April 1, 2001
by Edward Lowe, BSc., DMD
Resurrection of endodontically treated teeth has traditionally involved a post, core, and crown.1 The use of a cast metal post and core has been the standard procedure taught in the curriculum of nearly all the dental schools. With the advent of pre-fabricated threaded posts and parallel-sided post systems, the use of the cast post in clinical situations is diminishing.2
The popular demand for esthetic materials and the ongoing advancements in metal-free material technology have allowed us the luxury of incorporating these progresses into endodontic posts. The metal free post should offer a flexure modulus similar to that of dentin, adhesive properties, and aesthetic advantages (Figs. 1 & 2). The bonded post and core has contributed to both the cast and pre-fabricated metal system’s decline in popularity. One of the weaknesses of a metal endodontic post is the frequency of root fractures3 and the decementation of the post,4 which continues to occur at an unacceptable clinical rate.
The dark appearance of the CEJ and the prescence of unesthetic dark soft tissue apical to endodontically treated sites has also been attributed to the use of metal posts. The use of cast post and cores is often both time consuming for the dentist and expensive for the patient.5
Numerous studies have confirmed that a post placed in an endodontically treated tooth does not strengthen the tooth but is used as a means to retain a core or foundation.6,7 This core, in turn, provides adequate retention for the final restoration.
The purpose of this article is to review the criteria for restoring an endodontically treated tooth. A look at the various metal free post systems and a clinical example follows.
The moisture content of coronal and root dentin in endodontically treated teeth is histologically different than vital teeth.8 It remains to be seen whether these differences contributes to tooth brittleness.9
Some controversy exists as to whether endodontic therapy itself causes the tooth to become more brittle. It’s possible that the active tooth structure removal in endodontic access preparation and post preparation weakens and compromises the strength of the tooth making it more susceptible to fracture.10 The amount of remaining tooth structure and how it is treated directly affects its mechanical properties and structural integrity.11
The level of the occlusal force placed on endodontically treated teeth during function is often not detected due to a decrease of proprioceptive response. This factor is especially significant in posterior teeth where the forces of mastication and cuspal interdigitation are magnified, exposing them to the increased possibility of fracture.12
The relative amounts of tooth reduction in a full crown preparation, both anterior and posterior, involves additional aspects in comparison to vital teeth. The bulk of remaining dentin used to retain the crown may be minimal when the presence of the access cavity and any existing restorations are considered.
It is prudent to visualize the possibility of a complete fracture of the crown at the base of the tooth occurring if a dentin conserving crown or veneer preparation does not incorporate a margin design which minimizes the removal of valuable supportive dentin.
Preliminary analysis of periapical radiographs is compulsory in the restoration of endodontically treated teeth. The root length and shape, amount of tooth structure lost, periodontal status, and quality of endodontic treatment all contribute to the success or failure of the final outcome.
Post placement should be as long as possible for stress distribution and better retention along the dentin. However, the position of the post within the root plays a major role in the long term outcome.
Placement of posts into the curve of the root or into thin roots mesial-distally may lead to fracture. Placement of long posts into short roots may result in the disruption of the apical root canal seal. Placement of long posts in roots periodontally compromised and lacking proper bony support causes the concentration of forces on the tooth to be focused at the apex of the post. This may result in root fracture and possible tooth loss.13
In order to prevent wedging of the post during occlusal function, half of its length should be placed in a root surrounded by alveolar crestal bone. This is provided that the nature of the root morphology and the quality of the root canal filling has been assessed.
One of the goals of root canal treatment is to fill three-dimensionally, to the apical extent of the root canal system. To achieve this, 4 to 5 mm of the root canal filling material should be retained in the apical portion. Any less than this amount may predispose the apical seal to leakage from accessory canals, undetected canal exits, or apical resorption.
If there is not sufficient length for a 4 mm apical gutta percha seal, retreatment of the canal may be warranted. If the criteria of adequate root length for apical seal or post retention is not met, the clinician may have to consider treatment alternatives.14
It is no longer necessary to wait for extended periods of time following root canal treatment to restore the tooth. The time interval between end treatment and post placement should be minimized in order to prevent microleakage and the potential of endodontic failure.15 The fracture of tooth structure which could render the tooth unrestorable is a distinct possibility.
A healthy gingival attachment complex consists of 2.5 to 3.0 mm between the restoration margin and the crest of bone. This measurement is called the biological width.16,17 Adequate tooth structure coronal to the restoration margins is needed in order to restore the tooth.
The importance of the ferrule effect has been well documented in the literature.18,19 A ferrule is an encircling collar of the metal or porcelain crown, which acts like a reinforcing band around the parallel walls of the dentin extending coronally from the preparation margin. A minimum of 1.5 mm of coronal tooth structure should be present apical to the post in order to resist dynamically loading and reduce stress concentration at the post and core junction.
Lack of available tooth structure can lead to leakage in the post space. The absence of a ferrule in the anterior region coupled with a low modulus composite resin core puts the tooth at risk when subjected to high shear forces, the forces of bruxism, or clenching.
Clinical evidence seems to indicate that the angle of incidence of occlusal forces applied to a tooth may be more influential than the amount of force itself. Heavy occlusal forces parallel to the long axis of the tooth are less damaging than lighter more inclined forces.20 This explains why more failures occur in anterior teeth restored with intraradicular anchorage when compared to endodontically treated posterior teeth.21
Posterior areas in the dental arch are subject to heavy occlusal wear and require cuspal coverage for enhanced long term results.6 However, closure of the endodontic access opening with a composite restoration is often all that is needed to restore intact anterior teeth; full coverage is not required: further esthetic enhancement can be easily achieved with tooth whitening or a conservative porcelain laminate veneer.
Post length should be as long as possible while maintaining an apical seal. Studies have shown that the longer the post is, the greater the probability of retention within the canal. It is safe to say that the length of the post extending into the root should equal the length of the crown.22
Post size should not exceed one third of the root diameter. Increases in post diameter do not increase post retention in the root, as the increased removal of the tooth structure to accommodate a larger post is generally accompanied by a proportional increase in stresses placed on the root. A 1 mm thickness of axial dentin measured from the inside of the canal wall to the outside surface of the ferrule seems to be the accepted standard minimum.
Increasing the thickness of axial dentin beyond 1 mm does not increase the strength of the restored tooth, however with thicker axial dentin the likelihood of failure due to post dislodgement was greater than that of root fracture.23
Direct Composite Plastic/Fiber Post and Core
In the last decade, several methods of direct fiber reinforcement of anterior endodontically treated teeth have been used. One of these involved using a clear plastic post (Luminex, Dentatus USA, NY) resin-bonded into a canal with dual cured resin cement.24 The theory was that the post would transmit light from the light curing unit down the post, thereby polymerizing the resin cement25 (Fig. 3).
Another popular method involves the resin cementation of woven polyethylene fibers (Ribbond, Seattle, WA) condensed tightly into the canal space with an endodontic plugger. Two separate strips of fibers are folded and adhesively placed in a canal. The excess extruding from the canal space is utilized to build a composite core.26
Yet another fiber material has been used to fulfil the requirements of endodontically treated teeth. This material consists of silicate E-glass ceramic fiber (Glasspan, Exton, PA) woven into different tube sizes.27 The tubes are cut to desired length, folded over, filled with flowable dual cured resin and introduced into the root canal space. Hybrid composite is used to build a core upon which the final restoration will sit.28
Pre-fabricated Zirconia Posts
The zirconium oxide all ceramic posts attract those who prefer a stiffer post and yet would like the esthetic advantages that a white post has to offer. Two of the posts, both conical in shape, that fit in this category are the Cerapost (Brasseler, Savannah, GA) and the Cosmopost (Ivoclar-Vivadent, Amherst, NY). The Cosmopost has an additional feature of being used alone in the preformed post and core buildup technique or it can be cast with a special IPS Empress Cosmo ingot to create a one piece cast ceramic post and core (Fig. 4).
Both posts are adhesively cemented with a dual-cure composite cement and dentin bonding agent.29 These posts may be difficult to remove if retreatment is necessary because of their dense ceramic composition. While the optical properties are attractive, these posts have a modulus of elasticity (~170 Gpa) which is closer to steel (~210 Gpa) than dentin (~20 Gpa).
Pre-fabricated Lab Processed Fiber Posts
An indirect post and core constructed with the fiber-reinforced, light — activated material Targis-Vectris (Ivoclar-Vivadent, Amherst, NY) falls into this category. The unidirectional longitudinal Vectris fibers demonstrate high tensile strength and tensile modulus, while the matrix exhibits durability. Chemically bonded, these components result in a material with enhanced functional and esthetic properties.
A polyvinylsiloxane impression of the endodontically treated tooth with a passive parallel sided post analog is sent to the laboratory for post fabrication. At the seat appointment, the silanated post and core is adhesively cemented with a dual-cure composite cement and dentin bonding agent.30
Pre-fabricated Direct Fiber Posts
Several fiber post systems fall into this category and their importance and popularity in the dental community has increased31 (Fig. 5). The carbon fiber posts were introduced commercially in 1990 and showed much promise. Their strong points are a lower modulus of elasticity (~120 Gpa) than steel posts (~210 Gpa) and a high fatigue resistance. They are also easily removed via endodontic retreatment.
The disadvantages of these posts include a lack of radiopacity and poor adhesion to composite resin cores. The modulus of elasticity is still significantly higher than that of dentin ( ~20 Gpa). While colour also limits esthetic expectations, coated posts like the Aestheti-Post (Bisco, Schaumberg, IL) offer a reasonable alternative.32
Posts made of glass fibers or quartz fibers embedded in a resin matrix addresses several of the carbon fiber post’s shortcomings (Fig. 5).
The main advantage is a modulus of elasticity (~40 Gpa) which is closer to that of dentin (~20 Gpa). Their enhanced bond strength to a variety of resin cements presents the clinician with the possibility of improved restorative predictability. They can be conical like the Luscent Anchor (Dentatus, USA, NY) or parallel like the FibreKor Post System (Jeneric Pentron, Wallingford, CT) and Parapost Fiber White (Coltene/Whaledent, Mahwah, NJ) system.
In addition to the benefit a tapered design, resulting in less tooth structure removal in post preparation, the Luscent Anchor’s translucent nature allows it to transmit light into the root canal. This feature enhances the light polymerization of the dual cure composite resin cement used to seat it.33
The FiberKor post and Parapost Fiber White post both incorporate serrated edges in the larger sizes, which aid in post retention. The Parapost Fiber White also features a round anti-rotational head with double undercuts to reduce stress and enhance retention of the composite core.
A 32-year-old female presented with a 20-year-old PFM crown on tooth #11 (Fig. 6). Clinical and radiographic examination revealed that the tooth has had successful endodontic therapy and a short cast gold post and core existed. The patient requested the replacement of her crown on #11 with a restoration that would be durable and yet maximize esthetics.
To achieve this objective, the placement of a fiber-reinforced post and an all ceramic crown was planned. An orthodontic and esthetic consultation was suggested as part of her long term treatment plan as time was a factor. The patient consented to the plan and treatment was initiated.
After anesthesia, #11 was isolated with a rubber dam. The defective PFM crown and the old gold cast post and core were removed (Fig. 7). Gutta percha was removed with a depth marked drill, taking care to maintain a 4mm apical seal (Fig. 8). In this case, the Parapost Fiber White red post was chosen and tried in for fit (Fig. 9).
The post was shortened to the appropriate length, wiped with alcohol, and silane was applied. The canal was conditioned with the Parapost Cement self etching, self curing primer and adhesive (Fig. 10). The excess adhesive was removed with a mini-brush and paper points (Fig. 11).
After evaporating the solvent from the canal with a warm air tooth dryer, (Fig. 12) the Parapost self curing cement was mixed and backfilled into the canal space with a Centrix syringe tip (Fig. 13). Cement was also applied to the post and seated immediately (Fig. 14). After setting, a composite core of A3 hybrid ceromer (Teric Ceram, Ivoclar-Vivadent, Amherst, NY) was built on the post (Figs. 15 & 16).
After removal of the rubber dam and laser gingival recontouring, the finished tooth preparation was ready for impression taking. The prep shade (Fig. 17) and tooth shade (Fig. 18) was recorded and a Luxatemp provisional crown from a pre-op polyvinylsiloxane impression was fabricated (Fig. 19).
The patient returned in one week for the adhesive cementation of her IPS Empress crown. The final result demonstrates an improvement in her esthetics as well as a better balance between her cental incisors (Fig. 20).
It is important to note that following sound principles and adhering to clinical protocols enhances the success of these new post and core systems. The issues of remaining tooth structure and a ferrule are paramount to the longevity of these materials. With any new materials one must proceed with caution and be on the lookout for clinical and laboratory evaluations on the literary horizon. OH
Dr. Edward Lowe is a clinical instructor and co-director of the Pacific Aesthetic Continuum (P.A.C~live) at the University of the Pacific, San Francisco, CA. He has lectured internationally on all facets of aesthetic dentistry and has published articles in most of the leading dental journals. He maintains a private practice devoted to functional aesthetic restorative dentistry in Vancouver, BC.
Oral Health welcomes this original article.
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