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a. To have corrosion strength, no matter the processing types; the alloy compounds to be chemically stable, without any influence of various factors or the pH-alteration in the mouth.

b. The mechanical characteristics of the devices or dentures to be optimal for the stress developed during the functions of dento-maxillar apparatus (elasticity, tearing strength, wear, etc)

The fixed orthodontic devices appeared due to the necessity of solving complicated cases of dental-maxillary anomalies, as fast as possible and with a permanent control of dental movements. These devices are helpful both for the coronal or radicular movement and the entire dento-parodontal units movement.

Lately, a new era was opened in the Orthodontic appliance, which is in connection with the placement of the orthodontic device depending on the minimum visibility.

ORIGINAL ARTICLES

abstract

rEZUMat

MEtallographic aspEcts of wEldings bEtwEEn orthodontic rings and wirEs

Cristina Bortun, Florica Glavan, Sorin Porojan, Liliana Sandu, Otilia Gombos

Introduction: Fixed orthodontic devices are frequently used in Dental Medicine. Quad Helix/Crib Therapy is an MIA (Mobile Intraoral Arch) with palatal or lingual appliance, used for Dento-skeletal anomalies like: constricted maxillary, open bite. It uses orthodontics rings that can be fixed on molars. Objectives: The main objective of the study is to determine the optimal parameters of microplasma-welded joints, for the compounds of the orthodontic devices. Material and methods: We used worn-out orthodontic rings and Wipla and CoCr wires, in order to weld the arches to the rings. Microplasma Welding Device (Schutz Dental- Rosbach, Germany) was used for welding. In order to define the optimal welding parameters (Power 3; time/ welding point 40msec.; time between impulses 1 sec.; time for Argon running before pointing 1 sec.), the weldings underwent a metallographic analysis and a microhardness evaluation. Results: The structural analyses of welding zone between lingual arches and orthodontic rings revealed typical alloys structure from the base material, welding material and heat affected zone. The microhardness of Wipla wire is twice as that of the orthodontic ring, which imposes some special welding conditions. If this fact is not considered, the orthodontic rings can be destroyed. Conclusions: 1. It is recommended to weld the orthodontic rings using Wipla wire. 2. The tangential welding is much more comfortable compared with the point-like one and has better results. Key Words: orthodontic rings, microplasma welding, CoCr wire, wipla wire, metallographic analysis, hardness

Introducere: Aparatele ortodontice fixe sunt frecvent utilizate n practica medicinei dentare. Quad Helix/Crib Therapy este unul din aparatele tip MIA (Mobile Intraoral Arch) cu arcuri aplicabile palatinal sau lingual, sudate de inele ortodontice aplicabile pe molari, ce este utilizat n anomalii dentomaxilare, cum ar fi: endogna]ia maxilar\, ocluzia deschis\. Obiective: Obiectivul principal al studiului este cel de a determina parametrii optimi de imbinare sudat\ cu microimpulsuri a componentelor aparatului ortodontic. Material [i metode: S-au utilizat inele ortodontice (Ormco, Ormco Corporation, West Collins USA) uzate, srma de CoCr Finolloy (Fino, Bad Bocklet, Germany) sau de Wipla Osteofix (Zelezarny, Chomutov, Cehia) pentru sudarea arcurilor la inele. A fost folosit aparatul de sudur\ cu microimpulsuri Welder (Schutz Dental). Sudurile au fost analizate metalografic [i li s-au evaluat microdurit\]ile, pentru a defini parametrii optimi de sudur\ (putere 3; timp /punct sudur\ 40msec.; interval de timp ntre impulsuri 1 sec.; timp de curgere a argonului nainte de sudare 1 sec.). Rezultate [i discu]ii: Analizele microscopice ale sudurilor arcurilor linguale la inele ortodontice au relevat structurile caracteristice ale aliajelor metalice din zona de baz\, zona sudat\ [i zona de influen]\ termic\. S-a constatat c\ duritatea srmei de wipla este dubl\ fa]\ de cea a inelului ortodontic, ceea ce impune anumite condi]ii de sudur\. Dac\ nu este luat n calcul acest fapt [i experien]a profesional\, la mbin\rile sudate se deterioreaz\ inelul ortodontic. Concluzii: 1. Se indic\ s\ se sudeze inelele ortodontice cu srm\ de wipla. 2. Sudura tangen]ial\ este mult mai confortabil\ dect cea punctiform\ [i d\ rezultate mai bune. Cuvinte cheie: inele ortodontice, sudura cu microimpulsuri, srm\ de CoCr, srm\ de wipla

Received for publication: Jul. 10, 2007. Revised: Dec. 12., 2007.

Department of Removable Partial Dentures Technology, Faculty of Dentistry, Dental Technology Specialization, Victor Babes University of Medicine and Pharmacy Timisoara

Correspondence to:Prof. Cristina Bortun, 9 Revolutiei din 1989 Blvd., Timisoara, RomaniaEmail: cristinabortun@yahoo.com

introdUction

The alloys used in Orthodontics and in the Denture Technology are metallic structures, obtained through the melting of many chemical metallic compounds (Co, Cr, Mo, Ni, Ti, etc).1-8 Generally, the alloys used in dentistry have to fulfill specific conditions, in order to be used in the oral cavity:

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Figure 1. Orthodontic device of type Quad Helix: a. situated on the cast;

b. detached from the cast; c. ring-arch welding detail

The coming out of a new series of devices like MIA (Mobile Intraoral Arch) satisfies the patients not only orthodontic but also esthetically. We insisted on the Quad Helix, an orthodontic device with orthodontic rings, on which an oral arch is attached.5,9 The orthodontic rings are placed on the molars and the palatal or lingual arch is welded on these rings.

The arch, which is orally placed, moves symmetrically or asymmetrically various groups of teeth and can solve complicated anomalies (maxillary disjunction, open bite, posterior cross bite), without being noticed by the entourage. From this group of devices, we present a QUAD HELIX/CRIB variant, created at the Orthodontic Department in Timisoara and which was tested. (Fig. 1)

Generally, Orthodontics uses voltaic arch point soldering which often is followed by failures and/or the deterioration of the orthodontic ring. The concept of joints welded using microimpulses or laser welding was tested in Prosthodontics, (but not frequently) and only rarely for the orthodontic devices.3,4,10-17 For this reason, we tested the system in order to indicate the right solution: no matter the welding procedures or the material precipitate, always structures (rings and arches) modification takes place, especially in the heat-affected zone (HAZ), namely near the welded zone. Here, due to the overheating, precipitates of some compounds can appear and hardens the zone.

MatErial and MEthods:

We studied 15 joint samples, welded with material addition, on worn out orthodontic rings (Ormco, Ormco Corporation, West Collins USA). For these joints, two types of wire were used, Wipla Osteofix (FeCrNi) (Zelezarny Chomutov, Chomutov, Cehia) wire and CoCr Finoloy (Fino, Bad Bocklet, Germany) wire. The welding was performed with a microimpulse device (Schütz Dental, Rosbach, Germany). (Fig. 2) The welding were point-like when the wire position was vertically (perpendicular on the ring) and linear, in case of an oblique position of the welded joints.

The welding parameters were those indicated (for these types of stainless steels) by the company that produces microplasma welder (Schütz Dental): Power 3; time/ welding point 40msec.; time between impulses 1 sec.; time for Argon running before pointing 1 sec.

We used the following sequence of the experimental program, in order to evaluate the welded joint:

- Evaluation of the chemical composition of the alloys, through metallographic analysis (structure, defects) and hardness tests.

- Evaluation of the welded joints, probes loading, metallographic analysis (structure, defects), and hardness attempts on specific zones: base material, heat affected zone, welding zone.

- Establishment of optimal possibilities for welding and loading, followed by analysis and tests.

Microscopic observations were made using an inverted metallographic microscope Reichert MeF2 (Reichert, NY, USA). Therefore the samples were incorporated into an acrylic resin, cut perpendicularly to the weld axis, grinded using silicon carbide abrasive paper and polished with a series of abrasives rubbers. The surfaces to be analyzed were chemically attacked using acid solution of ferric chloride for 2-3 seconds at room temperature. The images magnification was 100x on polarized light.

a

b

c

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Figure 2. Welder Schűtz Dental - a. welding equipment; b. Ormco orthodontic ring (Ormco Corporation, West Collins USA); c. Welded joining on an orthodontic ring with two types of wire (wipla- thick wire and CoCr- thin wire)

a

b

c

a

b

c

d

e

f

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Figure 3. Metallographic structures of ring, welding area and wire: a. Martensite and fine carbids - Osteofix Wipla; b. Solid solution of CoCr; c. Orthodontic ring; d,e. Details - welded joint zones: BM,HAZ,WM; f. Ring with Wipla wire welded zone; g. Ring with Finolloy CoCr wire (Fino, Bad Bocklet, Germany) welded zone; h. Mixed zone of ring-wipla alloys; i. Mixed zone of ring-CoCr alloys.

Microhardness (HV) was measured on the polished surface of the samples at room temperature using a Zwick 3212 (Zwick, Ulm, Germany) microhardness unit.

Individual tests with a peak load of 9.8 N (HV1) or 49 N (HV5), depending on probe thickness, and a loading time of 20 seconds were performed for each determination.

rEsUlts

Accomplishment of welded joints and analysis of the results prove that the great majority of the samples point out the existence of a joint with austenitic and/or martensitic structure, with ferrite and carbides complex. This is the way the characteristic structures of the melted alloys are noted. 18

Some metallographic aspects of alloys structures resulted after analysis are shown in Figure 3 (those of wipla wire, CoCr wire and orthodontic ring, those of welded joints and those where the alloys miscibility can be observed).

The VICKERS hardness tests were realized with 1kg load [HV1] (CoCr wire and rings) and 5kg [HV5] (wipla wire). Table 1 presents the results of the hardness tests performed on the samples.

It has been noted that the hardness of wipla wire (with approximately 100 HV units) is twice as that of the orthodontic ring, which imposes some welding precaution.

Certainly, there were also joining failures, which resulted especially from the lack of experience in welding and from the inadequate welding parameters, like those presented in Figure 4. These failures can be visible: deterioration/melting of the orthodontic ring, the welding itself being resistant at tearing. More failures can be noticed in case of wipla wire welding, which has a bigger microhardness.

Figure 4. Failures in the welding of orthodontic rings arches

discUssion

The laser and microplasma welding are performed using through rapid, economic and highly accurate techniques.12,13 Time is saved, because the frameworks are welded directly on the working cast. Distortions, which are caused by the transfer from the working cast, disappear; the welding can be done very close to the acrylic resins or the ceramics, without any damage or chromatic modification. Theoretically, all metals can suffer joining processes.16,17,19,20 Sometimes, when the cooling is to fast, we observe cracks in the base material.14,15

a

g

h

i

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The modern lasers, like Girrbach Dental System (Neolaser L 126500), Vision Industry (LWI 4th

Generation), Interdent Laser System (Herculea), Manfredi, can be electronically programmed; they have a microscopic visualization system and the parameters can be set depending on the welded alloy. The welding has a maximum stability, gives a biocompatible zone, which is mechanically resistant and can be realized rapidly and with maximum precision, also at room temperature.

Experimentally, some alloys used in Dental Technology have been welded using laser techniques. Their quality can be estimated through destructive and nondestructive methods.5,14,21-23 The metallographic analysis and microhardness evaluation belong to the destructive methods, while Spectrographic and Radiologic analyses are nondestructive. Our studies regarding laser Nd:Yag welding were made with help of Welding Department from Politehnica University Timisoara and researchers from ISIM/ WMTI Timisoara (Welding and Materials Testing Institute); they demonstrated a good compactness of the studied samples and the lack of defects in the joining area.14

The microhardness evaluation was realized with loads of 100g; it pointed out a light increase of the hardness in HAZ (heated affected zone). The hardness values of the weld metal (WM) are situated between those in the base material (BM) and heat affected zone (HAZ). Regarding the chemical composition of the joint, a small diminution of the main reactive elements has been noticed. In the testing base material of over 1 mm thickness cracks appear, which can be observed not only macroscopically but also by metallographic study That is why, at 1 mm thickness, we recommend welding without material addition, while, for the rest, material addition being necessary and mandatory.

The microplasma welding offers good quality at a reasonable price and rapidly. The (effective) welding takes place also under the microscope, in a way the spot overlapping should be optimal, namely more than ½. Different types of welding on dental alloys used in Removable Partial Denture Technology were experimented before; this is the reason why we considered interesting this experience in Orthodontics. On this type of welding, the alloy’s lowest thickness is 0.4-0.5 mm, while in that of the orthodontic ring the ring thickness is 0.3 mm. Therefore, there were some

Table 1. Microhardness of some welding areas

Track no Material

Orthodontic ring CoCr wire Wipla wire

Vickers HV1 HV5

BM HAZ WM BM HAZ WM BM HAZ WM

0 1 2 3 4 5 6 7 8 9 10

1

RING 1

193 229 210 368 - - 454 - -

2 210 228 210 378 - - 466 - -

3 214 229 210 389 - - 480 - -

1

RING 2

239 - 224 368 - - 460 - -

2 219 - 229 378 - - 460 - -

3 214 - 234 358 - - 454 - -

1

RING 3

205 219 197 358 - - 473 - -

2 201 218 201 378 - - 488 - -

3 202 210 224 348 - - 473 - -

1

RING 4

201 215 225 360 476

2 209 224 226 354 484

3 198 213 236 367 463

1

RING 5

204 220 235 358 459

2 189 211 232 362 469

3 207 210 227 369 456

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welding difficulties when the power degree was too high, because the orthodontic ring effectively melted.

conclUsions

1. It is recommended to weld the orthodontic rings using Wipla wire.

2. The tangential welding is much more comfortable in comparison with the point-like one and has better results.

acKnowlEdgEMEnt

This study was supported by the CNCSIS Grant Type A, No. 744/2006

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