Keywords
resin restoration - shelf life - expiration date
Introduction
Dental manufacturers publish a material expiration date intended to assure clinicians
that purchased materials will maintain efficacy over a stated time period.[1] Manufacturers may use methods to help establish product shelf life, which is identified
as the period between product formulation/manufacture and the time that the material
no longer has the mechanical and physical properties required to accomplish its intended
purpose.[1] Stability, somewhat synonymous with shelf life, is also described by the amount
of the same characteristics which a product retains at the time of manufacture and
throughout its storage period.[2] Various criteria were proposed to delineate stability assessment methods. For instance,
pharmaceutical industry methods exemplify testing of biologic, mechanical, and optical
properties.[3] However, other testing factors have been suggested that include the consideration
of nonideal environmental conditions prior to and during product delivery.[4] Unfortunately, there is no established standard for the determination of dental
product stability and shelf life.
For dental restorative products, stability and shelf-life determinations are frequently
accomplished using accelerated aging protocols that expose materials to increased
and/or more frequent heat and humidity conditions than the normally recommended ones.[4]
[5]
[6] Accelerated aging protocols usually follow the Arrhenius collision theory model
that is known as the “10-degree rule,”[1] which presumes that a product’s reaction rate during storage will double for each
10°C increase above standard room temperature.[1] The Arrhenius model is based on the formula of r = Q10 (RT-ET/10), where r = accelerated
aging rate, RT = room temperature (22°C), ET = elevated temperature (usually 37°C),
and Q10 = reaction rate coefficient. Under this model 12 months of simulated ambient
storage at 22°C would require 17 weeks storage at 37°C. However, hastened evaluations
may not always be appropriate, as full-time, ambient storage shelf-life studies may
be used for new materials not previously evaluated.[4]
[5]
[6]
Resin composite shelf life determinations are complicated, as these materials contain
many components in which individual constituent degradation could cause a potential
myriad impact on the polymeric composite’s functional properties.[1] Furthermore, this individual degradation may be obscured by other structural components,
until sufficient degradation accumulates, that affect material property.[3]
[7] Likewise, clinicians attempting to determine product shelf-life by assessing clinical
handling characteristics alone is also not advised, as unobserved individual component
degradation may not be discerned but could have an intense impact on the material’s
functional longevity.[8] Hence, clinicians are advised to discard expired products. However, when based solely
on arbitrary assigned criteria, this may represent additional costs to the dental
profession. Due to a lack of standards, manufacturers are not necessarily required
to divulge shelf-life determination methods. Therefore, some clinicians may be tempted
and choose to use a product past its expiration date.[9]
Studies involving expired resin composite performance reported various methodologies
including flexure strength,[4]
[10] flexural modulus,[4]
[10] hardness,[9]
[11] diametral tensile strength,[12] surface roughness,[101] filler distribution,[13] thermal analysis,[10] infrared spectroscopy,[11] electron paramagnetic resonance (EPR) spectroscopy,[14] and X-ray diffraction.[14] Furthermore, accelerated and/or actual storage times differ, varying from 6 months
ambient aging,[9]
[11] 9 months simulated due to accelerated aging,[10]
[13]
[14] 15 months,[12] and 7 years of ambient storage.[4]
The purpose of this study was to investigate the postexpiration flexural strength
and modulus of five, ambiently stored, visible light-cured direct restorative resin
composites. The null hypothesis was that there would be no difference in the individual
material’s flexure strength and flexural modulus as compared with that obtained 1
month prior to the expiration date.
Materials and Methods
The products evaluated are listed in [Table 1]. The materials used were in excess after other material testing and near manufacturer
recommended expiration date. All five materials were stored in a laboratory storage
drawer freely exposed to ambient conditions (23 ± 2°C, 52 ± 6% relative humidity)
that were within manufacturer specified storage ranges. Baseline data was obtained
with sample fabrication and testing 1 month prior to manufacturer’s supplied expiration
date, followed by product testing at 3, 6, 9, 12, 15, 18, 24, and 30 months after
which supplies were exhausted. Twenty specimens were fabricated for each test (n = 20) with the chosen sample size designed to provide more accurate mean values and
lower standard error. Flexural strength specimens were fabricated as per ISO 404919
using standardized, 2 × 2 × 25 mm stainless steel molds (Sabri Dental Enterprises;
Downers Grove, IL, USA). Molds were placed onto a polyester film on the dorsal surface
of a glass slab. The mold was filled with the resin composite, a second polyester
strip placed, and pressure exerted using a second glass slide to form a flat and uniform
surface. The resin composite was then cured with a light-emitting diode (LED) curing
unit (Bluephase G2, Ivoclar-Vivadent; Amherst, NY, USA) for 20 seconds as overlapped
on both sides. The VLC output was periodically assessed (~ 1000 mw/cm2) using a hand-held
radiometer (BluePhase Meter II; Ivoclar Vivadent). Specimens were further refined
with flash material removed using surgical scalpel blades and stored in physiologic
fluid (0.2M phosphate buffered saline) in a light-proof container at 37°C and 98 ±
1% humidity. After 24 hours, specimens were tested until failure in three-point bend
using a universal testing machine (Alliance RT/5; MTS Corporation, Eden Prairie, MN,
USA) at 0.5 mm/min. Flexure strength (FS) was determined using the formula FS = 3FI/2bh2,
where F was the maximum load recorded in Newtons, I represents the millimeter distance
between supports, while b and h describe millimeter specimen width and height, respectively.
Young’s modulus (flexural) was defined by linear slope of the stress/strain curve.
Mean data were found to contain both an abnormal distribution and variance regularity
using Shapiro–Wilk and Bartlett’s testing, respectively. Mean data were then compared
using Kruskal–Wallis/Dunn’s with additional correlation analysis (Pearson’s) to identify
possible similar trends between materials within each mechanical property over the
study duration. All statistical analysis was performed at a 95% level of confidence
(α = 0.05) using GraphPad Prism 8 (GraphPad Software; San Diego, CA, USA).
Table 1
Materials evaluated
Product
|
Classification
|
Constituents
|
Lot number
|
Abbreviations: Bis-GMA = Bisphenol A diglycidyl ether dimethacrylate.
a = content % not provided; content obtained from manufacturer literature.
|
Beautifil II
Shofu Dental Corporation, San Marcos, CA USA
|
Giomer
|
Bis-GMA: ~ 70%
Triethylenglycol dimethacrylate: < 5%
Aluminofluoro-borosilicate glass: 70%
Al2O3
a
DL-Camphorquinonea
|
021250
|
Esthet X HD
Dentsply–Sirona/
Dentsply Caulk
Milford, DE, USA
|
Nano hybrid
|
Hydrophobic amorphous fumed silica: < 5%
Silica (amorphous): < 5%
Fluoroaluminoborosilicate glass: < 50%
Urethane modified Bis-GMA dimethacrylate: < 10%
Polymerizable dimethacrylate resins: < 20%
|
111007
|
Filtek Supreme Ultra
3M Oral Care, St. Paul, MN, USA
|
Nanofill
|
Silane treated ceramic: 60–80%
Silane treated silica: 1–10%
Diurethane dimethacrylate (UDMA): 1–10%
Bisphenol A polyethylene glycol diether dimethacrylate: 1–10%
BISGMA: 1–10%
Silane treated zirconia: 1–5%
Polyethylene glycol dimethacrylate: < 5%
Triethylene glycol dimethacrylate < 1%
|
N357235
|
TPH3
Dentsply-Sirona
|
Nano hybrid
|
Bariumaluminofluorosilicate glass: 49.7%
Fluoroaluminoborosilicate glass: 24.6%
Hydrophobic amorphous fumed and amorphous Silica: < 5%
Urethane modified Bis-GMA dimethacrylate
resin: 2.5 < 10%
Ethoxylated bisphenol A dimethacrylate 2.5 < 10%
2,2'-Ethylendioxydiethyldimethacrylat 2.5 < 10%
|
1110031
|
Z250
3M Oral Care
|
Hybrid
|
Silane treated ceramic: 75–85%
Bisphenol A polyethylene glycol diether dimethacrylate (BISEMA6): 1–10%
Diurethane dimethacrylate (UDMA): 1–10%
BISGMA: 1–6%
Triethylenglycol dimethacrylate (TEGDMA): < 3%
Aluminum oxide: 1%
|
N34012
|
Results
Results are displayed in [Table 2]. All materials demonstrated similar flexure strength as compared with baseline for
up to 15 months, with TPH3 and Z250 mean flexure values remaining similar for the
entire 30-month period. Both Beautifil II and Filtek Supreme Ultra were noted to have
significantly lower mean flexure strength at 18 months, with Esthet X having significantly
lower values at 30 months. At 12 months, Z250 displayed a nonsignificant flexure strength
increase at 12 months which decreased afterward, while both Beautifil II and Esthet
X demonstrated the same trend at 15 months. Nevertheless, although some variation
within the study timeframe was noted, Beautifil II, Esthet X, and TPH3 did not display
significantly lower modulus values until 30 months. Z250 and Filtek Supreme Ultra
both demonstrated significantly lower modulus values at 15 and 18 months, respectively.
The correlation analysis results can be seen in [Tables 3]
[4]. The flexure strength behavior between TPH3 and Esthet X HD was identified as having
a strong correlation (p = 0.008, r2 = 0.71) with Beautifil II flexure strength behavior also identified as having a strong
correlation with both TPH3 and Esthet X HD (p = 0.004, r2 = 0.71). Furthermore, a significant and strong correlation was found between Filtek
Supreme Ultra and Esthet X HD (p = 0.008, r2 = 0.64) but not between Filtek Supreme Ultra and TPH3. Interestingly, correlation
was not identified between Filtek Supreme Ultra and Z250 (p = 0.09, r2 = 0.33), both of which are produced by the same manufacturer. Filtek Supreme Ultra
modulus behavior was found to have a significant correlation with Z250 (p = 0.006, r2 = 0.67) as well as TPH3 (p = 0.045, r2 = 0.45), but not Beautifil II (p = 0.07, r2 = 0.38). Similar to flexure strength, TPH3 and Esthet X HD maintained a strong correlation
(p = 0.028, r2 = 0.65), but each were not as strongly correlated with Beautifil II (r2 = 0.5 and r2 = 0.42, respectively).
Table 2
Mean flexure strength and modulus results (MPa)
|
|
Baseline
|
3M
|
6M
|
9M
|
12M
|
15M
|
18M
|
24M
|
30M
|
n = 20; capital letters identify similar groups for each row compared with row baseline
data (Kruskal—Wallis/Dunn’s, p < 0.05).
|
Beautifil II
|
Flexure strength
|
131.7 (12.6) A
|
131.6 (10.0) A
|
128.6 (12.6) A
|
125.9 (10.5) A
|
127.2 (14.1) A
|
131.3 (13.3) A
|
113.3 (22.4) B
|
112.3 (19.9) B
|
114.3 (17.4) B
|
Modulus
|
11778 (1120)
A
|
12608 (1042) A
|
11558 (574) A
|
10825 (862) B
|
11167 (956) A
|
12306 (618) A
|
11055 (746) A
|
11259 (1169) A
|
10196 (1105) B
|
Esthet X
|
Flexure strength
|
139.7 (15.9) A
|
142.4 (14.4) A
|
139.6 (7.8) A
|
134.5 (12.2) A
|
130.1 (19.8) A
|
143.0 (11.5) A
|
126.1 (23.4) A
|
129.7 (16.2) A
|
116.6 (16.2) B
|
Modulus
|
8894 (949) A
|
9390 (877) A
|
8777 (607) A
|
9507 (417) A
|
9426 (590) A
|
9748 (694) B
|
8675 (464) A
|
8478 (534) A
|
7584 (481) B
|
Filtek Supreme Ultra
|
Flexure strength
|
149.9 (22.5) A
|
147.0 (12.2) A
|
144.6 (14.5) A
|
146.4 (17.5) A
|
137.1 (13.6) A
|
136.4 (20.6) A
|
133.3 (18.2) B
|
128.2 (16.9) B
|
121.3 (23.3) B
|
Modulus
|
12393 (1535) A
|
11452 (731) A
|
11551 (952) A
|
11453 (1057) A
|
11629 (615) A
|
11729 (672) A
|
10702 (665) B
|
10559 (474) B
|
10145 (413) B
|
TPH3
|
Flexure strength
|
146.0 (21.0) A
|
156.5 (22.8) A
|
152.7 (20.7) A
|
149.7 (13.7) A
|
152.0 (23.2) A
|
164.5 (21.9) A
|
141.7 (18.8) A
|
136.8 (23.4) A
|
135.9 (22.1) A
|
Modulus
|
9439 (1321) A
|
9299 (897) A
|
9158 (492) A
|
8638 (388) A
|
9798 (515) A
|
10016 (606) B
|
9174 (466) A
|
8935 (600) A
|
8225 (535) B
|
Z250
|
Flexure strength
|
147.0 (34.8) A
|
163.1 (19.4) A
|
155.7 (16.8) A
|
158.9 (16.4) A
|
170.9 (15.8) B
|
151.8 (21.2) A
|
152.2 (22.1) A
|
141.2 (23.0) A
|
134.5 (33.1) A
|
Modulus
|
13903 (1680) A
|
13071 (691) A
|
13304 (1313) A
|
11623 (947) B
|
12565 (844) A
|
12235 (1313) B
|
11833 (931) B
|
11199 (900) B
|
11347 (718) B
|
Table 3
Correlation matrix of mean flexure strength results (Pearson’s)
|
Beautifil II
|
Esthet X
|
Filtek Supreme Ultra
|
TPH3
|
Z250
|
n = 20; r = Pearson’s correlation coefficient; r2 = determination coefficient Correlation matrix represents comparison of all materials’
results with each other using all data points (baseline through 30 months).
|
Beautifil II
|
1.0
|
r = 0.84
p = 0.004
r2 = 0.71
|
r = 0.81
p = 0.007
r2 = 0.65
|
r = 0.84
p = 0.004
r2 = 0.71
|
r = 0.58
p = 0.099
r2 = 0.33
|
Esthet X
|
r = 0.84
p = 0.004
r2 = 0.71
|
1.0
|
r = 0.80
p = 0.008
r2 = 0.64
|
r = 0.84
p = 0.008
r2 = 0.71
|
r = 0.47
p = 0.198
r2 = 0.22
|
Filtek Supreme Ultra
|
r = 0.81
p = 0.007
r2 = 0.65
|
r = 0.84
p = 0.008
r2 = 0.71
|
1.0
|
r = 0.56
p = 0.109
r2 = 0.31
|
r = 0.58
p = 0.095
r2 = 0.33
|
TPH3
|
r = 0.84
p = 0.004
r2 = 0.71
|
r = 0.84
p = 0.008
r2 = 0.71
|
r = 0.56
p = 0.109
r2 = 0.31
|
1.0
|
r = 0.64
p = 0.06
r2 = 0.41
|
Z250
|
r = 0.58
p = 0.099
r2 = 0.33
|
r = 0.47
p = 0.198
r2 = 0.22
|
r = 0.58
p = 0.095
r2 = 0.33
|
r = 0.64
p = 0.06
r2 = 0.41
|
1.0
|
Table 4
Correlation matrix of mean flexural modulus results (Pearson’s)
|
Beautifil II
|
Esthet X
|
Filtek Supreme Ultra
|
TPH3
|
Z250
|
n = 20; r = Pearson’s correlation coefficient; r2 = determination coefficient
Correlation matrix represents comparison of all materials’ results with each other
using all data points (baseline through 30 months).
|
Beautifil II
|
1.0
|
r = 0.65
p = 0.057
r2 = 0.42
|
r = 0.62
p = 0.073
r2 = 0.38
|
r = 0.71
p = 0.030
r2 = 0.50
|
r = 0.61
p = 0.078
r2 = 0.37
|
Esthet X
|
r = 0.65
p = 0.057
r2 = 0.42
|
1.0
|
r = 0.71
p = 0.031
r2 = 0.50
|
r = 0.72
p = 0.028
r2 = 0.65
|
r = 0.34
p = 0.36
r2 = 0.11
|
Filtek Supreme Ultra
|
r = 0.62
p = 0.073
r2 = 0.38
|
r = 0.71
p = 0.031
r2 = 0.50
|
1.0
|
r = 0.65
p = 0.045
r2 = 0.42
|
r = 0.52
p = 0.006
r2 = 0.67
|
TPH3
|
r = 0.71
p = 0.030
r2 = 0.50
|
r = 0.72
p = 0.028
r2 = 0.65
|
r = 0.67
p = 0.045
r2 = 0.45
|
1.0
|
r = 0.52
p = 0.15
r2 = 0.27
|
Z250
|
r = 0.61
p = 0.078
r2 = 0.33
|
r = 0.34
p = 0.36
r2 = 0.22
|
r = 0.82
p = 0.006
r2 = 0.67
|
r = 0.52
p = 0.15
r2 = 0.27
|
1.0
|
Discussion
Dental direct restorative composite resins are polymers possessing both clinical and
laboratory performance largely related to the configuration of the polymer’s structure
and time-related degradation.[3]
[15] Polymer stability and its association with shelf life involve intertwining processes
(e.g., chemical aging and physical aging) that are difficult to fully understand.
International standards distinguish between chemical and physical degradation/aging
processes, although, in practice, the processes occur simultaneously. The German Institute
for Standards (DIN) 50035 identifies chemical aging as irreversible changes with molecular
weight, physical structure, and/or chemical composition.[16] DIN 50035 further defines physical aging as processes that involve structural changes,
organization of the molecular state, and/or changes in quotients of multicomponent
systems on measurable mechanical performance that does not lead to chemical degradation.[16] However, the interactions between chemical and physical aging processes are very
complex, as components of both processes intertwine and occur simultaneously with
the produced degradation products also affecting the total results.[16] To wit, it can be easily envisioned where resin composite shelf life, determined
by polymerization function, can be affected by both processes–illustrated by tertiary
amine breakdowns (chemical aging) as well as inhibitor loss, resulting in microenvironment
polymer polymerization and crystallization (physical aging).[15] Furthermore, degradation may also not be dependent on individual constituent concentration,
as silane coupling agents are not a major percentage of resin composites, but silane
coupling agent interaction at filler/matrix interfaces greatly influences the material
performance and long-term stability.[13]
[14] Shelf-life determinations are based on ambient real-time testing or accelerated
aging protocols, which both contain advantages and disadvantages. This evaluation
followed an ambient condition and real-time protocol, as all materials were stored
together in a controlled laboratory setting. This evaluation used five resin composite
restorative materials that were in excess from previous research evaluations and were
close to the manufacture recommended expiration date. Beautifill II is classified
as a Giomer with aluminofluoro–borosilicate glass used as a filler material. Both
Esthet X HD and TPH3 are classified by the manufacturer as nanohybrids, with the filler
content of TPH3 and Esthet X HD being said to contain fillers consisting of barium
boron fluoroalumino silicate glass, amorphous silica, and hydrophobic amorphous-fumed
silica. The one nanofilled material, Filtek Supreme Ultra, is described to contain
fillers listed as silane-treated ceramic and silica as well as silane treated zirconia.
Z250 is classified as a hybrid consisting of silane-treated ceramic fillers. A generalized
decreasing trend with all materials can be observed. Filtek Supreme Ultra is observed
to follow a general decreasing flexure strength behavior, while at 12 months Z250
demonstrates a momentary flexure strength increase at 12 months. The flexure strength
behavior existed at 15 months with TPH3, Esthet X HD, and to a lesser extent, Beautifil
II. The flexure strength behavior between some of the materials were suggested to
correlate. TPH3 and Esthet X HD were identified to have a strong correlation (r2 = 0.71), with this relationship attributed to a function of similar polymer and filler
components. However, Beautifil II flexure strength behavior is also identified as
having a strong correlation (r2 = 0.71) with both TPH3 and Esthet X HD but the resin component differs. Moreover,
strong correlation was found between the flexure strength performance between Filtek
Supreme Ultra and Esthet X HD (r2=0.71) but not between Filtek Supreme Ultra and TPH3 (r2 = 0.31). Interestingly, correlation was not identified between Filtek Supreme Ultra
and Z250 (r2 = 0.33), both of which are produced by the same manufacturer. While both Z250 and
Filtek Supreme Ultra essentially contain the same resin backbone and filler components,
this could possibly identify a function of the mean filler size difference between
the nanofilled and hybrid particles. Although material specific, the flexural modulus
performance largely mirrored the behavior observed with flexure strength. Significant
modulus changes were noted, but the low covariance might helped identify significant
difference within each group that could unlikely be of clinical significance. Unlike
flexure strength behavior, Filtek Supreme Ultra modulus behavior was found to have
a significant correlation with Z250 (r2 = 0.67), while TPH3 and Esthet X HD (r2 = 0.65) maintained a strong correlation. The overall behavior between different material
correlations when considering both flexural strength and modulus is indeed puzzling.
While some reports suggest a weak correlation between flexure strength and modulus
performance,[17]
[18] the behavior noted with the correlations noted between flexure strength and modulus
may merit further investigation. Comparison of this current study with that reported
in the literature is difficult due to the lack of comparative studies. Hondrum and
Fernandez[4] did report shelf life flexure strength results, but it only involved two restorative
resins that were not comparable with currently marketed materials. D’Alpino et al[10] evaluated Filtek Z250 and Filtek Z350XT with an accelerated aging protocol comparable
to 9 months ambient storage and reported similar Z250 results of loss of flexural
strength and modulus.
The null hypothesis was rejected, as changes in both flexure strength and modulus
were noted over the course of the evaluation. However, an important overall consideration
with evaluating data of this nature is identifying when a selected mechanical property
degradation indicates that the material is no longer suitable for clinical use. For
flexural strength, ISO 4049 recommends a minimal flexure strength of 80 MPa for clinical
function,[19] which all materials in this study surpassed. However, recent work of Heintze et
al[20] reported the results of a systematic review of 74 clinical experiments from 45 studies
involving 31 different materials. While this work did not identify a correlation between
flexure strength and clinical material fracture, it did suggest a strong correlation
between significant resin composite surface wear and a minimal flexure strength of
130 MPa. [Fig. 1] demonstrates the flexure strength results compared with suggested limits of ISO
4049[19] and that suggested by Heintze et al.[20] While all materials in this study surpassed the agreed minimal ISO 4049 flexure
strength functional requirements, Beautifil II might be expected to demonstrate significant
surface wear beyond 3 months after the expiration date, and Esthet X HD and Filtek
Supreme Ultra might demonstrate the same after 15 and 18 months, respectively. While
it would be tempting to relate the beginning of wear due to silane degradation,[13]
[14]
[21] resin composite surface wear is a multifactorial phenomenon and is usually material
dependent and cannot be predicted from resin composite category, filler loading, and
resin matrix.[22]
[23] Other effects of degradation may include reduction of fracture toughness,[24] flexure strength,[25]
[26] surface roughness,[27]
[28] all of which may cause early restoration failure.
Fig. 1 Mean flexure strength results compared with suggested minimum performance standards.
n = 20; red dotted line annotates lower limit of 130 MPa, suggested to prevent surface
wear and degradation, as suggested by Heintze et al.[20] Blue dotted line annotates minimal flexure strength required for clinical function,
as agreed upon in ISO 4049[19]
Limitations of this study include that the materials were not thermally stressed before
testing, as Rutterman et al[29] reported that flexural properties were affected by thermocycling. A further limitation
was the materials evaluated were not stored in strictly controlled conditions but
rather in a general laboratory environment that may represent daily temperature and
humidity fluctuations as that of a clinical situation. Furthermore, baseline data
was obtained approximately 1 month prior to stated expiration date, which assumes
that material stability demonstrated at that time would be comparable to that obtained
at material formulation. Importantly, this study identifies the complex interrelationships
between resin composite constituent components and the difficulty involved with the
determination of resin composite instability due to selected mechanical property evaluations.
Accordingly, no guidance exists from international dental standards that detail testing
methodologies for resin composite shelf-life determination. Furthermore, it is not
common knowledge which manufacturer protocols are used for material shelf-life determination,
and transparency with specific testing methodologies would be welcomed.
Conclusion
Under the conditions of this study, five resin composite direct restorative materials
were found to maintain flexure strength and modulus in most cases up to 15 months
after the manufacturer’s recommended expiration date. However, clinicians are still
advised to follow manufacturers’ recommended expiration dates, as resin composite
degradation mechanisms are complex, and constituents may degrade that seriously affect
restoration longevity but are not overtly identified by clinical handling characteristics
and selected mechanical property testing. As no dental standards for shelf-life determination
exist, manufacturers are requested provide transparency with protocols used in assigning
shelf-life expiration dates.