temperature of 820 °C, using a Kota Evo furnace (Brazil, São Paulo, Rua Iris Memberg-Dorjen Company). The results obtained were as follows: A microhardness of 6.78 ± 0.18 GPa and a fracture toughness of 1.40 ± 0.12 KIC [40].

3.3.2. Zirconium Oxide Materials

As can be seen in Table 7, the results of three studies were analyzed, revealing that Dentsply Sirona’s InCoris TZI exhibited remarkably high FS values, reaching 1.183 ± 204 MPa, when adhering to the manufacturer’s recommended treatment [37].
In Table 8, a comparative analysis reveals that the Katana STML block (Kuraray, Noritake, Japan) achieved significantly higher ranges in FS when using the SpeedFire oven (Dentsply Sirona) at a temperature of 1600 °C for 30 min [34].
When examining the results detailed in Table 9, it is highlighted that Razor Zirconia from the commercial company U&C International exhibited superior values in FS, reaching 659 ± 39 MPa, compared to IPS e.max ZirCAD from Ivoclar Vivadent [41,42].
Three studies [31,34,43] analyzed the thermal influence on FS over a range of 7 to 8 h at 1550 °C. The results indicate that Zpex Smile from TOSOH outperformed Katana STML Block/Kuraray Noritake and Prettau Anterior/Zirconzahn, as presented in Table 10.
Regarding MH and FT, a study evaluated the Razor Zirconia ceramic from the manufacturer U&C International by applying different heat units. The results favored the 1430 °C condition [42], detailed exhaustively in Table 11 and Table 12.

4. Discussion

The present systematic review encompasses data from 15 studies investigating the impact of heat treatment on the mechanical properties of CAD/CAM ceramics. For analysis, these studies were categorized into two groups and further subdivided based on FS, MH, and FT, considering various times and units of heat application for a systematic interpretation of the results. Within the silicate ceramics group, e.max CAD demonstrated exceptional performance in FS, while Vita Suprinity exhibited good performance in MH and FT. In the zirconium oxide group, Incoris excelled in FS, and Razor Zirconia delivered a solid performance in MH and FT.
Silicates/disilicates must crystallize in a sintering furnace. Multiple in vitro studies and clinical trials claim that heat treatments are favorable because they improve optical characteristics and optimize mechanical properties, especially for single-unit prosthetic restorations [46,47,48,49]. However, the type of furnace, time, and heating units significantly influence the results [31,32,33,34,35,36,37,38,39,40,41,42,43,44,45].
For example, e.max CAD in the metasilicate state presents an FS of 130–150 MPa, an FT of 0.9 to 1.25 MPa-m1/2, and an MH of 5400 MPa. A heat treatment protocol that generally ranges from 800 °C to 870 °C, for 5–30 min, achieving complete crystallization, significantly improves its mechanical strength, also determining its final color [50,51].
In the context of the present SR, it is observed that four studies focused on evaluating the FR of e.max CAD, applying various heat treatment protocols [35,36,37]. Of particular note is the study by Lawson et al. [34], which exhibited the highest values, achieving an FS of 471.22 ± 87 MPa at a temperature of 840 °C for 7 min using the Ivoclar Vivadent Programat P500 furnace. These findings could be attributed to the kinetics of the phase transformation. Previous studies [52,53,54] indicate that the crystallization of metasilicate (Li2SiO3), which exhibits reduced mechanical properties, such as an FS of 130–150 MPa and an FT of 0.9–1.25 MPa m1/2 [55], to reach the pentasilicate phase (Li2SiO25) starts at approximately 820 °C in 9 min in systems with similar composition. In this temperature and time range, both the diffusivity and the remaining glass composition undergo changes, which complicates the formation of Li2Si2O5 crystals. Although the researchers evidenced a microstructural change in the glass ceramic material, short durations at 820 °C were insufficient for the growth of elongated lithium disilicate crystals. However, increasing the temperature to 840 °C for 7 min resulted in pronounced crystal growth as observed in the anisotropic pentasilicate with a high aspect ratio, interlocked in the glassy matrix and randomly oriented [35]. This significantly improved its mechanical properties to an FS of 360 MPa and an FT of 2.25 MPa m1/2 [56].
In support of these findings, the study by Bellini et al. (2022) provides additional evidence, concluding that a temperature of 850 °C for 10 min results in an RF of 443.84 MPa [57]. These results are at variance with those found by Juntavee et al. [37], who conducted a study on Vita Suprinity, stating that the crystallization process, using the Ivoclar Vivadent Programat 310 Furnace at a temperature of 840 °C for 8 min, achieves an FS of 267.15 ± 32.7 MPa. Likewise, another investigation [45] opted for a prolonged heat treatment approach, 25 min at 850 °C, obtaining significantly lower FS results of 134.7 ± 29.5 MPa. This discrepancy highlights the importance of material sensitivity to variations in the thermal protocol, suggesting that indiscriminate increases in time and temperature do not necessarily guarantee improvements in desired mechanical properties. This was demonstrated by Ordonez et al. who analyzed and compared in a recent study the FS of four CAD/CAM glass ceramics, showing that the AHT of CEREC Tessera at 760 °C for 2 min obtained the best results of 437,462 ± 69.17 MPa [58].
Celtra Duo (Dentsply Sirona, Germany), according to the manufacturer, presents an initial flexural strength of 210 MPa. Once subjected to an AHT of 840 °C for 8 to 10 min, its FS can increase to 379 MPa [53]. The results presented by Riquieri et al. [38] reported that by using Dentsply Sirona’s Multimat furnace at a temperature of 830 °C for 10 min, an FS of 251 ± 0.59 MPa was obtained. In a similar vein, a study conducted by F. Schweitzer et al. [36] indicates that using a different furnace (DEKEMA, Austromat 624i) at a temperature of 820 °C for 7 min yields similar results of 252 ± 53.78 MPa.
The variability in results could be attributed to differences in heat treatment protocols, including temperature and exposure time. It is evident that factors such as the choice of furnace and the specific conditions of the thermal process can significantly influence the mechanical properties of the ceramic. These findings underscore the importance of precise control of thermal parameters to achieve the desired properties in ceramic restorations.
In relation to MH and FT, several researchers opted for different heat treatment protocols [38,39,40] with different furnaces. However, the most outstanding results were observed when using the Programat Ep 510 furnace (Ivoclar Vivadent) with MH values of 7.6 GPa and FT of 2.8 KIC, as observed in 5 and 6. Mavriqi et al. (2022) reinforce the findings presented by evaluating the mechanical properties of three varieties of ceramics. They conclude that, within a temperature range of 810 to 840 °C for 8 to 10 min, the Vita Suprinity ceramic exhibited specific values of 7.6 GPa for MH and 4.7 Pa m1/2 for FT [27]. On the other hand, Alkadi et al. determined and compared the FT between e.max CAD and Press using a furnace of the same commercial house (Programat P500; Ivoclar Vivadent AG) at 840 °C, following the manufacturer’s instructions. Their results were significantly lower for e.max CAD at 1.79 KIC than those presented in our SR [59].
In summary, the silicate-based group of ceramics, IPS e.max CAD, subjected to a 7 min heat treatment at 840 °C in an Ivoclar Vivadent furnace, showed superior mechanical parameters. In contrast, Vita Suprinity showed better MH and FT when treated in the same furnace, albeit with an extended time of 8 min.
From a mechanical standpoint, diverse outcomes have been observed concerning the influence of temperature and sintering time on the properties of zirconium oxide, particularly in type 3Y-TZP. A study observed that a substantial increase in temperature and sintering time did not yield statistically significant differences in MH or FS. Nevertheless, these parameters did notably enhance the translucency and color rendering fidelity [60]. In contrast, another investigation yielded different results. For example, it was identified that the grain size in 3Y-TZP zirconia increases as the sintering temperature rises, reaching its maximum at 1700 °C. In addition, a significant negative correlation was established between sintering temperature and FS, as well as the contrast ratio (p < 0.001). This relationship is attributed to alterations in the grain structure, which could lead to a decrease in FS [61]. The results found by Killinc et al. evidenced a lower FS 480 ± 111 MPa in e.max ZirCAD Ivoclar Vivadent using an AHT of 1550 °C for 2 h with a furnace of the same commercial company [41].
Interestingly, Ersoy et al. proposed a different perspective, suggesting that a combination of high sintering temperature with reduced time can enhance the FR of zirconium [62]. Although previous research on 5Y-ZP zirconia has not shown significant effects on FR with high-speed sintering schedules [63], this finding aligns with results obtained for the Katana STML Block using fast sintering protocols [34]. In the context of this RS, positive results on FS were achieved in a key study by Juntavee et al. in 2020, reaching an FS of 1183 ± 204 MPa for InCoris TZI/Dentsply Sirona. They used a specific heat treatment of 1510 °C for 15 min with a furnace of the same brand [37]. This theory is supported by a study that evaluated FR through a compressive test on InCoris TZI from Dentsply Sirona, employing two different heat treatments: One with the InFire furnace at 1650 °C for 8 h and another with SpeedFire at 1600 °C for 18 min. The results favored SpeedFire, registering a value of 1222.80 ± 136.90 N [44].
In contrast, an analysis by Know et al. [42] on Razor Zirconia from U&C International showed an FS of 304 ± 21 MPa with an AHT 1230 °C protocol for 2 h, indicating a considerable difference. This contrast can be attributed to variations in zirconia composition, specifically its microstructure, and the importance of following the manufacturer’s recommendations regarding equipment and heat treatment protocols. Coinciding with previous trends, it is observed that the FR of 5Y-ZP is approximately half that of 3Y-TZP [64,65]. Furthermore, an additional study by Juntavee et al., in 2022, analyzed the impact of post-sintering processes on the FR of different monolithic zirconium oxides. The results revealed significant differences: For classically glazed (CzAG) zirconium oxide, the mean ± standard deviation value of σ (MPa) was 1626 ± 184, while for those with high translucency (HzAG), it was 671 ± 96 [66].
In a complementary investigation, the same authors conducted a study in 2021 that focused on evaluating the impact of post-sintering processes on FT. The results indicated that CzAG presented values of 5.40 Gpa MH and 7.27 KIC FT, while HzAG recorded notably higher values of 5.53 Gpa MH and 10.91 KIC for FT [67]. These observations disagree with the data obtained by Kwon et al. in 2023, providing relevant information by performing a comparative analysis on various thermal effects over a 2 h period for Razor Zirconia/U&C International. Specifically, when applying a temperature of 1430 °C, values of 9.18 GPa MH and 6.28 KIC FT were obtained [42]. However, they are consistent with the observations of Guazzato et al. [68], who, when studying 3Y-TZP zirconia-based materials at 1450 °C for 1 h, found values varying between 4.8 ± 0.5 KIC and 7.4 ± 0.6 KIC for FT and between 11 ± 0.9 GPa and 13 ± 0.3 GPa. These data reaffirm the relevance of considering multiple factors, including material specifications, when evaluating mechanical properties in dental ceramics.
Ultimately, Incoris TZI excelled with short sintering cycles of 15 min at elevated temperatures without compromising the mechanical properties of the material. However, although U&C International’s Razor Zirconia showed higher MH and FT, it used longer cycles.
It is important to highlight that the analysis of the mechanical properties of ceramic materials is also directly related to the design of the dental preparation and crown preparation, milling of the block, type of cementation, and mechanical forces that occur during mastication that can lead to catastrophic failure in the short term. For this reason, they are variables that cannot be isolated and should be evaluated [69,70,71]. For example, it has been reported that the occlusal force will depend on age, gender, and the strategic position of the teeth in the dental arch [72], marking a great difference in values [73]; however, a dental restoration should support 2000 N, considering the occlusal force in parafunction [74]. Several studies have shown that the chemical interaction that occurs in the cementation strategy significantly increases the mechanical behavior of ceramic restorations [75,76,77].
Occlusal morphology also has a significant relevance in the FR of CAD/CAM ceramics. A study by Passos et al. investigated the RF of ceramic crowns in the posterior sector with and without manual enhancement of occlusal morphology (MEOM). The results indicated that performing MEOM resulted in a significant decrease in the RF of ceramics and is, therefore, considered detrimental and should be avoided [78].
Time optimization and energy savings are crucial factors in CAD/CAM technology, which is why furnace manufacturing companies have developed equipment that follows this philosophy. However, despite the diversity of heat treatment protocols available, our systematic review revealed that there are specific protocols for sintering and crystallization of CAD/CAM ceramic materials that can significantly improve mechanical properties. It is important to consider the microstructure of these materials, and it is essential to follow the manufacturer’s indications to avoid alterations, optimizing the biomechanical behavior of the restorations with clinical longevity as the oral cavity is influenced by various factors [79].
As outlined in the methodology, QUIN tools, tools specifically designed for assessing quality and the risk of bias in in vitro dental studies, were employed [31]. These tools constitute a fundamental basis for clinical decision making in evidence-based dental practice [80,81]. It is relevant to note that question 10 could not be considered as it was not applicable to the studies included in our SR. This may be attributed to the awareness among researchers in in vitro studies examining the effects of AHT on the mechanical properties of CAD/CAM ceramics. They understand the importance of adhering to the protocols recommended by the material’s manufacturer to avoid alterations in microstructure or optical properties [44,82,83].
Finally, it is important to highlight the limitations inherent to this SR. A notable heterogeneity was observed in the selected studies, attributable to the diversity of furnaces, commercial brands, times, and heat units used. This variability was also reflected in the mechanical properties analyzed, preventing the performance of a consolidated meta-analysis. Additionally, some excluded studies had discrepancies in the sample or in the presentation of data. For example, certain studies did not segment the data related to the impact of AHT on mechanical properties, while others included additional variables such as adhesion to core build-up or were limited to CAD/CAM resins. The latter generated confusion which justified their exclusion from this SR.
Research that focused on different conditions, such as cavity preparation designs or surface analysis with tribochemical treatments, was also omitted. Studies not focusing on monolithic materials were excluded. Therefore, we emphasize the imperative need for more rigorous investigations, with more representative samples and standardized protocols, to better understand the effect of AHT on the mechanical properties of CAD/CAM ceramic materials. This methodological clarity will guide not only clinicians but also technicians in fabricating ceramic restorations.

5. Conclusions

It can be concluded that both reinforced glass ceramics and zirconium oxide ceramics, when subjected to high temperatures for short periods of time, significantly improve their mechanical properties, favoring the biomechanical behavior of the restorations, since they are subjected to constant changes in temperature, forces of different nature, intensity, or direction, changes in acidity and presence of acidity, and presence of humidity, among other factors that make this environment difficult for their clinical survival.
This finding highlights how crucial the post-production processes of CAD/CAM ceramic restorations are. It is important to note that further research, such as non-randomized controlled trials and retrospective and prospective clinical studies, is needed to establish more conclusive results.

Author Contributions

Conceptualization: A.O.B., C.A.-C., P.P. and B.J.M.B.; writing and preparation of the original draft: A.O.B. and P.P; writing, revising, and editing: A.O.B., C.A.-C., P.P. and B.J.M.B.; supervision: C.A.-C.; fundraising: A.O.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Universidad Bolivariana del Ecuador 045010125.

Acknowledgments

To the Universidad Bolivariana del Ecuador. The authors thank Karyn Munik Lehmkuhl for her support with the search strategies.

Conflicts of Interest

The authors declare that they have no conflicts of interest. The funders had no role in the design of the study; in the collection, analysis, and interpretation of the data; in the writing of the manuscript; or in the decision to publish the results.

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Applsci 14 05188 g001
Figure 1. Flow Diagram of Literature Search and Selection Criteria.
Figure 1. Flow Diagram of Literature Search and Selection Criteria.
Applsci 14 05188 g001
Table 1. Scoring sheet for Quality Assessment Tool for In Vitro Studies (QUIN) Tool.
Table 1. Scoring sheet for Quality Assessment Tool for In Vitro Studies (QUIN) Tool.
No.CriteriaKongkiatkamon S et al. 2022 [31]Romanyk DL et al. 2020 [32]Lu Y, Dal Piva AMO et al. 2023 [33] Lawson NC et al. 2020 [34]Simba B, Ribeiro M et al. 2019 [35]Schweitzer F et al. 2020 [36]Juntavee N, Uasuwan P. 2020 [37]Riquieri H, Monteiro JB et al. 2018 [38]Traini T, Sinjari B et al. 2016 [39]Alves M et al. 2019 [40]Kilinc H, Sanal FA et al. 2021 [41]Kwon WC et al. 2023 [42]Hatanaka GR, Polli GS et al. 2017 [43]Ordonez A, Abad C et al. 2022 [44]Aurélio IL et al. 2018 [45]
1Clearly stated aims/objectives222222222122222
2Detailed explanation of sample size calculation000000220122000
3Detailed explanation of sample technique222111120122212
4Details of comparison group222122221221222
5Detailed explanation of methodology222222221222222
6Operator details010000010000010
7Randomization222222122111222
8Method of measurement of outcome211122221222222
9Outcome assessor details100011001102111
10BlindingNANANANANANANANANANANANANANANA
11Statistical analysis222222222222222
12Presentation of results222222222222222
Score 17(100)/2216(100)/2215(100)/2213(100)/2218(100)/2216(100)/2216(100)/2219(100)/2212(100)/2215(100)/2217(100)/2218(100)/2217(100)/2217(100)/2217(100)/22
%77.27%72.72%68.18%59.09%81.81%72.72%72.72%86.36%54.54%68.18%77.27%81.81%77.27%77.27%77.27%
Table 2. FS results of the silicate ceramics group over a time range of 2–5 min.
Table 2. FS results of the silicate ceramics group over a time range of 2–5 min.
Flexural Strength (MPa) 2–5 min
AuthorsCeramics/ManufacturerHeat Teatment/Furnace ManufacturerTimeFS (MPa)FS
(MPa) SD
Romanyk DL et al. 2020 [32]Celtra Duo
Dentsply Sirona 		820 °C
Programat EP 5000 Ivoclar Vivadent		4:30 s22016
Lu Y, Dal Piva AMO et al. 2023 [33]CEREC Tessera
Dentsply Sirona		790 °C
Programat P100
Ivoclar Vivadent		2 min195 44
SD: Standard deviation; MPa: Megapascals; FS: Flexural strength.
Table 3. FS results of the silicate ceramics group in the time range 6–7 min.
Table 3. FS results of the silicate ceramics group in the time range 6–7 min.
FS (MPa) 6–7 min
AuthorsCeramics/ManufacturerHeat Teatment/Furnace ManufacturerTimeFS (MPa)FS
(MPa) SD
Simba B, Ribeiro M et al. 2019 [35]IPS e.max CAD
Ivoclar Vivadent		840 °C
Kota Press
Kota Evo		7 min303 58
Simba B, Ribeiro M et al. 2019 [35]IPS e.max CAD
Ivoclar Vivadent		820 °C Kota Press
Kota Evo		7 min 192 29
Lawson NC et al. 2020 [34]IPS e.max CAD
Ivoclar Vivadent		840 °C
Programat P500 Ivoclar/Vivadent		7 min47187
Schweitzer F et al. 2020 [36]IPS e.max CAD
Ivoclar Vivadent		840 °C Dekema Austromat 624i/Dekema6 min34451
Schweitzer F et al. 2020 [36]Celtra Duo
Dentsply Sirona 		820 °C
Dekema Austromat 624i/Dekema		7 min25253
Juntavee N, Uasuwan P. 2020 [37]IPS e.max CAD
Ivoclar Vivadent		850 °C Programat P310 Ivoclar/Vivadent6 min39237
SD: Standard deviation; MPa: Megapascals; FS: Flexural strength.
Table 4. FS results of the silicate ceramics group in a time range of 8–10 min.
Table 4. FS results of the silicate ceramics group in a time range of 8–10 min.
FS (MPa) 8–10 min
AuthorsCeramics/ManufacturerHeat Teatment/Furnace ManufacturerTimeFS (MPa)FS
(MPa) SD
Riquieri H, Monteiro JB et al. 2018 [38]Vita Suprinity
VITA Zahnfabrik 		840 °C Vita Vacumat 6000 MP Vita Zahnfabrik8 min191x
Riquieri H, Monteiro JB et al. 2018 [38]Celtra Duo
Dentsply Sirona 		830 °C Multimat
Dentsply Sirona		10 min251 x
Lu Y, Dal Piva AMO et al. 2023 [33]IPS e.max CAD
Ivoclar Vivadent		850 °C
Programat P100
Ivoclar Vivadent		10 min358 73
Juntavee N, Uasuwan P. 2020 [37]Vita Suprinity
VITA Zahnfabrik		840 °C Programat P310
Ivoclar Vivadent		8 min26732
SD: Standard deviation; MPa: Megapascals; FS: Flexural strength.
Table 5. MH results in the silicate ceramics group in a time range of 8–10 min.
Table 5. MH results in the silicate ceramics group in a time range of 8–10 min.
MH (GPa) 8–10 min
AuthorsCeramics/ManufacturerHeat Teatment/Furnace ManufacturerTimeVickers (GPa)Vickers (GPa) SD
Alves M et al. 2019 [40]Vita Suprinity
VITA Zahnfabrik		840 °C Kota Evo
Dorjen Company		8 min6.434
Alves M et al. 2019 [40]IPS e.max CAD
Ivoclar Vivadent		850 °C Kota Evo
Dorjen Company		10 min6.522
Traini T, Sinjari B et al. 2016 [39]Vita Suprinity
VITA Zahnfabrik		840 °C Programat EP 510/Ivoclare Vivadent8 min7.60.7
Riquieri H, Monteiro JB et al. 2018 [38]Vita Suprinity
VITA Zahnfabrik		840 °C Vita Vacumat 6000 MP Vita Zahnfabrik8 min6.816
Riquieri H, Monteiro JB et al. 2018 [38]Celtra Duo
Dentsply Sirona 		830 °C Multimat
Dentply Sirona		10 min6.910
SD: Standard deviation; GPa: Gigapascals; MH: Microhardness.
Table 6. FT results in the silicate ceramics group in a time range of 8–10 min.
Table 6. FT results in the silicate ceramics group in a time range of 8–10 min.
FT (MPa) 8–10 min
AuthorsCeramics/ManufacturerHeat Teatment/Furnace ManufacturerTimeFT
KIC		FT
KIC (SD)
Alves M et al. 2019 [40]Vita Suprinity
VITA Zahnfabrik		840 °C Kota Evo
Dorjen Company		8 min1.150.13
Alves M et al. 2019 [40]IPS e.max CAD
Ivoclar Vivadent		850 °C Kota Evo
Dorjen Company		10 min1.300.16
Traini T, Sinjari B et al. 2016 [39]Vita Suprinity
VITA Zahnfabrik		840 °C Programat EP 510
Ivoclar Vivadent		8 min2.80.9
Riquieri H, Monteiro JB et al. 2018 [38]Vita Suprinity
VITA Zahnfabrik		840 °C Vita Vacumat 6000 MP
Vita Zahnfabrik		8 min2.630.14
Riquieri H, Monteiro JB et al. 2018 [38]Celtra Duo
Dentsply Sirona 		830 °C Multimat
Dentsply Sirona		10 min2.51 0.59
SD: Standard deviation; FT: Fracture toughness.
Table 7. FS results of the zirconium oxide ceramics group in a time range of 15–18 min.
Table 7. FS results of the zirconium oxide ceramics group in a time range of 15–18 min.
FS (MPa) 15–18 min
AuthorsCeramics/ManufacturerHeat Teatment/Furnace ManufacturerTimeFS
(MPa)		FS
(MPa) SD
Lawson NC et al. 2020 [34]Katana STML Bloc Kuraray Noritake1600 °C SpeedFire Dentsply Sirona18 min859110
Kongkiatkamon S et al. 2022 [31]Katana STML Bloc Kuraray Noritake1560 °C SpeedFire Dentsply Sirona 15 min 500234
Juntavee N, Uasuwan P. 2020 [37]InCoris TZI
Dentsply Sirona		1510 °C Infire Dentsply Sirona15 min 1183204
SD: Standard deviation; MPa: Megapascals; FS: Flexural strength.
Table 8. FS results of the zirconium oxide ceramics group in a 30 min period.
Table 8. FS results of the zirconium oxide ceramics group in a 30 min period.
FS (MPa) 30 min
AuthorsCeramics/ManufacturerHeat Teatment/Furnace ManufacturerTimeFS
(MPa)		FS
(MPa) SD
Lawson NC et al. 2020 [34]Katana STML Bloc Kuraray Noritake1600 °C SpeedFire
Dentsply Sirona		30 min788 28
Lawson NC et al. 2020 [34]Prettau Previous
Zirconzahn		1600 °C SpeedFire
Dentsply Sirona		30 min 55746
Lawson NC et al. 2020 [34]TOSOH
Noritake KATANA 		1600 °C SpeedFire
Dentsply Sirona		30 min49365
SD: Standard deviation; MPa: Megapascals; FS: Flexural strength.
Table 9. FS results of the zirconium oxide ceramic group in a period of 2 h.
Table 9. FS results of the zirconium oxide ceramic group in a period of 2 h.
FS (MPa) 2 h
AuthorsCeramics/ManufacturerHeat Teatment/Furnace ManufacturerTimeFS (MPa)FS
(MPa) SD
Kilinc H, Sanal FA et al. 2021 [41]IPS e.max ZirCAD
Ivoclar Vivadent		1550 °C Programat p500
Ivoclar Vivadent		2 h480111
Kwon WC et al. 2023 [42]Razor Zirconia
U&C International 		1230 °C Zircom Plus, KDF
Denken High Dental 		2 h304 21
Kwon WC et al. 2023 [42]Razor Zirconia
U&C International		1330 °C Zircom Plus, KDF
Denken High Dental 		2 h57444
Kwon WC et al. 2023 [42]Razor Zirconia
U&C International		1430 °C Zircom Plus, KDF
Denken High Dental 		2 h659 39
Kwon WC et al. 2023 [42]Razor Zirconia
U&C International		1530 °C Zircom Plus, KDF
Denken High Dental 		2 h62317
SD: Standard deviation; MPa: Megapascals; FS: Flexural strength.
Table 10. FS results of the zirconium oxide ceramics group in a time range of 7–8 h.
Table 10. FS results of the zirconium oxide ceramics group in a time range of 7–8 h.
FS (MPa) 7–8 h
AuthorsCeramics/Manufacturer Heat Teatment/Furnace ManufacturerTimeFS
(MPa)		FS
(MPa) SD
Lawson NC et al. 2020 [34]Katana STML Bloc
Kuraray Noritake		1550 °C Noritake KATANA
Kuraray		7 h 761 6.80
Lawson NC et al. 2020 [34]Prettau Previous
Zirconzahn		1550 °C Noritake KATANA
Kuraray 		7 h 78750.48
Lawson NC et al. 2020 [34]Zpex Smile
TOSOH		1550 °C Noritake KATANA
Kuraray 		7 h 789 6.56
Hatanaka GR, Polli GS et al. 2016 [43]Lava Frame
3M		1500 °C Lava Furnace 200
Dekema Dental-Keramiköfen GmbH 		8 h642699.3
Kongkiatkamon S et al. 2022 [31]Katana STML Block
Kuraray Noritake		1550 °C Infire
Dentsply Sirona 		7 h 466 22.89
SD: Standard deviation; MPa: Megapascals; FS: Flexural strength.
Table 11. MH results in the group of silicate ceramics in a 2 h time period.
Table 11. MH results in the group of silicate ceramics in a 2 h time period.
MH (GPa) 2 h
AuthorsCeramics/Manufacturer Heat Teatment/Furnace ManufacturerTimeVickers
(GPa)		Vickers
(GPa) SD
Kwon WC et al. 2023 [42]Razor Zirconia
U&C International 		1230 °C Zircom Plus, KDF
Denken High Dental 		2 h7.3637.14
Kwon WC et al. 2023 [42]Razor Zirconia
U&C International		1330 °C Zircom Plus, KDF
Denken High Dental 		2 h9.0437.65
Kwon WC et al. 2023 [42]Razor Zirconia
U&C International		1430 °C Zircom Plus, KDF
Denken High Dental 		2 h9.1839.42
Kwon WC et al. 2023 [42]Razor Zirconia
U&C International		1530 °C Zircom Plus, KDF
Denken High Dental 		2 h8.3531.50
SD: Standard deviation; GPa: Gigapascals; MH: Microhardness.
Table 12. FT results in the group of silicate ceramics in a period of 2 h.
Table 12. FT results in the group of silicate ceramics in a period of 2 h.
FT (KIC) 2 h
AuthorsCeramics/Manufacturer Heat Teatment/Furnace ManufacturerTimeFT
(KIC) 		FT
(KIC) SD
Kwon WC et al. 2023 [42]Razor Zirconia/U&C International 1230 °C Zircom Plus, KDF/Denken High Dental 2 h4.99 0.14
Kwon WC et al. 2023 [42]Razor Zirconia/U&C International 1330 °C Zircom Plus, KDF/Denken High Dental 2 h6.28 0.27
Kwon WC et al. 2023 [42]Razor Zirconia/U&C International 1430 °C Zircom Plus, KDF/Denken High Dental 2 h6.48 0.31
Kwon WC et al. 2023 [42]Razor Zirconia/U&C International 1530 °C Zircom Plus, KDF/Denken High Dental 2 h6.09 0.20
SD: Standard deviation; FT: Fracture toughness.
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Balladares, A.O.; Abad-Coronel, C.; Pauletto, P.; Martín Biedma, B.J. Thermal Influence on the Mechanical Properties of CAD/CAM Ceramics: A Systematic Review. Appl. Sci. 2024, 14, 5188. https://doi.org/10.3390/app14125188

AMA Style

Balladares AO, Abad-Coronel C, Pauletto P, Martín Biedma BJ. Thermal Influence on the Mechanical Properties of CAD/CAM Ceramics: A Systematic Review. Applied Sciences. 2024; 14(12):5188. https://doi.org/10.3390/app14125188

Chicago/Turabian Style

Balladares, Andrea Ordoñez, Cristian Abad-Coronel, Patrícia Pauletto, and Benjamín José Martín Biedma. 2024. "Thermal Influence on the Mechanical Properties of CAD/CAM Ceramics: A Systematic Review" Applied Sciences 14, no. 12: 5188. https://doi.org/10.3390/app14125188

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