Jakub Mateusz Gryko Fifth year, OF group 15
Analysis of biomechanics of glass fiber posts and ferrule, using finite elements method as analytical tool. Systemic review.
Master’s Thesis
Supervisor Lecturer Rimantas Ožiūnas
Kaunas, year (12 pt)
LITHUANIAN UNIVERSITY OF HEALTH SCIENCES MEDICAL ACADEMY
FACULTY OF PROSTHODONTICS
Analysis of biomechanics of glass fiber posts and ferrule, using finite elements method as analytical tool. Systemic review.
Stiklo pluošto kaiščių ir "movos" efekto biomechanikos analizė baigtinių elementų metodu.
Sisteminė litaratūros apžvalga.
Master’s Thesis
The thesis was done
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Kaunas, year (12 pt)
EVALUATION TABLE OF THE MASTER’S THESIS
OF THE TYPE OF SYSTEMIC REVIEW OF SCIENTIFIC LITERATURE
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No. MT parts MT evaluation aspects
Compliance with MT requirements and
evaluation Yes Partially No
1 Summary
(0.5 point)
Is summary informative and in compliance withthe
thesis content and requirements? 0.3 0.1 0
2 Are keywords in compliance with the thesis
essence? 0.2 0.1 0
3 Introduc- tion, aim and tasks (1 point)
Are the novelty, relevance and significance of the
work justified in the introduction of the thesis? 0.4 0.2 0 4 Are the problem, hypothesis, aim and tasks formed
clearly and properly? 0.4 0.2 0
5 Are the aim and tasks interrelated? 0.2 0.1 0
6
Selection criteria of the studies,
search methods and
strategy (3.4 points)
Is the protocol of systemic review present? 0.6 0.3 0 7
Were the eligibility criteria of articles for the
selected protocol determined (e.g., year, language, publication condition, etc.)
0.4 0.2 0
8
Are all the information sources (databases with dates of coverage, contact with study authors to identify additional studies) described and is the last search day indicated?
0.2 0.1 0
9
Is the electronic search strategy described in such a way that it could be repeated (year of search, the lastsearchday;keywordsandtheircombinations;
number of found and selected articles according to the combinations of keywords)?
0.4 0.1 0
10
Is the selection process of studies (screening, eligibility, included in systemic review or, if applicable, included in the meta-analysis) described?
0.4 0.2 0
11
Is the data extraction method from the articles (types of investigations, participants, interventions, analysed factors, indexes) described?
0.4 0.2 0
12
Are all the variables (for which data were sought and any assumptions and simplifications made) listed and defined?
0.4 0.2 0
13 Are the methods, which were used to evaluate the
risk of bias of individual studies and how this 0.2 0.1 0
information is to be used in data synthesis, described?
14 Were the principal summary measures (risk ratio,
difference in means) stated? 0.4 0.2 0
15
Systemiza- tion and analysis of
data (2.2 points)
Is the number of studies screened: included upon assessment for eligibility and excluded upon giving the reasons in each stage of exclusion presented?
0.6 0.3 0
16
Are the characteristics of studies presented in the included articles, according to which the data were extracted (e.g., study size, follow-up period, type of respondents) presented?
0.6 0.3 0
17
Are the evaluations of beneficial or harmful outcomes for each study presented? (a) simple summary data for each intervention group; b) effect estimates and confidence intervals)
0.4 0.2 0
18
Are the extracted and systemized data from studies presented in the tables according to individual tasks?
0.6 0.3 0
19
Discussion (1.4 points)
Are the main findings summarized and is their
relevance indicated? 0.4 0.2 0
20 Are the limitations of the performed systemic
review discussed? 0.4 0.2 0
21 Does author present the interpretation of the
results? 0.4 0.2 0
22 Conclusions (0.5 points)
Do the conclusions reflect the topic, aim and tasks
of the Master ’s thesis? 0.2 0.1 0
23 Are the conclusions based on the analysed material? 0.2 0.1 0
24 Are the conclusions clear and laconic? 0.1 0.1 0
25
References (1 point)
Is the references list formed according to the
requirements? 0.4 0.2 0
26
Are the links of the references to the text correct?
Are the literature sources cited correctly and precisely?
0.2 0.1 0
27 Is the scientific level of references suitable for
Master ’s thesis? 0.2 0.1 0
28
Do the cited sources not older than 10 years old form at least 70% of sources, and the not older than
5 years – at least 40%? 0.2 0.1 0
Additional sections, which may increase the collected number of points 29 Annexes Do the presented annexes help to understand the
analysed topic? +0.2 +0.1 0
30
Practical recommen-
dations
Are the practical recommendations suggested and
are they related to the received results? +0.4 +0.2 0 31
Were additional methods of data analysis and their results used and described (sensitivity analyses, meta-regression)?
+1 +0.5 0
*Remark: the amount of collected points may exceed 10 points.
Reviewer ’scomments:
32
Was meta-analysis applied? Are the selected statistical methods indicated? Are the results of each meta-analysispresented?
+2 +1 0
General requirements, non-compliance with which reduce the number of points 33
General require- ments
Is the thesis volume sufficient (excluding annexes)?
15-20 pages (-2 points)
<15 pages (-5 points)
34 Is the thesis volume increased
artificially? -2 points -1 point
35 Does the thesis structure satisfy the
requirements of Master’s thesis? -1 point -2 points 36 Is the thesis written in correct language,
scientifically, logically and laconically? -0.5 point -1 points 37 Are there any grammatical, style or
computer literacy-related mistakes? -2 points -1 points 38 Is text consistent, integral, and are the
volumes of its structural parts balanced? -0.2 point -0.5 points
39 Amount of plagiarism in the thesis. >20%
(not evaluated) 40
Is the content (names of sections and sub- sections and enumeration of pages) in compliance with the thesis structure and aims?
-0.2 point -0.5 points
41
Are the names of the thesis parts in compliance with the text? Are the titles of sections and sub-sections distinguished logically and correctly?
-0.2 point -0.5 points
42 Are there explanations of the key terms
and abbreviations (if needed)? -0.2 point -0.5 points 43
Is the quality of the thesis typography (quality of printing, visual aids, binding) good?
-0.2 point -0.5 points
*In total (maximum 10 points):
TABLE OF CONTENTS
SUMMARY ... 7
INRODUCTION ... 8
Aim and Tasks ... 10
SELECTION CRITERIA OF THE STUDIES. SEARCH METHODS AND STRATEGY ... 10
2. Methods ... 11
2.1. Inclusion/Exclusion Criteria ... 12
2.2. Online search of databases ... 12
SYSTEMIZATION AND ANALYSIS OF DATA ... 13
3. Results ...13
DISCUSSION ... 19
CONCLUSION ... 23
CLINICAL IMPLICATIONS... ...24
REFERENCES ... 25
Summary
Objectives: The aim of this study is to present a review of the contemporary literature regarding the finite element method used in dentistry, the positive effect of ferrule and glass fiber restoration and type of software that was used to create 3D models.
Material and Methods: Three different databases were investigated during literature search. A total amount of 407 articles from PubMed (2009-2018), ScienceDirect (2009-2018) and Google Scholar (2014-2018) were investigated by title and abstract as shown in study flow below. (Fig. 1) In addition to the electronic search, a hand search of reference lists, of full articles was made.
Results: This study compared glass fibre posts with other types of posts, usually metal and the positive effect of ferrule preparation over the restored tooth. The final results show that GFP and ferrule effect have positive influence on stress distribution. The only exception were mandibular lateral incisors which show worse fracture resistance properties when additional preparation of the endodontically treated tooth was performed.
Conclusions: Ferrule effect seem to have very good prognosis for teeth restored with glass fibre posts. Endodontically treated teeth with higher and uniform ferrule show lower stresses at adhesive interfaces, that may lead to decrease the chance of clinical failure.
The most important is first mm of ferrule because the stresses are mainly ditributed in this region.Restorations with glass fiber posts without ferrule effect had much higher Maximum principal stress values (Mpa) compared to teeth restored with ferrule effect. Teeth restored without ferrule were more prone to increased stresses and risk of fracture. The absence of a ferrule was directly related with increased strain values on buccal and proximal surfaces for cast post and core. Post length did not have influence for stress
distribution in the dental tissues. So far we can terminate that there are several correct ways to perform the FEA if the results obtained are reliable. It can be suggested to create a protocol to follow when analysing tissues with FEA, this would provide more transparent and understandable records.
Keywords: "finite elements method", "post and core", "glass", "fiber", "ferrule effect" and
additionally, to precise the search in Google Scholar "dentistry" as keyword was added.
INTRODUCTION
Dentistry since many years is struggling not only with oral diseases, but also with limitations to the mechanical properties of materials used during restorations. The clinicians are looking for new methods to investigate the abilities of adaptation of materials used in daily dentist's routine and now as finite element (FE) analysis is widely used in dental biomechanical studies we should consider if the method is beneficial.
The finite element method (FEM) is a numerical method for solving problems of engineering and mathematical physics. It is also called as finite element analysis (FEA). The method is possible to use in structural analysis of heat transfer, fluid flow, mass transport, and electromagnetic
potential. When the problem is formulated by the finite element method the results are presented in a system of algebraic equations. To solve the problem, the programme subdivides a large problem into smaller, simpler parts that are called finite elements. Finite element method then uses different options from the calculus of variations to approximate a solution by minimizing an associated error function. With this method, in computer mechanics (CAE - computer aided engineering) the structural strength of the structure is tested, it simulates strains, stresses, displacements, heat flow and liquid flow. Dynamics, kinematics and statics of machines as well as electrostatic, magnetostatic and electromagnetic interactions are also investigated in different branches of medicine, like cardio surgeries, maxillofacial surgeries or even planning surgical excisions of tumors.
FEM calculations can be carried out in two-dimensional space (2D), where discretization comes to dividing the area into triangles. This solution allows you to calculate the values appearing in the cross-section of a given system. There are, however, some limitations related to the specificity of the problem being solved.
Due to the progress of computer technology in recent years, the majority of simulation packages are equipped with the ability to solve issues in three-dimensional space (3D).
Discretization usually involves the division of the area into tetrahedrons. Such modeling is devoid of
the fundamental limitations of 2D technology, but is much more demanding in terms of memory and
computing power of the computer. [1]
Prosthetic dentists are using this technique more, and more often to understand stresses and strains apearing in the tooth after endodontic restoration. Endodontically treated teeth are considered more prone to fracture than vital teeth [2, 3]. Loss of dental tissue structure during instrumentation of the canal or the crown is causing the change in the biomechanical behavior of the tooth [2]. The presence of the ferrule, protection of healthy, intact, radicular and coronal tooth structures have huge influence in strengthening weakened teeth [4,5]. By ferrule it is meant - parallel walls of dentin extending above the step of the preparation [6]. When the crown is casted with designed ferrule it provides a protective collar around the tooth which is reducing stresses within a tooth, such action is called the ferrule effect [5].
It is suggested to use glass fiber post when a prefabricated post is about to be applied [7]. The posts are showing homogeneous stress distribution to the dentins external surface alike dental tissues and reduce the stresses in the tooth. It's because of low elastic modulus materials which are used in GFP [8]. It has been claimed that, ferrule incorporation improves the fracture resistance by
distributing forces to the saved structures when it comes to restoring an endodontically treated tooth with a post and core, and crown [9].
To be able to understand the behaviour of the particles 2D or 3D models are being created using different methods of scanning and proper software eg. PRO Engineer software
(Parametric Technology Corporation
,USA
)for modeling [7], Algor Fempro software (ALGOR, Inc. Pittsburgh, USA) for stress analysis [10], for numerical simulations ABAQUS/CAE
(SIMULIA, Version 6.10, Providence, RI, USA) [11], for tissue identification Mimics 16.0
(Materialise, Leuven,Belgium) [10, 12]. Von Mises stresses are then being analyzed which allows
the researchers to know the limitations of the materials (such as glass-fiber posts) which later got
under tension of miscellaneous forces depending on the study. Von Mises proposed values used to
determine if a given material will yield or fracture. It is mostly used for flexible materials, such as
metals, glass-fiber, carbon fiber but also bone and other anatomical structures. The von Mises yield
criterion states that if the von Mises stress of a material under load is equal or greater than the yield
limit of the same material under simple tension then the material will yield [13]. Authors to better
determine the properties of the object are also using Young's modulus, to understand the relationship
between stress (force per unit area) and strain (proportional deformation) in a material [14]. The role
of elastic properties in the post and core restorations have been extensively studied over the years.
Literature includes numerous researches with different outcomes regarding the elastic modulus.
Aim:
The aim of this study is to present a review of the contemporary literature regarding the finite element method used in analyzing biomechanics of ferrule effect and glass fiber posts as well as metal posts and type of software that was used to create 3D models.
Tasks:
1. To analyze the importance of uniformity and height of ferrule using FEA.
2. To investigate biomechanical behavior of glass fiber post and core complex in case of absent ferrule, using FEA.
3. To see if the ferrule effect is important for cast post and core complex, using FEA.
4. To verify influence of posts length on biomechanics of the complex, using FEA.
5. To see what kind of software was most likely used for investigation of biomechanics of post and core complex.
SELECTION CRITERIA OF THE STUDIES. SEARCH METHODS AND STRATEGY
METHODS
This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines [15].
Three different databases were investigated during literature search. A total amount of 407
articles from PubMed (2009-2018), ScienceDirect (2009-2018) and Google Scholar (2014-2018)
were investigated by title and abstract as shown in study flow below. (Fig. 1) In addition to the
electronic search, a hand search of reference lists, of full articles was made.
Figure 1, Study flow.
T
he Population, Intervention, Comparison, Outcome, Study (PICOS) framework was used to form the following search strategy as seen in the table. (Fig. 2)
Figure 2. PICOS Table
The inclusion criteria were:
1. Clinical studies.
2. Studies related to restorative treatment of endodontically treated teeth.
3. Studies using finite elements method to investigate the problem.
4. Full-text.
5. Not older than 10 years and human filters were applied.
Keywords used in the search were: "finite elements method", "post and core", "glass",
"fiber", "ferrule effect" and additionally, to precise the search in Google Scholar "dentistry" as keyword was added.
Articles were analyzed by title and abstract, and excluded according to the following criteria:
1. Language different than english, 2. Older than 10 years,
3. Title not matching 'fiber glass' and 'ferrule', 4. Animal study.
5. Nine studies were also removed because of the duplication.
Filters application can be seen in Fig. 3.
Figure 3. Filters application
Database and date of search Filters and results
Keywords order: No filter Human 10
years 5 years
English languag e PubMed 02.23.2018
Keywords order: ((((((finite elements method) AND post) AND core) AND glass) AND fiber) AND ferrule)
10 6 6 3 N/A
GoogleScholar 02.23.2018
Keywords order: finitie, elements, method, post, core, glass, fiber, ferrule effect, dentistry
925 N/A N/A 366 334
ScienceDirect 02.23.2018
Keywords order: finite elements method, post and core, glass fiber, ferrule
72 N/A 36 N/A N/A
If the title and abstract had not enough information according the inclusion criteria, the full article was read. The full text reading of related publications and the data extraction were carried out.
SYSTEMIZATION AND ANALYSIS OF DATA
RESULTS
The initial electronic search resulted in 407 publications, of which 399 were excluded after
title and/or abstract assessment and foreign language. Based on the inclusion criteria, 8 papers were
selected for full-text reading. Of these, all were accepted for the review. (Fig. 1). Total of 8 in-vitro
studies were analyzed.
Authors Scanning method
Programs used
2D or 3D
Type of posts
Load and angle
Ferrule height and uniformity
Results
Zhang et al Laser- based 3D digitizing system
Finite element (FE) models with different ferrule configuratio n
were obtained using ScanIP1 module
All numerical simulations were performed using ABAQUS
3D Quartz fiber post
350N , 45 angle degree with respect to the long axis of the tooth
6 models
Group 1: no ferrule
Group 2: 1 mm labial ferrule and 0 mm palatal ferrule.
Group 3: 2 mm labial ferrule and 0 mm palatal ferrule.
Group 4: 0 mm labial ferrule and 1 mm palatal ferrule.
Group 5: 0 mm labial ferrule and 2 mm palatal ferrule.
Group 6:
uniform 2mm ferrule
Group 6 presented the highest fracture resistance, whereas, the lowest
fracture resistance appeared in the Group 1.
The study showed that there were no significant differences among Group 1, 2 or 3, in which the teeth have no palatal ferrule.
With the increase of palatal ferrule height in Groups 4 and 5 the ultimate loads increased.
A clear stress concentrations noticed around the palatal margin of cement layer in groups 1, 2 and 3.
The bigger the palatal ferrule the more anti-fracture ability of adhesive cement. The labial ferrule had less influence in this case.
In all of the FE models the loads which caused the initial damage in the glass ionomer cement were similar.
Rodrigues et al
CBCT Identifying software Mimics 16.0
STL design and meshing software 3Matic 8.0, remeshed- REMESH
3D Smooth conical fiber post
155N Right maxillary incisor - uniform
Left maxillary incisor - not uniform
Stresses in the root dentin and fiber post of the left maxillary central incisor were higher than the right maxillary central incisor.
Highest stress concentration - on the distal region of the left maxillary central incisor, where the ferrule was smaller.
Jelena Juloski, Davide Apicella, Marco Ferrari
Laser- based 3D digitizing scan of a plaster cast
The scanned profiles were assembled in a 3D wire frame structure using a 3D CAD, Auto- cad 12 and parametric solid modeler Pro-
Engineering
4 solid models were imported to the FEA software ANSYS
3D Glass fiber posts
200N load, 45 degrees
1 model simulated a premolar restored with a crown in absence of dentin ferrule.
3 models simulated teeth with different uniform ferrule heights.
Comparing to the model without ferrule it was presented that tensile stress observed in the composite abutment is significantly higher than those observed in models with ferrule.
FE-models with different height shared similar stress values and distribution patterns in the 1st mm of the ferrule area.
Considering this finding, it may be stated that ferrule height does not influence the stress state neither in the ferrule structure nor the root. Still the presence of ferrule effect is crucial for good prognosis for restored tooth.
Tensile stress state in the ferrule structure of 3 FE-models with designed ferrule does not reach the dentin tensile failure limit ranging from 105 to 135 MPa.
Beata Dejak, Andrzej Młotkowski
Dental 3D Scanner D250, a CT of the tooth was made with a GXCB- 500/i-CAT system.
3Shape Dental Designer CAD software used to process obtained the scans.
FEA software – ANSYS 14.
3D GFP and NiCr alloy
100N, 130 degrees to the long axis of the tooth
3 models with ferrule effect, glass fiber post.
2 models without ferrule effect glass fiber post.
3 models with ferrule effect, cast NiCr posts
2 models without ferrule effect, cast NiCr posts
In models with ferrule stresses in dentin were reduced to 11 MPa, while without ferrule stresses in dentin increased to 17.6 MPa. Ferrule effect in teeth reduces mvM stresses in dentin by 42–60% and in posts by 20–
33%, depending of of posts materials.
The mvM stresses in post and core were concentrated around the core- post junction, in the tooth cervix.
In GFP posts in teeth with ferrule according Tsai-Wu ratio presented no risk of fracture.
The highests stresses were observed in metal post and core without ferrule 99.7 MPa.
No matter if the posts are made of metal or FRC, are not prone to fractures under physiological loads, if they are ideally cemented in anterior teeth.
Post length did not have an influence on stresses in crowns and dentin.
Savychuk et al
CT scans 3D computer- aided design
FEA - Algor.
3D Gold cast post and core
Fiber post
140N oblique loading, angled at 135 degrees from the vertical tooth axis
A total of 10 models were created.
5 models of lateral incisors:
1 vital 1 CP with uniform ferrule 1 CP without ferrule 1FP with uniform ferrule 1 FP without ferrule
5 models of canine:
1 vital 1 CP with uniform ferrule 1 CP without ferrule 1FP with uniform ferrule 1 FP without ferrule
Glass fiber posts showed a not clear ferrule effect on the MnPS parameters in different teeth, lowering the stress values in the canine (33%) but increasing them in the mandibular lateral incisor (42%), if compared with the models without ferrule - no impact on the MxPS parameters was noticed.
The ferrule preparation infuenced the stress generated in the dentin of the mandibular lateral incisor and mandibular canine.
The ferrule effect in lateral incisor increased MPa in the dentin when in the canine the dentin stresses were decreased.
Ferrule effect in teeth restored with GFP for lateral incisor and canine reducied MPa in the core.
Lateral incisor without ferrule experienced reduced maximum principal stress (MPa) for gold cast post and core (70.6 MPa) when for the model with ferrule it was higher (73.4 MPa).
Roscoe et al The 3D numerical models of an intact maxillary canine were possessed from DT3D.
The 3D models were created with computer- aided design software Rhinoceros 3D
FEA software NeiNASTR N
3D Cast post and core Glass fiber post
Load from 0 to 100N
4 models with uniform ferrule were used;
1 GFP with uniform ferrule
1 GFP without ferrule effect
1 cast post and core with uniform ferrule effect
1 cast post and core without ferrule effect
The study showed significant
difference between teeth restored with cast post and core, restored with glass fiber post and teeth restored
with/without ferrule.
Restorations with glass fiber posts with ferrule effect had much lower Maximum principal stress values (Mpa) compared to teeth restored without ferrule and the traditional casted NiCr with or without ferrule effect.
The absence of a ferrule was directly related with increased strain values on buccal and proximal surfaces for all groups.
Rajambigai et al
CT The
modeling was done using software called PRO Engineer,
Stress analysis - ANSYS
3D Parallel sided prefabri cated titaniu m post
parallel sided prefabri cated glass fiber post
Load of 100N was applied with an angle of 45 degrees to the long axis of the tooth
4 models were created:
-Titanium post with uniform ferrule
-Titanium post without ferrule -Glass fiber post with uniform ferrule -Glass fiber post without ferrule
Glass fiber post with a ferrule of coronal dentin had the lowest stress value.
The highest stress value was obtained for Titanium post without a ferrule of coronal dentin.
Santos et al A contact scanner was used for
acquisition of the external morpholog y data (MDX-40;
Roland Co, Osaka, Japan) by scanning an extracted sound maxillary central incisor.
The external geometry was obtained by 3D Bundle
STL files imported to computer- aided design (CAD) soft ware Rhinoceros 3D
Meshing - Femap
3D Glass fiber post
Casted post and core
100N oblique loading
8 models
2 models of CPC with uniform ferrule and different post length
2 models of CPC without ferrule and with different post length
2 models of GFP with uniform ferrule and different post length
2 models of GFP without ferrule and with different post length
Relined GFP models showed
homogeneous stress distribution to the dentin external surface and related structures which was similar to the behavior found for a healthy tooth.
CPC models elevated stress in the root canal regardless of ferrule presence.
Increased tensile stress inside the root canal for CPC models.
The presence of ferrule was important for both post systems used in this study for the rehabilitation.
The worst condition for stress
distribution was observed for the CPC with ferrule.
The stress distribution of relined GFP models were not influenced by the post length, no matter if the ferrule was presented.
Although the researchers used different types of loadings they came up with similar results.
Most of them noticed that usage of glass fiber posts and ferrule effect had positive outcome for stress distribution over dentine and restorative materials that were used.
Surprising data were presented by Savychuk et al. [10] they analyzed a total of 10 3D models
under 140N oblique loading, angled at 135 degrees from the vertical tooth axis. During their
principal stresses in mandibular lateral incisors were bigger in model with designed ferrule, than in models without ferrule or canines with or without ferrule. The MPa increased in lateral incisor by 42% when in canine it decreased by 33%.
Santos et al. [8], by creation of 8 3D models they claimed that the stress distribution of
relined glass fiber posts models were not influenced by the post length, no matter if the ferrule was present or not (similar results were presented by
Dejak and Młotkowski [16]). On the other hand, thepost length was more important for the cast post and core models in which stresses were
concentrated in a smaller area. Better stress distribution in GFP is probably achieved because of the materials elastic modulus which is similar to dentine.
Zhang et al. [11] and Rodrigues et al. [12], noticed that the most important part of the
ferrule design is in palatal region. The bigger the palatal ferrule the more effective was increase of anti-fracture ability of adhesive cement. On the other hand, the labial ferrule had less influence in this case.
Roscoe et al. [9] showed significant difference between teeth restored with cast post and core, glass fiber post and teeth restored with/without ferrule. Restorations with glass fiber posts with ferrule effect had much lower Maximum principal stress values (Mpa) compared to teeth restored without ferrule. The
traditional casted NiCr with or without ferrule effect had bigger Mpa than GFP. The absence of a ferrule was directly related with increased strain values on buccal and proximal surfaces for all groups.
As well as Roscoe et al., Juloski et al. [17] state that the presence of ferrule effect is crucial for good prognosis for restored tooth. Comparing to the model without ferrule it was presented that tensile stress observed in the composite abutment was significantly higher than those observed in models with ferrule. FE- models with different height shared similar stress values and distribution patterns in the 1st mm of the ferrule area. Considering this finding, it may be stated that ferrule height does not influence the stress state neither in the ferrule structure nor the root. However, stress state in the ferrule structure of 3 FE-models with designed ferrule does not reach the dentin tensile failure limit ranging from 105 to 135 MPa.
Computer analyzes performed by the authors are presented in the annexes.
DISCUSSION
Endodontically treated teeth with severe loss of coronal tissues are most favorably restored by using cast posts, prefabricated glass fiber and metal posts supported with composite cores.
Prefabricated posts made of titanium are becoming more and more popular among metal posts because of their favorable physical properties and resistance to corrosion [18]. When it comes to non-metal posts, glass fiber reinforced composite posts are increasing in popularity due to their translucency and flexibility [7].
Uncontrolled preparation or caries-caused loss of tooth structure decreases the retention of future restorations, making them more problematic to apply. It is well known that after root canal treatment, dentin faces changes not only in its physiologic characteristics but also in its physical properties, leading to decrease of immature collagen levels. This can proceed to reduced hardness and resistance to shearing [19]. Dehydration of dentine tubules because of the use of etching, air drying and drilling, also occurs. It is causing a decrease in the value of Young ’s modulus, in accordance a tooth is more prone to fractures [20]. The problem primarily appears as a result of tensile and shear stresses [16]. The development of computers and increase of their calculating abilities, allow that the finite element analysis can be used for prognosis of fracture (or even fracture pattern). It is possible by analyzing the models of the tissue scans achieved by processing them in 3D software. The experiments lead under experimental loading provide crucial data about tensile and shear strength values of a material, tissue or an interface between them. This allows practitians to choose the materials for restorations more accurate.
When restored with elastic posts, the tooth, cement and post will all deform during mastication. Most favorably, the failure appears at the post-cement-dentin interface as this is an adhesive connection and the weakest point at the same time. The more the coronal structure of the tooth is destroyed the bigger will be the stress on the adhesive joint [21]. When the endodontic treatment of a tooth is completed, the whole structure must be protected due to risk of fracture during functional loading. As other studies showed, FE analysis has been often used to analyze stress and strain distributions in teeth restored with different types of post and core, adhesive
techniques or ferrule designs [22-24]. A ferrule is defined as a 360 degrees metal band of the crown
which is surrounding matching preparation of dentin walls [4]. The current study evaluated stress
height and uniformity of it taking into consideration load application on the crown surfaces. Inside failures of endodontically treated teeth may be due to the fracture of a component or debonding at the adhesive connections between the parts [11].
Dejak and Młotkowski [16] in their study, using finite element method, stated that the
presence of ferrule in a tooth treated with a post and core restoration reduces stresses in dentin, post and luting cement applied around the post. As proved in their analysis the usage of posts and cores in teeth with ferrule effect makes teeth more resistant, but increases stresses in dentin in teeth if the supragingival structure is missing. High stresses in the cement were mainly spotted around short fiber glass reinforced posts without ferrule. Not preparing a ferrule leads to significant tensile contact stresses around posts and the area around the root may predispose to micro-leakage in the borders of restoration and debonding of the post. The calculations performed by Dejak and
Młotkowski [16] allowed to determine that the higher the Young’s modulus of the post materials, the
smaller modified von Mises failure criterion (mvM) stresses applied on dentin and cement around posts. Simillar results of FE analysis of ferrule effect and post and core restorations were achieved by other researchers [22, 25-28] As specified by Pereira [29], ferrule causes a statistically
significant increase in tooth resistance to fracture. An interesting study was as well presented by
Lima et al. [30] The research tested the maximum loading that can be sustained by teeth restoredwith and without ferrule. It showed that teeth with ferrule effect The maximum load for tooth with ferrule at which it failed was 573.3 N, when without ferrule the force that the tooth was capable to resist was 275,3 N [31]. According to a study performed by Zhang et al. [11], the best for support and retention, if the amount of tissues allow for it, is a 2mm uniform ferrule. Group 6 containing a model with 2mm uniform ferrule presented in the reasearch had the highest fracture resistance.
However, the lowest fracture resistance appeared in the Group 1which had 0mm ferrule.
Rodrigues et al. [12] came up with more precise statement. When the load is applied to a
structure, deformation and stresses are initiated. To simulate the masticatory forces, this study used
contact forces applied through the opposing incisal teeth. After analyzing the anatomic finite
element model, which they received by transferring cone-beam computed tomography image into
the software, they found that ferrule uniformity is more important than ferrule height unlike claimed
by Zhang et al[11]. The stress investigation verified that the ferrule design is an important factor in
the distribution of stresses in glass fiber posts and root dentin. They also agreed that ferrule
the right maxillary central incisor had more uniform ferrule and was approximately 1.6 +/- 0.2mm high. In second model the ferrule of the left maxillary central incisor was higher medially - 3.5 mm and left with no ferrule distally - 0.3 mm. Stresses in the root dentin and fiber post of the left maxillary central incisor were higher than the right maxillary central incisor. Authors observed highest stress concentration on the distal region of the left maxillary central incisor in root dentin and also in the root canal, where the ferrule was smaller and less uniform. Authors proved that ferrule had positive effect on distribution of tensile contact stresses around posts in teeth without supragingival structure.
In choosing a post system not only ferrule and physical/physiological changes have important meaning. One of the most determining factors is related to the post length. A couple of studies have reported positive results in the rehabilitation of endodontically treated teeth when the post length was the same or higher than the crown length [30, 31], when for a shorter posts, a high rate of root fractures was found [33]. Santos-Filho et al. [28, 34] has stated that the length of
metallic posts is directly related to the fracture resistance abilities of endodontically treated teeth. In presented study of Santos et al., [8] finite element analysis showed that the stress distribution of cast posts and cores was negatively affected by the reduction of post length. The stiffness of the material is transmitting higher stress concentration in the thin dentin wall for the weakened tooth. When finally the whole restored complex fails because of the fatigue, the stresses are transferred to dentin, causing even vertical root fractures [11]. In the absence of palatal ferrule, the root canal associated with the use of a glass fiber post showed a stress concentration at the connection between the resin composite and the post. This situation may cause that this adhesive interface will lead to post debonding and resin core fractures, resulting in bacterial colonization and periapical lesions [11].
On the other hand
Dejak and Młotkowski [16] claimed in their study that post length had aninsignificant effect on mvM stresses in roots, no matter of post material [ANNEX 2]. Widely spreading finite element analysis allows other authors to perform their tests more homogenously.
Some of them also stated that they found no significant differences between post length, distribution
of their stresses and the values obtained in the study [35]. Also Rodriguez-Cervantes et al. observed
there was no connection between length of posts (4-14 mm) and fracture force [36]. Similar to
Zhang et al. [11], Nissan et al. validated that post length does not affect tooth strength where there isa 2 mm ferrule [37]
.So, when choosing a correct post for restoration the post length should be considered and the post:crown ration should be lower than 1:1.
Rajambigai et al. [7] comparing the titanium post with, the glass fiber post agreed that post-
cement-dentin interface is the area where failure is more likely to initiate, so the stresses were measured in this region. Glass fiber post with a ferrule of coronal dentin had the lowest stress value, when the highest stress value was obtained for titanium post without a ferrule of coronal dentin.
What is more, Santos et al. [38] reported that the risk of fracture occurs in the composite core and/or it is higher in the post than in the root, which makes teeth restored with a glass fiber post less likely to fail.
What's surprising, not every tooth is acting similar to its neighbor. Data collected by
Savychuk et al. [10] provided very interesting outcome. Namely, the mechanical properties ofmandibular incisors are different than those of bigger mandibular teeth. They admitted that no matter if the ferrule was present or what type of post and core material was used, models of lateral incisors showed higher tensile and compressive stress values than mandibular canines. This was not the only study revealing such behavior. Gluskin et al. [339] wrote that the characteristics of
mandibular lateral incisors separate them from teeth with bigger and uniform roots. This finding was also proved by Deutsch et al. [40], who found that the smaller was the root diameter the more prone to fractures it was, no matter what was the type of post used. In this case all potst had standardized 1.5 mm diameter. There is a close relationship between the fracture resistance of a tooth and thickness of the dentin covering the post as verified in another studies [41-42]. The same studies also found that preparation of ferrule had a role in stress distribution in dentin and also depended on a a type of tooth. Savychuk et al. [10] concluded that the preparation of ferrule
increased tensile stress in dentin, almost to the critical level of dentin withstanding limit [43], when for mandibular canine the reaction to stresses was opposite. As proposed by the above autor, this phenomenon might be described by the anatomy of mandibular lateral incisors. Narrow
buccolingual and mesiodistal root dimensions of these teeth, allowing to prepare smaller chamfer width and ferrule thickness which will lead to expansion of high stress fields. It is necessary to keep this results in mind when designing a prosthetic restoration of mandibular incisors.
The software and methods of scanning used by the researchers were the most varied part in
models with different ferrule designs when Juloski et al. [17], decided to apply three programs, to obtain four models with different ferrule heights and Roscoe et al. [9] used only two. There is no agreement on one diagnosing method which could be helpful in simplifying the results. Also, the situation may be dictated by the years when the studies were performed or/and hardware capabilities.
Finite element analysis is a handful tool in calculating stress distribution in complex structures. The results are provided without variation. The accuracy of the study depends on how the model approaches reality. The huge advantage is that, it provides detailed information concerning stress in a non-homogenous structure such as tooth. There is no need for human material, the measurements can be corrected easily and simulation can be performed without risk of failing and starting from the begining [17].
CONCLUSIONS
Based on the results of the present systematic review it can be concluded that glass fibre post are the best choice for most of the clinical cases. However, the preparation in mandibular incisors must be as least traumatic as possible to provide safe restoration without risk of fracture.
1. Endodontically treated teeth with higher and uniform ferrule show lower stresses at adhesive interfaces, that may lead to decrease the chance of clinical failure.
The most important is first mm of ferrule because the stresses are mainly ditributed in this region.2. Restorations with glass fiber posts without ferrule effect had much higher Maximum principal stress values (Mpa) compared to teeth restored with ferrule effect. Teeth restored without ferrule were more prone to increased stresses and risk of fracture.
3. The absence of a ferrule was directly related with increased strain values on buccal and proximal surfaces for cast post and core. On the other hand CPC models presented elevated stress in the root canal regardless of ferrule presence.
4. Post length did not have influence for stress distribution in the dental tissues when minimal 1:1
(root:crown) ratio was preserved.
5. The researchers used different types of software, provided by companies from all around the world, to obtain their results. 2D or 3D models were being created using different methods of scanning and proper software eg. PRO Engineer software (Parametric Technology Corporation, USA) for modeling [7], Algor Fempro software (ALGOR, Inc. Pittsburgh, USA) for stress analysis [10], for numerical simulations ABAQUS/CAE (SIMULIA, Version 6.10, Providence, RI, USA) [11], for tissue identification Mimics 16.0 (Materialise, Leuven,Belgium) [10, 12] So far we can terminate that there is several correct ways to perform the FEA if the results obtained are reliable. It can be suggested to create a protocol to follow when analysing tissues with FEA, this would provide more transparent and understandable records.
Clinical Implications
A ferrule has a positive impact on the mechanical stress values of restorative elements, especially teeth restored with glass fiber posts. It is important to keep proper length of the post, ratio at least 1:1 (root:crown), not for stress distribution but for proper retention of the complex. Glass fiber posts should be a considered as main choice during restoring of endodontically treated teeth.
However, it is important to keep in mind that additional preparation for ferrule in mandibular incisors
leads to a considerable rise in stress in the dentin.
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ANNEX 1 Rodriguez et al. (Modified von Mises stress results).
ANNEX 2 Dejak and Młotkowski (Maximal mvM stresses in materials in the models of teeth
restored with different posts and cores).
ANNEX 3 Dejak and Młotkowski (Distribution of stresses in different materials of the tooth
restored with FRC, F +).
ANNEX 4 Dejak and Młotkowski (Distribution of stresses in different materials of the tooth
restored with NiCr post 13mm length, F +)
ANNEX 5
Juloski et al. (Distribution of tensile and compressive stress with the interfaces).
ANNEX 6 Juloski et al. (Distribution of tensile and compressive stress within the restored tooth components).
ANNEX 7 Roscoe et al. (Stress distribution within dentine and bone structure).
ANNEX 8 Rajambigai et al. (Distribution of von Mises stress in model restored with glass fiber post, F -)
ANNEX 9 Rajambigai et al. (Distribution of von Mises stress in model restored with titanium post, F - and F +.
Distribution of von Mises stress in model restored with glass fiber post, F +).
ANNEX 10 Santos et al. (Stress distribution in teeth).
ANNEX 11 Savychuk et al. (Von Mises stress distribution patterns or mandibular incisors F + and F -),
ANNEX 12 Zhang et al. (The maximum principal stress ditribution in cement layer).
