“Traditional thinking that a post is only placed to retain a core and serves no other purpose may no longer be valid.”1
The preservation of dentin during access opening, shaping the canal, preparing the root for placement of a post, and during restoration with an onlay or full coverage preparation is critical to the clinical longevity and success of the final restoration.2 It is now well recognized that excess removal of dentinal support, not only in the root, but also coronally, changes the flexural behaviour and resistance to failure, and that over-flaring the canal for straight line access to the canals weakens the dentinal complex.3-6 Dentin coronally must be maintained, not only to give support to the core buildup,7,8 but as well, because clinical and in vitro studies support the fact that survival of endodontically-treated teeth restored with posts is directly proportional to the residual coronal dentin that remains.9,10 Post preparation of the root canal space must not remove additional dentin, as this contributes to a reduced fracture toughness. (Fig. 1) Ree et al. state that “no additional dentin should be removed beyond what is necessary to complete the endodontic treatment”.11 If this concept is to be adhered to clinically, then of course the use of parallel sided posts must be eliminated from our clinical protocol, as these posts usually require removal of sound apical radicular dentin, creating sharper internal line angles, resulting in a weakened root and a higher root fracture risk (Fig 2).12 As well, the parallel post does not complement the tapered shape of the prepared canal, resulting in excess luting composite in the coronal aspect of the canal, which can decrease bonding efficacy and decrease dislocation resistance (Fig. 3).13 If we adhere to the concept of minimal dentin removal in the root, and if we recognize that most root canals are ovoid in shape, then a wholly different treatment approach than what we have been taught in the past is indicated. Boksman et al. have recommended utilizing a tapered master quartz fiber post (Macro-Lock™ Post Illusion X-RO Clinical Research Dental, London, Ontario) with additional Fibercones (Clinical Research Dental, London, Ontario) placed into the irregularity (lateral spaces) of the canal (Figs. 4 & 5).14 This technique is similar to using a master gutta percha point with accessory gutta percha points, which is well-understood. Utilizing this approach provides several clinical advantages15-19 including more anti-rotational resistance, decreased volume of composite or cement lateral to the post to decrease the C and S Factor constraints (volumetric shrinkage), better adhesion to the root canal walls resulting in decreased microleakage and increasing resistance to dislodgement, as well as decreased likelihood for lateral perforation. The combination of a post, or multiple posts, that transmit light efficiently, with sufficient extended light curing time/output, results in better composite polymerization.
The indirect cast gold/metal/zirconia post and core has been largely replaced with a one appointment restoration of a direct post and core. Fiber posts such as the Ultradent Unicore Fiber Post (Clinical Research Dental, London, Ontario), the quartz fiber posts manufactured by RTD (St. Egreve, France), the Macro-Lock X-RO (Clinical Research Dental, London, Ontario), and the DT Light-Post (Bisco Canada, Richmond, BC) have many physical characteristics that make them more desirable clinically, rather than metal and zirconia posts:
1: The elastic modulus (or a material’s stiffness) of fiber posts more closely approximates that of dentin (18.6 Gigapascals -GPa) allowing some slight flex in function, dissipating stress and reducing the likelihood of damage to the root.20,21 Stainless steel has an elastic modulus of about 200GPa, titanium alloy 110GPa and Zirconia 300GPa.22 The stiffness of metal and zirconia posts creates more internal stress, zones of tension and shear during function and parafunction,23 which can result in un-restorable catastrophic root fractures.
2: Fiber posts have a high flexural strength, and in a study by Stewardson, “the flexural strength of fiber reinforced composite endodontic post materials exceed the yield strength of gold and stainless steel and two of the FRC (fiber reinforced composite) posts were comparable to the yield strength of titanium”.24 It must be noted here that not all fiber posts are created equal. There are differences in fracture load, flexural strength, fiber diameter, fiber/matrix ratio, type of fiber (with quartz fiber posts having higher failure resistance), light transmission, shape, post surface adhesion, quality of fiber, structural defects/voids and manufacturing quality, which all affect the clinical outcome and longevity.25-29,16 The clinician must make an informed choice for choosing a fiber post – looking for the best attributes above – in order to select the post with superior properties based on independent research. The dental practitioner must also be aware of the best adhesive combinations and techniques as there are some incompatibilities between dual-cure core materials and simplified acidic adhesives due to residual acidity. There is a variation in the results of the scientific literature when evaluating fiber posts, not only because of the differences in the posts themselves, but also because of the cementing/bonding/adhesive systems used. To date, multiple articles in the scientific literature support the statement that “only specific combinations of dentin adhesives and luting cements prove efficient, with total etch adhesives combined with dual-cure cement (composite) appearing to be the best choice”.30,31
3: Fiber posts are not subject to galvanic or corrosion activity. The corrosion of base metals predisposes to a high percentage of failures with cast posts which can also create a negative esthetic outcome of a dark root and darkening of the gingival collar (Fig. 6A).32,33 Milnar and others have published excellent papers showing that the use of a light transmitting post can eliminate this common esthetic challenge, allowing not only light transmission down the canal eliminating the dark gingival colour, but also the creation of superb clinical esthetics with translucent ceramics over a composite core (Fig. 6B).34-36
4: Clinically, heavily restored teeth may hold up to normal occlusal function but many fail in cyclic fatigue–repeated functional stress and torque.21 Fiber posts are more fatigue resistant than metal posts, and the quartz fiber post is found to be more than twice as fatigue resistant as the stainless and titanium alloy posts.37 During repeated fatigue loading, the flexural strength of metal posts can decrease by 40%, whilst there is only a 14% decrease in a fiber composite post.38
5: Endodontic procedures do fail, either due to faulty technique, the inability to access or completely debride a canal, micro-leakage/bacterial contamination/exposure to endotoxins after endodontic therapy is performed, but before a final restoration is placed (all endodontic procedures should be followed by immediate restoration),39,40 or due to failure and microleakage of the coronal restoration. It has been estimated that 25% of re-treatments involve the presence of a post. Fiber posts are atraumatically removed in a matter of a few minutes with available proprietary removal drill systems.41-43
No discussion of the restoration of a badly broken down endodontically-treated tooth would be complete without discussing the concept of the circumferential ferrule, which is defined as “a metal band or ring that encircles the tooth in order to provide retention and resistance form, as well as protect the tooth from fracture”.44 Most of the published articles, based on in vivo and in vitro data, suggest that a 2mm ferrule is best for improving resistance to fracture with significant decreases when the ferrule is 1mm or non-existent.45-47 However, it is not only the height of the remaining dentin that is critical for creating the ferrule, but just as important is the width of the remaining dentin and the number of walls. As shown in Figures 7 and 8, there is a drastic difference in outcomes when preparing a ferrule in a modestly flared canal versus a wide flare. As can be seen, when a wide flare exists, the preparation of a ferrule actually removes the dentinal lateral walls creating a stand-alone core that essentially has no ferrule at all. It is important to note here that glass ionomer cements and resin modified glass ionomers lack the physical properties to function as a core material.48,49 Jotkowitz et al. in their article on “Rethinking the Ferrule”, provides one of the best regression analyses and clinical guidelines in the literature, evaluating the effects of the height, number of walls remaining, thickness of the walls, and whether a mesial/distal or buccal/lingual wall is remaining in relationship to the functional stresses involved.50 A simple example would be the difference of losing a lingual wall on an upper central – even if three walls remain – which can be catastrophic due to the torque placed on the lingual in function, as opposed to losing an interproximal wall which has little weakening effect when lingual stress is applied. Their conclusion is that no ferrule equals un-restorable. “Clinical protocols should feature well-defined inclusion criteria, including delineation of the number of residual coronal walls, for a clearer assessment of the influence of the remaining tooth structure on treatment outcomes”.51 As the number of remaining walls decrease, the fracture resistance decreases when no post is used, but the fracture resistance is increased significantly when fiber posts are placed – except when there is no wall left.52 “The success rate for all posts decreases drastically in the absence a residual coronal wall.”51
The literal definitions of reinforcement from various sources includes:
A device designed to provide additional strength;
To strengthen by adding extra support;
To make stronger;
To strengthen with some added piece, support or material;
To make a structure stronger.
Much of the dental literature and texts from the 1970s to the early 1990s indicate that a post is placed when there is insufficient structure left to retain a core/crown, and that metal posts do not reinforce the root.53-56 Retrospectively looking at research on endodontically-treated teeth utilizing metal posts certainly support this finding.57,58 However, more recent research articles and publications are creating a body of work that fiber posts do indeed make the root more resistant to fracture and may strengthen the root. What follows is only a partial list with short summaries of some of the more recent relevant studies supporting the notion of reinforcement by using fiber posts:
D’Arcangelo et al.59 studied the fracture resistance and deflection of teeth restored with a fiber post and prepared for veneers. Seventy-five human maxillary central incisors with similar anatomic crowns were included: no preparation, veneer preparation, endodontic access filled with composite, endodontic access with composite and veneer preparation, and fiber post placement (RTD Endo Light Post) followed by veneer preparation. All specimens were thermo-cycled and submitted to fracture strength tests by using a displacement measurement system. Preparation for veneers increased the deflection values of the specimens, but the fiber reinforced post restoration with veneer preparations did not show statistically significant differences from the intact unprepared incisor.
When investigating the fracture resistance and failure mode of premolars restored with composite resin and various prefabricated posts Hajizadeh et al.1 utilized 60 extracted teeth with four subgroups: no cavity preparation, endodontics with an MOD and no post, endodontics with a DT Light Post (RTD) and MOD, and the last group with endodontics, Filpost (Filhol Dental, Gloucestershire, UK) and an MOD composite restoration. The teeth restored with the DT Light Post and composite were as strong as the control (the unprepared tooth) and stronger than those teeth restored with composite alone without a post, and those restored with a Titanium post and composite. In the DT Light Post group, 86% of the fractures were “restorable”, which was much higher than any of the other three groups. According to the authors, “There is growing evidence that fiber posts provide the additional benefit of increased fracture resistance.”
The effect of placing fiber posts under zirconia-ceramic crowns was studied by Salameh et al.60 Ninety mandibular second molars were divided into three test groups representing various extents of coronal damage, endodontically-accessed and obturated with warm vertical condensation. Half of the specimens were restored with composite, the other half with a translucent FRC post (Rely-X Fiber Post 3M/Espe) with a composite core. The insertion of the fiber post improved the support under the zirconia crowns, which resulted in higher fracture loads and favourable failure type compared to a composite core build-up.
Maccari et al.61 utilized thirty single rooted endodontically-treated teeth to evaluate the fracture resistance of different prefabricated esthetic posts. Included in the study were Aestheti-Post (RTD), FibreKor Post (Jeneric Pentron, Wallingford, CT) and CosmoPost (a ceramic post system)(Ivoclar Vivadent, Schaan, Liechtenstein). They summarized that the mean fracture resistance of the glass fiber prefabricated esthetic posts proved a higher fracture resistance than the ceramic post which was less than one half of the fiber posts.
The fracture resistance and failure pattern of endodontically-treated maxillary incisors restored with composite resin, with and without fiber reinforced composite posts under different types of full coverage crowns, was studied by Salameh et al.62 One hundred twenty maxillary incisors were endodontically-treated and divided into four groups of 30 each and further divided into two sub-groups of restoration with or without a fiber post (Postec Plus, Ivoclar Vivadent, Schaan, Liechtenstein). Restorations placed were PFM, Empress II, SR Adoro crowns and Cercon crowns with all preparations including a 2mm ferrule. Fracture tests showed that the type of crown was not a significant factor affecting the fracture resistance, but the presence of a post was. The authors state that “although prosthodontic textbooks do not generally advocate the placement of fiber posts in endodontically-treated incisors, the results of this study indicate that the use of fiber posts in such teeth increases their resistance to fracture and improves the prognosis in case of fracture.”
In a study of 80 endodontically-treated maxillary premolars treated with or without fiber posts, and MOD cavity preparations restored with different types of crowns including porcelain fused to metal, lithium disilicate, fiber reinforced composite or zirconia crowns, Salemeh et al,63 loaded the restorations until failure recording the maximum breaking loads. Under vertical loading conditions, the fracture loads of teeth restored with fiber posts were significantly greater than those without posts, and the fiber posts significantly contributed to the reinforcement and strengthening of pulpless teeth by supporting the remaining tooth structure against vertical compressive stresses.
There are many more studies showing the reinforcement of tooth structure with fiber posts.64-73 It is impossible to summarize them all, but it seems obvious that our concept of restoring endodontically-treated teeth is continually advancing as new products and bonding techniques evolve. Even when there are variations in the types of fiber posts used in the studies, and different cementation and adhesive protocols, there is compelling evidence that fiber posts can reinforce tooth structure.
To create balance in this overview of the literature, it must be said that there are of course some published scientific articles that do not show a reinforcing effect of fiber posts.7,74,75
In addition to the traditional definition of mechanical reinforcement: restoring a compromised tooth to a fracture strength equal to or greater than its original “untreated” fracture resistance, we clinicians perhaps should be more focused on the predictability of outcomes, particularly in worst-case scenarios. That is the contribution of the post versus no post, or composite only, to the remaining structures. The most predominant conclusion emerging from the growing body of in vitro76-79 (and clinical) data is that failures of fiber posts in situ are more likely to be described as “non-catastrophic” or “repairable” which is usually not the case with high modulus posts.
Furthermore, recently published clinical trials correlate the success rate to the number of remaining dentin walls.51,80,81 Variations in the literature on fiber posts are the results of: use of natural teeth or bovine teeth, in vivo versus in vitro results, the effect of the periodontal ligament in distributing some of the stresses, loading technique (vertical, horizontal or at an angle), the type and quality of the post, the recognition of the “secondary smear layer” and how it affects adhesion, the type of radicular dentin that is to be bonded, the adhesive used, the light carrying or transmission capability of the post, the type of composite used to cement the post, the amount of composite lateral to the post, the filler loading of the composite, and the amount of critical dentin that is removed to place the post.
Adhesive bonding in the root canal has its unique challenges due to dentinal structure in the canal (coronal dentin bonds better than apical dentin), the “secondary smear layer” of debris from gutta percha and sealer that compromises the ability of simplified systems to actually bond to the root surface (results in mainly frictional resistance), C and S Factor polymerization effects, curing to depth when using dual-cured composite (all dual-cured composites have a higher polymerization percentage when exposed to sufficient light) resulting in better overall physical properties, and material incompatibilities.
Fiber post restoration techniques require a meticulous protocol and the clinician is urged to scour the literature, not only for the best fiber post available but also the best techniques for placement. Materials and techniques for fiber post restoration of endodontically-treated teeth are continuously evolving with the inevitable outcome of better clinical results for our patients.OH
Dr. Len Boksman graduated as a DDS from the Faculty of Dentistry, University of Western Ontario in 1972. After 7 years in private practice, he joined the Faculty of Dentistry at Western as an assistant professor of operative dentistry, shortly thereafter attaining the tenured position of Associate Professor. He has authored more than 100 articles and several chapters in textbooks.and was awarded the Ontario Dental Association Award of Merit in 2005. He has recently been appointed as adjunct professor in the University of Technology Dental School, Jamaica, where he donates his time. He can be contacted at firstname.lastname@example.org.
Dr. Gary Glassman graduated from the University of Toronto, Faculty of Dentistry in 1984 and was awarded the James B. Willmott Scholarship, the Mosby Scholarship and the George Hare Endodontic Scholarship for proficiency in Endodontics. A graduate of the Endodontology Program at Temple University in 1987, he received the Louis I. Grossman Study Club Award for academic and clinical proficiency in Endodontics. The author of numerous publications, Dr. Glassman lectures globally on endodontics, is on staff at the University of Toronto, Faculty of Dentistry in the graduate department of endodontics, and is Adjunct Professor of Dentistry and Director of Endodontic Programming for the University of Technology, Jamaica. Gary is a fellow of the Royal College of Dentists of Canada, and the endodontic editor for Oral Health dental journal. He maintains a private practice, Endodontic Specialists in Toronto, Ontario, Canada. He can be reached through his website www.rootcanals.ca.
Dr. Santos received his DDS (1986) and MSc in dental clinics (1999) from Federal University of Bahia, and PhD in prosthodontics (2003) from University of São Paulo. Dr. Santos was appointed as assistant professor, Division of Restorative Dentistry at the University of Western Ontario Schulich School of Medicine and Dentistry in 2006 and in 2011 was appointed chair of the Division of Restorative
He has several publications in international journals and has trained and mentored graduate students in the area of biomaterials research throughout his career. He can be reached at via e-mail at email@example.com.
Dr. Manfred Friedman graduated from the University of Witwatersrand and Johannesburg in 1971 and then obtained his BChD Honours at the University of Pretoria in 1980. He immigrated to Canada in 1987 where he took up a full-time position at the University of Western Ontario(UWO) and was appointed as director of dentistry at The Southwestern Regional Center for developmentally challenged adults from 1987 to 1994. Dr. Friedman was also the director of the undergrad endo program from 1997 to 2002. He currently has a full time practice in London, ON, restricting his practice to endodontics, and is a major part-time adjunct professor at Schulich School of Medicine and Dentistry at UWO, where he directs the endodontic lab course as well as acts as clinical instructor. Dr. Friedman has given numerous courses on endodontics since joining UWO, with particular interests in rotary instrumentation, endodontic materials, apex locators, and restoring the endodontically treated tooth. He can be reached at (519) 673-5293 or at firstname.lastname@example.org.
Oral Health welcomes this original article.
Dr. Leendert (Len) Boksman is a paid part time consultant to Clinical Research Dental and Clinician’s Choice.
Dr. Gary Glassman has no disclosures.
Dr. Gildo Santos is a part-time consultant (Research and Development) for Clinical Research Dental and Clinician’s Choice.
Dr. Manfred Friedman has no disclosures.
The authors would like to thank Mrs. Laura Delellis for her work creating the figures used in this article.
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