ARTÍCULOS ORIGINALES
Titanium alloy orthodontic mini-implants: scanning electron microscopic and metallographic analyses
Paola F. P. Burmann1, Henrique C. Ruschel2,4, Ivana A. Vargas1, José C. K. de Verney3, Paulo F. Kramer4
1 Department of Orthodontics. Lutheran University of Brazil (ULBRA), Canoas, RS, Brazil.
2 Department of Oral Histology. Lutheran University of Brazil (ULBRA), Canoas, RS, Brazil.
3 Department of Engineering. Lutheran University of Brazil (ULBRA), Canoas, RS, Brazil.
4 Department of Pediatric Dentistry. Lutheran University of Brazil (ULBRA), Canoas, RS, Brazil.
CORRESPONDENCE Dr. Paola Flach Perim Burmann Rua Antunes Ribas, 2016, Meller Sul Santo Angelo – RS – Brazil Phone/Fax: +55 55 9101 470 e-mail: ortopfperim@hotmail.com
ABSTRACT
Anchorage control is one of the determining factors of successful orthodontic mechanics. In mini-implants, fractures due to placement and removal have been related to implant design and titanium alloy quality. This study assessed the topography and microstructure of five brands of mini-implants (Neodent, SIN, Morelli, Conexao, Foresta Dent). Scanning electron microscopic analyses of the head, transmucosal neck, threaded body, and tip were performed to assess implant design and manufacturing defects (n=3/group). Metallographic analysis of longitudinal sections (n=15) and cross-sections (n=15) was performed under conventional light microscopy according to international standards of "American Society for Testing and Materials". The results showed significant differences in miniimplant design. Surface irregularities in the threaded body and tip were observed. Microstructural analyses revealed an alpha/beta-phase grain structure, in compliance with the ETTC-2 ("Technical Committee of European Titanium Producers" –2nd edition). No structural defects were detected. We conclude that differences in mini-implant design and the presence of surface irregularities may influence the effectiveness of orthodontic anchorage.
Key words: Orthodontics; Dental implantation; Titanium.
RESUMO
Mini-implantes ortodônticos de liga de titânio: análises ao microscópio eletrônico de varredura e metalográfica
O controle da ancoragem e um dos fatores decisivos no sucesso da mecanica ortodontica. Fraturas devido ao estresse de insercao e remocao de mini-implantes sao associadas ao design das pecas e a qualidade da liga de titanio. O presente estudo analisou a topografia e a microestrutura de cinco marcas de mini-implantes (Neodent, SIN, Morelli, Conexao, Foresta Dent). Analise ao microscopio eletronico de varredura da cabeca e perfil transmucoso, porcao rosqueavel e ponta ativa foi realizada com o proposito de avaliar o design e defeitos de fabricacao (n=3/grupo). A analise metalografica baseou-se nas normas internacionais da "American Society for Testing and Materials" e revelou a microestrutura em cortes longitudinais (n=15) e transversais (n=15) por meio do microscopio optico. Os resultados demostraram que os mini-implantes apresentam diferencas significativas no design. Irregularidades superficiais na porcao rosqueavel e na ponta ativa foram tambem observadas. A analise da microestrutura revelou uma estrutura de graos fases alfa e beta distribuidas de acordo com os padroes definidos pelas normas ETTC-2 ("Technical Committee of European Titanium Producers" – 2a edicao). Alem disso, nao foram detectados defeitos na estrutura interna das ligas. Conclui-se que diferencas no design dos mini-implantes e a presenca de irregularidades superficiais podem influenciar na efetividade da ancoragem durante o tratamento ortodontico.
Palavras-chave: Ortodontia; Implante dentario; Titanio.
INTRODUCTION
Orthodontics is based on the exertion and control
of forces acting on the teeth and supporting structures.
Therefore, control of anchorage is essential
for the success of orthodontic treatment1.
Traditional orthodontic anchorage depends on patient
compliance. Furthermore, the number or quality of
teeth is often insufficient for effective anchorage2.
Therefore, several anchorage devices have been used
in recent decades. Prosthetic implants, plates, and
onplants have been replaced with mini-implants
because they eliminate the need for invasive surgical
procedures, high cost, placement site limitations, and
considerable time for osseointegration3.
Mini-implants are popular because of their ease of
insertion and removal, less discomfort for patients,
possibility of immediate loading, high versatility,
and low cost4-6. Clinical and laboratory outcomes,
however, have shown failure rates of 10 to 30%,
mostly related to inflammation of peri-implant tissues,
characteristics of soft tissues, and miniimplant
placement site7-9. Screw diameter, length,
thread form, presence of flutes, and screw material
have also been implicated in poor primary stability
of these devices10-13.
The optimal material of mini-implants would exhibit
excellent corrosion resistance, biocompatibility,
and sufficient mechanical strength to resist placement
and removal14. Titanium alloys have been used
in these devices. The use of vanadium and aluminum
have significantly enhanced their performance
and mechanical properties15. Nevertheless,
studies on the internal microstructure of miniimplants
rare in the literature14-16.
Because of the diameter and length restrictions of
mini-implants, optimal shape design is important for
primary stability. The strength resistance of a titanium
alloy depends on its microstructure, which is
influenced by the composition, heat treatment, and
machining processes of the mini-implant17. Thus,
studies analyzing the topography and microstructure
of mini-implants are essentially important. The
objective of the present study was to analyze the topographical
and microstructural features of miniimplants
used for orthodontic anchorage.
MATERIALS AND METHODS
Topography and microstructure were analyzed on
Ti-6Al-4V (Grade 5 titanium alloy) self-drilling
orthodontic mini-implants from five different dental
implant manufacturing companies (four Brazilian
and one imported). They were allocated into five
groups: Group 1 – NeodentR (Curitiba, Parana,
Brazil); Group 2 – SINR (Sao Paulo, Sao Paulo,
Brazil); Group 3 – MorelliR (Sorocaba, Sao Paulo,
Brazil); Group 4 – ConexaoR (Aruja, Sao Paulo,
Brazil); and Group 5 – Foresta DentR (Pforzhein,
Baden-Wurttemberg, Germany).
Scanning electron microscopy (SEM) analysis was
conducted to obtain a descriptive analysis of
implant design and detect potential manufacturing
defects. Three mini-implants of each of the five
brands were analyzed. The implants were bonded
to aluminum stubs (Sigma Chemical Co., St. Louis,
Missouri, USA) with cyanoacrylate adhesive
(Super Bonder Gel, Loctite, Diadema, Sao Paulo,
Brazil) and immediately analyzed under high-vacuum
SEM (Philips XL 20, FEI, Eindhoven, The
Netherlands) at 20kV accelerating voltage. Images
of the screw head, transmucosal neck, threaded
body, and tip were analyzed at 50x, 100x, and 200x
magnification.
Metallographic analysis was conducted to detect discontinuities
and to assess the presence of alpha- and
beta-phase titanium. The methodology was based on
the "American Society for Testing and Materials"
(ASTM International). The standards applied were
ASTM E3-01 (Standard Guide for Preparation of
Metallographic Specimens)18, ASTM E407-99 (Standard
Practice for Microetching Metals and Alloys)19,
and ASTM E7-03 (Standard Terminology Relating
to Metallography)20. Six mini-implants (three in longitudinal
section, three in cross section) from each of
the five brands were analyzed.
For longitudinal sections, the samples were coldembedded
in methyl methacrylate polymer before
being sectioned lengthwise. The mini-implants were
ground and polished with a series of silicon carbide
abrasive sheets - 220, 320, 400, 600, and 1200 grit
(3M, Sumare, Sao Paulo, Brazil) under water. The
samples were polished in a DP-10 sander (Panambra,
Sao Paulo, Sao Paulo, Brazil), using diamond
abrasive compound (3M, Brazil) with 1- and 2-mm
grains. The longitudinally sectioned and polished
mini-implants were etched in a solution of 10mL
HF, 5mL HNO3, and 85mL H2O (Kroll's reagent) for
20 seconds, dried with hot air, and analyzed under
light microscopy (Union MC 85800, OptiTec Ltd.,
Japan) at x50 and x400 magnification.
Cross sections were obtained at TORK(Controle Tecnologico
de Materiais Ltda, Sao Paulo, Sao Paulo,
Brazil). Samples were cold-embedded in acrylic
resin. Metallographic analysis was based on the ISO
5832-3 standard (Implants for surgery - Metallic
materials - Part 3), and the alpha/beta phases were
compared with the European Technical standards
(ETTC-2) published by the Technical Committee of
European Titanium Producers. The mini-implants
were sectioned with a circular table saw (Arotec, Sao
Paulo, Sao Paulo, Brazil) and a cutting disc (Norton,
Worcester, Massachusetts, USA). The specimens
were sanded in a round device with silicon carbide
abrasive sheets - 150, 220, 320, 400, and 600 grit (3M,
Brazil), on a tabletop grinding machine, using water
as a lubricant to obtain a flat, homogeneous surface.
The specimens were then polished in a sander with 6-
μm and 3-μm diamond abrasive compound and
buffed with 1-μm diamond compound. The cross-sectioned
specimens were etched with a solution composed
of 6 g NaOH, 60 mL H2O, and 10 mL H2O2 for
20 seconds and dried with hot air. This process
revealed the microstructure of the mini-implants, in
which an effective contrast between alpha and beta
phases was observed. The cross sections were examined
under light microscopy (OPTON / TNM-07 PL,
Cotia, Sao Paulo, Brazil) at x200 magnification.
RESULTS
The mini-implants exhibited significant differences between brands in screw head and transmucosal neck; pitch and shape of threads; and active tip design (Fig. 1). Surface irregularities can result from machining process, polishing defects, crystal growth deposits, and areas of detritus. The greatest amount of surface irregularities and detritus was found on the tips of Groups 1, 2, and 3. The best surface finish along the threaded body was found in Groups 1 and 5. Mini-implants in all groups had adequate surface finish and no evidence of irregularities on the screw head or transmucosal neck (Fig. 1).
Fig. 1: Head and transmucosal neck (a) threaded body, (b)
and active tip, (c) of mini-implants in the NeodentR (1), SINR
(2), MorelliR (3), ConexaoR (4), and Foresta DentR (5) groups.
Despite the differences in the size of the orthodontic accessory on the screw head, they all had uniform structure and good surface finish. In Groups 1, 2, 3 and 4, the accessory was in the shape of an orthodontic button, whereas Group 5 mini-implants had bracket-shaped screw heads (Fig. 1). Mini-implants in Group 4 had a greater number of threads and flutes at the tip and a screw head diameter equal to that of the transmucosal neck. In Groups 1, 2, 3, and 5, the screw head and transmucosal neck had different diameters. Although all mini-implants were of the self-drilling variety, those in Groups 1, 3 and 5 had sharper tips (Fig. 1). Longitudinal sections were assessed to detect defects in the internal microstructure of each miniimplant, whereas cross-sections were compared against the ETTC-2 regarding the distribution of alpha and beta phases in the alloy. There were no visible imperfections in the inner structure of any mini-implants, and no internal defects were detected on the longitudinal sections. The mini-implants had a fine microstructure composed of an alpha matrix into which spheroidal beta-phase particles were dispersed. On cross sections, the internal microstructure of the alloys was consistent with ETTC-2 standard class A1. Alpha phase titanium appears light, whereas beta-phase granules appear darker. The small granule size of both phases and balanced alpha/beta ratio are indicative of high internal structure quality (Fig. 2).
Fig. 2: Photomicrograph showing the microstructure of the
mini-implants (cross section)
DISCUSSION
We found significant differences in screw head,
threaded body, and tip design between the five
brands of mini-implants. Furthermore, SEM analysis
showed surface irregularities and detritus, particularly
at the implant tip. Metallographic analysis
did not show any defects in the microstructure. All
mini-implants tested met the European standard for
titanium alloy production.
Mini-implants are an effective and very well tolerated
tool for skeletal anchorage, and have become
the gold standard for orthodontic biomechanics in
adults2. They are available in a variety of shapes,
diameters, lengths, and titanium alloy compositions.
However, it bears stressing that failure has
been reported during mini-implant placement and
removal. Mini-implant fractures are usually due to
torsional strain caused by their small diameter7,15,21.
Reicheneder et al.22 reported that different miniimplant
systems showed comparable elementary
composition. They stressed that differences in
mechanical properties can be attributed to miniimplant
design, and that implant morphology plays
an essential role in ensuring primary stability.
In our study, the surface defects found in most samples,
particularly at the active tip, may be caused by
the machining process. These defects may be a
starting point for electrochemical degradation
processes that can alter the surface finish of the
implant and its resistance and other material properties23.
According to Sebbar et al.24 improvements
in the surface treatment of mini-implants could
improve their corrosion resistance. Mini-implants
in Group 5 had fewer surface irregularities at the
tip and better polish along the threaded body.
The machining process determines the surface finish
of the piece. Machining leads to a rough surface.
Therefore, the biocompatibility of the surface texture
has major influence on the type and progression
of reactions in the tissues adjacent to the
implant surface. Furthermore, changes in surface
morphology that may occur during the sterilization
process and mechanical damage that may be sustained
during mini-implant placement and removal
may induce changes in osteoblast growth and differentiation25,26.
Studies have also analyzed changes in mini-implant
design that might lead to improvements in the
mechanical properties11. A greater number of threads
and a finer pitch in the implant are associated with
greater mechanical locking ability, enhanced resistance
during mini-implant placement, improved
resistance to displacement, and improved primary
implant stability8. Mini-implants in Group 4 may
improve the distribution of applied forces because
they have a greater number of threads. Furthermore,
the presence of flutes may be linked to greater fracture
resistance, as it prevents concentration of excessive
strain in the adjacent tissues8,26. Conversely,
thread design may also interfere with the distribution
of strain under load11. Hence, further studies are
required to ascertain the influence of design on the
mechanical properties of mini-implants.
Lee et al.27 found that many undesirable outcomes
are attributable to the design of mini-implants. They
claim that a coarsely finished or poorly designed
mini-implant active tip may compromise final
implant placement and primary stability. All miniimplants
we tested were of the self-drilling variety,
and those in Groups 1, 3 and 5 had the finest and
sharpest tips, which suggests greater ease of placement
without pilot hole drilling. The diameter of
the mini-implant head is an important design factor.
It should be wider than the transmucosal neck
to prevent overgrowth of soft tissues. All implants
had this design feature, except for those in Group 4.
Casaglia et al.12 showed that small transmucosal
neck diameter is a site of increased fragility. The
authors detected microfissures and grooves on the
surface and concluded that these irregularities may
predispose to mini-implant fracture.
Most mini-implants are made of Ti-6Al-4V (ASTM
Grade 5 titanium alloy). This alloy has greater
mechanical resistance than pure titanium and is
more appropriate for small-diameter devices. Furthermore,
its lower bioactivity facilitates implant
removal because of less osseointegration18.
Titanium alloys must be free of external irregularities
and internal imperfections to avoid interference
with fracture resistance, mechanical retention, displacement
resistance, and primary stability16. Our
metallographic analysis did not reveal any internal
defects, corroborating the findings of Cotrim-Ferreira
et al.15 and Eliades et al.16.
The alpha phase of titanium alloys is a soft alloy
showing high resistance and tensile strength, but low
ductility. Alpha-stabilizing elements increase the
temperature range at which the alpha phase remains
stable. The beta phase, in turn, has superior forming
and fatigue resistance, but is highly vulnerable to
atmospheric contamination. Beta stabilizers make
the beta phase stable at low temperatures18.
The matrix of all mini-implants assessed contained
beta and alpha titanium, which indicates that the
small amount of vanadium in the alloy was sufficient
to retain significant amounts of beta-phase
titanium, thus enhancing the properties of the alloy,
as demonstrated by Iijima et al.14.
All mini-implants complied with the ETTC-2 standards,
namely in class A1. Cotrim-Ferreira et al.15 found
that the microstructure of ConexaoR brand implants was
class A9, showing differences in hardness, resistance,
and elastic modulus because of different alpha-beta
phase ratio. However, this may not interfere with
mechanical resistance, as alloys of any class from A1
through A10 can be used for mini-implant manufacturing
according to the ETTC-2 standards.
Despite the advantages of titanium alloy miniimplants,
practitioners must be aware of the topographic
and microstructural features of mini-implants,
since they influence the effectiveness of orthodontic
anchorage. Orthodontic treatment depends on reliable
and effective anchorage. Primary stability, mechanical
resistance, and clinical performance of miniimplants,
in turn, depend on their topographical and
microstructural characteristics.
The present study concluded that the orthodontic
mini-implants assessed exhibited significant differences
in the design of the screw head, transmucosal
neck, threaded body, and active tip.
Furthermore, surface irregularity and debris were
found in all groups, particularly at the active tip.
Conversely, no internal defects were detected, and
all groups complied with the international standards
for mini-implant manufacturing. Further
studies of orthodontic mini-implants should prioritize
topographic and microstructural analysis combined
with mechanical testing.
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