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Post and Core: Clinical Factors
Howard P. Fraiman, DMD
After root canal therapy is completed on a tooth, several treatment options exist to restore the tooth’s form and function. The treatment choices include restorations that range from root canal access fillings to full-coverage restorations. Many times, in order to complete a full-coverage restoration, a post and core is necessary to provide structure. For a crown to be successful, there should be enough existing tooth structure to provide retention and resistance form, and enough sound tooth structure at the margin to create a ferrule effect and provide an adequate seal of the crown margin. When the structure does not exist to ensure retention and resistance form, then a core restoration or post-and-core restoration is required to replace missing tooth structure.
There needs to be enough remaining tooth structure to retain the core restoration, taking into account that the tooth will be prepared for a full-coverage restoration. When adequate retention does not exist for the core restoration, a post is used. It is important to remember that the purpose of the post is to retain the core restoration only. The post will not strengthen the tooth but rather weaken it by potentially leading to fractures within the remaining tooth structure. The exception might be the use of a post in a tooth that has a thin remaining coronal structure at risk of fracture. In this specific situation, it could be argued that a rigid post is actually preventing the thin coronal structure from fracturing.
When a post and core is necessary, many factors are involved in determining the type used and how the procedure is completed. One of the oldest and most predictable techniques involves the use of an indirect one-piece post and core. This type of post and core is typically cast from a metal, which is frequently a precious metal such as gold. The pattern is created directly in the mouth using either a resin or wax pattern. After the canal is prepared for the post, a starter post is used in the canal and surrounded by the pattern material to create an intimately fitting post. Then, the coronal section is created in the pattern material and shaped to the desired contour. The entire post-and-core pattern is removed from the tooth. This is then cast into a one-piece post and core and cemented to the tooth.
The advantages of this type of post and core include the fact that the post will fit intimately into the canal space even if the canal is not round. The fact that the post and core is one piece ensures that the core cannot break away from the post. However, the indirect post and core requires multiple patient visits and involves the use of a nonesthetic metal material that may show through ceramic restorations. The current trend is using a direct one-visit post-and-core technique.
A direct post and core involves placing the post-and-core restoration in one visit, using a prefabricated post. These posts come in several forms: parallel-wall smooth cylindrical shape, parallel-wall screw type, and screw type. The posts are made from stainless steel, titanium, composite, and fiber. Prefabricated posts can be divided into two basic categories: passive and active. A passive post does not impart any stress on the root on insertion, whereas an active post stresses the root on insertion.
The use of a post has been associated with fractures and ultimate failure of the tooth: This occurs in many more teeth with posts than without. With that in mind, it must be considered that the purpose of the post is to retain the core while minimizing the risk for fracture of the tooth. The parallel-walled passively fitting post is believed to minimize the chance of fracture versus a screw-type post that is actively screwed into a tooth and places forces on the remaining tooth structure, increasing the risk of fracture. When selecting a post, one must remember that roots taper as the canal preparation moves apically. Care should be exercised to not overprepare the apical portion and to conserve as much tooth structure as possible.
Once the post type has been selected, the technique involves preparing the root canal space and cementing the post, followed by the core buildup. The depth of the post preparation should be based on the length of the root canal and the amount of root structure in bone. Ideally, the post should be placed to a length that is at least half of the remaining root structure in bone while retaining 3 mm to 5 mm of gutta percha at the apical area. While all canals are not spherical, the post space will be prepared to a round shape without overpreparation of the root, which can lead to root fracture. Consequently, it is important that the depth and width of the canal are prepared to allow the prefabricated post to fit the prepared space as closely as possible. The post is cemented in the post space using an adhesive-type cement.
This process has improved as technology and research have advanced the dental cements available. Once, posts were cemented with zinc phosphate cement; now, an adhesive-type cement is used, which increases the retention of the post and strengthens the entire system. While metal posts can be cemented with various cements, it is advantageous to bond fiber posts within the canal because this increases the system’s strength.
Other improvements in post systems include the material from which the post is made. Traditionally, posts have been fabricated using different metals; however, current technology is allowing the fabrication of posts from resin and fiber materials. These posts allow cements to be cured through the posts themselves, are more esthetic, and can more closely replicate the modulus of the tooth’s elasticity. In addition, some tout the flexibility of the post system.
In any situation, the key to success in a post-and-core restoration revolves around certain essential factors. The post should passively fit in a well-prepared proper-length post preparation that conserves the most tooth structure possible. The post space should be cleaned and dried, and the post properly cemented with an adhesive system. The core should be built up to meet the requirements of the preparation of the tooth being restored. Even with a tooth being restored with a post and core, critical to the success of the full-coverage restoration is ensuring an adequate ferrule exists on the remaining sound tooth structure. While the post and core is important for retention of the full-coverage restoration, ultimately the remaining tooth structure will determine the success of the restoration.
Post and cores have many variables in terms of which systems, materials, and techniques are used. Research supports varying techniques and is somewhat ambiguous in determining what the ideal post and core would entail. This is understandable because, ultimately, the post and core will be determined by each clinical situation that is presented. The dentist can use the information available about the techniques and materials on hand to create the post and core that will best help retain the full-coverage restoration and minimize detrimental forces on the tooth. As materials continue to develop, our ability as dentists to create better post-and-core systems will improve.
About the Author
Howard P. Fraiman, DMD
Assistant Clinical Professor
Department of Periodontics
University of Pennsylvania School of Dental Medicine