Contemporary Restoration of Endodontically Treated Teeth. Nadim Z. Baba
on a radiographic image. Therefore, the guideline for appropriate length is to retain not 3.0 or 4.0 mm but rather 5.0 mm of apical gutta-percha, as determined radiographically, and to extend the post fully to the point where the gutta-percha is located.
Influence of root selection
This 5.0-mm apical guideline for gutta-percha length works well for all teeth except molars. In 1982 Abou-Rass et al88 prepared post spaces to depths of 4.0 mm and 7.0 mm into a canal space using No. 2 (diameter of 0.8 mm), No. 3 (diameter of 1.0 mm), and No. 4 (diameter of 1.2 mm) Peeso instruments.88 The thickness of remaining dentin was then measured at the 4.0- and 7.0-mm depths along with the incidence of root perforation or thinning to the dentin-cementum border. The authors concluded that the distal roots of mandibular molars and the palatal roots of maxillary molars are the roots best suited for post preparation. They also contended that these roots can be safely prepared to a depth of 7.0 mm using the No. 2 and No. 3 Peeso instruments. In contrast, perforations or root thinning occurred in the mesial roots of mandibular molars as well as the facial roots of maxillary molars.88 Of the 75 mandibular molars included in the study, perforations or instrumenting to only a thin layer of root structure occurred in 27.7% (20) of the prepared mesial roots.88
Based on these results, it is proposed that posts with diameters between 0.8 and 1.0 mm extend no more than 7.0 mm into a molar canal space. This depth guideline should only be applied to the distal roots of mandibular molars and the palatal roots of maxillary molars. The mesial roots of mandibular molars and the facial roots of maxillary molars should not be used for post placement.
Guidelines for prevention of post loosening
By design, threaded posts provide greater resistance to post loosening than do cast root-form posts. As mentioned previously, however, posts with a thread design are not recommended because of their potential for generating high levels of internal root stresses and the associated higher incidence of root fracture. Prevention of post loosening is best accomplished by following the recommended guidelines for length and selecting the appropriate primary root. The clinician should leave 5.0 mm of intact apical gutta-percha and extend a post the length of the root space right to the level of the gutta-percha. The exception is in molar teeth, where the post length should be limited to 7.0 mm into the primary roots (palatal root of maxillary molars and distal root of mandibular molars). Care should be exercised to avoid preparing canal spaces in secondary roots (buccal roots of maxillary molars and mesial root of mandibular molars).
Root fracture and root perforation
Influence of threaded posts
If the use of threaded posts is avoided, the threat of high internal root stresses is reduced along with the accompanying risk of root fracture.74–76 Multiple clinical studies comparing threaded and cemented posts have shown higher root fracture rates with threaded posts.77–89 One meta-analysis of clinical studies determined that cemented posts had a 10% higher tooth survival rate than did threaded posts.79 An evaluation of 95 ETT where vertical root fractures had occurred identified threaded posts as the most common type of post (64 of 95 teeth) present in teeth that fractured.89
Influence of post length
Studies have determined that short posts increase the stress in teeth90, 91 while increases in post length enhance the resistance of a tooth to root fracture.92 Additionally, posts of optimal length offer the greatest rigidity and produce the least amount of root deflection.93 In an examination of 154 teeth that had been extracted because of vertical root fractures, 95 teeth had posts, and 66 of the posts ended in the coronal third of the root.89
Influence of post diameter
In addition to post length, post diameter affects the potential for root fracture by increasing stress91, 94 and decreasing resistance to fracture.92 With large-diameter posts (those 1.5 mm or greater), root fracture increased sixfold for every millimeter of decreased root diameter.76 Unfortunately, there have been no clinical studies comparing fracture resistance of teeth restored with several different post diameters to determine the most appropriate proportional relationship between post diameter and root diameter. However, in 2011 Du et al95 reported results, based on finite-element analysis of the mandibular first premolar, showing that posts with a diameter that was 50% of the root diameter exhibited the most favorable stress distribution.95 Nevertheless, the clinical guideline that has been used successfully is not to prepare post spaces that exceed one-third the root diameter in its smallest cross-sectional dimension.
Influence of residual dentin thickness
The amount of dentin thickness remaining after root canal treatment is an important factor because overenlargement of the root canal space for placement of a post can compromise tooth strength.
It has been shown that only five teeth have more than 1.0 mm of dentin wall thickness remaining after conventional endodontic therapy.96 Those five teeth are the (1) maxillary central incisors, (2) maxillary lateral incisors, (3) maxillary canines, (4) mandibular canines, and (5) maxillary first molars (palatal root only).96 According to Ouzounian and Schilder,96 all other teeth have less than 1.0 mm of dentin thickness after endodontic cleaning and shaping.
A study of maxillary first premolars with posts prepared to a depth equal to the clinical crown height determined that 1.0 mm of residual dentin thickness was only present when a 0.7-mm diameter rotary instrument was used to prepare the post space.97 It has been shown that any amount of root canal preparation in mandibular first and second molars following endodontic treatment decreased the residual dentin thickness to less than 1.0 mm.98 Additionally, a study of residual dentin thickness in the distal root of 26 mandibular molars after endodontic treatment alone determined that the canal wall on the furcal side of the root was less than 1.0 mm thick 82% of the time and less than 0.5 mm thick 17.5% of the time.99
Influence of instrument diameter
Because of the potential for root perforation or root thinning that can lead to tooth fracture or perforation, appropriate post diameters have been identified in relation to average root diameters. Establishing appropriate post diameters helps to determine the maximum diameter of instruments that can be used safely in each type of tooth to prepare post spaces.
One study measured 50 teeth of each type and recommended selection of a post provided its diameter was (1) no greater than one-third the root diameter at the cementoenamel junction; (2) at least 2.0 mm less than the root at the midpoint of the post; and (3) 1.5 mm smaller than the root at the apical end of the post.100 For mandibular incisors, it was proposed that posts be 0.7 mm in diameter. However, the authors proposed a diameter of 1.1 mm for the distal roots of mandibular molars, 1.3 mm for the palatal root of maxillary molars, and 0.9 mm for maxillary first premolars. The largest recommended diameter was 1.7 mm, for maxillary central incisors.100
Another study measured average root dimensions of 125 teeth of each tooth type. The authors recommended that post diameters not exceed one-third of the average root width,101 using a 95% confidence level.
Based on these post diameters, certain instruments should not be used to prepare post spaces. For instance, No. 5 and No. 6 Peeso instruments, No. 6 Gates Glidden instruments, No. 4 and No. 6 round burs, and prefabricated post drills such as the No. 6 and No. 7 ParaPost drills (Coltène/Whaledent) are too large and should not be used. The No. 4 Peeso, No. 5 Gates Glidden, and No. 5 ParaPost drills have diameters that are slightly larger than 1.1 mm and, therefore, are only recommended for teeth with large-diameter roots. Even a No. 4 Gates Glidden drill has a diameter that is too large for the distal root of mandibular molars.99
Influence of post depth and length
The previously cited study of root thinning and perforations in molars by Abou-Rass et al88 determined that posts can safely be placed to a depth of 7.0 mm when instruments with diameters between 0.8 and 1.0 mm