An in vitro assessment of prefabricated fiber post systems
Erica C.N. Teixeira, DDS, MSc,
Fabricio B. Teixeira, DDS, MSc, PhD,
Jeffrey R. Piasick and
Jeffery Y. Thompson, PhD
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ABSTRACT
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Background. The authors conducted a study to characterize the in vitro retention, fracture and light transmission behavior of four different fiber-reinforced resin-based composite root canal posts.
Methods. The authors divided 44 teeth into four groups according to the type of post they would receive: parallel fiberglass posts, double-tapered fiber quartz posts, tapered fiberglass posts and two different types of parallel fiberglass posts. They prepared teeth and cemented posts with dual-cured resin cement. After the specimens aged, the authors conducted pull-out tests. For fracture testing, they loaded posts at 45 degrees in a universal testing machine. They determined load-to-fracture values and relative stiffness. They measured percentage of light transmission by means of a light microscope coupled with a spectrometer.
Results. Both tapered posts showed lower retention than did the parallel posts. Scanning electron microscropic analysis indicated that more cement adhered to the surfaces of the parallel fiberglass post than to those of the tapered fiberglass post. All posts demonstrated some plastic behavior, with the double-tapered fiber quartz post being stiffer than the others.
Conclusion. Parallel fiber-reinforced composite posts showed better retention than did tapered posts when a dual-cured resin-based cement was used. Translucent prefabricated posts have limited light transmission.
Clinical Implications. Parallel prefabricated fiber posts luted with dual-cured resin-based cement seem to be suitable for clinical application in endodontically treated teeth that require post-and-core restoration, showing good retention and acceptable fracture resistance.
Key Words: Prefabricated fiber posts; retention; light transmission; mechanical properties
Awide variety and number of prefabricated posts—made of stainless steel; zirconia; or carbon, glass or quartz fiber—are available in dentistry, in different geometries and sizes.1–3 Clinically, the necessity of placing a post arises when too little tooth structure is present to sustain a coronal restoration. One of the critical factors that can influence the survival of the restoration is the retention capacity of the post.4–6 The post must be cemented to the root canal walls in such a way that it cannot be dislodged by external forces.4
Some aspects that can influence the retention of a post include its length in the root canal, its size, its shape1–3 and the type of luting cement used to affix it.7–12 If a light-cured cement is used, the amount of light that can be transmitted by the post also can be a factor in its retention.9 The posts should transmit light to permit curing of the cement throughout the apical region of the tooth. Few reports have discussed the efficacy of light-transmitting posts.13,14 The use of dual-cured or self-curing resin-based cements has been recommended to bond fiber-reinforced, resin-based composite posts to root canal walls. The long-term performance of restorations in endodontically treated teeth with intracoronal posts depends on the retention of the post.7,15
Translucent and white fiber posts have increased in popularity in the last few years, mainly due to the fact that they can be used in high-demand cosmetic procedures, such as with all-ceramic restorations. Translucent posts are not visible through these types of restorations, thus yielding better esthetic results than metal and carbon fiber posts. Ceramic posts also offer good esthetics and are stronger and stiffer than fiber posts, but they are more difficult to bond to root canal walls.7–9 Several modifications have been made to fiber post composition, radiopacity and shape. However, most fiber posts are relatively radiolucent and have a different radiographic appearance than do traditional posts.4,16 These changes made to post configuration will, in part, affect their mechanical properties.17–19
It is critical that the clinician consider the mechanical properties of fiber-reinforced composite posts when designing or using a post restoration in an endodontically treated tooth. For example, the quality of the support of the coronal restoration can be reflected by the stiffness of the post, being related to loss of retention of a crown.17 It also has been shown in some studies that carbon posts are less stiff and have lower resistance to fracture when loaded than are metal posts.18,19 Posts with low strength and elastic limits have an increased risk of failure due to distortion or fracture.20,21 However, posts with elastic modulus similar to that of dentin induce less stress in the root. Some studies have claimed that the occlusal forces are better distributed through the length of the root if the rigidity of the post is closer to that of dentin.15,20,21
A restoration lacking resistance, form and retention is not likely to have long-term success. Less research has been published regarding the newer prefabricated fiber posts than regarding metal and carbon fiber posts. Therefore, we conducted a study to compare the retention, mechanical behavior and light-transmission capacity of four different prefabricated posts—tapered and parallel—cemented to the root canal with a dual-cured resin-based cement.
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MATERIALS AND METHODS
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We tested four fiber-reinforced resin-based composite posts according to retention behavior, relative stiffness, load to fracture and light transmission capacity:
- – double-tapered fiber quartz (D.T. Light-Post, Bisco, Schaumburg, Ill.);
- – tapered fiberglass (FibreKleer Tapered Post, Pentron Clinical Technologies, Wallingford, Conn.);
- – parallel fiberglass (FibreKleer Parallel Post, Pentron Clinical Technologies);
- – parallel fiberglass (FibreKor, Pentron Clinical Technologies).
Figure 1
shows the posts tested and its radiographic images.
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Figure 1. Fiber posts tested (A) and correspondent radiographic images (B). From left: FibreKor (Pentron Clinical Technologies, Wallingford, Conn.), D.T. Light-Post (Bisco, Schaumburg, Ill.), FibreKleer Parallel and Tapered posts (Pentron Clinical Technologies). Image of D.T. Light-Post reproduced with permission of Bisco. Image of FibreKleer and FibreKor posts reproduced with permission of Pentron Clinical Technologies.
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Evaluation of pull-out behavior.
We horizontally sectioned 44 extracted single-root canal human teeth at the cementoenamel junction with a diamond disk (Brasseler USA, Savannah, Ga.). We measured the roots to ensure that each was at least 12 millimeters in length. We instrumented root canals by using a high-torque (350 revolutions per minute) motor (Aseptico Electric Motor, Dentsply International, York, Pa.) and ProFile Series 29 rotary and ProFile .04 hand files (Dentsply International). We instrumented all roots up to a no. 40 master apical file, 1 mm from the anatomical apex. We irrigated the root canals with 1 percent sodium hypochlorite, followed by 17 percent ethylenediaminetetraacetic acid (EDTA), which we left in place for three minutes to remove the smear layer. After that, we flushed the teeth with saline solution. We carried out irrigation using a 10-milliliter plastic syringe with a 27-gauge needle to ensure that the irrigant approached the apex.
After we completed instrumentation, we randomly distributed the teeth into four experimental groups containing 11 specimens each. We prepared standardized post spaces in all root canals to a depth of 8 mm, using the manufacturers’ corresponding post drill system. After post space preparation, we rinsed each canal with 17 percent EDTA and distilled water (in that order) and dried it with paper points. We etched the root canal walls with phosphoric acid gel, rinsed them with water and dried them with paper points. We applied a primer/adhesive system (Bond 1 Primer/Adhesive, Pentron Clinical Technologies) and used a dual-cured resin based cement system (Cement-It, Pentron Clinical Technologies) to cement the posts into the prepared teeth. We mixed and handled the cement and used the bonding agent precisely according to the manufacturers’ instructions. We applied the cement in the root canal using a lentulo spiral and cemented the post to a length of 8 mm. After 15 minutes, we placed the specimens in a high-humidity environment at 37 C for 48 hours before pull-out testing.
After aging, we embedded the specimens individually in phenolic rings (Buehler, Lake Bluff, Ill.) with self-curing tray resin (Trayresin, Dentsply International). We placed the cylinders with the vertically aligned roots in a holding fixture in a universal testing machine (Evolution, MTS Systems, Eden Prairie, Minn.) one at time. We grasped the posts and pulled them out from the roots at a cross-head speed of 10 mm/minute. We placed a small amount of tray resin on the exposed portion (head or top) of each post to enhance gripping by the test fixture. We recorded the force required to dislodge each post.
Evaluation of fracture behavior.
To evaluate the posts’ facture behavior, we used a method described by Asmussen and colleagues.17 We drilled artificial root canals into aluminum blocks, the canals having a diameter (approximately 1.9 mm) corresponding to the diameters of the posts (and accounting for cementation). We seated posts to a depth of 10 mm and cemented them in place using a resin-modified glass ionomer luting cement (RelyX Luting Plus, 3M ESPE, St. Paul, Minn.). Prior to placing the posts, we adjusted (cut) them so that in the cemented state each protruded 5 mm from the canal. After hardening of the luting cement, we put the posts in an incubator for 24 hours at 37 C. We placed the testing assembly in a universal testing machine (Model 4411, Instron, Norwood, Mass.) and loaded each specimen to fracture on a 45-degree angle at a cross-head speed of 5 mm/minute (Figure 2). For each specimen, we recorded the load to fracture (in newtons) and the relative stiffness (N/mm) from the load-displacement curve.
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Figure 2. Testing assembly of the aluminum block with a prefabricated post (FibreKleer Parallel Post [Pentron Clinical Technologies, Wallingford, Conn.]) seated at 10 millimeters into the artificial canal used for the mechanical testing. Image of FibreKleer post reproduced with permission of Pentron Clinical Technologies.
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Evaluation of light transmission behavior.
Light transmission was measured using a transmission optical light microscope coupled with a spectrometer (Olympus BX1/BX2-RFA, Olympus America, Melville, N.Y.). The spectrometer used a 400-micrometer fiber-optic cable that measured transmittance on the focal plane of the microscope. For this test, we cut the posts to a standard length of 10 mm. We mounted the samples (cut end down) and took the measurements from the bottom end of each post. We transmitted a referenced light source up through the cut end and measured the intensity percentage of light (compared with the reference of 100 percent) for each post. Values are given as the percentage of incident light measured at the opposite length of the post.
Statistical analysis.
We analyzed all groups of specimens for means and standard deviations. Our comparison of the groups involved analysis of variance (ANOVA) and post-hoc Tukey test (P 
.05) (performed with SPSS software, release 9.0, SPSS, Chicago).
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RESULTS
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Pull-out testing.
The mean pull-out load values for FibreKor and FibreKleer Parallel were significantly greater than those for D.T. Light-Post and FibreKleer Tapered (Table 1). The mean pull-out load values of the two tapered post groups were not statistically different. Scanning electron micrographs (SEMs) showed more cement adhered to the surface of the FibreKleer Parallel Post surface after testing than to that of the FibreKleer Tapered Post (Figure 3). Figure 4 (page 1011) is an SEM of the post surface of a random FibreKor post (after pull-out testing).
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Figure 3. Scanning electron micrographs of post surface after pull-out test, with arrows indicating presence of cement. A. FibreKleer Parallel Post (Pentron Clinical Technologies, Wallingford, Conn.). B. FibreKleer Tapered Post (Pentron Clinical Technologies).
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Figure 4. Scanning electron micrograph of FibreKor post (Pentron Clinical Technologies, Wallingford, Conn.) before cementation and after pull-out testing. The serrated surface of the post is covered by resin-based cement after pull-out testing.
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Fracture testing.
The FibreKleer Parallel Post had the highest mean load to fracture, and this value was significantly higher than those of any of the other groups (Table 2, page 1011). FibreKor and FibreKleer Tapered Post had similar load-to-fracture values lower than the others. The D.T. Light-Post had an intermediate value. That group had the highest mean relative stiffness, which was significantly greater than that of the other post groups. FibreKor and FibreKleer Parallel Post had intermediate stiffness values, with the FibreKleer Tapered Post group having the lowest mean stiffness value. We should note that the absolute values for these measurements are a function of each post’s geometry, which is clearly different for each post type.
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TABLE 2 Means and standard deviations correspondent to load to fracture and relative stiffness of the fiber posts tested.
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Light transmission behavior.
D.T. Light-Post, FibreKleer Tapered Post and FibreKleer Parallel Post—all translucent posts—showed some light transmission capacity, but with values lower than 40 percent of incident light. The lowest value observed was for FibreKor, which demonstrated less than 1 percent light transmission (Figure 5, page 1011). The percentage values in Figure 4
derive from the curves at 470 nanometers (the approximate wavelength of a visible curing light).
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Figure 5. Light transmission capacity for the four posts tested. The percentage of light transmitted according to a reference point (100 percent) at 470 nanometers is shown. nm: Nanometers. D.T. Light-Post is manufactured by Bisco, Schaumburg, Ill. FibreKleer Parallel Post, FibreKleer Tapered Post and FibreKor are manufactured by Pentron Clinical Technologies, Wallingford, Conn.
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DISCUSSION
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The results showed parallel posts to have better retention than tapered posts. This observation has been demonstrated in other studies.1–3
The retention of a post can depend on its shape size and post length.2,8 In this study, we set the post length at an 8-mm depth, eliminating seating depth as a variable. In a clinical procedure, the length of the post should be equal to or bigger than the crown length, thus optimizing its retention. FibreKor showed the highest values in the pull-out test, not only because it is a parallel post but also owing to its different design. Although it is considered a passive post (because it does not have threads that engage in the dentin and does not induce internal stresses on insertion into the canal), the FibreKor post incorporates a serrated surface geometry that can increase the surface’s mechanical retention of luting cement. This can be observed in the SEM taken after pull-out testing (Figure 4), in which the serrations in the tail part of the post surface are covered by cement.
The retention of passive posts relies mostly on the cement used for luting, especially in the case of tapered posts.10 It has been demonstrated that resin-based cements have greater retention than do conventional cements, such as zinc phosphate.7,10,12 The newest cements depend on light activation or are dual-cured. Although the manufacturers claim that translucent posts can transmit light for enhancement of curing, few studies have been conducted to explore this issue. Our results showed that some light is transmitted through the posts at a depth of 10 mm. Roberts and colleagues14 showed that the hardness of a resin-based composite at 6 mm from the head of a post is much less than at 2 mm, even with the use of a translucent post. Although we observed that FibreKor had the least amount of light transmission, we observed that it showed the highest values in the pull-out test. For tapered posts evaluated in this study that have lower retention and only moderate light transmission through the post, the use of light-cured cements may not represent the best clinical scenario. In addition, clinicians should consider other factors related to the mechanical properties of endodontic posts when choosing a post.
Each prefabricated post system is manufactured in multiple widths and lengths to enable its ultimate selection for a given clinical situation. All the posts we chose for the study were adequate for similar clinical situations or root canal and tooth size. However, D.T. Light-Post is a double-tapered post, and its diameter varies throughout its length. In the case of the post type evaluated here (size no. 1, according to manufacturer), its diameter can reach 1.5 mm. FibreKleer Parallel Post and FibreKor post have a constant diameter throughout the post, though they are characterized by different initial lengths and head configurations. The FibreKleer Tapered Post is a single-tapered post, and the diameter varies from approximately 0.6 to 1.2 mm. The considerable differences in the mechanical properties of these posts are closely related to their configuration. Normalization of data is not trivial, and it would provide limited additional insight regarding expected clinical behavior, because the absolute geometry of the post, as provided by the manufacturer, is fixed.
We evaluated the fracture behavior of the posts at a 10-mm depth, with 5 mm of each post extending out of the artificial canal in the aluminum blocks. The FibreKleer Parallel Post group showed the highest mean load to fracture, probably due to the fact that at that depth the load was applied to a thicker cross-section of the post. In the FibreKleer Tapered Post group, the post configuration after 10 mm (bottom to top) is slightly straightened, with the post containing two recessed grooves in the neck region. This unique geometry likely is responsible for the lower mean load to fracture of this group when compared with that of the FibreKleer Tapered Post group.
The D.T. Light-Post group showed higher fracture resistance than FibreKleer Tapered Post and FibreKor. This post has a cross-section diameter of 1.5 mm, while that of the FibreKleer Tapered Post and FibreKor groups is approximately 1.2 mm. Because the diameter and the head part of the posts extending from the aluminum block were not the same, a direct comparison between the posts may be difficult. However, it is important to know how the fracture behavior of these posts is reflected in this type of situation. Newman and colleagues19 tested the fracture resistance of FibreKor post in human teeth and obtained an average value of 9.8 kilograms for a 1.5-mm diameter post. In our study, the fracture resistance observed for the same type of post with 1.2 mm diameter was 4.7 kg (45.8 N). The shape, configuration and design seem to have an important role in the mechanical behavior of fiber-reinforced posts.
Some in vitro studies have shown that the strength of fiber-reinforced posts is lower than that of metal posts.19 However, it also has been observed that a fiber post of larger diameter than a titanium post can be stiffer and more resistant to fracture.17 Regarding the stiffness of the posts in this study, D.T. Light-Post displayed higher stiffness than the other posts, which may be related to the fact that this post contains crystalline quartz fibers, which are more rigid than the glass fibers present in the other posts we tested. D.T. Light-Post also has a greater volume of fibers in its composition than does FibreKor (60 percent versus 42 percent, respectively).
Although there are different views regarding the required stiffness of posts,17 it seems that in the case of fiber posts, one should want a post that is as stiff as possible. Fiber posts are less stiff than metal posts and have less risk of causing root fracture. Also, the load values observed in this study to fracture the posts are less than those needed to cause a tooth root fracture. However, too much flexure produced under crown and core restorations may produce microcracks in the core material or in the resin cement, leading to the restoration’s failure.19,20 There seems to be some consensus that posts effectively bonded to the root canal walls might increase the strength of the tooth.4,5 Regarding stress distribution, the most important factor seems to be the configuration of the posts and not their stiffness.
Apart from the mechanical properties of posts, adequate radiopacity should be noted as a clinically relevant criterion for the selection of fiber-reinforced posts. High radiopacity makes radiographic verification and visualization easier and may reduce unwarranted retreatment of the tooth. Some of the fiber posts we evaluated had limited radiopacity, as shown in Figure 1. Therefore, many factors are involved in the selection of a prefabricated post for a clinical procedure, some of them assessed in this study.
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CONCLUSIONS
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Based on the results of this study, prefabricated parallel fiber posts were found more retentive than tapered posts. Some light transmission could be measured through the translucent posts, with the white posts having the least transmission. The mechanical behavior of different posts can vary significantly, being influenced significantly by post geometry.
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FOOTNOTES
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DISCLOSURE: Pentron Clinical Technologies (Wallingford, Conn.) provided the materials used in the study described in this article, including the posts made by Bisco (Schaumburg, Ill.).
Dr. Erica Teixeira is a teaching fellow, University of North Carolina School of Dentistry, Department of Diagnostic Sciences and General Dentistry, CB #7450, Chapel Hill, N.C. 27599-7450, e-mail "teixeire{at}dentistry.unc.edu". Address reprint requests to Dr. Teixeira.
Dr. Fabricio Teixeira is an associate professor, Department of Endodontics, School of Dentistry, University of North Carolina, Chapel Hill.
Mr. Piascik is a PhD candidate, Curriculum in Applied and Materials Sciences, University of North Carolina, Chapel Hill.
Dr. Thompson is a professor and the interim chairman, Department of Biomedical Engineering, University of Texas at San Antonio.
Research was supported by National Institutes of Health/National Institute of Dental and Craniofacial Research grant DE013511-04.
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ABSTRACT
MATERIALS AND METHODS
