Barry Musikant
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 ALLACIOUS
CONCEPTS encouraging the use of fiber-reinforced posts are not supported
by a growing number of research articles and clinical experience.
Unless the weaknesses of these concepts are well understood, they have
the potential to propagate as valid approaches. The purpose of this article
is to shed light on these misconceptions.
Posts in teeth serve only one purpose: to supply
extra support for a core when sufficient tooth structure does not exist
to do it alone. Posts placed into teeth with sufficient dentin to support
a core serve no purpose and are, at best, redundant. In fact, removing
dentin in order to place a post may actually weaken the root. The placement
of a post may give support to the core, but it does not strengthen the
root.
Having the same modulus of elasticity as a root
in no way assures that the post will bend to the same degree as the tooth
in which it is embedded. Materials having the same modulus of elasticity
will bend the same only if they have the same cross-sectional area. A
post with the same modulus of elasticity as tooth, yet 1/15 the diameter
of the tooth in which it is placed, will bend about 15 times more than
the surrounding root, creating stresses in the supporting cement, the surrounding
core buildup, and the post itself. In short, a post with greater flexibility
than the tooth compromises the longevity of the overlying crown. Fiber
posts are significantly more flexible than the roots in which they are
embedded (1-6).
The concept that the core and post join together
and create a monobloc stronger than either two of these components is a
false notion. This is easily realized by making a post-and-core combination
in which the post is no thicker than the diameter of a thread of hair.
In this case, the post bends in the air and offers no support to the core.
If such a post were placed in a root without the support of circumferential
dentin, there would be virtually no resistance to lateral forces.
In this absurd example, it becomes clear that the resistance to lateral
movement is defined by the weakest element in the construct, namely the
hair-thin post. As the posts become stronger, the resistance to lateral
displacement increases. This resistance is always limited by the flexibility
of the post, which is not enhanced by bonding to a stronger core material.
In order for a post to bend like tooth in spite
of its thinner cross-sectional area, the modulus of elasticity must be
much higher than that of the surrounding root. In fact, because the
post is about 1/15 the diameter of the root, the modulus of elasticity
should be about 15 times higher than the tooth’s. Stainless
steel and titanium fall into this category and will therefore bend much
more similarly to the bending of the tooth in which they are embedded.
Another false concept implies that bonding will
increase the retention of a post beyond the cohesive strength of the cement
holding the tooth in place. SEMs are often shown with fibrils of cement
infiltrating the dentinal tubules by the millions as proof of the greatly
increased retention. While this type of adhesion increases retention
more than that of a non-adhesive cement, these millions of penetrating
fibrils provide no additional strength to the bond beyond the strength
of the cement. To date, this strength has never exceeded 90 pounds of
tensile resistance (7-8).
Research has demonstrated not only that retention
is limited to the cohesive strength of the cement, but also that when subjected
to thermal cycling fiber-reinforced composite posts degrade over time significantly
more than metal posts do (9).
Many studies conclude that metal posts offer more
support for restoration than fiber-reinforced composite posts do.
Metal posts are more resistant to bending and are far more resistant to
thermal cycling. Those who support fiber-reinforced metal posts have attempted
to turn a weakness into a strength by saying that fiber-reinforced posts
are less likely to cause root fracture if subjected to excessive forces.
The research has again clearly demonstrated that while this is true, it
takes forces beyond human capacity to produce these fractures when metal
posts are placed (10). On the other hand, the forces necessary to
displace cores supported by fiber-reinforced composite posts fall clearly
within the capabilities of human function. The best rationale for the
use of fiber-reinforced posts would be to place them in order to avoid
the increased chances of fracture when a tooth is subjected to a traumatic
blow. This would, however, leave the restored tooth open to gap formation
from normal function, an everyday occurrence.
Once the strong case for the preference of metal
posts over fiber-reinforced composites is established, an equally strong
case can be made for the design of a split-shank parallel-thread post.
(See Figure 1.)
The split-shank design is the only threaded post
design that produces the degree of retention that only threads embedded
in dentin can provide while simultaneously minimizing the insertional stresses
to those of a passive post (11). In effect, the split-shank design is
a graduated tap that deepens the threads in a sequential fashion, never
cutting more than .02 mm of dentin at any one time (Figure 2).
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| FIGURE 2: The split-shank
design is a graduated tap that deepens the threads in a sequential fashion,
never cutting more than .02 mm of dentin at any one time. |
By limiting the removal of dentin, the stresses associated
with that removal are also limited. The result is a post with retention
of about 340 pounds, about four times higher than that of the most retentive
passive post, but with stress levels no higher than that of a passive post.
The retention of a natural crown to that of a natural
root is at least 220 pounds, a result extrapolated from the research of
Shimon Friedman. He demonstrated that it took at least 220 pounds to split
a tooth in half along its long axis. Two hundred twenty pounds represents
the weakest vector of strength for a tooth. As such, it is reasonable
to expect that a natural tooth would have at least that much retention
to a natural crown. The 90 pounds that the best passive post provides
is inadequate to duplicate nature’s design. The split-shank design,
on the other hand, is far more comparable to nature’s design. Not only
does it supply 340 pounds of retention, it does so without generating high
insertional stresses. Just as important, a parallel threaded post also
distributes its functional stresses evenly around each of the threads.
A passive parallel post distributes a good portion of the functional stresses
in the apical region because the non-threaded parallel design offers no
other area of resistance to these forces. Like all stresses, they are
handled better when distributed evenly over a large area than they are
when concentrated into a small area, as is the case with the passive parallel
post design.
From a restorative point of view, it is only the
lack of coronal tooth structure that defines the need for a post. Therefore,
once a post is required, external support by the crown is also required.
The best way for the crown to supply this support is through the incorporation
of a circumferential ferrule ending on the dentinal surfaces. The longer
this ferrule, the greater the support the crown offers. To place
a butt joint restoration on a tooth where most, if not all, of the axial
wall is composed of a post supported by a composite core is to dramatically
increase the functional stresses that will be directed against the axial
wall. Knowing that this post-and-core buildup is subject to degradation,
it is important to create a ferrule onto solid tooth structure that redirects
most of these functional forces away from the axial wall and toward the
external root surface.
Summary
Establishing the greatest stability and longevity for restorations requires
building a highly retentive and stable substructure. In turn, this requires
the placement of a parallel threaded metal post. The split-shank
design provides high retention with minimal stress, as well as even distribution
of functional stresses. The crown should incorporate a ferrule and
end on a long beveled dentin margin for maximum support.
References
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King PA, Setchell DJ, Rees JS. Clinical evaluation of a carbon fibre reinforced
carbon endodontic post J Oral Rehabil. 2003 Aug; 30(8):785-9.
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Drummond JL, Bapna MS. Static and cyclic loading of fiber-reinforced dental
resin. Dent. Mater. 2003 May;19(3): 226-31.
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Drummond JL In vitro evaluation of endodontic posts. Am J. Dent. 2000 May;13
(Spec No): 5B-8B.
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Sidoli GE, King PA, Setchell DJ. An in vitro evaluation of a carbon-based
post and core system. J Prosthet Dent. 1997 Jul;78(1):5-9.
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Torbjorner A, Karlsson S, Syverud M, Hentsen-Pettersen A. Carbon fiber
reinforced root canal posts. Mechanical and Cytotoxic properties. Eur
J Oral Sci. 1996 Oct-Dec;104(5-6):605-11.
-
Yang HS, Lang LA, Guckes AD, Felton DA. The effect of thermal change on
various dowel-and-core restorative materials. J Prosthet Dent. 2001 Jul;86(1):74-80.
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Saunders, RD, Lorey RE, Powers JM, Sloan KM. A comparison of five post-cement
systems for tensile retentive capacity. J Den Res 1988;67: IADR Abstract
304.
-
Stockton LW, Williams PT. Retention and shear bond strength of two post
systems. Oper Dent 1999;24:210-216.
-
Yang HS, Lang LA, Guckes AD, Felton DA. The effect of thermal change on
various dowel-and-core restorative materials. J Prosthet Dent 2001;86:74-80.
-
Wong EJ, Ruse ND, Greenfeld RS, Coil JM. Initial failure of post/core systems
under compressive-shear loads. J De Res (IADR abstract #2269) 1999;78:389.
-
Ross RS, Nicholls JI, Harrington GW. A comparison of strains generated
during placement of five endodontic posts. J Endodon 1991;17:450-456.
July-September 2005
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FIGURE 1: Split-shank parallel-thread
posts.
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