Re: Cast post and core

[sushantpatel_doc]

Factors in Restoration Decision Making
In a case where the tooth has an access preparation only, no post is necessary. The final restoration can be an adhesive composite resin. For premolars and anterior teeth when an access and only one proximal surface is compromised, an adhesive restoration should be sufficient with the composite resin expending into the pulp chamber. Also, there are other factors that contribute to success when restoring the endodontically treated tooth. It has been demonstrated that endodontically treated teeth with two proximal contacts have substantially better survival than teeth with fewer than two proximal contacts. Also, alveolar bone loss from periodontal disease may lead to a decreased risk of endodontically treated tooth survival.

In cases where significant tooth structure is missing, consideration should be given to restoring the coronal portion of the tooth as a foundation for a full-coverage crown. The restoration of the coronal portion of the tooth can be with a direct restorative material, amalgam or composite resin, or an integrated cast post-and-core. Studies have shown a strong correlation between crown placement and survivability of endodontically treated teeth where tooth structure has been compromised. Overall survival rates of endodontically treated molars without crown placement at 1, 2, and 5 years had survival rates of 96%, 88%, and 36% respectively. In a University of Iowa study evaluating 600 endodontically treated teeth, teeth that were not crowned had a six times greater rate of loss than endodontically treated teeth that were crowned.

In consideration of the crown preparation, there needs to be sufficient tooth structure remaining to create a ferrule of at least 1.5 mm to 2 mm. Ferrule refers to an extension of the tooth preparation beyond the core restorative so that there is a continuous ring at least 1.5 mm to 2 mm in length of sound tooth structure 360° around the crown preparation. This ensures that the tooth has a rigidity to resist coronal leakage and fracture. This stiffness of the ferrule will ensure clinical success. An evaluation of the remaining dentin thickness at the cementoenamel junction (CEJ) is important to determine if there is sufficient tooth structure to support a crown. In the case of a thin-walled root canal, it may be necessary to plan a porcelain-metal crown to conserve the remaining tooth structure when compared to an all-ceramic crown that requires a minimum 1-mm shoulder preparation at the CEJ. If an adequate ferrule is not developed in the crown preparation, the tooth and core are at risk of fracture (Figure 6A through Figure 6C).

While posts are used extensively in the restoration of the endodontically treated tooth, their use should be limited to those clinical cases in which the core needs to be retained by the post. For endodontically treated molars, there is usually sufficient tooth structure remaining and the presence of the internal walls of the pulp chamber to retain a core. The use of a post is usually not necessary for molars. If a post is necessary because of significant loss of tooth structure, the consideration should be that the post is placed in the straightest, largest canal. Small-diameter, curved canals are at risk for perforation during post placement. Post placement should be in the distal canal of a mandibular molar and palatal canal of a maxillary molar.

In some cases, the root canal is enlarged. This may be because of the need for endodontic treatment of a young anterior tooth because of trauma (Figure 7), a tooth that has had the root canal enlarged during endodontic treatment, or any tooth that has significant caries in the pulp chamber. For the young child, maintaining the tooth is critical and in some cases an apexification procedure is necessary. For an adult patient, treatment recommendations for an enlarged root canal or caries in the pulp chamber should include replacement with an implant. For the young, immature tooth where there is an enlarged root canal and the root and crown are at risk of fracture, an adhesive-resin cement can be used for root reinforcement. In a widely flared canal, the use of a clear, light-transmitting post with a fourth-generation etch-and-rinse adhesive and a light-cure composite is recommended. The technique with the light-transmitting post involves restoring the enlarged, overprepared root canal with an adhesive restorative composite resin with the post placed centered in the root canal. The post extends 2 mm from the canal and is light-cured at right angles for 1 to 2 minutes. The post is removed leaving a composite resin “canal” that can then be restored with either a prefabricated post or a custom-cast post-and-core. The composite reinforces the overflared canal (Figure 8A and Figure 8B).

Although there have been trends to use prefabricated post systems, there are specific clinical situations where a morphological, custom-cast post-and-core with a high-noble alloy would be beneficial. A cast post-and-core is the restoration of choice for clinical circumstances where there is limited tooth structure available to develop a ferrule when preparing the tooth for a crown or when there is insufficient tooth structure available to create an anti-rotational design to the core on a prefabricated post. Mandibular incisors, maxillary incisors, and canines that have minimal amounts of coronal tooth structure remaining are good candidates for a custom, morphological cast-gold post and core. There are two techniques that can be used when fabricating a custom-made, laboratory-fabricated post. A custom-made post can be fabricated directly in the mouth on the tooth preparation using a plastic post that is burnt-out during the metal-casting or ceramic-pressing process. A more predictable technique uses an impression of the tooth preparation and the root canal (Figure 9A and Figure 9B). This saves chairtime. The impression is poured by the laboratory and casting patterns are made on the master cast by the laboratory technician for casting in either metal or ceramic (Figure 10). The final custom-made post is then cemented into the root canal (Figure 11).

Risk of Root Fracture
A major risk when using post systems to restore endodontically treated teeth is that of vertical root fracture. A review of the literature reveals that vertical root fractures most frequently occur in posterior teeth in patients between ages 45 and 60. The average elapsed time between the endodontic treatment and the subsequent diagnosis of vertical fracture was found to be approximately 10 years. The evidence and symptoms most often found are mild pain in the area of the fractured tooth often accompanied by swelling and fistula, along with a deep pocket in just one area of the attachment surrounding the tooth. Radiographic evidence of the fracture demonstrates a radiolucent area at the site of the fracture. The sign most often revealed by x-ray is a radiolucent peri-radicular band. The major etiologic factors that contribute to vertical root fracture are metal-post placement and over-enlarged root canals during endodontic treatment and restoration with a metal post.

In recent years, there has been major interest in fiber-reinforced composite posts. In vitro studies have demonstrated that a metal post transmits energy down the post to the apical end of the post within a root canal leading to catastrophic fracture. When a fiber-reinforced post is bonded within the root canal it dissipates functional and parafunctional forces reducing the stress on the root. In these cases, the post or crown will fracture instead of the post transmitting the energy of force down the root creating a vertical root fracture.