Keywords bioactive - casein phosphopeptide–amorphous calcium phosphate - fracture resistance
- reattachment technique
Introduction
Crown fractures of the permanent dentition are prevalent among school children (26–76%)
as a part of traumatic injuries.[1 ] Generally, the anterior teeth are the most affected ones (80% central incisors and
16% lateral incisors) due to the position of the maxilla and protrusion of teeth.[2 ] Restoration of the fractured teeth through reattachment of the intact tooth is the
treatment of choice due to its total esthetical recovery, color stability over time,
and wear at similar rate as other teeth. Besides being safe and simple, this conservative
procedure requires less chair-time, which might reduce the treatment cost.[3 ]
[4 ]
[5 ]
[6 ]
[7 ]
[8 ] Yet, this method has several inherent drawbacks, the most important of which is
the lower fracture resistance of the reattached tooth. It is reported that the mechanical
strength of reattached tooth is 20 to 60% lower, depending on the bonding agent and
restorative technique.[8 ]
[9 ]
[10 ]
[11 ]
Based on a couple of clinical follow-ups, additional tooth preparations may not provide
clinical success.[9 ]
[10 ]
[12 ]
[13 ]
[14 ] Meanwhile, some others believe that simple reattachment by using adhesive systems
without additional preparation does not recover the original mechanical strength.[6 ]
[15 ] Moreover, the reattached fragment is highly susceptible to detachment in case of
a new trauma or excessive masticatory forces.[1 ]
The fragment retention may be affected by the restorative materials and the adhesive
system used. The 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) monomer present
in self-etch adhesives is capable of forming strong ionic bonds with the calcium in
hydroxyapatite crystals, remaining around the collagen fiber due to the partial demineralization
effect of self-etch adhesive system.[16 ] According to Yoshida et al’s study, the interaction of 10-MDP monomer and calcium
generates a stable monomer–Ca salt, which is capable of producing strong bond to dentin.[11 ] On the other hand, casein phosphopeptide–amorphous calcium phosphate (CPP-ACP) promotes
the formation of hydroxyapatite crystals around the exposed collagen fibers. The (CPP-ACP)
mechanism of action is to provide a reservoir of free calcium and phosphate ions to
maintain a supersaturation mineralization state on the enamel surface.[17 ] Therefore, applying CPP-ACP adds higher amount of these ions on the tooth surfaces,
and consequently increases the bond strength.[18 ]
Activa BioActive-Restorative is a flowable resin-based composite, which incorporates
components of glass ionomer and resin composite. Fluoroaluminum silicate particles
and polyacid components undergo the acid-base setting reaction.[19 ] The smart material of Activa has the potential of releasing and recharging significant
amounts of calcium, phosphate, and fluoride from the saliva or in the presence of
a source like CPP-ACP paste, and therefore stimulates the apatite formation. This
dynamic role acts against the marginal microleakage and discoloration of the restoration
margin and improves the mechanical properties.[20 ]
[21 ] According to the manufacturer, Activa contains rubberized resin, which is tougher
with shock-absorbing property and is claimed to be more fracture resistant than the
conventional restorative materials.[22 ]
So far, studies on reattachment technique have merely focused on the conditions of
the storage medium of the fragment and preparation of the tooth.[8 ]
[23 ] To the authors’ knowledge, no study has ever investigated the effect of pretreatment
of the fractured teeth fragments with calcium-containing materials along with application
of bioactive composite materials. The present study was conducted to evaluate the
effect of CPP-ACP and self-adhesive bioactive flowable composite on the fracture resistance
of fractured teeth restored through reattachment technique. The null hypothesis is
that: (1) reattachment of fractured incisors with bioactive composite would not yield
higher fracture strength compared with the conventional composite, and (2) CPP-ACP
treatment prior to reattachment would not increase fracture strength.
Materials and Methods
This experimental in vitro study was done on 60 extracted bovine central mandibular incisors (average age 2.5–3
years), free of any crack, defect, or caries. The teeth were first disinfected in
0.5% chloramine-T for 1 week, and then stored in saline till used. The teeth were
marked on the labial surface 4 mm apical to the incisal edge. Each tooth was cut perpendicular
to its long axis by using a diamond disk ([Fig. 1 ]). Then, they were randomly divided into six groups (Groups 1–6; n = 10) according to the reattachment technique.
Fig. 1 Preparation of the specimens.
In Group 1 (conventional composite), the enamel of the tooth and the fractured segment
was selectively etched for 15 seconds with phosphoric acid (Etch-Rite; Pulpdent, Watertown,
Massachusetts, United States), rinsed with water for 30 seconds, and gently air-dried
for 15 seconds. Then, the primer of Clearfil SE Bond (Kuraray; Okayama, Japan) was
applied on the enamel and dentin with slight agitation (20 seconds) and air-dried
for 10 seconds. The bonding agent of Clearfil SE Bond (Kuraray) was applied (15 seconds),
air-thinned (10 seconds), and light-cured (20 seconds). For reattachment of the two
segments, a thin layer of conventional flowable composite (Pulpdent; Watertown, Massachusetts,
United States) was applied on both parts. The fractured segment was adapted on the
tooth part and light-cured for 20 seconds from the buccal and lingual aspects. The
excess composite on tooth surfaces was removed by using a scalpel.
In Group 2 (bioactive composite with bonding), the specimens were prepared just like
Group 1, except for the conventional flowable composite, which was replaced with bioactive
flowable composite (Activa BioActive; Pulpdent, Watertown, Massachusetts, United States).
The reattachment technique in Group 3 (bioactive composite without bonding) was like
Group 2, but Activa was used as self-adhesive composite without etching and bonding.
Reattachment in Groups 4 (CPP-ACP + conventional composite), 5 (CPP-ACP + bioactive
composite with bonding), and 6 (CPP-ACP + bioactive composite without bonding) was
similar to Groups 1, 2, and 3, respectively; they were only different in CPP-ACP pretreatment;
that is, CPP-ACP (Recaldent, GC, Japan) was actively rubbed on both tooth fragments
by using a brush for 3 minutes prior to adhesion. [Table 1 ] summarizes the application of materials for groups of the study.
Table 1
Summary of the materials used for the groups of the study
Groups
Adhesive
Composite
Pretreatment
Abbreviation: CPP-ACP, casein phosphopeptide–amorphous calcium phosphate.
Group 1
Clearfil SE Bond (Kuraray, Okayama, Japan)
Conventional flowable composite (Pulpdent, Watertown, Massachusetts, United States)
None
Group 2
Clearfil SE Bond (Kuraray, Okayama, Japan)
Bioactive flowable composite (Activa BioActive, Pulpdent, Watertown, Massachusetts,
United States)
None
Group 3
None
Bioactive flowable composite (Activa BioActive, Pulpdent, Watertown, Massachusetts,
United States)
None
Group 4
Clearfil SE Bond (Kuraray, Okayama, Japan)
Conventional flowable composite (Pulpdent, Watertown, Massachusetts, United States)
CPP-ACP (Recaldent, GC, Japan)
Group 5
Clearfil SE Bond (Kuraray, Okayama, Japan)
Bioactive flowable composite (Activa BioActive, Pulpdent, Watertown, Massachusetts,
United States)
CPP-ACP (Recaldent, GC, Japan)
Group 6
None
Bioactive flowable composite (Activa BioActive, Pulpdent, Watertown, Massachusetts,
United States)
CPP-ACP (Recaldent, GC, Japan)
The specimens were embedded in acrylic blocks up to 1 mm below the fracture line,
and the long axis of the tooth was parallel to the central axis of the block. The
specimens were loaded on a universal testing machine (Zwick/Roell Z020; Zwick GmbH
& Co., ulm-Einsingen, Germany). The force was applied with a chisel-shaped tip, which
was perpendicularly positioned on the fracture line on the facial surface of the crown,
with the speed of 1 mm/min until fracture occurred ([Fig. 2 ]). The force leading to fracture was measured in Newton. The area of the fracture
surface was measured through planimetry to convert the Newton to Megapascal (MPa =
F/mm2 ).[23 ]
Fig. 2 Reattachment of fragment and specimens loaded on the universal testing machine.
The failure modes were assessed by using a ×10 magnifier and classified as:
Type I: adhesive failure in the tooth composite interface
Type II: cohesive failure in the composite
Type III: mixed adhesive and cohesive failure in the composite
Type IV: mixed adhesive and cohesive failure in the tooth
Data were statistically analyzed using SPSS—Statistical Package for the Social Sciences—software
(version 22, SPSS Inc.; Illinois, Massachusetts, United States). One-way analysis
of variance (ANOVA) was used to find the effect of each variable, and Tukey post-hoc
test was used for pairwise comparison of the study groups. The confidence level was
95%, so p < 0.05 was considered statistically significant.
Results
[Table 2 ] displays the results of the fracture test. The results of one-way ANOVA revealed
significant differences among the study groups. The highest fracture resistance was
recorded for Group 5 (CPP-ACP + bioactive composite with bonding: 15.96 MPa), which
was significantly higher than the other groups. The lowest fracture resistance was
observed in Group 6 (CPP-ACP + bioactive composite without bonding: 1.95 MPa). The
results of Tukey post-hoc test showed that Groups 3 (bioactive composite without bonding)
and 6 (CPP-ACP + bioactive composite without bonding) had significantly lower fracture
strengths than the other groups (p < 0.05); however, these two groups were not significantly different in this regard
(p = 0.206). No significant difference existed between Groups 1 (conventional composite),
2 (bioactive composite with bonding), and 4 (CPP-ACP + conventional composite). The
prevalence of the failure modes is shown in [Table 2 ]. The most commonly occurring failure mode was adhesive failure.
Table 2
The mean fracture strength (MPa) and failure modes of the study groups
Groups
Mean (SD) fracture strength
Failure mode
I
II
III
IV
Abbreviations: CPP-ACP, casein phosphopeptide–amorphous calcium phosphate; SD, standard
deviation.
a Same lowercase letters indicate lack of statistical difference (Tukey post-hoc test).
b Failure modes: I, adhesive failure in the tooth composite interface; II, cohesive
failure in the composite; III, mixed adhesive and cohesive failure in the composite;
IV, mixed adhesive and cohesive failure in the tooth.
Conventional composite
11.86 (3.03)a
4
2
4
0
Bioactive composite with bond
12.14 (1.46)a
4
0
5
1
Bioactive composite without bond
4.54 (2.79)b
8
0
2
0
CPP-ACP + Conventional composite
12.29 (1.36)a
1
2
6
1
CPP-ACP + Bioactive composite with bond
15.96 (3.55)c
3
0
5
3
CPP-ACP + Bioactive composite without bond
1.95 (0.49)b
10
0
0
0
Discussion
Successful reattachment of fractured tooth depends on several factors like the storage
media, preparation mode, adhesive system, and the materials used for reattachment.[8 ]
[12 ]
[15 ]
[23 ] The present study evaluated the fracture strength of fractured incisors restored
through fragment reattachment technique with conventional and bioactive flowable composites.
The bioactive agent of Activa has dynamic interaction with the tooth structure, and
mimics the natural teeth physically and chemically. Therefore, it was hypothesized
that this material can establish the integrity and continuity between the two fragments.
The current findings showed that the fracture resistance of the teeth reattached with
Activa was comparable to the conventional flowable composite. Thus, the first hypothesis
was accepted. However, this material is smart and moisture-friendly with dynamic role
in the mouth, so it can substitute the older conventional composites without bioactive
potential.
Activa is a hybrid material with biologic properties similar to glass ionomer, so
it can be used without bonding agent.[24 ] Application of the bonding agent is recommended in nonretentive preparations. The
present study compared the use of Activa with or without bonding to find if selective
etching and self-etching bonding agent could improve the bonding of Activa to dentin
and enamel. For specimens without bonding agent, the fracture resistance decreased
significantly. Therefore, using adhesive system with Activa is suggested, which is
in accordance with the results of Poitevin et al.[25 ] Another study also recommended the use of bonding agent in conjunction with self-adhesive
composite.[26 ]
If the exposed dentin is close to the pulp chamber, like what is seen in most cases
of crown fracture, self-etch adhesive is more appropriate.[27 ] Bonding of the self-etch system involves two adhesion mechanisms: micromechanical
interlocking and chemical bond. As formerly mentioned, the 10-MDP functional monomer
is capable of forming strong ionic bonds with the calcium of hydroxyapatite crystals
remaining around the collagen fibers.[11 ] Since mild self-etch adhesive system is not efficient for enamel etching, some researchers
suggest selective enamel-etching technique.
In the present study, selective enamel etching was done before bonding agent application.
Clearfil SE Bond was selected as bonding agent because of the 10-MDP content and its
compatibility with dual-curing composite.[27 ]
Controversies exist among the results of the investigation about using CPP-ACP on
the tooth surface before adhesive application. Some studies found that pretreating
the enamel with CPP-ACP before etching increases the surface roughness, provides a
greater surface area for the adhesive bonding, and improves the interaction of enamel
surface with the adhesive system.[28 ]
[29 ]
[30 ] Applying the CPP-ACP also increases the wettability of the dentin surfaces, thereby
creating a high-energy surface and low contact angles, which are usually in favor
of mechanical lock and adhesion.[30 ]
The adhesives that contain phosphoric acid esters have shown better bond strength
on the CPP-ACP-treated substrates. The reason is the increased availability of calcium
ions, which can chemically bond with the phosphoric acid ester monomers.[11 ]
[31 ]
[32 ] The hydroxyl groups in these monomers can chelate with the calcium ions.[18 ] The present study detected that CPP-ACP application prior to Clearfil SE Bond significantly
increased the fracture strength of bioactive composite, but the increase was not statistically
significant for the conventional composite. Therefore, the second hypothesis was accepted
only for the conventional composite. Similar results were achieved by Araújo et al,[33 ] who reported that application of the CPP-ACP paste for 10-MDP-based adhesive system
did not increase the bond strength for conventional composite.
In contrary, Adebayo et al[34 ] observed that upon removing the smear layer, the CPP-ACP-containing materials increased
the bond strength of self-etch adhesive system.
Jalannavar and Tavargeri[32 ] also found that the bond strength increased when the broken fragment was kept in
CPP-ACP paste (12–24 hours). These contradictory results can be attributed to the
different pretreatment times (3 minutes in the present study as recommended by the
manufacturer), variety of bonding agents, and presence of the smear layer. In this
regard, Yang et al[35 ] believed that application time of 3 minutes was not long enough to induce sufficient
remineralization with CPP-ACP by forming the substances that occlude the dentinal
tubules. Therefore, they suggested longer pretreatment time.
In this study, when Activa was used without bonding in self-adhesive mode, the fracture
strength was not influenced by CPP-ACP (p < 0.05). According to a study by Shafiei et al,[36 ] pretreatment of the dentin with CPP-ACP positively affected the bonding ability
of self-adhering materials (resin-modified glass ionomer and self-adhering flowable
composite resin). In their study, use of ethylenediaminetetraacetic acid conditioning
before the CPP-ACP facilitated deeper penetration of the calcium and phosphate ions
into the dentin, without the interfering penetration of acidic monomers of the self-adhesive
composite.[36 ] Seemingly, the presence of smear layer interferes with the beneficial effect of
CPP-ACP on the adhesion capabilities of the self-adhesive composite.
Regarding the failure mode, adhesive failure was the most prevalent failure pattern
in the groups without bonding, which correlates with the lower bond strength. The
fewer adhesive failures in the pretreated groups might be attributed to the stronger
interaction of the tooth substrate and the adhesive.
This study was designed to be done on intact teeth of approximately equal size and
age. To overcome this limitation bovine teeth were used, which are proper substitutes
for human teeth.[37 ]
[38 ] Breaking and cutting methods can be used to simulate tooth fracture. In breaking
method, adaptation between the remaining fragment and the broken fragment increases
the fracture resistance values.[37 ] In this study, diamond disc was used to cut the fragments. Thus, the evaluations
were exclusively focused on the pure effect of employed reattachment materials without
the intervention of adaptation. Another limitation of the study was regarding the
mechanical test, which was desirable to be dynamic. Due to lack of access to computerized
systems for calculation of surface area, fracture resistance test was done.
The current study was a preliminary study to compare the effect of bioactive and conventional
composites on the fracture strength of reattached teeth. However, an ongoing study
is being performed to investigate the effect of ion release and the recharging capacity
of Activa on improvement and preservation of the bond strength with load cycling and
interval application of CPP-ACP on reattached specimens.
Conclusion
With respect to the present findings, it was concluded that bioactive composite was
not superior to the conventional composite for fragment reattachment. However, pretreatment
with CPP-ACP improved the resistance to fracture of bioactive composite in case of
applying bonding agent, but not the conventional composite. Using bioactive composite
in self-adhesive mode without bonding agent led to decreased fracture resistance.
