Vat photopolymerization is the additive route that dominates resin-based definitive restorations. Technical reviews converge in describing stereolithography and digital light processing as the processes with the resolution, speed and surface quality required for fixed prostheses, whereas material extrusion and material jetting occupy secondary positions in this application because they yield rougher surfaces and lower mechanical isotropy.(6,7) The two vat processes share the same chemistry but differ in how they cure each layer, which has practical consequences for the laboratory: stereolithography traces the layer with a laser, while digital light processing projects a full image of the layer at once, so the choice between them trades resolution against build speed and affects how the manufacturer calibrates exposure and shrinkage compensation.(8) The same sources distinguish the printable resins marketed as permanent from those used for models and provisionals: the permanent products are photopolymerizable composites formulated with methacrylate matrices and inorganic fillers, and they require washing and post-curing steps that complete conversion before the restoration can be delivered.(9)

The printed part is therefore an incomplete material until it is finished. A crown leaves the printer attached to supports and covered in uncured resin; it must be detached, washed to remove residual monomer, and post-cured with light or heat to reach its final conversion. Each of these steps has variables that laboratories set independently: the orientation and number of supports, the solvent and duration of washing, and the dose, wavelength and temperature of the post-curing cycle. Because the manufacturer specifies a protocol rather than a fixed property, the same resin can yield restorations with different degrees of conversion and different mechanical behavior, a dependence documented repeatedly in the studies reviewed below. This workflow dependence is the clearest difference between printed and milled materials: a milled block arrives fully polymerized, whereas a printed restoration is finished in the laboratory, where quality control is less standardized.

The material base has evolved in three directions. Early printable systems adapted polymethyl methacrylate (PMMA) to stereolithography; Hata et al.( 12) developed such a system to reduce brittleness and improve printability, an adaptation that shows how difficult it is to transfer a well-known dental polymer to a photopolymerization process. Later formulations introduced printable and machinable resin composites in which filler loading and monomer chemistry were adjusted so that the printed part approached the properties of machinable blocks, a comparison that established the milled composite as the natural benchmark for printed permanent resins.(10) Two further directions illustrate the search for stronger materials: printable polymer-infiltrated ceramic networks that mimic the mechanical behavior of enamel and could narrow the gap with glass ceramics,(13) and resins reinforced with modified zirconia nanoparticles intended for long-term service in the oral environment.(14) Low-shrinkage formulations addressed the dimensional change that accompanies polymerization and can compromise fit, particularly in large restorations.(15)

Across these products there is no harmonized definition of what makes a resin permanent. The label reflects the intended use declared by the manufacturer more than a regulated material category, and guidance on biocompatibility certification and post-processing remains product-specific.(9,11) This heterogeneity conditions the interpretation of the mechanical data reviewed in the next axis, because products sold under the same name differ in filler content, matrix chemistry and processing requirements, and because the post-processing protocol is part of the material system rather than an optional step. In practice, the term permanent should be read as a statement of intended service rather than a guarantee of performance, and any laboratory evaluation must name the exact product, its layer settings and its post-curing regimen before the result can be compared with another study.

Comparative in vitro studies place printed permanent resins below milled ceramics and roughly within, or slightly below, the range of milled resin composites. Grzebieluch et al.( 10) measured printable restorative composites that approached the flexural strength of machinable composites, and Sahin et al.( 16) found additive and subtractive composite resins within a comparable strength interval, which supports the claim that printing itself does not impose an insurmountable penalty when the formulation is designed for it. When the comparison extends to hybrid or ceramic restorations, printed resins fracture at lower loads: Suksuphan et al.( 17) documented lower fracture resistance for printed hybrid crowns than for milled hybrid materials at reduced occlusal thickness, and Khosravani et al.( 18) described the fracture of printed dental resin as brittle and dependent on the layer interfaces, which points to the interlayer boundary as a weak plane. Abad-Coronel et al.( 19) reported, in the opposite direction, that a printed CAD/CAM material reached fracture resistance comparable to milled PMMA for single units, which indicates that the gap depends on the specific resin rather than on the printing principle itself.

Failure modes reinforce the same conclusion. Printed resins tend to fail through the layer plane, producing fractures that split the restoration rather than chip it, whereas milled composites and ceramics fail predominantly by crack propagation from the surface or from the margins.(18,20) A single load-to-fracture value cannot express this behavior, because it says little about the weak plane that will dominate in a connector or a thin margin. The dispersion of results across parameter studies(21,22) further complicates the comparison, since the same nominal strength can carry different failure risks depending on how the part was oriented and cured. This situation recalls the early literature on milled resin composites, which also relied on monotonic tests before standardized fatigue protocols were adopted.

Printing parameters modify these values materially. Layer thickness, printing orientation and post-curing time and temperature change flexural strength and hardness, sometimes by a magnitude larger than the difference between competing resins.(21,22) This finding matters for clinical reproducibility, because two laboratories using the same commercial resin but different post-curing units can produce parts with different strength and surface properties, a source of variability that does not exist for prefabricated milled blocks. Park et al.( 23) found that crown thickness and post-curing influenced strength and surface characteristics of printed resin crowns for primary molars, and Kim et al.( 24) showed that resin composition determined biaxial flexural strength and fracture resistance in the same application, which suggests that pediatric restorations, where loads are lower, tolerate printed resins more easily than adult posterior crowns.

None of these comparisons settles whether printed permanent resins meet the minimum requirements expected of polymer-based crowns. Standards for polymer restorative materials define threshold values for flexural strength, and the values reported for printed permanent resins tend to cluster near that threshold rather than comfortably above it, which leaves little reserve for fatigue, aging and processing variation. The practical consequence is that a printed crown may pass a single-load requirement in ideal conditions and still be marginal under the combined effect of water, load cycling and a suboptimal post-curing protocol, the very combination that defines clinical service. This is why monotonic strength alone is an insufficient basis for recommending a material as permanent, and why the aging data reviewed next carry more weight than the strength data reviewed here.

The ordering of materials follows filler content and matrix type. Ceramics are fully inorganic and reach flexural strengths several times higher than polymer-based materials, which is why printed resins, however well filled, cannot be expected to replace ceramics in high-load posterior sites, a gap documented when printed hybrids were tested against milled ceramic-containing materials.(17) Within the polymer family, the difference between printed and milled composites is smaller because both are methacrylate-based systems with comparable filler fractions; the residual difference comes from the layer structure of the printed part and from the conversion that the post-curing cycle achieves.(16,25) This framing explains most of the apparent contradictions in the literature: studies that compare printed resins with ceramics report large gaps, whereas studies that compare them with milled composites report small or no gaps, and the two conclusions are not in conflict.

Evidence for multi-unit and implant restorations is thinner and less favorable. Skienhe et al.( 20) reported lower fracture resistance for printed three-unit fixed dental prostheses than for milled composite equivalents, and Bein et al.( 26) documented lower fatigue and fracture values for printed than for milled interim implant-supported crowns, a comparison that warns against transferring the single-unit results to connectors and cantilevered spans. Fatigue data for definitive printed resins remain scarce; most studies apply monotonic loading, which does not capture the slow degradation expected under intraoral function, and the few available fatigue reports use different numbers of cycles and loads.(17) Reinforcement with glass silica or zirconia nanoparticles raises strength and hardness, but the benefit depends on dispersion and interfacial adhesion and has been demonstrated mostly in experimental formulations rather than in commercial permanent resins.(14,27) Two systematic syntheses of provisional printed materials(28,29) show that the provisional literature is larger and more homogeneous than the definitive one, and their estimates should not be transferred to permanent resins, which are exposed to longer service and higher demands.

Artificial aging widens the gap between printed and milled or ceramic materials. Güven et al.(30) found that additively manufactured resin crowns showed greater dimensional change and occlusal wear than subtractive ones after thermomechanical aging, and Cha et al.(31) reported that 3D-printed denture tooth resin wore more than zirconia and metal antagonists under simulated function, which has implications for the maintenance of the opposing dentition. Surface studies agree that printed permanent resins are more sensitive to thermocycling and brushing than resin-based CAD/CAM blocks, with roughness and microhardness deteriorating faster.(32,33) The consistency across these studies is notable because they use different resins, aging regimens and measurement methods, yet they converge on the same ordering: printed resins degrade more, and the degradation is visible in wear, roughness and hardness rather than in color alone.

The chemical basis of this sensitivity lies in incomplete polymerization. Berghaus et al.( 25) measured a lower degree of conversion and higher residual monomer elution for printed composites than for milled or self-cured materials, and Hwangbo et al.( 34) showed that washing time and washing solution modify both biocompatibility and mechanical properties. Kim et al.( 35) linked the choice of photoinitiator to cytotoxicity, color stability and dimensional accuracy, which connects processing chemistry to clinical tolerability. These findings indicate that the properties of a printed restoration are not fixed at printing but are determined by the entire post-processing chain, from the solvent used for washing to the dose and wavelength of the post-curing light.

Biocompatibility assessments give a generally acceptable but product-dependent picture. Aydın et al.( 36) found that printed permanent resins, resin-based CAD/CAM blocks and conventional composites elicited comparable cytotoxic responses on gingival and fibroblast cell lines when processing was complete. Wuersching et al.( 37) reported, by contrast, that some novel resins for printed fixed prostheses failed initial biocompatibility screening, a divergence that reflects the influence of formulation and post-processing and that cautions against assuming all permanent resins are interchangeable. Optical properties follow the same logic: translucency of printable permanent composites is lower than that of ceramics and varies with shade and thickness,( 38) and color stability under coffee staining and aging is material-specific.( 33) Water sorption and solubility of printed denture resins also exceed those of heat-polymerized acrylic,( 39) which matters for the dimensional stability of large prostheses. Retention of printed permanent crowns to prepared teeth is adequate after artificial aging when surfaces are pre-treated, but the evidence rests on a single material comparison.( 40)

Surface finish adds a further dimension. Printed restorations carry the layered texture of the build and, unless polished, retain roughness that is higher and less stable than that of milled blocks.(32) Polishing restores a smoother surface, but the degree of recovery varies with the resin and the finishing protocol, and the roughness returns under thermocycling and brushing faster than in subtractive materials.(33) Because roughness conditions biofilm accumulation and stain retention, the surface behavior of printed permanent resins is inseparable from their aging behavior, and the two should be evaluated together rather than as separate properties. This coupling between surface and bulk degradation is a recurring feature of the axis and one of the strongest arguments for standardized aging protocols.

The aesthetic record follows the same pattern. Printed permanent resins offer a narrower shade range than ceramics and lower translucency, which limits their use in the anterior region where color matching is decisive.(38) Staining under coffee and repeated thermocycling changes color in a product-dependent way, and the change is larger than that of resin-based CAD/CAM blocks in most comparisons.(33) Together with the surface and sorption data, these optical limitations suggest that the natural indication for printed permanent resins is the posterior single unit or the implant-supported crown where strength and fit dominate and where aesthetic demand is moderate, rather than the anterior restoration where optical fidelity is the primary requirement.

Laboratory fit data support clinical feasibility. Kakinuma et al.(41) and Dönmez et al.(42) reported marginal and internal gaps for printed resin crowns within clinically accepted thresholds, with implant-supported printed definitive composite crowns performing similarly to milled equivalents. Build angle and the number of supports modify internal fit, so accuracy is controllable but protocol-dependent, and a laboratory that does not control these variables can produce restorations that fall outside the acceptable range even with a validated resin.(43) For endocrowns, Arafa et al.(44) found printed and milled restorations comparable in marginal adaptation before and after thermomechanical loading. The picture for printed ceramics is less uniform: Refaie et al.(45) observed larger marginal gaps for printed than for milled monolithic zirconia crowns, a divergence explained by the additional sintering steps required for ceramic green bodies rather than by any limitation of the resin systems.

Direct clinical evidence on printed permanent resins is limited. A randomized trial in primary molars by Al-Halabi et al.(46) compared printed resin crowns with CAD/CAM crowns and reported acceptable short-term clinical performance, which remains the principal in vivo datum for resin definitive crowns. For adults, clinical series concentrate on printed zirconia rather than resin: Kao et al.(47) described favorable short-term outcomes of selective laser melted zirconia crowns, and a systematic review with meta-analysis concluded that printed and milled zirconia prostheses have comparable clinical effectiveness over short observation periods.(48) In removable prosthodontics, randomized and follow-up studies document that printed complete dentures perform acceptably over up to 18 months,(49,50) but these findings concern denture base resins and cannot be transferred to fixed single crowns, where the loads and the cement interface differ.

No prospective study with survival endpoints exists for printed permanent resin single crowns in adults, and no comparative trial has followed such restorations beyond short observation periods. The clinical axis therefore rests on laboratory fit data and on evidence borrowed from neighboring applications, a limitation developed in the Discussion. This absence is itself a finding: the market adoption of permanent resins has run ahead of the evidence that would normally precede the routine use of a new definitive material. The consequence for practice is that the clinician who prescribes such a restoration accepts an evidentiary position closer to that of a provisional material than to that of a milled ceramic, even though the restoration is intended to be definitive.

Despite this thin evidence, printed definitive resins are adopted for practical reasons. A single printer can produce crowns, provisionals, splints and surgical guides, which amortizes equipment costs, and the workflow removes the milling bur, the block inventory and much of the manual finishing that subtractive methods require. These advantages are real and explain why laboratories and clinicians have moved toward printed definitive restorations before survival data are available. They do not, however, substitute for evidence on longevity, and they should be weighed against the lower mechanical margins and the higher aging sensitivity documented in the previous axes when a posterior load-bearing site is planned. The same logic applies to the minimal evidence that would be needed: a prospective series of printed permanent resin single crowns with at least two or three years of follow-up, survival and marginal integrity would begin to answer the question that the current literature leaves open.

Laboratory fit data should not be over-read. A marginal gap within laboratory thresholds is a necessary but not sufficient condition for clinical success, because cement wear, biofilm ingress and occlusal loading act on the margin over time and are not reproduced by a single seating measurement. The favorable in vitro fit of printed resin crowns(41,42) therefore predicts early seating rather than long-term integrity, and cannot compensate for the absence of survival data. The same caveat applies to the fracture values reviewed earlier: they describe the restoration on the day of delivery, whereas the clinical question concerns its behavior years later.