Contemporary Restoration of Endodontically Treated Teeth. Nadim Z. Baba
is ovoid to hourglass in shape.102 Because of the small mesiodistal root dimension, these teeth are not suitable for canal space enlargement beyond that resulting from endodontic treatment itself. Because prefabricated posts are larger than the maximum diameter recommended for mandibular incisors, these teeth are best treated with custom cast posts and cores made to fit the existing root canal morphology; the optimal post diameter is 0.6 mm.101
Canines. Mandibular canines have an oval cross-sectional shape and prominent developmental depressions.102 They also generally have root dimensions sufficient to permit safe post placement provided that the maximum diameter does not exceed 1.0 mm.
Premolars. Mandibular first premolar roots typically are larger faciolingually than they are mesiodistally. Developmental root depressions are frequently found on both the mesial and distal surfaces of the root.102 Nonetheless, the root dimensions generally are large enough to permit the safe placement of posts when they adhere to the recommended diameter of 0.9 mm.
Mandibular second premolars too have a root that is ovoid in cross section and have sufficient dimensions to permit placement of a post with the recommended diameter of 0.9 mm. However, mandibular premolars with oval or ribbon-shaped canals should not be subjected to any preparation of the root canal space beyond that produced during endodontic treatment because it is likely to leave less than the desired 1.0 mm of residual dentin thickness.116
Molars. Data on mandibular first molar root morphology indicate that both the mesial and distal surfaces of the mesial root have developmental root depressions.108, 111 The depression is deep on the mesial surface but even deeper on the distal surface.108 Nevertheless, the distal root of the mandibular first molar is better suited for post placement because the distal root is straighter than its mesial counterpart and rounder in cross-sectional form. Also, the mesial developmental depression on the distal root often is not very deep, and the distal surface may not even have a depression or may possess only a slightly concave surface.108 The distal roots of mandibular molars should not be enlarged beyond the preparation produced by endodontic treatment because the residual dentin thickness after root canal treatment typically is less than 1.0 mm.98 Similar characteristics are present on mandibular second molars.
Guidelines for prevention of root fracture and root perforation
Fracture is more likely to occur when roots are weakened by posts that are short because short posts also increase root stress. Therefore, the same guidelines used to minimize post loosening should be followed to minimize, if not prevent, root fracture. As stated previously, the recommended post length for most teeth (except molars) is what remains after 5.0 mm of apical gutta-percha is left in place. For molars, post length should not exceed a maximum length of 7.0 mm in the primary roots (the palatal root of maxillary molar and distal root of mandibular molars).
Root fracture is more likely to occur when teeth are weakened by post spaces of an excessive diameter. A good guideline to follow clinically is to prepare post spaces to no greater than one-third the root’s original diameter. In some teeth, any root canal enlargement beyond that produced during root canal treatment will create excessive space because the amount of residual dentin remaining after endodontic treatment typically is 1.0 mm. As mentioned previously, only five maxillary teeth have more than 1.0 mm of root wall thickness remaining after root canal treatment: (1) maxillary central incisors, (2) maxillary lateral incisors, (3) maxillary canines, (4) mandibular canines, and (5) the palatal root of the maxillary first molar. Teeth with less than 1.0 mm of residual dentin thickness following endodontic treatment should not be subjected to further canal enlargement for placement of a post.
Overenlargement of a canal space and subsequent fracture can occur when instruments with diameters that exceed the recommended dimensions are used. Therefore, most teeth should be prepared with instruments that do not exceed 0.9 to 1.0 mm in diameter. The largest recommended instrument diameter is 1.1 mm, and that is reserved for use in maxillary central incisors. Teeth with small roots, such as mandibular incisors, should be prepared with instruments that do not exceed 0.6 mm in diameter. Instruments used to prepare the distal roots of mandibular molars and the palatal roots of maxillary first molars should be between 0.8 and 1.0 mm in diameter, provided that the post does not extend more than 7.0 mm into the root canal.
As stated previously, root perforation is more likely to occur for any one of the following three reasons: (1) instruments with diameters that cannot be accommodated by the morphology of the root are used; (2) the post is extended too far into a root with developmental root depressions and/or root curvature; and (3) a rotary instrument cuts on an incorrect angulation, and the instrument does not follow the root canal. Root perforations can be avoided if the same guidelines as those recommended for preventing root fracture are followed. That is, post spaces should be prepared so that they do not exceed one-third the root diameter, and instruments with diameters that can be accommodated by the dimensions of the roots being prepared should be used. One of the best means for clinicians to avoid root perforation is to have an understanding of root morphology.
Although posts technically can be placed in any root, the maxillary roots most suitable for posts include maxillary central incisors as long as the post has the recommended diameter of 1.1 mm; maxillary lateral incisors when the post diameter does not exceed the proposed diameter of 0.9 mm; maxillary canines as long as the post diameter does not exceed 1.0 mm; maxillary second premolars as long as the post diameter does not exceed 0.9 mm; and the palatal roots of the maxillary first and second molars provided that their diameters are 0.8 to 1.0 mm. Posts should be avoided in the facial roots of maxillary molars.
Maxillary first premolars have root morphology that has to be carefully respected if a post is required. There should be no enlargement of the root canal space beyond that produced by root canal treatment due to root dimensions and the depths of developmental root depression. With two-rooted first premolars, the lingual root is preferred because it is generally straighter and has a more suitable surface form. Two-rooted premolars frequently have a furcal depression in the facial root that compromises the facial root when the bifurcation occurs in the cervical one-third of the root because a post extended into the facial root would approximate this depression. When a single-rooted maxillary first premolar with one root canal requires a post, the post diameter should be 0.7 mm or less because the mesial and distal developmental root depression depths restrict the amount of available tooth structure peripheral to the centrally located single root canal.
The mandibular roots best suited for posts include mandibular canines, provided the post diameter does not exceed 1.0 mm, and mandibular first and second premolars when the post diameter is no greater than 0.9 mm. However, mandibular premolars with oval or ribbon-shaped canals should not be subjected to any preparation of the root canal beyond that produced during endodontic treatment because it will result in less than 1.0 mm of dentin. Posts can be placed in the distal roots of mandibular molars but there should be no enlargement beyond that produced by the endodontic treatment because the residual dentin thickness after root canal treatment is less than 1.0 mm. Posts should be avoided in the mesial roots of mandibular molars.
Posts also should be avoided in mandibular incisors, if possible. When posts are absolutely needed, there should be no enlargement beyond that produced during root canal treatment. Posts in mandibular incisors should not exceed 0.6 mm in diameter; this dimension precludes the use of prefabricated posts because most commercially available posts have a diameter that exceeds this dimension.
Requirements for Remaining Sound Tooth Structure
Sound dentin tooth structure should extend cervically beyond any type of core material. In fact, it is essential that sound tooth structure remain circumferentially to produce a cervical ferrule. The minimum amount of sound tooth structure grasped by a crown is important to optimize a tooth’s resistance to fracture. It is generally recognized that more than 1.0 mm of circumferential tooth structure should be encompassed by the overlying crown,117–120 with some studies121–123 indicating that the minimum should be 2.0 mm. Furthermore, maximum potential resistance can be achieved when that 2.0-mm ferrule encompasses all four of the axial surfaces.123
Post