Prepared by

Esraa Sayed Ahmed

Demonstrator of Bio dental material
Faculty of dentistry, Mina University

Under supervision of
Prof/Yasser Fathi Hussien
Head of Biomaterials Department
Dean of faculty of dentistry, Minia University
Dr. / Asmaa Abdel-Hakeem Metwally
Lecturer in Biomaterials Department

Minia University
2019
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Introduction
Traditional belief was that endodontically treated teeth were weaker or
more brittle than vital teeth. Their moisture content was reduced and clinical
fracture occurred. It was assumed that, for this reason the tooth had to be
strengthened by removing part of the root canal filling and replacing it with a post.
Studies concluded that neither dehydration nor endodontic treatment caused
degradation of physical or mechanical properties of dentin. These studies indicate
that the restorations that enhance structural integrity would be expected to increase
the prognosis of endodontically treated teeth exposed to heavy masticatory loading
forces. The endodontically treated tooth must be strengthen in such a way that it
will withstand both vertical and lateral forces and not be liable to fracture. Vertical
support must be added to the restorations so that they may be strong enough to
protect the treated tooth from horizontal fracture. (1)
Studies suggests that many endodontically treated teeth are not reinforced
with the use of a post.Further, some studies have indicated that minimally damaged
endodontically treated teeth without posts are more resistant to fracture than teeth
restored with posts and cores. Resistance to fracture is directly related to the
thickness of remaining dentin, especially in the buccolingual direction.Therefore
excessive flaring during endodontic treatment or over preparation of the canal
space for a post can increase the risk of failure.(3) So accurate diagnosis and
indication, successful selection of post and core type and accurate placement into
the root canal of the abutment tooth prolong the lifetime of the fixed prosthetic
work on the endodontically treated tooth. The post and core must ensure good and
lasting retention of the crown or bridge and provide for adequate stress transfer on
the entire root and the surrounding supporting tissue.(2)

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Biomechanical behavior of endodontically treated teeth (1)
-Differences between healthy and root canal treated teeth
There is substantial literature stating that endodontically-treated teeth differ from
teeth with a viable pulp. Three main aspects can be analyzed regarding these
differences after root canal therapy: 1) changes in the physical properties and in the
chemical composition, 2) changes in the morphology and in the biomechanical
behavior of teeth under stress, 3) possible elevation of pain threshold and loss of
presso-receptors.
-EFFECTS OF ENDODONTICS ON THE TOOTH
Endodontic manipulation further removes important intracoronal and intraradicular
dentin, changes the actual composition of the remaining tooth structure, increases
fracture susceptibility and decreased translucency of the nonvital tooth. pulpless
teeth have 9% less moisture than vital teeth. With aging, greater amounts of
peritubular dentin are deposited, which decreases the amount of organic materials
that may contain moisture indicated that dentin from endodontically treated teeth
shows significantly lower shear strength and toughness than dentin from vital
teeth. The degree of work required to fracture dentin may be less in endodontically
treated teeth because the collagen intermolecular cross-links may be weaker.
The major changes in the endodontically treated tooth are:
A. Loss of tooth structure
endodontic procedures reduce tooth stiffness by only 5% whereas mesio-
occlusual-distal preparation reduces stiffness by 60%. Endodontic access into the
pulp chamber destroys the tooth’s structural integrity, and allows greater flexing of
the tooth under function. In cases with significantly reduced remaining tooth
structure, normal functional forces may fracture undermined cusps or fracture the
tooth in the area of the smallest circumference, frequently at the CEJ.

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B. Altered physical characteristics
endodontically treated teeth exhibit changes in collagen crosslinking and
dehydration of the dentin that result in a 14% reduction in strength, toughness and
structural integrity. Maxillary teeth are stronger than mandibular teeth, and
mandibular incisors are the weakest. the hardness of the cervical dentin was 3.5%
lower in endodontically treated teeth; which indicate that teeth do not become
more brittle following endodontic therapy; but their failure is due to high stress
concentration at the narrow cervical diameter.

Effect of Biomechanical factors of post systems

The biomechanical factors are particularly important for root canal
preparation of post and cores. The form and length of the post are the most
important factors affecting the retention and tooth stress distribution.
Unfortunately, the morphology of the post and core surface with reinforced
retention causes more intensive strains in the root during the post placement and in
its later functioning. This particularly relates to the prefabricated posts with sharp
threads, which has been proven by photoelastometric analysis. Depending on the
direction of masticatory forces, the retention weakens as a consequence of shear
stress on the post and core, cement or dentine surface. (4)

Factors to be considered while planning posts: (1)

1. Retention and resistance form.
2. Mode of failure.
3. Preservation of tooth structure.
4. Ferrule effect.
5. Retrievability.

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IMPORTANT PRINCIPLES FOR POSTS
Conservation of Tooth Structure
Coronal and radicular tooth structure should be conserved whenever
possible In creating post space, only minimum tooth structure from the canal is
removed. Further enlargement only weakens the root. Adequate apical seal,
minimal canal enlargement, adequate post length, positive horizontal stop (to
minimize wedging), vertical walls to prevent rotation and extension of the final
restoration margin onto sound tooth structure are six important features for
successful design.(5) Bonded posts are reported to strengthen the root initially but
this strengthening effect is lost over time as the tooth is exposed to functional
stresses and the resin bond to dentin weakens. Minimal enlargement of a post
space means the post must be made of a strong material that can withstand
(1)
functional and parafunctional forces.

Mode of failure (1)

All post systems have some percentage of failure. Some posts have a higher
percentage of failure that result in teeth that are non restorable. Teeth restored with
less rigid posts (fiber posts) tend to have failures that are more likely to be
restorable. Teeth prepared with a ferrule also tend to fail in a more favorable mode.
Composite resin cores tend to fail more favorably than amalgam or gold.

The ferrule effect

Ferrule is defined as a vertical band of tooth structure at the gingival aspect
of crown preparation. It primarily provides resistance form and enhances
longevity.Tthe ferrule prevents independent flexure of the different components of
these supra-ferrule-margin structures in response to occlusal forces, which would
normally occur if the structures were not ferruled and the ferrule has the effect of

5
transferring the occlusal forces from these supra-ferrule-margin structures to the
margin of the ferrule. (7)

A ferrule with (1.5 -2) mm of vertical tooth structure doubles the resistance
to fracture versus teeth restored with out a ferrule. It was reported that there was no
difference in fracture resistance with or with out a 2 mm ferrule using prefabricated
posts and resin cement. But fracture patterns were more favorable when a ferrule
was present. In some cases, especially in anteriors, it is necessary to perform crown
(1)
lengthening/orthodontic eruption of a tooth to provide an adequate ferrule.

Retrievability
Endodontic treatment can fail. Therefore, it is important that posts can be
retrieved if re-treatment becomes necessary. Unfortunately, the retrievability of a
metal post, especially the cast post and core system is difficult and involves
removal of tooth structure around the post, which could further weaken the tooth.
fiber posts is easy to retrieve. In contrast ceramic and zirconium posts are
(1, 6)
considered to be very difficult and sometimes impossible to retrieve.

Retention and resistance form (1, 5)

Post retention refers to the ability of a post to resist vertical dislodging
forces. Retention is influenced by post length, diameter, taper, luting cement used
and whether a post is active or passive. Increasing the length and diameter of the
post can increase retention of the post. Parallel posts are more retentive than
tapered posts. Active posts are more retentive than passive posts. Diameter is the
least important of all the factors.
Resistance refers to the ability of the post and tooth to withstand lateral and
rotational forces. Factors influencing resistance form are post length, rigidity,

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presence of anti-rotational features and the presence of a ferrule. A restoration
lacking in resistance form is not likely to be a long-term success regardless of the
retentiveness of the post.

Stress Distribution
-Should distribute stresses over larger areas as possible.
- Increasing post length decreases stresses.
- Parallel sided post distributes force more than tapered post.
- Avoid sharp line or point angles.
- Excess cement increases stresses.
-Rotational Resistance
- Preparation of vertical coronal wall similar to a box.
- A small groove is placed in the canal wall in the bulkiest area cervically with 2-
3mm length.
- Placing an auxiliary pin in the root face.
Ferrule
It is the extension of the crown margin into sound tooth structure helps binding the
remaining tooth structure together which prevent root fracture during function.

Multiple Factors Which Influence Post/Dowel resistance and retention

form:(6)
-Amount of coronal tooth structure
-Tooth anatomy
-Position of the tooth in the arch
-Root length
-Root width
-Canal configuration

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-Functional requirements of the tooth
-Torquing force
-Stresses
-Development of hydrostatic pressure
-Post design
-Post material
-Post length
-Post diameter
-Material compatibility
-Bonding capability and luting agents
-Core retention
-Retrievability -Crown material
1-Root Length & post length
The length and shape of the remaining root determines the length of the post .
It has been demonstrated that the greater the post length, the better the retention
and stress distribution. It may not always be possible to use a long post, especially
when the remaining root is short or curved.it is important to preserve 3 to 5 mm of
apical gutta percha to maintain the apical seal.(6) Studies have found that posts
with a length of at least 3-quarters of the length of the root offered the greatest
rigidity and least root deflection (bending) when compared with posts that were a
half or a quarter the root length. Short posts are especially dangerous and have a
much higher failure rate. (3)
When the root length is short, the clinician must decide whether to use a
longer post or maintain the recommended apical seal and use a parallel-sided
threaded post. The use of reinforced composite luting agents may compensate for
the reduced post length. (6, 8). For molars with short roots the placement of more

8
than one post will provide additional retention for the core foundation
restoration.(6)
The post length should reach two-thirds of the entire root length. A crown-
length/post-length ratio of at least 1:1 should be provided. Post length influences
the stress load along the root. Whereas the enlargement of the canal increases
cervical stress, post placement will decrease stress in this region. Short, wide posts
lead to elevated stress concentrations in the cervical region. Post placement beyond
two-thirds of root depth does not further decrease cervical stress, but tends to
increase stress in the apical region. (8)

2-Tooth Anatomy
Root anatomy such as root curvature, mesio-distal width, and labio-lingual
dimension dictates post selection. A consideration of the root size and length is
important, because improper post space preparation and use of large diameter posts
present the risk of apical or lateral perforation. Moreover, an active post can
initiate cracks in the thin dentinal wall.
Studies reported that anatomic considerations stated that roots of maxillary centrals
and laterals, and also mandibular premolars have sufficient bulk to accommodate
most post systems. (6)

3-Canal Configuration and Post Adaptability

Canal configuration aids in making a choice between a custom designed post
and a prefabricated post. Often a dilemma arises in funnel-shaped canals, whether
to use a parallel sided post and fill the remaining post spaced with cement or to use
a tapered post that closely adapt to the canal wall. A third option is to use large
prefabricated parallel-sided posts. It has been suggested that if a canal requires
extensive preparation a well adapted cast post and core restoration will be more

9
retentive that a prefabricated post that does not match the canal shape. In addition,
root reinforcement with composite is suggested for wide canals. A well adapted
tapered post provided increased resistance to fracture, however on fracture it
resulted in an extensive loss of tooth structure. 44% of the cast posts were less than
one half to one quarter the length of the clinical crown and that the failure rate
reported was the result of compromised length rather than post type.(3, 6)

4-POSITION OF TEETH IN THE DENTAL ARCH( 6, 8)

The location of the tooth in the dental arch necessitates different restorative
requirements to ensure the longevity of endodontically treated teeth.
-Molar teeth
Molar teeth receive predominately vertical rather than shear forces, unless a
large percentage of coronal tooth structure is missing, posts are rarely required in
endodontically treated molars. More conservative methods of core retention
include chamber retention, threaded pins, amalgam pins, and adhesive retention.
When a post is required because of lack of adequate remaining coronal tooth
structure, if more than 60% is missing, it should generally be placed only in the
largest canal; that is the palatal canal in the maxillary molar and the distal in the
mandibular molar. When the molar is to be used as an abutment tooth, a post is
commonly used.
- Anterior teeth
Because of the shearing and torqueing forces that act on them, anterior
endodontically treated teeth are restored with posts more often than posterior teeth.
Clinically, a post in a maxillary anterior tooth is subjected to compressive, tensile,
shear, and torqueing forces. At the dentin-post interface, the forces that tend to
dislodge the post are predominately labially inclined shear forces, and studies have
suggested resistance form can be increased with the use of a beveled preparation.

10
The maxillary anterior region is considered to be a high risk area for failure, which
may be due in part as a result of unfavorable directional loading during function. (3)
- Premolar teeth
All endodontically treated maxillary premolars and most mandibular second
premolars should receive cuspal coverage to protect the remaining cusps during
occlusion. Studies found that lateral excursive forces can shear the remaining cusp
or cause vertical root fracture. If an endodontically treated premolar has increased
functional stresses acting on the crown due to loss of the periodontuim and is to
serve as an abutment for a removable partial denture a post may be indicated.
Conversely, if a premolar has a relatively short crown and functions more like a
small molar, then a post is not indicated. When a post is indicated for placement in
a maxillary premolar the delicate morphologic anatomy must be considered . Post
systems that require minimal enlargement and reshaping of the canal space, such as
tapered posts, are best suited for maxillary premolars.

5-Stress
Post and core restored endodontically treated teeth are subjected to various
types of stresses: compression, tensile and shear. Of these stresses, shear stress is
most detrimental to the restored tooth so an increase in the post length with the
diameter kept to a minimum will help to reduce shear stresses and preserve tooth
structure.(6)
6- Torsional Force
Intra-orally, post and core restored teeth are subjected to various types of
forces. Torsional forces on the post-core-crown unit may lead to loosening and
displacement of the post from the canal. The importance of an antirotational
feature and concluded that resistance to torsional forces is integral to the survival

11
of the post-core-crown unit. Active post designs provide greater torsional
resistance than a passive post. (6)

7-Role of Hydrostatic Pressure

Cementation plays a significant role in enhancing retention, stress

distribution and sealing irregularities between the tooth and the post. During
cementation an increase in stress within the root canal has been reported because of
the development of hydrostatic pressure that will affect the complete seating of the
post and may also cause root fracture. The fitting stresses can be reduced by
careful placement of the post and by using a proper post design with a cement vent
to permit escape of the luting agent and thus reduce the hydrostatic pressure.
Pressure is also dependant on the viscosity of the cement. The more viscous the
cement, the greater the development of the hydrostatic pressure. (6)

8-Coronal Structure
The amount of remaining coronal tooth structure is also a critical factor in
determining the post selection. Studies determined that the amount of tooth
structure present is more important than the material from which the post and core
is fabricated (amalgam, resin, or cast gold).The bulk of the tooth above the
restorative margin should be at least 1.5mm to 2mm to achieve resistance form. (6)

9-Material Compatibility
Corrosion of the post and fracture of the root has been reported in the dental
literature. studies attributed 72% of longitudinal and oblique root fractures to
prolonged electrolytic reaction between dissimilar post and core metals (stainless
steel, silver, or brass posts reacting with the tin in the amalgam core). They

12
hypothesize that the products of this reaction deposited in the root canal induced
volumetric changes and caused root fracture.
Ideally post and cores are made of the same alloy. Dissimilar alloys may create
galvanic action, which may lead to corrosion of the less noble alloys. Corrosion
of the post may be initiated because of the access of the electrolyte to the post
surface, through cementum, dentin, microleakage around the coronal restoration,
and through the accessory canals, which may be opened during post space
preparation, or through undiagnosed root fracture. Between the various alloys used
for posts, titanium alloys are the most corrosion resistant. Alloys containing brass
have lower strength and lower corrosion resistance and hence are less desirable.
Noble metal alloys are corrosion resistant, but their cost is higher. With the
availability of nonmetallic post materials, the corrosion factor is eliminated.

10-Post fixation
Adhesive systems seem to be able to stabilize the tooth.studies have shown
that composite restorations with dentin enamel etching provide a stability similar to
that of the intact tooth. The use of composite in the entrance of the root canal
stabilizes the root-filled tooth, whereas an additional post is unable to contribute
further stabilization. Roots in which the posts were adhesively cemented were
significantly more fracture
resistant than those using zinc phosphate cement. Based upon this evidence, the
present study recommends adhesive fixation for any kind of post. (8)
The bonding of a post to the tooth structure should improve the prognosis by
increasing post retention and by reinforcing the tooth structure. This is due to stress
distribution characteristics of the bonding materials.

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11- Core Retention
The primary reason for using a post is to retain the core and the post head design is
an important factor and should provide adequate retention and resistance to
displacement of the core material.
Studies have reported that prefabricated metal posts with direct cores made of glass
ionomer, composite, or amalgam are less reliable than a one-piece cast post and
core because of the interface between the post and core. (6)

12- Post Material

To achieve optimum results, the material used for the post should have physical
properties similar to that of dentin, can be bonded to the tooth structure, and
biocompatible in the oral environment. It should also act as a shock absorber by
transmitting only limited stress to the residual tooth structure.
Recently introduced carbon fiber posts are reported to have mechanical properties
that closely match that of the tooth. The presence of parallel fibers in the resin of
carbon fiber posts enable it to absorb and dissipate stresses as it has a modulus of
elasticity that is nearly identical to that of dentin, by comparison the modulus of
elasticity for stainless steel is roughly 20 times greater than dentin; for titanium,
the modulus of elasticity is 10 times greater than dentin. Posts with a high modulus
of elasticity do not flex with the tooth under loading and are empirically believed
to cause root fractures. (6, 8)
13-Post Diameter
Several in vitro studies have confirmed the importance of the remaining
bulk of tooth structure with regard to strength and resistance to root fracture.
Increasing the diameter of the post does not provide a significant increase in the
retention of the post. However, it can increase the stiffness of the post at the
expense of the remaining dentin and the fracture resistance of the root. Therefore
14
post diameter must be controlled to preserve radicular dentin, reduce the potential
for perforations, and permit the tooth to resist fracture. (3)
suggested that post diameters should not exceed one third of the root diameter at
any location.
There have been different approaches regarding the selection of post diameter and
categorized into: conservationist, preservationist, and proportionist. proportionist
approach suggest the post width should not be greater than one third of the root
width at its narrowest dimension. Preservationist proposed that the post should be
surrounded by a minimum of 1mm of sound dentin. Conservationist approach
proposed that minimal canal preparation and maintaining as much residual dentin
as possible.(6,8)
Figure 2: Cross section of the mandibular
13- Post Design
The available post designs can be classified according to their shapes and
surface characteristics. They may be parallel, tapered, or parallel and tapered
combination. According to their surface characteristics, the posts are active or
passive. The active posts mechanically engage the dentin with threads, whereas the
passive post depends on the cement and its close adaptation to the canal wall for its
retention.
Studies proposed that the active post design as a cause of failure of the post
and core restored teeth. The tapered post conforms to the natural root form and the
canal configuration, thus permitting optimal preservation of tooth structure at the
post apex; however it produces a wedging effect, stress concentration at the
coronal portion of the root, and lower retentive strength.
Parallel sided post designs have been shown to increase retention and
produce uniform stress distribution along the post length and resiste tensile, shear,
and torqueing forces better than tapered posts. Concentration of stress has been
15
reported to occur at the apex of the post in a narrow and tapering root end. This
stress is caused by unnecessary removal of the tooth structure at the apical end of
the root and sharp angles of the post.
In the parallel tapered design, the post is parallel throughout its length
except for the most apical portion, where it is tapered. It permits preservation of
the dentin at the apex achieves sufficient retention because of parallel design).
The surface characteristics of the post also change the retentive values, the
highest retention is observed in the threaded post, followed by the post with a
serrated surface. The least retention is seen with smooth surface posts. But
threaded post engages in dentin and may lead to increased undesirable stresses
within the root. The performance of a threaded post is inferior to that of a custom
cast post and it exerts a greater amount of stress and was considered the least
desirable. The parallel sided, serrated, and vented posts were found to exert the
least amount of stress.(4,6)
Robbins (2002) reported that if the available post space is short 5 to 6 mm,
a more retentive active post is indicated. If the available post space is 8 to 9 mm
and the canal is not funnel shaped, a tapered post may be a better choice, because
the available post space is long enough to provide adequate axial retention and it
does not require canal enlargement during post space preparation.(6)
Standlee et al investigated tensions in dentine and surrounding bone after post and
core placement, by using the polarized light method.
They concluded that:
1. Reduction of stress and shear forces concentrated around the post and core is
achieved by increase in the length of the root part of the core.
2. If a shorter post needs to be used, a threaded post should be used because it
results in better tension distribution.

16
3. Threaded post stress increases when the thread fully engages the dentine
notches.
4. Conic posts cause longitudinal root fracture and the stress is concentrated in the
coronal third of the root.
5. Tension is induced when the post is incorrectly screwed.
6. The smooth post and core without notches for drainage of surplus cement during
cementing cause, because of the hydrostatic pressure, dentine strains in the apical
third of the root. The torsion caused by the conic post in the tooth root walls is
more intensive than the tension caused by the parallel-sided threaded post, so root
fracture may be avoided by correct selection of the prefabricated post shape.(4)

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References
1- Nandini V.V, Venkatesh V.: Current concepts in the restoration of
endodontically treated teeth; The Journal of Indian Prosthodontic Society; 2006; 6:
63-67.
2- Kulkarni G, Shetty N, Moogi P, Kumar GV, Prashanth BR. Choice of post and
core systems in endodontics. J Int Oral Health 2016; 8(3): 409-411.
3- Stockton L.W.: Factors affecting retention of post systems: A literature review ;
J Prosthet Dent 1999; 81: 380-5.
4-Adnan Δatovi. Davor Seifert, Renata Poljak-Guberina, Boris Kvasni
Eka:Influence of Biomechanical Factors on Restoration of Devitalized Teeth; Acta
Stomatol Croat ;2000 ; 67-72.
5-Luthra RP, Gupta R, Sirohi R, Kumar N, Mehta S.: Management of mutilated
teeth with Post and Core: A Review; J Adv Med Dent Scie Res; 2015; 3(4):99-101.
6- Reem Al-Dhalaan, PROSTHODONTIC MANAGEMENT OF
ENDODONTICALLY TREATED TEETH; Factors Determining Post Selection,
Foundation Restorations and Review of Success & Failure Data1-35.
7- Mamoun J.: Post and core build-ups in crown and bridge abutments: Bio-
mechanical advantages and disadvantages; J Adv Prosthodont; 2017; 9:232-7.
8- Peroz I., Blankenstein F., Lange K.P., Naumann M., Restoring endodontically
treated teeth with posts and cores—A review; Quintessence Int; 2005; 36:737–
746.

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