Stereolithography: A new method for processing dental ceramics by additive computer-aided manufacturing

Abstract

Objectives

The aim of this study was to compare the physical and mechanical properties of stereolithography (SLA)- manufactured alumina ceramics of different composition to those of subtractive- manufactured ceramics and to produce suitable dental crown frameworks.

Methods

The physical and mechanical properties of a control and six experimental SLA ceramics prepared from slurries with small (S) and large (L) particles (0.46 ± 0.03 and 1.56 ± 0.04 μm, respectively) and three dry matter contents (70%, 75%, 80%) were evaluated by dynamic rheometry, hydrostatic weighing, three3-point flexural strength measurements, and Weibull analyses, and by the micrometrics measurement of shrinkage ratio before and after the heat treatments.

Results

S75 was the only small particle slurry with a significantly higher viscosity than L70. The viscosity of the S80 slurry made it impossible to take rheological measurements. The viscosities of the S75 and S80 slurries caused deformations in the printed layers during SLA manufacturing and were excluded from further consideration. SLA samples with low dry matter content had significantly lower and densityflexural strengths. Only SLA samples with a large particle size and high dry matter content (L75 and L80) were similar in density and flexural strength to the subtractive- manufactured samples. The 95% confidence intervals of the Weibull modulus of the L80 ceramic were higher (no overlap fraction) than those of the L75 ceramic and were similar to the control (overlap fraction). The Weibull characteristics of L80 ceramic were higher than those of L75 ceramic and the control. SLA can be used to process suitable crown frameworks but shows results in anisotropic shrinkage.

Significance

The hH High particle size and dry matter content of the L80 slurry allowed made it possible to produce a reliable ceramic by SLA manufacturing with an anisotropic shrinkage, and a density, and flexural strength similar to those of a subtractive-manufactured ceramic. SLA allowed made it possible to build up a dense 3D alumina crown framework with controlled shape. Further studies on the marginal adaptation and shrinkage model of alumina crown frameworks will be required to optimize the process.

Introduction

Subtractive computer-aided design (CAD) and computer-aided manufacturing (CAM) technologies have led to major improvements in dentistry . They make it possible to produce reliable restorations with accurate dimensions and to reduce manufacturing time . However, these processes are limited by the waste of raw material (loss of unused portions of blocks and difficulty in recycling excess material) and heavy wear of milling tools. Microscopic cracks, which can weaken restorations, can also appear in objects due to the milling process .

Additive manufacturing processes avoid these limitations by building objects layer-by-layer. While such processes are already being used to manufacture metal and polymer prototypes , the shaping of ceramics for dental applications is still in its infancy . Ceramics can be additively manufactured by polymerizing an inorganic binder in a ceramic powder bed (3D printing) , selective laser sintering (SLS) , ceramic slurry printing (direct inkjet printing) , or stereolithography (SLA) . SLA produces scaffolds in calcium phosphate hydroxyapatite with the higher accuracy, surface quality, and mechanical properties than the other processes by selectively curing a photosensitive ceramic slurry using an ultraviolet beam. Curing with ultraviolet light makes it possible to work under artificial light without causing polymerization during the slurry preparation process.

Alumina and zirconia ceramics, the gold standard for crown frameworks in polycristalline ceramic, possess good mechanical resistance and are thus the focus of SLA additive manufacturing research and development . Even though additive-manufactured zirconia has the highest flexural strength , its high refractive index limits curing depth compared to alumina. This parameter is of utmost importance for SLA manufacturing , making alumina a good material for optimizing additive processes for preliminary dental research.

Since the viscosity of a slurry influences the reliability of complex structures, this parameter must be adapted to the SLA-manufacturing process. The incorporation of ceramic particles in curable resins modifies their rheological behavior. Increases in viscosity depend on the nature, dry matter content, and particle size of the ceramic . The dry matter content of slurries has to exceed 50% (v/v) to avoid deformation and crack formation during thermal treatments. However, the higher the dry matter content the higher the viscosity . As such, a compromise between a high dry matter content and low viscosity that is compatible with SLA manufacturing must be found.

The photosensitive cured polymer used to assemble ceramic particles during SLA manufacturing is burnt out during debinding. This increases the porosity and reduces the mechanical strength of sintered objects , preventing their use as crown frameworks, which require the highest density possible.

In addition, crown frameworks require high flexural strength to ensure the durability of restored teeth that are subjected to masticatory strains . The 3-point flexural strength of these materials should be at least 300 MPa (ISO 6872:2008) if they are to be employed for clinical uses. Ceramics used for dental restorations must also be very reliable. Structural reliability is generally based on two Weibull distribution parameters: the Weibull modulus and the Weibull characteristic strength . The Weibull modulus allows lower range strength values whose distribution is typical of ceramic materials (do not follow Gaussian distribution) to be used . A high Weibull modulus indicates a high degree of homogeneity and smaller variations in flexural strength. Both parameters have a major impact on the clinical success of ceramic restorations .

Most reports in the literature dealing with ceramic SLA manufacturing focus on the impact of slurry composition on polymerization and the optimization of photo-initiator, diluent, and polymerizable monomer concentrations in ceramic slurries. However, no data is available with regard to the impact of variations in particle size and dry matter content on the flexural strength and Weibull characteristics of SLA-manufactured dense ceramics. While SLA-manufactured temporary crowns are commercially available , there are no reports in the literature on the feasibility of using SLA-manufactured crown frameworks composed of dense ceramic.

The aim of the present study was thus to compare the physical and mechanical properties of SLA-manufactured alumina ceramics of different composition (dry matter content, particle size) and viscosity to those of subtractive-manufactured ceramics, and to demonstrate the feasibility of producing suitable dental crown frameworks by the SLA process.

The four null hypotheses tested were: (1) variations in the composition of the ceramic slurry used in the SLA-manufacturing process do not influence viscosity; (2) there is no difference in density between SLA-manufactured and subtractive-manufactured alumina ceramics; (3) there is no difference in flexural strength between SLA-manufactured and subtractive-manufactured alumina ceramics with high Weibull characteristics; (4) there is no difference in shrinkage between SLA-manufactured and subtractive-manufactured alumina ceramics; and (5) it is possible to produce dental crown frameworks by SLA manufacturing.

Materials and methods

Specimen preparation

Experimental groups

Slurries were prepared using small (S) (CT3000SG; Almatis, PA, USA) and large (L) (CT1200SG; Almatis, PA, USA) particle size ( Table 1 ) alumina powders. The powders were dried for 1 h in an oven (Beschickung loading model 100-800; Memmert, Germany) at 90 °C and were stored in hermetically sealed containers (TROP-500-010; Labbox, Spain) at room temperature. Particle sizes were measured (n = 3/group) by laser scattering (Malvern 2000; Mastersizer, UK) ( Table 1 ). Six experimental groups were formed based on particle size and dry matter content (S80, S75, S70, L80, L75, L70). A photosensitive ( λ = 350–400 nm) acrylic resin (C1-alumina; CryoBeryl Software, France) was placed in jars (01.462.0220; Retsch, Germany) using graduated pipettes (pip-0806-250; Labbox, Spain) for the SLA manufacturing of the slurries. The powders were weighed (PS8000/X; Radwag, Poland) (linearity ± 0.03 g) and were incorporated into the liquid. The slurries were poured into agate jars containing zirconia milling balls and were homogenized at 150 rpm by planetary milling for 30 min (PM100; Retsch, Germany). The absence of media contamination during the milling process was confirmed by inductively coupled plasma-optical emission spectrometry (720-ES, Varian Inc., PA, USA).

Table 1
Materials used in the different experimental groups with their particle sizes and dry matter content.
Group CAD type Dry phase Photosensitive resin Dry matter content (%weight)
Compo sition Material Manufacturer Batch no. D50 particle size (μm) Mean (sd) Material Manufacturer Batch no.
L80 SLA Al 2 O 3 CT1200SG Almatis 1446533255A 1.58 (0.03) C1-Alumina Cryoberyl Software 01 80
L75 SLA Al 2 O 3 CT1200SG Almatis 1446533255A 1.58 (0.03) C1-Alumina Cryoberyl Software 01 75
L70 SLA Al 2 O 3 CT1200SG Almatis 1446533255A 1.58 (0.03) C1-Alumina Cryoberyl Software 01 70
S80 SLA Al 2 O 3 CT3000SG Almatis 200412442 0.46 (0.04) C1-Alumina Cryoberyl Software 01 80
S75 SLA Al 2 O 3 CT3000SG Almatis 200412442 0.46 (0.04) C1-Alumina Cryoberyl Software 01 75
S70 SLA Al 2 O 3 CT3000SG Almatis 200412442 0.46 (0.04) C1-Alumina Cryoberyl Software 01 70
Cont Sub. Al 2 O 3 In-Ceram AL Vita Zahnfabrik 32510

The parallelepipedic samples (1.3 × 4 × 22 mm, oversized by the shrinkage ratios given in Table 3 , ISO 6872:2008) with different densities (n = 10/group) and flexural strengths (n = 15/group) were designed using CAD software (Catia; Dassault Systèmes, France) ( Fig. 1 ). The 3D objects were supported by pillars (end diameter: 0.2 mm, base diameter: 0.7 mm, height: 2 mm) during the design phase and were sliced (width = Z-direction, layer thickness = 50 μm) using Creation Workshop software (Datatree3D, TX, USA). The data were transferred to SLA software (CryoCeram; CryoBeryl Software, France) and equipment (CryoCeram Printer; CryoBeryl Software, France) for additive manufacturing (curing energy = 10 mW cm −2 ; resolution = 50 μm). The SLA process used integral SLA methods that make it possible to polymerize the entire surface of a layer at once using a dynamic mask of digital micro-mirror devices. Pillars were hand-sectioned following the SLA-manufacturing phase, and the microstructures of the samples were assessed by scanning electron microscopy (S-3500N; Hitachi, Japan) (magnification ×90, secondary electron imaging). The samples were thermally debound (0.1 °C/min, 450 °C, 2-h dwell time) in an oven (LHT; Carbolite, UK) before sintering (15 °C/min, 1690 °C, 3-h dwell time) at atmospheric pressure in a furnace (HT16/17; Nabertherm, Germany).

Table 2
Physical and mechanical properties of dental alumina ceramics manufactured using the stereolithography (L80, L75, L70, S70) and subtractive methods (Cont): viscosity, density, 3-point flexural strength ( σ ), and Weibull characteristics ( σ 0 and m ).
Sample name Viscosity at 2 s −1 retention in mPa s Mean (sd) Density in %
Mean (sd)
σ in MPa
Mean (sd)
σ 0 in MPa
[CI range 95%]
m
[CI range 95%]
L80 113 (5.54) ab 98.1 (0.4) cd 367.9 (52.4) e 396 [385.7–409.5] g 10.2 [7.9–12.5] jk
L75 77.11 (3.59) ab 98.2 (0.8) c 363.7 (74.6) e 392.5 [379.9–412] g 05.5 [4.5–6.4] l
L70 65.73 (4.94) a 97 (0.7) d 273.8 (41.9) f 293.1 [281.5–301.9] h 07.5 [6–9.1] kl
S80 Na
S75 218,9 (12.78) b
S70 137.7 (12.68) ab 96.9 (0.8) d 271.7 (44.5) f 273.9 [269.6–280;3] i 11.7 [9.8–13.5] j
Cont 99.4 (0.5) c 350.4 (49.5) e 371.5 [360–386.5] g 08.3 [6.4–10.1] jkl
p -Value 0.0091 0.0009 <0.0001
sd: standard deviation; σ : 3-point flexural strength; m : Weibull modulus; σ 0 : Weibull characteristic strength. In the viscosity, density and σ columns, there were no statistical differences between groups with the same superscript in. In the σ 0 and m columns, there was an overlap fraction of confidence intervals between groups with the same superscript letter.

Table 3
Shrinkage after the heat treatment of alumina ceramics manufactured using the stereolithography (L80, L75, L70, S70) and subtractive methods (Cont) in terms of thickness, width, and length. Statistical analyses by column and by line are separated by a comma.
Sample name Thickness shrinkage (%)
Mean (sd)
Width shrinkage (%)
Mean (sd)
Length shrinkage (%)
Mean (sd)
p -Value Overall shrinkage (%)
Mean (sd)
L80 14.0 (1.8) Aa 19.1 (0.7) Bd 17.2 (0.3) Ch <0.0001 16.9 (2.3) ln
L75 19.6 (0.7) Db 23.1 (1.1) Ee 19.3 (1.4) Di 0.0002 20.6 (2.1) lm
L70 23.0 (1.1) Fc 28.2 (1.5) Gf 22.9 (0.7) Fj <0.0001 24.5 (2.6) m
S70 21.2 (1.9) Hbc 27.2 (0.5) If 22.7 (0.6) Hj <0.0001 24;0 (2.6) m
Cont 15.6 (1.6) Ja 14.7 (1.2) Jg 14.6 (0.4) Jk 0.3925 15.0 (1.2) n
p -Value <0.0001 <0.0001 <0.0001 <0.0001
sd: standard deviation; There were no statistical differences between groups with the same superscript.

Fig. 1
Stereolithography manufacturing process.

Control group

Samples prepared using the subtractive method were used as controls (Cont). In-Ceram Al blocks (Vita Zahnfabrik, Germany) were serially cut into sticks using a Setocom-15 cutting machine (Struers Ltd., UK) equipped with a 0.8-mm-thick abrasive cut-off wheel. The samples were sintered using the manufacturer’s standard procedure (17 °C/min, 1530 °C, 2-h dwell time) in a Zycromat sintering furnace (Vita Zahnfabrik, Germany).

After sintering, the sample sizes manufactured by the SLA and subtractive methods were determined using an analog micrometer (Mitutoyo; Japan) (precision ± 0.01 mm). Based on the ISO 6872:2008 standard, the standard deviations of specimen dimensions should not exceed 0.20 mm in thickness and width, with minimal values of 1.20 and 4.00 mm, respectively. In the present study, the sample dimensions were 1.35 ± 0.14 mm in thickness, and 3.89 ± 0.09 mm in width and passed the D’Agostino Pearson normality test, indicating that they had good dimensional homogeneity.

Dynamic viscosities of the slurries

The dynamic viscosities of three slurries from each group were evaluated by dynamic rheometry using parallel plates (cone-plate 2°/55 mm) at 25 °C (Haake, France). The slurries were placed on the lower plate of the rheometer, and the instantaneous viscosities (Pas) were measured at a constant shear rate of 2 s −1 .

Densities of the sintered ceramic samples

The densities of the ceramic samples were determined by hydrostatic weighing on a high-precision balance (ALT-310-4 AM; Kern, Germany) using the Archimedes principle under water (3 vacuum cycles, n = 10/group).

Three-point flexural strengths of the sintered ceramic samples

The samples (n = 15/group) used to determine the 3-point flexural strength were evaluated using a bending machine (DY30; Adamel Lhomargy, France) equipped with a 100 N cell (Adamel Lhomargy, France) set to a maximum force of 1 kN. A progressive central vertical load was applied at a constant speed of 0.5 mm/min (ISO 6872:2008 standard). The distance between the lower supports was 20 mm. The flexural strength in MPa ( σ ) was calculated using following equation (Eq. (1) ):

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='σ=(3×F×L)/(2×W×H2)’>σ=(3×F×L)/(2×W×H2)σ=(3×F×L)/(2×W×H2)
σ = ( 3 × F × L ) / ( 2 × W × H 2 )

where F is the maximum force to failure (N), L is the distance between the lower supports (mm), W is the sample width (mm), and H is the sample height (mm).

Weibull characteristics

The structural reliability of ceramics is commonly assessed by a Weibull distribution analysis using flexural strength data for two parameters. The Weibull modulus ( m ) and the Weibull characteristic strength ( σ 0 ) were calculated using the following equation (Eq. (2) ):

<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='Pf=1−exp[−(σσ0)m]’>Pf=1exp[(σσ0)m]Pf=1−exp[−(σσ0)m]
P f = 1 − exp [ − ( σ σ 0 ) m ]

where P f is the probability of failure between 0 and 1, σ is the flexural strength in MPa, and σ 0 is the characteristic strength in MPa.

Shrinkage

The shrinkage of the samples was assessed by anisotropic retraction (thickness, width, length). The sample (n = 10/group) sizes ( d ) were measured before and after the thermal treatment step using an analog micrometer (Mitutoyo, Kawasaki, Japan). The shrinkage ( S ) ratios in each direction were calculated as follows (Eq. (3) ):

<SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='S(%)=[1−(daftersinteringdbeforesintering)]×100′>S(%)=[1(daftersinteringdbeforesintering)]×100S(%)=[1−(daftersinteringdbeforesintering)]×100
S ( % ) = [ 1 − ( d after sintering d before sintering ) ] × 100
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Nov 22, 2017 | Posted by in Dental Materials | Comments Off on Stereolithography: A new method for processing dental ceramics by additive computer-aided manufacturing
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