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OCCLUSION AND TMJ
ANALYSIS OF STRESS GENERATED IN THE ENAMEL OF
Original Articles
AN UPPER FIRST PREMOLAR: A FINITE ELEMENT STUDY
Andreea Stănuşi1a , Veronica Mercuţ1b , Monica Scrieciu1c , Sanda Mihaela Popescu2d ,
Monica Mihaela Iacov Crăiţoiu1e , Luminiţa Dăguci1f , Ştefan Castravete3g, Marina Olimpia Amărăscu1h
1
Department of Proshodontics, Faculty of Dentistry, University of Medicine and Pharmacy of Craiova, Craiova, Romania
2
Department of Oral Rehabilitation, Faculty of Dentistry, University of Medicine and Pharmacy of Craiova, Craiova, Romania
3
Caelynx Europe SRL, Craiova, Romania
a
DMD, PhD Student; e-mail: andreeacazan22@yahoo.com; ORCIDiD: https://orcid.org/0000-0003-2520-7967
b
DDS, PhD, Professor; e-mail: veronica.mercut@yahoo.com; ORCIDiD: https://orcid.org/0000-0003-3573-7128
c
DMD, PhD, Professor; e-mail: scrieciu_monica@yahoo.com; ORCIDiD: https://orcid.org/0000-0003-0696-7100
d
DMD, PhD, Professor; e-mail: sm_popescu@hotmail.com; ORCIDiD: https://orcid.org/0000-0003-3011-6322
e
DMD, PhD, Professor; e-mail: mcraitoiu@yahoo.com; ORCIDiD: https://orcid.org/0000-0003-4998-8198
f
DMD, PhD, Professor; e-mail: daguciluminita@yahoo.com; ORCIDiD: https://orcid.org/0000-0003-4327-2738
g
Eng; e-mail: scastravete@caelynx.ro
h
DMD, PhD; e-mail: marinaamarascu@yahoo.com; ORCIDiD: https://orcid.org/0000-0002-3394-1486
ABSTRACT https://doi.org/10.25241/stomaeduj.2020.7(1).art.4
Introduction This study investigated the distribution and magnitude of stress generated in the enamel
of an upper first premolar, after applying normal and excessive occlusal loads in a vertical and horizontal
direction, using Finite Element Analysis (FEA).
Methodology A 3D virtual model of an upper first premolar was analyzed. The CT images of the tooth were
converted into 3D data using the program MIMICS and Finite Element Analysis (FEA) was used for the stress
study. To better understand the distribution of stress generated by occlusal loading, the situation of the
enamel in various 3D virtual models was presented. 14 scenarios for the occlusal loading of the virtual
models of the upper first premolar were obtained and the areas with the highest concentration of stress
were emphasized.
Results In the model with the tooth intact, stress values were higher than the admissible ones in the
simulation of the excessive vertical loading, normal horizontal loading and excessive horizontal loading.
Stress was found in the buccal cusp area and in the cervical area, mainly on the buccal side of the tooth. In
the models with horizontal occlusal tooth wear, stress values were higher than the admissible ones in the
simulation of the excessive vertical loading. Stress was found in the cervical area. In the models with oblique
occlusal tooth wear, stress values were higher than the admissible ones in the simulation of the normal and
excessive horizontal loading. Stress was found mainly in cervical area, on the buccal side of the tooth.
Conclusions The most harmful loads were the heavy vertical ones and the horizontal ones, no matter the
magnitude.
KEYWORDS
Tooth Wear; Finite Element Analysis; Stress, Non-Carious Cervical Lesions.
1. INTRODUCTION [3], and in an advanced stage they can determine
endodontic pathology and even the fracture of
Non-carious cervical lesions involve the loss of hard the teeth involved [4]. Furthermore, non-carious
dental tissues in the cervical areas of the teeth, cervical lesions are important because of their high
without carious activity [1,2]. These lesions are prevalence (5-85%) [5]. Initially, the toothbrush/
important because, in a first stage, they can produce dentifrice abrasion was considered to be the cause
gingival retractions associated with teeth sensitivity for the loss of dental tissues in the cervical area, as
OPEN ACCESS This is an Open Access article under the CC BY-NC 4.0 license.
Peer-Reviewed Article
Citation: Stănuşi A, Mercuţ V, Scrieciu M, Popescu SM, Iacov Crăiţoiu MM, Dăguci L, Castravete Ş., Amărăscu MO. Analysis of stress generated in the enamel
of an upper first premolar: a finite element study. Stoma Edu J. 2020;7(1):28-34.
Received: February 20, 2020; Revised: February 26, 2020; Accepted: March 16, 2020; Published: March 18, 2020
*Corresponding author: Prof. Veronica Mercuţ; Prof. Sanda Mihaela Popescu, Faculty of Dentistry, University of Medicine and Pharmacy of Craiova
Craiova, Romania
Tel./Fax: +40/0251 - 524 442
e-mail: veronica.mercut@yahoo.com; e-mail: sm_popescu@hotmail.com
Copyright: © 2020 the Editorial Council for the Stomatology Edu Journal.
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Original Articles
Table 1. Dental materials properties.
Properties Enamel Dentin Pulp Periodontal ligament
Elastic Modulus of Young (GPa) 80.35[32-34] 19.89 [22,23] 0.002 [22] 0.069 [22,23]
Poisson Ratio 0.33[32-34] 0.31 [22,23] 0.45 [22] 0.45 [22,23]
Table 2. Ultimate tensile and compressive strength of dental enamel.
Ultimate Properties Enamel
Tensile Strength (MPa) 24 [8]
Compressive Strength (MPa) 62 [13]
Figure 1. Von Mises Stress in enamel:
a) intact premolar; b) horizontal tooth wear
version 1; c) horizontal tooth wear version 2.
Figure 2. Von Mises Stress in enamel:
a) intact premolar; b) horizontal tooth wear
version 1; c) horizontal tooth wear version 2.
stated by Zsigmondy in 1894 [6]. He described these the non-carious cervical lesions, away from that
lesions as being wedge-shaped and distributed point [12]. The effects of stress in teeth depend on
on the buccal surface of the upper frontal teeth. the magnitude, direction, frequency, point of force
In 1991, Grippo named the non-carious cervical application, but also on the axis of the teeth, their
lesions produced by the mechanism of occlusal form, structure and mobility [12,17].
loads as “abfraction lesions”, so that they could be The areas most affected by abfraction lesions are
easily differentiated from the erosion and abrasion located on the buccal surfaces of teeth, in the cervical
lesions [7]. Recently, a series of studies [7-16] claim zone, with the highest prevalence amongst upper
that many factors are involved in the genesis of and lower premolars [8,15,16,18]. Initially, abfraction
non-carious cervical lesions: heavy occlusal loads, lesions appear at the enamel-cement junction in the
biocorrosion, and abrasion. enamel structure, as a result of reduced thickness of
These factors could be encountered individually enamel in that area, the absence of enamel rods and
or in association. It was suggested that heavy low frequency of Hunter Schreger Bands [19].
occlusal loads from mastication, swallowing and The aim of this study was to investigate the
parafunctional movements determine tooth flexure distribution and magnitude of stress generated
and high stress concentration in the cervical area by normal and heavy occlusal loads, which were
with the development of enamel cracks. In 2012 directed vertically and horizontally in the enamel of
Grippo et al. described the mechanism via which an upper first premolar, using Finite Element Analysis
heavy occlusal loads generate stress in the cervical (FEA) and the virtual model from a previous study
area [12]. [20]. Finite Element Analysis (FEA) is a numerical
Depending on the direction of forces, in the tooth procedure used in dentistry for the simulation of
structure there appear forces of compression, various clinical situations, in order to study the
tension, flexion and shear, all known as stress, which distribution of stress in the tooth structure and the
generate fatigue at the enamel-dentine junction restorative materials, as a result of occlusal loading
at the point of force application, and in the case of [21].
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Original Articles
Figure 3. Tensile stress in enamel: a) intact
premolar; b) lateral tooth wear version 1; c)
lateral tooth wear version 2; d) lateral tooth
wear version 3.
Figure 4. Compressive stress in enamel: a)
intact premolar; b) lateral tooth wear version
1; c) lateral tooth wear version 2; d) lateral
tooth wear version 3.
2. MATERIAL AND METHODS normal horizontal load (F=180 N) and an excessive
horizontal load (F=532 N) [35] were applied on the
The 3D virtual model of the upper first premolar was virtual model with the intact premolar and on the
made using the CT images of a 14 year-old patient three models with oblique occlusal tooth wear,
[20]. The CT images were converted into 3D data there resulting eight scenarios for the horizontal
using the MIMICS program (Materialise NV, Leuven, occlusal loading. The study was approved by the
Belgium, 1992) and the 3D virtual model obtained Ethics Committee of the Faculty of Dental Medicine
was analysed using Finite Element Analysis (FEA) of University of Medicine and Pharmacy of Craiova
and the Abaqus/CAE program (ABAQUS Software, (no. 257/12.10.2019). An informed consent for using
S.A.R.L., Versailles, France, 1994). The 3D virtual model the CT was obtained from the patient.
consisted of all the tissues of the given premolar:
enamel, dentine, pulp and periodontal ligament. The 3. RESULTS
alveolar bone was reconstructed as a parallelepiped,
to which the tooth was not rigidly fixed, and between In the simulations with horizontal occlusal tooth
these two structures the periodontal ligament wear the Von Mises Stress, which is a combination
was simulated. The finite element model was a 3D of compressive, tensile and shear stress, the
model with tetrahedral elements resulting in 47548 compressive stress was dominant. In the simulations
elements and 68504 nodes. The average size of an with oblique occlusal tooth wear, the dominant
element was approximately 0.5 mm per surface. stress was both compressive (Min Principal Stress,
For the elastic modulus of Young and Poisson’s ratio the maximum values are negative) and tensile (Max
of the dental tissues, data from the literature were Principal Stress, the maximum values are negative).
used as shown in Table 1. The values obtained for the
tensions generated in enamel were compared to the a - Simulation of normal vertical load, F=180 N
ultimate values of tensile and compressive strength In the simulation of a normal vertical loading
for enamel used in other studies (Table 2) [8,13]. (F=180 N), in the model with the intact premolar, a
To better understand the distribution of stress compressive stress higher than 62 MPa, located in
generated by occlusal loading, the situation of the superficial layer of enamel, at the buccal cusp
enamel in various 3D virtual models was presented. was observed (Fig. 1a). In the models with horizontal
A virtual model of the upper first premolar with no occlusal tooth wear, the values of compressive stress
lesion was used to create five more models with were lower. The highest ones were found in the
different degrees of occlusal tooth wear. T palatal cervical area, but they did not exceed 62MPa
he occlusal tooth wear models used in this study (Fig. 1 b, c).
represent simulations of some clinical conditions
encountered in clinical practice as a consequence b - Simulation of excessive vertical load, F=532 N
of the occlusal load in static mandibular position In the simulation of an excessive vertical loading
(occlusal horizontal tooth wear) - 2 models, and (F=532 N), in all models, the compressive stress was
the occlusal load in lateral movements of the jaw higher than 62 MPa (Fig. 2 a, b, c), the maximum
(oblique occlusal tooth wear) -3 models. Using these value being found in the model with the intact
models, various scenarios emerged for the occlusal premolar (Fig. 2a). Regarding the distribution of the
loading applied as a flat surface of a block, in a compressive stress in enamel, in the model with the
vertical and horizontal direction. A normal vertical intact premolar, it was located at the buccal cusp
load (F=180 N) and an excessive vertical load (F=532 affecting the whole enamel layer and in the cervical
N) [24] were applied on the virtual model with area (Fig. 2a). In the models with horizontal occlusal
the intact premolar and on the two models with tooth wear, the maximum values for the compressive
horizontal occlusal tooth wear, there resulting six stress were located in the cervical area of the enamel
scenarios for the vertical occlusal loading. Also, a (Fig. 2 b, c).
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Original Articles
Figure 5. Tensile stress in enamel: a) intact
premolar; b) lateral tooth wear version 1; c)
lateral tooth wear version 2; d) lateral tooth
wear version 3.
Figure 6. Compressive stress in enamel: a)
intact premolar; b) lateral tooth wear version
1; c) lateral tooth wear version 2; d) lateral
tooth wear version 3.
c - Simulation of normal horizontal load, F=180 N occlusal tooth wear, the stress values were higher
In the simulation of normal horizontal loading than the admissible ones in the simulation of the
(F=180 N), in the model with the intact premolar, the normal and excessive horizontal loading. The stress
tensile stress (Max Principal Stress) exceeded 24 MPa was found mainly in cervical area, on the buccal
in the occlusal area and in the cervical lingual distal side of the tooth. The most harmful forces were the
area (Fig. 3a). In the models with oblique occlusal excessive vertical ones and the horizontal ones, no
tooth wear, the highest values for the tensile stress matter the magnitude.
were found at the palatal cusp (Fig. 3 b, c, d). 4. DISCUSSIONS
The values for the compressive stress generated
exceeded 62 MPa in the buccal cervical area and in the This study aimed to analyse the distribution and
occlusal area in the model with the intact premolar magnitude of stress generated by normal (F=180
(Fig. 4a). In the models with oblique occlusal tooth N) and excessive (F=532 N) occlusal loads, directed
wear, these small values for the compressive stress vertically and horizontally. The stress obtained repre-
were found at the palatal cusp and in the cervical sents the effect of the cumulative action of the
palate-distal area (Fig. 4 b, c, d). compressive and tensile stress in the dental tissues.
For this study, a 3D virtual model of an upper first
d - Simulation of excessive horizontal load, F=532 N premolar was used. Clinical experience and data from
In the simulation of an excessive horizontal loading the literature [8,16,18] showed that the upper first
(F=532 N) in the model with the intact premolar, the premolar had a great susceptibility towards abfra-
tensile stress (Max Principal Stress) exceeded 24 MPa ction lesions. Meanwhile, its form and the way the
in the occlusal area and in the cervical disto- linguo- occlusal forces are distributed in its structure make
mesial area (Fig. 5a). In the models with oblique it liable for fractures. For the conversion of the CT
occlusal tooth wear, these high values for the tensile images into 3D data the MIMICS program (Materialise
stress were found in the occlusal and palatal area NV, Leuven, Belgium, 1992) was used as it was used in
and less in the cervical area (Fig. 5 b, c, d). The values prior studies [20, 25-27]. For the stress analysis Finite
for the compressive stress (Min Principal Stress) Element Analysis (FEA) was applied as it was used in
generated exceeded 62 MPa in the occlusal area and prior studies [28-39]. A similar study was performed
in the cervical buccal area in the model with the intact by Lee et al. in 2002 [8], also using the Finite Element
premolar (Fig. 6a). In the models with oblique occlusal Analysis (FEA) in which a normal vertical force (F=170
tooth wear, these small values for the compressive N) was applied at the buccal cusp of an upper second
stress were found in the occlusal and palatal area, premolar. The highest value of the tensile stress was
towards the distal part of the tooth (Fig. 6 b, c, d). observed in the cervical area, namely 13.55 MPa [8].
14 scenarios for the occlusal loading of the virtual In our study, for the normal vertical force of 180 N,
models of the upper first premolar were obtained the values for the Von Mises Stress in the cervical
and the areas with the highest concentration of area were around 10-15 MPa. The results of our
stress were enhanced. In the model with the tooth study are similar to those found by Lee et al in their
intact, the stress values were higher than the admi- study in 2002 [8]. In both studies the value of the
ssible ones in the simulation of the excessive vertical tensile stress obtained in the cervical area was lower
loading, normal horizontal loading and exce- than the admissible one. In 2003, Rees and Jagger
ssive horizontal loading. Stress was found in the [5] performed a study using the Finite Element
buccal cusp area and in the cervical area, mainly Analysis (FEA) in order to find an explanation for
on the buccal side of the tooth. In the models with the high prevalence of non-carious cervical lesions
horizontal occlusal tooth wear, the stress values were in maxillary teeth. In that study they used three 2D
higher than the admissible ones in the simulation of models, representing an upper incisive, upper canine
the excessive vertical loading. The stress was found and upper premolar and through the Finite Element
in the cervical area. In the models with oblique Analysis (FEA) recorded the magnitude of stress in
Stoma Edu J. 2020;7(1): 28-34 pISSN 2360-2406; eISSN 2502-0285 31
Stănuşi A. et al.
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the cervical area, obtained after applying a force of between bruxism and non-carious cervical lesions
Original Articles 500 N. In the case of the upper premolar, force was [42]. In 2017, Duangthip et al. [47] performed a
applied at the buccal cusp and angulated 45 degrees review to search for correlations between excessive
in a buccal and apical direction. They recorded the occlusal loads and abfraction lesions and found such
tensile stress and compared the values obtained on an association in 81% of cases.
the buccal surfaces with the ones obtained on the
palatal surfaces. The values obtained were similar. 5. CONCLUSIONS
Our study had results in accordance with the results
obtained in the Rees and Jagger study. However, This study showed the distribution and magnitude
practice has proven that abfraction lesions were of stress obtained in the virtual 3D model of an upper
rarely found on the palatal surfaces of the teeth first premolar, after applying normal and excessive,
[5] and this phenomenon could be correlated with vertical and horizontal forces. The most harmful
stress corrosion, meaning that the association forces were the excessive vertical ones and the
between stress and acidic substances cause more horizontal ones, no matter the magnitude. Regarding
lesions than if these factors act alone. Acidic the tooth morphology, the least favourable models
substances are removed by saliva from the palatal for the distribution of stress were the ones with the
surfaces of teeth 6 times more rapidly than the ones intact premolar and with lateral tooth wear.
located on the buccal surfaces [5]. Furthermore, the In current practice, these situations are found in the
enamel affected by demineralization can regain its case of occlusal interferences, parafunctions and
mechanical resistance as a result of the action of bruxism.
different substances and even saliva [14]. Clinically,
these excessive forces or the ones with a horizontal CONFLICT OF INTEREST
direction can be identified in the case of occlusal
interferences [39-41], any other clinical situation The authors declare no conflict of interest.
that modifies occlusion, such as parafunctions,
bruxism [42,43] and treatment procedures [44,45]. AUTHOR CONTRIBUTIONS
Other studies have tried to find evidence for the AS, VM: Concept, protocol, manuscript writing;
causal relationship between occlusion and non- MS, SMP, MMIC, LD, AMO: Manuscript writing, critical review of
carious cervical lesions [41,46]. In these studies, manuscript; SC: concept, data entry, data analysis, critical review
the presence of non-carious cervical lesions was of manuscript.
associated with the presence of occlusal wear ACKNOWLEDGMENTS
facets. If excessive occlusal forces can cause non-
carious cervical lesions, it would be expected that Acknowledge persons who have made substan-
parafunctional habits such as bruxism and clenching tive contributions to the study. Specify grant or other
financial support, citing the name of the supporting organization
would increase the progression of non-carious and grant number.
cervical lesions. Two studies reported a correlation
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Stoma Edu J. 2020;7(1): 28-34 pISSN 2360-2406; eISSN 2502-0285 33
Stănuşi A. et al.
www.stomaeduj.com
Original Articles Andreea STĂNUŞI
DMD, PhD Student, Prosthodontic Medical Resident
Department of Proshodontics
Faculty of Dentistry
University of Medicine and Pharmacy of Craiova
Craiova, Romania
CV
Andreea Stănuşi (DOB: September 22, 1992) graduated from the Faculty of Dentistry, University of Medicine and Pharmacy
of Craiova in 2017 and became Medical Resident in Prosthodontics. As a student, she held various positions: President of the
Craiova Stomatology Students Society (2013-2014), Member of the Faculty Council (2016-2017), Member of the University
Senate (2016-2017). As a postgraduate her activity is still connected with the academic life. In 2017 she became a PhD student
under the guidance of University Professor Veronica Mercuț, her research interest being the etiology and treatment of tooth
wear. In 2019 she published the results of an experiment in which the stress involved in the etiology of tooth wear was
examined (“Effects of occlusal loads in the genesis of non-carious cervical lesions – a Finite Element Study”).
Questions
1. Non-carious cervical lesions can produce in a first stage:
qa. endodontic pathology;
qb. gingival retractions associated with teeth sensitivity;
qc. fracture of the teeth involved;
qd. periodontal pockets.
2. The areas most affected by abfraction lesions are located on:
qa. enhanced thickness of enamel in that area;
qb. reduced thickness of dentine in that area;
qc. the high frequency of Hunter Schreger Bands;
qd. the low frequency of Hunter Schreger Bands.
3. Abfraction lesions appear at the enamel-cement junction in the enamel structure, as a
result of:
qa. less than 20% in the three groups with no statistical difference among groups;
qb. over 10% in the three groups with no statistical difference among groups;
qc. over 80% in the three groups with no statistical difference among groups;
qd. less than 10% in the three groups with no statistical difference among groups.
4. The excessive forces or the ones with a horizontal direction involved in the etiology of
NCCL, can be identified in the following clinical situations apart from:
qa.occlusal interferences;
qb. parafunctions;
qc. bruxism;
qd. procedures that do not modify occlusion.
34 Stoma Edu J. 2020;7(1):28-34 pISSN 2360-2406; eISSN 2502-0285