Introduction
A post and core is a dental restoration used to sufficiently build-up tooth structure for future restoration i.e crown when there is not enough tooth structure to properly retain the crown
Dowel- a post, usually made of metal that is fitted into a prepared root canal of a tooth that has had endodontic therapy
Core-The core is the part of the restoration that shows out in the mouth that help anchor a cap or crown.
Objectives of post and core treatment-
The main objective of the post-and-core is to replace the missing coronal tooth structure and to provide the retention and resistance form for the final crown. The main function of the post is to anchor the post-and-core complex within the radicular portion of the remaining tooth. The post does not reinforce the root, nor does it extend any strength to the fragile remaining dentin. Therefore, it is important to select a post system that provides maximum retention, yet removes as little as possible of the remaining subgingival tooth structure.
Basic Principles in the Restoration of Endodontically Treated Teeth -
The preponderance of literature supports the following guiding principles: n Posterior teeth with root canal treatment should receive cuspal coverage restorations. Bonded restorations, once thought to obviate the need for cuspal coverage, provide only short-term strengthening of the teeth, according to recent studies. Anterior teeth with minimal loss of tooth structure can be restored conservatively with bonded restorations. Preservation of coronal and radicular tooth structure is desirable. The purpose of a post is to retain core buildup. A ferrule is highly desirable when a post is used. An adequate ferrule is considered a minimum of 2 mm of vertical height and 1 mm of dentin thickness.
The Ferrule Effect-
The successful rehabilitation of any endodontically treated tooth using the post-retained system requires the consideration of one specific structural design characteristic: the ferrule effect. The stability of the crown is influenced by the preparation design for endodontically treated teeth. Preserving tooth structure during preparation is paramount in preventing stress concentrations at the cementoenamel junction of the endodontically restored tooth and provides resistance to tooth fracture. In cases where there is insufficient sound tooth structure for a ferrule design, it is necessary to obtain this dimension through periodontal crown lengthening and/or forced-tootheruption procedures
TYPES OF POSTS
Posts are categorized a number of different ways. For the purpose of this review, they will be classified as active or passive, parallel or tapered, and by material composition
Most active posts are threaded and are intended to engage the walls of the canal, whereas passive posts are retained strictly by the luting agent. Active posts are more retentive than passive posts, but introduce more stress into the root than passive posts . They can be used safely, however, in substantial roots with maximum remaining dentin . Their use should be limited to short roots in which maximum retention is needed.
Parallel Versus Tapered Posts -Parallel-metal posts are more retentive than tapered posts and this also is reported tobe true for fiber posts . Parallel posts induce less stress into the root, because there is less of a wedging effect, and are reported to be less likely to cause root fractures than tapered posts. Tapered posts, on the other hand, require less dentin removal because most roots are tapered. They are primarily indicated in teeth with thin roots and delicate morphology
Prefabricated Post and Cores- Prefabricated posts are typically made of stainless steel, nickel chromium alloy, or titanium alloy. They are very rigid, and with the exception of the titanium alloys, very strong. Because they are round, they offer little resistance to rotational forces. This is not a problem if adequate tooth structure remains, but if minimal tooth structure remains, antirotation features must be incorporated into the post preparation with slots or pins. A bonded material should be used for the core
Custom Cast Post and Cores- Cast post and cores were the standard for many years and are still used by some clinicians. They have fallen from favor because they require two appointments, temporization, and a laboratory fee. A cast post and core may be indicated when a tooth is misaligned and the core must be angled in relation to the post to achieve proper alignment with the adjacent teeth. Cast post and cores also may be indicated in small teeth such as mandibular incisors, when there is minimal coronal tooth structure available for antirotation features or bonding. Cast post and cores are generally easy to retrieve when endodontic retreatment is necessary. Perhaps the biggestdisadvantage for cast post and cores is in areas that require an esthetic temporary restoration
Ceramic and Zirconium Posts One factor that has reduced the use of metal posts is esthetics. Metal posts are visible through the more translucent all-ceramic restorations and even with less translucent restorations may cause the marginalgingiva to appear dark. These concerns have led to the development of posts that are white and/or translucent. Among the materials used for “esthetic” posts are zirconium and other ceramic materials. Zirconium posts can not be etched, therefore, it is not possible to bond a composite core material to the post, making core retention a problem . Retrieval of zirconium and ceramic posts is very difficult if endodontic retreatment is necessary or if the post fractures. Some ceramic materials can be removed by grinding away the remaining post material with a bur, but this is a tedious and dangerous procedure. It is impossible to grind away a zirconium post. For these reasons, ceramic and zirconium posts should be avoided.
Fiber Posts Carbon fiber posts gained popularity in the 1990s. Their main proposed advantage was that they were more flexible than metal posts and had approximately the same modulus of elasticity (stiffness) as dentin. When bondedin place with resin cement, it was thought that forces would be distributed more evenly in the root, resulting in fewer root fractures.
Preparing post space-
A number of authors make recommendations about post length. A review article by Goodacre and Spolnik recommends post length equal to 3⁄4 of root canal length, if possible, or at least equal to the length of the crown. They caution that 4 to 5 mm of gutta-percha should remain apically to maintain an adequate seal. According to traditional teachings, a minimum of 3 to 5 mm of gutta-percha should remain in the apical portion of the root to maintain an adequate seal. A recent study by Abramovitz et al. (100) demonstrated that 3 mm of gutta-percha provides an unreliable apical seal, therefore, 4 to 5 mm is recommended.
Luting Cements Any of the current luting cements can be used successfully with a post if the proper principles are followed. The most common luting agents are zinc phosphate, resin, glass ionomer, and resinmodified glass-ionomer cements.
CONCLUSIONS- If certain basic principles are followed in the restoration of endodontically treated teeth, it is possible to achieve high levels of clinical success with most of the current restorative systems. These principles include: 1. Avoidbacterial contamination of the root-canal system 2. Provide cuspal coverage for posterior teeth 3. Preserve radicular and coronal tooth structure 4. Use posts with adequate strength in thin diameters 5. Provide adequate post length for retention 6. Maximize resistance form including an adequate ferrule 7. Use posts that are retrievable.
References-
1) Restoration of endodontically treated teeth;the endodontist’s perspective, part 1 2004
2) Post-and-cores: past to present; international dentistry sa vol. 12, no. 2
3) Prefabricated post and core material versus custom-cast post and core in a maxillary first premolar tooth: review of literature and management of a clinical casecairodentaljournal, 14 (1) : 23-26,1998.
4) Post Placement and Restoration of Endodontically Treated Teeth: A Literature Review, Journal of endodontics, Vol. 30, No. 5, May 2004