Effect of three post systems on the fracture resistance of teeth with extensive coronal structure loss and internal resorption

Abstract

PURPOSE

The purpose of this study was to evaluate the effect of three different post systems on the fracture resistance of maxillary incisors with simulated internal root resorption and extensive coronal structure loss.

MATERIALS AND METHODS

A total of 100 freshly extracted maxillary incisors were randomly divided into five groups (n = 20): glass fiber post, polyetheretherketone (PEEK) post, Ribbond post, positive control, and negative control. Internal resorption cavities were prepared 6 mm above the apex in each tooth, and the canals were filled using the single-cone technique with Sure-Seal Root bioceramic sealer. PEEK posts were digitally designed based on the dimensions of the glass fiber posts and fabricated in a dental laboratory. All posts were cemented using Panavia V5 resin cement according to the manufacturer’s instructions. The specimens were tested under static loading at a 45° angle using a universal testing machine until fracture occurred. Data were analyzed using one-way ANOVA and Tamhane’s post hoc test (P < .05).

RESULTS

The highest mean fracture resistance was observed in the negative control group (1559.4 ± 209.1 N), while the lowest was in the positive control group (492.9 ± 73.4 N). Among the post-applied groups, the PEEK post group exhibited a significantly higher fracture resistance (907.4 ± 104 N) compared to the others (P < .001).

CONCLUSION

Although all post systems increased the fracture resistance, none reached the strength of intact teeth. PEEK posts yielded the highest fracture resistance under the present experimental conditions.

INTRODUCTION

Internal root resorption is a pathological process that leads to the progressive destruction of dentin, resulting in the deterioration of the tooth structure.¹ In teeth affected by internal resorption, the root canal walls become thinner, making the roots more susceptible to fracture. Therefore, root canal treatment is required for teeth with internal root resorption to stop active dentin degradation.²,³ Particularly in teeth with severe structural loss and internal root resorption, both coronal and radicular strength are significantly reduced, making these cases among the most challenging clinical problems that threaten endodontic success.⁴ During root canal filling, materials with reinforcing capacity should be preferred to prevent fractures, especially in areas where dentin walls are thin.⁵

Furthermore, endodontic treatment may also be required when the coronal portions of teeth have suffered extensive structural loss due to caries, trauma, abrasion, attrition, or erosion. The restoration of such endodontically treated teeth with substantial substance loss remains one of the most common clinical challenges. In the prosthetic restoration of endodontically treated teeth with severe coronal tissue loss, the placement of an intraradicular post and the construction of a core structure supported by the post to provide retention and reinforcement for the final restoration are among the most frequently preferred methods.⁶ Today, various post systems are used for the restoration of teeth with extensive substance loss.⁷ In the present study, three currently used post systems—glass fiber post, PEEK post, and polyethylene fiber-reinforced Ribbond post—were selected for comparison.

PEEK is a high-performance polymer known for its excellent flexibility and mechanical strength. One of the remarkable features of PEEK is its ability to distribute stress, thereby preventing both restoration and root fractures. By directing and dissipating stress, PEEK reduces the forces transmitted to both the restoration and the root.⁸ Its tooth-colored appearance enhances its applicability in esthetic restorations. PEEK has been proposed for the fabrication of various fixed and removable dental prostheses using CAD-CAM technology. In addition, it has been recommended for temporary restorations, implant abutments, customized healing abutments, intraradicular posts, and occlusal splints.⁹

The PEEK system possesses high chemical and thermal resistance, low solubility in water, and excellent biocompatibility. Although the elastic modulus of PEEK is relatively low, its mechanical strength is remarkably high. Therefore, it has been demonstrated that PEEK is suitable for long-term restorative applications.¹⁰,¹¹ Due to its favorable mechanical properties and biological inertness, PEEK has been found appropriate for use in various dental applications. The elastic modulus of PEEK is lower than that of dentin and enamel (3 – 4 GPa),¹² which significantly reduces the risk of root fracture in endodontically treated teeth.¹³

Another material commonly preferred in post applications for the restoration of endodontically treated teeth is the prefabricated glass fiber post.¹⁴ The elastic modulus of fiber posts (16 – 40 GPa) is close to that of dentin (18.⁶ GPa). They exhibit resistance to tensile forces, reduce stress formation within the root dentin, and help prevent root fractures.¹⁵ Glass fiber posts do not undergo corrosion and do not induce allergic reactions. They can also be used in combination with esthetic restorations.¹⁶ However, glass fiber posts do not completely adapt to the shape of the root canal, which may increase the risk of debonding.¹⁷

The polyethylene fiber-reinforced Ribbond post system is a type of fiber post composed of ultra-high-strength, bondable, reinforced polyethylene with a unique woven structure.¹⁸ It has a high modulus of elasticity (117 GPa), which makes it highly resistant to bending, distortion, and tensile stress (3 GPa). Each post possesses an interlaced polymer network and an elastic modulus similar to that of dentin (15 – 20 GPa), providing uniform distribution of occlusal stresses.¹⁹

Teeth with severe structural loss and internal root resorption are among the cases in which both coronal and radicular strength are markedly reduced, thus representing challenging clinical conditions that threaten endodontic success.⁴ In the literature, studies addressing this topic have primarily evaluated either the fracture resistance of teeth with extensive structural loss after post placement or the fracture resistance of teeth with simulated internal resorption cavities filled with various root canal filling materials.²⁰,²¹ Only one study, which considered the coexistence of both conditions in the same tooth, evaluated the bond strength of the post.²² Numerous case reports describing the treatment of severely damaged teeth with internal resorption have emphasized the need for root canal filling materials and post systems that enhance fracture resistance in order to strengthen such teeth and prevent potential fractures.²³,²⁴,²⁵,²⁶

The present study aimed to compare the fracture resistance of three currently available intraradicular post systems in teeth with simulated internal resorption and extensive coronal structural loss under standardized in vitro conditions. The null hypothesis of this study was that “different post systems applied to teeth with internal resorption and extensive coronal structural loss do not affect the fracture resistance of the teeth.”

MATERIALS AND METHODS

Specimen Selection and Preparation

Based on a power analysis conducted for a one-way analysis of variance (ANOVA) with five groups and according to the methodology reported in a similar study,²⁷ a minimum of 16 specimens per group (n = 16) was determined to be necessary to detect a mean difference of 200 N between at least one group and the others. This provided a statistical power of 80% at a significance level of α = 0.05, assuming a standard deviation of 200 N in each group (effect size d = 0.40). To obtain results closer to real clinical conditions, the sample size was increased to 20 specimens per group (n = 20). Ethical approval for this study was obtained from the Non-Interventional Clinical Research Ethics Committee of Bolu Abant Izzet Baysal University (Decision No. 2023/447, dated 19 December 2023). This study was conducted in accordance with the principles of the Declaration of Helsinki. Informed consent was obtained from all participants and/or their legal guardians.

In this study, 100 freshly extracted, single-rooted, single-canal maxillary incisors were collected from patients who presented to the Faculty of Dentistry and were required extraction for orthodontic, traumatic, or periodontal reasons. No teeth were extracted solely for research purposes. All adherent soft tissue remnants and debris were removed with a curette, and the teeth were stored in sterile saline solution until testing began. The specimens were examined under a dental operating microscope (×20 magnification; Zumax Medical Co., Ltd., Suzhou, China). Teeth with intact crowns, mature roots, and no evidence of caries, cracks, internal or external discoloration were included. Teeth with morphological defects, existing restorations, a history of bleaching, previous root canal treatment, fractures, cracks, or resorption were excluded from the study. Randomization was performed using an online statistical tool (Randomizer.org) to allocate specimens into five groups: Glass fiber post group, PEEK post group, Ribbond post group, Positive control group (only root canal filled, no post placement), Negative control group (no treatment applied).

Before testing, periapical radiographs were taken to confirm the presence of a single root canal in all teeth. The specimens were stored at 37℃ in 100% humidity for two weeks until testing. To ensure standardization, each specimen was measured using a digital caliper (accuracy = 0.01 mm; Schulzz, Germany). The buccolingual and mesiodistal dimensions were standardized at 5 ± 1 mm, and the root length from the cementoenamel junction (CEJ) to the apex was standardized at 15 ± 1 mm. Teeth with similar root lengths and diameters were selected. Except for the negative control group, all specimens were sectioned at 2 mm coronal to the CEJ under water cooling using a sterile diamond disc to create a standardized ferrule effect.²⁸

Endodontic Treatment Procedure

After preparing endodontic access cavities using sterile diamond burs, the pulp tissue was extirpated with a #10 K-file (Dentsply Maillefer, Ballaigues, Switzerland). To determine the working length, a #15 K-file was inserted into the canal until it was visible at the apical foramen, and the working length was established 1 mm short of this measurement. The root canals were prepared using the ProTaper Next rotary system (Dentsply Maillefer, Ballaigues, Switzerland) in sequence with X1, X2, X3, X4, and X5 files (50/0.6) operated with an endomotor device (VDW Silver, VDW, Munich, Germany). During preparation, canals were irrigated with 2 mL of 5.25% sodium hypochlorite (Wizard, Rehber Kimya, Istanbul, Türkiye) between each file change using a 30-G side-vented needle (Endo-Top, Cerkamed, Stalowa Wola, Poland). After completion of instrumentation, the smear layer was removed by irrigating with 5 mL of 17% EDTA (Imicryl, Konya, Türkiye) for 1 minute, followed by 5 mL of 5.25% sodium hypochlorite. Finally, the canals were rinsed with 5 mL of distilled water and dried using sterile paper points (DiaDent, Chungcheongbuk-do, Republic of Korea).

Preparation of Internal Resorption Cavities

Topçuoğlu et al.² and Gençoğlu et al.²⁹ previously described a method for creating internal root resorption cavities, which was modified in the present study. The roots were sectioned perpendicularly to their long axis at a distance of 6 ± 0.2 mm from the apex using a fine diamond disc (Sunshine, Langenhagen, Germany), resulting in two separate halves. In both halves of each specimen, simulated resorption cavities were created 6 mm above the anatomical apex with a depth of 0.8 mm and a diameter of 1.6 mm. A 1.6 mm diameter diamond round bur (G&Z Instrumente) was marked at the halfway point (0.8 mm), and resorption cavities were prepared on the opposing internal surfaces of the two root halves, surrounding the root canal space, at a depth of 0.8 mm and a diameter of 1.6 mm. Keskin et al.³⁰ described a procedure for acid demineralization, which was followed in the present study to create an irregular resorption surface simulating clinical conditions. The cavity surfaces were etched with 37% phosphoric acid (Dia-Etch; DiaDent, Seoul, Korea) for 30 seconds, rinsed with distilled water, and dried with paper points. After demineralization, the root halves were reassembled at the reference alignment lines using a cyanoacrylate adhesive (Derby, Istanbul, Türkiye). To prevent the adhesive from leaking into the canal space during reassembly, a paper point matching the canal size was temporarily placed inside the canal.

Root Canal Obturation and Post Space Preparation

The root canals were filled up to the resorption cavity using the single-cone technique with a bioceramic sealer (Sure-Seal Root; Sure Dent Corporation, Seongnam, Korea). To prevent the root canal filling material from overflowing into the simulated internal resorption cavity and affecting adhesion, the gutta-percha cones were cut back to the base of the artificial cavity using the down-packing technique with a heated electric plugger. After obturation, the coronal access cavities were sealed with a temporary restorative material, and the specimens were stored in 100% humidity at 37℃ for one week.

After one week, post spaces were prepared in all experimental groups, leaving 5 mm of apical gutta-percha intact and extending to the level of the internal resorption cavity. For the glass fiber post group, the post space was prepared using #2 drills (Reforpost™, Angelus™, Londrina, PR, Brazil). For the PEEK post group, digital scans were obtained from glass fiber posts of identical size using a laboratory scanner (7 Series; Dental Wings^®, Montreal, QC, Canada). PEEK posts were fabricated in the Set Dent Dental Laboratory (Fig. 1). Before cementation, both PEEK and glass fiber posts were cleaned with 70% ethanol, rinsed with distilled water, and air-dried without any surface treatment. The PEEK post surfaces were then air-abraded with 50 µm Al₂O₃ particles (Basic quattro IS; Renfert GmbH, Hilzingen, Germany) to enhance resin cement adhesion. Air abrasion was performed for 20 seconds at 35 psi pressure from a 10 mm distance, applied from four directions. Both PEEK and glass fiber posts were subsequently etched with 37% phosphoric acid gel. PEEK, glass fiber and Ribbond posts were cemented into the post spaces using the Panavia V5 primer system and resin cement (Kuraray, Okayama, Japan), following the manufacturers’ instructions. A core build-up of 4 mm in height was then created cervico-incisally using a nanohybrid composite resin (Filtek Ultimate; 3M ESPE, St. Paul, MN, USA).

Fig. 1. Digital scanning and design process of PEEK post fabrication: (A) Extraoral scanner (7 Series; Dental Wings^®, Montreal, QC, Canada) used to digitize the post space model. (B) Master model with simulated internal resorption cavity positioned in the scanner for digital impression acquisition. (C) Three-dimensional virtual model obtained after scanning, showing the post space and tooth morphology designed in Exocad (Exocad GmbH, Darmstadt, Germany).

To simulate the periodontal ligament, the roots of the restored teeth were coated with a thin layer of wax up to 2 mm apical to the cervical line and then embedded in rectangular acrylic resin blocks with a 45° inclination base. Fracture testing was performed using a universal testing machine (TSTM 02500; Elista Ltd. Co., Ankara, Türkiye). In the positive control group, the entire root canal, including the internal resorption cavity, was filled using the thermoplastic injection technique with the same bioceramic sealer via the single-cone method. No treatment was applied to the negative control group.

RESULTS

A statistically significant difference was found among the mean maximum fracture resistance values of all groups (P < .001) (Table 1). The mean values were 492.9 N for the positive control group, 907.4 N for the PEEK post group, 683.4 N for the Ribbond post group, 640.7 N for the glass fiber post group, and 1559.4 N for the negative control group (Fig. 2). There was a statistically significant difference between the positive control group and the PEEK post, Ribbond post, glass fiber post, and negative control groups. The PEEK post group showed a statistically significant difference compared with the Ribbond post, glass fiber post, and negative control groups. No significant difference was found between the Ribbond post and glass fiber post groups, whereas both groups differed significantly from the negative control group. A statistically significant difference was also observed between the glass fiber post and negative control groups.

Table 1. Comparison of maximum fracture resistance (N) among the experimental groups.

		Mean ± SD	Test Statistic	P
Group	Positive Control	492.9 ± 73.4a	139.113	< .001x
	PEEK post	907.4 ± 104b
	Ribbond post	683.4 ± 169c
	Glass Fiber post	640.7 ± 127.5c
	Negative Control	1559.4 ± 209.1d

^xOne Way Anova; groups sharing the same letter (a – d) are not significantly different.

Fig. 2. Mean and standard deviation of maximum fracture resistance values by group. Different letters (a – d) above the bars indicate statistically significant differences between groups (P < .05); groups sharing the same letter are not significantly different.

Statistical Analysis

All data were analyzed using the R statistical software (version 4.4.1). The Shapiro-Wilk test was used to assess the normality of data distribution. For variables showing normal distribution, One-Way ANOVA was performed. Multiple comparisons were analyzed using the Tamhane post hoc test. Quantitative data were expressed as mean ± standard deviation, and the level of statistical significance was set at P < .05.

DISCUSSION

In teeth with extensive coronal structure loss and internal resorption leading to thinning of the dentin walls, conventional post-core restorations may fail to maintain long-term structural integrity.⁵,³¹ Internal resorption reduces circumferential dentin thickness and compromises fracture resistance; therefore, the use of post materials with an elastic modulus close to dentin may help reduce the risk of failure.²² Previous studies have reported that preserving residual dentin and using materials with dentin-like elasticity enhances fracture resistance and promotes more repairable fracture patterns.³²,³³,³⁴

In the restoration of severely compromised anterior teeth, contemporary approaches emphasize the use of fiber post systems due to their esthetic properties and mechanical compatibility with dentin.³⁵,³⁶ Recent systematic reviews have shown that Ribbond posts tend to produce repairable fractures in structurally weakened endodontically treated teeth, and that their performance is highly influenced by cavity geometry.¹⁸ Meanwhile, PEEK material has recently gained attention as a post material because of its favorable biocompatibility and stress distribution characteristics.³⁷ Therefore, in the present study, we evaluated the fracture resistance of three different post systems in a simulated model combining both severe coronal tissue loss and internal root resorption.

The findings of this study are consistent with previous reports demonstrating that the mechanical properties of the post material play a crucial role in determining fracture resistance.³³,³⁴ A significant difference was found between the positive control group and all other experimental groups, indicating that although post systems enhanced fracture resistance, none could fully restore it to the level of sound teeth. The high fracture resistance recorded in the negative control group supports the inherent mechanical advantage of natural tooth structure, as similarly reported in previous studies.³¹

Among the tested post systems, PEEK posts exhibited the highest fracture resistance (907.4 ± 104 N). This can be attributed to the dentin-like elastic modulus and damping capacity of PEEK, which enables more uniform stress distribution, particularly in roots with thin and asymmetrical resorption walls.²⁰,²³,³⁸ This finding is in agreement with the results of Lima et al.,³⁹ who reported that although the difference between PEEK and glass fiber posts was not statistically significant, PEEK posts demonstrated superior mechanical performance. Similarly, biomechanical analyses by Sugano et al.⁴⁰ and Kermanshah and Estedlal⁴¹ indicated that PEEK posts may provide favorable stress distribution characteristics and potentially reduce stress concentration within the root dentin. Benli et al.⁴² also found that when appropriate surface treatments are applied, PEEK posts can be considered a promising alternative to conventional fiber or cast metal posts, especially in anterior restorations. Özarslan et al.⁴³ compared the fracture resistance of zirconia, glass fiber, and PEEK post systems and emphasized that fractures observed in the PEEK group caused minimal additional damage to the tooth structure, allowing for easier re-restoration.

Gontijo et al.³⁴ reported that in structurally weakened roots, custom-designed PEEK and glass fiber post-core systems demonstrated higher compressive strength and more repairable fracture patterns than prefabricated systems under non-aged mechanical conditions. Similarly, Rakotoaridina et al.⁴⁴ emphasized that PEEK posts may exhibit comparable or even superior mechanical performance to glass fiber posts, particularly in structurally compromised roots and customized post designs. Zoidis et al.⁴⁵ stated that single-piece PEEK post-core restorations fabricated with CAD-CAM technology provide excellent adaptation to the root canal and that the dentin-like elastic modulus of this material may contribute to a reduced stress transmission to the root. In the present study, the PEEK posts were fabricated to match the exact dimensions of the glass fiber posts to prevent variations due to post size, canal geometry, or cement thickness. This approach was intended to eliminate potential measurement errors originating from post-space adaptation, ensuring a more standardized comparison among post systems.

The absence of a statistically significant difference between the Ribbond post and the glass fiber post groups is consistent with previous reports describing comparable performance between conventional fiber and polyethylene-reinforced systems. In the meta-analysis conducted by Vartak et al.,¹⁸ Ribbond posts demonstrated slightly lower fracture resistance than prefabricated fiber posts; however, the authors emphasized that differences in bonding mechanisms and structural integrity may influence outcomes.

In the present study, the exclusion of coronal crown fabrication aimed to isolate and emphasize the effect of post and core materials on the fracture resistance of teeth exhibiting internal resorption and severe coronal tissue loss.²⁰ Static loading tests were performed using a universal testing machine, and the load was applied at a 45° angle to the long axis of the tooth. This angulation was chosen to simulate the oblique forces that typically occur during function or traumatic loading in maxillary anterior teeth. Previous studies have stressed that the biomechanical behavior of dentin and post systems should be evaluated not only under axial loading but also under lateral forces that more closely represent clinical conditions.⁴⁶ Bhaktikamala et al.⁴⁷ compared different post materials using 45° oblique loading and reported that this approach allows for a clinically relevant prediction of both fracture load and fracture pattern. However, these results should be interpreted cautiously in terms of clinical extrapolation, as fracture resistance was assessed under static loading without thermomechanical aging or fatigue simulation. Since clinical failures of post-core restorations are often fatigue-driven, the higher fracture resistance of PEEK posts observed here reflects performance under non-aged conditions.

Additionally, only maximum fracture resistance values were assessed in this study. Fracture modes were not systematically classified and recorded, which limits the clinical interpretation of the failure behavior of the tested post systems. Future studies should include standardized fracture mode analyses alongside fatigue loading and aging protocols to better predict clinical outcomes.

Another limitation is that the bonding performance of PEEK posts may vary depending on the surface conditioning protocol; although air-abrasion and phosphoric acid etching were used in this study, alternative approaches such as sulfuric acid etching or dedicated PEEK primers were not evaluated.

As with all in vitro studies, the present research cannot fully replicate intraoral conditions, since most clinical fractures result from fatigue rather than static compressive loads. Moreover, the exclusion of full-crown restorations limits clinical extrapolation, as adequate ferrule and coronal coverage significantly enhance fracture resistance in endodontically treated teeth. Therefore, the fracture resistance results should be interpreted within the context of a crownless restoration design without ferrule, and future studies incorporating full-coverage crowns with standardized ferrule heights are warranted to improve clinical relevance. Nevertheless, the present study highlights the influence of post selection on the mechanical reinforcement of structurally compromised and internally resorbed teeth, and shows that PEEK posts achieved higher fracture resistance under the present experimental conditions, warranting further investigation under thermomechanical aging and cyclic fatigue protocols.

CONCLUSION

Within the limitations of this in vitro study and static loading conditions, PEEK posts demonstrated higher fracture resistance compared with glass fiber and polyethylene-based Ribbond posts in the restoration of teeth weakened by both extensive structural loss and internal resorption. However, none of the tested post systems were able to fully replicate the fracture resistance of sound natural teeth.