Quasistatic Mechanical Testing for Computer-Aided Design and Manufacturing Occlusal Veneers Cemented to Milled Dentin Analog Material

Summary

A quasistatic load-to-fracture test with a non-fixed stainless steel ball was developed to determine the fracture strength of minimally invasive posterior computer-aided design and manufacturing restorations cemented to dentin analog materials. This test models the typical loading regime responsible for the fracture of dental restorations.

Abstract

Under current minimally invasive treatment regimes, minor tooth preparation and thinner biomimetic ceramic restoration are used to preserve the restored tooth's vitality, aesthetics, and function. New computer-aided design and computer-aided manufacturing (CAD/CAM) ceramic-like material are now available. To guarantee longevity, a dental clinician must know these newly launched product's mechanical strength compared to the relatively brittle glass-matrix ceramic. Furthermore, a tooth substitute has been promoted for laboratory investigation, especially after the pandemic, and more evidentiary support is required for its application.

This study developed a laboratory protocol for a monotonic load-to-fracture test to determine the fracture strength of 1 mm-thick CAD/CAM occlusal veneers. Master dies were milled from high-pressure fiberglass laminate, which has similar elastic modulus and bond strength as hydrated dentin. They were mounted into polyvinyl chloride (PVC) end caps with cold-curing epoxy resin. Occlusal veneers, also called tabletop restorations, were milled from lithium disilicate (LD) and resin nanoceramic blocks (RNC) and cemented to prepared master dies using dual-cured adhesive resin cement. They were allowed to cure fully by storing in distilled water for 48 h at 37 °C.

All samples were then placed in a universal testing machine and loaded via a non-fixed 5.5 mm stainless-steel ball that allows lateral movement as would occur against the antagonist teeth. Compression was applied at a 1 mm/min rate, and the load-displacement graph was generated. The average maximum load-bearing capacity of restorations in the RNC group (3,212.80 ± 558.67 N) was significantly higher than in the LD group (2727.10 ± 472.41 N) (p < 0.05). No debonding was found during the test. Both CAD/CAM materials may have a similar flaw distribution. Hertzian cone crack was found at the loading site, whereas radial cracks propagating from the cementation surface were found close to the margin in both groups.

Introduction

Metal-free restorations are now highly preferred in anterior and posterior dentition due to their excellent optical characteristics and biocompatibility¹. However, the major drawback of such materials is their susceptibility to fracture². Most ceramics are vulnerable to cracks generated by tensile stresses, even under low strain³. Fractures of dental ceramic prostheses usually develop from slow radial crack growth due to long-term exposure to the tensile stresses generated during chewing⁴. Their weaknesses escalate with intrinsic flaws or defects within the materials and extrinsic flaws from fabrication and postprocessing⁵. Flexural strength, the capability to withstand tensile stress, of dental CAD/CAM materials can be achieved and compared through standard tests such as uniaxial (3-point or 4-point bending) and biaxial bending tests (ball-on-ring, ring-on-ring, and piston-on-three balls). Meanwhile, fracture toughness, the capability of a material to resist crack growth, can be derived from a single-edged notch beam and an indentation test. However, these tests cannot entirely predict and represent the behavior of the cemented prostheses with different anatomical configurations⁶. Other monotonic or dynamic mechanical tests have been introduced to justify their performance with various clinical aspects^7,8.

A load to fracture or "crunch the crown" test has been used extensively in dentistry to investigate and compare the strengths of ceramic restorations with complex geometries^9,10. Monotonic uniaxial compression is quasistatically exerted on the restorations in a vertical or lateral direction until a catastrophic fracture occurs. The fracture strength of the material can be determined from the maximum loading force, whereas the modes of fracture, including the site and direction of the crack(s), can be examined microscopically. A good restoration should be able to withstand both compressive and tensile stress from the voluntary maximum bite force, the highest masticatory force generated by jaw-elevator muscles under the influence of craniomandibular biomechanics and the reflex pathway^11,12, which could be up to 900 N in the posterior teeth³. Furthermore, bruxism can involuntarily increase the force to 1,200 N in the same region¹³. In addition to material properties (i.e., elastic modulus), geometries, thicknesses, adhesive cement, and defect distributions influence any prosthesis's strength¹⁴. However, arguments have been raised on the clinical relevance of such tests due to the nonclinical high forces and the failure mechanisms being dissimilar to clinical situations^6,14. A fatigue resistance test involving step-stress analysis and intraoral condition may be a more realistic approach for predicting the longevity of dental restorations⁷. Nevertheless, the load to fracture is still a quick, simple, and repeatable in vitro test to compare the strengths of new CAD/CAM ceramics materials launched onto the market where the manufacturer's data may not be dependable^15,16,17. The result may reflect the prosthesis's tolerance to extreme forces from parafunction activities and unexpected clinical situations such as biting on hard seeds or gravels, which also causes failure in dental prostheses^18,19,20,21.

With their increasing use to rehabilitate posterior teeth, the mechanical performance of occlusal veneers made from milled and printed CAD/CAM materials has been investigated for various aspects, including types of materials, prosthetic designs, tooth abutment preparation design, thicknesses, surface treatments, adhesive bonding, and luting cement system^22,23. However, the data are still limited, and the test materials are from glass matrix ceramic and conventional CAD/CAM composite materials. An alternative hybrid material, resin nanoceramic, is now available. It claims to incorporate the strength of nanoceramic fillers and resiliency from resin matrix, which may be suitable for thin, minimally invasive restoration. However, its mechanical performance, especially in the molar region, requires more supporting evidence for clinical implications.

Until now, researchers have not had materials that can substitute for natural teeth in laboratory testing. High-pressure fiberglass laminate (National Electrical Manufacturers Association; NEMA grade G10) with the tradename of Garolite has been proposed as a dentin analog material for the mechanical testing of dental ceramics since 2010¹⁴. It is a thermoset composite material comprising multilayers of fiberglass soaked in epoxy resin under high pressure. It can withstand high-stress conditions with similar elastic properties, fatigue behavior, and adhesive bond strengths as hydrated dentin^14,24. It provides advantages over natural teeth regarding specimen preparation, standardization, and ethical authorization, with time savings due to reduced biosafety concerns²⁴. Surface treatment can be performed by etching with 5% or 10% hydrofluoric acid from 60 s to 90 s and applying a silane coupling agent^14,24. Nevertheless, studies on cemented prostheses with this material are limited, and the reliability of the existing evidence is still questionable^24,25.

In this study, a laboratory protocol for a monotonic load-to-fracture test of 1 mm-thick occlusal veneers cemented to the master dies milled from dentin analog material against a nonfixed stainless steel ball was developed. The maximum load-bearing capacities of two dental CAD/CAM materials: lithium disilicate (LD) - IPS e.max CAD and resin nanoceramic (RNC) - Lava Ultimate, with n = 15 per group, were quantified and statistically compared through a two-sample independent t-test and Weibull statistical analysis. The fracture patterns were also investigated under optical stereomicroscopy and scanning electron microscopy. The study hypothesis was that this was an appropriate method of modeling the failure of occlusal veneers in clinical applications. The statistical null hypothesis was that there should be no difference in the maximum load-bearing capacities between the occlusal veneers made from the two materials.

Protocol

1. Tooth analog fabrication

1. Anatomically reduce the occlusal surface of a typodont mandibular first molar (with bifurcated root) by 1 mm and bevel the margin using coarse and fine diamond burs.
2. Scan the prepared typodont with a dental laboratory scanner.
3. Open the scanned file with OrthoAnalyzer in the CAD software. On the Sculpt toolkit window, click on the Wax knife tool and set its diameter and level as 2.6 mm and 63 μm, respectively. Gradually pull each root surface toward each other to merge the bifurcated roots into a single root to facilitate the milling process (Figure 1).
4. Mill the tooth analog dies from a high-pressure fiberglass laminate (n = 30) (Garolite, National Electrical Manufacturers Association [NEMA] grade G10) using a five-axis milling machine (Figure 2).

2. Mounting

1. Use any appropriate CAD software (e.g., Autodesk Inventor Professional 2025) to design a jig that fits the root section of the model tooth and the space inside the PVC end caps to ensure standardized position and orientation of the test specimen.
2. Print one jig per test toot in PMMA or similar modulus material using a 3D printer.
3. Combine the root parts and dies with the PVC end cap, typically 25 mm in inner diameter, 21.5 mm high, and 5.5 mm in wall thickness.
4. Mix the cold-curing low-viscosity epoxy resin and pour it up to the cementoenamel junction region of the model teeth. Be careful not to contaminate the occlusal surface of the model teeth with the flowing resin. Leave to fully set at room temperature for at least 24 h (Figure 3).

3. Occlusal veneer fabrication

1. Import the scanned file of tooth analog to the CAD software.
2. Under Directions, determine the insertion direction of the occlusal veneer.
3. Under Interfaces, select Margin line and mark the margin line of the scanned tooth analog. Then, select Die interface | Advance settings and adjust the cement gap to 0.025 mm and the extra cement gap to 0.050 mm.
4. Under Anatomy design, design a 1 mm-thick occlusal veneer using a template from Smile Library and adjust it with tools in Sculpt.
5. Mill the lithium disilicate (IPS e.max CAD) and resin nanoceramic (Lava Ultimate) blocks (n=15 per group) using a five-axis milling machine and following the manufacturer's instructions (Figure 4).

4. Bonding and cementation

1. Clean all master dies for 90 s in an ultrasonic machine and air dry. Then, apply 5% hydrofluoric acid to the occlusal surface for 60 s, thoroughly rinse with water, and air dry.
2. For the LD group, etch the internal surface with 5% hydrofluoric acid for 20 s, thoroughly rinse with water, and air dry.
3. For the RNC group, air-abrade with aluminum oxide powder grain size 50 µm at 2 bars (200 kPa, 30 psi) for 10 s. Remove excess sand with alcohol and air dry.
4. Apply silane, a universal bonding agent, and load dual-cured adhesive cement to the restoration intaglio. Place the restoration onto the prepared master dies by loading them under a silicone-filled compression head in an universal testing machine with a 40 N load.
5. Cure with a light-emitting diode (LED) using light intensity in the normal mode of 1,000-1,200 mW/cm² for 1-2 s. Remove excess cement, continue the curing light on each surface for 20 s, and leave it in the universal testing machine for 5 min.
6. Remove from the universal testing machine and leave in distilled water at 37 °C for 48 h to allow the cement to fully cure.
7. Before testing, use fine permanent markers to draw three medial-lateral reference lines (sinistral, central, and dextral) and three anterior-posterior reference lines (upper, central, and lower) in different colors using fine permanent markers.

5. Quasistatic mechanical testing

1. Position the test specimen in the center of the lower platen of a mechanical testing machine with a 5 kN load cell set up for compression testing.
2. Place a 5.5 mm diameter stainless-steel ball in the central fossa of the restoration at the intersection of the central reference lines (Figure 5).
3. Place a protective acrylic ring around the specimen and a debris shield in front of the testing machine to limit flying debris.
4. Bring the crosshead down until nearly in contact with the steel ball and zero load and displacement.
5. Apply compression at 1 mm/min until the restoration fractures, indicated by a sudden drop in the load. Record this load (Figure 5).
6. After fracture, remove the shield and the acrylic ring, and carefully collect the test specimen and its fragments. The colored lines assist in placing the ceramic fragments into their original positions for subsequent analysis (Figure 5).
7. Place the next test specimen and follow steps 5.2-5.6 until the groups are complete.

6. Statistical analysis

1. Fill in Minitab worksheets with material codes and maximum loading values (N) retrieved from the experiment in the first and second columns, respectively.
2. Conduct an independent t-test for two samples by selecting Stat | Basic Statistics | 2-sample t. Set the confidence level at 95% and assume equal variances.
3. Create the Weibull distribution plot by selecting Stat | Reliability/Survival | Distribution Analysis (Right Censoring) | Distribution ID Plot. Choose maximum loading values in the Variable box, tick and choose material names in the By variable box, specify distribution as Weibull, and click OK.
4. Conduct the Weibull statistical analysis by selecting Stat | Reliability/Survival | Distribution Analysis (Right Censoring) | Parametric Distribution Analysis | Weibull | Graphs. Choose the Probability plot | Display confidence intervals on above plots and click OK.

7. Fractographic analysis

1. For stereo microscopy, mount the eyepiece camera and capture images (20x) of the samples' aerial and side views via the stereomicroscopy software.
2. For scanning electron microscopy, cut the specimen to the cementoenamel junction (CEJ), put it in an acetone bath in an ultrasonic cleaner and then air dry. Gold-coat and capture images (250-300 magnification) of the samples' aerial and side views.

Results

The sample size calculation was performed using the referenced software, which generated an effect size of 0.39 and suggested a minimum sample size of n = 13 per group. However, a sample size of n = 15 was chosen in this study to detect the difference of 5%. The null hypothesis was rejected. Despite having greater flexural strength, the mean values of the maximum loading force of the 1 mm-thick occlusal veneers (n = 15 per group) made from the LD (lithium disilicate: 2,727.10 ± 472.41 N) group were significantly low...

Discussion

In recent years, minimally invasive occlusal veneers have increasingly received attention in contemporary restorative dentistry. These restorations are usually fabricated from monolithic CAD/CAM glass-matrix ceramic, polycrystalline, and hybrid materials²⁶. Conservative tooth preparation, the ease of access and visibility during tooth preparation, impression taking and cementation, and preservation of the marginal gingiva have been promoted as advantages^26,

Materials

Name	Company	Catalog Number	Comments
3D printing (SLA)	Formlabs, Somerville, MA, USA		Form3+
3Shape Dental Designer CAD software	3Shape A/S, Copenhagen, Denmark		CAD software for tooth analog and veneers
5% hydrofluoric acid	Ivoclar Vivadent, Schaan, Liechtenstein		IPS Ceramic Etching Gel
Alumina powder	Ronvig Dental Mfg. A/S, Daugaard, Denmark
Bluehill Universal materials testing software	Instron Mechanical Testing Systems, Norwood, MA, USA
CamLabLite software	Bresser UK Ltd, Kent, UK		Stereomicroscopy Software
Cold-curing low-viscosity epoxy resin	Struers SAS, Champigny-sur-Marne, France
Dual-cure resin cement	3M, Saint Paul, MN, USA		Rely X Ultimate Adhesive Resin Cement
Eyepiece camera	ToupTek Photonics Co., Ltd., Hangzhou, China
High-pressure fibreglass laminate discs  (G10)	PAR Group Ltd, Lancashire, UK
IPS e.max CAD	Ivoclar Vivadent, Schaan, Liechtenstein	YB54G9/605330	Low translucency, A3, C14
Laboratory scanner	3Shape A/S, Copenhagen, Denmark		D900L
Lava Ultimate	3M ESPE, Saint Paul, MN, USA	9541467/3314A3-LT	Low translucency, A3, 14L
Light-emitting diode (LED) curing light	Woodpecker Medical Instrument, Guilin, China
Milling machine	VHF camfacture AG, Amnnerbuch, Germany		VHF S2
Minitab 18	Minitab Inc, State College, PA, USA
nQuery Advisor Version 9.2.10	Statistical Solutions Ltd., CA, USA		Statistical Software
Polyvinyl chloride end cap	Plastic Pipe Shop Ltd, Stirling, UK		25 mm X 21.5 mm;
Scanning electron microscope	Tescan, Brno, Czech Republic		Tescan Vega
Silane coupling agent	3M, Saint Paul, MN, USA		RelyX Ceramic Primer
Autodesk Inventor Professional 2024	Autodesk, San Francisco, CA, USA		CAD software for jig
Sputter vacuum coater	Quorum, East Sussex, UK		MiniQS Sputter Coater
Stata18	StataCorp LLC, College Station, TX, USA
Stereomicroscope	Carl Zeiss AG, Oberkoche, Germany		Zeiss Stemi 508
Typodont mandibular first molar	Frasaco GmbH, Tettnang, Germany		ANA-4 Z3RN-36
Universal dental bonding agent	3M, Saint Paul, MN, USA		Scotch Bond Universal Adhesive
Universal testing machine	Instron Mechanical Testing Systems, Norwood, MA, USA		Intron 5900-84