Three-dimensional-printed endocrowns: Does increasing intracoronal depth improve strength at the expense of fracture severity?

Abstract

Aim:

To evaluate fracture strength (FS) and failure mode (FM) of additively manufactured endocrowns fabricated from two three-dimensional-printed resin composites with distinct inorganic filler contents, tested at two intracoronal preparation depths following thermomechanical aging.

Materials and Methods:

Twenty-eight endodontically treated human mandibular molars were restored using additively manufactured endocrowns according to material (Bio Crown or Voxelprint) and preparation depth (3 mm or 5 mm). After adhesive cementation and thermomechanical aging (300,000 cycles; 5°C–55°C), FS was measured under axial loading (0.5 mm/min), and FM were classified into four categories. Data were analyzed by two-way analysis of variance and Tukey’s test (α = 0.05).

Results:

Restorative material significantly affected FS (P = 0.0063), whereas preparation depth did not (P = 0.403). Voxelprint showed higher FS than Bio Crown, independent of depth, with a significant difference between Voxelprint 5 mm and Bio Crown 5 mm (P = 0.034). Preparations with greater depth were associated with a higher frequency of catastrophic vertical fractures. No interaction effect was observed.

Conclusion:

Resin composition influenced FS more than preparation depth. Increased depth did not improve load capacity but shifted failure toward more severe patterns.

INTRODUCTION

Prosthetic rehabilitation of endodontically treated teeth remains a biomechanical challenge in contemporary restorative dentistry.[1,2,3] The loss of coronal tooth structure due to access cavity preparation and endodontic instrumentation reduces the stiffness of the remaining dental substrate,[1,4,5] increasing cuspal deflection under occlusal loading and favoring irreparable failure modes (FMs).[3,4] Although intraradicular posts have traditionally been used, their role is primarily retentive and does not necessarily increase the intrinsic strength of the radicular structure.[6] Thus, preservation of coronal tooth structure remains the main determinant of mechanical prognosis.[5]

Endocrowns have emerged as a conservative adhesive alternative for molars with extensive coronal destruction,[7] using the pulp chamber as a retentive feature and eliminating intraradicular preparation.[7] Traditionally fabricated from glass ceramics or zirconia-reinforced materials,[8,9] these restorations exhibit high fracture strength (FS) but also high elastic modulus and brittle behavior.[8] Hybrid resin composites and ceramic–polymer materials have expanded restorative options, presenting an intermediate elastic modulus and potentially less severe FMs.[8]

Additive manufacturing by photopolymerization has expanded the use of resin composites for definitive indirect restorations.[10,11] These materials show mechanical properties comparable to Computer-aided Design and Computer-aided Manufacturing (CAD/CAM) composites,[11] although their application in endocrowns remains insufficiently explored, particularly regarding preparation design variables.[12]

Intracoronal preparation depth is a critical biomechanical variable.[13,14] Increasing pulp chamber extension may increase restorative volume and adhesive surface area,[14] potentially influencing FS.[13] However, greater tooth structure removal may reduce substrate stiffness and alter stress distribution, affecting FM.[14,15] While partially investigated in ceramic systems,[16] this relationship remains unclear for three-dimensional (3D)-printed resin-based endocrowns.[17]

Despite the growing clinical use of additive manufacturing,[18] evidence evaluating the combined effect of resin composition and preparation depth on the mechanical behavior of endocrowns after thermomechanical aging is limited.[11]

Therefore, this study evaluated FS and FM of endocrowns fabricated by additive manufacturing using two 3D-printed resin composites with different inorganic filler contents at two intracoronal depths (3 mm and 5 mm) after thermomechanical aging. The null hypothesis was that material type and preparation depth would not significantly influence FS or FM.

MATERIALS AND METHODS

This in vitro laboratory study was conducted using a 2 × 2 factorial design, considering two factors: restorative material (two 3D-printed resins indicated for definitive indirect restorations) and intracoronal preparation depth (3 mm and 5 mm).[13] The sample size (n = 7 per group) was determined by an a priori power analysis based on a pilot study, assuming a power of 0.80 and a significance level of 0.05. The primary outcomes were FS (N) and FM (types I–IV).[14]

The protocol was approved by the Research Ethics Committee of the Faculty of Dentistry of Ribeirão Preto, University of São Paulo, and the teeth were obtained from the Tooth Bank of the Faculty of Dentistry of Ribeirão Preto, University of São Paulo. Teeth with incomplete root formation, fractures, perforations, or previous endodontic treatment were excluded.[15]

After endodontic treatment, specimens were stored in distilled water at 37°C ± 2°C. Restorations were cemented 14 days after endodontic filling and pulp chamber lining, and specimens were stored for an additional 7 days prior to thermomechanical cycling, totaling 21 days before fracture testing.[19,20]

Endodontic treatment and preparations for endocrown

Endodontic access was prepared using diamond burs under water cooling. Canal patency was confirmed with a size #10 K-file to the apical foramen, and working length was established 1 mm short of the full canal length. Biomechanical preparation was performed using a Reciproc 50/0.05 instrument (VDW GmbH, Munich, Germany). Irrigation was performed with 2.5% sodium hypochlorite, followed by 17% EDTA and a final rinse with sodium hypochlorite. Obturation was completed using a single-cone technique with gutta-percha and AH Plus sealer (Dentsply DeTrey GmbH, Konstanz, Germany).

For intracoronal standardization, the pulp floor was conditioned with 37% phosphoric acid, followed by application of a universal adhesive (Ybond Universal; Yller Biomateriais SA, Brazil) and lining with flowable bulk-fill resin (Tetric N-Flow Bulk Fill; Ivoclar Vivadent AG).

Teeth were randomly allocated and prepared for intracoronal depths of 3 mm or 5 mm using a tapered diamond bur, with depth confirmed using a periodontal probe. The cervical finish line consisted of a 1 mm chamfer with 8° axial divergence.[14] The restoration margin was standardized 2 mm above the cementoenamel junction, with a 2 mm ferrule.[13] Specimens were embedded in PVC cylinders and stabilized with autopolymerizing acrylic resin up to the region below the furcation.[13]

Digital workflow and additive manufacturing

For digital acquisition, specimens were stabilized in a typodont and scanned using an intraoral scanner.[21] The datasets were exported via Cerec Connect, and restorations were designed using InLab CAD 22 software (Dentsply Sirona, Germany). A cement space of 0.08 mm and a minimum thickness of 2 mm were established, with proximal and occlusal contact intensity set at 0.025 mm.[15] The hardware systems are described in Table 1.

Table 1.

Specifications of intraoral scan, tridimensional printer, and postpolymerization units

Equipment	Type	Manufacturer	Description
PrimeScan	Intraoral scanner	Dentisply Sirona Inc., Germany	Smart Pixel Sensor technology, dynamic depth scanning up to 20 mm, data acquisition rate: >1,000,000 3D points/s, field of view: 22.5 mm × 20.7 mm, output formats: STL and PLY, dimensions: 253 mm × 58.8 mm × 50.9 mm, weight: 457 g
PrimeScan Acquisition Center	Built-in computer	Dentisply Sirona Inc., Germany	Acquisition center integrated with an Intel Core i7-9700K processor (3.60 GHz), 32 GB of DDR4 RAM, a 1 TB NVMe SSD, and a dedicated NVIDIA GeForce RTX 2070 graphics card, operating on a 64-bit Windows 10 IoT Enterprise system
Hunter	3D Printer	Flashforge Corporation, China	DLP technology with a 405 nm LED light source, featuring an XY resolution of 62.5 μm, adjustable layer thickness between 12.5 and 50 μm, and a build volume of 120 mm × 67.5 mm × 150 mm
dOne Cure	Postpolymerization unit	dOne 3D, Brazil	Multi-wavelength UV-LED technology (365–405 nm) with digital thermal control up to 60 °C, a 360° rotating turntable for uniform exposure, and an internal capacity of 145 mm × 145 mm × 100 mm

3D: Tridimensional, STL: Stereolithography, PLY: Polygon file format, DDR: Double data rate, RAM: Random access memory, NVMe: Nonvolatile memory express, SSD: Solid state drive, IoT: Internet of things, DLP: Digital light processing, LED: Light emitting diode, UV: Ultraviolet

STL files were processed using FlashDental software. Two 3D-printable resin composites indicated for definitive restorations were used, with parameters described in Table 2. Printing orientation was standardized at 0°, with cusp-directed positioning and supports placed at cusp tips. Printing and postcuring were performed under controlled temperature (33°C).

Table 2.

Compositions of tridimensional printed resins

3D printed resins	Shade	Lot number	Components	Manufacturer	3D print parameters
Bio crown diamond	A1	111624	Silanized ceramic and zirconia fillers (50–52 wt%), methacrylate monomers and oligomers (45–48 wt%), stabilizers (<2 wt%), photoinitiator system (<1 wt%), and pigments (<1 wt%).	Marketech Labs Ltd., Brazil.	Layer thickness of 0.05 mm, curing time of 5 s, adhesive layer curing time of 20 s, 6 adhesive layers and 6 transition layers, light intensity of 100%. Postpolymerization curing of 30 min
Voxelprint ceramic	A1 LT	130825	Silanized radiopaque glass particles (57–63 wt%), methacrylate monomers (35–45 wt%), stabilizers (<2 wt%), photoinitiator system (<2 wt%), and pigments (<1 wt%)	FGM Dental Group Ltd., Brazil.	Layer thickness of 0.10 mm, curing time of 4.2 seconds, adhesive layer curing time of 35 s, 6 adhesive layers and 6 transition layers, light intensity of 100%. Postpolymerization curing of 30 min

Adhesive cementation

Internal surfaces were airborne-particle abraded with 50 µm aluminum oxide (1 bar, 10 s), ultrasonically cleaned, and treated with silane, followed by application of a universal adhesive.[22,23] A dual-core resin cement (SetPP; SDI Limited, Australia) was applied, and restorations were seated under digital pressure. After excess removal, an oxygen-impermeable gel was applied, and photoactivation was performed on all surfaces.

Thermomechanical aging and fracture testing

Thermomechanical aging was performed for 300,000 cycles (≈1 year simulation) at 1.8 Hz, with alternating baths at 5°C ± 2°C and 55°C ± 2°C.[19,20] FS testing was conducted using a universal testing machine (EMIC DL-2000; ITW Test and Measurement, USA). Specimens were positioned axially, and load was applied using a 6 mm spherical indenter at 0.5 mm/min until failure.[19] Calibration followed ISO 4049:2019.[24] FM was classified into four categories (Types I–IV). Normality was assessed using the Shapiro–Wilk test. FS data were analyzed using two-way analysis of variance (ANOVA) followed by Tukey’s test (α = 0.05).

RESULTS

The mean FS values ± standard deviation for each group analyzed are presented in Figure 1. Overall, the Voxelprint groups demonstrated higher mean FS values than the Bio Crown groups at both preparation depths.

Figure 1.

Mean and standard deviation of fracture strength for the groups evaluated. Uppercase letters denote statistically significant differences among preparation depths, whereas lowercase letters indicate statistically significant differences among the three-dimensional-printed resin materials (α = 0.05)

Two-way ANOVA revealed a statistically significant main effect of restorative material on FS (F [1, 24] = 8.63; P = 0.0063). Voxelprint exhibited significantly higher FS than Bio Crown, regardless of preparation depth.

The main effect of preparation depth was not statistically significant (F [1, 24] = 0.72; P = 0.403), indicating that increasing intracoronal depth from 3 mm to 5 mm did not significantly influence FS. No statistically significant interaction was observed between restorative material and preparation depth (F [1, 24] = 1.33; P = 0.257).

Post hoc pairwise comparisons (Tukey’s test) demonstrated a statistically significant difference between Voxelprint 5 mm and Bio Crown 5 mm (P = 0.034), with higher FS observed for Voxelprint 5 mm [Figure 1]. The other pairwise comparisons showed no statistically significant differences (P > 0.05).

Only two FM were observed: Type II (fracture involving limited coronal tooth structure with cohesive failure of the restoration) and Type IV (catastrophic vertical fracture involving both crown and root). No Type I or Type III failures were identified. The distribution of FM is presented in Figure 2.

Figure 2.

Distribution diagram of specimens from each analyzed group, classified by failure mode

DISCUSSION

The present study demonstrated that the restorative material influenced FS, whereas its effect on FM appeared limited. The maximum fracture load represents the resistance to the compressive event,[12,17] while FM reflects how stresses are transmitted in the tooth–restoration relationship and whether the structural compromise remains clinically manageable.[6,15,16]

Although the material effect reached statistical significance, the absolute difference in FS between the two resin systems remained moderate. Both materials performed within a range considered compatible with posterior occlusal function.[9,19] Therefore, the relevance of this material effect should not be interpreted as an isolated categorical difference,[10] but rather as a difference in structural behavior within the same class of resin-based restoratives.[10,11] In this context, moderate differences may become significant when associated with distinct stress distribution patterns and variation in FM.[15,16]

The higher FS observed for Voxelprint should be interpreted in light of its formulation. Although zirconia-containing fillers possess high intrinsic stiffness and fracture toughness,[9] reinforcement in resin matrices depends on effective silanization, particle distribution, and compatibility with the organic matrix.[10,11] A higher filler weight fraction, as seen in Voxelprint, can increase the elastic modulus and reduce the dominance of the polymer matrix,[10] potentially improving resistance under compressive loading.[11] The quality of the filler–matrix interface and the degree of polymer conversion influence crack initiation and propagation.[11]

In additively manufactured polymers, these compositional factors interact with processing parameters.[10,11] Differences in layer thickness, exposure time, and postcuring protocols influence interlayer adhesion and the distribution of internal defects.[11,12] As polymerization occurs incrementally, incomplete interfacial bonding between layers can act as structural discontinuities.[12]

Intracoronal preparation depth, on the other hand, did not significantly increase the fracture load,[15,17] but was associated with more severe FM.[15,17] From a biomechanical standpoint, a deeper extension increases intracameral engagement and restorative volume,[14,17] which may favor compressive load accommodation.[14] Nevertheless, a greater extension also modifies the stress transfer pathways within the remaining dentin.[6,15,16] The higher incidence of catastrophic radicular fractures with deeper preparations suggests that a higher stress concentration may overload the cervical and radicular regions,[15,16] particularly when dentin thickness is reduced.[15] In this context, an apparent increase in load-bearing capacity does not necessarily correspond to an improved biomechanical prognosis.[14]

Clinically, catastrophic radicular fractures compromise restorability,[2] while coronal failures may permit retreatment.[2] The present data reinforce that cavity design can exert a decisive influence on failure reparability.[14,15,16,17] When compared with CAD/CAM ceramic endocrowns, the reported FS values for reinforced glass-ceramics frequently range from approximately 1200 to 2000 N,[8,19] and zirconia restorations can exceed 2000 N under axial loading conditions.[8] However, ceramic systems often demonstrate a higher proportion of irreparable failures involving the root structure.[19] Resin-based materials, due to their lower elastic modulus relative to ceramics,[10,11] and closer proximity to dentin, are often considered more modulating.[10] Nevertheless, the present findings indicate that preparation geometry may compromise this potential advantage,[15,17] particularly when deeper preparations alter FM and radicular involvement.[15]

Several limitations must be considered for the present study. Monotonic axial loading does not replicate cyclic fatigue or multidirectional occlusal forces.[19,20] Future investigations should incorporate cyclic fatigue protocols and computational approaches to clarify stress distribution mechanisms.[6,15,16]

Within the study configuration, the findings support one interpretation: resin composition influences FS, but preparation depth is determinant in failure severity. Deeper intracoronal preparations should not be assumed to be advantageous solely because they numerically increase load capacity. Clinical decision-making must integrate the mechanical behavior of the material, the cavity geometry, and the prognostic implications of FM.

CONCLUSION

Based on the present in vitro findings, the following conclusions were drawn:

1. Restorative material significantly influenced FS, with Voxelprint demonstrating higher values than Bio Crown

2. Intracoronal preparation depth did not affect FS; however, increased depth was associated with a higher incidence of catastrophic (Type IV) failures, whereas 3 mm preparations were predominantly associated with more favorable, reparable FM (Type II)

3. The interaction between preparation design and material properties is clinically relevant, as greater FS does not necessarily correspond to a more favorable biomechanical outcome

4. Within the limitations of this study, material selection and preparation depth should be considered jointly, as deeper preparations may increase load-bearing capacity at the expense of a higher risk of irreparable radicular fracture.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

The financial support statement has been carefully checked and revised to include the appropriate funding information. The text now indicates that this study was financed in part by CAPES, Finance Code 001, and by FAPESP, grant no. 2023/10471-2, as correctly specified, and we kindly request that it be maintained in this form. The descriptions provided in parentheses indicate the meaning of the acronyms CAPES and FAPESP and should remain untranslated, as they refer to funding agencies from our country. We sincerely appreciate each careful comment and hope that the revisions have addressed the expectations. We remain at your disposal for any further clarification.