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Clinical Predictability Using Fiber Posts

hs-leendert-boksman

hs-glenn-as

article and clinical photos by Leendert Boksman, DDS, BSc, FADI, FICD,
and Glenn A. van As, BSc, DMD

The restoration of endodontically treated teeth is often compromised by the minimal amount of residual tooth structure remaining to support and retain the final restoration. This may be due to endodontic overinstrumentation and flaring of the root canal during endodontic therapy, or to extensive structural defects from previous restorations, or the deleterious effects of the carious process.1 Many of these teeth require the placement of a post-and-core restoration for retention, whether the tooth is restored directly or indirectly with full or partial coverage. The dental literature supports the fact that a circumferential ferrule in combination with an endodontic post results in better long-term clinical success for crowned, endodontically treated teeth.2-4 However, many of these studies looked at cemented posts, while different ferrule designs have little, if any, influence on the fracture resistance of teeth with fiber posts, with no significant change in the resistance of teeth with fiber posts regardless of which ferrule design is incorporated. This unique property of these types of posts is an additional advantage in clinical practice.5

The gold standard for many years was the cast gold post-and-core; however, “given the scientific literature that has appeared, the creation of a conventional cast post/post core, even if passively cemented, is useless.”6 Goto et al., when looking at crown retention, have shown that fiber-reinforced dowels (posts) and bonded composite cores under fatigue loading provided significantly stronger crown retention than cast gold dowels and titanium alloy dowels with composite cores.7 A four-year clinical study comparing fiber posts and traditional cast posts showed that the cast post group had root fractures in 9 percent of the sample, while fiber posts had no root fractures.8 In a literature review by Heydecke, it has been shown that cast post-and-cores do not have a better performance than direct post-and-core restorations.9 Therefore, it seems that the current use of cast post-and-cores is difficult to justify.

The use of metallic posts for retention has been studied extensively. Stiff metallic posts work against the natural function of the tooth, creating zones of tension and shear, both in the dentin and at the interfaces of the luting cement and post.10 For teeth restored with stainless steel posts, a significantly lower load resulted in failure, as compared with those teeth restored with glass-fiber posts.11 There is a worse mechanical performance in teeth restored using stainless steel posts, with a high stress concentration due to the significant difference between the elastic moduli of the steel and surrounding materials.11,12 The use of fiber posts is preferable to titanium posts as well, with teeth restored with fiber posts exhibiting a significantly higher resistance to fracture than with titanium posts.13 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 of teeth restored with metallic posts.14 The more rigid metallic post can transfer more stress to the root than fiber posts, which increases the probability of vertical root fracture.15 Because fiber-reinforced posts have an elastic modulus that more closely approaches that of dentin,16 fiber posts produce less stress on the root dentin around the post tip than do metal posts.17 The nonmetallic 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 biometric behavior, reducing the risk of failure or fracture of the root.18 If fracture does occur, fracture with metal posts is usually catastrophic, while fracture of teeth restored with fiber posts show fractures that would allow repeated repair.19 In a literature search looking at 1,984 abstracts on the failure modes of post systems, Fokkinga et al. concluded that the fiber-reinforced post system more frequently showed favorable failure modes compared to metal posts.20

Many metallic posts are parallel, which can require the removal of significant amounts of root dentin in the apical half, while not adapting well to the coronal aspect of the root structure. In contrast, an anatomical fiber post requires minimal tooth structure removal during canal reshaping, and allows for greater post-to-canal adaptation in the apical and coronal half of the canal, with an esthetic nature. This provides a favorable foundation for eliminating discoloration caused by a metallic post placed under all-ceramic crown systems.21

Because fiber-reinforced posts have an elastic modulus that more closely approaches that of dentin, fiber posts produce less stress on the root dentin around the post tip than do metal posts.

As an added clinical benefit, fiber posts can have twice the fatigue resistance of ceramic or metal posts.22 Statistically, as well, resin-supported polyethylene fiber and glass-fiber dowels show the lowest coronal leakage when compared with stainless steel and zirconia dowels at all time periods.23-25

Not all fiber posts are created equal. In a study by Seefeld et al., looking at eight types of fiber posts, the fracture load of the tested systems ranged from 60 to 96 N and the flexural strength from 565 to 898 MPa. The differences in fiber diameter ranged from 8.2 to 21 µm and the fiber:matrix ratio from 41 to 76 percent.26 These fiber posts can be: zirconia enriched, glass fiber, quartz fiber, glass fiber-zirconia enriched, and glass fiber and carbon fiber,27 with quartz fiber posts recording significantly higher failure loads.19 They can be translucent, white, dentin or color-changing translucent,27 and for light transmission they can be excellent, good, fair or poor.28 Posts with a high translucency facilitate excellent esthetic results, eliminating show-through and discoloration while allowing transmission of light polymerization energy.29 Fiber posts can be tapered, double-tapered, parallel, parallel with a tapered end, parallel-tapered and serrated,27 with the double-tapered post closely imitating the post-endodontic shape of a radicular canal, leaving a thin and uniform thickness of cement at the post/canal interface. This improved adaptation of the post promotes the mechanical properties of the quartz fiber/epoxy material, instead of the weaker composite resin cement.30 The quality, type and volume of fibers, the way the fibers are silanated, and the type of resin used affects the clinical performance of these fiber posts, with some failing in cyclic fatigue to fracture in a few cycles and others in over two million cycles (DT Double Taper Light-Post radiopaque [RTD]).31

A new addition to the RTD family of fiber posts is the Macro-Lock Illusion post, which is fabricated with a high-percentage loading (80 percent vol) of pretensed long continuous translucent unidirectional quartz fibers with epoxy resin, and is radiopaque. Quartz crystal is the world’s purest form of silica. It is homogeneous and non-porous,32 and has an interface treatment of the fibers to achieve a perfect match between the thousands of fibers and the organic resin matrix. Color-changing technology allows for the post to disappear when seated at oral temperature and for it to be visible under water spray if retrieval is ever necessary. The Macro-Lock Illusion post has high translucency for good light transmission, an elastic modulus as low as 13 GPa (depending on the angle), and a flexural strength of 1600 MPa. It is unique in that it allows for micro and macro retention of the bonded core. A recent study by Dallari et al. showed that the Macro-Lock post with its macro retention reached one of the highest results when testing the retention of quartz fiber posts.33 No surface treatment of the post (etch, silane, sandblasting) is necessary before cementation.

Case Presentation

A patient presented to the office with a failed bridge from tooth #9 to tooth #11. The patient was advised that #11 was unrestorable and that one treatment plan option was to place a new 5-unit bridge from #8–12 with double abutments on the anteriors. The preoperative radiograph (Fig. 1) shows a short endodontic filling requiring retreatment, prior to placement of a post-and-core.

Figure 1: Preoperative radiograph of tooth #9.

Figure 2: Clinical presentation of loose bridge.

Access was difficult to negotiate at the canal terminus. The use of ultrasonic tips eventually removed the calcification deep into the canal at the apex and the tooth was re-treated endodontically. Figure 2 shows the preoperative photograph of the loose bridge before removal. At removal of the bridge (Fig. 3), it was confirmed that #11 was unrestorable, necessitating its extraction and the preparation of #12. Figure 4 shows the rubber dam application on tooth #9, with the occlusal view of the access into the canal in Figure 5. Twisted rotary endo files were used to remove the gutta-percha (Fig. 6), and ultrasonic tips were used to open the apical part of the canal (Fig. 7).

Figure 3: Clinical presentation of failed abutments.

Figure 4: Rubber dam application prior to beginning endodontic treatment.

Figure 5: Occlusal view of endo access.

Figure 6: Removal of existing gutta-percha.

As an added clinical benefit, fiber posts can have twice the fatigue resistance of ceramic or metal posts. Statistically … resin-supported polyethylene fiber and glass-fiber dowels show the lowest coronal leakage when compared with stainless steel and zirconia dowels at all time periods.

The trial cone was inserted and verified with a trial cone radiograph (Fig. 8). A warm vertical downpack was achieved with the System B™ (SybronEndo; Orange, Calif.) and the canal was backfilled with the Obtura II (Obtura Spartan; Algonquin, Ill.). The green Macro-Lock Illusion post drill #4 (RTD) (Clinical Research Dental; London, Ontario, Canada) was used to create the post space (Fig. 9). The post space depth was measured at 15 mm on a 19 mm root (Fig. 10). The post was inserted to verify fit and position (Fig. 11).

Figure 7: Ultrasonic tips used to open apical area of canal.

Figure 8: Trial cone radiograph.

Figure 9: Macro-Lock Illusion post drill #4 used to create post space.

Figure 10: Measurement of post at 15 mm.

Figure 11: Try-in of Macro-Lock Illusion post.

Figure 12 shows a magnified view of the Macro-Lock Illusion post seated in the canal. The post was trimmed with a diamond disc (Fig. 13). The post space was etched with Ultra-Etch® (Ultradent; South Jordan, Utah) for 15 seconds using an Endo-Eze™ tip (Ultradent) (Fig. 14) to reach the apex of the post preparation and inject from the apex to the incisal, so as not to entrap any air. The post space was thoroughly rinsed and a purple Capillary tip (Ultradent) was used to lightly dry the canal. A light-cure adhesive, proven to be compatible with dual-cure cements, was applied to the canal with a tapered brush long enough to reach the bottom of the post space (Fig. 15), and at the same time the light-cure adhesive was placed on the Macro-Lock Illusion post (Fig. 16).

Figure 12: High-power view of post try-in.

Figure 13: Trimming of the post with a diamond disc.

Figure 14: Ultra-Etch inserted into the canal from the apex coronally.

Figure 15: Placement of the adhesive into the canal. Remove excess.

Figure 16: Placement of adhesive on the Macro-Lock Illusion post. Air-dry and light-cure.

Figure 17: Light-cure of adhesive after evaporating solvent.

The solvent was evaporated with a light airflow and the bond cured in the post hole and the post (Fig. 17). A dual-cure resin cement was injected into the canal space from the bottom up (Fig. 18), and the post was seated into the dual-cure cement (Fig. 19). After curing the cement (Fig. 20), CosmeCore™ (Cosmedent, Inc.; Chicago, Ill.) was used to create the core buildup (Fig. 21), followed by a cure of 20 seconds (Fig. 22). CosmeCore can be used as the dual-cure cement and the core buildup simultaneously. Figure 23 shows preparation of the CosmeCore/Macro-Lock Illusion post-and-core for full coverage (note the color of the post when exposed to cold water). Teeth #8 and #12 were prepared and tooth #11 extracted (Fig. 24). A provisional bridge was fabricated using Temptation® (Clinical Research Dental) and cemented with Cling2 Temporary Cement (Clinical Research Dental) (Fig. 25). A final postoperative radiograph (Fig. 26) shows the opacity of the Macro-Lock Illusion post.

Figure 18: Placement of the dual-cure resin cement from apex coronally.

Figure 19: Post inserted into the uncured dual-cure resin cement.

Figure 20: Light-curing the resin cement.

Figure 21: CosmeCore buildup around the post.

Figure 22: Light-curing of the CosmeCore buildup.

Figure 23: Preparation of the post-and-core for full-coverage crown (note appearance of post when bathed in cold water).

Figure 24: Tooth #11 extracted and abutments prepared.

Figure 25: Temporization with Temptation and Cling2 Temporary Cement.

Figure 26: Postoperative radiograph showing radio-opacity of the Macro-Lock Illusion post.

Dr. Boksman is director of clinical affairs for Clinical Research Dental/Clinician’s Choice. He maintains a private practice in London, Ontario, Canada. Contact him at lboksman@clinicalresearchdental.com.

Dr. van As has lectured and published on lasers and the dental-operating microscope in practice. He maintains a private practice in North Vancouver, British Columbia. Contact him at glennvanas@mac.com.

References

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Reprinted by permission of Oral Health, November 2008.

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