July 1, 2004
by Bruce Glazer
Although journal coverage of this subject is ubiquitous, confusion22 still exists regarding the original if, how, and when formula.1 A recent Medline search yielded over one hundred articles between 1999 and 2004. All of the articles were peer reviewed, some were bench top in vitro studies and unfortunately, many were contradictory. It is this paucity of statistically significant clinical studies which “leads to less than optimal treatment selections”.2
“Endodontically treated teeth form a unique subset of teeth requiring restoration due to several factors”.2
Moisture Loss–There has always been controversy about the moisture content between the dentin in teeth with vital pulps and the dentin in teeth treated endodontically. Recent work by Huang et al3 and Papa et al4 showed no statistical difference between the two.
Dentinal Toughness–Papers by Huang et al3 and Sedgley et al5 concluded that endodontically treated teeth are not “brittle as a result of an alteration in dentinal toughness.
Collagen Alteration–early research indicated dentin consisted of type 1 collagen and its alteration might lead to brittleness. Rivera and Yamauchi6 found no significant differences in the cross-link content between normal and treated pulpless teeth. Endodontically treated teeth are not brittle as a result of an alteration in the collagen matrix.
Architectural Changes–Reeh et al7 discovered a relationship between the amount of central tooth structure lost during cavity preparation and the amount of deformation under load. This research suggested the following: access opening (5% reduction in stiffness), occlusal preparation (20%), loss of one marginal ridge (46%), loss of two marginal ridges (63%); thus, the conclusion to preserve marginal ridges whenever possible. Gutmann8 concluded that the cumulative loss of tooth structure from caries, trauma, restorative and endodontic access resulted in susceptibility to fracture.
Sensory Apparatus–The loss of pressoreceptors and an elevated pain threshold leads to decreased protection of endodontically treated teeth during mastication.9
LONGEVITY OF ENDODONTICALLY TREATED TEETH
One of the most important factors in the success or failure of the post endodontic restoration not reported in clinical studies is the amount of coronal structure remaining before the final restoration. Many of the clinical studies included teeth with less than 50% loss of coronal structure, thus the success rate included teeth which would have had a favourable prognosis regardless of the methodology. If this anatomic qualifier were added to a concept of biological or mechanical failure, statements such as “The primary cause of failure is inadequate restorative therapy followed by failure due to periodontal reasons”10 would be enlightened and expanded.
Aquilino SA and Caplan DJ11 confirmed an earlier study, 20 years ago, by Sorenson and Martinoff12 that recommended cuspal coverage of posterior endodontically treated teeth. As well research in the past decade has identified failure caused by the orthograde transport of salivary contaminants through an open access preparation or a faulty margin.13-15 Intact endodontically treated teeth are three times more fracture resistant when compared to teeth restored with dowels.16 Many times the long term results of treatment were directly dependent on the preoperative status of the pulp and the periapical tissue. Vital and non vital teeth without apical pathology (AP) enjoyed a 96% success rate17,18 whereas only 86% of the cases with pulpal necrosis and AP showed apical healing. Previously treated teeth with AP showed only a 62% success rate after re-treatment. Cross sectional examinations of populations around the world have demonstrated failure in 20% to 40% of root filled teeth.19,20 Teeth in which the restoration was defective but the root filling was adequate had a higher incidence of failure than teeth with inadequate root fillings and sound restorations. Teeth in which both the root fillings and restoration were adequate had only a 9% failure, whereas teeth where both were inadequate had an 82% failure.21
Tronstad17 was not alone in pointing out that endodontically treated teeth overall do not have a 100% success rate. In Kirkevange’s paper22 52.2% of endodontically treated teeth had apical pathology (AP) and AP tended to increase with increasing age. Hoen and Pink23 reviewed 1,100 failing endodontically treated teeth and concluded that 65% of the cases demonstrated poor radiographic obturation quality and only 13% noted coronal leakage. Poor technical quality was a recurring theme in papers by Boucher Y24 and De Moor.25
If however, we accept the ultilmate goal of endodontics as therapy to prevent or heal disease–as well as functional retention–and we apply the concept of evidence- based health care, then reviewing studies that provide the best evidence reveal that the chance of complete healing is reasonably high, and the chance for the tooth remaining asymptomatic and functional over time is excellent, provided that the tooth is promptly and well restored.a
Unless a large percentage of coronal structure is missing, posts are rarely placed in molars. Work by Nayyar and Walton26 has clearly shown that a corono-radicular core is close to 100% successful. Threaded pins have been advocated by Robbins, Christiansen and Kane in both a horizontal and vertical direction in addition to adhesive techniques due to the long-term concern for breakdown of the bond. If posts are required, they should be placed only in the largest canals–upper palatal and lower distal. Hachmeister et al27 concluded that endodontic access through an existing complex amalgam, which is subsequently refilled, compromises the fracture strength of the original restoration.
After completion of the endodontic procedure, all the old amalgam and resin should be removed and replaced with either a bonded amalgam or resin.
Posts are used more often in the anterior (cuspid to cuspid) due to the shearing forces generated by the envelope of function. Custom cast and prefabricated metal posts do not provide increased fracture protection whereas carbon fiber posts have been tested both in vitro and vivo with differing results. Mannocci28 et al suggest that the use of a resin based three steps adhesive system can be strongly recommended to obtain a strong link between composite cement, composite core, and the root canal walls. In an earlier study, Ferrari29 suggests that a one-bottle system can also create a mechanical interlocking with the root etched dentin under clinical conditions. When quartz posts with composite cores and full coverage are compared to titanium, glass and zirconia with matching cores and crowns, the quartz shows significantly higher failure loads.30
Post placement is based on the remaining coronal structure, the functional requirements of the tooth and an evaluation of the forces which will be acting on the tooth. A post may be indicated if a premolar will function as an abutment for an RPD or suffers from an attachment loss. In a prospective in vivo study by Glazer8 failures in all but one tooth were premolars. The canal anatomy of premolars is delicate and in order to succeed, the post system chosen must require minimal reshaping and enlargement of the canal space.
Fanned by the work of Kantor, Pines,31 Trabert and Caputo32 it was tradition that every pulpless tooth received a dowel to reinforce it and a crown to protect it from fracture. However today most studies suggest that this is not the case. But even with literature saying the opposite, a survey in Sweden revealed that a high proportion of prothodontists and GPs still believe that a post re-enforces.33
TYPES OF POSTS
The custom cast post has a long history of clinical success but when compared to parallel prefabricated, both in vitro and in vivo, its superiority is questionable. There are circumstances where custom cast posts would still be the post of choice. These include medically co
mpromised patients where reduced chair time is the imperative as well as multiple posts, restoration of small teeth and changes in angulations. The greatest disadvantage of a cast post is the inability of the temporary post and crown to seal and prevent bacterial contamination of the root canal.
Passive tapered posts are less retentive but more conservative as they idealize the existing shape of the root canal. It is an excellent choice for premolars but needs longer length to make up for lack of parallelism. This tapered design works because the length is sufficient to provide axial retention with no canal enlargement.
Passive Parallel has a long history of success and it is the post by which all others are measured. However, it is not kind to the retention of structure in the apical third of the root canal.
Active Posts imply that the threads actually engage the pericanal dentin. Its primary indication is a short root or canal space. This post is the most retentive but also the most destructive.
Carbon Fiber and Quartz is an important enough category to occupy its own sub- heading. It is available in many different configurations to assure the least amount of canal destruction.
There are three primary methods of gutta percha removal: instruments, heat, and solvents. Whichever method is chosen, care must be taken not to damage the periodontal ligament. Injudicious use of mechanical reamers may cause a significant temperature increase on the root surface.34,35 Peeso reamers show the greatest rise in temperature, Gates Glidden the least. Similarly a hot instrument may damage the periodontal ligament. Post space preparation may take place on the same appointment as the canal is obturated or it can be delayed twenty-four hours or more. The in vitro data does not indicate that one method is superior to another.
Spinning the cement into the canal with a lentulo has been shown to be the most effective method of placement. Needles and tubes are also effective as long as they reach the bottom of the canal. The post should be coated with the cement before placement.
Some of the reviews suggest problems with bonding when zinc oxide is used as the cement for obturation. The claim is that bonding will not occur unless significant removal of canal dentin takes place before cementation. However a study by Mannocci et al36 suggests that the use of endodontic sealers containing ZOE have no effect on the marginal seal of carbon fiber post/composite resin core restorations. The movement today is toward resin ionomer and composite cements to create a monobloc between the post, radicular dentin and the core. Further research is needed to create a self-curing one step etch and bonding agent matched with a high strength core material. The key is to be able to use the same material for the cementation of the post as well as the core build up.
This monobloc would have the strength needed to function under load, would increase fracture resistance of the tooth and would be re-treatable.
PREVALENT PHILOSOPHIES OF RESTORATIVE TECHNIQUES
According to the work of Morgano et al37 there is no consensus as to the best method of how to restore this unique subset of teeth. The majority of dentists in the United States use either cast posts exclusively or both cast and prefabricated. Forty per cent of GP’s use prefabricated all the time. The most popular post is the prefabricated serrated parallel sided. Composite core is the most popular core for GP’s (45%) and educationally qualified prosthodontists (43%) whereas board certified prosthodontists use amalgam (52%).
The longevity of endodontically treated teeth depends on the quality of the endodontic procedure, a good seal while the final restoration is arranged and a concept of minimal invasive dentistry. From this strict beginning the final restoration is developed taking into consideration the amount of coronal destruction, tooth position in the arch, parafunctional activity, fracture history and functional load. If a post is required, carbon, quartz or glass-reinforced epoxy resin posts are quickly becoming state of the art. According to the manufacturer’s information carbon and quartz fiber-reinforced posts have equivalent physical mechanical characteristics. Quartz is the first choice when esthetics is the driving force of the restoration
The shape of the post must be determined by the morphology and the size of the endodontic preparation which must acquiesce to the principal of preservation of remaining dentin. Most of the canal space should be taken up by the post to prevent micro porosities inside the cement and undue polymerization shrinkage.
Recently, Resilon Research LLC, (Madison, CT) has introduced Resilon obturating points and a resin sealer which when used in combination with a self etching primer after smear layer removal, allows for the creatation of a solid monobloc (a material which is contiguous from its resin tags in cleared dentinal tubules through sealer to the core canal filler).38
In light of all the scientific literature available on the subject it is becoming more logical for the restoration of the endo-coronal complex to be completed by the endodontist.2 Obviously, this will mean close co-operation and treatment planning between the generalist and specialist involved in the restoration.
Bruce Glazer is the Prosthodontic consultant to the editorial board of Oral Health. He maintains a private practice in Toronto limited to Prosthodontics and Anaesthesia.
Oral Health welcomes this original article.
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