Chapter 18 Esthetic Posts
The increasing predictability and popularity of endodontic procedures, attributable in no small measure to the decreased discomfort associated with the process, have encouraged patients to treat and maintain their dentitions for their entire lifetimes. Largely responsible is a very successful long-term public education process by the dental profession. Increased endodontic coverage by insurance carriers also motivates more patients to choose restoration rather than extraction. In today’s information-based society, patients are more aware of dental treatment options and more actively involved in co-diagnosis and co–treatment planning. Their concerns include the biocompatibility of the post-endodontic restorative materials (posts, cores, and cements), the invasiveness of the restorative procedure, and, most important, the overall functional and esthetic result.
Dental professionals’ experience over the past half-century has confirmed that most, if not all, endodontically treated teeth require a comprehensive restoration such as a full crown or onlay to restore the tooth structure destroyed by decay, fracture, or endodontic access. The clinical aspects of the restoration, given the variety of post and core materials along with the remaining tooth structure, can be a challenge. Endodontically treated teeth seen in a dental practice have lost appreciable amounts of coronal tooth structure to caries and/or the access preparation. The objective of the post and core buildup is primarily to replace the missing coronal tooth structure which will provide retention and resistance for the crown that will ultimately restore the tooth’s function and esthetics. Although some controversy surrounds the absolute need for post and core treatments, the issue can be reduced to mechanical terms. When much of the coronal tooth structure remains, a post may be indicated but not required. When the remaining root exhibits little or no remaining coronal tooth structure, the foundation provided by the post and core buildup is an absolute precondition for crown preparation. The post anchors the restoration to the remaining radicular dentin without necessarily strengthening the root.
The prognosis is often directly proportional to the bulk of the remaining dentin: the greater the remaining dentin thickness, the greater the fracture resistance. Posts are selected to provide maximal retention for the overlying restoration while minimally invading remaining dentinal tissue. Adhesively bonded posts increase the retention of the post and core system and improve the restored tooth’s prognosis. Bonded non-metallic posts also tend to distribute functional stresses over larger internal radicular surfaces, decreasing the force per area of root and thus the possibility of root fracture. The core platform is the accessible and visible supragingival extension of the post that supports the crown. Because the core is the physical link between the remaining subgingival dentin and the overlying crown, its shape and position are critical in managing the direction and magnitude of forces transferred to the remaining tooth. The core material may be exposed through partially translucent or ceramic crowns, and thus the ideal core coloration is the dentinal shade.
The post-endodontic complex forms a monobloc which comprises the multi-layered tooth-to-restoration structure with no inherently weak interlayer interfaces. Sequential bonding of the dentin to the post resin cement, to the post, to the core, to the crown resin cement, and to the crown is critical. The adhesive strength at each interface must be greater than the bond of the natural tooth to itself. Successful treatment offers strength and resistance to the post-endodontic continuum from the residual dentin to the final restoration that approach the strength and resistance of the original non-decayed tooth.
Post and core materials are divided into three separate classes: post material, core material, and cement. The criteria that dental professionals must use for determining which materials to use are based mostly on scientific data, the dentist’s clinical experience, and, to a lesser extent, patient preference. The first two would seem to be self-evident. The material must work in the realm of scientific predictability and it must be successful clinically in terms of time spent, results achieved, and comfort of both patient and practitioner. Patient preference is largely a matter of esthetics. As more and more all-ceramic restorations find their way into dental treatment, the nature of the sub-structure that supports the crown is increasingly relevant to the ultimate esthetics of the final restoration. If metal shows through a tooth-colored ceramic and creates a shadow on the surface of the restored tooth, the esthetics of the restoration will not be acceptable. Therefore, of all the criteria for post and core treatment, esthetics is among the most important.
Dental posts first began as gold pins inserted into teeth, possibly at considerable pain to the patients. They were used about 2500 years ago in the Etruscan lands of ancient Italy. These retentive pins were inserted into the radicular canals, which were probably untreated, and anchored carved ivory chunks to restore the patient’s function and appearance. Over the past 150 years or so, cast gold has played a significant role in restoring endodontically treated teeth. Gold posts were cast using a lost wax process and fit the post space more or less accurately. About 30 years ago, the prefabricated stainless steel post was introduced. It was stronger and easier to fit, in some respects, than gold. The titanium post, manufactured from an even stronger material, was next to be used in dentistry. Subsequently, the carbon fiber post was introduced, noted less for its strength than its ability to be slightly bendable. However, carbon fibers have the black coloring of carbon and were not esthetically acceptable. They were relatively quickly replaced by various glass and fiber rod posts. Core materials have also progressed from gold to amalgam to glass ionomer, and finally to composite resin materials. Cements began with zinc phosphate materials, and have been relatively unchanged until recently, when composite resin cements became available.
Typically, the post becomes virtually invisible when it is covered by a porcelain-fused-to-metal crown. Areas that may pose a visible liability include around the gingival margin where a shine-through of the metal—gold, stainless steel, or titanium—through thin dentinal root walls and thin gingival attached tissue covering the bone on the buccal of anteriors, can give the entire tooth a grayish appearance. This is particularly problematic when patients have high lip lines and tend to smile a lot.
An esthetic post should assume the same coloration as the underlying dentin, ideally becoming indistinguishable. Thus, whether working with a porcelain-fused-to-metal or a ceramic crown, there is no shine-through. The only shade that is actually visible is of the same hue as the dentin and blends into the tooth structure. A slight discrepancy is easy to cover up with current ceramic restoration technologies. It is also very important to use a cement that is either dentin colored or preferably translucent when developing the esthetics of the post and core. Translucent cement materials allow the underlying tooth coloration to shine through and to blend with the ceramic margin. Tinted cements often create the potential for an additional color mismatch that further complicates the marginal esthetics of the restoration.
The function of the post and core as the intermediate restoration between the remaining root and the crown depends on a strong adhesion among the various restored components. The dentin adheres to the cement, the cement adheres to the post, the post adheres to the core, and the core adheres to the crown. All adhesive strengths exceed the natural adhesive strength of the tooth to itself. With an adhesively retained post and core, and subsequently an adhesively retained crown, the tooth can actually be as strong as the natural dentition prior to any decay. Earlier cements such as zinc phosphate and polycarboxylate provided no bonding strength to any of the substrates to which they attached—enamel, dentin, metal, or ceramic. Resin cements attach adhesively to all of these materials effectively.
Esthetic posts are clinically similar to and generally more conservative of tooth structure than other prefabricated posts. They can be used for virtually any post-endodontic situation. They are more conservative and easier to place than cast metal restorations and typically require one chairside appointment versus a minimum of two sessions for indirect procedures.
There are many posts available to the dental practitioner, as well as many core materials and cements. The best way to evaluate the most suitable material in each category is to review the scientific data, check the research support, and evaluate the materials. The dentist’s clinical experience, including ease of clinical use, predictability, and ease of placement, are significant factors as well. The patient, of course, expects and should be receiving a long-lasting solution for their post-endodontic condition that is essentially better, faster, and easier than the other procedures available.
Cast metal posts (Box 18-1), whether in gold or base metal, have certain advantages. They are laboratory fabricated, eliminating chairside technique sensitivity. Cast as metal objects, they have great transverse strength and are very unlikely to break. The negatives of cast metal posts include difficulty taking impressions; it is hard to place hydrophobic impression materials into the narrow confines of the post-endodontic canal and to ensure the absence of air or water bubbles. The pour of the impression at the dental laboratory is perhaps even more difficult; it is virtually impossible to orient the direction of the post precisely when pouring the stone impression, so the angulation of the post with respect to the tooth and/or core is often slightly off the mark. This minute discrepancy can very easily compromise the fit of the entire post and core complex.
Box 18.1 Pros and Cons of Cast Metal Post
Most dentists will identify with the difficulty of seating the cast metal post and core into the residual root. In fact, adjustments are often needed either on the restorative material or on the remaining tooth structure in order to actually develop an acceptable fit. However, the great advantage of the cast metal post is its supposedly tight fit into the remaining canal system. Therefore, in modifying this tight fit in order to be able to seat the restoration, the major advantage of the cast metal post is eliminated. The increased cost of the cast metal post arises from the expenses of the technician’s work and transportation to and from the laboratory. If gold or similar precious metals are used, the cost to the dentist and patient can be even higher. The greatest problem of cast metal posts is that they are typically manufactured of noble or non-reactive materials and thus do not bond to the tooth structure—either enamel or dentin. Furthermore, these materials do not bond to the overlying crown through the cement. All the interfaces in the cast metal post system are luting interfaces which provide a filling of the space between materials, but no adhesion.
Prefabricated metal posts (Box 18-2) have the advantage of being inserted chairside, thus eliminating the need for a second appointment and incremental laboratory and transportation costs for the practitioner. The metal itself has good transverse strength. Unless the prefabricated metal post is abused during or after insertion, it is unlikely to break or bend during clinical use. More than 30 years of clinical experience with these materials have generally been very positive.
Box 18.2 Pros and Cons of Prefab Metal Post
The negative aspect is that the post hole must be made to fit the shape of the post. Because this fit is an approximation at best, the adaptation between the remaining tooth structure and the post itself is not precise. The intervening space is ultimately filled by cement, typically less strong than the post. The prefabricated metal does not bond to either the underlying tooth structure or the overlying core or crown. Prefabricated metal posts tend to have the same poor esthetics as cast metal posts. They cast a gray shadow through esthetic restorations if there is not enough masking material in the core cement or the ceramic to overcome this esthetic liability.
Resin fiber posts (Box 18-3) were introduced to the dental profession in the early 1990s. The advantages of these posts include the fact that they are placed chairside in a single appointment and they bond to the underlying tooth structure whether enamel or dentin, to the overlying core, and subsequently to the overlying crown if suitable resin cements and techniques are used. They have excellent transverse strength; they are composed of many fibers that are bundled together with BIS-GMA, the basic component of composite dentistry. The fibers tend to bend under load rather than break. As they bend, they also act as shock absorbers. This means that as forces are placed on the crowned tooth, the underlying post can actually absorb most of the shock rather than transmitting it to the remaining tooth structure. Resin fiber posts are available in a variety of colors. The earliest ones were black (carbon fiber), very well researched, and highly regarded by the profession but posed an esthetic liability in visible anterior areas. The more recent ones are tooth colored, white, or translucent, making them much more adaptable to an esthetic objective.
Box 18.3 Pros and Cons of Resin Fiber Post
Data from Mannocci F, Ferrari M, Watson TF: Intermittent loading of teeth restored using quartz fiber, carbon-quartz fiber, and zirconium dioxide ceramic root canal posts, J Adhes Dent 1:153-158, 1999.
The advantages of the cast metal core (Box 18-5) are that it is laboratory fabricated and involves little chairside work for the dentist. The thickness of the core provides great transverse strength to this part of the restoration. It is extremely unlikely to ever fracture. The cast metal post and core are actually cast as a single unit, together providing a very strong substructure for restorations. The major disadvantage of the cast metal core is its esthetic appearance under ceramic crowns. The bright yellow of the gold metal core or the darkness of the base metal shine through ceramic restorations and making esthetic results rather difficult. The color is quite intense and the porcelain thickness is limited. Masking cements are difficult to use when the discoloration is intense, s/>