Resin viscosity determines the condition for a valid exposure reciprocity law in dental composites

ObjectiveTo provide conditions for the validity of the exposure reciprocity law as it pertains to the photopolymerization of dimethacrylate-based dental composites.MethodsComposites made from different mass ratios of resin blends (Bis-GMA/TEGDMA and UDMA/TEGDMA) and silanized micro-sized glass fillers were used. All the composites used camphorquinone and ethyl 4-dimethylaminobenzoate as the photo initiator system. A cantilever beam-based instrument (NIST SRI 6005) coupled with NIR spectroscopy and a microprobe thermocouple was used to simultaneously measure the degree of conversion (DC), the polymerization stress (PS) due to the shrinkage, and the temperature change (TC) in real time during the photocuring process. The instrument has an integrated LED light curing unit providing irradiances ranging from 0.01 W/cm² to 4 W/cm² at a peak wavelength of 460 nm (blue light). Vickers hardness of the composites was also measured.ResultsFor every dental composite there exists a minimum radiant exposure required for an adequate polymerization (i.e., insignificant increase in polymerization with any further increase in the radiant exposure). This minimum predominantly depends on the resin viscosity of composite and can be predicted using an empirical equation established based on the test results. If the radiant exposure is above this minimum, the exposure reciprocity law is valid with respect to DC for high-fill composites (filler contents >50% by mass) while invalid for low-fill composites (that are clinically irrelevant).SignificanceThe study promotes better understanding on the applicability of the exposure reciprocity law for dental composites. It also provides a guidance for altering the radiant exposure, with the clinically available curing light unit, needed to adequately cure the dental composite in question.     

通用混合型复合树脂收缩应力的光弹分析

目的:分析临床常用的3种通用混合型复合树脂Charisma、TPHSpectrum和Esthet-X收缩应力大小。方法:制备外径为80mm、厚度为4mm的光弹材料圆盘,中心制备直径为4mm的孔。分别用3种复合树脂各充填8个样本,光照固化40s。于固化后1、2、3、4、5、10、20、30min和1、24、48h记录光弹图,根据光弹图中条纹的直径计算收缩应力大小。结果:3种材料在固化后10min内收缩应力迅速增加,1h和24h时Charis-ma收缩应力最大,分别为(2.8936±0.1)MPa和(4.1904±0.1)MPa,Esthet-X最小,分别为(2.2917±0.1)MPa和(3.1439±0.3)MPa。固化后48h有79%的样本出现应力释放现象。结论:复合树脂固化后收缩应力变化速度是不同的。     

Polymerization stress related to radiant exposure and its effect on microleakage of composite restorations

Objectives

Verify the influence of radiant exposure (dose) on polymerization stress and microleakage of composite restorations using two adhesive systems.

Methods

An experimental composite (1BisGMA:1TEGDMA, 75 wt.% filler) was subjected to 3, 6, 12 or 24 J/cm² under 500 mW/cm². Stress was measured in 1-mm height specimens placed between two glass rods (Ø = 5 mm) attached to a testing machine. Microleakage was assessed in cylindrical restorations (Ø = 5 mm, h = 2 mm) with enamel margins, using Adper Single Bond 2 (SB) or Adper Prompt (PR). After storage, specimens were subjected to dye penetration (50% AgNO₃) and sectioned twice, perpendicularly. Microleakage of the 8 tooth/composite interfaces was recorded in millimeters. KHN was measured at both surfaces of cylindrical specimens (Ø = 5 mm, 1 or 2 mm height). ANOVA/Tukey test were used (α = 0.05). KHN top and bottom were compared using paired t-tests.

Results

Stress increased significantly with dose (3 J/cm²: 1.4 ± 0.3^c; 6 J/cm²: 4.3 ± 0.5^b; 12 J/cm²: 9.3 ± 0.6^a; 24 J/cm²: 9.9 ± 0.7^a, in MPa). For both average and maximum microleakage, interactions were not statistically significant (p > 0.05). Tooth average microleakage was influenced by the adhesive system (SB < PR) and increased with dose (3 J/cm² < 6 J/cm² = 12 J/cm² = 24 J/cm²). Tooth maximum microleakage was influenced by dose only (3 J/cm² < 6 J/cm² = 12 J/cm² = 24 J/cm²). A direct association between stress and microleakage was suggested with SB, while with PR showed severe dye penetration at all doses evaluated. KHN showed significant differences among all groups, except at bottom of 2 mm specimens cured with 3 and 6 J/cm². Differences between top and bottom were found for 2 mm specimens cured with 3 and 6 J/cm².

Conclusions

Stress and microleakage increased significantly with the applied radiant exposure.     

The effect of filling technique on the cuspal strain,polymerization shrinkage stress,enamel crack formation and depth of cure of restored molars

ObjectiveEvaluate the effect of different restorative filling techniques on the residual shrinkage stress (ShrS), cuspal strain (CS), depth of cure (DC), and enamel crack formation (Ec) in molars with MOD restorations.MethodsPost-gel shrinkage, elastic modulus, compressive and diametral tensile strength of the Filtek One Bulk Fill composite were calculated. Sixty molars with MOD preparations were restored using four filling techniques: Bulk; Horizontal; Oblique; Natural enamel and dentin substitution (NEDS) technique. CS was measured using a strain gauge (n = 10). The DC (n = 5) was measured using Knoop hardness. Shrinkage stress/strain was analyzed using 3D finite element analysis. The Ec analysis was carried out by transillumination. Two-way ANOVA with repeated measures and Tukey’s HSD test (α = 0.05) was performed for the CS data. Two-Way ANOVA and Tukey’s HSD test was performed for the DC data (α = 0.05).ResultsCS was higher at the lingual cusp for the horizontal and NEDS technique. No statistical difference was found between the buccal and lingual CS values for the Bulk (p = 0.367) or Oblique techniques (p = 0.192). CS values were lower for the Bulk. More enamel cracks were found for the Bulk. DC was lower at 4 mm regardless the filling technique. The Horizontal showed the highest ShrS values. The Bulk generated the lower ShrS values.SignificanceA Bulk technique caused the lowest shrinkage stress/strain. An Oblique technique yielded the best balance between stress, strain and crack formation. NEDS technique is a good alternative to decrease the number of increments while maintaining the stress levels nearby the Oblique technique.     

A simple method for the measurement of polymerization shrinkage in dental composites

OBJECTIVE: In this study a simple non-contact method was developed to measure the polymerization shrinkage of dental composites. METHODS: A gas pycnometer was used to determine the volumes of specimens prior to and after photopolymerization and from which the total volumetric shrinkage could be determined. RESULTS: Four commercial composites were studied and were found to have polymerization shrinkages varying from 1.6 to 2.5%. The method was found to be labour efficient and produced reproducible results with a standard deviation of approximately 10%. SIGNIFICANCE: This method is appropriate for shrinkage measurements where only the total amount shrinkage is required and in particular for the measurement of shrinkage of photocured materials which are sensitive to water absorption.     

Influence of resin cement polymerization shrinkage on stresses in porcelain crowns

Objective

The aim of this study was to analyze the influence of polymerization shrinkage of the cement layer on stresses within feldspathic ceramic crowns, using experimentally validated FEA models for (1) increasing occlusal cement thickness; and, (2) bonded versus non-bonded ceramic-cement interfaces.

Methods

2-D axial symmetric models simulated stylized feldspathic crowns (1.5 mm occlusal thickness) cemented with resin-cement layers of 50–500 μm on dentin preparations, being loaded (500 N) or not. Ceramic–cement interface was either bonded or not. Cement was bonded to the dentin in all models. Maximum axial shrinkage of 0%, 1%, 2%, 3%, 4% and 4.65% were simulated. The first principal stresses developing in the cementation surface at the center and at the occluso-axial line-angle of the crown were registered.

Results

Polymerization shrinkage of the cement increased tensile stresses in the ceramic, especially in loaded non-bonded crowns for thicker cement layers. Stresses in loaded non-bonded crowns increased as much as 87% when cement shrinkage increased from 0% to 4.65% (100–187 MPa), for a 500 μm-thick cement. Increasing polymerization shrinkage strain raised the tensile stresses, especially at the internal occlusal-axial line-angle, for bonded crowns.

Significance

Changes in the polymerization shrinkage strain (from 0% to 4.65%) have little effect on the tensile stresses generated at the cementation surface of the ceramic crowns, when the occlusal cement thickness is thin (approx. 50 μm for bonded crowns). However, as the cement becomes thicker stresses within the ceramic become significant.     

Spatially resolved assessments of composite shrinkage in MOD restorations using a digital-image-correlation technique

Objectives

To analyze the influence of cavity geometry and lining materials in MOD composite restorations by characterizing the polymerization shrinkage and cusp deflection.

Methods

Eighty intact molars with similar sizes were collected and randomly divided into eight groups. MOD cavities with various widths and depths were prepared on these teeth: Group I, 2_(W) × 2_(D) mm; Group II, 4_(W) × 2_(D) mm; Groups III, IIIf, IIIg, 2_(W) × 4_(D) mm; and Groups IV, IVf, IVg, 4_(W) × 4_(D) mm. In Groups IIIf and IVf, flowable composite liner was placed prior to composite restoration, while glass-ionomer liner was used in Groups IIIg and IVg. Deformations of restorations resulted from composite shrinkage were recorded for 30 min following light irradiation using a digital-image-correlation (DIC) method to subpixel level. The displacements at the boundaries of the restorations were analyzed using one-way ANOVA and the post hoc test at a 5% significance level. The correlation between the geometric factors and the displacements was also analyzed.

Results

The inward displacements on free surfaces were greater than those on the bonded surfaces. Groups with flowable composite linings showed greater amount of displacements on free and bonded surfaces compared to the unlined and glass ionomer lining groups. The correlation analysis showed that the free surface shrinkage was related with the cavity width and C-factor, while cusp deflections were correlated with the cavity depth and the cusp compliance.

Significance

The DIC technique measures composite shrinkage on different boundaries of restorations to facilitate the investigation of polymerization kinetics. Using flowable composite lining and increased cusp depth may aggravate the cusp flexure.     

Consistency in the amount of linear polymerization shrinkage in syringe-type composites

OBJECTIVES: The purpose of this study was to investigate whether the composite resin in a syringe showed a consistent shrinkage through its content. Additionally, the amount of linear shrinkage was compared between materials. METHODS: Five brands of syringe-type and one brand of carpule-type composite resins were used in this study. To each brand, two to three syringes were assigned. In the carpule-type composite, 15 carpules were used. The linear polymerization shrinkage was measured using a custom-made linometer. In this linometer, the amount of displacement of an aluminum disk, which was caused by the linear shrinkage of composite resin, was recorded by a computer every second for 90 s. RESULTS: The syringe-type composites showed similar consistencies in the amount of linear shrinkage except one. The linear shrinkage of the carpule-type Tetric Ceram showed more consistency compared with syringe-type composites. The amount of linear polymerization shrinkage varied between materials. SIGNIFICANCE: This investigation demonstrates that the use of carpule-type composites is recommended instead of syringe-types, because of the consistency in its linear shrinkage. The custom-made linometer provides an effective way to study polymerization shrinkage.     

Photoinitiator type and applicability of exposure reciprocity law in filled and unfilled photoactive resins

Objectives

To test the influence of photoinitiator type and filler particle inclusion on the validity of exposure reciprocity law.

Materials and methods

50/50 wt% Bis-GMA/TEGDMA resins were prepared with equimolar concentrations of camphorquinone/DMAEMA (0.20/0.80 mass%) (CQ) or Lucirin-TPO (0.42 mass%), and were used either unfilled or filled to 75 mass%. Specimens were cured with a halogen Swiss Master Light (EMS, Switzerland) using four different curing protocols: 400 mW/cm² for 45 s as reference protocol (18 J/cm²), 1500 mW/cm² for 12 s (18 J/cm²), 3000 mW/cm² for 6 s (18 J/cm²) and 3 s (9 J/cm²). Degree of conversion (DC) was measured in real time for 70 s by FT-NIRS and temperature rise using a thermocouple. Depth of cure was determined with a penetrometer technique.

Results

With respect to DC and depth of cure, exposure reciprocity law did not hold for any tested material, except for the depth of cure of filled CQ-based materials. At similar radiant exposure, DC was significantly higher (p < 0.05) for all unfilled and filled TPO-based materials compared with CQ-based materials. As exposure time was reduced and irradiance increased, TPO-based materials exhibited higher DC whilst an opposite trend was observed for CQ-based materials (p < 0.05). For similar curing regimes, depth of cure of CQ-based materials remained significantly greater than that of TPO-based materials. Adding fillers generally reduced DC, except at higher irradiance for CQ-based materials where a positive effect was observed (p < 0.05).

Significance

The validity of exposure reciprocity law was dependent on several factors, among which photoinitiator type and filler content were important. Lucirin-TPO is a highly reactive and efficient photoinitiator, which may allow the potential for a reduction in curing time of TPO-based photoactive materials in thin sections.     

复合树脂聚合收缩应力的研究进展

复合树脂作为常用的牙色充填材料,具有美观、操作方便等优点,但也存在聚合收缩、不耐磨、变色等问题.其中,聚合收缩可引起充填复合树脂与牙体之间形成间隙、釉质裂纹和牙尖移动,导致充填失败.影响收缩应力大小的因素包括复合树脂的组成、窝洞形态和临床操作.通过减小聚合收缩应力可提高复合树脂充填的成功率.     

不同充填方式对复合树脂在盒形洞壁产生收缩应力的影响

目的:观察不同充填固化方式对复合树脂在盒形洞壁产生收缩应力的影响.方法:在厚度为4 mm、边长4 cm的聚碳酸酯板上制备直径4 mm、深3 mm的窝洞,涂布粘结剂后分别以4种充填固化方式充填复合树脂(卡瑞斯玛):①整体充填,照射40 s;②水平2层充填,每层照射20 s;③水平3层充填,每层照射20 s;④斜向3层充填,每层照射20 s.分别于照射后3 min、24 h在显微光弹仪上观测窝洞周围应力等差条纹的直径,计算出洞壁承受的收缩应力.结果:所有充填方式组固化后24 h的收缩应力均显著大于其固化后3 min的应力(P＜0.05);不同充填方式组间比较,无论是固化后3 min,还是24 h,均以斜向3层充填组的收缩应力最小,分别与其他各组相比差异均有统计学意义(P＜0.05);其次是水平3层充填固化组,显著小于整体充填固化组和水平2层充填组(P＜0.05);整体充填固化组收缩应力最大,但与水平2层充填组相比无统计学差异(P＞0.05).结论:分层充填能够降低盒形洞壁的收缩应力,其中斜向分层充填的效果最好.     

弱光启动固化对后牙树脂聚合收缩和表面硬度的影响

目的探讨弱光启动固化对三种临床常用的光同化后牙树脂（EcuSphere—Carat，EC；Fihek P60，P60；Tetric Ceram HB．HB）聚合收缩和表面硬度的影响。方法将树脂充填入直径0．7cm、高0．4cm的圆柱状容器中．每种树脂实验组和对照组各10个样本．采用扫描电镜测量树脂样本与充填容器的边缘裂隙宽度，评价弱光启动对聚合体积收缩的影响；同法制备样本，每种树脂实验组和对照组各7个样本．采用显微硬度计测量两种固化方式处理的样本表面的维氏硬度．探讨弱光启动固化对树脂表面硬度的影响．结果EC、P60、HB三种树脂传统光固化后边缘裂隙宽度分别为（15．32±1．78）μm、（15．72±1．97）μm和（16．14±1．74）μm，差异无统计学意义（P〉0．05）；弱光启动固化后的边缘裂隙宽度分别（14．83±3．34）μm、（13．64±1．92）μm、（14．18±2．01）μm，弱光启动固化后的边缘裂隙宽度减小＋差异有统计学意义（P〈0．05）。树脂表面硬度传统光固化后分别为（116．76±12．00）MPa、（159．14±5．90）MPa、（119．59±5．00）MPa：弱光启动固化后分别为（109．74±5．77）MPa、（154．81±3．99）MPa、（114．11±6．42）MPa，表面硬度较传统光固化低，差异有统计学意义（P〈0．05）。结论弱光启动固化模式能有效降低后牙树脂的聚合收缩．但树脂的表面硬度有所下降．三种树脂中P60的表面硬度最高。     

Effect of pulse width modulation-controlled LED light on the polymerization of dental composites

Objectives

The purpose of this study was to investigate the effects of radiant emittance and cure time of pulse width modulation (PWM)-controlled LED light on polymerization shrinkage/stress kinetics and surface hardness of dental composites.

The use of an elastomeric methacrylate monomer (Exothane 24) to reduce the polymerization shrinkage stress and improve the two-body wear resistance of bulk fill composites

ObjectivesCharacterize the chemical structure of an elastomeric monomer (Exothane 24) and evaluate the degree of conversion (DC), polymerization shrinkage stress (PSS), rate of polymerization (Rp), flexural strength (F_Strenght), flexural modulus (F_Modulus), Vickers hardness (V_Hardness) and two-body wear resistance of dental bulk fill composites (BFCs) containing Exothane 24.MethodsThe Exothane 24 was characterized using mass spectroscopy, elemental analysis, ¹³C- and ¹H NMR. BFCs were formulated containing Exothane 24 (E10, E25, and E50). Similar BFCs containing regular UDMA (U10, U25, and U50), commercial conventional, and BFCs were used as control groups. ATR-FTIR spectroscopy was used to measure DC and the Rp of the composites. The PSS was measured using the universal testing machine method. Specimen bars were used to assess the F_Strenght, F_Modulus, and V_Hardness. RBCs were submitted to a two-body wear test using a chewing simulator machine; the rate and volumetric wear loss were evaluated using an optical scanner. Data were analyzed statistically with α = 0.05 and β = 0.2.ResultsExothane 24 is a urethane isophorone tetramethyl methacrylate monomer with polymerization stress-relieving properties. No differences were found in the DC up to 4 mm in depth for E25. All BFCs had similar F_Strenght, except for E50. E25 had the lowest volumetric wear loss and wear rate. E25 had approximately 30% lower PSS and slower Rp than commercial BFCs with similar wear resistance to conventional commercial composites.SignificanceThe Exothane 24 reduced the PSS and increased the wear resistance of BFCs; however, the formulation is important to optimize the properties of the BFCs.     

The effect of framework fabrication technique on the fit accuracy of full arch screw retained implant supported prostheses

PurposeThe purpose of this study was to compare the accuracy of fit of Co-Cr full arch screw-retained implant-supported fixed dental prosthesis fabricated among three different methods: conventional casting, milling, and additive manufacturing technology.Materials and methodsA master model of a completely edentulous mandible with five internal connection implants was utilized. Thirty full arch Co-Cr screw-retained implant-supported frameworks were fabricated by three different methods: conventional casting, milling, and additive manufacturing (AM) technology. The marginal fit was measured using a coordinate measuring machine in x-, y-, and z-axes, as well as the three-dimensional discrepancy. The casting group were measured twice: before the adaptation procedure and again after the adaptation procedure (sectioning and laser welding). For comparisons of marginal fit of frameworks between different groups one-way analysis of variance and Games Howell test was used. Paired t-test was used to compare cast frameworks before and after adaption.ResultsThere were statistically significant differences in marginal fit and width distortion between groups (P <.05). The mean of total distortion of each group was 94.6 µm (SD 50.5 µm) for casting group before adaptation, 92.44 µm (SD 49.6 µm) for casting group after adaptation, 71.4 µm (SD 37.2 µm) for additive manufacturing group, while for the milling group the total distortion was 50.1 µm (SD 27.5 µm).ConclusionFull arch screw-retained implant-supported frameworks fabricated with any of the three fabrication techniques using cobalt-chromium material exhibited acceptable marginal fit. Milling fabrication technique showed the most accurate marginal fit. Adaptation procedure for the cast group has significantly improved the marginal fit.