THE JOURNAL OF - Dentsply

Braem
Table 2 Results for the microshear strength (τ, MPa) and μSFR (MPa)
Brand name μ-shear ± SD μSFR 25% 75% β0 1 β1 1 p-value 2 τ/μSFR
strength quartile quartile (%)
Fuji II LC Caps 10.1 ± 3.1 4.1 3.1 5.1 -4.5580 1.1195 0.0000 41
Syntac 55.5 ± 5.1 30.4 28.4 32.4 -16.6834 0.5492 0.0000 55
Optibond FL 56.9 ± 3.2 17.2 13.4 21.0 -4.9682 0.2893 0.1056 30
Optibond Solo Plus 49.2 ± 10.5 11.9 9.7 14.1 -6.1994 0.5188 0.0001 24
Prime & Bond NT 47.0 ± 2.7 18.3 13.9 22.7 -4.5571 0.2491 0.3987 39
XP Bond 56.4 ± 2.0 30.3 26.7 33.8 -9.3190 0.3081 0.0000 52
Adper ScotchBond 1 XT 51.7 ± 4.8 17.9 15.7 20.1 -8.7526 0.4895 0.0585 35
Clearfil SE Bond 35.0 ± 8.4 20.7 16.4 24.9 -5.3315 0.2577 0.5672 65
Clearfil Protect Bond 42.8 ± 12.3 32.6 28.9 36.3 -9.7454 0.2991 0.0000 76
Xeno III 46.2 ± 3.5 25.2 25.1 25.4 -175.7207 6.9613 0.0073 55
Adper Prompt-L-Pop 53.5 ± 10.7 22.7 22.5 22.9 -132.0255 5.1610 0.0585 42
G-Bond 54.9 ± 3.7 15.3 8.5 22.1 -2.4908 0.1625 0.0318 28
iBond 51.0 ± 8.3 12.3 8.7 15.9 -3.7347 0.3042 0.0003 24
1) β0 and β1 are parameters of the logistic regression function, determining the probability of failure in terms of stress applied to the interface (S): Y = exp(β0 + β1 •S) / (1 + exp(β0 + β1 •S))
2) p-value of estimate of the multiple logistic regression
evance. Assuming 3 periods of 15 min of chewing per day
at a chewing rate of 1 Hz, the number of chews is 2700 per
day. 14 This amounts to roughly 10 6 times per year. At this frequency,
a fatigue test on 1 sample takes about 12 days to
accomplish 10 6 cycles. Increasing the rate to 2 Hz will offer
a time-saving approach well within the frequency dependency
range of the tested visco-elastic materials. Furthermore,
if the noncontact events are eliminated from the
chewing cycles, it can be calculated that 10 4 cycles represents
about 1.2 months of continuous real-life contact. This
approach is justified, since restorations tend to fail either
early, before 10 4 cycles, or very late, after 10 5 cycles. 9 Moreover,
a test run on a sample that does not fail now takes
about 1.5 h to complete, as opposed to about 1 week for
10 6 cycles.
A common standard in the analysis of fatigue data is the
value at which 50% of the specimens fail after a pre-set number
of cycles. 2,4,5 However, in the present type of fatigue testing,
considerable errors can be made as to the determination
of the so-called pre-set stress levels due to geometry errors. 4
Therefore, the probability of failure at each applied stress level
was approximated by logistic regression where the μSFR
represents the value at which 50% of the specimens fail. Calculating
the 25% and 75% quartiles further illustrates the
distribution of the obtained data (Table 2). It can be seen
from these data that the products with the steepest slopes
(β 1 ) show the lowest distribution around the μSFR.
Some of the adhesives therefore show a so-called type 2
fatigue behavior 10 and appear to have a rather well-defined
fatigue stress level or threshold above which they fail rapidly
and below which they will survive (Xeno III, Adper Prompt-
L-Pop, Fuji II LC). In other products, such as G-Bond, the opposite
behavior is found and a wide spread of data around
the μSFR can be noted. Most of the materials show a mixed
type of behavior, indicating that imperfections and defects
probably determine the actual lifespan of the adhesive
bond. Fractographic analysis needs to be carried out to test
this hypothesis.
The main advantage of the present method is that with
the same setup, both shear strength and fatigue resistance
can be measured and compared. It is also advantageous
that the method requires no processing of the sample once
the interface has been formed, thereby avoiding operator-induced
defects. The results show that the μSFRs are consistently
lower than the respective microshear strength for the
same adhesive (Table 2). This is not surprising, since it is
known that internal interfacial defects and imperfections
shorten the lifespan of a joint under cyclic fatigue. Hence,
fatigue data generally vary substantially more than static
bond strengths. 8 The present results, however, show in
some cases that the variation in the fatigue data is lower
than that present in the quasi-static data, thereby emphasizing
the need for nontraumatic sample preparation.
It is beyond the scope of the present article to discuss in
full the possible mechanism of each individual adhesive related
to the measured μSFR. The present results emphasize
that within each type of adhesive, products exist that show
a high fatigue resistance. Furthermore, some few-step adhesives
perform better than other multi-step ones, while for
others it is just the opposite. Thus, not only the reduction in
252 The Journal of Adhesive Dentistry