Simulating the shrinkage-induced interfacial damage around Class I composite resin restorations with damage mechanics

Accurate assessment of the three-dimensional shrinkage stress evolution for photopolymerized dental filling materials: Mechano-chemo-thermo-coupled finite element modeling and experimental validation

OBJECTIVE

Photopolymerized resin composites are widely used as dental filling materials. However, the shrinkage stress generated during photopolymerization can lead to marginal microcracks and eventual restoration failure. Accurate assessment of the stress evolution in dental restorations, particularly in complex cavity geometries, is critical for improving the performance and longevity of the dental filling materials. This study aims to develop a novel mechano-chemo-thermo-coupled finite element method (FEM) to accurately capture three-dimensional (3D) shrinkage stress of resin-based photopolymerized filling materials.

METHODS

The FEM was established with consideration for the evolution of mechanical properties, thermal effects, and polymerization shrinkage during photopolymerization. Real-time material property evolution was derived from measurements of degree of conversion and temperature changes, and these were integrated into the FEM alongside thermal expansion/contraction effects. The FEM was parameterized through mechanical, chemical, and thermal experiments, then applied to simulate different photocuring protocols and boundary conditions. The accuracy of the predicted shrinkage stress was validated through three experiments: uniaxial shrinkage stress measurement, full-field optical measurement, and acoustic emission analysis using typical dimethacrylate-based dental filling materials.

RESULTS

The coupled FEM model achieved predictive stress magnitudes in quantitative agreement with the experimental measurements (relative error ∼1 %), significantly improving upon existing methods (∼22.5 %). Furthermore, the FEM accurately predicted spatial debonding based on stress distribution, providing insights unattainable through current methods.

SIGNIFICANCE

This experiment-modeling-combined study provides a valuable tool for accurately predicting the spatial and temporal evolution of the shrinkage stress in resin-based dental filling materials, thereby providing new insights for optimizing their clinical applications and enhancing durability.

Adhesive damage of class V restorations under shrinkage stress and occlusal forces using cohesive zone modeling

OBJECTIVE

This study aims to investigate adhesive damage caused by the synergistic effects of polymerization shrinkage and occlusal forces via finite element analysis (FEA), based on damage mechanics with the cohesive zone model (CZM). The objective is to obtain the adhesive damage distribution and investigate how the material properties of resin composite impact adhesive damage.

METHODS

A 3D reconstruction model of an mandibular first molar was constructed through CBCT imaging, and a Class V cavity was prepared using computer-aided engineering (CAE) software. Common clinical resin composite and an universal adhesive were selected for restorative filling. A 3D FEA was performed, incorporating the pre-stress induced by polymerization shrinkage of the resin composite, followed by occlusal forces. The cohesive zone model (CZM) was employed to represent the adhesive damage. To emphasize the impact of synergistic loading on adhesive damage, three types of loads were separately applied to the model: polymerization shrinkage, occlusal forces, and combined loading. Subsequently, three clinical resin composites with varying polymerization shrinkage and elastic modulus were used as restorative materials. Sensitivity analysis was conducted on dozens of hypothetical materials to provide definitive results.

RESULTS

Polymerization shrinkage was undergone by the cured resin composite, resulting in extensive adhesive damage. Occlusal forces induced microdamage in regions already damaged by shrinkage stress, especially in the gingival wall. Predictably, the regions with severe adhesive damage were prone to marginal microleakage. The properties of the resin composite can affect adhesive damage. The adhesive damage with bulk-fill resin composite was milder than that with flowable and conventional resin composite. The extent of adhesive damage correlated markedly positively with the polymerization shrinkage of the resin composite and mildly positively with its elastic modulus.

SIGNIFICANCE

Adhesive damage has been directly implicated in marginal microleakage. The cohesive zone model (CZM) can effectively elucidate the distribution of adhesive damage and provide a clear representation of the impact of varying material properties of resin composite on adhesive damage.

Elucidating interfacial failure of cervical restorations using damage mechanics: A finite element analysis

Background/purpose

Although clinical studies have suggested a link between non-axial forces and reduced longevity of cervical restorations, the underlying mechanisms require further numerical investigation. This in-silico study employed a cohesive zone model (CZM) to investigate interfacial damage in a cervical restoration subjected to different load directions.

Materials and methods

A plane strain model of a maxillary premolar was established, with a wedge-shaped buccal cervical restoration. To simulate debonding, the restoration-tooth interface was modeled by the CZM, which defines the strain-softening damage behavior based on interfacial stress and fracture energy. Occlusal loads were applied in three different directions: (1) obliquely on the buccal triangular ridge, (2) obliquely on the palatal triangular ridge, and (3) equal magnitude axially on both ridges. Damage initiation and progression were analyzed, and stress distribution in damaged models was compared with the corresponding perfect-bond models.

Results

Non-axial oblique loads initiated damage at lower forces (100 N for buccal and 120 N for palatal) compared to axial loads (130 N on both ridges). After debonding, buccal oblique loading caused higher stress at the central groove (42.5 MPa at 150 N). Furthermore, buccal oblique loading resulted in more extensive debonding than that caused by the palatal oblique load (88.3% vs. 43.3% of the bonding interface at 150 N).

Conclusion

The study provides numerical evidence supporting the tooth flexure hypothesis, that non-axial forces are more detrimental to the bonding interface of the cervical restoration. The results highlight the necessity of damage mechanics in deriving stress distribution upon debonding.

Direct ink writing with dental composites: A paradigm shift toward sustainable chair-side production

OBJECTIVES

To evaluate the dimensional accuracy of occlusal veneers printed using a novel direct ink writing (DIW) system and a clinically approved dental composite.

METHODS

A novel three-dimensional printer was developed based on the extrusion-based DIW principle. The printer, constructed primarily with open-source hardware, was calibrated to print with a flowable resin composite (Beautifil Flow Plus). The feasibility of this technology was assessed through an evaluation of the dimensional accuracy of 20 printed occlusal veneers using a laboratory confocal scanner. The precision was determined by pairwise superimposition of the 20 prints, resulting in a set of 190 deviation maps used to evaluate between-sample variations.

RESULTS

Without material waste or residuals, the DIW system can print a solid occlusal veneer of a maxillary molar within a 20-minute timeframe. Across all the sampled surface points, the overall unsigned dimensional deviation was 30.1 ± 20.2 µm (mean ± standard deviation), with a median of 24.4 µm (interquartile range of 22.5 µm) and a root mean square value of 36.3 µm. The pairwise superimposition procedure revealed a mean between-sample dimensional deviation of 26.7 ± 4.5 µm (mean ± standard deviation; n = 190 pairs), indicating adequate precision. Visualization of the deviation together with the nonextrusion movements highlights the correlation between high-deviation regions and material stringing.

SIGNIFICANCE

This study underscores the potential of using the proposed DIW system to create indirect restorations utilizing clinically approved flowable resin composites. Future optimization holds promise for enhancing the printing accuracy and increasing the printing speed.

Assessments of polymerization shrinkage by optical coherence tomography-based digital image correlation analysis-Part II: Effects of restorative composites

OBJECTIVES

To examine the polymerization shrinkage of different resin-based composite (RBC) restorations using optical coherence tomography (OCT) image-based digital image correlation (DIC) analysis.

METHODS

The refractive index (RI) of three RBCs, Filtek Z350XT (Z350), Z350Flowable (Z350F), and BulkFill Posterior (Bulkfill), was measured before and after polymerization to calibrate their axial dimensions under OCT. Class I cavities were prepared in bovine incisors and individually filled with these RBCs under nonbonded and bonded conditions. A series of OCT images of these restorations were captured during 20-s light polymerization and then input into DIC software to analyze their shrinkage behaviors. The interfacial adaptation was also examined using these OCT images.

RESULTS

The RI of the three composites ranged from 1.52 to 1.53, and photopolymerization caused neglectable increases in the RI values. For nonbonded restorations, Z350F showed maximal vertical displacements on the top surfaces (-16.75 µm), followed by Bulkfill (-8.81 µm) and Z350 (-5.97 µm). In their bonded conditions, all showed increased displacements. High variations were observed in displacement measurements on the bottom surfaces. In the temporal analysis, the shrinkage of nonbonded Z350F and Bulkfill decelerated after 6-10 s. However, Z350 showed a rebounding upward displacement after 8.2 s. Significant interfacial gaps were found in nonbonded Z350 and Z350F restorations.

SIGNIFICANCE

The novel OCT image-based DIC analysis provided a comprehensive examination of the shrinkage behaviors and debonding of the composite restorations throughout the polymerization process. The flowable composite showed the highest shrinkage displacements. Changes in the shrinkage direction may occur in nonbonded conventional composite restorations.

Szabó V, Szabó B, Vincze-Bandi E, Braunitzer G, Lassila L, Vallittu P, Garoushi S, Fráter M. Mechanical Performance of Extensive Restorations Made with Short Fiber-Reinforced Composites without Coverage: A Systematic Review of In Vitro Studies

In recent years, composite resin materials have been the most frequently used materials for direct restorations of posterior teeth. These materials have some clinically relevant limitations due to their lack of fracture toughness, especially when used in larger cavities with high volume factors or when utilized as direct or indirect overlays or crown restorations. Recently, short-fiber-reinforced composite materials have been used in bi-structure restorations as a dentine substituting material due to their superior mechanical properties; however, there is no scientific consensus as to whether they can be used as full restorations. The aim of our review was to examine the available literature and gather scientific evidence on this matter. Two independent authors performed a thorough literature search using PubMed and ScienceDirect up until December 2023. This study followed the PRISMA guidelines, and the risk of bias was assessed using the QUIN tool. The authors selected in vitro studies that used short-fiber-reinforced composite materials as complete restorations, with a conventional composite material as a comparison group. Out of 2079 potentially relevant articles, 16 met our inclusion criteria. All of the included studies reported that the usage of short-fiber-reinforced composites improved the restoration's load-bearing capacity. Fifteen of the included publications examined the fracture pattern, and thirteen of them reported a more favorable fracture outcome for the short-fiber-reinforced group. Only one article reported a more favorable fracture pattern for the control group; however, the difference between groups was not significant. Within the limitations of this review, the evidence suggests that short-fiber-reinforced composites can be used effectively as complete restorations to reinforce structurally compromised teeth.