Contemporary Restoration of Endodontically Treated Teeth. Nadim Z. Baba
in the distal roots of mandibular molars and the palatal roots of maxillary molars.88 In another investigation where No. 4 Gates Glidden instruments were used to prepare post spaces in the distal roots of 26 mandibular molars, perforations occurred 7.3% of the time and even more frequently when larger drills were used.99 As a result, the authors recommended that Gates Glidden drills larger than No. 3 not be used in the distal roots of mandibular molars. The authors went on to state that “post spaces in such teeth should be limited to the endodontically prepared canal.”99
Findings from other research also support the recommendation that the root canal space not be enlarged after endodontic treatment in the distal roots of mandibular molars.98 Again, it was determined that the residual dentin thickness after root canal treatment was less than 1.0 mm,98 making root fracture or perforation more likely even with self-limiting Gates Glidden mechanical instruments. Similar characteristics and guidelines exist for mandibular second molars as well.
To be optimally successful with posts, it seems reasonable to adopt the more conservative diameter recommendations reported in the studies cited. Therefore, the following maximal post diameters are recommended: 0.6 mm for mandibular incisors; 0.9 mm for most teeth (maxillary lateral incisors, maxillary and mandibular premolars, the distal root of mandibular molars, and the palatal root of maxillary molars); 1.0 mm for canines; and 1.1 mm for maxillary central incisors.
When molar canals are prepared, the previously recommended guideline of 7.0 mm for post length must be followed in the primary roots (palatal roots of maxillary molars and distal roots of mandibular molars). However, as already mentioned, posts should not be placed in the mesial roots of mandibular molars; as one study showed, 20 of 75 teeth with 7.0-mm-long posts had only a thin layer of remaining dentin or were perforated.88
Influence of root morphology
Root morphology has been well described,102 and an understanding of external root morphology is helpful in identifying the teeth and roots that are best suited for posts and less likely to be subjected to root thinning or root fracture.
Maxillary root morphology. The nature of the root morphology of teeth in the maxillary arch has been characterized well in the literature. An understanding of this information, coupled with proper root selection for post placement, aids clinicians in negotiating these delicate spaces when placement of a radicular post is planned.
Incisors. Maxillary central incisors have a cross-sectional root anatomy that is triangular or ovoid with a lingual taper.102 Their form and dimensions usually allow placement of posts with the proposed optimal diameter of 1.1 mm.101 Maxillary lateral incisors also possess a single root with a circular, oval, or ovoid cross-sectional form. The root canal is triangular in cross section in the cervical area and round apically.102 However, the smaller root diameter makes it desirable to follow the recommended optimal post diameter of 0.8 mm.101
Canines. Maxillary canines have an oval cross-sectional shape with prominent developmental depressions,102 making their form less ideal than that of maxillary incisors for post placement. However, the root dimensions are usually sufficient to permit posts with the recommended optimal diameter of 1.0 mm.101
Premolars. In a 10-year retrospective study of both metal prefabricated posts and custom posts, posts in maxillary first premolars had the highest failure rate (30%).103 The canals in these teeth are not well suited for enlargement beyond the root canal diameter present after endodontic treatment. In fact, one study of residual dentin thickness validated the negative effect of post preparation on dentin thickness. When the first premolars were prepared using rotary instruments with diameters of 0.9 and 1.0 mm, 61% of the lingual roots and 77% of the buccal roots had less than the desired 1.0 mm of dentin remaining.104
Maxillary first premolars have prominent mesial and distal developmental depressions on the root trunk as well as a relatively narrow mesiodistal root dimension.105 Additionally, in two-rooted first premolars, the developmental root depressions on both the mesial and distal aspects of the root trunk deepen progressively from the cervical line to the furcation.106 When measured, the depth of the mesial furcal concavity increased to slightly more than 1.0 mm at a distance of 4.7 mm apical to the cementoenamel junction.107
In two-rooted maxillary first premolars, the palatal root is the more desirable location for a post because it is usually straighter and does not have the distal root curvature frequently present (66%) in the facial root.108 The palatal root of two-rooted premolars also has a surface form that is more conducive to placement of a post because the facial root frequently has a concavity on the furcal aspect of the root.109 In one study of 100 maxillary first premolars, 37 of the teeth had bifurcated roots, and 62% of these teeth had a concavity (with a mean depth of 0.46 mm) on the furcal aspect of the buccal root.106 Another study found that 35 (78%) of 45 maxillary first premolars with two roots had this groove on the palatal aspect of the buccal root.110
A third study of 97 bifurcated first premolars determined that the residual dentin thickness after root canal treatment, at 6.0 mm apical to the cementoenamel junction, was less than 1.0 mm in 53% of the buccal roots on the palatal surface. After post preparation, the dentin thickness in the area of the furcal groove was less than 1.0 mm in 77% of the teeth. The authors recommended that the lingual root be used instead of the facial root when posts are necessary.104
When a single-rooted maxillary first premolar requires a post, the diameter of the canal should be 0.7 mm or less because the mesial and distal developmental root depressions restrict the amount of available tooth structure in the centrally located single root canal.97
Maxillary second premolars are better suited for post placement then are maxillary first premolars because they usually have one root with a slightly larger mesiodistal dimension at the cervical line and slightly greater root length than the first premolar.105 While mesial and distal developmental root depressions are typically present on the root of maxillary second premolars,108 the mesial depression is shallower than the one present on the first premolar.111
Molars. In maxillary first molars, only the palatal root is well suited for post placement. This canal is ovoid in cross-sectional shape102 and has greater cross-sectional dimensions than the facial roots.105 Additionally, while developmental depressions can be present on the facial and palatal surfaces of the lingual root, they are generally shallow.102 One complicating factor for the palatal root is the frequent presence of a facial curvature in the apical third.112 However, when the recommendation of a 7.0-mm palatal post length is followed, the presence of facial curvature does not create a problem because the root curvature occurs apical to the end of an ideal post. The mesiofacial root of the first molar is relatively thin mesiodistally and also has prominent depressions or flutings on both the mesial and distal surfaces,110, 111, 113–115 making it unsuitable for post placement.
The distofacial root is rounded or ovoid in cross section102 and usually does not have a distal developmental depression,108 but it does contain a developmental depression on its mesial surface.102 It is also smaller faciolingually than the other roots and narrower mesiodistally at its attachment to the root trunk, making it a poor candidate for post placement.111
Maxillary second molars are similar to first molars and, therefore, only the palatal root is suitable for post placement. Their facial roots are even less suitable than the facial roots of first molars because of their distal curvature.
Mandibular root morphology. The root morphology of teeth in the mandibular arch is important to understand so that appropriate treatment recommendations are made and post loosening and root fracture are averted.
Incisors. Mandibular central and lateral incisors have roots that are broad faciolingually but narrow mesiodistally with substantial longitudinal depressions on both the mesial and distal surfaces.102 These depressions are usually deeper at the junction of the middle and apical thirds of the root.111 The roots’ cross-sectional