Physical and Mechanical Properties of 3D-Printed Provisional Crowns and Fixed Dental Prosthesis Resins Compared to CAD/CAM Milled and Conventional Provisional Resins: A Systematic Review and Meta-Analysis

Newly introduced provisional crowns and fixed dental prostheses (FDP) materials should exhibit good physical and mechanical properties necessary to serve the purpose of their fabrication. The aim of this systematic literature review and meta-analysis is to evaluate the articles comparing the physical and mechanical properties of 3D-printed provisional crown and FDP resin materials with CAD/CAM (Computer-Aided Designing/Computer-Aided Manufacturing) milled and conventional provisional resins. Indexed English literature up to April 2022 was systematically searched for articles using the following electronic databases: MEDLINE-PubMed, Web of Science (core collection), Scopus, and the Cochrane library. This systematic review was structured based on the guidelines given by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). The focused PICO/PECO (Participant, Intervention/exposure, Comparison, Outcome) question was: ‘Do 3D-printed (P) provisional crowns and FDPs (I) have similar physical and mechanical properties (O) when compared to CAD/CAM milled and other conventionally fabricated ones (C)’. Out of eight hundred and ninety-six titles, which were recognized after a primary search, twenty-five articles were included in the qualitative analysis, and their quality analysis was performed using the modified CONSORT scale. Due to the heterogeneity of the studies, only twelve articles were included for quantitative analysis. Within the limitations of this study, it can be concluded that 3D-printed provisional crown and FDP resin materials have superior mechanical properties but inferior physical properties compared to CAD/CAM milled and other conventionally fabricated ones. Three-dimensionally printed provisional crowns and FDP materials can be used as an alternative to conventional and CAD/CAM milled long-term provisional materials.

. Inclusion and exclusion criteria.

Inclusion Criteria Exclusion Criteria
Literature in English language Literature in a language other than English Human clinical studies Animal studies In vitro studies Letters to the editor, case reports, technical reports, cadaver studies, dissertations, incomplete trials, unpublished abstracts, reports, commentaries, and review papers. Studies comparing the physical properties of the 3D-printed provisional crowns and fixed dental prosthesis (FDP) materials with other materials and methods used for the fabrication of provisional crowns and FDP.
Studies comparing properties other than physical and mechanical properties.
Studies comparing mechanical properties of 3D-printed provisional crowns and FPD materials with other materials and methods used for the fabrication of provisional crowns and FPD.
Studies discussing properties of only 3D-printed provisional materials but do not compare them with other types of provisional materials Studies comparing accuracy, marginal, and internal adaptation of 3D-printed provisional materials with other types of provisional materials. Studies discussing effects of various 3D-printing parameters (printing orientation, resin color setting, layer thickness, degree of conversion, etc.) on mechanical properties and accuracy of 3D-printed crown and bridge provisional restorative material. Studies discussing materials under trial

Study Selection and Data Extraction
Duplicate articles were removed. The titles and abstracts of the identified articles were screened based on the pre-set inclusion and exclusion criteria (by S.G.G. and M.E.S.). Later, S.J. and M.S. cross-checked the shortlisted articles after reviewing the full texts, and disagreements related to conflicting articles were resolved after a discussion between the four authors (S.J., M.S., M.E.S., S.G.G.). S.J., M.S., M.E.S., A.A.A.O., and S.M.A. used self-designed tables to tabulate the relevant data. The information extracted was divided into two categories; Table 3 was a common table for all the selected articles giving infor-mation about the author's name, year of publication, study type, studied characteristic  and property, sample size, trade name and main composition of the evaluated materials,  specimen fabrication technique, shape and dimensions of the tested samples, and layer  thickness and orientation of the 3D-printing. Quality analysis results of the included studies  are listed in Table 4. Moreover, Tables 5-15 gave comprehensive information about each physical or mechanical property tested. Details in these tables were related to the exposure agent/aging technique, testing machine, results of the property tested for each type of material, and authors' conclusions and suggestions.

Quality Assessment of Included Studies
As all the selected studies were in vitro studies, so the Modified CONSORT scale for in vitro studies given by Faggion C. [33,34] was used to assess the quality of the included studies. The fourteen items included in this scale were as follows: Item 1: Structured abstract. Items 2a and 2b are related to the introduction. Item 2a: scientific background and explanation of rationale; Item 2b: Introduction should have specific objectives and/or hypotheses). Items 3 to 10 are related to Methodology. Item 3: intervention for each group; Item 4: Completely defined, pre-specified primary, and secondary measures of outcome; Item 5: sample size determination; Item 6: Method used to generate the random allocation sequence; Item 7: Mechanism used to implement the random allocation sequence; Item 8: Who generated the random allocation sequence; Item 9: If done, who was blinded after assignment to intervention and how; Item 10: Statistical methods used to compare groups for primary and secondary outcomes; Item 11: For each primary and secondary outcome, results for each group and the estimated size of the effect and its precision (for example 95% confidence interval); Item 12: Trial limitations; Item 13: Sources of funding and other support, role of funders; Item 14: Where the full trial protocol can be accessed, if available (Table 4).

Quantitative Assessment
Review Manager 5.4.1 was used to perform a Meta-analysis in Non-Cochrane Review mode [35]. Since all the physical and mechanical properties were measured and reported in studies on a continuous scale, inverse variance was used as the statistical method. The fixed-effect model was used under the assumption that all effect estimates are estimating the same underlying intervention effect. Since the measurement tools and scales varied among different studies, standardized mean difference was used. A 95% confidence interval was used to express the results of individual studies and the pooled result. Chi-square was used to measure heterogeneity, and a p-value < 0.05 was considered significant. I 2 was also calculated and reported in the results. Statistical significance was calculated for the overall effect; if p was less than 0.05, the null hypothesis was rejected.

Identification and Screening
This literature review compared the physical and mechanical properties of resins used for fabricating provisional crowns and FDPs by 3D-printing with those provisional resins used for CAD/CAM milling and other conventional techniques. For ease of understanding, the results of each physical and mechanical property were tabulated in separate tables (Tables 5-15).
Eight hundred and ninety-six titles were recognized from the primary search on the selected electronic databases. On checking, 107 titles were found to be duplicates and were excluded. After reviewing the titles and abstracts, 710 articles were rejected as they did not meet the inclusion and exclusion criteria. Full texts of the remaining 79 articles were reviewed, and secondary articles were searched manually from the references of these articles, but no more relevant articles were found. Out of the selected 79 articles, 15 were rejected, as they were discussing the properties of provisional 3D-printed resins without comparing them with CAD/CAM milled and other conventional provisional resins. Thirtyfour articles were rejected as they compared other properties (other than physical and mechanical), and four were rejected as they were comparing provisional 3D-printed resins with definitive restorative materials. Finally, one article was rejected as it discussed the properties of 3D-printed resins under the trial phase. Thus, 25 articles were finally included in this systematic review for qualitative analysis. Out of 25 articles, only 12 provided comparative data and were included for quantitative analysis (Figure 1).

Quality Assessment of Included Studies
All twenty-five studies included in this review were in vitro studies. A total of 221 out of 375 (58.93%) entries were positively reported. All studies reported items related to abstract, introduction, intervention, outcome, statistical method, and results (Items 1-4, 10, and 11). Fifteen studies addressed the trial limitations (Item 12) and provided information related to funding sources (Item 13). Only six studies mentioned the procedure of calculat-ing the sample size of the specimens (Item 5), while five studies gave details related to the accessibility of the full trial protocol (Item 14). Only four studies described the method used to generate random allocation sequence (Item 6), with one of them reporting the allocation concealment mechanism briefly (Item 7). Details related to the blinding of the examiners and the details of the researcher who generated the random allocation were not reported by any of the studies (Item 8 and 9) ( Table 4).

Study Characteristics
The majority of the studies (21 out of 25) included in this review were published between 2020 and 2022, while four were published between the years 2016 and 2019. All the included articles were in vitro studies. Nineteen articles analyzed and compared the mechanical properties, four analyzed physical properties, and two articles analyzed both physical and mechanical properties. Some of the studies focused on one particular character, while others studied multiple characteristics at the same time (Table 3).

Color Change
Five studies compared the change in the color values of 3D-printed interim resins with other materials (Table 5).
(i) Comparing the change in color values of MMA-based 3D-printed provisional resins: Three studies reported a greater change in the color values of MMA-based 3D-printed resins when compared to CAD/CAM milled PMMA resins [28,44,45].
Two studies provided data for the meta-analysis to compare color changes between 3D-Printed MMA Resins and CAD/CAM Milled PMMAs. There was a statistically significant heterogeneity between the studies, with I 2 = 94%. The results were inconclusive, favoring 3D-Printed MMA (p = 0.23) (Figure 2). (ii) Comparing the change in color values of hybrid composite-based 3D-printed provisional resins: Studies by Atria et al. [42] reported a greater change in color for hybrid composite-based 3D-printed provisional resins when compared to conventional bisacrylic and PMMA resins. On the contrary, Taşın et al. [48] and Song et al. [44] reported greater change in color for conventional resins. Compared to CAD/CAM milled PMMA resins, a greater change in color was reported in 3D-printed hybrid composite resins [15,19].
Two studies provided data for the meta-analysis to compare color changes between 3D-printed hybrid resin and conventional PMMA resin. There was a statistically significant heterogeneity between the studies, with I 2 = 96%. The results were inconclusive, favoring conventional PMMA resin (p = 0.40) (Figure 3).
Two studies provided data for meta-analysis to compare color changes between 3Dprinted hybrid resin and conventional bBis-acrylic resin. There was a statistically significant heterogeneity between the studies, with I 2 = 96%. The results were inconclusive, favoring conventional bBis-acrylic resin (p = 0.12) (Figure 4).

Water Sorption and Solubility
Two studies compared the water sorption and solubility of 3D-printed interim resins with other materials ( Table 6). The water sorption of 3D-printed PMMA resins was reported to be higher than conventional polycarbonate resins and lower than conventional PMMA resins [28]. For 3D-printed photopolymer resins, the water sorption was reported to be higher than conventional bis-acrylic and CAD/CAM milled PMMA resins and lower than conventional PMMA resins [44]. The solubility of the 3D-printed PMMA resins was reported to be higher than conventional polycarbonate and PMMA resins [28]. For 3D-printed photopolymer resins, the solubility was higher than conventional PMMA, conventional bis-acrylic, and CAD/CAM milled PMMA resins [44].

Fracture Strength
Eight studies analyzed and compared the fracture strength of 3D-printed resins with CAD/CAM milled and/or conventionally fabricated resins used for the fabrication of provisional crowns and FDPs (Table 7).
(i) Comparing the fracture strength of PMMA-based 3D-printed provisional resins: Three studies reported higher FS when compared to PMMA-based CAD/CAM milled resins [38][39][40]. One study reported contrasting results of lower FS when compared to PMMA-based CAD/CAM milled resins [50], and one study each reported higher FS when compared to conventional MMA [39] and bis-acrylic resins [40]. A study by Reeponmaha et al. [16] reported higher FS MMA-based 3D-printed resins when compared to PMMA-based CAD/CAM milled and conventional resins.
Five studies provided data for the meta-analysis to compare the fracture strength between 3D-printed PMMA resin and CAD/CAM milled PMMA resin. There was a statistically significant heterogeneity between the studies, with I 2 = 93%. The results were inconclusive, favoring 3D-printed PMMA (p = 0.18) ( Figure 5).
Two studies provided data for the meta-analysis to compare the fracture strength between 3D-Printed PMMA resin and conventional PMMA resin. There was a statistically significant heterogeneity between the studies, with I 2 = 87%. However, both the studies favored 3D-printed PMMA resin, and the 95% confidence interval did not include 0, i.e., no effect. Thus, the pooled estimate favored 3D-printed PMMA resin with p < 0.0001 ( Figure 6).  (ii) Comparing the fracture strength of bis-acrylic and other photopolymer hybrid 3Dprinted provisional resins: the FSs of 3D-printed bis-acrylic resin [48], micro-hybrid resin [46], photopolymer resin [52], and UDMA-based resins [50] were reported to be lower than PMMA-based CDA/CAM resins. A study by Henderson et al. [51] reported that bis-acrylic-based 3D-printed resins have lower FS when compared to bis-acrylic-based conventional resins.
Two studies provided data for the meta-analysis to compare the fracture strength between 3D-printed PMMA resin and conventional bis-acrylic resin. There was a statistically significant heterogeneity between the studies, with I 2 = 90%. The results were inconclusive, favoring 3D-printed PMMA resin (p = 0.09) (Figure 7).
(i) Comparing the hardness of MMA-based 3D-printed provisional resins: Two studies reported lower hardness values of MMA-based 3D-printed resins when compared to conventional MMA [49,53] and conventional bis-acrylic interim resins [53], respectively. Moreover, a study by Revilla-León et al. [49] reported higher hardness values for 3D-printed MMA-based interim resins when compared to conventional bis-acrylic interim resins. (ii) Comparing hardness of micro-filled and polylactic-acid-based 3D-printed provisional resins: Digholkar et al. [36] reported higher hardness values for 3D-printed microfilled resins when compared to conventional PMMA-based interim resins, whereas Crenn et al. [29] reported PMMA-based conventional resins to have higher hardness values when compared to 3D-printed polylactic-acid-based interim resins.

Surface Roughness
Four studies compared the surface roughness of 3D-printed interim resins with other materials (Table 9).
(i) Comparing the surface roughness of MMA-based 3D-printed provisional resins: Myagmar et al. [47] reported lower surface roughness values for MMA-based 3Dprinted resins compared to PMMA-based conventional resins and CAD/CAM milled interim resins.
Two studies provided data for meta-analysis to compare Surface Roughness between 3D-printed PMMA resin and conventional PMMA resin. There was a statistically significant heterogeneity between the studies, with I 2 = 78%. Both studies favored the conventional PMMA with a 95% confidence interval. The pooled estimate favored conventional PMMA resin with a p-value < 0.0001 ( Figure 8). Two studies provided data for the meta-analysis to compare the surface roughness between 3D-printed PMMA resin and conventional bis-acrylic resin. There was a statistically significant heterogeneity between the studies, with I 2 = 90%. The results were inconclusive, favoring conventional bis-acrylic resin (p = 0.09) (Figure 9). (ii) Comparing the surface roughness of hybrid and other 3D-printed provisional resins: One study [42] showed that hybrid 3D-printed resins have a higher surface roughness when compared to conventional PMMA, conventional bis-acrylic, and CAD/CAM milled PMMA-based resins. However, the results of a study by Taşın et al. [48] gave contradictory results, with hybrid 3D-printed resins displaying a lower surface roughness when compared to conventional PMMA, conventional bis-acrylic, and CAD/CAM milled PMMA-based resins. Simoneti et al. [53] reported that the surface roughness of SLS 3D-printed resins was higher, and that of SLA-based 3D-printed resins was lower when compared to conventional PMMA and bis-acrylic-based interim resins.
Two studies provided data for the meta-analysis to compare the surface roughness between 3D-printed hybrid composite resins and conventional PMMA resins. There was a statistically significant heterogeneity between the studies, with I 2 = 98%. The results were inconclusive, favoring 3D-printed hybrid composite resin (p = 0.09) ( Figure 10). Two studies provided data for the meta-analysis to compare the surface roughness between 3D-hybrid composite resin and conventional bis-acrylic resin. There was a statistically significant heterogeneity between the studies, with I 2 = 97%. The studies showed varied results, one favoring each side. The pooled estimate favored 3D-printed hybrid composite resin with a p-value = 0.04 ( Figure 11). Two studies provided data for the meta-analysis to compare the surface roughness between 3D-hybrid composite resin and CAD/CAM milled PMMA resin. There was a statistically significant heterogeneity between the studies, with I 2 = 97%. The studies showed varied results, one favoring each side. The pooled estimate favored 3D-printed hybrid composite resin with a p-value = 0.02 ( Figure 12).

Wear Resistance
Four studies compared the wear resistance of 3D-printed interim resins with other materials (Table 10).
Comparing the wear resistance of MMA-based 3D-printed provisional resins: The wear resistance of MMA-based 3D-printed provisional resins was reported to be higher than the wear resistance of PMMA-based conventional and CAD/CAM milled 3D-printed interim resins [26,27,47,50].
Two studies provided data for the meta-analysis to compare the wear resistance between 3D-printed PMMA resin and CAD/CAM milled PMMA resin. There was a statistically significant heterogeneity between the studies, with I 2 = 89%. Both studies favored the 3D-printed PMMA resin with a 95% confidence interval. The pooled estimate favored 3D-printed PMMA resin with a p-value < 0.00001 ( Figure 13).  (Table 11).
(i) Comparing the flexural strength of MMA-based 3D-printed provisional resins: Two studies reported higher flexural strength values of MMA-based 3D-printed resins when compared to conventional MMA [31,43] and CAD/CAM milled PMMA resin [43].
Two studies provided data for the meta-analysis to compare the flexural strength between 3D-Printed PMMA resin and CAD/CAM milled PMMA resin. There was a statistically significant heterogeneity between the studies, with I 2 = 80%. Both studies favored the 3D-printed PMMA resin with a 95% confidence interval. The pooled estimate favored 3D-printed PMMA resin with a p-value < 0.0001 ( Figure 14). than the wear resistance of PMMA-based conventional and CAD/CAM milled 3D-printed interim resins [26,27,47,50].
Two studies provided data for the meta-analysis to compare the wear resistance between 3D-printed PMMA resin and CAD/CAM milled PMMA resin. There was a statistically significant heterogeneity between the studies, with I 2 = 89%. Both studies favored the 3D-printed PMMA resin with a 95% confidence interval. The pooled estimate favored 3Dprinted PMMA resin with a p-value < 0.00001 ( Figure 13).  (Table 11).
(i) Comparing the flexural strength of MMA-based 3D-printed provisional resins: Two studies reported higher flexural strength values of MMA-based 3D-printed resins when compared to conventional MMA [31,43] and CAD/CAM milled PMMA resin [43].
Two studies provided data for the meta-analysis to compare the flexural strength between 3D-Printed PMMA resin and CAD/CAM milled PMMA resin. There was a statistically significant heterogeneity between the studies, with I 2 = 80%. Both studies favored the 3D-printed PMMA resin with a 95% confidence interval. The pooled estimate favored 3D-printed PMMA resin with a p-value < 0.0001 ( Figure 14).  Contrasting results were reported when the flexural strengths of SLA 3D-printed resins were compared with conventional PMMA and bis-acrylic resins. Crenn et al. [29] reported higher flexural strength values for 3D-printed resins, while Simoneti et al. [53] reported higher values for conventional resins.

Elastic Modulus
Five studies compared the elastic modulus of 3D-printed interim resins with other materials (Table 12).
(i) Comparing the elastic modulus of MMA-based 3D-printed provisional resins: Two studies reported higher elastic modulus values of MMA-based 3D-printed resins when compared to conventional MMA [31,37], whereas a study by Simoneti et al. reported lower elastic modulus values when compared to conventional PMMA-based resins [53]. Two studies reported lower elastic modulus values of MMA-based 3Dprinted resins compared to conventional bis-acrylic resins [37,53]. (ii) Comparing the elastic modulus of composite-based, ester-based, and polylactic-acidbased 3D-printed provisional resins: Crenn et al. [29] reported higher elastic modulus values for ester-based and polylactic-acid-based 3D-printed resins when compared to conventional PMMA and bis-acrylic-based resins. Taşın et al. [30] reported higher elastic modulus values for composite-based 3D-printed resins compared to conventional PMMA, CAD/CAM PMMA, and conventional bis-acrylic-based resins.

Toughness, Peak Strain, and Resilience
Two studies compared the peak strain values, and one each studied toughness and resilience of 3D-printed interim resins with other materials (Tables 13-15).
Taşın et al. [30] reported that the resilience and toughness of 3D-printed composite resins is higher than conventional PMMA and bis-acrylic resins but lower than CAD/CAM milled PMMA resins.
When peak stress values were compared, Tahayeri et al. [37] reported higher values for 3D-printed PMMA when compared to conventional bis-acrylic and PMMA-based resins. The study by Simoneti et al. [53] reported that peak stress values for conventional resins (bis-acrylic and PMMA) were higher than 3D-printed SLA resins but lower than 3D-printed SLS resins.

Discussion
The introduction of CAD/CAM technology in the field of fixed prosthodontics has improved the quality of treatment provided to the patients [54]. This systematic review and meta-analysis is the first of its kind to analyze and document all the available studies comparing the mechanical and/or physical properties of the 3D-printed provisional crown and FPD materials with CAD/CAM milled and/or conventional provisional resins. All twentyfive papers included were in vitro studies [16,[26][27][28][29][30][31][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52]. The overall findings reveal that the mechanical and physical properties of the provisional crown and FDP materials are affected by the technique of fabrication and composition of the tested materials. Threedimensionally printed provisional materials have shown significantly different mechanical and physical properties. Thus, the tested null hypothesis is rejected. The mechanical and physical properties of 3D-printed provisional resins in comparison to conventional and CAD/CAM milled will be discussed.

Physical Properties
Three physical properties (color stability, solubility, and water sorption) were evaluated in the selected articles. In general, most of the studies reported that, irrespective of the composition, the 3D-printed provisional crown and FDP materials displayed poor physical properties when compared to CAD/CAM milled and conventionally processed provisional restorative materials. Three studies [28,44,45] that compared the color stability of 3D-printed PMMA resins reported that they have poor color stability when compared to CAD/CAM milled PMMA resins. The studies by Atria et al. [15] and Taşın et al. [19] reported a poor color stability of 3D-printed hybrid composite resins compared to CAD/CAM milled PMMA, conventional PMMA, and conventional bis-acrylic provisional resins. However, two studies [44,48] reported better color stability for 3D-printed hybrid composite resins compared to conventional PMMA and bis-acrylic resins.
The poor color stability of 3D-printed provisional resins has been attributed to multiple reasons: CAD/CAM milled PMMA resins have a high polymerization rate, undergo industrial manufacturing, and have high crosslinking, thus making them dense in comparison to 3D-printed PMMA resins, which have low polymerization rates leading to poor surface integrity and color stability [14,36,37,42,[55][56][57][58][59][60]. Studies reported that CAD/CAM milled and conventionally processed PMMA resins have MMA (methylmethacrylate)-based monomers that are hydrophobic, whereas HDMA (hexamethylene glycol dimethacrylate), which is the monomer used in light polymerized resins, is hydrophilic in nature. Thus, the higher polarity of 3D-printed PMMA resins could also be a reason for the poor color stability [48,[61][62][63][64][65]. Studies by Atria et al. [42] and Yao et al. [45] evaluated the optical properties of 3D-printed hybrid composite resins. The poor color stability could be attributed to a lack of filler particles in these resins, thus leading to an increase in surface roughness. Song et al. [44] attributed the poor color stability to the presence of an uncured layer on the 3D-printed resins. The quantity of residual monomers, high solubility, and water sorption are also additive factors that influence the color stability of 3D-printed materials [28,66].
Myagmar et al. [47] and Atrial et al. [42] tested the color stability after artificial aging by thermocycling, whereas Shin et al. [28], Song et al. [44], and Taşın et al. [48] immersed the test specimens in different staining solutions (coffee, grape juice, curry, black tea, cola, and red wine). In general, as the immersion duration increases, the extent of discoloration increases for the tested specimens. The extent of color change also varied depending upon the type of staining solution. Studies [45,[58][59][60][61] have shown that the application of surface glaze/sealant materials significantly improves the color stability and decreases the surface roughness of 3D-printed materials.
Water sorption by acrylic resins can affect the dimensional stability and can lead to failure of the prosthesis [67][68][69], whereas a high solubility of acrylic resins can lead to the presence of more unreacted monomers, which can adversely affect oral tissues. Thus, for a material to be successful, it should have minimal water sorption and solubility [70]. Two studies evaluated the sorption and solubility of 3D-printed provisional resins [28,44]. They reported that the water sorption and solubility of 3D-printed PMMA and photopolymer provisional resins were higher than CAD/CAM milled PMMA and conventional bisacrylic resins, while the water sorption is less than in conventional PMMA provisional resins. Perea-Lowery et al. [71] and Berli et al. [68] correlated the high water sorption and solubility of 3D-printed resins to the polymerization technique. The 3D-printed materials are printed in layers, and water can enter in these layers, causing movement in the polymer chains, which can cause dimensional changes. In addition to this, the presence of free monomers in 3D-printed materials due to the low polymerization degree increases the water sorption [68,71,72].

Mechanical Properties
Mechanical properties discussed in the articles included in this systematic review and meta-analysis are fracture strength, microhardness, surface roughness, wear resistance, flexural strength, elastic modulus, peak stress, toughness, and resilience.
Fracture strength, flexural strength, peak stress, elastic modulus, and wear resistance are some of the mechanical properties which were found to be better for 3D-printed resins when compared to conventional and CAD/CAM milled provisional materials.
Three-dimensionally printed materials are fabricated by a layering technique; thus, there is a chemical bond between the layers [38]. The technique of fabrication affects the mechanical properties of 3D-printed resins. The authors of [38,73] reported that orientation during printing affects the mechanical properties. Vertical build orientation causes layers to be deposited perpendicular to the direction of the load application. So, these materials display superior mechanical properties compared to those printed in horizontal orientation (as layer deposition is parallel to load direction). The layer thickness during the printing process also effects the mechanical properties of these materials. Ibrahim et al. [38] and Tahayeri et al. [37] stated that the lower the layer thickness of printing is, the more layerto-layer interfaces that will be available; thus, each layer will be polymerized in a better way, which will increase the mechanical properties of these materials. After fabrication, 3D-printed materials are subjected to post-curing, which increases the degree of conversion, thus leading to lower residual monomers and increased mechanical properties [36,41]. Park et al. [26] and Mayer et al. [50] reported that 3D-printed provisional resins contain multiple different methacrylate resins and further additives. This difference in composition can be the reason for their superior wear resistance properties.
In conventional provisional resins, which are mixed manually or by using automixing units, there are high chances of incorporating air bubbles and porosities, which can be a reason for their poor mechanical properties [16,39]. Studies reported inferior mechanical properties of CAD/CAM milled provisional resins compared to 3D-printed resins. Monomer release from PMMA Blank after aging [55], the presence of fine grooves and lines on the surface of milled resins (due to milling process) [37,74], and the presence of higher weight percentages of carbon and oxygen (representing organic part) in CAD/CAM milled provisional resins [38] can be some of the possible reasons for such behavior. On the contrary, the shrinkage of specimens during the building and post-curing processes can be a reason why few studies reported the poor mechanical properties of 3D-printed resins compared to others [36,38,75].
Toughness, resilience, and microhardness are some of the mechanical properties that are poor for 3D-printed composite-based resins compared to CAD/CAM milled PMMA resins. For long-term provisional restorations, the resiliency should be higher to avoid failures. The dense cross-linking and homogenous structure of CAD/CAM milled PMMA resins make them less prone to hydrolytic degradation when compared to conventional and 3D-printed resins [30]. In addition, the difference in composition and manufacturing technique [29,36,49,53] are some of the causes for 3D-printed resins to have these properties inferior to other tested groups.
Studies have shown contrasting results when comparing the surface roughness of 3D-printed materials and other provisional materials. Atria et al. [15] reported high surface roughness of 3D-printed hybrid resins compared to conventional and CAD/CAM-printed PMMA resins. They stated that while printing these resins factors such as curing time, orientation, and the post-curing process may play an important role. In addition to that, they used unfilled 3D resins. Contrary to this, Taşın et al. [48] found that 3D-printed hybrid resins have less surface roughness when compared to conventional and CAD/CAM PMMA resins. They stated that due to the milling and polishing process, there could be additional surface defects that can increase the surface roughness. In general, it can be stated that the surface roughness of 3D-printed resins is affected by the composition of tested resin and printing orientation [47].
This systematic review employed a comprehensive search strategy, and independent assessments of the reviewers were used during article selection to avoid bias. These are the highlights of this review. All the articles discussing the physical and mechanical properties of 3D-printed provisional materials were evaluated to ensure that no relevant article is missed.

Limitations
Studies included in this systematic review had medium-to-high-quality methodologies, but the risk of bias was high. High heterogeneity was observed in all meta-analyses, and most of the meta-analyses had contributions from two studies only. Most of the pooled estimates showed inconclusive results. Thus, more studies with uniformity in material and measurement techniques are needed to make conclusive statements from meta-analysis regarding the physical and mechanical properties of 3D-printed provisional resin materials. This systematic review and meta-analysis focused only on physical and mechanical properties. However, there are other parameters, such as accuracy, dimensional stability, marginal adaptation, internal adaptation, etc., which play essential roles in decision making while selecting the best material to be used for provisionalization of crowns and FDPs. Further systematic reviews are recommended to cover these aspects of the materials.

Conclusions
The following conclusions can be drawn from this systematic review and meta-analysis: • When compared to conventional and CAD/CAM milled provisional resin materials, 3D-printed provisional crown and FDP resins have: (a) superior mechanical properties in terms of fracture strength, flexural strength, elastic modulus, peak stress, and wear resistance; (b) inferior mechanical properties in terms of toughness, resilience, and microhardness; (c) contrasting results in terms of surface roughness; and (d) inferior physical properties in terms of color stability, water sorption, and solubility.

•
In vitro studies should follow blinding protocols to avoid bias. • Three-dimensionally printed provisional crowns and FDP materials can be used as an alternative to conventional and CAD/CAM milled long-term provisional materials.  Informed Consent Statement: Not applicable.

Data Availability Statement:
The data that support the findings of this study are available from the corresponding author upon reasonable request.