Fracture patterns in endodontically treated teeth restored with or without horizontal fiber posts: An in vitro study

Abstract

Context:

Endodontically treated maxillary premolars are more susceptible to fracture because of the loss of tooth structure following caries removal and endodontic procedures. Various composite restorative systems and reinforcement strategies, such as horizontal fiber posts (HFPs), have been proposed to improve fracture behavior and promote more favorable failure patterns.

Aim:

The aim of this study was to evaluate fracture patterns in endodontically treated maxillary premolars restored with different composite systems, with and without horizontal fiber posts (HFP).

Materials and Methods:

Sixty extracted maxillary premolars were prepared with standardized mesio-occlusal cavities and endodontically treated. Specimens were divided into six groups (n = 10) based on restorative material (Filtek Z350 XT, Multicore Flow, Smart Dentin Replacement (SDR)) and presence or absence of HFP. Horizontal slots were prepared for post placement in designated groups. Restorations were completed accordingly, and specimens were subjected to compressive loading using a universal testing machine. Fracture patterns were evaluated under a stereomicroscope and classified as adhesive/cohesive and favorable/unfavorable. Statistical analysis was performed using an unpaired t-test and one-way ANOVA (P < 0.05).

Results:

HFP groups showed a higher incidence of cohesive (0.70) and favorable fractures (0.70) compared to nonpost groups (0.50 and 0.40). Adhesive and unfavorable fractures were reduced in HFP groups. The differences were statistically significant (P = 0.019). SDR with HFP demonstrated the most favorable outcomes, although intermaterial differences were not significant (P > 0.05).

Conclusion:

HFP significantly improve fracture patterns by promoting favorable and cohesive failures. Their reinforcing effect is more influential than the type of restorative material used.

INTRODUCTION

The longevity of endodontically treated teeth (ETT) is determined more by their resistance to fracture than by endodontic success alone. Although advancements in endodontic techniques have significantly improved disinfection and obturation outcomes, the structural integrity of the remaining tooth continues to be the primary determinant of long-term survival.[1] Loss of tooth structure resulting from caries, cavity preparation, and endodontic access procedures compromises the biomechanical behavior of teeth by altering stress distribution under functional loading.[2]

This structural vulnerability is particularly evident in maxillary premolars. Their anatomical configuration – characterized by steep cuspal inclines and relatively thin dentinal walls – predisposes them to cuspal deflection and fracture, especially following mesio-occluso-distal cavity preparation.[3,4] Consequently, restoring such teeth in a manner that enhances fracture resistance while preserving residual structure remains a significant clinical challenge.[5]

Traditional restorative approaches have relied on intraradicular posts to improve retention of the core material.[6] However, these posts do not inherently reinforce the tooth; instead, they often necessitate additional removal of radicular dentin, which may weaken the root and increase the likelihood of catastrophic fractures.[7] This limitation has prompted a paradigm shift toward minimally invasive restorative strategies that prioritize conservation of tooth structure and aim to restore biomechanical harmony.

Within this evolving framework, horizontal fiber posts (HFP) have emerged as a promising alternative. Unlike vertical posts, HFPs are placed intracoronally and extend horizontally between cusps, thereby acting as a splint. This configuration reinforces weakened cusps and facilitates a more uniform distribution of occlusal stresses without compromising the radicular dentin. By limiting cuspal deflection and redistributing stresses, HFP may reduce the risk of unfavorable fractures.[8]

In addition to reinforcement strategies, the choice of restorative material plays a critical role in determining fracture behavior. Composite resins differ in their elastic modulus, polymerization shrinkage, and adhesive properties, all of which influence stress generation and distribution within the tooth restoration complex. Bulk-fill composites, such as Smart Dentin Replacement (SDR), have been reported to exhibit reduced polymerization stress and improved cavity adaptation, potentially contributing to enhanced fracture performance.[9]

While numerous studies[10,11,12] have focused on fracture resistance, fracture pattern analysis is arguably of greater clinical significance. The mode of fracture determines whether a tooth can be restored following failure. Favorable fractures are confined above the cementoenamel junction (CEJ) and are typically restorable, whereas unfavorable fractures extend below the CEJ and often necessitate extraction.

Accordingly, this experimental study was designed to assess patterns of fracture in root canal-treated bicuspids restored with various composite systems, with or without HFP, under compressive loading conditions.

The following statistical hypotheses were evaluated:

1. HFP do not influence fracture patterns in ETT

2. There is no significant variation in patterns of fracture among the restorative materials used.

MATERIALS AND METHODS

The present laboratory-based invitro experimental investigation was carried out in the Department of Conservative Dentistry and Endodontics on a sample of sixty recently extracted human maxillary premolars following approval from the Institutional Ethics Committee of Baba Farid University of Health Sciences, Faridkot (Approval No. BFUHS/2025/TH/C/9198). Specimens were carefully chosen according to predefined selection standards, ensuring that all teeth were free from carious lesions, fractures, prior restorations, or any anatomical irregularities. Teeth exhibiting congenital anomalies, previous endodontic treatment, visible fracture lines, or prior restorations were excluded from the study. To minimize variability, only teeth with similar dimensions (20–24 mm in length) were included [Figure 1a].

Figure 1.

(a) Freshly Extracted Maxillary Premolar, (b) Illustration of the MO tooth arrangement, preparation and endodontic treatment, (c) Tooth after MO placement, (d) Occlusal view of horizontal glass fiber post, (e) Tooth sample under Universal Testing Machine (f) stereo microscope for fracture pattern analysis

After debridement, the samples were preserved in distilled water at ambient temperature until further procedures were performed. Standardized mesio-occlusal cavity preparations [Figure 1b] were created using a #245 carbide bur with continuous water irrigation. To ensure consistency, all cavity dimensions were standardized: the occlusal width was set at 4 mm, the isthmus width at 3 mm, and the gingival floor depth at 2 mm. These measurements were confirmed using a digital caliper.[13]

Endodontic treatment was performed using a rotary instrumentation system following standardized protocols. The root canal system was chemically cleansed using a combination of sodium hypochlorite solution and normal saline. Following irrigation, moisture within the canals was eliminated with absorbent paper cones. The canals were then sealed by placing gutta-percha cones along with a resin-based endodontic cement (Adseal) employing a lateral condensation approach at room temperature. After obturation, the access openings were thoroughly rinsed and cleared of any remaining material before proceeding with the restorative phase [Figure 1c].

The collected samples were allocated into six experimental groups, with 10 specimens assigned to each group. The classification was determined according to the type of restorative material used [Table 1] as well as whether horizontal fiber reinforcement was incorporated or not.

Table 1.

List of the restorative materials used in the investigation

Material	Type	Resin matrix	Filler
MultiCore Flow (IVOCLAR VIVADENT, Schaan, Liechtenstein)	Dual-cure core build-up material	DMA	Barium glass, ytterbium trifluoride, Ba-Al-fluorosilicate glass (70 wt %; average: 0.04–25 µm)
Filtek Z350 XT (3M ESPE, Seefeld Germany)	Nanohybrid composite resin	Bis-GMA, UDMA, TEGDMA, PEGDMA	Nonagglomerated/nonaggregated silica (20 nm), zirconia (4–11 nm), zirconia/silica clusters (0.6–10 µm); total filler loading: 72.5 wt %
SDR Bulk Fill (DENTSPLY, Caulk, Milford, DE, USA)	Flowable bulk-fill composite	Modified UDMA-based monomer	Barium–alumino–fluoro–borosilicate glass, strontium aluminum fluorosilicate glass, fumed silica (68 wt %)
Single Bond Universal (3M ESPE, Seefeld Germany)	Universal adhesive	MDP, Bis-GMA, HEMA, dimethacrylates	Silica nanofiller (10 nm) at 10 wt %
Monobond N (IVOCLAR VIVADENT, Schaan, Liechtenstein)	Silane coupling agent	Silane methacrylate, phosphoric acid methacrylate, sulfide methacrylate	No fillers (homogeneous clear solution)

SDR: Smart dentin replacement, DMA: Dimethacrylate, Bis-GMA: Bisphenol A-glycidyl methacrylate, UDMA: Urethane dimethacrylate, TEGDMA: Triethylene glycol dimethacrylate, PEGDMA: Polyethylene glycol dimethacrylate, MDP: 10-Methacryloyloxydecyl dihydrogen phosphate, HEMA: 2-Hydroxyethyl methacrylate

• Group Ia: Filtek Z350 XT without post

• Group Ib: Multicore Flow without post

• Group Ic: SDR without post

• Group IIa: Filtek Z350 XT with HFP

• Group IIb: Multicore Flow with HFP

• Group IIc: SDR with HFP.

Restorative procedure

In the specimens assigned to the fiber-reinforced groups, a horizontal glass fiber post (Tanex Fiber Trans, Coltene, Switzerland) with a diameter of 1.3 mm was used as intracoronal reinforcement. Following cavity preparation and endodontic treatment, shallow grooves were prepared at the midpoint of the buccal and palatal cavity walls using a #3 round bur (MANI Inc., Japan) to accommodate the fiber post. The post was sectioned to the required length to ensure passive adaptation between the buccal and palatal walls. Before placement, the glass fiber post surface was treated with a silane coupling agent (Monobond N, Ivoclar Vivadent, Schaan, Liechtenstein) according to the manufacturer’s instructions. Subsequently, a universal adhesive system (Single Bond Universal, 3M ESPE, St. Paul, MN, USA) was applied to the post surface and light-cured for 10 s. The post was then positioned horizontally within the prepared grooves extending between the buccal and palatal walls and stabilized with a resin composite. The remaining cavity was restored with the designated composite restorative material according to the respective study group [Figure 1d].

Incremental placement was employed for Filtek Z350 XT and Multicore Flow to minimize polymerization shrinkage stress, whereas SDR was placed in bulk owing to its enhanced depth of cure and reduced shrinkage properties. All restored specimens underwent finishing and polishing procedures in accordance with established clinical guidelines.

Testing procedure

Before mechanical evaluation, all samples were immersed in distilled water and maintained at 37°C for 24 h to approximate intraoral conditions. Fracture strength was then assessed using a universal testing apparatus by applying a compressive load at a crosshead speed of 1 mm/min until structural failure occurred [Figure 1e].

Following testing, the fractured samples were analyzed under a stereomicroscope at ×20 magnification [Figure 1f]. The failure patterns were categorized as follows:

• Interfacial failure (Adhesive): Separation occurring at the junction between the tooth substrate and restorative material [Figure 2a and b]

• Internal failure (Cohesive): Fracture confined within either the dental tissue or the restorative material itself [Figure 2c and d]

• Restorable fracture (Favorable): Fracture line located coronal to the CEJ, indicating a favorable prognosis [Figure 2e]

• Nonrestorable fracture (Unfavorable): Fracture extending apical to the CEJ, indicating an unfavorable outcome [Figure 2f].

Figure 2.

(a and b) Samples with adhesive failure, (c and d) Sample with cohesive failure, (e and f) Favorable versus Unfavorable fracture patterns

All observations were systematically recorded and subjected to statistical evaluation. Differences between groups were analyzed using an independent (unpaired) t-test, whereas comparisons within groups across different materials were performed using one-way analysis of variance (ANOVA). A P < 0.05 was considered statistically significant.

RESULTS

The results demonstrated a definitive influence of HFP on fracture patterns [Table 2]. Specimens restored with HFP exhibited a higher proportion of cohesive fractures (mean = 0.70) compared to those without posts (mean = 0.50), indicating enhanced internal integrity of the restorative complex.

Table 2.

Distribution of fracture type and pattern across all groups

Groups	n	Mean (cohesive)	Mean (adhesive)	Mean (favorable)	Mean (unfavorable)	±SD
Ia (Z350, no post)	10	0.40	0.60	0.30	0.70	0.48
Ib (Multicore, no post)	10	0.50	0.50	0.40	0.60	0.52
Ic (SDR, no post)	10	0.60	0.40	0.50	0.50	0.53
IIa (Z350, post)	10	0.70	0.30	0.70	0.30	0.48
IIb (Multicore, post)	10	0.60	0.40	0.60	0.40	0.52
IIc (SDR, post)	10	0.80	0.20	0.80	0.20	0.42

SD: Standard deviation, SDR: Smart dentin replacement

A similar trend was observed in fracture patterns. Favorable fractures were significantly more frequent in the HFP groups (mean = 0.70) compared to the nonpost groups (mean = 0.40). Conversely, adhesive fractures decreased from 0.50 in nonpost groups to 0.30 in post groups, whereas unfavorable fractures were reduced from 0.60 to 0.30.

Intergroup comparison [Table 3] revealed that these differences were statistically significant (P = 0.019), thereby rejecting the first null hypothesis and confirming the reinforcing effect of HFP.

Table 3.

Inter-group comparison (with post vs. without post)

Group	n	Cohesive mean	Adhesive mean	Favorable mean	Unfavorable mean	Mean difference	Percent change (%)	P	S/NS
Without post	30	0.50	0.50	0.40	0.60	0.30	75.00	0.019	S
With post	30	0.70	0.30	0.70	0.30

S: Significant, NS: Not significant

Among the restorative materials, SDR combined with HFP demonstrated the most favorable outcomes, with the highest incidence of cohesive (0.80) and favorable (0.80) fractures. However, intermaterial comparisons did not reveal statistically significant differences (P > 0.05), indicating that the presence of HFP had a greater influence on fracture behavior than the type of composite material used [Tables 4 and 5].

Table 4.

Intermaterial comparison (adhesive vs. cohesive)

Groups	n	Cohesive mean	Adhesive mean	Mean difference	Percent change (%)	P	S/NS
Z350 XT	20	0.55	0.45	0.15	27.27	0.434	NS
Multicore	20	0.55	0.45
SDR	20	0.70	0.30

S: Significant, NS: Not significant, SDR: Smart dentin replacement

Table 5.

Intermaterial comparison (favorable vs. unfavorable)

Groups	n	Favorable mean	Unfavorable mean	Mean difference	Percent change (%)	P	S/NS
Z350 XT	20	0.50	0.50	0.15	30.00	0.402	NS
Multicore	20	0.50	0.50
SDR	20	0.65	0.35

S: Significant, NS: Not significant, SDR: Smart dentin replacement

DISCUSSION

The present study evaluated fracture patterns in ETT restored with different composite systems, with and without HFP. Unlike conventional studies[14,15] that focus primarily on fracture resistance, this investigation emphasized fracture patterns, which are more clinically relevant as they determine the restorability of the tooth following failure. The findings clearly demonstrated that HFP significantly influence fracture behavior. The increased prevalence of cohesive fractures in HFP groups suggests improved integrity of the tooth restoration complex. Cohesive failures indicate that the adhesive interface remains intact, with stresses being absorbed within the restorative material or tooth structure. This is clinically advantageous, as adhesive failures are often associated with debonding and restoration failure.[16]

The reduction in adhesive failures observed in the HFP groups may be attributed to improved stress distribution. HFP act as intracoronal splints, linking the buccal and lingual cusps and limiting cuspal deflection under occlusal loading.[17] This splinting effect redistributes stresses more evenly and reduces tensile forces at the adhesive interface.[18] A key finding of this study was the significant increase in favorable fractures in the HFP groups. Favorable fractures, confined above the CEJ, are restorable and thus represent a desirable mode of failure. The reduction in unfavorable fractures suggests that HFP not only enhances mechanical performance but also improves the clinical prognosis of restored teeth. Among the materials tested, SDR demonstrated superior performance in terms of favorable and cohesive fracture patterns. This may be attributed to its lower polymerization shrinkage stress, improved flowability, and enhanced cavity adaptation. These properties reduce internal stress concentration and may act synergistically with fiber reinforcement.[19] However, the absence of statistically significant differences among materials suggests that structural reinforcement provided by HFP plays a more dominant role than material properties. This supports the concept that in structurally compromised teeth, macro-mechanical reinforcement strategies are more critical than micro-mechanical variations in restorative materials.[20] The findings are consistent with previous studies[10,12,21] that have reported improved fracture resistance and favorable fracture patterns with fiber reinforcement. However, conflicting evidence exists, with some studies[19,22] reporting no significant effect of HFP on fracture behavior. These discrepancies may be attributed to variations in study design, cavity configuration, and testing protocols.

Compared to vertical posts, which may weaken the root by requiring additional dentin removal, HFP offer a conservative alternative. They reinforce the coronal structure without compromising radicular integrity, aligning with the principles of minimally invasive dentistry.[23] Although the findings are encouraging, several limitations should be considered. As a laboratory-based study, it does not entirely reproduce the dynamic conditions present in the oral cavity. Variables such as temperature variations, humidity changes, and repetitive loading were not incorporated into the experimental design. Moreover, the application of a single, static compressive force does not adequately mimic the complex nature of functional chewing forces. The lack of periodontal ligament simulation may have further affected the distribution of stresses within the specimens. Future research should aim to include thermomechanical cycling, fatigue loading protocols, and finite element modeling to achieve conditions closer to the clinical scenario. In addition, well-designed long-term clinical studies are necessary to confirm the practical efficacy of HFP in everyday dental practice.

CONCLUSION

Considering the constraints of this laboratory-based investigation, HFP significantly improved fracture patterns in ETT. Their incorporation increased the incidence of cohesive and favorable fractures while reducing adhesive and unfavorable failures. Although SDR exhibited the most favorable performance, differences among materials were not statistically significant. The use of HFP represents a conservative and effective strategy for reinforcing structurally compromised teeth and enhancing their clinical prognosis.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.