Does Hybridization of Intraradicular Dentin Really Improve Fiber ...

Does Hybridization of Intraradicular Dentin Really Improve
Fiber Post Retention in Endodontically Treated Teeth?
Chiara Pirani,* Stefano Chersoni,* Federico Foschi,* ,† Gabriela Piana,* Robert J. Loushine, ‡
Franklin R. Tay, § and Carlo Prati*
Abstract
This study tested the hypothesis that hybridization of
intraradicular dentin eliminates interfacial gaps,
thereby improving the coronal seal and retention of
teeth restored with fiber posts. Post spaces were
bonded with two types of fiber posts, using the corresponding
etch-and-rinse adhesives and dual-cured resin
cements. Longitudinal sections of the interfaces were
examined for dentin hybridization in the coronal- and
middle-thirds of the root canals. Resin replicas of these
sections were evaluated for interfacial gap formation.
Although intraradicular dentin hybridization was not
compromised irrespective of whether the adhesives
were light-cured before cementation, the universal occurrence
of interfacial gaps along the hybrid layer surface
or the post-cement interface reflects the challenge
in bonding to post spaces with low compliance and
high C-factors. The clinical success associated with
bonded fiber posts is probably due predominantly to
frictional retention.
Key Words
Hybrid layer, resin tag, adhesion, fiber posts, SEM.
From the *Endodontic Unit, Department of Dental Sciences,
Alma Mater Studiorum, University of Bologna, Via San
Vitale 59, Bologna, Italy; † Department of Cytokine Biology,
The Forsyth Institute, Boston, Massachusetts; ‡ Department of
Endodontics, School of Dentistry, Medical College of Georgia,
Augusta, Georgia; § Pediatric Dentistry and Orthodontics, Faculty
of Dentistry, The University of Hong Kong, Pokfulam, Hong
Kong SAR, China.
Address requests for reprint to Dr. Franklin R. Tay, Prince
Philip Dental Hospital, The University of Hong Kong, Pokfulam,
34 Hospital Road, Hong Kong SAR, China. E-mail address:
kfctay@netvigator.com.
Copyright © 2005 by the American Association of
Endodontists
Basic Research—Technology
Fiber posts are considered as alternatives to cast or prefabricated metal posts in
restoring endodontically treated teeth (1–5). These fiber-reinforced, epoxy resinbased
posts are used with methacrylate-based dentin adhesives and resin cements.
Together, they are being marketed as technologically advanced restorative materials.
Conceptually, hybridization of intraradicular dentin that results from the application of
dentin adhesives should eliminate interfacial gaps that are indicative of a breach of
coronal seal or compromised retention (6–8). Thus, it is reasonable for clinicians who
are using these materials to demand better performances from fiber post-retained
restorations, over those that are luted with nonbonding dental cements (9, 10).
Microtensile bond strength studies on bonding of composites/fiber posts to intact
post spaces (i.e., without presectioning to facilitate laboratory bonding) yielded disappointing
results (11, 12) that were contrary to their acclaimed clinical success (13,
14). A recent push-out test study further highlighted that retention of bonded fiber posts
was contributed predominantly by friction (15). To date, techniques for bonding to
intraradicular dentin are based upon the knowledge acquired from bonding to crown
dentin. However, the bonding characteristics of root-treated dentin may differ in terms
of the optimal moisture content required for application of some etch-and-rinse adhesives
(16, 17), the ability to achieve a reasonable degree of conversion when light-cured
adhesives are polymerized from the top of the post spaces (18, 19), and the potential for
relief of polymerization shrinkage stresses during the setting of resin cements (20, 21).
Depending on the time when bonding is performed, exposure of root-treated dentin to
sodium hypochlorite (NaOCl) (22), eugenol-containing sealers (23), and heat generated
from warm compaction techniques (24) may or may not influence the hybridization
quality of intraradicular dentin. Thus, there is a need to evaluate whether compromised
bonding within post spaces, as indicated by the occurrence of interfacial gaps, is
caused by the ineffectiveness of dentin adhesives to adequately hybridize root-treated
dentin.
Materials and Methods
Endodontic Procedures
Forty extracted single-rooted human incisors were employed in this study. Straight
line access was achieved through the crown of each tooth under an operating microscope
(Karl Kaps, Wetzlar, Frankfurt, Germany), using a tapered diamond bur in a
high-speed handpiece with water cooling. Canal patency was established using a size 10
Flex-o-file (Dentsply Maillefer, Tulsa, OK). Instrumentation was performed with a
crown-down technique, using ProTaper nickel-titanium rotary instruments (Dentsply
Maillefer). All canals were prepared to ISO size 30, 0.09 taper and 1-mm short of the
apex. They were irrigated in between instrumentation with the alternate use of 5% NaOCl
and 17% EDTA for 1 min each before obturation with Thermafil (Dentsply TulsaDental,
Tulsa, OK) according to the manufacturer’s instructions. AH Plus endodontic sealer
(Dentsply DeTrey, Konstanz, Germany) was introduced using a paper point, followed by
a size 30 Thermafil obturator. A heated instrument was used to remove the excess
Thermafil carrier at the level of the cementoenamel junction. Each root-filled tooth was
temporized with a noneugenol provisional dressing (Coltosol F, Colténe AG, Altstätten,
Switzerland) and immersed in water at 37°C.
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Basic Research—Technology
TABLE 1. Comparison of (A) the hybrid layer thickness in root-treated intraradicular dentin in the four experimental groups (measured directly from specimens)
and (B) the maximum interfacial gap width measured along the dentin-cement and post interface of resin replicas obtained from the four experimental groups
A. Hybrid layer thickness
Group
designation
Fiber post/adhesive/resin
cement
Adhesive
polymerization
mode
Bonding of Fiber Posts
The Thermafil was removed using Gates Glidden drills from the
coronal- and middle-thirds of each root, leaving behind at least 5 mm of
intact gutta-percha. Standardized 15-mm long post spaces were prepared
with size #2 post-hole drills supplied by the manufacturers. Preparation
was performed under the operating microscope to ensure that
the post space was free of cutting debris. The specimens were randomly
divided into four groups (n 10) based on the types of post/adhesive/
resin cement employed, and whether the corresponding dentin adhesive
was light-cured before the placement of the fiber post.
In groups 1 and 2, the fiber post Tech 2000 (Isasan, Rome, Italy)
was employed with Single Bond (3M ESPE, St. Paul, MN) adhesive and
RelyX ARC (3M ESPE) resin cement. For both groups, the post spaces
were etched with 35% phosphoric acid for 15 s, rinsed with water for
10 s, and dried with four paper points. The adhesive was applied with a
microbrush to the post space and on the post. In group 1, the adhesive
was not light-cured before resin cement insertion. In group 2, the adhesive
was light-cured for 20 s (Visilux Command II, 3M ESPE) by
placing the light source on top of the post-space. For both groups,
additional light-curing was performed for 60 s after bonding of the fiber
post with the resin cement.
In groups 3 and 4, Endopost (RTD, St. Egève, France) was placed
with All-Bond 2 (Bisco Inc., Schaumburg, IL) adhesive and Duo-Link
(Bisco Inc.) resin cement. The mixed primer was applied in 5 to 6 coats
with a microbrush to the post-space. In group 3, D/E Resin was mixed
with Pre-Bond from the All-Bond 2 kit to render the resin dual-curable.
In group 4, only D/E Resin was used and light-cured for 20 s from the
top of the post-space before applying the resin cement. For both groups,
additional light curing was performed for 60 s after fiber post placement.
Scanning Electron Microscopy
After water storage at 37°C for 1 wk, each specimen was sectioned
longitudinally into two halves using a water-cooled diamond saw
(REMET, Fossombrone, Italy). The sections were polished with 120 to
4000 grit silicon carbide papers under water irrigation, and subsequently
treated with 5% NaOCl to dissolve the collagen fibrils that were
Hybrid layer thickness (m)
at 5–10 mm (middle-third)
from apex (mm)*
1 Tech 2000/Single Bond/RelyX ARC Not-cured 6.5 2.3 A
2 Tech 2000/Single Bond/RelyX ARC Light-cured 6.5 1.3 A
3 EndoPost/All-Bond 2/Duo-Link Auto-cured 4.3 1.2 B
4 EndoPost/All-Bond 2/Duo-Link Light-cured 5.8 2.3 B
Hybrid layer thickness
(m) at 10–15 mm
(coronal-third) from
apex (mm)*
6.3 2.2 A
7.2 2.3 A
5.7 1.6 B
6.4 1.3 B
*For each fiber post/adhesive/resin cement, a two-way ANOVA was employed to examine the influence of adhesive polymerization mode and location of fiber post on hybrid layer thickness. Subgroups with the same
superscripts are not statistically significant (p 0.05).
B. Maximum interfacial gap width
Group
designation
Fiber post/adhesive/resin
cement
Adhesive
polymerization
mode
Gap width (m) at 5–10
mm (middle-third) from
apex (mm)*
1 Tech 2000/Single Bond/RelyX ARC Not-cured 5.5 5.7 A
2 Tech 2000/Single Bond/RelyX ARC Light-cured 8.1 3.9 A
3 EndoPost/All-Bond 2/Duo-Link Auto-cured 5.5 3.7 C
4 EndoPost/All-Bond 2/Duo-Link Light-cured 3.1 1.6 C
Gap width (m) at 10–15
mm (coronal-third) from
apex (mm)*
4.2 5.0 A
1.4 3.1 B
5.6 12.0 C
0.3 0.6 D
*For each fiber post/adhesive/resin cement, data were converted into ranks. A two-way ANOVA was employed to examine the influence of adhesive polymerization mode and location of fiber post on maximum
interfacial gap width, followed by the use of Tukey multiple comparison tests. Subgroups with the same superscripts are not statistically significant (p 0.05).
not enveloped by resin. Impressions of these interfaces were taken using
a low viscosity polyvinyl siloxane (Affinis LightBody; Colténe, Altstätten,
Switzerland). Epoxy resin replicas were fabricated from these impressions
for interfacial gap evaluation.
The specimens were fixed in 4% glutaraldehyde in 0.2 M sodium
cacodylate buffer for 4 h, rinsed in cacodylate buffer, dehydrated in an
ascending ethanol series (40–100%), critical point dried, gold-sputtered
and examined with a scanning electron microscope (JSM-5200,
JEOL, Tokyo, Japan).
Each section was evaluated at the middle (5–10 mm measured
from root apex) and the coronal-third (10–15 mm) of the root for the
quality of hybrid layer and its thickness. For each adhesive, a two-way
ANOVA was performed to evaluate the influence of polymerization mode
(uncured/auto-cured versus light-cured) and location (middle-third
versus coronal-third) on the thickness of the hybrid layer. Statistical
significance was set in advance at 0.05.
The resin replicas were also gold-sputtered and examined along
the same sites for the occurrence of interfacial gaps. For each adhesive,
a two-way ANOVA and Tukey tests were employed to evaluate the influence
of polymerization mode and location on the maximum gap width,
with 0.05.
Results
For Tech 2000/Single Bond/RelyX ARC (groups 1 and 2) and EndoPost/All-Bond
2/Duo-Link (groups 3 and 4), the thickness (Table
1A) of the hybrid layers were independent of the adhesive polymerization
modes and the location of the intraradicular dentin (p 0.05).
Hybrid layers formed by Single Bond (Fig. 1A) and All-Bond 2 (Fig. 1B)
after laboratory NaOCl challenge did not demonstrate features that were
indicative of inferior quality, such as incomplete resin infiltration.
For both systems, gap widths were significantly influenced only by
the location of the intraradicular dentin (Table 1B). However, interfacial
gaps could be universally observed in all sections. An example of a
gap at the cement-dentin interface is depicted in Fig. 2A. When gaps
were absent from the cement-dentin interface, delamination of the fiber
post from the resin cement could frequently be identified (Fig. 2B).
892 Pirani et al. JOE — Volume 31, Number 12, December 2005

Figure 1. Hybridization of intraradicular dentin observed in specimens of post
spaces that were bonded with fiber posts. (A) Representative SEM micrograph
of the hybrid layer that was seen in the middle-third and coronal-third of the root
in group 1 (Single Bond, uncured). As the adhesive was not cured before the
application of the resin cement, a core of resin could be seen inside the resin
tags (open arrowhead). (B) Representative SEM micrograph of the hybrid layer
that was seen in the middle-third and coronal-third of the root in group 3
(All-Bond 2, auto-cured). [C: resin cement; H: hybrid layer; RD: root dentin.]
Discussion
When a light-cured dentin adhesive is employed for bonding to
root canals, the depth of the post space usually exceeds the depth of cure
of most light-curing units (19). A pragmatic but earnest question is
whether light-curing from the top of a post space, which may be 10 to 15
mm away from the most vulnerable site that requires a coronal seal (i.e.,
the severed end of the gutta-percha), is sufficient to optimally polymerize
a light-cured adhesive. It is important to determine whether such a
technique results in a better degree of conversion than what may be
attained via diffusion of free radicals from the adjacent dual-cured resin
cement (25). Although the use of light-transmitting posts may improve
the polymerization of adhesives and resin composites in deep post
spaces (26), such a concept was challenged in a recent study (27).
Clinicians are often confronted with advertisements that portray
excellent micrographs of hybrid layers and profuse resin tag formation
in root canal dentin. Although resin tag penetration was also examined
initially, the authors felt that the method of investigation was not robust
enough to permit a cause-effect correlation, and have refrained from
including such data in this work. Nevertheless, the present study dem-
Basic Research—Technology
Figure 2. Interfacial gaps that were identified from resin replicas of the specimen
sections. (A) Representative SEM micrograph of a gap (arrow) along the
surface of the hybrid layer (pointer) that was seen in group 4 (All-Bond 2,
light-cured). (B) Representative SEM micrograph illustrating the delamination
(arrow) that occurred between the fiber post surface and the resin cement that
was identified from group 2 (Single Bond, light-cured). The interface between
the hybrid layer (H) and resin cement remained intact in this specimen. [C:
resin cement; RD: root dentin.]
onstrated that hybridization of intraradicular dentin, even in the absence
of deterioration after laboratory challenge, is merely a phenomenological
manifestation of resin infiltration following smear layer
dissolution and partial demineralization of the root dentin (28). As
such, it has no bearing on the integrity of the coronal seal or resistance
to dislocation of fiber post-retained restorations.
This work was performed in the absence of eugenol-containing
endodontic sealers. A recent study also demonstrated that heat generated
from warm compaction techniques did not result in disappearance
of collagen cross-banding in intraradicular dentin (29). However, as
the fiber posts were bonded 24 h after endodontic treatment, the residual
oxidizing effect of NaOCl on resin polymerization could not be neglected.
Even under the circumstances when this residual oxidizing
effect is managed with reducing agents (30), the most relevant issue
related to the occurrence of interfacial gaps is the problem of low
compliance that one encounters in endodontics.
In pulpal inflammation, the minimal capacity for pulpal tissues to
expand accounts for the augmentation in pulpal hydrostatic pressure
associated with vasodilatation and increase in vascular permeability
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Basic Research—Technology
(31). Likewise, in bonded root canals, the low compliance of the hard
tissue cage renders it impossible to contract to accommodate resin
polymerization shrinkage (32). It is not possible to rely on layering
techniques to reduce polymerization shrinkage in post spaces. Relief of
the shrinkage stresses thus generated is further hampered by the high
cavity-configuration factors encountered in these long narrow channels,
the nonfitting nature of circular prefabricated posts in irregular
post spaces such as those with canal fins, and the light-curing that is
initiated from the top of the post space (21, 33). As a result, interfacial
gaps appeared either along the cement-dentin interface or the cementpost
interface. It has also been assumed that epoxy resin-based posts
bond to methacrylate-based adhesive resins. However, as the two materials
differ completely in resin chemistry, adhesion must rely on the
silanization of the glass fibers within the post, if indeed glass-fiber posts
are utilized.
This work makes no attempt to deter clinicians from using fiber
posts, or to challenge the excellent clinical track record of these systems.
Rather, the concurrent identification of subclinical interfacial
gaps and hybridization of intraradicular dentin should alert clinicians
that the clinical success associated with fiber posts may have little to do
with the actual benefits derived from dentin bonding, and may simply be
the result of resistance to dislocation contributed by frictional retention
(16). Thus, it would be desirable to examine ways to improve the adhesion
of fiber posts to root canals, such as relining them with resin
composites to minimize the cement space (32–34), and etching of fiber
posts with hydrogen peroxide to increase micromechanical retention
(35). While the former technique has been brought to fruition with the
commercialization of the “anatomic fiber post system” (RTD, St. Egève,
France), we are currently investigating the combination of the two techniques
to enhance the adhesion of fiber posts in clinically relevant,
intact post spaces.
Acknowledgments
This study was based on a dissertation to be submitted by Dr.
Chiara Pirani in partial fulfillment of the requirements of the degree
of Doctor of Philosophy in the University of Bologna, Italy. The
fiber posts examined in this study were generously sponsored by
Isasan, Italy and RTD, France. The work was supported by 60%
UNIBO 2002 and 2003, Italy, and by RGC CERG grant 10204604/
07840/08004/324/01, Faculty of Dentistry, University of Hong Kong.
The authors are grateful to Anna Prati and Anna Tay for secretarial
support.
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