Operative dentistry aje qualtrough, jd satterthwaite la morrow, pa brunton

Design requirements
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Length
There is a direct relationship between post length and retention and in
addition, the post must also be long enough to minimise internal
stresses on the root. The exact dimensions of the ideal post have been
considered in detail and these include:
• Equal to the length of the clinical crown
18,19
• Half the length of the root
20
• Longer than the final clinical crown
8
• Midway between the apex and the height of the alveolar crest
21
• Leaving 3–5 mm of root filling material
22
While a disparity of opinions exists as to the length of post prepara-
tion, most investigators and clinicians agree that the post length must
not interfere with the apical seal. Microleakage is considerably
reduced if an excess of 4 mm of gutta-percha remains in the canal 
apical to the post
23,24
.
Diameter
With respect to retention, diameter is not as important as post length
with little or no increase in retention seen with increasing diameter.
Post diameter should therefore not be increased in the hope of gaining
retention but should solely be dictated by the physical properties of
the post and canal morphology, in order to avoid removing an excess
of sound tooth structure. Increasing the diameter unnecessarily 
weakens the remaining root and may predispose it to fracture.
Shape
Posts may be generally classified as tapered or parallel, threaded or
non-threaded and with a variety of surface finishes. The post shape is
an important factor in determining retention and stress distribution,
and selection of post type is usually dictated by the clinical situation.
With respect to retention, tapered posts usually fare poorly; the
greater the taper, the lower the retention. Due to the tapered design,
these posts allow cement to flow and vent easily and hence perform
favourably during cementation and before loading (as shown in 
photoelastic stress studies). However, during loading tapered posts
demonstrate unfavourable stress distribution compared with parallel-
sided posts
25
, and are more likely to split the root due to wedging
Endodontics – further considerations


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forces. Parallel-sided posts offer increased retention compared with
tapered posts along with a more favourable stress distribution pattern
on loading.
To alleviate hydrostatic pressure during cementation, cylindrical
posts usually incorporate vents along their long axes. The resistance
of cylindrical posts to torsional/rotational forces is low and hence
whenever possible an anti-rotational feature should be included in
the final post-core unit to improve resistance to these forces. A circular
cross-sectional configuration of the prepared post space will not offer
any significant resistance to rotation of the post, thus the normal
eccentricity of the coronal portions of the canal should be accentuated
to provide an anti-rotational feature. This may also act as a positive
seat for the core at the coronal portion of the canal, thus preventing
over seating and wedging within the root that could lead to vertical
fracture.
Indirectly constructed intra-radicular posts are made from a replica
or ‘pattern’ of the desired shape: this pattern may be custom made 
or constructed from preformed shapes. Custom posts are essentially
posts made from a pattern of a freehand preparation of the canal
space: this pattern is usually made indirectly on a cast (obtained from
an impression of the preparation) but may be made directly in the
mouth. Preformed shapes usually entail the use of a drill and match-
ing post components. Such components may consist of ‘blanks’ used
for impression taking, burnout patterns for construction of a pattern
(either indirectly on a model or directly in the mouth) or may be 
prefabricated posts with a core either built up directly or a core cast
onto the prefabricated post.