nastavno – naučnom veću stomatološkog fakulteta u beogradu

NASTAVNO – NAUČNOM VEĆU
STOMATOLOŠKOG FAKULTETA U BEOGRADU
Nastavno-naučno veće Stomatološkog fakulteta, Univerziteta u Beogradu je na VII redovnoj
sednici u školskoj 2014/2015. godini, održanoj 15.09.2015. godine, donelo odluku zavedenu pod
brojem 3/67 o imenovanju komisije za ocenu i odbranu završene doktorske disertacije dr Nataše
Milošević pod nazivom
„POLIMORFIZMI U MTHFR, GSTM1, GSTT1, MMP9 GENIMA KAO FAKTORI
PREDISPOZICIJE ZA POJAVU TEMPOROMANDIBULARNIH DISFUNKCIJA“.
Na osnovu pregleda priloženog materijala, komisija u sastavu Prof. dr Ljiljana Tihaček-Šojić,
Doc. dr Aleksandra Špadijer-Gostović i Prof. dr Ivana Novaković sa Medicinskog fakulteta,
Univerziteta u Beogradu, podnosi Nastavno-naučnom veću sledeći
IZVEŠTAJ
Doktor stomatologije Nataša Milošević rođena je 30.12.1983. godine u Tuzli. Osnovnu
školu i XIII Gimnaziju u Beogradu, završila je sa odličnim uspehom. Stomatološki fakultet
Univerziteta u Beogradu upisala je 2002. godine, a diplomirala 2008. godine sa prosečnom
ocenom 8,88. U toku akademske godine 2007/2008 učestvovala je u izvođenju praktične nastave
kao demonstrator na predmetu Bolesti zuba – pretklinika i klinika, kao i u studentskim naučnoistraživačkim radovima. Pripravnički staž obavila je na Vojnomedicinskoj akademiji (VMA),
nakon čega je 2010. godine položila stručni ispit za doktora stomatologije.
Doktorske studije iz naučne oblasti Stomatološka protetika, upisala je školske 2009/2010. godine
i položila sve ispite predviđene nastavnim planom i programom doktorskih studija, sa prosečnom
ocenom 9,75. Od 2011. godine, angažovana je na projektu Ministarstva za nauku i tehnološki
razvoj Republike Srbije, pod nazivom “ Genetička kontrola i molekularni mehanizmi u
malignim, inflamatornim i razvojnim patologijama orofacijalne regije, rukovodioca prof.dr
Jelene Milašin (broj projekta 175075). Na klinici za stomatološku protetiku Stomatološkog
fakulteta u Beogradu od 2013. godine obavlja i zdravstvene specijalističke studije.
1 Nastavno-naučno veće Stomatološkog fakulteta, Univerziteta u Beogradu je na IV redovnoj
sednici u školskoj 2012/2013. godini održanoj 05.03.2013. godine zavedenu pod brojem 3/26
usvojilo pozitivan izveštaj stručne komisije za ocenu predloga teme i imenovalo Prof.dr Vojkana
Lazića i Prof.dr Jelenu Milašin za mentore ove doktorske disertacije. Doktorska disertacija dr
Nataše Milošević je napisana na 163 strane, raspoređene u 7 poglavlja: Uvod, Ciljevi, Materijal i
metode, Rezultati, Diskusija, Zaključak i Literatura. Dokumentovana je sa 92 tabele i 27 slika.
U Uvodu kandidat opisuje kliničke karakteristike temporomandibularnih disfunkcija (TMD)
i njihovu klasifikaciju sa osvrtom na dosadašnju etiologiju. Istaknuta je važnost detaljnog
sagledavanja uzročnih faktora TMD i pregled literature ukazao je na značaj naslednih faktora
koji su i predmet samog istraživanja. Opisano je kako prisustvo određenih polimorfizama može
da utiče na smanjenu ili povećanu predispoziciju pojedinca za nastanak različitih oboljenja kao i
na specifičan odgovor na terapiju. Nosioci određenog genotipa imaju veći/manji rizik za
nastanak oboljenja, pa čak i veću mogućnost ispoljavanja simptoma karakterističnih za dato
oboljenje. Ovim je objašnjeno time da razlike u genotipovima mogu značajno da utiču na
fenotip, odnosno težinu kliničke slike. U ovom poglavlju je objašnjena uloga polimorfizama u
genima koji menjaju katalitičku aktivnost, stabilnost i nivo ekspresije odgovarajućih enzima, a
samim tim mogu potencijalno da doprinesu nastanku TMD.
U poglavlju Ciljevi istraživanja kandidat jasno navodi ciljeve studije: -
Ispitati povezanost polimorfizama u genima uključenim u kontrolu sinteze DNK i
metabolizam folata (MTHFR), detoksikacije (GSTM1, GSTT1) i remodelacije
ekstraćelijskog matriksa (MMP-9) sa nastankom temporomandibularnih disfunkcija,
-
Ustanoviti eventualnu asocijaciju između genotipova različitih polimorfizama i
kliničkih karakteristika pacijenata sa TMD.
U okviru ciljeva istraživanja doktorske disertacije definisani su sledeći zadaci:
1. Utvrđivanje osnovnih epidemioloških i kliničkih parametara u grupi sa TMD.
2. Utvrđivanje učestalosti alela i genotipova za polimorfizam pojedinačnog nukleotida
MTHFR 677C>T, delecioni polimorfizam u GSTM1 i GSTT1 genima i promotorski
polimorfizam pojedinačnog nukleotida MMP9 -1562 C>T u grupi pacijenata sa
temporomandibularnim disfuncijama i u kontrolnoj grupi.
3. Ispitivanje asocijacije polimorfizama navedenih gena i rizika za nastanak
temporomandibularnih disfunkcija, na osnovu razlike u distribuciji učestalosti alela i
genotipova između TMD grupe i kontrolne grupe.
2 4. Utvrđivanje povezanosti genotipova sa kliničkim fenotipom u okviru TMD grupe.
U poglavlju Materijal i metode kandidat navodi sve segmente studijskog dizajna i opisuje
metode koje su primenjene u okviru kliničkog i molekularno-genetičkog ispitivanja. Detaljno
opisuje kriterijume za odabir pacijenata, standardni dijagnostički protokol za TMD kao i
pomoćne dijagnostičke procedure. Istraživanje je sprovedeno kao prospektivna klinička studija u
koju su bile uključene dve grupe ispitanika. Studijsku grupu činilo je 100 pacijenata, koji su došli
na Kliniku za stomatološku protetiku i Kliniku za maksilofacijalnu hirurgiju Stomatološkog
fakulteta Univerziteta u Beogradu, sa nekim od simptoma temporomandibularnih disfunkcija, i
kod kojih je utvrđen ovaj poremećaj. Kontrolnu grupu činila su 182 zdrava ispitanika kod kojih
nije prisutan nijedan znak niti simptom TMD. Za potrebe molekularno-genetičkih ispitivanja
uzimani su uzorci brisa sa obrazne sluzokože, obostrano, u eksperimentalnoj i u kontrolnoj
grupi iz kojih je izolovana DNK. Nakon izolacije DNK molekula primenjivana je PCR/RFLP
analiza (lančana reakcija polimeraze i restrikciona analiza) u cilju genotipizacije obe grupe
ispitanika za polimorfizme u MTHFR, GSTM1, GSTT1 i MMP9 genima. Statističkom obradom
podataka analizirane su razlike u učestalosti alela i genotipova između TMD i kontrolne grupe
Hi kvadrat testom. Logističkom regresionom analizom utvrđivana je povezanost genskih
varijacija sa rizikom za nastanak TMD. Korišćen je statistički program SPSS 18.0 (SPSS Inc,
Chicago, IL, USA).
U poglavlju Rezultati prvo su analizirani klinički rezultati i tabelarno su prikazane
distribucija ispitanika prema polu i starosti i distribucija grupa TMD i karakterističnih simptoma.
Nakon toga je tabelarno prikazano poređenje učestalosti TMD disfunkcija (mišićna, zglobna,
kombinovana) i sva četiri karakteristična simptoma (bol, devijacija/defleksija, ograničene
kretnje, zvuk) među polovima i osobama različite starosne dobi. Takođe je u okviru ovih
rezultata, korišćenjem univarijantne i multivarijantne analize, prikazana i procena veće/manje
verovatnoće za pojavu pomenutih disfunkcija i simptoma u odnosu na pol i godine. Tabelarno su
dati i podaci molekularno-genetičkih ispitivanja, odnosno pokazana je zastupljenost različitih
genotipova i alela među grupama kao i njihov uticaj na predispoziciju za nastanak TMD. Pored
analize polimorfizama kao faktora rizika za pojavu TMD, poređenjem učestalosti pojave
različitih simptoma i disfunkcija kod različitih genotipova za sva četiri navedena gena dobijeni
su i podaci o povezanosti genotip- fenotip.
Kandidat u Diskusiji koja je podeljena na klinički i na molekularno-genetički aspekt, izlaže
saznanja i činjenice koje doprinose ukupnom razumevanju postavljenog problema. Tumači
dobijene rezultate i poredi ih sa rezultatima drugih, relevantnih studija i daje smernice za dalja
istraživanja. Kandidat je u naučnoj literaturi pronašao mali broj radova koji se bave ispitivanjem
ovih genskih polimorfizama i njihove uloge kao predisponirajućeg faktora u nastanku TMD, pa
3 tako rezultati dobijeni u ovoj nedovoljno obrađenoj oblasti disertaciji Nataše Milošević daju
posebnu težinu.
Zaključci su jasno formulisani i pružaju odgovore na postavljene ciljeve. Kandidat navodi da
su TMD značajno zastupljenije kod osoba ženskog pola u odnosu na osobe muškog pola kao i
da su zglobne disfunkcije znatno učestalije u odnosu na mišićne i kombinovane. Pokazano je
da starosna dob utiče na pojavu zvučnih efekata u zglobu kao i na mišićne i zglobne disfunkcije
kod pacijenata sa temporomandibularnim disfunkcijama. Osobe mlađe od 30 godine imaju tri
puta veću verovatnoću za pojavu zglobne disfunkcije i zvučnih efekata u zglobu od osoba starijih
od 30 godina. Osobe mlađe od 30 godina imaju dva i po puta manju verovatnoću za pojavu
mišićne disfunkcije. Iz ovoga proizilazi da je pojava zglobnih disfunkcija i zvučnih efekata
karakteristika mladih, a da stariji imaju značajno veću predispoziciju za pojavu mišićnih
disfunkcija. U okviru genetičkih parametara polimorfizam u genu za MTHFR se nije pokazao
kao faktor predispozicije za nastanak TMD. Delecioni polimorfizam u genu za GSTM1 i GSTT1
igra ulogu modifikatora predispozicije za nastanak TMD, tačnije ima protektivni efekat. Nosioci
nultog GSTT1 genotipa i kombinacije GSTM1 M1 i GSTT1 T0 genotipova imaju šest puta manji
rizik za pojavu TMD. Nosioci GSTM1 M1 genotipa su pokazali češću pojavu
devijacije/defleksije u poređenju sa nosiocima M0 genotipa. Polimorfizam u genu za MMP9 se
pokazao kao modulator rizika za nastanak TMD. Nosioci CT genotipa ispoljavaju dva puta veći
rizik za nastanak TMD u odnosu na osobe divljeg genotipa (CC homozigoti). Kandidat takođe
zaključuje da su glavni faktori rizika za nastanak TMD: ženski pol (tri puta veći rizik za pojavu
TMD) i GSTT1 T1 genotip (pet puta veći rizik za pojavu ovog poremećaja.
Literatura je sveobuhvatna i savremena. Sadrži 165 bibliografskih jedinica iz domaće i
strane relevantne literature.
Zaključak
Doktorska disertacija dr Nataše Milošević predstavlja dobro dokumentovanu eksperimentalnu
studiju koja daje doprinos u oblasti proučavanja polimorfizama gena uključenih u kontrolu
sinteze DNK i metabolizam folata, detoksikacije, i remodelacije ekstraćelijskog matriksa sa
nastankom temporomandibularnih disfunkcija. Disertacija predstavlja doprinos u rasvetljavanju
povezanosti polimorfizama pomenutih gena sa rizikom od nastanka temporomandibularnih
disfunkcija, kao i vezu polimorfizama sa izraženošću kliničkih simptoma. Poznavanje genetičkih
polimorfizama koji mogu da se smatraju faktorima predispozicije za oboljevanje od TMD može
značajno da poboljša prevenciju TMD i drugih bolnih stanja u orofacijalnoj regiji.
Kompleksnost teme ove disertacije, njena jedinstvenost, kao i dobijeni rezultati ukazuju na
naučnu zrelost kandidata. Postavljeni ciljevi istraživanja su sistematično proučeni primenom
4 adekvatnih naučnih metoda. Rezultati su precizno obrađeni, i logično interpretirani u skladu sa
postavljenim ciljevima. Diskusija je detaljna i sveobuhvatna.
Nakon uvida u dostavljeni tekst, komisija je jednoglasno ocenila da doktorska disertacija
pod nazivom „POLIMORFIZMI U MTHFR, GSTM1, GSTT1, MMP9 GENIMA KAO
FAKTORI
PREDISPOZICIJE
ZA
POJAVU
TEMPOROMANDIBULARNIH
DISFUNKCIJA“ dr Nataše Milošević predstavlja originalno, samostalno i dobro
dokumentovano istraživanje. Ispitivanje povezanosti genetičkih polimorfizama i TMD do sada
nije obrađeno u domaćoj literaturi.
Doktorska disertacija ispunjava sve kriterijume propisane Zakonom o Univerzitetu i
statutima Univerziteta i Stomatološkog fakulteta u Beogradu.
Na osnovu iznetog, predlažemo Nastavno-naučnom veću Stomatološkog fakulteta
Univerziteta u Beogradu da prihvati izveštaj i odredi datum javne odbrane ove doktorske
disertacije.
U Beogradu, 03.11.2015
____________________________
Prof. dr Ljiljana Tihaček Šojić
Stomatološki fakultet, Univerzitet u Beogradu
____________________________
Doc.dr Aleksandra Špadijer Gostović
Stomatološki fakultet, Univerziteta u Beogradu
____________________________
Prof. dr Ivana Novaković
Medicinski fakultet, Univerziteta u Beogradu
5 UDK 678.7:66.017:539.412
Original scientific article/Izvirni znanstveni ~lanek
ISSN 1580-2949
MTAEC9, 48(6)811(2014)
V. LAZI] et al.: MECHANICAL PROPERTIES OF THE MATERIALS FOR BRUXOGUARDS
MECHANICAL PROPERTIES OF THE MATERIALS FOR
BRUXOGUARDS
MEHANSKE LASTNOSTI MATERIALOV ZA NO^NE ZOBNE
[^ITNIKE
Vojkan Lazi}1, Aleksandra [padijer Gostovi}1, Neboj{a Rom~evi}2,
Igor \or|evi}1, Ana Todorovi}1, Nata{a Milo{evi}1, Rebeka Rudolf3
1Faculty of Dental Medicine, University of Belgrade, Dr. Suboti}a 8, 11000 Belgrade, Serbia
2Institute for Physics, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia
3University
of Maribor, Faculty of Mechanical Engineering, Smetanova 17, 2000 Maribor, Slovenia
anatod2004@yahoo.com
Prejem rokopisa – received: 2013-07-09; sprejem za objavo – accepted for publication: 2014-01-31
The aim of this study is to investigate the mechanical properties of polymethyl methacrylate (PMMA) and thermoplastic polycarbonate (TPC) materials in order to produce night bruxoguards. For this purpose we used a static tensile test. In the next step
the microstructures of PMMA and TPC were observed. Within this framework special attention was paid to the examination of
the tensile-test tube fracture surfaces for both materials. This approach revealed that PMMA is a brittle and TPC is a plastic
material. Certain mechanical properties and a review of the crucial areas confirmed that the TPC material is extremely
favourable for making occlusal splints.
Keywords: bruxoguards, polymethyl methacrylate (PMMA), thermoplastic polycarbonate (TPC), mechanical properties, characterization
Namen te {tudije je preiskava mehanskih lastnosti polimetil metakrilata (PMMA) in termoplasti~nega polikarbonata (TPC) za
izdelavo no~nih zobnih {~itnikov. Za ta namen je bil uporabljen stati~ni natezni preizkus. V naslednjem koraku je bila preiskana
mikrostruktura PMMA in TPC. Posebna pozornost je bila namenjena preiskavi povr{ine preloma na ceveh za natezni preizkus,
izdelanih iz obeh materialov. Tako je bilo ugotovljeno, da je PMMA krhek material, TPC pa plasti~en. Nekatere mehanske
lastnosti in pregled klju~nih podro~ij potrjujejo, da je za izdelavo okluzalne opornice izredno primeren material TPC.
Klju~ne besede: no~ni zobni {~itnik, polimetil metakrilat (PMMA), termoplasti~ni polikarbonat (TPC), mehanske lastnosti,
karakterizacija
1 INTRODUCTION
Sleep bruxism (SB) is associated with a rhythmic
masticatory muscle activity, characterized by repetitive
jaw-muscle contractions, occurring during sleep in the
NREM stage.1 Sleep bruxism is reported by 4.4–8.0 % of
the adult population, 14 % of children and is regulated
mainly centrally, not peripherally. In terms of etiology,
stress and psychosocial variables have been associated
with SB. The strongest agreement among several authors
is that SB patients are anxious personalities (but not
having an anxiety disorder) and are more task-oriented
(e.g., focused on a successful performance) in comparison with others.2 During nocturnal bruxism, the bite
forces are much greater than those exerted during mastication. The mean maximum bite force among Europeans
and Americans is in the range of 600–750 N, while
masticatory forces are much lower (about 60–100 N).
During clenching, the forces of about 1000–1500 N, or
higher, have been reported. In some individuals, the
forces generated and the duration of repeated bruxing
episodes can produce considerable loads which are potentially harmful for all the components of the orofacial
system.3,4 The consequences of SB may include tooth
destruction, implant fracture, jaw pain, headaches or a
limitation of mandibular movement, as well as toothMateriali in tehnologije / Materials and technology 48 (2014) 6, 811–816
grinding sounds that disrupt the sleep of bed partners.5
Sleep bruxism alone, as long as it is performed subconsciously by the patient and no apparent damage to the
oral structure is caused, does not require any therapy. In
the case of damage, a multitude of therapeutic
approaches is possible, such as drugs (benzodiazepine,
tricyclic antidepressants), occlusal adjustment, bruxoguards (occlusal splints) or even occlusal rehabilitation
and anti-stress therapy (biofeedback and self-awareness).6
Occlusal splints have been advocated to reduce the
harmful consequences of bruxism7, especially when the
goal is to protect large restorations and implants. Splints
may reduce the total amount of nocturnal activity of
bruxers by decreasing the number of bruxing events per
night and may also reduce daytime postural muscle
activity.8 However; it is not actually known whether
splints can absorb some of the stresses transmitted to the
teeth. There is no universal agreement as to the effect of
hard and soft splints on bruxism.9 It was reported that
hard stabilization splints decrease muscle activity significantly10, while the vacuum-formed soft splints increase
muscle activity.11,12
The aim of this study was to investigate the mechanical properties of two different, most commonly used
811
V. LAZI] et al.: MECHANICAL PROPERTIES OF THE MATERIALS FOR BRUXOGUARDS
materials, hard polymethyl methacrylate (PMMA) and
thermoplastic polycarbonate (TPC), with respect to using
them for the production of customized bruxoguards. In
this way we wanted to provide an explanation of the
plastic-elastic behaviour of SB materials which could be
a guideline for dentists when choosing the correct bruxoguard material. PMMA is possibly more widely known
as acrylic is a superb optical material with high levels of
both visible and UV light transmission. It is often used
for a multitude of point-of-sale display applications and,
due to its compatibility with human tissue, it has found
use in a number of medical applications. The characteristics of PMMA include low density, excellent light
transmission, good electrical properties and compatibility with human tissue (medical grades). TPC is a type
of thermoplastics that features polymers linked together
by carbonate groups, forming a long molecular chain.
TPC is similar to PMMA, but TPC is stronger and has
exceptional physical properties and it is usable over a
larger temperature range. The advantages of TPC include
impact resistance, dielectric strength, excellent formability, superior optical clarity, outstanding dimensional
stability, UV stabilization, etc.
2 EXPERIMENTAL WORK
PMMA and TPC testing probes were produced from
plates of 2 mm – the optimum guard thickness. The
Figure 1: a) PMMA plate (Galenika, Serbia) and b) TPC plate (Erkodent, Germany)
Slika 1: a) Plo{~a iz PMMA (Galenika, Srbija) in b) plo{~a iz TPC
(Erkodent, Nem~ija)
812
PMMA material was made in Galenika (Serbia) by a
dental technician from a heat-curing acrylic resin (Figure 1a). PMMA is a strong and lightweight material. It
has a density of 1170–1200 kg/m3, which is less than
half that of glass. It also has good impact strength, higher
than both glass and polystyrene; however, PMMA’s
impact strength is still significantly lower than those of
polycarbonate and some engineered polymers. PMMA
ignites at 460 °C and burns, forming carbon dioxide,
water, carbon monoxide and low-molecular-mass compounds, including formaldehyde. PMMA transmits up to
92 % of visible light (a thickness 3 mm) and gives a
reflection of about 4 % from each of its surfaces on
account of its refractive index (1.4914 at 587.6 nm). It
filters ultraviolet (UV) light at the wavelengths below
about 300 nm (similar to ordinary window glass).13,14
The TPC material was produced by Erkodent (Germany) (Figure 1b) and it is composed of PETG: polyethylene terephthalate-glycol-modified ethylene-1.4cyclohexylene dimethylene terephthalate copolymer.
PETG is a thermoplastic polymer that turns into liquid
when heated and its hardness is very near the glassy state
when cooled. TPC is highly impact resistant, exhibiting
excellent clarity and having the density of 1380 kg/m3 (at
20 °C).15
All the prepared samples were subjected to several
tests to investigate their mechanical properties, using the
following methodology: the static tensile test on a universal testing machine (Zwick/Roell ZO 10). For each
material the measurements were done in a series that
consisted of six samples in the form of a tensile-test specimen. The standard dimensions of a tensile-test specimen are shown in Figure 2. Experimental settings and
conditions, shape and dimensions of the tension specimens were in accordance with standard SIST EN
1562:2000 (chapter 6.2). The task was to apply a static
tensile load in order to measure the magnitude of the
strain at the moment of fracture of an individual tensile
specimen. The measurements were conducted at a constant strain-rate increase, which amounted to v = 1.5
mm/min. This speed was selected on the basis of the
Figure 2: Schematic presentation of a standard tensile-test specimen
with dimensions
Slika 2: Shematski prikaz in dimenzije standardnega vzorca za natezni
preizkus
Materiali in tehnologije / Materials and technology 48 (2014) 6, 811–816
V. LAZI] et al.: MECHANICAL PROPERTIES OF THE MATERIALS FOR BRUXOGUARDS
theory of viscoelastic behaviour, which enabled the chain
slip and plastic deformation of both materials. The
following parameters were determined by analysing the
measured values: yield point (stress) Rp0.2, ultimate tensile strength Rm and elongation at break of tensile
specimen A.
The aim of this part of the investigation was to obtain
the real data for the mechanical properties of the PMMA
and TPC materials which are generally not known. In
this manner we want to predict a possible use of these
materials in the special field of dentistry (SB).
The examination of the microstructures of the
PMMA and TPC plates included observations of the
cross-sections of both materials and a qualitative
micro-chemical analysis carried out at the characteristic
points or stages. The samples were cleaned with an ultrasonic vibration instrument for cleaning where the medium was alcohol. The cleaned samples were mounted
into a special holder, which was placed in the chamber of
an electron microscope, Sirion NC 400.
We also observed the surfaces of the broken tensiletest tubes with a scanning electron microscope, Quanta
200 3D, which features a dual-beam system – electronic
and ionic (a dual beam). The samples were observed at
low vacuum, which allowed an observation of the conductive and non-conductive samples without a prior
preparation. Through a review of the fractured surfaces,
we wanted to determine the type of fracture and, in this
way, the behaviour of the materials during the operation
of occlusal forces during nocturnal bruxism that had
surpassed the maximum desired biting force by almost
30–60 %.
An XRD analysis was performed on the PMMA and
TPC materials. Diffractograms were recorded on a
PANalyticalX’pert PRO MPD diffractometer using the
reflection technique with the Cu Ka1 radiation in the range
of 30° to 120° 2q. The diffractograms were compared
with ProgrammeX’pertHighScore (PANalytical) (ICDD,
2006, ICSD, 2007).
3 RESULTS AND DISCUSSION
The results of the mechanical-property measurement
were shown in a graphical form with the typical s – e
curve for the PMMA material (Figure 3a). The curve
describes the flow of the changes in the voltage dependence of the measured deformation of the sample.
On the a-axis in all the diagrams the tension was
marked in MPa, and on the ordinate axis the strain of the
sample was in μm. From the s – e curves and from the
results given in Table 1, it can be concluded that these
PMMA polymers have a pretty high modulus of elasticity; they were broken in a non-linear elastic range, but
the stretch in the elastic field reached only a few percent
(1.05 %). It can be seen that the tensile strength is 72.65
MPa and the measured elongation is 1.85 mm, which
means that the elongation at the break is A = 2.48 %. The
average value of the measured elastic module for PMMA
is 2.04 GPa.
Table 1: Average values of (measured) mechanical properties for
PMMA and TPC materials
Tabela 1: Srednje vrednosti (izmerjenih) mehanskih lastnosti materialov PMMA in TPC
EngineerMaterial ing stress
s/MPa
PMMA
5.21
TPC
4.15
Young’s
modulus
E/GPa
2.04
1.94
Ultimate tensile Fracture
strength
strain
Rm/MPa
e/mm
72.65
1.85
67.97
74.86
Figure 3b shows a typical s – e curve for the TPC
material, from which the material creep was clearly observed. The average values of the results from the tensile
tests for the TPC material are given in the second line of
Table 1. Here the value for the tensile strength is 67.97
Figure 3: Typical s – e curves: a) for PMMA and b) for TPC
Slika 3: Zna~ilna krivulja s – e: a) PMMA in b) TPC
Materiali in tehnologije / Materials and technology 48 (2014) 6, 811–816
Figure 4: Elongation of the TPC tensile specimens before tensile testing and after the break
Slika 4: Natezni preizku{anec iz TPC pred preizkusom in po poru{itvi
813
V. LAZI] et al.: MECHANICAL PROPERTIES OF THE MATERIALS FOR BRUXOGUARDS
MPa which is lower than that of PMMA. However, the
measured elongation is 74.86 mm, which makes the
value of elongation at break to be A = 99.81 %. The
average value of the elastic module is 1.94 GPa. From
the typical curves, and based on the average of the
measured results of the mechanical tests for the TPC
material, it can be concluded that this material has a
lower modulus of elasticity and tensile strength than the
PMMA material, but its stretching ability is almost a
hundred times greater. Greater stretching is clearly seen
in Figure 4, where parallel tensile TPC specimens are
shown – the tensile specimen before and after the break.
At the beginning of TPC the material deformation was
elastic, and then there was distinct tension strength,
ending the tension. The consolidating factor in the
plastic region was small and cracking occurred when the
stress exceeded the cohesion strength. It should be noted
that the TPC material in the sphere of sliding was, on
average, loaded for 45 min before the fracture of the
tensile specimen took place.
Figure 5a shows a SEM microstructure of PMMA
without crystal grains and this means that the material
belongs to the amorphous group. A detailed examination
showed that within the volume there are micropores with
Figure 5: Electron micrographs of: a) PMMA and b) TPC
Slika 5: SEM-posnetka: a) PMMA in b) TPC
814
a size of less than 1 μm. In a detailed review of the
PMMA structure the presence of the particles with the
size of a few μm was identified. Most of the particles
were round and had a non-homogeneous distribution
within the volume. The results of the qualitative analysis
of these particles were confirmed with the increased
amounts of chlorine (Cl), potassium (K) and sulphur (S)
– this data is not shown. The review of the PMMA
material’s surface showed an inhomogeneity of the
structure, which reflected a high degree of roughness.
A SEM microstructure of the TPC material can be
seen in Figure 5b, showing typical characteristics of the
amorphous state. The observations and studies demonstrated that this material is quite homogeneous, with
some lamellar structure and with almost no defects. No
other inclusions or faults were observed.
By inspecting the fracture surface of the PMMA
material it was found that it had the typical characteristics of a brittle fracture. The lines of sliding, the
so-called "rivers", were also present in the figure with a
lower magnification (Figure 6a) and, normally, in the
microstructure of the PMMA samples with a higher
magnification (Figure 6b). The resulting "rivers" were
about 10 μm in length and extended at right angles in the
direction of the load. Brittle materials did not deform
plastically under the mechanical loading; therefore, the
fracture strain was extremely reduced at the eventual
fracture. As some particles were found in this material,
they are probably one of reasons responsible for the
brittleness. Namely, the particles acted as obstacles to
plastic deformation leading to a higher value of the
elastic modulus and tensile strength, which means that
some kind of strengthening effect is achieved in PMMA.
As a consequence, at the top of the fracture a concentration of stress was detected because the progression of
the fracture was not necessary to achieve the cohesive
strength along the entire cross-section of the sample but
on both its surfaces. When, in the brittle material, the
fracture began to move forward, it was going to break
and reduce the external loading. This phenomenon
makes the use of the PMMA material for splints extre-
Figure 6: Electron micrographs of the PMMA fracture surface: a)
general impression and b) detail
Slika 6: SEM-posnetka povr{ine preloma PMMA: a) videz in b) detajl
Materiali in tehnologije / Materials and technology 48 (2014) 6, 811–816
V. LAZI] et al.: MECHANICAL PROPERTIES OF THE MATERIALS FOR BRUXOGUARDS
Figure 7: Electron micrographs of the TPC fracture surface: a) general impression and b) detail
Slika 7: SEM-posnetka povr{ine preloma TPC: a) videz in b) detajl
mely unfavourable. Based on the investigations of the
structure of the fractured surface of the PMMA material,
we can hypothesize that only on the lower part was there
a certain degree of plastic deformation.
For the TPC material, the inspection of the fracture
surfaces revealed that it behaves elastically (Figure
7a).16 In the structure of the entire intersection sliding
lines are present, that is "rivers", which originate from
the area where the shooting material continues to spread
like feathers on its turning over the entire surface (Figure 7b). The sliding lines are at an angle of 45° relative
to the direction of the loading during the tensile test, in
the direction of the action of the shear stress. A proper
selection of technological parameters (the power of prestress, the strain rate) caused a sliding of the chains and a
large plastic deformation. Due to a low strain rate the
polymer chains had the time to slide next to each other.
In the s – e diagram it was observed that in a relatively
short time of the tensile loading in the TPC material
there was a drop in the stress from 75 MPa to 55 MPa,
which was a constant all the time during the sliding to
the fracture of the material. The cause can be found in
the stress relaxation. At the beginning the defined stress
in the polymer was necessary in order to achieve a deformation. As the polymer chains flowed viscously into
each other, the stress in the polymer material was not
preserved. As a result, a decrease in the stress was
observed in the s – e diagram. This phenomenon can be
attributed to the viscous behaviour of the polymer. During the sliding of the TPC polymers there was no consolidation of the materials, except just before the fracture
completion, when the stress was increased again to 50
MPa. Certain mechanical characteristics and the inspection of the fracture surfaces confirmed that the usage of
the TPC material for splints was favourable.
The results of the XRD analysis indicated that the
structures of both materials were amorphous, so that the
backgrounds obtained at all the spectra were wide (Figures 8a and 8b). The diffractograms of the PMMA and
TPC samples were recorded from the lower and upper
surfaces because the aim of the analysis was to examine
the influence of the polycarbonate-making technology on
Materiali in tehnologije / Materials and technology 48 (2014) 6, 811–816
Figure 8: Diffractograms of the samples taken (recorded) from both
sides of the plate (rough and smooth) for: a) PMMA and b) TPC
Slika 8: Difraktogram vzorca z obeh povr{in (hrapave in gladke) pri:
a) PMMA in b) TPC
the structure. The obtained diffractograms did not show
any deviation in the phase structure in comparison to the
surface. It can be concluded that because of different
spectra the samples differ one from the other. Indirectly,
this was confirmed by, and can be also seen from, the
measured mechanical characteristics.
During an application in the mouth cavity, the
materials for splints would be exposed to the stresses that
should not cause a fracture. However, a certain degree of
previously allowed plastic deformation would be possible. Therefore, it would be necessary to know the mechanisms that influence the yield stress as well as the
plasticity of the materials. The investigations of the
PMMA and TPC materials have shown that they possess
the characteristics of elasto-plastic behaviour so, after a
certain degree, the elastic deformations partly change
into the plastic deformation. A much higher degree of
chain sliding and, consequently, a higher plastic deformation, were found in the TPC material. Moreover, the
medium value of the TPC elongation during the tensile
test was approximately 100 %. Certain mechanical characteristics and the inspection of the fracture surfaces
confirmed that the usage of the TPC material for occlusal
splints was very favourable.
The use of splints in the treatment of bruxism ensures
a reduction in the wearing out of the tooth and leads to a
better force distribution in the dental arch supporting the
splint.17 Moreover, a splint increases cognitive awareness
and leads to a behavioural modification via a reduction
in the muscular activity.18 With soft splints, there are
815
V. LAZI] et al.: MECHANICAL PROPERTIES OF THE MATERIALS FOR BRUXOGUARDS
contact areas with the opposing tooth because of the
resilient properties of the material. Therefore, there is a
better distribution of the occlusal force, better energy
absorption19 and almost no change in the compliance,
which leads to a decreased bending effect on the teeth. In
the oral cavity the materials for night guards are exposed
to voltages that may not directly cause their tearing, but
can cause a degree of deformation to a predetermined
allowable value. Certain mechanical properties and an
examination of broken surfaces confirmed that the use of
the TPC material for occlusal splints is extremely
favourable and has clinical advantages.
4 CONCLUSIONS
Investigations of PMMA and TPC materials showed
that they have the characteristics of elasto-plastic behaviour. A comparison of the obtained results demonstrated
a significantly higher level of the TPC plastic deformation (A = 99.81 %). PMMA has higher Rm0.2 and E,
but these do not guarantee an efficient usage in the SB
dentistry.
The elasto-plastic behaviour of TPC was confirmed
by detecting many sliding lines in its fracture microstructure. The position of the sliding lines was almost at
an angle of 45° relative to the direction of the loading
during the tensile test. This finding can be compared to
the situation in the mouth where the teeth forces act in
similar directions. On this presumption it can be concluded that the TPC material would have good elastoplastic behaviour also in the oral environment involving
higher biting stresses.
Acknowledgements
This study was supported by the research project
Optoelectronic nanodimensional systems – path to
implementation – III 45003 (Ministry of Education and
Science of the Republic of Serbia).
816
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(1999), 63–73
12
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M. H. Kryger, T. Roth, W. C. Dement (editors), Principles and
practice of sleep medicine, W. B. Saunders, Philadelphia 2000,
229–241
13
M. F. Ashby, Materials Selection in Mechanical Design, 3rd ed.,
Elsevier, 2005
14
W. F. Smith, J. Hashemi, Foundations of Materials Science and
Engineering, 4th ed., McGraw-Hill, 2006
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M. Kutz, Handbook of Materials Selection, John Wiley & Sons,
2002, 341
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17
K. Niemella, M. Korpela, A. Raustia, P. Ylostalo, K. Sipila, J. Oral,
Rehabil., 39 (2012) 11, 799–804
18
M. Lotze, C. Lucas, M. Domin, B. Kordass, Hum. Brain. Mapp., 33
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Materiali in tehnologije / Materials and technology 48 (2014) 6, 811–816
Na osnovu člana 49. Statuta Stomatološkog fakulteta Univerziteta u
Beogradu, Nastavno naučno veće Stomatološkog fakulteta, na III redovnoj
sednici u školskoj 2015/16. godini, održanoj 29.01.2016. godine, donelo je
sledeću
O D L U K U
Usvaja se pozitivan izveštaj Komisije za ocenu završene doktorske
disertacije dr Nataše Milošević, pod nazivom „POLIMORFIZMI U MTHFR,
GSTM1, GSTT1, MMP9 GENIMA KAO FAKTORI PREDISPOZICIJE ZA
POJAVU TEMPOROMANDIBULARNIH DISFUNKCIJA“.
Imenovani/a će javno braniti doktorsku disertaciju, ukoliko dobije pozitivno
mišljenje Veća naučnih oblasti medicinskih nauka Univerziteta u Beogradu,
pred komisijom u sastavu:
1. prof. dr Ljiljana Tihaček Šojić
2. doc. dr Aleksandra Špadijer Gostović
3. prof. dr Ivana Novaković, Medicinski fakultet u Beogradu.
O b r a z l o ž e nj e
Veće naučnih oblasti medicinskih nauka, na sednici od 28.05.2013.
godine, dalo je saglasnost na predlog teme doktorske disertacije dr Nataše
Milošević, pod nazivom „POLIMORFIZMI U MTHFR, GSTM1, GSTT1, MMP9
GENIMA
KAO
FAKTORI
PREDISPOZICIJE
ZA
POJAVU
TEMPOROMANDIBULARNIH DISFUNKCIJA“.
Imenovani/a je u časopisu „Journal of Oral and Facial Pain and
Headache“, objavio/la rad pod nazivom: „Association of Functional
Polymorphisms in Matrix Metalloproteinase-9 and Glutathione S-Transferase
T1 Genes with Temporomandibular Disorders“ (2015).
Imajući u vidu napred navedeno, Nastavno naučno veće Stomatološkog
fakulteta Univerziteta u Beogradu, rešilo je kao u dispozitivu.
Odluku dostaviti: Imenovanom/oj, Univerzitetu u Beogradu, Odseku za
nastavu, Veću, Komisiji (3) i Pisarnici.
Referent kadrovskog odseka
Violeta Rastović
Dekan
Stomatološkog fakulteta
Prof. dr Miroslav Vukadinović
Obrazac 1.
Fakultet
STOMATOLOŠKI
UNIVERZITET U BEOGRADU
Stručno veće za medicinske nauke
Broj zahteva
(naziv stručnog veća kome se zahtev upućuje , shodno
čl.6 Statuta Univerziteta u Beogradu i čl. 7. st.1
ovog pravilnika)
.god.
.Datum)
ZAHTEV
za davanje saglasnosti na izveštaj o urađenoj doktorskoj disertaciji
Molimo da, shodno članu 68. st.3. Zakona o univerzitetu ("Službeni glasnik RS" br. 20/98), date saglasnost na
izveštaj o urađenoj doktorskoj disertaciji kandidata
MILOŠEVIĆ VLADO NATAŠE
(ime, ime jednog od roditelja i prezime)
KANDIDAT
MILOŠEVIĆ VLADO NATAŠA
prijavila je doktorsku disertaciju pod nazivom
(ime, ime jednog od roditelja i prezime)
„POLIMORFIZMI U MTHFR, GSTM1, GSTT1, MMP9 GENIMA KAO
PREDISPOZICIJE ZA POJAVU TEMPOROMANDIBULARNIH DISFUNKCIJA“
Univerzitet je dana
28.05.2013
svojim aktom pod br.
61206-1666-2-13
FAKTORI
dao saglasnost na predlog teme
doktorske disertacije koja je glasila
„POLIMORFIZMI U MTHFR, GSTM1, GSTT1, MMP9 GENIMA KAO FAKTORI
PREDISPOZICIJE ZA POJAVU TEMPOROMANDIBULARNIH DISFUNKCIJA“
Komisija za ocenu i odbranu doktorske disertacije kandidata
MILOŠEVIĆ VLADO NATAŠE
(ime, ime jednog od roditelja i prezime)
obrazovana je na sednici održanoj
15.09.2015
odlukom fakulteta pod br.
u sastavu:
ime i prezime člana komisije:
zvanje:
naučna oblast:
Ljiljana Tihaček Šojić
profesor
Kliničke stomatološke nauke
Aleksandra Špadijer Gostović
docent
Kliničke stomatološke nauke
Ivana Novaković
profesor
Humana genetika
Nastavno-naučno veće fakulteta prihvatilo je izveštaj Komisije za ocenu i odbranu doktorske
disertacije na sednici održanoj dana
29.01.2016.
DEKAN FAKULTETA
_____________________________
Prof. dr Miroslav Vukadinović
Prilog:
1. Izveštaj komisije sa predlogom
2. Akt Nastavno-naučnog veća fakulteta o usvajanju izveštaja
3. Primedbe date u toku stavljanja izveštaja na uvid javnosti,
ukoliko je takvih primedbi bilo.