Factors Influencing Vertical Radicular Fractures in Teeth Supported by Metallic Dental Core: A Scoping Review

Introduction

A vertical root fracture (VRF) is defined as a longitudinal fracture of the root, where the fracture lines run parallel to the long axis. Vertical root fractures (VRFs) typically initiate within the root canal and extend outward toward the root’s surface. Studies suggest VRFs occur in 11% to 20% of endodontically treated teeth. While their exact frequency is uncertain, VRFs are commonly encountered in clinical practice, primarily affecting treated teeth, particularly those with or without posts, and are more prevalent in posterior teeth of patients over 40. During the early stages, symptoms of VRFs may be subjective or minimal, posing challenges for diagnosis. As the fracture progresses, patients typically experience discomfort, mild pain, dull pain while chewing, gingival swelling, sensitivity, and localized periodontal probing defects. Radiographic images may reveal bone loss resembling periodontal damage, a fractured root, periradicular radiolucency, widening of the periodontal ligament, and sudden changes in root canal space density or width.¹

Various factors have been proposed to contribute to vertical root fractures (VRFs) in teeth that have undergone endodontic treatment. These factors include excessive root canal preparation, unnecessary removal of tooth structure during instrumentation, applying excessive force during root canal filling, wedging forces, corrosion and expansion of root canal posts, and intracanal restorations. Other risk factors include a worn occlusal surface and thin root morphology.¹

Metallic post and core restorations are a widely accepted method to restore endodontically treated teeth with compromised tooth structure. The most common complications associated with post and core restorations are loss of retention (5%), root fracture (3%), and secondary caries lesion (2%).²

Radicular fractures are influenced by various parameters, such as the length and width of the post, the ferrule effect, post cement material, and others. Evaluating these parameters is crucial for determining the future prognosis of the tooth.

Therefore, the objective of this study is to conduct a scoping review to highlight all the parameters involved in sub-prosthetic radicular fractures of teeth supported by metallic dental cores.

Materials and Methods

The following research was based on the interrogation of two bibliographic, scientific, and medical databases, with a language restriction limited to French and English, over a period from 2013 to 2023.

PubMed/Medline: 2013–06/2023.

Scopus: 2013–06/2023.

It was then supplemented by manual searching within the bibliographies of pre-selected articles from the initial search.

Keywords

Endodontically treated teeth, root fracture, radicular fracture, tooth fracture, dental restoration failure, biomechanical factor, stress factor, cast post, metal post, post and core technique, dental posts, metallic coronoradicular restoration.

These keywords were combined to formulate the following search equations:

(“Endodontically treated teeth”) AND ((“root fracture”) OR (“radicular fracture”) OR (“Tooth fracture”)) OR (“Dental restoration failure”) AND ((“biomechanical factor”) OR (“Stress factor”)) AND ((‘Cast posts’) OR (“Metal post”) OR (“Post core technique”) OR (‘Dental posts’) OR (“Metallic coronoradicular restoration”))

AND

((“root fracture”) OR (“radicular fracture”)) OR (“Dental restoration failure”) AND ((“Cast posts”) OR (“Metal post”) OR (“Dental posts”) OR (“Metallic coronoradicular restoration”))

Inclusion and Exclusion Criteria

This step involves specifying the criteria for study selection. In this context, inclusion criteria are those that lead to the retention of a study, while exclusion criteria result in the inevitable rejection of the study.

Inclusion criteria: Prospective studies - Retrospective studies -Book chapters - Grey literature- Publications that are neither in French nor in English - Experimental studies

Exclusion criteria: Case serie - Literature reviews - Systematic reviews - Opinion and review articles – Encyclopedias - Animal studies - Studies with titles and/or abstracts that do not match the research question

Selection Strategy is Carried Out in Four Essential Steps

1st Step: Preselection of articles based on their titles.

2nd Step: Provisional inclusion or exclusion of the article based on predefined selection criteria after reading its abstract.

3rd Step: Reading the article in its entirety with definitive inclusion or exclusion based on the criteria.

4th Step: Analysis of the retained articles.

Data Extraction

From the full text of the included articles, the following information is collected: Author names, Publication date, Study type, Number of participants, Study objective, Interventions.

Results

Flowchart

To present the results of our literature review, we have created a flowchart to illustrate the various stages of the research and study selection process leading to the final body of work. (Figure 1) The baseline characteristics of the included studies are shown in Table 1.

Figure 1 Flow chart of study selection.

Table 1 Characteristics of the Studies Included in the Scoping Review

Discussion

The main objective of our review was to first understand vertical root fractures in terms of diagnosis, dental predispositions, and all the parameters influencing sub-prosthetic root fractures in teeth with cast metal posts. Various factors associated with root fracture include intracanal preparation, ferrule effect, characteristics of the root post (elastic modulus, length, diameter, and clinical situation), inlay-core assembly materials, and occlusal forces.

Diagnosis

The diagnosis of vertical root fractures is based on the triad: subjective, objective, and radiographic evaluation. From the retrospective cohort by Wan-Chuen Liao and al., subjective or objective clinical symptoms and signs of root fractures may be absent or minimal in the early stages, making precise diagnosis of root fractures challenging.¹ As root fracture exacerbation develops, patients often suffer from discomfort, mild pain near the fractured tooth, dull pain during chewing, gingival swelling with a fistula, sensitivity to percussion and palpation, and localized deep periodontal probing defects. Radiographic images can indicate bone loss and bone defects resembling periodontal destruction with a fractured root, periradicular radiolucency, and widening of the periodontal ligament space.¹

Dental Predisposition

According to the study by Sarkis-Onofre R. and al. and Cohen S. and al., the most commonly affected teeth are maxillary premolars and mandibular molars.^16,20 According to Wan-chuen et al, mandibular first molars followed by maxillary second premolars and maxillary central incisors are the teeth most predisposed to vertical root fracture, whether or not they have undergone endodontic treatment. The roots most likely to fracture are those with narrow mesiodistal diameters and a thin, flat morphology. The root depression of the mesial root of mandibular molars and the buccal root of bifurcated maxillary premolars presents an anatomical entity that may predispose to fractures and root perforations in cases of excessive dentin removal. These areas should be considered “danger zones”.¹

All studies, including finite element studies and experimental studies, have taken this predisposition into account when selecting the test samples.

Intracanal Preparation

According to Rippe MP. and al., manual canal preparation seems to have induced irregularities, cracks, and more frequent wear on the cervical dentin compared to rotary canal preparation. Prosthetic preparation does not seem to eliminate these irregularities, especially when using a fiber post, which results in a higher concentration of stress in the cervical and middle thirds. These cracks can become areas of high stress concentration when forces are applied during restorative procedures or due to occlusal stresses, evolving into fractures over time.⁷

Ferrule Effect

The ferrule effect is the circumferential portion of the prosthetic crown that rests on the cervical dentin, providing the gripping and protection of the tooth. It is considered as the area of remaining dentin at the cervical margin.

According to a finite element study conducted by Dejak B. and al., they demonstrated that the ferrule effect has a critical influence on reducing stress in anterior teeth restored with metal posts.⁸

According to a finite element study by Mosharaf R. and al., increasing the height of the ferrule to 2 mm significantly increases dental fracture resistance. In incisors, retaining coronal dentin on the palatal side doubles their fracture resistance. In a three-year clinical follow-up study, only 6.67% of incisors restored with 2 mm ferrule posts were damaged, while the rate for teeth without ferrule reached 26.20%.¹⁹ However, in the study by Samran A. and al. conducted on mandibular premolars, the specific location of the ferrule had no significant effect on fracture resistance under the study’s conditions.¹³

According to Iemsaengchairat R. and al., metal inlay-cores provided the highest fracture resistance for the reconstruction of thin walls in endodontically treated teeth without a ferrule.¹⁸

Ferrule effect can improve resistance to occlusal forces applied to the tooth. It also reduces the forces applied to the tooth, including shear or bending moments during post placement, contributing to improved crown marginal fit. Ferrule is a key protective factor in preventing root fractures.

Root Posts and Tooth Position

According to the study by Dejak B and al. on 3D central incisors, post length has a minor effect on stress levels in dental structures.⁵

However, according to the study by Kainose K., stress in cast metal posts in mandibular premolars is primarily generated in the end of the post, and this increases with decreasing post length.⁹

The study by Sugaya T. found no association between post length and fractures originating in the cervical region, and there were fewer fractures originating from the apical region with longer posts. Thus, longer posts may be preferable to prevent vertical fractures.¹¹ These findings are consistent with the study by Misharaf R. and al., which indicates that a reduced post length reduces stress in the cervical region of the tooth and increases stress at the post’s end.¹⁹ An interplay exists between the height of the ferrule and the post length, and in the presence of high residual coronal height, a shorter post length might be feasible.

According to the study by Kainose K. and al., in the presence of cervical encirclement, post length and the mechanical properties of post and core materials do not affect fracture resistance and stress distribution in the restored tooth. However, in the absence of cervical encirclement, materials and post length significantly affect stress distribution in the restored tooth. The metallic post model produced lower shear stress at the interface in the cervical area than the fiber post model. However, shear stress at the post and intracanal housing interface was primarily generated in the cast post and increased with decreasing post length. This raises the risk of cast post debonding, especially if the post is shorter. Fiber post models produced lower stress at the root surface, and the effect of post length in this case was less pronounced.⁹

Vertical fractures in the cervical region can occur at the vestibular/lingual and mesial/distal sites, while fractures originating in the apical region were found at the vestibular/lingual location.¹¹

According to the study by Veeraganta S. K. and al., larger-diameter posts exhibited greater fracture resistance than smaller-diameter posts, regardless of post material or remaining dental substance. However, using thinner posts has been recommended, as it preserves more radicular dentin, reinforcing the dental structure and enhancing fracture resistance. It is also suggested to limit post diameter to less than 50% of root diameter and preserve dentin as much as possible.¹⁷

The study by Uzun İ. and al. found that oval posts did not increase fracture resistance in premolars with oval canals compared to circular posts.¹²

According to the study by Ona M. and al., using high modulus of elasticity alloys leads to nearly complete transmission of stresses to dentin due to the limited deformability and absorption capacity associated with a high modulus of elasticity. However, reducing the modulus of elasticity of the post material increased stress on the root surface in anterior teeth subjected to bending.³

The study by Madfa AA. and al. conducted on 3D models of maxillary central incisors observed the highest stresses in stainless steel posts, followed by zirconia and titanium posts compared to fiberglass posts.⁸ These findings are consistent with the studies by Rippe MP and al., the study by Oyar P and the study by Torres-Sánchez C and al. conducted on single-rooted premolars.^4,7,10

Fractures Occurring Due to the Use of Glass Fiber or Quartz Posts were Repairable, unlike fractures resulting from Zirconia or Titanium posts, which were irreparable. This could explain why glass and carbon fiber posts exhibited a higher average von Mises stress at the crown-cement/post interface than stainless steel, zirconia, and titanium posts. This concentration of high stresses in stainless steel, zirconia, and titanium posts could potentially lead to post debonding from radicular walls.⁸

According to the study by Veeraganta SK and al., titanium posts on mandibular premolars showed statistically higher fracture resistance than fiberglass posts.¹⁷

Fractures with a guarded prognosis occurred only under loads greater than physiological force values. This implies that these fractures may not occur under physiological loads.⁷ This is in line with the study by Barcellos RR and al.⁶

Indeed, posts with higher modulus of elasticity amplify stresses within the post itself and reduce stress distribution in radicular dentin. Posts with an elastic modulus similar to dentin formed a mechanically homogeneous unit (monobloc) with better biomechanical performance.^6,7

According to the study by Dejak B and al. on 3D central incisors, using rigid materials for posts results in reduced stresses in dental tissues. Teeth restored with cast posts need to demonstrate higher fracture resistance than teeth with fiber posts.⁵

Finally, according to the randomized controlled trial by Sarkis-Onofre R. and al., cast metal posts had an annual failure rate of 1.2%, while fiberglass posts had an annual failure rate of 1.7%.¹⁶

Post Cement Material

As per the study by Oyar P.¹⁰ and al., using a post material with a lower modulus of elasticity and a cement material with a higher modulus of elasticity reduced deformation in the residual root, cement, and post, as well as reduced stress in the post. The combination of ceramic-metal crown/Ni-Cr (nickel-chrome) post/Au-Pd (gold-palladium) zinc phosphate or Panavia cement could be favorable for post and core restorations.¹⁰

According to the study by Torres-Sánchez C., the highest fracture resistance was observed in the group with posts sealed using resin-modified glass-ionomer cement.⁴

According to Abdulmunem M. and al., using zinc phosphate cement offered relatively higher fracture resistance. Composite resin as a luting cement resulted in more restorable failure modes. However, the adhesive technique makes cleaning the smear layer from the root canal space and removing moisture from the root challenging.¹⁴ Adhesion is less certain in the radicular than in the coronal region, primarily due to structural dentin changes and a decrease in the number of dentinal tubules. Therefore, it is best not to attempt bonding more than 5 to 7mm into the root canal.²¹

According to Veeraganta S. K. and al.¹⁷ customizing the post increases its adaptation to root walls and reduces resin thickness. Closer contact between the cement type and dentin is also crucial for enhancing post frictional retention, with frictional retention being directly proportional to the contact surface (larger contact surfaces result in better retention).¹⁷

The sealing technique can affect the fracture resistance of restored teeth. Residues in the post space, bubbles in the cement layer, and excessive seating pressure can lead to stress concentration in the root and predispose it to fracture.

Occlusal Loads

Cast metal posts have a high modulus of elasticity, and when subjected to occlusal forces, they exert forces against radicular dentin walls, which have a lower modulus of elasticity, increasing the risk of coronoradicular fracture.⁶

Bruxism imposes significant non-physiological mechanical stresses but does not seem to be as determinant in causing radicular fractures as it might be for coronal cracks. However, it remains a risk factor that could increase cases of vertical fracture.^20,22

Occlusal factors, in the form of repetitive stresses, are responsible for propagating cracks from surface defects. This is known as fatigue fracture.²³

Conclusion

Understanding vertical radicular fractures in teeth supporting cast metal post restorations relies on a thorough diagnosis, encompassing subjective, objective, and radiographic evaluations. These findings have significant implications. Practitioners should be aware of the predisposition of certain teeth, the importance of preserving the ferrule effect, the choice of root post materials, and the role of occlusal forces in managing and preventing vertical root fractures. This knowledge can guide clinical decision-making, leading to more effective and tailored treatment approaches, ultimately benefiting patients’ oral health and long-term outcomes.