Polymerization Shrinkage Stresses of Different Flowable and Universal Bulk-Fill Composites
Marlen Robles Moreno1*, Yuwein Fan2 and Dan Nathanson2
1Department of Restorative Sciences, The Dental College of Georgia at Augusta University, USA
2Department of Restorative Sciences and Biomaterials, Boston University, USA
Submission: March 01, 2019; Published: March 19, 2019
*Corresponding author: Marlen Robles-Moreno, DDS, CAGS, MSD, Former Prosthodontic Resident at Boston University, School of Dental Medicine; present Assistant Professor, Department of Restorative Sciences, The Dental College of Georgia at Augusta University. Georgia, USA, 1430 John Wesley Drive GC 4224, Augusta, GA 30912
How to cite this article: Marlen R M, Yuwein F, Nathanson. Polymerization Shrinkage Stresses of Different Flowable and Universal Bulk-Fill Composites.
Adv Dent & Oral Health. 2019; 10(4): 555794.
Objective: To analyze and compare the shrinkage stress properties of flowable and universal bulk-fill resin composites over five minutes after photopolymerization.
Materials and Methods: Eleven resin composites, seven flowable and four universals, were tested (n=5). Two glass cylinders were affixed end-to-end in a vertical position on an INSTRON universal testing machine, leaving a 1mm gap between them. Each resin was injected until filling the gap and photopolymerized for 20s. The force curve was recorded over 5 minutes after photopolymerization. The stress values were calculated in MPa at 20s, 60s, 180s, and 300s. Statistical analysis using ANOVA and All-Pairs Tukey-Kramer Test (JMP Pro 12.0. SAS, Cary, NC) were done to compare the polymerization shrinkage stresses among the groups at α = 0.05.
Results: All the resins showed a continuous increase in shrinkage stress up to the 300 seconds, following photopolymerization. Three flowable composites Dentex, G¬aenial Universal, and Prime-Dent Flow, obtained the highest shrinkage stresses, while another flowable resin SureFil SDR demonstrated the lowest shrinkage stress from all the materials tested. In general, universal composites demonstrated lower shrinkage stress values than the flowable composites.
Conclusions: In general, flowable resin composites presented higher shrinkage stresses compared to universal composites.
It is clinically important to understand how polymerization shrinkage stress affects the interface between resin restorations and tooth structure. These have the potential to damage both enamel and resin composite in the form of cracks, and produce marginal gaps, which can lead to postoperative sensitivity and secondary caries [1,2]. In some cases, fracture lines attributable to polymerization stresses through the enamel cusps have been observed by clinicians following photopolymerization of resins in posterior teeth .
Shrinkage is a characteristic of the polymerization of resins, which once constrain by the walls of a cavity preparation will result into shrinkage stress. The amount of material and the material’s formulation play important roles in stress development, as well as, in the transmission of these stresses to the tooth structures [4,5]. Any stress that approaches or exceeds the local adhe
sive force or any structural strength, resulting in gap formation, is considered a high shrinkage stress .
In recent years, many manufacturers have introduced “bulk-fill” resin composites that are said to have more light-sensitive photoinitiation systems. There are two types of bulk-fill composites available, low viscosity (flowable) and high viscosity (universal). Studies have shown that low viscosity bulk-fill composites have shrinkage values like regular flowable composites [7-9].
The amount of polymerization stress transferred to the resin/tooth interface is not always proportional to the shrinkage and is influenced by the composition of the resin. While an increase in filler content results in decreased shrinkage, flowable materials with a lower filler content may plastically deform and transfer less stress to margins than stiffer resin composites.
Much of the effort among manufacturers to decrease the
shrinkage stresses produced by resin restorations has been based
on changing the chemical structure of resin monomers as well as
the type and percentage of their fillers . Recent studies of experimental
composites have demonstrated that higher shrinkage
values and higher polymerization stresses are observed when resin
composite viscosity is reduced with increasing ratios of TEGMA/
Polymerization shrinkage of resin composites is most commonly
measured with the sample unconstrained [12-14], while
polymerization shrinkage stresses are usually measured via uniaxial
testing in a universal testing machine [15,16], like the method
used in this study. In this testing design, the resin composite is
shaped into a cylinder between two flat surfaces, one of which is
fixed and the other of is attached to the load cell. When the resin
composite starts the polymerization, the contraction force pulls
the two bonded surfaces together, and the signal of the load cell is
monitored continuously by a computer program. Then, the shrinkage
stress is calculated by dividing the contraction force by the
area of the bonding substrates [17,18].
Despite efforts to document the chemical and mechanical
properties of resin composites, there is little information regarding
the polymerization shrinkage stress produced by the recent
class of bulk-fill resin composite materials. Therefore, the purpose
of this study is to analyze and compare the different polymerization
shrinkage stress properties and characteristics of some recently-
introduced commercial flowable and universal bulk-fill
The shrinkage stress properties of eleven resin composites
were examined. Seven flowable resins: Filtek Supreme Ultra Flow,
NOVAPRO Flow, SureFil SDR, G-aenial Universal Flo, Herculite Ultra
Flow, Dentex Flow and Prime Dent Flowable. And four universal
resin composites: Filtek Supreme Ultra, NOVAPRO Fill, Beautifil
II, and finally, Herculite Ultra. Five specimens from each group
(n=5) were tested for polymerization shrinkage.
The composition, manufacturer, place of manufacture, and the
LOT number for all included resin composites tested in this study
are presented on Table 1.
EBADMA: Ethoxylated BisPhenol A Dimethacrylate, UDMA: urethane dimethacrylate, Bis-GMA: bisphenol A glycidyl methacrylate, Bis-EMA: ethoxylated
bisphenol A glycol dimethacrylate, EDMAB: ethyl-4-dimethylaminobenzoate. BISEMA6: diether Dimethacrylate. (Information disclosed by
manufacturers on MSDS).
A universal mechanical tester Instron Model 5566A (Instron Corp.,
Norwood, Massachusetts, USA) was used to analyze the polymerization
The load of contraction force was recorded from 100N load
cell. One fused silica cylinder with 10mm in diameter and one
glass cylinder 5mm in diameter, had one of their flat ends treated
with 100 grit sand paper to obtain a rough surface necessary
for resin composite bonding. The surfaces were then rinsed and
air dried, the two rods were fixed in a vertical position with two
chucks attached to the universal testing machine. The working
load cell was mounted at the top. A bonding agent (Optibond
Solo Plus, Kerr Restoratives Orange, CA.) was applied on the
surface of the lower glass cylinder, and light cured for 10 seconds
with a light curing diode (10 Wats). (Blue phase 16i, Ivoclar, HIP
mode, ~1200mW/cm2). The upper and lower glass cylinders were
placed into close contact together, this contact was controlled by
a minimal load recorded by software from the Instron computer.
The distance between the upper and lower cylinders was set to
1.5-1.8mm. The resin composite was then injected on the flat surface
of the lower glass cylinder, until filling the 1mm space (Figure
1). The 1mm space was adjusted using the crosshead controller.
The excess of composite was cleaned off (Figure 2).
After a 30 seconds baseline was acquired, the resin was photo
initiated for 20 seconds (Figure 3). The tip of the curing light
was placed perpendicular to the junction at 2mm, approximately.
A radiometer, Demetron Model 100 (Demetron Research Corp.
Danbury, CT) was used to monitor the adequacy of the curing light
throughout the entire experiment. Finally, the peak load vs. time
curve was recorded by the attached 100N load cell in Bluehill 3
software until five minutes (300s) after the photo-initiation and
calculated into polymerization shrinkage stress.
Five specimens for each resin were analyzed following the
same method described (n=5). The shrinkage stress values of each
specimen were calculated in MPa from the division of contraction
force recorded at 20s, 60s, 180s, and 300s. The entire experiment
was conducted at room temperature of 73° F.
Statistical analysis of the data was performed using the JMP
Pro 12.0.1. SAS, Cary, NC software. One-way ANOVA and All-Pairs
Tukey-Kramer Test were used to compare the polymerization
shrinkage stress differences among the groups with α = 0.05.
All the resins evaluated in this study showed a continuous increase
in shrinkage stress over the 300 seconds, following photopolymerization.
Three flowable resin composites Dentex Flow,
G¬aenial Universal Flow, and Prime-Dent Flow, demonstrated significantly higher levels of shrinkage stress than the other materials,
while another flowable resin composite (SureFil SDR-flow)
demonstrated the lowest polymerization shrinkage stress from all
the materials tested in this study. In general, universal resin composites
demonstrated lower polymerization stress values than the
flowable resin composites.
*Levels not connected by same letter are significantly different.
The mean shrinkage stress values and MPa’s for all the resin
composites at different times are presented in Table 2 and Figures
4 & 5. Table 2 shows the mean polymerization shrinkage stresses,
and four overlapping Tukey groups categorizing the shrinkage
stresses, for each resin composite until five minutes after polymerization,
which was the peak stress in each case. Figure 4 shows
the polymerization shrinkage stress curves produced by each of
the materials over the entire five minutes. Figure 5 shows the
shrinkage stresses of the resins at different times in bar graph
form, with the materials grouped into flowable and universal resin
Pooled data demostrated that the flowable composites group
presented higher polymerization shrinkage stress values in general.
However, SureFil SDR flow had the lowest values among both
universal and flowable resin composites groups. The universal
resin with the lowest shrinkage stress values was Filtek Supreme
Malhotra et al. have concluded that it is important to minimize
the polymerization stress in order to improve the survival
and success rate of resin restorations . For this reason, effective
methods of investigating the polymerization shrinkage stress
properties of resin composite materials are neccesary to predict
their clinical behavior. The use of a universal testing machine with
either extensometers that register movement caused by polymerization
stresses or load cells which measure the forces produced has been widely used in various studies before [5,16,19-24]. One
limitation of these techniques is that they record stress along one
axis, whereas stresses occur along three axes [25,26].
In this study, all the resins had differing levels of polymerization
shrinkage stress. Moreover, the highest shrinkage stress values
among all tested materials was observed with Dentex Flow
(Sino-Dentex), this could be a result of filler content as low as 34%
w/w and use of monomers such as Bis-GMA, which produces a
stiffer polymer chain that is unlikely to undergo plastic deformation
and dissipate shrinkage stresses. The resin composite demonstrating
the lowest polymerization shrinkage stress, SureFil SDR
flow (Stress Decreasing Resin, Smart Dentin Replacement), is
filled 68% w/w, but employs a modified urethane dimethacrylate
monomer, which produces a more flexible polymer, chain and is
said by the manufacturer to polymerized more slowly, allowing
the resin to dissipate more stress by plastic deformation. The results
of this study on polymerization shrinkage stress of SureFil
SDR flow are confirmed in a prior study by Manhart J et al. 
and Abe Y et al. .
Based on the results of this study, the authors believe that the
capabilities of the resin composites in preventing the tooth margins
from polymerization shrinkage stresses is product specific
and does not result solely from the consistency of the material.
Thus, is important for a clinician to learn about the chemical properties
of the resin composites of their choice to understand and
predict its clinical behavior [29-31].
In general, flowable resin composites demonstrated higher
polymerization shrinkage stresses values than universal resin
composites, except for SureFil SDR flow showed the lowest polymerization
shrinkage stress properties among all the materials tested.
With the recent demand of flowable bulk-fill resin composites,
among dental professionals, an increase of research is necessary
in order to compare their different properties and understand
how these may clinically affect the longevity of the restorations.