The Effect of Fiber Dowel Heights in Resin Composite Cores on Restoration Failures of Endodontically Treated Teeth

Restored teeth weaken after endodontic treatment because of the loss of tooth structure and the necessary restorative and endodontic procedures.1,^–10 Cast gold posts and cores are used extensively in clinical practice because of their favorable physical properties.11^,12 Despite its popularity, the cast post and core restoration has a number of disadvantages, including tooth weakening as a result of the removal of tooth structure to accommodate the post and core,13,^–16 corrosion risks,11^,17^,18 poor stress distribution that can cause root fracture,1 difficulty in post removal, and the necessity of scheduling two appointments for fabrication.19^,20 Prefabricated metal posts offer some advantages over cast posts and cores and have also attained a degree of popularity.20,^–22 

Metal posts can create significant esthetic issues as a result of their coloration and interference with natural light transmission through the tooth and the gingival complex.11^,20 Oxidation and corrosion by-products of metallic posts have been reported to diffuse into roots and lead to irreversible discoloration and damage.17 

Fiber-reinforced (fiber posts) and ceramic posts provide improved esthetics.11^,20^,23^,24 Zirconia-based ceramic posts have been reported to possess good mechanical properties and biocompatibility,^25–28 but the chemical bonding of bis-GMA–based composite resins to zirconia is difficult to achieve. Zirconia posts are also difficult to remove.^29–32

Fiber posts consist of mineral fibers (ie, carbon, glass or quartz) embedded in an epoxy or resin matrix. They are anisotropic materials and have high fatigue and tensile strength.23 Their elastic modulus is similar to that of dentin,^28–33 which contributes to a “monobloc” concept (uniform stress distribution throughout the restored tooth, resulting in lowered core-dentin interface stress and failure rates).26^,34^,35 Glass or quartz fiber posts offer significant esthetic advantages and some are highly light-conducting.11^,26 Over the past decade, in vitro and in vivo testing have demonstrated that some fiber posts can dramatically reduce root fracture and tissue discoloration.^21,36–38

Residual tooth structure, when compounded with a fiber post and a resin composite core, appears to create a monobloc effect. However, the height of the fiber post within the resin composite core may also affect the longevity of the tooth. The aim of this study was to compare the effect of varying fiber post heights in resin composite cores on the fracture resistance of endodontically treated teeth.

Forty extracted maxillary central incisors were stored in isotonic saline solution containing thymol39 at room temperature. They were cleaned by ultrasonic scaling, and then stored in isotonic saline solution in a refrigerator. Selected teeth had 13 ± 2 mm of root length (measured from the labial aspect of the cementoenamel junction to the root apex); no carious lesions, cracks, or microfractures visible under fiber optic examination; and had never experienced endodontic treatment.

The teeth were randomly divided into 2 control groups of 5 teeth each (Groups 1 and 2) and 3 experimental groups of 10 teeth each (Groups 3, 4, and 5) (Figure 1). The pulp chamber of each tooth was instrumented to a 30 file size and filled with low-temperature gutta-percha (Obtura II, Obtura, Earth City, Mo) and root canal sealer (AH Plus, Dentsply De Trey, Konstanz, Germany). Each root was mounted in acrylic resin (Neocryl, Harry J. Bosworth Company, Skokie, Ill) in an 18-mm diameter plastic ring. The resin material extended 1 mm below the labial aspect of the cementoenamel junction.

An impression of the crown of each tooth was made with vinyl polysiloxane impression material (Reprosil, Dentsply/Caulk, Milford, Del) and poured with die stone (Vel-Mix, Kerr Corporation, Orange, Calif). Each tooth was prepared for an all-ceramic crown with a 1.5-mm full-shoulder finish-line preparation at the cementoenamel junction level.

Group 1. Gutta-percha was removed 2 mm below the labial aspect of the cementoenamel junction. Each tooth was acid-etched with 32% phosphoric acid (Uni-Etch, Bisco, Schaumburg, Ill) for 15 seconds and dried with a cotton pellet. Bonding agent was applied following the manufacturer's instructions (All-Bond 2, Bisco). The access opening was then filled with a dual-cured resin composite (LuxaCore, DMG, Hamburg, Germany) and light cured for 40 seconds.

Group 2. Gutta-percha was removed 8 mm below the labial aspect of the cementoenamel junction. Each tooth was irrigated with 10 mL of 2.5% NaOCl solution and then dried with cotton pellets and paper points. An 18-mm quartz fiber post (D. T. Light-Post, Bisco) was fitted. Each pulp chamber wall was treated with 3 mL of 17% ethylenediaminetetraacetic acid (EDTA) solution for 60 seconds followed by 3 mL of 2.5% NaOCl solution for 30 seconds, and then dried with cotton pellets and paper points. Three coats of primer (All-Bond 2) were applied within the canal, followed by sprayed air for five seconds, and the excess removed with a paper point. Preparation of the post surface was accomplished by wiping with a gauze square soaked in 70% methyl alcohol. The prepared post was then soaked in silane coupling agent (Porcelain Primer, Bisco), air sprayed for 5 seconds, cemented with LuxaCore Automix Dual (DMG), and light cured for 60 seconds.

Custom molds were made with clear vinyl polysiloxane impression material to preserve the individual coronal dimensions. These were later used as the molds for the cores.

Groups 3, 4, and 5. Each tooth was reduced to a level 1 mm coronal to the most incisal point of the interproximal cementoenamel junction and perpendicular to its long axis. Then, 10 mm of gutta-percha was removed. Preparation and cementation of 14, 16, and 18 mm D. T. Light-Posts were performed, as with Group 2. The cores were built up with LuxaCore composite resin to their former dimensions by using the previously fabricated custom molds. Each tooth was stored in 100% humidity for 24 hours.

All-ceramic crowns (IPS Empress, Ivoclar Vivadent, East Amherst, NY) were individually fabricated by a commercial dental laboratory. The internal surfaces of the crowns were treated with 4% hydrofluoric acid (Porcelain Etchant, Bisco) for 3 minutes, rinsed, and dried. Porcelain Primer was applied and dried and was followed by the application of a bonding agent (All-Bond 2).

Tooth preparations were treated with Uni-Etch for 15 seconds, rinsed, and moistened. Five coats of primer (Porcelain Primer) were applied, air dried for 5 seconds, and followed by a bonding agent (All-Bond 2). The crowns were cemented with resin cement (Duo-Link, Bisco) and stored in 100% humidity for 24 hours.

Each specimen was positioned in the mounting device and aligned at a 130-degree angle to its long axis (Figure 2). A universal testing machine (LR 10K, Lloyd Instruments, Fareham, UK) was used to apply constant load at a crosshead speed of 0.5 mm/min until failure occurred. Failure was defined as the highest level of loading force required to effect fracture of the restoration and/or the tooth. The mode of failure was observed and recorded for each specimen (Figure 3).

All data were analyzed with ANOVA and Duncan's test (P ≤ .5).

Fracture resistance force (in Newtons) was expressed as the mean of failure load values. The fracture resistance force and standard deviations of each group are shown in Figure 4.

The fracture resistance force for 3 of the experimental groups (Groups 3, 4, and 5), all of which had 6-mm high composite resin cores, differed based on the height of the D. T. Light-Posts within the core. The longest post length experienced the highest value for fracture resistance. Further analysis with Duncan's test showed that Group 2 (18 mm D. T. Light-Posts) was significantly stronger than the other experimental groups (Table 1). A one-way ANOVA indicated that there was a statistically significant difference among the groups (P ≤ .05) (Table 2).

Figure 3 and Figure 5 show the mode of failure that occurred. Every specimen experienced fracture in the IPS Empress crown restoration. In the control groups (Groups 1 and 2), fracture was often found in the coronal third of the dentin; this differed from the experimental groups (Groups 3, 4, and 5), where fracture was often at the resin composite core-dentin interface.

Studies have reported that a key factor in the failure of post and core restorations was coronal extension of tooth structure above the crown margin (ferrule).^40–42 A ferrule height of at least 1.5 mm is required to ensure a favorable prognosis. In this investigation a 2.0-mm ferrule length was used in the experimental groups (Groups 3, 4, and 5).

The D. T. Light-Post is a slightly double-tapered post, which closely resembles root form and transmits light. The light-transmitting property enables polymerization of the primer and cement through the post.26 

The maxillary central incisors were loaded palatally at 130 degrees to their long axes to simulate the contact angle found in Class I occlusions between maxillary and mandibular anterior teeth.43 However, submitting specimens to cyclic loading and then establishing their reaction to fatigue more accurately simulates intraoral conditions compared with monotonic loading.

Results of this study demonstrated that the higher the post within the composite resin core, the greater the resistance to fracture. The amount of remaining tooth structure is also an important factor in fracture resistance, as suggested by the comparison between Group 2 and Group 5. The fracture resistance force of Group 2 was significantly higher compared with Group 5 (Table 2). In the experimental groups, fracture was often found at the composite resin core and dentin interface because the interface area is the weakest point.44 

An additional factor contributing to failure may have been the inherent qualities of the fiber posts themselves. The quality and quantity of the fibers within the post affect its strength, as do the porosity and the bond between the fibers and the epoxy resin.45 Some fiber post fracture was observed in this study. Perhaps weakness can be introduced from the addition of radiopaquer, from debonding, or from error in the manufacturing procedure.30 

Within the limitations of this in vitro study, the results suggest that endodontically treated teeth should be restored with the longest possible post height while preserving maximum tooth structure.