The primary disadvantage of the resin
bonded bridge (RBB) is that the longevity of the prosthesis is less than that
for conventional prosthesis.(1,2) Improvements in the
prosthesis design, preparation design and adhesive bond strength enhance the
survival rates of RBB but there are still an unacceptable number of clinical
failures mostly because of debonding.(3-5) The tensile peel
strength (TPS) test as a means of assessment of bonding of RBB was explored by
Northeast et al.(6) where Ni/Cr beams were bonded to
Ni/Cr blocks by adhesive luting cement. The loading conditions result in a
peeling action at the adhesive interface, providing a possible explanation for
failure of RBB rather than failure being attributable to poor clinical or
laboratory technique.(6) In this study, it was shown that the TPS is a function of the thickness of
the retainer i.e. the thicker the retainer, the higher tensile peel
force required to cause failure.(6) With thicker retainer the level of stress within the
adhesive layer was reduced, suggesting that the stress the adhesive layer has
to withstand is an important contributory factor to the clinical outcome and is
governed by enhancing the mechanical properties of the luting cements. It
is therefore proposed that the fracture toughness (KIC) of the luting cement
has a major role in determining the retention of RBB. KIC is defined as the
amount of energy required to propagate a surface flaw or a pre-existing crack
through a material, causing catastrophic fracture.(7) It is a
measure of the critical stress at the tip of a flaw that allows propagation of
a crack under tension.(8) KIC
is the lowest stress at which catastrophic crack propagation can occur.(8,9)
Although there were some investigations
that reported on the KIC of luting cements, little information is available on
the relationship between the KIC of the cements and their adhesive bond
quality. The purpose of this study was to measure the TPS of different types of
luting cements bonded to Ni/Cr alloy. Then compare the values of TPS of these
cements with their known KIC values from the dental literature(10,11)
to examine the possible correlation between them.
Methods
Six cements were investigated; two
chemically adhesive resin cements (Super-Bond C&B and Panavia 21), one
compomer cement (Dyract Cem), two resin-modified glass ionomer cements (Fuji
Plus and RelyX Luting), and one conventional glass ionomer cement (Ketac Cem). Twenty
Ni/Cr alloy beams (Talladium-V, Talladium, Bucks, UK) 22mm long, 5 mm wide and
0.5 mm thick were used. A 1mm diameter central hole was drilled 1.5mm from one
end of each beam. A 15 mm brass block (20 blocks) with a Ni/Cr alloy base
bonded to one of its surfaces was also used in this study. Ni /Cr beams and blocks were blasted with
fresh 50μm alumina grit, washed in distilled water in an ultrasonic cleaner for
5 minutes and then air-dried before bonding with the luting cements. Manufacturers’
specifications as to the correct mixing time, paste-to-paste and
powder-to-liquid ratios were carefully followed during mixing of luting cement.
After mixing the luting cement, it was applied to the fitting surface of the
beam. The beam was aligned perpendicular to the centre of the free edge of the
Ni-Cr block such that a 10 mm length of the beam was bonded to the block with
the aid of an alignment jig. A compressive load of 40 N was applied vertically
to the beam (at about the middle of the 10 mm bonded to the block) during
setting of the cement using a Lloyd universal testing machine to produce
consistent cementation procedure. Excess cement was removed. Twenty samples
were made for each type of the tested cements.
The samples were stored in the dry air at room temperature and tested
after 24 hours. The prepared specimens were mounted in a Lloyd tensile machine
(Lloyds Instruments. UK)
with the free end of the beam perpendicular to, and in line with, the load cell
(100N). Each beam was pulled off the block with a peeling action by applying a
tensile load using a small hook that engaged the hole on the free end of the
beam at a crosshead speed of 1mm/min and the force at failure point was
recorded. Statistical analysis for the TPS values was carried out using one-way
analysis of variance and Tukey's pair wise comparisons. Regression analysis was undertaken to
compare the TPS of luting cements with their known KIC values.
Results
The mean TPS values and the
standard deviations (in Newton) are as follows: Super-Bond
C&B 7.7 +/- 1.4, Panavia 6.1 +/- 1.3, GC Fuji plus 5.1 +/-
0.7,
RelyX Luting 4.5 +/- 0.8, Dyract
Cem 4.2 +/- 1.3, Ketac
Cem 2.4 +/- 0.4. All the
fractured surfaces of the tested samples were examined under a stereo zoom
microscope. The mode of failure of all
the tested cements was cohesive in nature. One-way analysis of variance (ANOVA)
showed significant differences between the mean TPS of the cements (P < 0.05) (Table I). Tukey's pairwise comparisons showed that the mean TPS
of Super-Bond was significantly greater than Panavia 21 as well as all other
luting cements. The mean TPS of Dyract Cem, Fuji Plus and RelyX Luting were not
significantly different. The mean TPS of Ketac Cem was significantly the
lowest. The KIC values for those cements that are used in this study were
obtained from literature and were measured after 24 hours and after 7 days.(10,11)
The mean KIC values and the standard deviation (MPa.m½) for those cements are summarised in Table II. The relationship between KIC and TPS shows a highly positive correlation. The linear regression analysis showed that the correlation coefficient (r) is equal to 0.94 after 24 hours and 0.98 after 7days (Fig. 1 and 2). The correlation coefficient after 7 days is better than that after 24 hours.
Discussion
The TPS of different luting cements were
measured in this study. The experimental apparatus used was similar to that
used by Northeast et al.(6) except that the thickness
was the same for all the beams (0.5 mm) and the only variable was the luting
cement. The design of TPS experimental
apparatus is somewhat similar to the design of RBB if we assume that the Ni-Cr beam
acts as the retainer of RBB, the block as the tooth structure, and the pull out
load as the load responsible for failure of RBB. Not all the tested cements are
used for bonding RBB although all of them can bond well to the metal. This is
to show that its not only the ability of the cement to bond to metal is the
only requirement for bonding RBB, it must also be able to resist stresses
generated in the cement layer and to resist propagation of cracks. All the KIC
values were obtained from Knobloch et al(10) except
for Dyract Cem, which was obtained from a study by Ryan et al.(11) Using a wide variety of luting cements with
different KIC values is better for studying the relationship between KIC and
TPS. Although Knobloch et al and Ryan et al used different
methods to measure the KIC (The mini-compact tension method and chevron notch short rod method respectively)
the measured values for the same type of cement were close in both studies. The loads required to cause
debonding in the TPS experimental apparatus are lower than those obtained from
tensile bond strength tests.(12,13) This observation is consistent
with Northeast et al.(6) The
tensile bond strength of Super-Bond and Panavia bonded to Ni-Cr alloy was
reported to be 28.5 MPa and 70 MPa respectively.(12) With retainers having a surface area of 10mm², the tensile force
required to cause debonding would have to be about 280 N - 700 N, and such high
loads are unlikely to occur clinically, nevertheless, debonding is a common
mode of failure of RBB.(3-5) Another surprising observation
is that the RBB most frequently fails at the resin-metal interface leaving a
layer of resin on the enamel.(4,5) This contrasts with the
observation that the tensile bond strength of resin-metal is generally higher
than that of resin-enamel.(14) Therefore, the load required for bond failure
to occur due to tensile peel stresses within the adhesive interface is
potentially more clinically relevant than measurement of tensile or shear bond
strengths.
The main disadvantage of the TPS test is that it is structural
dependent; it depends on the thickness of the beam.(6) That
means the values of tensile peel strength for the luting cements used in this
study will change if we change the thickness of the beams but the 0.5 mm beam
thickness is similar to the recommended retainer thickness used clinically. (15)
The mode of failure of all the studied
cements was virtually identical, this was cohesive in nature. The crack
initiation took place close to the cement-substrate where there are maximum
tensile peel stresses as was shown by the finite element analysis done by
Northeast et al.(6) The fracture subsequently
travelled close to the beam-cement interface leaving most of the cement on the
substrate. This is similar to the clinical failure where most of the cement is
left on the tooth surface after debonding.(4,5) Since the
fracture started from within the cement itself and not at the beam-cement
interface, whether or not one material bonds better than another becomes
irrelevant. This proves that it is the KIC of the cement itself that may play
an important role in the adhesive bond quality of the RBB. The regression
analysis of the relationship between KIC and TPS showed a positive linear
relationship. That means the material with higher KIC will result in higher
tensile peel forces to cause debonding compared with more brittle material
assuming both of them bond well to the metal surface. The statistical
regression analysis showed a better relationship between the TPS and the
measured KIC values after 7 days (correlation coefficient = 0.98) than that
after 24 hours (correlation coefficient = 0.94). This may be due to the effect
of water as the KIC test was done after water storage in order to simulate the
oral conditions.(10) Water storage will plasticize the luting
cement and delay the onset of fracture.(10) Super-Bond and
Panavia are adhesive resin cements that form chemical bonds with clean sand
blasted base metal surfaces.(16,17) The TPS and KIC of
Super-Bond are significantly greater than that of Panavia. This may be due to
differences in the chemical composition of those materials; Panavia 21 is
Bis-GMA-based resin cement, which contains high volume fraction of inorganic
fillers. While Super-Bond is an unfilled poly methyl methacrylate (PMMA) based
resin cement that contains long flexible chains of high molecular weight, which
tend to lead to higher KIC.(10) Plastic deformation delays
the onset of brittle fracture, resulting in higher KIC(7,18)
and so higher TPS. There was no significant difference between the TPS
of Dyract cem, Fuji Plus and RelyX Luting, which are compomer and resin-modified
glass ionomer cements. This may be due to the relatively close chemical
composition of those materials. Their KIC and TPS were significantly greater
than that of conventional glass ionomer cements, which may be due to resinous
components of those materials.(11) The KIC and TPS of Ketac
Cem, which is conventional glass ionomer cement are significantly lower than
all the other luting cements. Conventional glass ionomer cements are
susceptible to dehydration and crazing during the initial setting reaction.(10,19)
The resultant microcracks would act to initiate and facilitate crack
propagation within the cement matrix.(10, 19) The TPS test is
somewhat similar to the KIC test principle in that both of them measure the
ability of the material to resist crack propagation. That’s why it is not
surprising to find relationship between KIC and TPS. The high KIC and TPS of
the adhesive resin cements may explain their good clinical performance in RBB.
On the other hand, glass ionomer cement showed unacceptable rate of debonding
although it adheres well to tooth structure and metals.(20,21)
This could be due to their low KIC and TPS which may contraindicate their use
in RBB. It is unclear how valuable compressive strength data are in selecting
luting cement for RBB. The compressive strength of glass ionomer cement was
shown to increase over several weeks to about 200 MPa.(22) Panavia has a very high compressive strength compared
to Super-Bond, which exhibits too much plastic deformation to be tested in this
way.(23) In contrast, Super-Bond has higher fracture toughness
than Panavia.(10) So there is a need to know which material property is
responsible for the better clinical performance in order to help in material
selection. The absolute value of KIC is a material property, which should be
independent of the size and geometry of specimen and may be a more reliable
parameter to predict clinical performance than the compressive or diametral
tensile strength measurements.(11,19) The compressive or tensile strength measurement might not
provide accurate information on the likely performance of the material in
clinical use, due to influence of the specimen geometry and flaws introduced
during manufacture on the values obtained.(24) This does not
mean that the KIC of the luting cement is the only property that is needed to
enhance the outcome of the RBB. Other mechanical properties of the luting
cement are still important. We still need a material with a high diametral
tensile and compressive strength in order to resist stresses within the
adhesive layer and tolerate the masticatory forces. Elastic modulus is also
important to
prevent microleakage.(25) It has been suggested that luting
cement with an elastic modulus in the intermediate range between that of tooth
structure and the indirect restorative material is desirable because this can
reduce interfacial stress concentrations without causing excessive strains.(25)
Recommendation
To enhance
the clinical out come of the RBB it is suggested to use structural adhesive cement;
the cement must not only be able to bond well to the metal surface, it must
also be able to resist stresses generated in the cement layer and to resist
propagation of cracks.
Conclusion
The results
of this study showed a direct relationship between fracture toughness and
tensile peel strength. The fracture toughness of the cement is the mechanical
property that could help in material selection i.e. the material with a higher
KIC may predict a higher clinical performance of RBB compared to more brittle
material assuming both are able to produce an adequate and durable bond to the
metal.
Acknowledgements
Many thanks and deep
appreciation to Professor Richard van Noort for his invaluable guidance and
help that he gave throughout the whole study.
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