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 ex....

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 mer....

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