An ex vivo comparative study on the retention of custom and prefabricated posts.

AIM: This study was designed to comparatively evaluate the effect of cyclic loading on the retention of custom-fabricated fiber-reinforced composite (CF-FRC), prefabricated metal, and glass fiber posts.
MATERIALS AND METHODS: Thirty mandibular first premolars decoronated at the CE junction were divided into three groups (n=10). Groups A, B, and C were restored using Para Post (Whale dent), Reforpost (Angelus), and CF-FRC post (Ribbond-THM), respectively. Five specimens from each group were subjected to cyclic loading. Tensile bond strength (TBS) was evaluated.
RESULTS: Pre-loading TBS values were statistically, significantly higher for all posts (P<0.05). Before and after loading, there was a significant difference between group C as compared to groups A and B.
CONCLUSIONS: Cyclic loading reduced the retention of all posts but was comparatively lesser for the CF-FRC post. This system provides sufficient retention required for clinical success.

INTRODUCTION

An endodontically-treated tooth that has lost significant tooth structure may require a post and core for restorative rehabilitation. In comparison to metal posts, the use of fiber reinforced composite (FRC) posts are becoming increasingly common as it offers improved esthetics, good fatigue strength, and potential to reinforce a compromised root.[1] Its modulus of elasticity is close to that of dentin and hence the distribution of stresses is uniform.[2] Moreover, the FRC post would yield prior to root fracture,[3] which makes it possible to salvage the tooth. The FRC post can be prefabricated or custom fabricated. Prefabricated carbon and glass FRC posts are designed by the manufacturer to fit specific dimensions. The chair-side custom fabricated fiber-reinforced composite post (CF-FRC) are formed by the dentist.[4] This post system uses nonpreimpregnated polyethylene fibers or glass fibers to form a post. It does not require drills for post space preparation and thus maintains the natural shape and strength of the root and eliminates the possibility of canal perforation.[5] Another advantage is its ‘pliable’ state, due to which it conforms to natural contours and undercuts of the canal providing additional mechanical retention. They are indicated in teeth with very large diameter or flared root canals.[6]

Clinically, posts are subjected to repeated tension, compression, and torquing forces.[7] Ideally, these forces should be uniformly distributed to the remaining tooth structure and supporting periodontal tissues.[8] This and post retention depend on the shape, length, composition, modulus of elasticity, and bonding of a post to the root dentin all along the root surface.[9] The retention of a bonded endodontic post reflects the effectiveness of the dentin bonding procedure, physical properties, and the interaction of post material to luting resin cement.[10]

The most common cause of post failure is loss of retention due to decementation.[11] Cyclic stresses of mastication can affect the bond of post either at the cement–post or cement–dentin interface.[12] The effect of occlusal stresses on bonding of a post into the canal can be evaluated ex vivo by change in retention potential of posts before and after cyclic loading.[13] Studies in the literature have focused on the retention of metal, ceramic, and prefabricated FRC posts luted with different cements.[14-16] However, information is scarce regarding retention of the CF-FRC post. This study comparatively evaluated the retention of CF-FRC to that of prefabricated metal and FRC posts before and after cyclic loading.

MATERIALS AND METHODS

Thirty permanent mandibular first premolars (extracted for orthodontic purpose) were collected and decoronated 1 mm above the cemento-enamel junction. Serial instrumentation of the root canal was performed. In this technique, Gates Glidden drills were used to enlarge the coronal portion of the canal. To standardize canal preparation, apex was enlarged to an ISO size of 40 and stepping back with progressively larger instruments to an ISO size of 70. Twenty-four hours post obturation, a post space of 9 mm depth and a diameter of 1.5 mm were prepared, using No. 5 Peeso reamer. Specimens were randomly distributed into three experimental groups (n=10). For Groups A and B prefabricated metal (ParaPost; Whaledent, International, NY, USA) and glass FRC post (Reforpost; Angelus, Londrina, Brazil) having a diameter of 1.5 mm were used, respectively. The length of prefabricated posts was adjusted to 14 mm by a cutting apical end of the post. The metal post was surface-treated by airborne-particle abrasion using 50 μm aluminum oxide particles. No surface treatment was done for glass FRC posts. The post space was etched with 35% phosphoric acid for 1 min. Prime and Bond NT Dual Cure, DENTSPLY, was applied to the etched canal space/post and light cured for 10 s. Calibra; DENTSPLY/Caulk, Milford, was placed into a prepared canal using a lentulo-spiral and simultaneously coated on the post. This was immediately inserted into the canal to a depth of 9 mm, leaving 5 mm of post for core foundation and light cured for 20 s.

For Group C, after etching/bonding and coating canals with the calibra cement as described above, thinner high modulus (THM) polyethylene fiber strips (3 mm wide, Ribbond-THM; Ribbond, Inc, Seattle, Wash) were cut into 30 mm long pieces, i.e. excess of twice the length of the post space (9 mm) and core height (5 mm). For fabrication of CF-FRC post, two pieces of cut fibers were used. Using a special post and core instrument, the dentin bonding agent soaked fiber was inserted into the post space. Another piece of fiber was inserted at the right angle to first fiber leaving excess projecting out of the root for the core build up. This was light cured for 20 s.

Light cure composite resin (Esthet X, DENTSPLY International Inc., Milford, DE) cores were made over a portion of the post (5 mm) projecting out of the canal, with a specially designed metal ring so that the dimension of core on all test specimens was uniform.

Five specimens, from each group, were subjected to cyclic loading on the core at an angle of 45° to a long axis of tooth/post, using an indigenously built cyclic loading machine. This was designed to provide a force of 60–70 N at a frequency of 4–6 Hz. Loading of each specimen was done for 7 h to complete 1,50,000 cycles, simulating 6 months of clinical usage.

Specimens were subjected to pull-out the tensile bond test on an Instron universal testing machine (Zwick testing instrument, Model: 5582). A crosshead speed of 1.0 mm per min along the long axis of the post was set until post dislodged. The dislodged posts were examined visually to ascertain failure site and interface. Data were recorded and subjected to statistical evaluation using SPSS 11.5 for Windows.

RESULTS

Mean TBS values for prefabricated metal, fiber, and customized fiber posts are shown in Table 1. There was a statistically significant difference between ‘pre-cyclic loading’ and ‘post-cyclic loading’ samples in all the three groups (P<0.05).

Table 1

Mean tensile bond strength of all three groups

On applying the Kruskal–Wallis test, the overall significant difference between three groups with no cyclic loading was 0.007 (P<0.01) and after cyclic loading, it was 0.008 (P<0.01) [Table 2]. For multiple comparisons, the post-hoc test was applied. It was found that for both, without and with loading, Groups A and B were not significantly different from each other but Group C was significantly different from Groups A and B. The results indicated that cyclic loading reduces retention potential of all three types of post.

Table 2

Comparison of tensile bond strength values among the three groups

DISCUSSION

The results of this comparative study demonstrated significant lowering of tensile bond strength (TBS) pre- and postloading (P<0.05). This suggests that loading forces cause weakening of either the post–cement, cement-dentin interface or both, resulting in decreased retention of the post. Bolhuis et al.,[13] in a SEM study, found crack formation in the adhesive cement layer and a loss of adaptation of cement-to-metal and fiber posts after fatigue loading, indicating that posts had to carry a significant part of masticatory load. In this study, cyclic loading resulted in no visible movement or failure of cores of any of the samples. These findings were similar to that of Dietschi et al.[17] and Isidor et al.[18] who found no core failures even after 2,50,000 and 10,00,000 impacts of 55 N, respectively. After tensile testing, specimens were also analyzed visually for mode of failure. In Group A, all failures were mainly at the post–cement interface, both before and after loading, that suggests either weak or no chemical bonding between the metal post and resin cement.

In Group B, nonloaded specimens failed at dentin–cement and loaded specimens failed at the post–cement interface, suggesting a stronger bond between the post–cement interface before loading.[19] In the case of nonloaded specimens, the post–cement interface was not disrupted. Cement embedded along serrations of the post could have resulted in an increased mechanical retention and thus mode of failure was along the cement–dentin interface. This finding was in agreement with the previous study by Bonfante et al.[14] For loaded specimens, force transmission from the post to dentin along the post–cement interface was not smooth and homogenous; instead it was interrupted at the post–cement interface, causing its weakening, resulting in failure at this interface. Chemical bonding, even if present, is not sufficient to withstand loading.[15] This finding, however, contradicts concept of true mono-block between fiber post, resin cement, and dentin. Calibra (methacrylate based) luting cement and prefabricated fiber-reinforced composite posts ‘Reforpost’ (an epoxy resin based) were used in this study.[16] Adhesion of resinous material to an already polymerized substrate by free radical polymerization is minor, as no chemical reaction occurs between methacrylate-based luting cements and well-polymerized epoxy polymer.[20]

In Group C, all the specimens failed at the cement–dentin interface. The post–cement interface was virtually absent in this post system as the resin cement and fibers used to fabricate post got intermingled into each other to form one unit.

The mode of retention of prefabricated parallel post under masticatory load is primarily mechanical, with interfacial sliding frictional forces between the post and cement playing an important role in resisting its dislodgement.[21] Chemical bonding between post and cement plays a significant role in post retention. Johnson and Sakumura[22] reported similar findings with metal post where tensile forces in the range of or greater than that required to break the cement bond were required to remove parallel-sided posts.

Descending order of TBS with and without loading was as follows: Prefabricated metal, fiber, and customized fiber post. These results suggests that if prefabricated posts have similar surface retentive designs and all other factors which can influence retention of post are standardized, then post material (metal or fiber) does not have significant influence on retention of posts, both before and after loading.

Lower values obtained for customized posts were probably due to post space preparation which is usually not recommended, except for removal of gutta-percha using heat. However in this study, for standardization, parallel post space was prepared, which probably deprived additional mechanical retention provided by small undercuts present in the canal. Under present conditions, i.e. in the absence of crown and ferrule, customized fiber posts had mean retentive values of more than 200 N both before and after loading. This is considered to be the minimum desirable TBS for a post to show clinical success.[23] Moreover, prefabricated posts have mechanical retentive features on their outer surface such as serrations, which get embedded into the resin luting cement and resists dislodging forces on pulling. In customized fiber post, fibers and resin cement are mixed into the post space and they form one unit on curing which has its outer surface as smooth as the dentinal wall. Therefore, its retention was primarily due to micromechanical bonding between the resin matrix of the post and dentinal wall of the post space. Once this bond was ruptured on pulling, there was no additional mechanical retention to resist dislodgement of the post, except for some frictional resistance between post and dentin.

The percentage loss of retention for prefabricated metal and fiber posts was in the range of 33–35%, in comparison to 23% for customized fiber posts. Rigid materials have tendency to transfer functional stresses to comparatively less rigid material, causing it to fail first.[24] Therefore, it can be conjectured that since metal and prefabricated posts have a higher modulus of elasticity, they transferred higher load stresses to the post–cement interface, causing its weakening. Customized fiber posts with comparatively lower modulus of elasticity than dentin, either absorbed or distributed/transferred load stresses along the dentinal wall so well that the post–dentin interface was least stressed and the resultant loss of retention was minimal.

When placing a post, the best approach that provides maximum fracture resistance for the tooth and retention of the core must be chosen. It was concluded from this study that though retention of customized fiber post was less than prefabricated posts, it provided sufficient retention required for clinical success, i.e. minimum of about 200 N under the masticatory load. A future study comparing retention of customized fiber posts without any specific post space preparation with that of prefabricated parallel posts could evaluate actual comparative retention of these posts.

CONCLUSIONS

Cyclic loading reduced retention potential of all post systems. Pre- and post-loading retention of prefabricated metal and glass fiber posts were comparable; however, it was reduced for CF-FRC. The effect of cyclic loading on loss of retention was significantly lesser for CF-FRC.