The primary factor that determines the prognosis of endodontically treated and restored teeth is the preservation of sound dentin. The potential of these teeth to fracture is directly related to the amount of dentin removed. 1 Removal of dentin at the radicular level 2,3 and/or removal of dentin at the coronal portion of the remaining tooth structure 4,5 changes the tooth biomechanics with the result that a highly conservative approach is indicated. 6 Endodontic over instrumentation and flaring of the root canal during endodontic therapy, extensive structural defects from previous restorations, and the deleterious effects of the caries process all contribute to weakening of the tooth structure. 7 Boudrais8 states that because of the compromised state of the endodontically treated tooth, the ideal post system should: complement minimal conservative preparation, reduce or eliminate stresses transferred to the tooth, approximate the taper of the canal walls, be resistant to dislodgement during function, and be retentive and possibly micromechanically attached to the tooth. In a similar discussion, Gluskin9 states that in selecting an appropriate post system, treatment should attempt to establish: a) a stable post system that transfers the stress of mastication throughout the radicular root and into the periodontal attachment uniformly; b) a post system that does not focus stress in function or create it during placement; c) a stiff post system that resists deformation or permanent bending to protect the integrity of the crown margins and cement seal; d) a cementation process that provides optimal luting of the dowel to the radicular dentin; and e) a conservation of coronal tooth structure that allows adequate encasement of dentin by overcasting the “ferrule” effect. A high quality, adhesively bonded quartz fiber post fulfils all these criteria.
Cheleux10 states that post space preparation is generally performed by means of drills of increasing diameter according to defined rules: choose the largest root, do not exceed the beginning of a root curvature, define a superior or equal length to the height of the clinical crown without exceeding two thirds of the root length and preserve at least 4mm. of root canal filling material to assure the apical seal. Parallel sided posts do not conform to the shape of an endodontically treated canal. 11 The preparation of the canal for parallel sided posts with cylindrical instrumentation can result in over-preparation of the apical third to achieve post/canal adaptation requiring excessive removal of tooth structure that is already weakened and leaves a large coronal portion occupied by the cement, and thus in most cases requires excessive removal of tooth structure in already weakened teeth. 12 This excessive removal of radicular dentin compromises root strength, and creates significant reduction in radicular toughness. 13,14
In contrast, an anatomical fiber post like the Macro-Lock Illusion (RTD, Clinical Research Dental), requires minimal tooth structure removal during canal shaping, and allows for greater post-to-canal adaptation in the apical and coronal half of the canal.15 Fiber-reinforced dowels and bonded composite cores under fatigue loading, provide significantly stronger crown retention than cast gold dowels and titanium alloy dowels with composite cores. 16 In both static and fatigue fracture testing under vertical or oblique loadings, the fracture loads of the teeth restored with fiber posts were significantly greater than those teeth restored with metallic posts. 17 Because fiber reinforced posts have an elastic modulus that more closely approaches that of dentin, 18 fiber posts produce less stress on the root dentin around the post tip than do metal posts19 and the glass fiber post induces a stress field quite similar to that of the natural tooth. 20 These non-metallic posts comply more satisfactorily with the requirements necessary to provide a mechanical behavior more similar to that of the dental structure, the compatibility among the mechanical properties found in these systems and the dentin providing a biomimetic behavior, reducing the risk of failure, or fracture of the root. 21 Statistically, resin supported polyethylene fiber and glass fiber dowels (posts) show the lowest coronal leakage when compared with stainless steel and zirconia dowels at all time periods, 22-24 minimizing one of the major causes of endodontic failure. The total-etch dental adhesives show a better marginal seal than that obtained with self-etching primers. 25
Because of the ability to bond fiber posts, the post space preparation does not have to be as deep as conventionally cemented posts allowing for the preservation of the remaining root structure internally.26However, after preparing the post space, bonding to prepared root canal dentin requires meticulous attention to bonding protocol. Bond strength of adhesives to root canal dentin generally tend to be lower than crown dentin. 27 This may be due to the fact that there is less inter-tubular dentin, with a difference in collagen cross-linking making typical hybridization more difficult,28 with the result that dentinal tubule penetration is more critical for micro-mechanical retention. A separate acid-etch adhesive system results in significantly higher bond strengths than a self-etch adhesive system. 29 The dentin surface area available for bonding increased by 202% after phosphoric acid etching in the cervical third, 156% in the middle third and 113% in the apical third of the root dentin, with the increase in dentin surface area responsible for the enhanced bond strength. 30 This is critical, as this bonding configuration inside the canal has a very high C factor with many walls competing against the polymerization contraction. 31 It has been shown that when a micro-brush is used to apply the bonding agent and agitated, the bonding mechanism created between root canal dentin and the bonding system was uniform and more predictable. 32
It is critical to note that not all dental adhesive systems are compatible with dual-cure and self-cure cements or dual-cure and self-cure composite resins. A highly acidic bonding agent will interfere with the chemical amine reaction of self-cure cements thus interfering with the setting reaction. The three step etch-and-rinse systems are compatible, and of the etch-and-rinse one-bottle systems those that are not highly acidic such as One Step Plus (Bisco, Schaumberg, IL) and SEALBOND ULTIMA (RTD, Clinical Research Dental) are the most predictable. The self-etch primer systems should not be combined with chemical (self) or dual-cured cements due to the remaining acidic components of the primer33 and the all-in-one adhesives tend to be too acidic. A newly introduced bonding agent, MPa Direct, (Clinical Research Dental) shows high bond strengths to chemically (self) cured materials when the oxygen inhibited layer is removed with alcohol. 34 The highest values are obtained for adhesives tested when used with dual-cure cements.35 However, when evaluating adhesive post-endodontic restorations with fiber posts, bond strength tests and SEM observations show that composite resins perform better than resin cements. 36 The dual-cure hybrid composite Cosmecore (Cosmedent, Clinical Research Dental) has a high flow allowing it to be used as the dual-cured cement, and enough working time to allow for insertion of the fiber post, and the core build-up all at the same time, making fiber post placement easier and more time efficient. Hybrid composites are also better core build-up materials than flowable composite. 37
When using light-cured bonding agents and/or dual-cure cements/dual-cure composites in the prepared canal
, the amount of light that is delivered down the canal space is critical. Depending on the output of the light, the distance from the light source to the coronal aspect of the tooth, and the chemical reactivity of the materials used, the duration of light curing is often understated. Patyk when looking at light curing and translucency of glass fiber reinforced resin posts states that ” the crucial factor for the polymerization depth is the duration of exposure to light, which should be at least 100 seconds”.38
Biomimetics is a treatment approach that has as its ultimate goal, to retain as much of the natural tissue as practical and to also mimic the physics and structures of the human body. 39 Generally, the form of a root canal on cross section at the coronal side and/or the middle 1/3rd shows an ovoid shape, except for the upper central incisor and the palatal root of the upper first molar in the medio-distal view. 40 Anatomic variations such as narrow mesio- distal widths, proximal concavities, and developmental invag inations within the cervical and middle third of a high percentage of root forms must be considered during the selection of a post shape and size. 41 The endodontic and endo-restorative goal should be to mimic the pulp space that was present, 39 but a drill presents a round section which weakens the proximal walls in these types of canals during its passage. 10 In severely ovoid root shapes like lower anteriors and many upper and lower premolars, this type of preparation can significantly reduce the amount of remaining lateral dentin, 42 while leaving gutta percha in the wings of the ovoid canal, which severely compromises the ability to bond to those surfaces. This combination of weakening of the tooth structure and the inability to bond to the entire circumference of the root canal reduces retention, increases the likelihood of microleakage, and increases the propensity for failure.
To address the clinical challenges that ovoid canals present, and especially those ovoid canals that are essentially thin ribbons, a new fiber post of ovoid shape has recently been introduced that works in synchronicity with a piezo electric (Satelec) ultrasonic tip. Instead of a rotary instrument to prepare the post channel, a minimally invasive diamond coated ultrasonic tip is used to prepare the exact shaped post space for the Ellipson Fiber Post. The anatomical, oval Ellipson Fiber Post (RTD, Clinical Research Dental) is fabricated with a high percentage loading 64% vol. (80% wt.) of pre-tensed long continuous translucent unidirectional quartz fibers with epoxy resin, and is radiopaque. It has an interface treatment of the fibers to achieve a perfect match between the thousands of fibers and the organic resin matrix.
The Ellipson Post has high translucency for good light transmission, an elastic modulus of 13 GPa and a flexural strength of 1600 MPa, and will of course not corrode. The Ellipson tip for the minimally invasive oval preparation with ultrasonics, is stainless steel with a diamond coated tip of 76 micron grain, a length of 1.8mm and has depth gauge markings. This tip can be used to simultaneously remove gutta percha, clean the canal and shape the canal to receive the oval Ellipson Fiber Post. In a study evaluating the effect on post space debridement in oval-shaped canals of the oval ultrasonic tip compared with a circular ultrasonic tip, Coniglio found that “the oval ultrasonic tip resulted in a better post space debridement than a circular ultrasonic tip in oval-shaped canals.”43 Another significant finding in this study was that the Satelec group resulted in significantly better open tubule scores in the coronal and middle regions, which should facilitate acid etch protocols and increase the predictability of the bonding procedure.
The Technique
For ease of photographic purposes, the technique is presented on an extracted tooth. Figure 1 shows a lower molar, with some of the gutta percha at the distal canal orifice removed with a warmed instrument. The Ellipson oval ultrasonic preparation tip has easy to read depth markings with reference marks at 8mm and 10mm from the tip point (Fig. 2). The piezo-electric hand piece (P5 Newtron XS Acteon) is set in the blue power range of 10-15 or 70-80% of the built in generator, and the tip is activated without water so that the gutta percha is warmed and easily removed (Fig. 3). Figure 4 shows the preparation after the first pass and Figure 5 shows the preparation after using the tip with water lavage. The Ellipson Anatomical Ovoid Fiber Post (RTD, Clinical Research Dental) is tried in the prepared ovoid canal to verify fit and laid aside (Fig. 6). As the mesial buccal canal was filled with a carrier based obturation, a red size UniCore drill (Ultradent Products, South Jordan, UT) was
used to remove the carrier (Fig. 7). A ‘Pro-post’ drill (Tulsa Dental Specialties) can also be used to remove the carrier. A Stropko Adapter (John J. Stropko, DDS, Clinical Research Dental) with an EndoEze tip is used to remove the dentin remnants for easier visualization (Figs. 8 & 9). A red Macro-Lock Illusion Post (Clinical Research Dental London, ON) was verified for fit in this canal. Ultra-Etch (Ultradent South Jordan, UT) is delivered from the bottom of the post preparations to the orifice openings by mating a 22 gauge EndoEze tip to the syringe so as not to create an air lock below the acid etch, which always occurs if a regular tip is used (Figs. 10-12). Because the radicular dentine is more difficult to bond to, and because there is less inter-tubular dentin and less ability to create a typical hybrid layer, the acid etch is rubbed and agitated in the canals with a small brush, so as to maximize the number and, as well, create wide open dentin tubules for maximum penetration of the bonding agent (Figs. 13 & 14). The variations in studies of fiber post success indicate that etching from the bottom up and washing from the bottom up is critical so that all of the canal is etched and that all of the acid is removed. Absolutely no remnants of the acid may remain as it will negatively affect the luting resin or composite. Figures 15 and 16 show the initial rinse from the surface, which is then followed by inserting a 22 gauge EndoEze tip into the bottom of the canal space and rinsing from the bottom up. The canal is lightly dried from the bottom up to expel any water that may be left in the post space with the 22 gauge EndoEze tip (Fig. 17). The bonding agent is applied with a brush that goes to the bottom of the canal spaces and the bonding agent is agitated in the canal to maximize the tubule penetration (Fig. 18). As mentioned previously, the clinician must use a bonding agent that is compatible with a dual-cure composite or a self-cure composite as covered in the main body of the article. The bonding
agent is thinned from the bottom up again with a 22 gauge EndoEze tip, with the solvent gently evaporated (Fig. 19). It is a critical step here to use a paper point to verify that there is no pooling of the bonding agent at the base of the post space preparation which may compromise the seating of the fiber post. The bonding agent is light cured if a one bottle etch and rinse system is used, with the amount of time required for a thorough cure dependant on the distance from the canal orifice and the power of the light source. There is no such thing as over curing the bonding agent and more time is better than not enough. Cure for at least 40 seconds (Fig. 20). There are controversial results in the literature as to whether a bonding agent is required on fiber posts. There is no evidence that placing a bonding agent is detrimental, and in fact may enhance flow if a stiff core composite is used. The bonding agent is placed on the Ellipson Anatomical Oval Fiber Post, and the Macro-Lock Illusion Post, the solvent is evaporated, and the bonding agent is light cured for 20 seconds (Figs. 21-23). If MPa (Clinical Research Dental) is used with a self-cure resin
cement, the oxygen inhibited layer must be removed with alcohol after light curing. Cosmecore (Cosmedent Chicago, IL) is directly injected (after discarding the first tiny amount which comes out of the mixing tip) into a Skini Syringe (Ultradent South Jordan, UT) that is mated with a 22 gauge EndoEze tip (Fig. 24). Figure 25 shows the flow of this hybrid composite which makes it suitable for use as the luting composite and core material at the same time. The Cosmecore is injected from the bottom of the prepared post space to the orifice and the other canal is similarly filled (Figs. 26 & 27). The ovoid Ellipson Fiber Post is placed in the distal canal and the Macro-Lock Illusion is placed in the mesial buccal canal (Figs. 28 & 29). The dual-cure Cosmecore is cured through light transmission down the fiber posts, with 40-60 seconds of cure time depending on the light source (Fig. 30). As mentioned in the body of the article, one research report indicates that with some lights 100 seconds may be necessary. After the composite is cured in the canals, the rest of the coronal portion is built up with Cosmecore (a larger gauge needle may be used to facilitate easier expression of the material) (Figs. 31 & 32). The posts shown in this series were not shortened prior to insertion. There is evidence in the literature to support that the posts should be shortened with a diamond prior to cementation, and that they should be buried in the composite core. The dual-cure Cosmecore core is now cured for 40-60 seconds (Fig. 33). After the final cure of the core material, in this tooth, the fiber posts are reduced with diamonds and the tooth is ready for full coverage preparation (Figs. 34 & 35).
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Dr. Leendert (Len) Boksman is Adjunct Clinical Professor at the Schulich School of Medicine and Dentistry at the University of Western Ontario, and he is the Director of Clinical Affairs at Clinical Research Dental, London, ON. Dr. Boksman practices part-time in London, ON. He can be reached at lboksman@clinicalresearchdental.com.
Dr. Manfred Friedman graduated from the University of Witwatersrand Johannesburg in 1971 and obtained his B. CH. D. Honours at the University of Pretoria in 1980. Dr. Friedman is a past Director of the Undergraduate Endodontic Program at the Schulich School of Medicine and Dentistry and currently has a full-time practice in London, Ontario restricting his practice to Endodontics. He is a major part-time Adjunct Professor at the Schulich School of Medicine and Dentistry where he directs the Endodontic laboratory course and as well acts as Clinical Instructor.
Oral Health welcomes this original article.
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———
A high quality, adhesively bonded quartz fiber post fulfils all these criteria
———
Parallel sided posts do not conform to the shape of an endodontically treated canal
———
The total-etch dental adhesives show a better marginal seal than that obtained with self-etching primers
———
A new fiber post of ovoid shape has recently been introduced that works in synchronicity with a piezo electric (Satelec) ultrasonic tip
———
The Ellipson oval ultrasonic preparation tip has easy to read depth markings with reference marks at 8mm and 10mm from the tip point
———
Absolutely no remnants of the acid may remain as it will negatively affect the luting resin or composite
———
There is no such thing as over curing the bonding agent and more time is better than not enough
———
The posts shown in this series were not shortened prior to insertion
