New monolithic post-core concept: a comparison of prefabricated and CAD-CAM fabricated post-cores

Abstract

Purpose:

The aim of this in vitro study was to evaluate the fracture and pull-out strengths of various post–core systems.

Materials and methods:

Forty-eight endodontically treated maxillary central incisors were assigned to three post–core groups: fiber post–composite core (RelyX) (FC), prefabricated monolithic fiber post–core (FiberSite) (MF), and CAD-CAM fabricated customized monolithic zirconia post–core (InCoris ZI) (MZ). Post–core designs were standardized according to the FiberSite system. After cementation (RelyX U200), the samples were divided into two mechanical test groups—fracture strength and pull-out strength (n = 8 per subgroup). Tests were performed at a crosshead speed of 1 mm/min, and failure types were recorded.

Results:

For the fracture strength test, the highest values (N) were obtained in the MZ group. The difference between MF and FC was not statistically significant (p = 0.190). For the pull-out test, statistically significant differences (N) were found among all groups (p less than 0.001). The MZ group exhibited predominantly catastrophic failures in both tests, whereas the MF and FC groups mainly showed Type II and Type III failures.

Conclusion:

Within the limitations of this study, the FiberSite system appears to have clinical potential for restoring severely damaged teeth. This system offers a balance of mechanical strength and favorable failure modes compared with CAD-CAM zirconia post–cores, which demonstrated catastrophic failures, and fiber post–composite cores, which exhibited lower strength values.

Introduction

Teeth with excessive substance loss are commonly restored using postcore systems, which provide mechanical support and retention for prosthetic restoration and preservation of the remaining tooth structure (1). The appropriate selection of a post is crucial for minimizing the risk of root fractures. Additionally, the amount of remaining tooth structure, along with the characteristics of the post, significantly affects the fracture resistance of the restored teeth (2). The material types and production methods of post-core systems vary widely. Fiber posts, known to provide modulus of elasticity similar to that of dentin, have been shown to cause minimal risk of catastrophic root fractures (3, 4). However, when compared with custom-made posts, the prefabricated fiber post may not provide adaptability in wide, oval, or extremely tapered canals, which may also affect retention.

In addition to the conventional prefabricated post and composite core system, which is widely used for restoring endodontically treated teeth with severe crown damages (5), the FiberSite system (Mega Dental, Partanna, Italy) has been introduced as a novel concept (6). The FiberSite system is a monolithic post-core design with a preshaped abutment composed of fiberglass-reinforced epoxy resin. This prefabricated single-unit post-core system is designed to accelerate restoration workflows and reinforce the remaining tooth structure by distributing stress more evenly and reducing accumulation at the bonding interface between materials with differing elastic moduli (6, 7).

The growing adoption of digital dentistry has introduced technological advancements that enhance efficiency and precision. Digital workflows utilizing intraoral scanners (IOS) and CAD-CAM technology enable the fabrication of customized restorations, including individual posts and cores (8, 9). CAD-CAM systems allow for better adaptation of post-core structures to root canals with respect to conventional prefabricated post-core systems. These customized restorations can often be completed chairside, further enhancing clinical workflow (10, 11). Zirconia, a popular CAD-CAM material, is frequently used for customized post-core applications due to its biocompatibility, high flexural strength, enhanced teeth fracture resistance, and superior aesthetic qualities (12). However, zirconia’s high modulus of elasticity poses a potential risk for catastrophic tooth fractures, creating clinical challenges (13, 14).

The design and material properties of post-core systems play an important role in their fracture resistance and longterm clinical success (15, 16). In addition to the fracture strength tests, which evaluate material performance under applied forces, pull-out bond tests are valuable for assessing the adaptation and cementation of post-core systems to root canal walls (17, 18). This study evaluates the fracture and pull-out bond strengths of the recently introduced FiberSite system, comparing its performance to that of a prefabricated glass fiber-composite core system and a CAD-CAM fabricated custom zirconia post-core system. The results of the two mechanical tests (fracture strength and pull-out bond strength) are analyzed and discussed independently, as there is no direct correlation between the tests. The mechanical performance of these systems was evaluated based on several factors, including the post-core materials, fabrication methods (prefabricated or customized), post-adaptation to the root canal, post-to-canal wall gaps, and whether the post and core parts formed a monoblock structure.

Although prior studies 7, 19, 20) have compared the FiberSite system with prefabricated glass fiber posts and composite cores, limited data exist regarding its physical and mechanical properties. Additionally, no studies in the literature have compared the FiberSite system with CAD-CAM fabricated monolithic zirconia post-cores or evaluated both the fracture and pull-out strengths of the FiberSite system relative to other post-core systems. The aim of this study was to compare different monolithic post-core systems with conventional fiber post-core systems in terms of fracture and pull-out strengths. The null hypotheses of this study were as follows: the type of post-core system does not affect the fracture strength of the restored teeth, the type of postcore system does not affect the bond strength between post and root dentin, and failure types are not influenced by the different post-core systems.

Materials and methods

Ethical statement

This project has been reviewed and approved by the Ethical Committee of Kocaeli University (2022/140).

Sample size estimation

Power analysis was conducted using G-Power 3.1.9.4 (Heinrich, Heine University, Dusseldorf, Germany) based on data from a reference article (7). The analysis determined that a minimum of six specimens per subgroup was required (power=0.95; effect size=2.14). However, a sample size of eight specimens per subgroup (n=8) was used to further increase the study’s power. Forty-eight freshly extracted, sound human maxillary central incisors with similar mesiodistal and buccolingual dimensions were collected (N=48). Specimens were free of caries, previous treatment and root fractures. They were stored in distilled water.

Endodontic treatment and root preparation

All teeth were endodontically treated to replicate the clinical procedure of post-core systems. The access cavity was prepared, and working length was established 1 mm short from the root apex. Root canals were prepared up to F4 with the Protaper System (PTU; Dentsply Maillefer, Baillagues, Switzerland) according to the manufacturer’s instructions. The root canals were obturated with lateral compaction of gutta-percha using AHPlus root canal sealer (Dentsply Maillefer, Baillagues, Switzerland). The teeth were sectioned 1 mm above the cementoenamel junction under water cooling using a diamond bur. The roots were then embedded in auto-polymerizing acrylic resin (Meliodent, Heraeus Kulzer, Hanau, Germany) up to 2 mm below the cementoenamel junction (N=48). All procedures were carried out by the same operator.

Post-core system preparation

The roots were divided into two mechanical test groups, namely fracture strength and pull-out strength. Each test group was further divided into three subgroups (n=8/subgroup) based on the post-core systems: MF: Prefabricated monolithic fiber post-core (FiberSite, Mega Dental, Partanna, Italy), MZ: Zirconia monolithic post-core manufactured by CAD-CAM (In Coris ZI, Dentsply Sirona Dental Systems GmbH, Bensheim, Germany), FC: Fiber post-composite core (Rely X, 3M ESPE, Seefeld, Germany). The brand names, manufacturers, and chemical compositions of the materials used in this study are listed in Table 1.

Table 1.

The brand names, manufacturers, and chemical compositions of the materials used in the study.

Brand	Manufacturer	Chemical composition
inCoris ZI	Sirona Dental Systems GmbH, Bensheim, Germany	ZrO2+HfO2+Y2O3 (> 99.0), Al2O3 (< 0.5), Other oxides (< 0.5)
FiberSite	Mega Dental, Partanna, Italy	Glass fiber, epoxy resin matrix
Fiber glass post	Rely X, 3M Espe, Seefeld, Germany	Glass fibers embedded into a composite resin matrix
Rely X U200	3M ESPE, Seefeld, Germany	Base paste: Methacrylate monomers containing phosphoric acid groups, Methacrylate monomers, Silanated fillers, Initiator components, Stabilizers, Rheological additives
		Catalyst paste: Methacrylate monomers, Alkaline(basic) fillers, Silanated fillers, Initiator components, Stabilizers, Pigments, Rheological additives

Post and core preparation

In the MF group, post preparations were performed using an orange drill (Fibersite, Mega Dental) with a 10-mm depth, a 1.1-mm apical diameter, and a 5-mm core diameter, operated with a low-speed micromotor.

In the MF group, post preparations were performed using an orange drill (Fibersite, Mega Dental) with a 10-mm depth, a 1.1-mm apical diameter, and a 5-mm core diameter, operated with a low-speed micromotor.

In the MZ group, post preparations followed the same procedure using the Fibersite drill to ensure consistency in canal preparation and post dimensions. For the fabrication of the zirconia monolithic post-core, the prefabricated FiberSite post-core was scanned using a lab scanner (InEos X5, Sirona Dental Systems, NY, USA), and a digital impression was obtained in the CAD-CAM software (CEREC AC, Sirona InLab V4.2.5; Sirona Dental Systems, NY, USA). The postcore form was designed using the crown restoration mode, and the scanned data were used as a biogeneric reference. Then, the MZ post-cores were then milled in the milling unit (CEREC MCXL, Dentsply Sirona, NY, USA), using monolithic zirconia discs (In Coris ZI, Dentsply Sirona, NY, USA) with 20- 25% of enlarged volume to compensate for sintering shrinkage. The milled zirconia monolithic post-cores were sintered in a furnace (InFire HTC Speed, Sirona Dental Systems, NY, USA) according to the manufacturer’s instruction. In the FC group, post preparations were performed using a red drill from a glass fiber post kit (RelyX, 3M ESPE, MN, USA), with diameters ranging from 0.8 mm to 1.6 mm. Drill depth was standardized to 10 mm using a silicone stopper. Posts were cut to a total of 15 mm, including 10 mm for the root canal and 5 mm for the core. For core preparation, composite material was incrementally placed on the cemented glass fiber posts using a silicone index obtained from the MF specimen to ensure uniformity. The representative specimens according to each group can be seen in Figure 1.

Figure 1.

Schematic representation of the prefabricated monolithic fiber post-core (MF), CAD-CAM fabricated customized monolithic zirconia post-core (MZ), and fiber post-composite core (FC) systems used in this study, respectively.

Cementation procedure

Monolithic zirconia post-cores were air-abraded with 50- μm aluminum oxide particles at 2.5 bar pressure for 4 seconds, ultrasonically cleaned, and air-dried. The prepared root canals were cleaned with 2 ml of 2.5% sodium hypochlorite solution, followed by 17% EDTA and rinsed with distilled water, and then dried with paper points. A dual-cure self-adhesive cement (RelyX U200, 3M ESPE, MN, USA) was applied using a lentulo spiral, and monolithic post-cores and glass fiber posts were seated with finger pressure. Excess resin cement was removed, and polymerization was performed with a light-curing unit (Elipar Deep Cure-S, 3M ESPE, MN, USA) for 40 seconds.

Mechanical tests

Fracture and pull-out strength tests were performed using a universal testing machine (Autograph AG-500 kNG; Shimadzu, Kyoto, Japan) at a crosshead speed of 1 mm/ min until failure occurred. A vertical load was applied along the long axis of the teeth in both mechanical tests to standardize the testing conditions. Although oblique forces are generally more representative of the intraoral loading conditions for central incisors, vertical loading was preferred in this study to ensure consistent and accurate measurement of fracture strength, because of the monolithic post-core design and the absence of crown restorations. Failure types were classified as follows: Type I: Debonding of post-core, Type II: Fracture of post-core, Type III: Fracture of the postcore / tooth complex above the cementoenamel junction, Type IV: Fracture of the post-core / tooth complex below the cementoenamel junction. Types I, II and III were categorized as “repairable” failures, while Type IV was classified as “non-repairable” (21, 22).

Statistical analysis

Fracture and pull-out strength values (Mean ± SD; median, min, max) were analyzed separately in IBM SPSS 26 (Armonk, NY, USA). Normal distribution was confirmed using the Kolmogorov-Smirnov test. One-way ANOVA test was performed to investigate the effects of different post-core systems on fracture and pull-out strength. Multiple comparisons of the three different post-core systems were analyzed by using the post-hoc Tukey test. P values less than 0.05 were considered to be statistically significant.

Results

The fracture and pull-out strength values (Mean ± SD; min, max) are presented in Table 2, while data distribution and variability are illustrated with box plot graphs in Figure 2.

Table 2.

Fracture strength and pull-out strength test values (N) of post-core combinations prepared with different materials. The same superscript small letters represent no significant difference in the column.

Fracture Strength Test		Pull-out test
	Mean±SD (N)	(min-max)	Mean±SD (N)	(min - max)
Fiber-composite (FC)	392.95±81.50 a	(251.67-480.53)	141.16±36.28 a	(100.39-193.12)
Monolithic fiber (MF)	633.75±136.40 a,b	(419.36-786.59)	233.92±50.39 b	(180.20-313.24)
Zirconia post-core (MZ)	946.33±432.26 b	(325.20-1631.06)	50.72±19.82 c	(27.83-84.85)

Figure 2.

Boxplot graphs illustrating data distribution. (a) fracture strength test, (b) pull-out strength test.

For the fracture strength test, the highest values were observed in the MZ group (946.33 ± 432.26). The differences between MZ (946.33 ± 432.26) and MF (633.75 ± 136.40) (p = 0.253), and MF (633.75 ± 136.40) and FC (392.95 ± 81.50) (p = 0.190) were found statistically insignificant (p > 0.05), whereas fracture strength values between MZ and FC were statistically significant (p = 0.001).

For the pull-out test, the highest values were observed in MF group, whereas the lowest values were observed for MZ group. Statistically significant differences were found among all groups [(MF:233.92 ± 50.39), (FC:141.16 ± 36.28) and (MZ: 50.72 ± 19.82)] (p less than 0.001).

Following the fracture and pull-out strength tests, failure type distributions for each post-core system are presented in Table 3. For the fracture strength test, repairable failures (Type II and III) predominated in the FC and MF groups. Catastrophic failures (Type IV) were only observed in the MZ group, in which it was observed as the most common type of failure.

Table 3.

Failure types distributions of the groups. Type I: Debonding of post-core, Type II: Fracture of post-core, Type III: Fracture of the post-core/tooth complex above the enamel-cement junction, Type IV: Fracture of the post-core/tooth complex below the enamel-cement junction.

FRACTURE STRENGTH TEST			PULL-OUT TEST
	FC	MF	MZ	FC
Type I				2
Type II	6	5		6
Type III	2	3	2

For the pull-out strength test, debonding of post-core (Type I and II) was seen in the FC group, with Type II being the majority. Type III and Type IV failures were seen for both MF and MZ groups. However, catastrophic failures (Type IV) occurred frequently in the MZ group compared to the MF group.

Discussion

In this study, the fracture and pull-out strengths and fracture modes of various post-core systems were investigated. The first null hypothesis was rejected because the differences between the fracture strength of prefabricated fiber post-composite core and monolithic zirconia post-core were found statistically significant. The second null hypothesis was rejected due to significant differences in pull-out strength values among the three post-core systems. The third null hypothesis was also rejected because catastrophic failures (Type IV) were observed for only the CAD-CAM fabricated customized monolithic zirconia group for the fracture strength test and were more common in this group for the pull-out strength test.

The FiberSite system, a monolithic post-core combining both post and core components, has been evaluated in a limited number of studies on the comparison of fracture strength with prefabricated post applications. Eren et al. (20) reported that the differences between the fracture strength of FiberSite (254.5 ± 69.1 N) and fiber post-composite cores (281.9 ± 66.5 N) on mandibular premolars, which were endodontically filled with AH Plus, were found statistically insignificant. This statistical result was similar to our data, although the Fibersite group (633.75 ± 136.40 N) showed numerically higher fracture strength values than the fiber post-composite core group (392.95 ± 81.50 N), in our study. Özyürek et al. (7) found significant differences in fracture strength between FiberSite (458.8 ± 82.6 N) and fiber posts with two different composite core materials (528.6 ± 118.7 N and 732.4 ± 161.3 N). These results were attributed to the grooves and two-stage design of the monolithic FiberSite, which may weaken fracture strength compared to ungrooved glass fiber posts (7, 23). In our study, the monolithic structure of FiberSite, which avoids the use of materials with different elastic moduli, may explain its higher fracture strength compared to fiber post-composite cores. The differences in observed fracture strength values with respect to the literature could be potentially attributed to the differences in the force axis, which is 30º to the long axis of premolar in the literature (7), whereas the mechanical test was performed in the direction of the long axis of central teeth in our study. In the same study, it is reported that fiber posts provide improved adhesion to dentin, in parallel with the higher fracture strength values (7). However, due to the higher pull-out test values obtained in our study, it can be stated that FiberSite, which has more retentive surfaces than fiber posts, adhered stronger to the root canal walls and adhesive cement. In the literature, according to the fracture type analysis, it is reported that most favorable failures were seen in all groups. However, more catastrophic failures were observed in the FiberSite group with respect to the fiber post-composite core groups (7). In another study, it is stated that mixed non-catastrophic failures were observed for the fiber post-composite core group (24). As similar to these studies, only repairable failures were observed in both the FiberSite and the fiber post-composite core groups for the fracture strength test in our study.

With advancements in digital dentistry, CAD-CAM fabricated custom monoblock zirconia post-cores have been studied for comparison with the conventional or prefabricated systems. Anweigi et al. (25) reported no statistically significant differences in fracture strength among CADCAM fabricated zirconia post-core (1567.26 ± 317.66 N), cast metal post-core (1355.92 ± 621.56 N) and prefabricated fiber post-composite core structure (725.67 ± 251.05 N). Our study similarly observed higher fracture strength for CADCAM zirconia post-core (946.33 ± 432.26) compared to fiber post-composite cores (392.95±81.50), though the differences were statistically significant. Non-repairable failures, such as root fractures, are frequently reported for zirconia postcores (14, 26). Similarly, in our study, mostly catastrophic failures predominated in the CAD-CAM fabricated custom zirconia post-core group.

The low pull-out bond strength of CAD-CAM fabricated custom zirconia post-cores in our study likely reflects the weak bond strength of zirconia to cement (27, 28). Additionally, the use of self-adhesive cement without silane could potentially affect to the reduced bond strength. In contrast, the higher pull-out strength of FiberSite can be attributed to its retentive form, stemming from its two-stage design and monoblock structure. According to the fracture type distribution, catastrophic failures were mostly observed in the zirconia group, followed by the FiberSite group. The adhesive failures observed in the fiber post-composite core group are similar with the literature (29), which compares the pull-out strength of the fiber post-composite core with respect to the endocrowns. In the literature, no study has been found on the pull-out strength of FiberSite. Further in vitro and in vivo studies will be required to explore the mechanical and clinical performance of the FiberSite system.

High standard deviations were observed in both fracture strength test and pull-out test values across all groups. This variability may be attributed to several factors, including anatomical differences among human teeth, irregularities in cement layer thickness, and possible inconsistencies in specimen positioning during testing. Although maxillary incisors with similar dimensions were selected to promote standardization, identical tooth composition, dentin thickness, and root canal morphology may not be achieved, due to natural variation. Therefore, the cement gap formed between the post and root canal may have varied across specimens, especially since customized post-core systems were not used. These heterogeneities in the specimens may have affected both specimen orientation and mechanical behaviour during testing.

Some studies in the literature have preferred to mimic the periodontal ligament by using silicone impression materials, whereas other studies (29, 30) have not used the application of the periodontal ligament simulation. The reason for not using periodontal ligament simulation has been stated as that displacement of the samples may occur during mechanical tests, and the mechanical test values may not reflect the reality. The periodontal ligament mimicking was not applied in our study due to the same reasoning.

The samples were not subjected to thermal cycling to mimic clinical aging, and only one CAD-CAM material was used in the custom post-core group, which could be considered as the limitations of the present study. Usage of the vertical mechanical loading could also be a limitation of the study, as it does not fully imitate the chewing bite axis of central incisors. Moreover, the cementation procedure without silane, which may have negatively affected the bond strength of zirconia group, could be listed as another limitation of the study. Future in vitro studies may be planned to include more CAD-CAM materials for customized groups and perform thermal cycling applications, or an in vivo study may be conducted to examine the clinical performance of various post-core systems.

Conclusion

Monolithic zirconia post-cores demonstrated the highest fracture strength despite predominantly catastrophic failures, making them less conducive for repair due to their high elasticity modulus. Conversely, fiber post-composite cores resulted in the lowest fracture strength values. The monolithic fiber post-core showed superior fracture strength compared to the fiber post-composite cores, which may be attributed to the durable structure of the monolithic fiber post-core, which eliminates the connection of different materials. Moreover, the prefabricated monolithic fiber post-core exhibited the highest pull-out strength and more repairable failures than the zirconia post-core, suggesting significant clinical advantages. These findings highlight the promising potential of the new generation prefabricated monolithic fiber post-core system in enhancing outcomes in restorative dentistry.