Conservation Of Tooth Structure

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Preparation of the Canal (Fig. 12-8). When creating post space, great care must be used to remove only minimal tooth structure from the canal. Excessive enlargement can perforate or weaken the root, which then may split during cementation of the post or subsequent function. The thickness of the remaining dentin is the prime variable in fracture resistance of the root. Experimental impact testing of teeth with cemented posts of different diameters' showed that teeth with a thicker (1.8 mm) post fractured more easily than those with a thinner (1.3 mm) one.

Photoelastic stress analysis also has shown that internal stresses are reduced with thinner posts. Conversely, the root can be compared to a ring. The strength of a ring is proportional to the difference between the fourth powers of its internal and external radii. This implies that the strength of a prepared root comes from its periphery, not from its interior, so a post of reasonable size should not weaken the root significantly." Nevertheless, it is difficult to enlarge a root canal uniformly and to j udge with accuracy how much tooth structure has been removed and how thick the remaining dentin is. Most roots are narrower mesiodistally than faci-olingually and often have proximal concavities that cannot be seen on a standard periapical radiograph. Experimentally, most root fractures originate from these concavities because the remaining dentin thickness is minimal. Therefore the root canal should be enlarged only enough to enable the post to fit accurately yet passively while ensuring strength and retention. Along the length of the post space, enlargement seldom needs to exceed what would have been accomplished with one or two additional file sizes beyond the largest size used for endodontic treatment. Because of the more coronal position of the post space, a much larger file must be used to accomplish this (Fig. 12-9).

Preparation of Coronal Tissue. Endodontically treated teeth often have lost much coronal tooth structure as a result of caries, of previously placed restorations, or in preparation of the endodontic access cavity. However, if a cast core is to be used, further reduction is needed to accommodate a complete crown and to remove undercuts from the

Apical Seal

Fig. 12-8. Faciolingual cross section through a maxillary central incisor prepared for a post-and-core. Six features of successful design are identified: 1, Adequate apical seal;

2, minimum canal enlargement (no undercuts remaining);

3, adequate post length; 4, positive horizontal stop (to minimize wedging); 5, vertical wall to prevent rotation (similar to a box); 6, extension of the final restoration margin onto sound tooth structure.

Minimal enlargement Length Stop

A nti rotation Margin design

Fig. 12-8. Faciolingual cross section through a maxillary central incisor prepared for a post-and-core. Six features of successful design are identified: 1, Adequate apical seal;

2, minimum canal enlargement (no undercuts remaining);

3, adequate post length; 4, positive horizontal stop (to minimize wedging); 5, vertical wall to prevent rotation (similar to a box); 6, extension of the final restoration margin onto sound tooth structure.

Prefabricated Post

Fig. 12-9. Use of a prefabricated post entails enlarging the canal one or two file sizes to obtain a good fit at a predetermined depth. A, Incorrect; the prefabricated post is too narrow. B, Incorrect; the prefabricated post does not extend to the apical seal. C, Correct; the prefabricated post is fitted by enlarging the canal slightly.

Fig. 12-9. Use of a prefabricated post entails enlarging the canal one or two file sizes to obtain a good fit at a predetermined depth. A, Incorrect; the prefabricated post is too narrow. B, Incorrect; the prefabricated post does not extend to the apical seal. C, Correct; the prefabricated post is fitted by enlarging the canal slightly.

chamber and internal walls. This may leave very little coronal dentin. Every effort should be made to save as much of the coronal tooth structure as possible, because this helps reduce stress concentrations at the gingival margin." The amount of remaining tooth structure is probably the single most

1 mportant predictor of clinical success. If more than

2 mm of coronal tooth structure remains, the post design probably has a limited role in the fracture re-

Corona Chamber Tooth

Fig. 12-10. A, It is preferable to maintain as much coronal tooth structure as possible, provided it is sound and of reasonable strength. B, Extensive caries has resulted in the loss of all coronal tooth structure. This is less desirable than the situation in A, because greater forces are transmitted to the root.

Fig. 12-10. A, It is preferable to maintain as much coronal tooth structure as possible, provided it is sound and of reasonable strength. B, Extensive caries has resulted in the loss of all coronal tooth structure. This is less desirable than the situation in A, because greater forces are transmitted to the root.

Ferrule Endodonti
Fig. 12-11. Extending a preparation apically creates a ferrule and helps prevents fracture of an endodontically treated tooth during function. A, Prepared with a ferrule (arrows). B, Prepared without a ferrule.

sistance of the restored tooth . The once common clinical practice of routine coronal reduction to the gingival level before post-and-core fabrication is outmoded and should be avoided (Fig. 12-10). Extension of the axial wall of the crown apical to the missing tooth structure provides what is known as a ferrule (Fig. 12-11) and is thought to help bind the remaining tooth structure together, preventing root fracture during function 22-24 Although there is evidence that preserving as much coronal tooth structure as possible will enhance prognosis, it is less clear whether the prognosis will improve by creating a ferrule in an extensively damaged tooth by surgical crown-lengthening. In this latter circumstance, although the crown-lengthening allows a ferrule, it also leads to a much less favorable crown-to-root ratio and therefore increased leverage on the root during function (Fig. 12-12). One recent laboratory study showed that creating a ferrule through

Coronal Tooth Damage

Fig. 12-12. Effect of apical preparation on crown-to-root ratio. A, Schematic of extensively damaged premolar tooth. Apical extension of the gingival margin would encroach on the biologic width (p. • • e), This preparation has no ferrule. B, Creating a ferrule with orthodontic extrusion (see Fig. 6-21) reduces root length (R) while crown length (C) remains unchanged. C, Surgical crown lengthening also reduces root length (R) but increases crown length (C). This results in a much less favorable crown-to-root ratio, which may not in fact strengthen the restoration.

(Courtesy Dr AG Gegauff From GegauffAG:J Dent Res 78:223,1999 [abstract].)

Fig. 12-12. Effect of apical preparation on crown-to-root ratio. A, Schematic of extensively damaged premolar tooth. Apical extension of the gingival margin would encroach on the biologic width (p. • • e), This preparation has no ferrule. B, Creating a ferrule with orthodontic extrusion (see Fig. 6-21) reduces root length (R) while crown length (C) remains unchanged. C, Surgical crown lengthening also reduces root length (R) but increases crown length (C). This results in a much less favorable crown-to-root ratio, which may not in fact strengthen the restoration.

(Courtesy Dr AG Gegauff From GegauffAG:J Dent Res 78:223,1999 [abstract].)

Crown Root Ratio

Fig. 12-13. Comparison of forces needed to remove different prefabricated post systems. (Redrawn from Standlee JP, Caputo AA: J Prosthet Dent

Fig. 12-13. Comparison of forces needed to remove different prefabricated post systems. (Redrawn from Standlee JP, Caputo AA: J Prosthet Dent

Fig. 12-14. The use of a parallel-sided post in a tapered canal requires considerable enlargement of the post space, which can weaken the root significantly.

Fig. 12-14. The use of a parallel-sided post in a tapered canal requires considerable enlargement of the post space, which can weaken the root significantly.

crown lengthening resulted in a weaker, rather than a stronger, restored tooth.'-' Creating a ferrule with orthodontic extrusion may be preferred as, although the root is effectively shortened, the crown is not lengthened (see Fig. 12-12, B).

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