The rationale for the suggested use of fiber post segments in composite core buildups for endodontically treated teeth

Drs. Leendert Boksman and Gary Glassman discuss the choices and challenges involved in certain restorations

The restoration of teeth utilizing com-posites still presents a myriad of clinical challenges for the dental clinician. This is especially true for extensively broken down teeth as well as those teeth that have been accessed endodontically.

Fiber posts — such as the quartz Macro-Lock® Post® Illusion® X-RO® (RTD, St. Egreve, France); UniCore® Fiber post (Ultradent); and D.T. Light-Post® (RTD, St. Egreve, France) — are now the posts of choice for a direct one-appointment restoration of the severely compromised endodontically treated tooth. Current research supports the use of an etch and rinse bonding protocol, with a compatible bonding agent, utilizing a dual-cured composite cement that can be utilized for the core as well (CosmeCore™, Cosmedent®; CoreCem®, RTD, St. Egreve, France; Zircules™, Clinician’s Choice Dental Products, Inc.) for best results.1,2 Traditionally, minimally accessed endodontically treated teeth that are not extensively compromised by caries or fracture have been restored solely with a composite core, without the placement of a post. This decision must be based on the amount of tooth structure left and if a full coverage restoration is to be placed now or in the future. The width and height of the remaining ferrule are critical to restorative success (Figures 1A and 1B). Also,3-6 the number of tooth walls left and post preparation significantly affect the long-term restorative outcome (Figure 2).6,7,8

In a review of 41 articles published between 1969 and 1999 (the majority from the 1990s), Heling states that “the literature suggests that the prognosis of root canal-treated teeth can be improved by sealing the canal and minimizing the leakage of oral fluids and bacteria into the periradicular areas as soon as possible after completion of root canal therapy.”9 A similar review by Saunders, et al., also concluded that coronal leakage of root canals is a major cause of root canal failure.10 Sritharan states, “it has been suggested that apical leakage may not be the most important factor leading to the failure of endodontic treatment — but that coronal leakage is far more likely to be the major determinant of clinical success or failure.”11 Coronal microleakage can occur due to a deficient final restoration (due to resultant microleakage from polymerization contraction, cement wash out, poor full coverage, flex, etc.), and resultant secondary caries.12

Polymerization contraction (shrinkage)

Many different types of composites are now available to the practitioner, including microfills, macrofills, hybrids, and small particle hybrids, nanofills, nanohybrids, or microhybrids.13 Even though the formulations can be adjusted in handling to make these composites “packable,” “flowable,” or “sculptable,” polymerization shrinkage or contraction stress is still the most important clinical challenge or problem associated with their use.14,15 This shrinkage or contraction and the stress created vary from composite to composite and can be affected by the following:
Its filler type and loading content
The resin matrix and its molecular weight
The shade and opacity
The cavity preparation shape (C-Factor) width and depth
The composite thickness
The elastic modulus of the composite and tooth
The irradiance level and curing time
The spectral output of the curing light
The curing light placement
Bulk or incremental fill
The rate of force development (high-irradiance lights)
The initiator system used
The degree of conversion16-25

In published studies, shrinkage values for various composites have been reported from 2.00 to 5.63 vol. percent26, and 1.67 to 5.68 percent,27 with flowables demonstrating the highest shrinkage with contraction stress measurements ranging from 3.3 to 23.5 MPa.26 Not all composites advertised as low shrinkage actually have reduced polymerization shrinkage measurements. When evaluating seven low-shrink BisGMA-based composites, Aelite™ LS Posterior (Bisco, Inc.) and N’Durance® (Septodont) presented relatively high-shrinkage values.28

The polymerization contraction of the composite resin and contraction stress created, as discussed before, can produce tensile forces on the tooth structure and the bonding system that may not only disrupt the bond to the cavity walls29,30 but also fracture enamel along the prisms (white line margins).31 This failure can lead to caries, sensitivity in vital teeth, and microleakage, allowing the penetration of bacteria, fluids, and toxins, which can negatively affect the success of endodontic treatment (coronal leakage).32 Braga, et al., state that “shrinkage stress development must be considered a multi-factorial phenomenon” and that “the volume of the shrinking composite becomes a variable to be considered.”33 Unterbrink and Liebenberg in their publication state that shrinkage stress increases with increasing C-Factor and that the size of the restored cavity is an important factor when bulk filling.34 Their study35 also shows that incremental filling lowers the C-Factor and that it is better than bulk cure because of better adaptation to the cavity wall, decreasing microleakage and increasing the degree of conversion. In a study looking at micro-leakage and cavity dimensions, it was found that microleakage seemed to be related to a restoration’s volume, but not to its C-Factor.36 With bulk filling techniques, the hardness or conversion of composites are significantly lower than those of the same material placed with the incremental technique.37 Watts, et al.,38 recommend that the restorative mass must be equally considered when translating shrinkage science into specific clinical recommendations.

So where does this lead us in a suggested modification of our restorative technique for placing a core in an endodontically treated tooth? Currently, when there are enough walls and tooth structure left, many clinicians insert a bulk fill, dual-cure composite resin into the endodontic access opening (the same material as that used for cementing the fiber post) and then cure it all at once with an LED curing light. As mentioned previously, this bulk fill not only creates a challenge for proper depth of cure and maximum physical properties on polymerization, which will be addressed later in this article, but the large volume/amount of composite inserted negatively affects the integrity of adhesion and increases microleakage. The typical access opening, which is essentially a very deep Class I cavity preparation, not only requires a large amount of composite, but as well, places the composite in the highest C-factor cavity preparation configuration of five. Only when utilizing a composite deep in the prepared root canal has the C-Factor claimed to be higher at 200 to infinity.39

The suggested solution to the high polymerization and contraction stress caused by bulk filling the access opening is to reduce the mass or bulk of composite by placing multiple Fiber Post Segments into the composite mass before curing with the LED light. It has been conclusively shown that even when the C-Factor is at 200 or more in a prepared root canal, minimizing the thickness of the composite (the mass), results in less contraction stress (S-Factor), which increases the patency of the bond to the root canal walls decreasing microleakage.40-43 Of course, the placement of inserts into composite is not a new idea. Glass ceramic inserts and beta quartz have been used to decrease composite volume, and later silica glass and ceramics were introduced as a method for post-composite insertion bulk reduction.44-46 These techniques demonstrated increased marginal patency and less microleakage, but the inserts were difficult to contour and polish with adhesion between the inserts and the composite being a challenge.47,48 Composite megafillers were introduced later, as these were essentially the same as the matrix of the bulk filled composite, eliminating the inherent chemical differences between the materials.49,50 The authors suggest the insertion of multiple high-quality, high-capacity, light-conducting fiber post segments. (Not all fiber posts conduct light efficiently.51,52) This is not only to reduce the composite volume, thereby minimizing the potential for micro-
leakage, but is also equally as critical to use the light conductance of the fiber post segments to significantly increase the degree of polymerization of the dual-cure composite resin cements/core materials deep in the access opening, thereby increasing their physical properties.53

In their review of polymerization shrinkage, Cakir, et al., discuss the attenuation of light, which means that the deeper layers of composite resin are less cured with reduced mechanical properties, and that bulk filling shows significantly less hardness.54 Others have also shown that bulk placement and increased cavity depth result in a significant decrease in the effectiveness of polymerization, regardless of the exposure time.55 The ADA Professional Product review on Restorative Materials evaluated the depth of cure of 38 restoratives with ranges of 1.2 mm to 5 mm with a core material CompCore™ AF SyringeMix™ (Premier Dental Products Co.) (W) being the lowest depth of cure at 1.2 mm. Included in the study were measurements of maximum polymerization shrinkage stress showing that LuxaCore® Dual Smartmix™ (DMG America) (W) was the highest in stress MPa of the core materials tested, with Clearfil™ Photo Core (Kuraray America, Inc.) (T) showing the highest development of shrinkage stress rate.56

Dual cure composite materials show the best physical properties and best polymerization with sufficient light exposure, even though they are claimed to polymerize in the absence of light,57-61 and “there is no evidence for a substantial chemically induced polymerization of dual cure resins that occurs after light exposure is completed.”62 This reality is especially critical for dual-cure self- adhesive resin cements Maxcem™ and RelyX™ Unicem (3M ESPE), which show a better degree of conversion when they are light activated, with a lack of light activation decreasing the monomer conversion by 25% to 40%63; and even in their dual cure mode, the degree of cure at best among the self-etch adhesives is only 41.52%.64-66 Thus, the placement of a bulk filled dual cure composite into the endodontic access opening, followed by the placement of multiple fiber post segments that carry sufficient light energy to the depth of the occlusal floor of the access preparation, will increase the polymerization conversion, resulting in a composite that demonstrates superior physical properties.
As a final comment, it has been proven that immediate high-intensity light polymerization creates the greatest polymerization stress. Ilie, et al., state that “fast contraction force development, high contraction stress, and an early start of the stress build-up cause tension in the material with possible subsequent distortion of the bond to the tooth structure.”67 This finding has been collaborated by many others in the scientific literature with resultant recommendations for a soft-start or lower energy over a longer period of time.68,69 Miller states that “manufacturers continue to make outlandish claims of their curing capabilities, most of which fall into the “too good to be true” category,70 and Swift concludes that “the curing times recommended by a manufacturer might not deliver the amount of energy required to adequately cure composite, even under the ideal laboratory conditions,” that “very short curing times are not a good idea in most clinical situations,” and  that “longer curing times are required.”71 As well, Swift states that “instead of obtaining a boost, the ‘turbo’ tip actually will reduce the amount of light reaching the composite to initiate the polymerization process.”72

Clinical Case

A 64-year-old female presented to the endodontic office with an uneventful medical history. She complained of spontaneous pain on the lower left side of 1 week’s duration, which radiated up the ramus of the jaw and was causing headaches. She also complained of hot and cold sensitivity with pain on biting. Clinical tests revealed pain to cold, which lingered for 5 minutes, and a sharp electric-like pain when a Tooth Slooth® (Professional Results, Inc.) was placed over the DL cusp tip. A distal crack was visualized. There was no periodontal pocketing. All other mandibular left and maxillary left teeth tested vital and asymptomatic. The radiograph revealed a small shallow minimally invasive amalgam restoration (Figure 3). The diagnosis was Cracked Tooth Syndrome with an irreversibly inflamed pulp. The patient was advised of the questionable long-term prognosis with cracked teeth yet decided to try and retain it, understanding that if the crack extended in the root proper and a periodontal pocket developed, then extraction with an implant replacement would be a viable solution.

Due to the minimal invasiveness of the restoration, it was anticipated that after endodontic treatment, there would be enough coronal tooth structure left to allow for the preparation of a full-coverage restoration with a fully circumferential ferrule of at least 2+ mm in height, as well as width (Figure 4). Figure 5 is a magnified view of the distal vertical crack, with the wear facet on the lingual cusp indicating a working side contact interference. Endodontic therapy was initiated under the microscope, and after a thorough debridement and shaping of the root canal spaces (Figure 6), the roots were obturated with gutta percha using a continuous wave of condensation technique to a level 2 mm below the pulpal floor (Figure 7). Phosphoric acid etching was initiated with the placement of Ultra-Etch® Etchant (Ultradent), followed by micro-brush agitation to work the etchant into the dentin, a thorough rinse, and light air drying (Figure 8).

Figure 9 shows the application of MPa bonding agent (Clinician’s Choice Dental Products) with a micro-brush, which again was followed by agitation to facilitate deeper penetration of the bonding agent, followed by evaporation of the solvent for 10 seconds. The bonding agent was cured with a Valo® curing light (Ultradent) for 10 seconds utilizing a Valo Proxiball Lens (Figure 10). The Macro-Lock X-RO segments were verified for fit over the three canal orifices, and then coated with MPa bonding agent, which was cured for 10 seconds (Figure 11). CosmeCore A2 was injected into the pulp chamber one-half way up the occlusal height of the clinical crown (Figure 12). The Macro-Lock X-RO segments were inserted into the CosmeCore followed by a 10-second cure with the Valo (Figure 13). The rest of the occlusal access opening is filled with the CosmeCore and thoroughly cured with the Valo for 20 seconds. Figure 14 is the final postoperative radiograph showing the placement of the fiber segments into the core. The final restoration of the occlusal access opening is shown in Figure 15 after trimming and occlusal adjustment. The endodontically treated tooth is now ready for a final restoration.

This article has recommended restoring the teeth that meet the criteria for not needing the placement of fiber posts because of sufficient remaining tooth structure with the use of multiple fiber post segments placed into the dual-cure composite cores of endodontically treated teeth based on the preceding evidence. This will decrease the overall polymerization contraction and stress formation, thereby reducing occlusal microleakage, while at the same time, driving the dual-cure composite to a better overall cure or conversion for better physical properties.

Acknowledgments

The authors wish to thank Mrs. Laura Delellis for her work in creating the figures used in this article.

This article was reprinted with permission from Oral Health, Canada’s leading dental journal.

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