The effect of fiber post length on fracture resistance of structurally compromised teeth
Structurally compromised teeth (SCT) are still considered as a dilemma for both clinician and patient. It is well known that endodontically treated teeth are much weaker than vital teeth because of considerable loss of valuable tooth structure1. Restorations of such teeth usually need more structural resistance features that may be derived from intra-radicular posts to add more retention of the restorative material to the root portion2.
Nowadays, many post and core systems are available in the dental field. They may be classified into two basic types; custom-made and prefabricated posts.
Prefabricated posts were first constructed from metallic materials such as stainless steel, titanium, or precious alloy. Later, due to increased demand on esthetic prosthesis, fiber reinforced composite posts were introduced in the beginning of 90s. They started with the introduction of carbon fiber posts, which had black color and did not solve the esthetic problem3. Later on, glass fiber posts were introduced which solved both the esthetic and biological downsides of metal.
Recently, many types of fiber posts are available for restoring SCT, they have many shapes, lengths and diameters. They are mainly composed of resin matrix strengthened by fibers, which may be zirconia, glass, carbon or quartz4. The use of fiber posts offers many advantages, including biocompatibility, esthetic properties, dentin-like rigidity, resistance to corrosion, and better mechanical properties that closely match those of natural teeth5.
The most important feature of glass fiber post is its chemical adhesion with bonding cement and composite resin cores6. This advantage gives the ability for more conservative post space preparation.
A wide range of recommendations have been made, regarding cast post length needed to best serve its purpose without compromising its strength, which include the following: (a) The post should be equal to the clinical crown, (b) The post should be longer than the clinical crown, (c) The post should be half of the root length, (d) The post should be two-thirds the root length, (e) The post should be four-fifths the root length, and (f) The post should be as long as possible without disturbing the apical seal7,8,9.
In 2015, A study by Latrash et al10, demonstrated that nayyar core restored premolars has more fracture strength than fiber post restored ones. In addition, it has been reported that, using a short FRCP rather than a longer one may improve both fracture resistance and restorability of tooth upon fracture of post/core/crown assembly11.
The recommended length of the post was transmitted automatically from the metal post to fiber post, without bearing in mind different biomechanical and fractographic behavior of both, and without studies conducted covering that area.
A question worth answering; does the length of fiber post really affect the fracture resistance of the structurally compromised teeth?
The null hypothesis of the study is that; there will be no difference in fracture resistance between tested fiber post lengths.
Materials and methods
Thirty-two sound freshly extracted mandibular premolars of approximate sizes were collected from surgery clinics at the Faculty of Dentistry, Beirut Arab University. They were randomly divided into four equal groups of 8 teeth each. (Table 1)
Teeth were cleaned using air-scaler and brass wire brush to remove any remnants. Then sterilized in an autoclave at 121°C, 15 psi for 40 minutes, and stored in distilled water for 72 hrs12.
A small access hole was drilled through the occlusal surface allowing access to the pulp chamber and root canals for treatment. Canals were disinfected using 5.25% sodium hypochlorite, then root length was confirmed visually by locating the tip of the file from the apical foramen.
Teeth were mounted centrally and vertically at the level of 2mm below the cement-enamel junction (CEJ) in an auto-polymerized resin blocks (Vertex, Netherland) with a size of 12×12×20 mm.
Teeth were endo treated using eugenol-free root canal sealer. Then crowns were amputated horizontally 2 mm above the cemento-enamel junction.
The recommended post space diameter is 1.3 mm consequently; fiber post size 2# was chosen which has 1.35 mm diameter13.
Gates Glidden drills size #1 followed by #2 were used to the planned depth (according to groups) to create a path way for Peeso-reamers to straighten up canals and remove remaining gutta percha on the walls. After each drilling, root canals were irrigated using NaOCl. Finally post-matched drill was used to create final post hole channel. All samples were irrigated using distilled water with endo-tip and dried using air jet then paper points to prepare them for cementation.
Each post was then inserted into canal and tried for fit and corresponding length. (Fig. 1)
Using intracanal tip, self-adhesive dual cure resin cement was injected into each canal, in an apical-coronal direction to minimize any void formation. The posts were then cleaned with alcohol, dried with oil-free air, and inserted into the canal. Each fiber post was held under pressure of curing tip, and the cement was cured for 20 seconds. After full polymerization each post was shortened to have a 2 mm coronal part.
Standardization of tooth preparation
The teeth were prepared using a high-speed handpiece fixed to a paralellometer surveyor to standardize design. A 0.5 mm finish line was prepared at the cemento-enamel junction with ferrule of 2 mm to receive a metal crown.
The coronal core portion was built with a light-cured core composite for one sample to duplicate it for all samples to standardise its form. An impression was taken with C-silicon material then poured with stone. The base block of the stone was trimmed to allow the vacuum celluloid sheet to have all details of the core form. Using this custom made core former, all specimens were built up in a standardized form then finished with fine finishing bur.
All specimens were scanned using CAD scanner to standardize the wax pattern. Once the wax pattern was completed, a spruing, investing, and casting the pattern was performed and tried in to make sure it fit the corresponding prepared tooth.
Finally, metal crowns were cemented using resin cement. Cementation was done under load of 5 Kg. All specimens were stored in distilled water for 48 h before testing. Then thermo-cycled for 1000 Cycles between 5°C and 55°C with dwell time of 30 seconds.
To simulate the occlusal contact points, wax-up was made resembles both the shape and size of the opposing teeth, casted to metal. And soldered to a metallic rod. After that, all specimens were statically loaded by the costum made metallic rod, at cross head speed 0.5 mm/min, in the central fossa parallel to the long axis of the tooth until failure.
The fracture load of each specimen was recorded in Newton (N). The results were recorded, tabulated and statistically analyzed.
Fracture mode analysis
The results were classified according to the mode of failure into restorable or unrestorable.
A. Fracture resistance:
Viewing the results; Group D scored the highest fracture resistance values (2670 N) followed by Group A (2668 N) and Group B (2609 N). Group C scored the lowest fracture resistance values (2517 N).
One-Way ANOVA test revealed no significant difference between the studied groups.
B. Failure mode:
Failure mode was analysed in table 2
Although most fractures were restorable, group A scored the highest percentage of restoring possibility followed by Group B and D and the lowest percentage was for Group C (Fig.6).
Chi-squre test also revealed no signicant differences in restoring possibility between the studied groups.
This in-vitro study investigated the effect of fiber post length on fracture resistance of structurally compromised RCT teeth. An ideal post system should exhibit fracture resistance higher than the average masticatory forces, with a clear correlation between post material and fracture of roots.
In this study, extracted lower premolar teeth were chosen because it’s a single rooted tooth with a single canal14.
Teeth were preserved immediately after extraction and were cleaned to prevent the changes in the dentine compositions and were sterilized in an autoclave, and stored in distilled water without using any fixation materials which may affect the bonding protocols12.
The canals were irrigated using 5.25% sodium hypochlorite solution, then obturated using single cone gutta-percha. Eugenol-free root canal sealer was used in order not to inhibit the resin polymerization. Many researchers investigated the effect of phenolic substances (e.g. eugenol, wintergreen oil) and they found an effect of these materials in polymerization of resin15..
In this study, a 2mm ferrule length was maintained in all groups in order to eliminate additional variable to the study.
Dual-cure self-adhesive resin cement was used to minimize the problems related to the difficulty of light reaching the most apical portions of the root canal, and to reduce the technique sensitivity that is associated with multiple-step adhesive systems16.
All the specimens in this study received full metal crowns. Because the presence of a prosthetic restoration generates a different biomechanical effect17, full metal was used to avoid the false reading from fracture of crown itself.
CAD-CAM resin wax was used to standardise the size and shape of the occlusal surface for all samples to insure that it will have the same direction of forces.
Increasing post length improves retention; however, in the apical region of the canal, the bonding between the post and dentin is unpredictable18. The use of adhesive resin cement resulted in increased tensile strength between short FRC posts and root dentin, and improved fracture resistance19. It has been reported that using a short FRC post rather than a long post may yield to a higher fracture resistance with more restorable fracture pattern11.
Moving through the results, no significant differences were detected between the studied groups which means that the fiber post length does not have an impact on the fracture strength of the tooth. On the other hand, favourable fracture was observed in all studied groups. Most of the specimens showed failure pattern at the coronal part above the CEJ which means the stress concentrates in the same area. This may be due to the 360° ferrule of 2mm height in all the specimens that distributes the applied forces and concentrates them at the narrowest point around the circumference of the tooth20.
These results are in agreement with many studies which revealed that the fiber post length does not contribute with fracture strength of the teeth21 and those with ferrule showed more satisfactory stress distribution and fracture resistance22.
It should be noted that groups A, B and D showed 0-1 non-restorable failure mode which may be due to the minimal ability of short fiber post to bend in its middle portion in group A and B. Regarding group D, the length of the post may dissipated the force along the root. On the other hand, group C showed the highest non-restorable failure mode may be due to bending of the post in a weak area of the root.
As these studies were in vitro, it is difficult to achieve standardization with regard to functional age of teeth, morphologic variations of the pulp, and abnormalities in dentin composition before tooth extraction (Meng et al. 2009). Differences in dentin composition may affect the resilience of the dentin and thus, change the fracture pattern during compression. These variations were not controlled in this study which probably constitute a limitation of the present analysis23. Hence, it is suggested to carry out similar researches in more simulating conditions to obtain more precise results.