The Role of Posts in Restorative Dentistry
“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 on lay or full coverage preparation is critical to the clinical longevity and success of the final restoration. This is particularly relevant when discussing fiber posts and tooth reinforcement.
Dentin Preservation and Its Importance
Researchers now widely recognize that excessively removing dentinal support, both in the root and coronally, alters the flexural behavior and reduces resistance to failure. Over-flaring the canal for straight-line access weakens the dentinal complex (3-6). Maintaining coronal dentin is crucial not only for supporting the core buildup (7,8) but also because clinical and in vitro studies show that the survival of endodontically treated teeth restored with posts directly depends on the amount of residual coronal dentin.(9,10)
Impact of Post Preparation on Fracture Toughness
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. emphasize that clinicians should not remove additional dentin beyond what is necessary to complete endodontic treatment (11). To follow this principle, clinicians must eliminate the use of parallel-sided posts from their protocols, as these posts typically require the removal of sound apical radicular dentin, create sharper internal line angles, weaken the root, and increase the risk of root fractures (Fig 2). (12)
Challenges with Parallel-Sided Posts
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.
Tapered Master Quartz Fiber Post Approach
Boksman et al. have recommended utilizing a tapered master quartz fiber post (MacroLock™ 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 advantages (15-19) 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.
Direct Post and Core Restoration
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.
Elastic Modulus and Material Stiffness
1. The elastic modulus (or a material’s stiness) 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 stiness of metal and zirconia posts creates more internal stress, zones of tension and shear during function and parafunction, (23) which can result in unrestorable catastrophic root fractures.
High Flexural Strength of Fiber Posts
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 dierences 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 aect the clinical outcome and longevity. (25-29,16)
Selecting Fiber Posts and Adhesive Combinations
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.
Scientific Literature on Fiber Posts and Cements
There is a variation in the results of the scientific literature when evaluating fiber posts, not only because of the dierences 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)
Corrosion Resistance and Esthetic Benefits
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 color, but also the creation of superb clinical esthetics with translucent ceramics over a composite core (Fig. 6B). (34-36)
Fatigue Resistance and Clinical Durability
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 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)
Challenges in Endodontic Retreatment and Post Removal
5. Endodontic procedures fail due to faulty technique, inability to access or completely debride a canal, micro-leakage, bacterial contamination, or exposure to endotoxins after performing endodontic therapy but before placing a final restoration (all endodontic procedures require immediate restoration) (39,40). Researchers estimate that 25% of re-treatments involve the presence of a post. Dentists can remove fiber posts a traumatically within minutes using proprietary removal drill systems. (41-43)
Circumferential Ferrule in Endodontically Treated Teeth
Any discussion about restoring a badly broken-down endodontically-treated tooth must include the concept of the circumferential ferrule, defined as “a metal band or ring that encircles the tooth to provide retention and resistance form and 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 nonexistent. (45-47)
Height, Width, and Wall Considerations for Ferrule Preparation
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)
Research on Ferrule Effects and Restoration Outcomes
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. A simple example highlights the difference: losing a lingual wall on an upper central even with three walls remaining can be catastrophic due to the torque placed on the lingual during function, whereas losing an interproximal wall has little weakening effect under lingual stress.
Ferrule Necessity in Restoration Outcomes
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”. As the number of remaining walls decreases, the fracture resistance also decreases without a post. However, placing fiber posts significantly increases fracture resistance, except in cases where no walls remain. (52) “The success rate for all posts decreases drastically in the absence of a residual coronal wall.” (51)
Defining Reinforcement and the Role of Fiber Posts
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 supports this finding. (57,58)
Recent Research on Fiber Posts and Reinforcement
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.
Study Summaries on Fiber Post Effectiveness
D’Arcangelo et al. (59) studied the fracture resistance and deflection of teeth restored with a fiber post and prepared for veneers. Researchers included seventy-five human maxillary central incisors with similar anatomic crowns in the study. They prepared the specimens with various conditions: 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. They then thermo-cycled all specimens and subjected them to fracture strength tests 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.
Comparison of Posts in Composite Resin Restorations
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.”
Impact of Fiber Posts Under Zirconia Crowns
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 favorable failure type compared to a composite core build-up.
Esthetic Post Types and Their Fracture Resistance
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 Cosmo Post (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.
Fracture Resistance with Fiber Posts Under Various Crowns
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. Researchers endodontically treated 120 maxillary incisors, divided them into four groups of 30 each, and further subdivided them into two subgroups based on 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.”
Impact of Fiber Posts on Pulp less Teeth
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 pulp less teeth by supporting the remaining tooth structure against vertical compressive stresses.
Advances in Fiber Post Materials and Techniques
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.
Evaluating Fiber Post Effectiveness
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.
Conclusion on Fiber Post Outcomes
That is the contribution of the post versus no post, or composite only, to the remaining structures. The growing body of in vitro (76-79) and clinical data predominantly concludes that fiber post failures in situ are more likely to be “non-catastrophic” or “repairable,” unlike the failures typically associated with high modulus posts. Furthermore, recently published clinical trials correlate the success rate to the number of remaining dentin walls. (51, 80, 81)
Factors Affecting Fiber Post Performance
Variations in the literature on fiber posts arise from researchers using natural or bovine teeth, comparing in vivo and in vitro results, analyzing the periodontal ligament’s role in stress distribution, applying different loading techniques (vertical, horizontal, or angled), evaluating the type and quality of the post, recognizing the “secondary smear layer” and its impact on adhesion, bonding to different types of radicular dentin, selecting various adhesives, examining the post’s light transmission capability, using different composites to cement the post, determining the amount of composite lateral to the post, assessing the filler loading of the composite, and considering the amount of critical dentin removed to place the post.
Adhesive Bonding Challenges in Root Canals
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 succent light) resulting in better overall physical properties, and material incompatibilities.
Evolving Protocols for Fiber Post Restoration
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.
Figure1 Especially in ovoid canals (which are the norm) post preparation can needlessly remove dentin and result in weakening the remaining tooth structure, while leaving lateral gutta percha which compromises bonding/cementation.
Figure2 require more apical removal of vital dentinal structure needlessly weakening the root and creating an apical stress point.
Figure3 The taper of the Marco-lock post allows respect for the dentin and ensures a more even and minimal amount of surrounding composite resin, thereby reducing polymerization contraction forces.
Figure4 In irregular or ovoid canals the use of Fibercones lateral to the Macro-lock X RO has many clinical advantages increasing longevity.
Figure5 A clinical photograph showing the placement of Fibercones Laterally to the main Marco-Lock Post which decreases composite volume, adds anti rotational elements, and decreases microleakage.
Figure6A The common esthetic failure when using metallic posts discolouration of the tooth structure as well as the gingival collar.
Figure6B The result of placing a light transmitting fiber post with a translucent ceramic.
Figure7 The typical result of creating a full crown with a ferrule in a moderately tapered endodontic access opening.
Figure8 When preparing a ferrule on a tooth with a wide flare, the preparation removes all lateral dentin creating a stand alone core which drastically decreases the clinical success rate.
AUTHORS
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. Dr. Len Boksman recently been appointed as adjunct professor in the University of Technology Dental School, Jamaica, where he donates his time. He can be contacted at lenpat28@gmail.com.
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 https://www.rootcanals.ca/site/home.
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. In 2006, the University of Western Ontario Schulich School of Medicine and Dentistry appointed Dr. Santos as an assistant professor in the Division of Restorative Dentistry, and in 2011, they appointed him as 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 gildo.santos@schulich.uwo.ca.
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. In 1987, he immigrated to Canada, took up a full-time position at the University of Western Ontario (UWO), and served 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 friedmanm@rogers.com. The authors would like to thank Mrs. Laura Delellis for her work creating the figures used in this article.