Retention Form

Anterior Teeth. Dislodgment of a post-retained anterior crown is frequently seen clinically and results from inadequate retention form of the prepared root. Post retention is affected by the preparation geometry, post length, diameter, surface texture, and by the luting agent.

Preparation Geometry. Some canals, particularly in maxillary central incisors, have a nearly circular cross section (see Table 12-3). These can be prepared with a twist drill or reamer to provide a cavity with parallel walls or minimal taper, allowing the use of a preformed post of corresponding size and configuration. Conversely, canals with elliptical cross sections must be prepared with a restricted amount of taper (usually 6 to 8 degrees) to ensure adequate retention while eliminating unde-sired undercuts. This is analogous to an extracoro-nal preparation (see Chapter 7). With extracoronal preparations, retention increases rapidly as vertical wall taper is reduced (see Chapter 7). Although retention can be further increased by using a threaded post, which screws into dentin, this procedure is not recommended because of residual stress in the dentin. If the procedure is used, however, threaded posts must be "backed off" to ensure passivity; otherwise, the root will fracture.

Laboratory testing has confirmed that parallel-sided posts are more retentive than tapered posts and that threaded posts are the most retentive (Fig. 12-13). However, these comparisons are relevant

Tooth Root Structure
Fig. 12-15. Effect of the depth of embedding a post on its retentive capacity. (Data from Standlee JPet al: J Prosthet Dent 39:401, 1978.)
Tooth Root Perforation

Fig. 12-16. Faciolingual longitudinal sections through a maxillary central incisor. A, With a post of the correct length, a force (F) applied near the incisal edge of the crown will generate a resultant couple (R). B, When the post is too short, this couple will be greater (R'), leading to the increased possibility of root fracture.

Fig. 12-16. Faciolingual longitudinal sections through a maxillary central incisor. A, With a post of the correct length, a force (F) applied near the incisal edge of the crown will generate a resultant couple (R). B, When the post is too short, this couple will be greater (R'), leading to the increased possibility of root fracture.

only if the post fits the root canal properly, because retention is proportional to the total surface area.

Circular parallel post systems are only effective in the most apical portion of the post space because the majority of prepared post spaces demonstrate considerable flare in the occlusal half. Similarly, when the root canal is elliptical, a parallel-sided post will not be effective unless the canal is considerably enlarged, which would significantly weaken the root unnecessarily (Fig. 12-14).

Post Length. Studies have shown that as post length increases, so does retention. However, the relationship is not necessarily linear (Fig. 12-15). A post that is too short will fail (Fig. 12-16), whereas one that is too long may damage the seal of the root canal fill or risk root perforation if the apical third is curved or tapered (Fig. 12-17). Absolute guidelines for optimal post length are difficult to define. Ideally, the post should be as long as possible without jeopardizing the apical seal or the strength or integrity of the remaining root structure. Most endodontic texts advocate maintaining a 5-mm apical seal. However, if a post is shorter than the coronal height of the clinical crown of the tooth, the prognosis is considered unfavorable, because stress is distributed over a smaller surface area, thereby increasing the probability of radicular fracture. A short root and a tall clinical crown present the clinician with the dilemma of having to compromise the mechanics, the apical seal, or both. Under such circumstances, an apical seal of 3 mm is considered acceptable.

Apical Seal
Fig. 12-17. A, Correct post length. B, The post is too short; the consequences are inadequate retention and increased risk of root fracture. C, The post is too long, jeopardizing the apical seal.

Post Diameter. Increasing the post diameter in an attempt to increase retention is not recommended because it may unnecessarily weaken the remaining root. Although one group of investigators reported that increasing the post diameter increased retention, other groups do not confirm this. Empirical evidence suggests that the overall prognosis is good when post diameter does not exceed one third of the cross-sectional diameter of the root.

Post Surface Texture. A serrated or roughened post is more retentive than a smooth one," and controlled grooving of the post and root canal (Fig. 12-18) considerably increases the retention of a tapered post.

Luting Agent. When considering traditional cements, the choice of luting agent seems to have little effect on post retention or the fracture resistance of dentin.34 However, adhesive resin luting agents (see Chapter 31) have the potential to improve the performance of post-and-core restorations; laboratory studies have shown improved retention. Resin cements may be indicated if a post becomes dislodged. Resin cements are affected by eugenol-containing root canal sealers, which should be removed by irrigation with ethanol or etching with 37% phosphoric acid if the adhesive is to be effective. 37 Zinc phosphate and glass ionomer have similar retentive properties-polycarboxylate and composite resin have slightly less .38 Some resin and glass ionomer cements have demonstrated significantly higher retention in comparison to hybrid cements. Although the choice of luting agent may become more important if the post has a poor fit within the cana1, a post-and-core should be remade if any rotation or wobble is present.

Posterior Teeth (Fig. 12-19). Relatively long posts with a circular cross section provide good retention and support in anterior teeth but should be avoided in posterior teeth, which often have curved roots and elliptical or ribbon-shaped canals. For these teeth, retention is better provided by two or more relatively short posts in the divergent canals.

When amalgam is used as the core material, it can be condensed either around cemented metal posts or directly into short, prepared post spaces. If more than 3 to 4 mm of coronal tooth structure remains, use of the root canals for retention is not necessary, and this avoids the chance of perforation. Using the canals for retention can provide good results; z although the strength of the tooth once a complete

Teeth Structure

Fig. 12-18. Effect of horizontal grooving on the retention of tapered posts. NS, Not significant. (Modified slightly from Wood WW: I Prosthet Dent 49:504,1983.)

Fig. 12-18. Effect of horizontal grooving on the retention of tapered posts. NS, Not significant. (Modified slightly from Wood WW: I Prosthet Dent 49:504,1983.)

Fig. 12-19. When preparing posterior teeth for intra-coronal retention, the practitioner must be careful to avoid perforation, especially on the distal surface of mesial roots and the mesial surface of distal roots, where residual tooth structure is normally thinnest (arrozvs).

additional retention, which was compromised because of the missing tooth structure. In mandibular molars, the larger distal canal is recommended for post placement. In maxillary molars, the palatal canal is used (see Fig.12-2, C and D).

Although it is possible to restore a molar with three or more missing cusps with multiple posts and amalgam, the tooth's overall importance must be assessed. If retaining the tooth is critical, a cast core can be used (made in sections that have different paths of withdrawal) (Fig. 12-20). An alternative preparation method for a posterior tooth is selecting the canals that are widest (normally the palatal of maxillary molars and the distal of mandibular molars) for the major post and then preparing short auxiliary post spaces in the other canals with the same path of withdrawal (Fig. 12-21).

crown has been provided is not dramatically influenced by differences in technique.

Mandibular premolars and molars with a reasonable amount of remaining coronal tooth structure, when coupled with a circumferential cervical band of tooth structure with restricted taper of about 2 mm, can often be restored with amalgam directly condensed into the chamber. Core buildups in molars with one or more missing cusps will benefit from one or more cemented posts around which the amalgam can be condensed. The posts provide the

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