Name: Quasistatic Mechanical Testing for Computer-Aided Design and Manufacturing Occlusal Veneers Cemented to Milled Dentin Analog Material (Video) | JoVE Journal
Uploaded: 2024-12-20T14:50:00.000+00:00
Duration: 7 min 42 s
Description: Learn quasistatic mechanical testing for evaluating fracture resistance of CAD/CAM occlusal veneers cemented to dentin analog materials. This repeatable method helps researchers assess minimally invasive dental restorations.

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5 minutes

This study has received funding from the Faculty of Dentistry, Mahidol University, Bangkok, Thailand. The authors thank Dr Erica Di Federico from School of Engineering and Materials Science and Dr Thomas Kelly from School of Geography at Queen Mary University of London for their expert technical inputs &#38; guidance in this work.

1. Tooth analog fabrication
<ol>
	<li>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.</li>
	<li>Scan the prepared typodont with a dental laboratory scanner.</li>
	<li>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&#160;&#956;m, 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).</li>
	<li>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).</li>
</ol>
2. Mounting
<ol>
	<li>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.</li>
	<li>Print one jig per test toot in PMMA or similar modulus material using a 3D printer.</li>
	<li>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.</li>
	<li>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).</li>
</ol>
3. Occlusal veneer fabrication
<ol>
	<li>Import the scanned file of tooth analog to the CAD software.</li>
	<li>Under Directions, determine the insertion direction of the occlusal veneer.</li>
	<li>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.</li>
	<li>Under Anatomy design, design a 1 mm-thick occlusal veneer using a template from Smile Library and adjust it with tools in Sculpt.</li>
	<li>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&#39;s instructions (Figure 4).</li>
</ol>
4. Bonding and cementation
<ol>
	<li>Clean all master dies for 90 s in an&#160;ultrasonic machine and air dry. Then, apply 5% hydrofluoric acid to the occlusal surface for 60 s, thoroughly rinse with water, and air dry.</li>
	<li>For the LD group, etch the internal surface with 5% hydrofluoric acid for 20 s, thoroughly rinse with water, and air dry.</li>
	<li>For the RNC group, air-abrade with aluminum oxide powder grain size 50 &#181;m at 2 bars (200 kPa, 30 psi) for 10 s. Remove excess sand with alcohol and air dry.</li>
	<li>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&#160;universal testing machine with a 40 N load.</li>
	<li>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.</li>
	<li>Remove from the universal testing machine and leave in distilled water at 37 &#176;C for 48 h to allow the cement to fully cure.</li>
	<li>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.</li>
</ol>
5. Quasistatic mechanical testing
<ol>
	<li>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.</li>
	<li>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).</li>
	<li>Place a protective acrylic ring around the specimen and a debris shield in front of the testing machine to limit flying debris.</li>
	<li>Bring the crosshead down until nearly in contact with the steel ball and zero load and displacement.</li>
	<li>Apply compression at 1 mm/min until the restoration fractures, indicated by a sudden drop in the load. Record this load (Figure 5).</li>
	<li>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).</li>
	<li>Place the next test specimen and follow steps 5.2-5.6 until the groups are complete.</li>
</ol>
6. Statistical analysis
<ol>
	<li>Fill in Minitab worksheets with material codes and maximum loading values (N) retrieved from the experiment in the first and second columns, respectively.</li>
	<li>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.</li>
	<li>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.</li>
	<li>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.</li>
</ol>
7. Fractographic analysis
<ol>
	<li>For stereo microscopy, mount the eyepiece camera and capture images (20x) of the samples&#39; aerial and side views via the stereomicroscopy software.</li>
	<li>For scanning electron microscopy, cut the specimen to the cementoenamel junction (CEJ), put it in an acetone bath in an ultrasonic cleaner&#160;and then air dry. Gold-coat and capture images (250-300 magnification) of the samples&#39; aerial and side views.</li>
</ol>

Quasistatic Load-to-Fracture Test of CAD/CAM Occlusal Veneers on Dentin Analogs

The authors have no conflicts of interest to declare.

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 advantages26,27. The current guidelines recommend a 1.5-2.0 mm thickness at the supporting cusp and central fossa to ensure durability and mechanical performance28,29. However, due to favorable experimental survival rates, ultrathin occlusal veneers of 0.7-1.0 mm thicknesses are now broadly employed as a minimally invasive restorative approach26,30,31.
Differences in the microstructural components and processing techniques can affect the dental CAD/CAM material&#39;s properties, including physicomechanical properties, optical properties, coefficient of thermal expansion (CTE), and chemical solubility3. After machining, LD usually requires heat treatment to convert the metasilicate crystallites into fully crystallized lithium disilicate crystals with a flexural strength of ~377 MPa and an elastic modulus of 67.2 GPa32,33,34. In contrast, the RNC did not require postprocessing and had a flexural strength up to 200 MPa and an elastic modulus of 12.7 GPa34. Several studies found that flexural strength values from general standard tests using simple dimensions did not correlate with failure loads of dental restorations with different complex morphology and thicknesses15,16,17.
In this study, the fracture resistance of 1 mm occlusal veneers made from RNC was superior to that of the LD group, with similar results to previous studies performed using natural teeth35,36,37. The higher load-bearing capacity of this hybrid material may derive from the strength and resiliency of the ceramic fillers and resin composite matrix, which allow the harmonization of its elastic modulus with dentin (15 GPa) and dentin-analog material (16.5 GPa), enhancing stress distribution14,38. This study did not incorporate the aging process and&#160;chewing simulation into the calculation, hence&#160;could not represent the long-term strength values of these restorations. However, the survival rate of occlusal veneers made from early CAD/CAM composite resin is reported to be higher than that of LD39,40.
Unlike dental alloys, dental ceramics do not exhibit a Gaussian or normal distribution of strength values due to the variation in the distribution of flaw sizes41. Their failure is usually associated with the large-sized flaw population, which generally weighs the failure frequency to the right side of the distribution. Therefore, a parametric regression would be required to confirm the reliability of the material&#39;s strength. According to BS EN 61649: 2008, Weibull statistical analysis could be applied with a sample size as small as n = 10 but with limited confidence42. A large sample size, usually greater than 20, would be required to ensure statistical reliability and more minor estimation errors43. Materials with low&#160;Weibull modulus (m) values may fail or break in a broad region. In contrast, those with large values are likelier to fail at the highest stress region due to a narrow strength distribution. Most m-values of commercial dental ceramics fall between 5 and 15 43, while&#160;the result of both CAD/CAM materials used in this study is ~6.7, which could differ when more test samples are included.
Future dental research, when more concerns have been placed on bio-safety issues and human rights laws44, may rely more on substitute materials for in vitro experimental tests. This study promotes the use of milled dentin analog material made from woven fiberglass impregnated with epoxy resin over polymethyl methacrylate (PMMA), which has lower mechanical strength and debonding issues45. A glass-filled polyamide has also been suggested as another material substitution for laboratory experiments24. Nonetheless, these synthetic materials may not be completely able to replace natural teeth despite having similar elastic properties and bonding ability. A recent study found that the lithium disilicate crowns cemented with either epoxy glass laminate or glass-filled polyamide showed higher fatigue resistance than when bonded to natural dentin25.
This study cannot compare such results since it did not include natural teeth a&#160;control&#160;group. However, when comparing two or more different veneer materials, using an artificial model for natural teeth shows increased repeatability and has no supply, storage, sterilization, or disease transmission concerns. The only concern of epoxy glass laminate is its surface hardness, which hindered the milling process, especially at the root bifurcation. A simplification of using a single nonanatomic root with the absence of periodontal ligament does&#160;not alter the load-bearing behavior of the restoration46. Further studies are required to fully justify these substitute materials&#39; mechanical properties and bonding performances with other restorative materials via different approaches.
This study also highlights the feasibility of using a nonfixed stainless-steel ball as an indenter for a monotonic uniaxial compression test. Several types of fixed indenters have been used in laboratory tests, including blunt, piston, half-ball, and ball shapes14,47,48,49,50. However, these may create undesirable stress at one spot in the test materials. Furthermore, it does not model the clinical position, where the antagonist cusps can move up to 0.5 or 1 mm eccentrically51,52. Unlike a fixed indenter, the stainless steel ball used in this study can slightly move to contact and dissipate the stress into the center of the prosthesis, preventing stress accumulation at a single inclined surface. Like previous studies, the fractographic investigation revealed that the load-to-fracture with monotonic loading might not be able to fully depict the subsurface crack system originating from the bonding surface opposite to the loading site6,14. However, in this study, subsurface radial cracks at the margins could be observed in both restorative groups, marking the common failure of the ceramic prosthesis from long-term daily usage53,54. Therefore, paying attention to the machining process of the minimally invasive prostheses&#39; margin is essential in not creating extrinsic flaws or defects that could escalate the failure events.
Within this study&#39;s limitations, it can be concluded that the 1 mm occlusal veneers made from CAD/CAM resin nanoceramic have superior fracture resistance to&#160;conventional lithium disilicate. Still, both demonstrated adequate fracture strength against voluntary and involuntary maximum bite force and are promising materials for restoring posterior teeth under a minimally invasive scheme. The quasistatic mechanical testing in this study can give a clue to dental CAD/CAM restorative materials&#39; strength, probably in the early stage after placement, via maximum load-bearing capacity with clinically relevant crack systems being seen. Newer restorative materials such as advanced lithium aluminum silicate (LAS), zirconia-reinforced lithium disilicate (ZLS), and other tooth analog materials could be further investigated with this proposed laboratory setting under artificial aging and simulation of oral conditions.

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 aspects7,8.
A load to fracture or &#34;crunch the crown&#34; test has been used extensively in dentistry to investigate and compare the strengths of ceramic restorations with complex geometries9,10. 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 pathway11,12, 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&#39;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 situations6,14. 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&#39;s data may not be dependable15,16,17. The result may reflect the prosthesis&#39;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 prostheses18,19,20,21.
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 system22,23. 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 dentin14,24. 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 agent14,24. Nevertheless, studies on cemented prostheses with this material are limited, and the reliability of the existing evidence is still questionable24,25.
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&#160;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.

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

quasistatic mechanical testing for computer-aided design and manufacturing occlusal veneers cemented to milled dentin analog material

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

Under current minimally invasive treatment regimes, minor tooth preparation and thinner biomimetic ceramic restoration are used to preserve the restored tooth&#39;s vitality, aesthetics, and function. New computer-aided design and computer-aided manufacturing (CAD/CAM) ceramic-like material are&#160;now available. To guarantee longevity, a dental clinician must know these&#160;newly launched product&#39;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&#160;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&#160;allowed to cure fully by storing in distilled water for 48 h at 37 &#176;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&#160;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 &#177; 558.67 N) was significantly higher than in the LD group (2727.10 &#177; 472.41 N) (p &#60; 0.05). No debonding was found during the test.&#160;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.

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 lower than those of the RNC (resin nanoceramic: 3,212.80 ± 558.67 N) group (p &lt; 0.05). The scale parameter or characteristic strength (σ) of LD when the probability of failure is 63.2% is slightly lower than the RNC group. However, the shape parameter or Weibull modulus (m) depicted by the slope of the probability plot (95% CI) of the LD was similar to the RNC group, as shown in Figure 6, reflecting that both materials may possess a similar distribution of internal flaws or defects. All data are presented in Table 1.
Stereomicroscope images of fractured LD and RNC occlusal veneers in aerial and side views are presented in Figure 7 and Figure 8, respectively. Both test groups depicted the surface ring-shaped cracks at the intersection of the central reference lines at the central fossa. The crack was, in fact, a part of the Hertzian cone crack system that extensively expands into the deep layer of the restorations. It typically consisted of an outer crack (OC), inner crack (IC), and median crack (MC), as shown in Figure 9. The radial crack originating and propagating from the cementation surface opposite to the impaction site was difficult to identify due to the thinness of the restorations and the nature of the crunch-the-crown test. However, the secondary fracture origin caused by tensile stress presenting as radial cracks could be observed at the marginal area of the remaining restoration attached to the master dies. Fragments showing the fractographic features of the direction of radial crack propagation from the secondary fracture origin are presented in Figure 10. Complete bulk fractures were consistently located at the distolingual aspect of both restorative groups except a sample in the RNC group where the fracture occurred buccolingually at the distal aspect.
Representative higher magnification images under the scanning electron microscope in aerial and side views of both test groups are shown in Figure 11 and Figure 12. Due to the impregnation of 80 wt% nanoceramic fillers into the 20 wt% highly cross-linked polymer matrix, the fracture surfaces of veneers made from RNC presented rough and fibrous appearances, which are characteristic of plastic deformation in ductile fractures. In contrast, the restorations made from LD shattered into multiple pieces with cracks propagating from the fracture origins (FO) into the deeper layers straightforwardly with few crack disruptions or diversions. In addition, the original impaction area of a stainless-steel ball could be spotted at the upper right area approximately 1 mm from a fractured line in some specimens, regardless of the material type. This shows that the ball could slip into its final position when it was not perfectly aligned on the central fossa during the compressive loading.
<img alt="Dental morphology diagram showing different tooth aspects, crucial for anatomy studies." class="xfigimg" src="/files/ftp_upload/67511/67511fig01.jpg" data-altupdatedat="1788304762000" data-alt="Figure 1"> 
Figure 1: Digital images from the dental computer-aided design software showing the merged bifurcated root of a typodont mandibular first molar. <a href="https://www.jove.com/files/ftp_upload/67511/67511fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Dental plaster model with cavity detail, showcasing dental restoration technique." class="xfigimg" src="/files/ftp_upload/67511/67511fig02.jpg" data-altupdatedat="1788304762000" data-alt="Figure 2"> 
Figure 2: Milled master die from high-pressure fiberglass laminate disc using a 5-axis dental milling machine. <a href="https://www.jove.com/files/ftp_upload/67511/67511fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Three-step experimental setup: chromatography columns, samples loaded for protein purification." class="xfigimg" src="/files/ftp_upload/67511/67511fig03.jpg" data-altupdatedat="1788304762000" data-alt="Figure 3"> 
Figure 3: Master dies mounting. (A) The jigs are fabricated from printed PMMA filament. (B) They are securely placed into a PVC end cap to standardize the orientation and position of the tooth analog. (C) The cold-curing low-viscosity epoxy resin is poured up to the cementoenamel junction of the tooth analog and left at room temperature for at least 24 h to set. Abbreviations: PMMA = polymethyl methacrylate; PVC = polyvinyl chloride. <a href="https://www.jove.com/files/ftp_upload/67511/67511fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Dental crown manufacturing process; 3D printed models vs. final ceramic crowns; comparative diagram." class="xfigimg" src="/files/ftp_upload/67511/67511fig04.jpg" data-altupdatedat="1788304762000" data-alt="Figure 4"> 
Figure 4: CAD/CAM blocks after milling and finishing following manufacturer's instructions. (A) Lithium disilicate, (B) Resin nanoceramic. Abbreviations: CAD = computer-aided design; CAM = computer-aided manufacturing. <a href="https://www.jove.com/files/ftp_upload/67511/67511fig04large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Tribological test setup and results; equipment includes pin-on-disk tribometer and analyzed specimen." class="xfigimg" src="/files/ftp_upload/67511/67511fig05.jpg" data-altupdatedat="1788304762000" data-alt="Figure 5"> 
Figure 5: Quasistatic mechanical testing. (A) A 5.5 mm stainless-steel ball is placed on the central fossa of the restoration. (B) The specimen is placed at the center of the universal testing machine with a 5 kN load cell and compressed with a compression plate at 1 mm/min protected by the acrylic ring and debris shield. The extension (mm) and maximum loading force (N) will be recorded in the software. (C) Bulk pieces of fractured specimens are carefully collected and placed back into their original position following the colored lines from the permanent marker. <a href="https://www.jove.com/files/ftp_upload/67511/67511fig05large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Failure probability graph, maximum fracture load comparison, LD vs RNC, material strength analysis." class="xfigimg" src="/files/ftp_upload/67511/67511fig06.jpg" data-altupdatedat="1788304762000" data-alt="Figure 6"> 
Figure 6: Weibull probability plot of the maximum fracture load at 95% CI between occlusal veneers made from LD and RNC. The maximum fracture load (N) at 95% CI was between occlusal veneers made from LD (blue line) and RNC (red line). The shape parameter or the Weibull modulus (m) represents the slope of the plot, whereas the scale parameter or the characteristic strength (σ) represents the strength value of materials when the probability of failure is at 63.2% (green line). The parameters and fitted line show the maximum likelihood analysis. Abbreviations: LD = lithium disilicate; RNC = resin nanoceramic. <a href="https://www.jove.com/files/ftp_upload/67511/67511fig06large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Dental crack analysis with dye penetration; microscope image; structural examination over time" class="xfigimg" src="/files/ftp_upload/67511/67511fig07.jpg" data-altupdatedat="1788304762000" data-alt="Figure 7"> 
Figure 7: Lithium disilicate occlusal veneers under a stereomicroscope (20x). (A) Aerial view; (B) lateral view. Scale bars = 2 mm (A), 1 mm (B). <a href="https://www.jove.com/files/ftp_upload/67511/67511fig07large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Tooth enamel erosion progression, sequential high-res microscope images showing decay stages, 2mm, 1mm." class="xfigimg" src="/files/ftp_upload/67511/67511fig08.jpg" data-altupdatedat="1788304762000" data-alt="Figure 8"> 
Figure 8: Resin nanoceramic occlusal veneers under a stereomicroscope (20x). (A) Aerial view; (B) lateral view. Scale bars = 2 mm (A), 1 mm (B). <a href="https://www.jove.com/files/ftp_upload/67511/67511fig08large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Histological section analysis; labeled tissue structures; microscope image; anatomical study." class="xfigimg" src="/files/ftp_upload/67511/67511fig09.jpg" data-altupdatedat="1788304762000" data-alt="Figure 9"> 
Figure 9: Representative fractographic features of occlusal veneers. (A) LD and (B) RNC. The white arrows point at the ring crack where the primary fracture origin occurred and propagated from the ball compression, generating typical Hertzian cone cracks with outer cone cracks, inner cone cracks, and a median crack. The secondary fracture origin caused by the tensile stress was found close to the marginal region. The dotted arrows mark the direction of crack propagation where other features, such as arrest lines, can also be noticed depending on the imaging conditions. Scale bars = 2 mm (A), 1 mm (B). Abbreviations: LD = lithium disilicate; RNC = resin nanoceramic; 1° FO = primary fracture origin; 2° FO = secondary fracture origin; OC = outer cone crack; IC = inner cone cracks; MC = median crack; DCP = direction of crack propagation; AL = arrest lines. <a href="https://www.jove.com/files/ftp_upload/67511/67511fig09large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Cross-sectional diagrams of tooth structure, occlusal and bonding surfaces, with labeled features." class="xfigimg" src="/files/ftp_upload/67511/67511fig10.jpg" data-altupdatedat="1788304762000" data-alt="Figure 10"> 
Figure 10: Representative fractographic features of occlusal veneer fragments. (A) LD and (B) RNC. The secondary fracture origin was close to the occlusal veneer's margin. The dotted arrows mark the direction of crack propagation with other fractographic features such as mist zone, twisted hackle, Wallner line, and arrest line. Scale bars = 1 mm. Abbreviations: LD = lithium disilicate; RNC = resin nanoceramic; 2° FO = secondary fracture origin; DCP = direction of crack propagation; AL = arrest lines; M = mist zone; TH = twisted hackle; WL = Wallner line. <a href="https://www.jove.com/files/ftp_upload/67511/67511fig10large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="SEM images of fracture surfaces, highlighting crack propagation in material at 252x and 229x magnification." class="xfigimg" src="/files/ftp_upload/67511/67511fig11.jpg" data-altupdatedat="1788304762000" data-alt="Figure 11"> 
Figure 11: Representative SEM images of fractured LD occlusal veneers. (A) Aerial view;(B) side view. The white arrow represents the initial impaction of the stainless steel ball. Scale bars = 1 mm. Abbreviations: LD = lithium disilicate; SEM = scanning electron microscope. <a href="https://www.jove.com/files/ftp_upload/67511/67511fig11large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Scanning electron microscope image comparing surface morphology at different magnifications." class="xfigimg" src="/files/ftp_upload/67511/67511fig12.jpg" data-altupdatedat="1788304762000" data-alt="Figure 12"> 
Figure 12: Representative SEM images of fractured RNC occlusal veneers. (A) Aerial view; (B) side view. Scale bars = 1 mm. Abbreviations: RNC = resin nanoceramic; SEM = scanning electron microscope. <a href="https://www.jove.com/files/ftp_upload/67511/67511fig12large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Table 1. <a href="https://www.jove.com/files/ftp_upload/67511/67511_Table%201%20(Edited).xlsx" target="_blank">Please click here to download this Table.</a>

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.

Under current minimally invasive treatment regimes, minor tooth preparation and thinner biomimetic ceramic restoration are used to preserve the restored ...

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Research

JoVE Journal

Medicine

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Learn quasistatic mechanical testing for evaluating fracture resistance of CAD/CAM occlusal veneers cemented to dentin analog materials. This repeatable method helps researchers assess minimally invasive dental restorations.

Quasistatic Mechanical Testing for Computer-Aided Design and Manufacturing Occlusal Veneers Cemented to Milled Dentin Analog Material (Video) | JoVE Journal

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

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