Elasticke moduly v ortodoncii Elastic modules in orthodontics

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Elasticke moduly v ortodoncii

Elastic modules in orthodontics

MUDr. Alena FormaÂnkovaÂ, MUDr. Magdalena Kot'ovaÂ, Ph. D.

OddeÏlenõ ortodoncie arozsÏteÏpovyÂch vad Stomatologicke kliniky 3. LF UK FNKV Praha

Department of Orthodontics and Cleft Defects, Clinic of Stomatology, 3rd Medical Faculty of Charles University, University Hospital KraÂlovske Vinohrady, Prague

Souhrn

JednõÂm ze zaÂkladnõÂch prÏedpokladuÊ uÂspeÏsÏne ortodonticke leÂcÏby je kontrolovane puÊsobenõ ortodontickyÂch sil. CõÂlem praÂce je analyÂza silovyÂch charakteristik beÏzÏneÏ pouzÏõÂvanyÂch ortodontickyÂch elastickyÂch rÏetõÂzkuÊ. Byl zkou-maÂn vliv ruÊznyÂch parametruÊ (tvar rÏetõÂzku, vyÂrobce, prestretching) na produkci sil v zaÂvislosti na cÏase.

Byl vytvorÏen soubor 18 typuÊ beÏzÏneÏ pouzÏõÂvanyÂch elastickyÂch rÏetõÂzkuÊ od 7 ruÊznyÂch vyÂrobcuÊ. RÏetõÂzky byly rozdeÏ-leny do 3 skupin, podle deÂlky mezicÏlaÂnku. NaÂsledneÏ byly protazÏeny o 50 % a 100 % jejich puÊvodnõÂ deÂlky a byl meÏ-rÏen pokles sõÂly v cÏase ve zvolenyÂch cÏasovyÂch intervalech ve standardizovanyÂch podmõÂnkaÂch.

Studie prokaÂzala, zÏe sõÂly produkovane elastickyÂmi rÏetõÂzky jsou velke a znacÏneÏ se lisÏõ dle vyÂrobce. BeÏhem prvnõÂch 24 hodin dochaÂzõ k vyÂrazneÂmu poklesu sil, pote sõÂly klesajõ uzÏ jen velmi pomalu. Vliv prestretchingu na snõÂzÏenõ vyÂ-chozõÂch sil byl patrny pouze u naÂsledneÂho protazÏenõ rÏetõÂzku o 50 %, prÏi 100 % protazÏenõ tento vliv prokaÂzaÂn nebyl (Ortodoncie, 23, cÏ. 2, s. 97-108).

Abstract

Controlled delivery of orthodontic forces is one of the preconditions for successful orthodontic treatment. The work aims to give the analysis of common orthodontic elastic chains force characteristics. The study focuses on the effect of individual parameters (chain shape, manufacturer, prestretching) on time-related delivery of forces. Our sample included 18 types of common elastic chains produced by 7 manufacturers. The chains were sub-divided into 3 groups according to the length of their connecting link. Subsequently, the chains were elongated by 50% and 100% of their original length, and the decrease of force over a period of time was measured at given intervals under standardized conditions.

The study proves that forces delivered by elastic chains are big, and that products of individual manufacturers are different. Within the first 24 hours forces decrease significantly, afterwards they lessen very slowly. The effect of prestretching on the decrease of initial forces was evident only in subsequent stretching of a chain by 50%. In case of 100% stretching the effect was not proved(Ortodoncie, 23, No. 2, p. 97-108).

KlõÂcÏova slova:elasticky rÏetõÂzek, pokles sõÂly, mezicÏlaÂnek, prestretching

Key Words:elastic chain, force degeneration, connecting link, prestretching UÂvod

Elasticke rÏetõÂzky se staly neodmyslitelnou, trvalou abeÏzÏneÏ pouzÏõÂvanou soucÏaÂstõ nasÏich ortodontickyÂch praxõÂ. Elasticke rÏetõÂzky aligatury jsou vyrobeny z polyu-rethanuÊ. Je pro neÏ charakteristickeÂ, zÏe vykazujõÂelasticitu kaucÏuku. To znamenaÂ, zÏe v jednom ze svyÂch

prÏechodo-Introduction

Elastic chains have become an inherent and integral part of orthodontic practices. Elastic chains and ligatu-res are made of polyurethane. They are as elastic as natural rubber; after substantial deformation with a re-latively light force they rapidly return to their original

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di-mensions, i.e. the deformation is reversible. However, the elastic limit must not be exceeded [1, 2].

Elastic chains behaviour, especially decrease of for-ces occurring after they are stretched, has been the focus of many studies. There were reported significant differen-ces in force decrease between chains by individual pro-ducers. The reasons may be seen in various additives in-corporated into the resulting structure of elastomer, and in morphological variability of chains. The majority of stu-dies focusing on force degradation have been conducted in vitro. There are several reasons for the research to be done mostly in vitro, not in vivo, the primary one being that to achieve standard conditions for the experiment in oral cavity is very demanding, if not impossible. There is a number of parameters examined during experiments in vitro (humidity, pH, stretching of samples, etc.). Most studies agree in that within the first 24 hours there is sig-nificant amount of force degradation - 40-60%. Subse-quent force degradation, i.e. during the following two or three weeks, is significantly slower [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13].

One of the factors affecting delivery of chain force is the ¹ageingª of chains in vivo. Differences in the mouth cavity conditions are mainly due to oral flora and its side effects, as well as plaque accumulation. This arti-ficial environment can not be exactly duplicated in stu-dies in vitro. It has been proved that the exposure of different materials to mouth cavity environment can provoke significant changes in their structure and sur-face characteristics.

The aim of the presented study, performed in vitro, is the analysis of force characteristics of common orthodontic chains. We strive to answer the following questions: Are the initial forces delivered by chains identical or do they decrease at different rate? Do the presence and length of the connecting link affect for-ces induced by chains? Does prestretching affect the level of the initial force and its degradation in compari-son with chains without prestretching?

Material

First we analyzed and measured force decrease over aperiod of time after stretching in 18 types of orthodontic chains by 7 manufacturers: Dentaurum, Inspringen, Germany; American Orthodontics, She-boygan, WIS, USA; G&H Wire Company, Franklin, IN, USA; Ormco, Glendora, CA, USA; Highland Metals Inc., San Jose, CA, USA; Ortho-Care /UK/ Ltd., Sal-taire, UK; 3M/Unitek, Monrovia, CA, USA.

The sample of 18 chains was subdivided according to whether a chain has a connecting link or not, and ac-cording to the connecting link length. As the length of connecting links and description given by individual manufacturers are different and do not correspond to vyÂch stavuÊ - stavu kaucÏukoviteÂm, je lze pomeÏrneÏ malou

silou vratneÏ deformovat azÏ o stovky procent. Nesmõ se ale prÏekrocÏit mez elasticity, to by jizÏ vratna deformace elastickeÂho rÏetõÂzku nebylamozÏna [1, 2].

ChovaÂnõ elastickyÂch rÏetõÂzkuÊ azejmeÂnapokles sil, ktery nastane po jejich natazÏenõÂ, je oblastõÂ, na kterou bylazameÏrÏenapozornost v mnohastudiõÂch. Byly za-znamenaÂny signifikantnõ rozdõÂly v poklesu sil mezi rÏe-tõÂzky jednotlivyÂch znacÏek. DuÊvody, procÏ by tomu tak mohlo byÂt, spocÏõÂvajõ ve variabiliteÏ prÏidanyÂch aditiv, ktera se inkorporujõ do vyÂsledne struktury elastomeru, atake v morfologicke variabiliteÏ rÏetõÂzku. Mnoho studiõ poklesu sil produkovanyÂch rÏetõÂzky bylo provaÂdeÏno in vitro. DuÊvoduÊ, procÏ studie degradace sil probõÂhajõ veÏ-tsÏinou in vitro ane in vivo, je võÂce alze je shrnout do konstatovaÂnõÂ, zÏe standardizace podmõÂnek pro experi-ment v dutineÏ uÂstnõ je velmi obtõÂzÏnaÂ, ne-li nemozÏnaÂ. SÏõÂrÏe testovacõÂch podmõÂnek prÏi experimentech in vitro bylavelika (vlhkost, pH, protazÏenõ vzorkuÊ...). I prÏes vsÏechnatato rozdõÂlnaÂkriterialze rÏõÂci, zÏe dosÏlo k vyÂrazne shodeÏ zaÂveÏruÊ ruÊznyÂch studiõ v tom, zÏe beÏhem prvnõÂch 24 hodin dojde k strmeÂmu poklesu sil v rozsahu o 40-60 % vyÂchozõ hodnoty sõÂly. NaÂsledny pokles beÏhem dalsÏõÂch dvou azÏ trÏõ tyÂdnuÊ je jizÏ vyÂrazneÏ pomalejsÏõ [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13].

JednõÂm z faktoruÊ, ktery ovlivnÏuje produkci sil rÏetõÂzkuÊ, je fenomeÂn ªstaÂrnutõª rÏetõÂzkuÊ in vivo. Hlavnõ faktor od-lisÏujõÂcõ prostrÏedõ dutiny uÂstnõ je prÏõÂtomnost oraÂlnõ floÂry ajejõÂch vedlejsÏõÂch produktuÊ arovneÏzÏ akumulace zub-nõÂho plaku. V studiõÂch in vitro se v soucÏasne dobeÏ ne-dajõ tyto podmõÂnky simulovat. Je oveÏrÏeno, zÏe vystavenõ ruÊznyÂch materiaÂluÊ podmõÂnkaÂm dutiny uÂstnõÂmuÊzÏe vyvo-lat podstatne zmeÏny v jejich strukturÏe apovrchovyÂch vlastnostech.

CõÂlem teÂto praÂce, provedene in vitro, je analyÂzasilo-vyÂch charakteristik beÏzÏneÏ pouzÏõÂvanyÂch ortodontic-kyÂch elasticortodontic-kyÂch rÏetõÂzkuÊ. Chceme odpoveÏdeÏt naotaÂz-ky, zdajsou vyÂchozõ hodnoty sil produkovanyÂch rÏe-tõÂzky stejneÂ, nebo sõÂly klesajõ ruÊznou rychlostõÂ, zda ovlivnõ prÏõÂtomnost adeÂlkamezicÏlaÂnku sõÂly produko-vane rÏetõÂzky azdabude mõÂt provedenõ prestretchingu (¹prÏednatazÏenõª) vliv nauÂrovenÏ vyÂchozõÂch sil ajejich pokles ve srovnaÂnõ s rÏetõÂzky, u kteryÂch prestretching proveden nebyl.

MateriaÂl

Nejprve byl analyzovaÂn a meÏrÏen uÂbytek sõÂly v cÏa se po natazÏenõÂ u 18 typuÊ ortodontickyÂch rÏetõÂzkuÊ od 7 ruÊz-nyÂch vyÂrobcuÊ: Dentaurum, Ispringen, Germany; Ame-rican Orthodontics, Sheboygan, WIS, USA; G&H Wire Company, Franklin, IN, USA; Ormco, Glendora, CA, USA; Highland Metals Inc., San Jose, CA, USA; Ortho-Care /UK/ Ltd, Saltaire, UK; 3M/Unitek, Monro-via, CA, USA.

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each other, we set out the limit of 0.9 mm according to which a chain was classified as the one with short or long connecting link.

Seven chains were in the group of chains without connecting link: Dyna-Link Continuous (G&H), Plastic Chain Closed (AO), Memory Chain Closed (AO), Dura Chain Close Link (Ortho-Care), Power Chain Closed Space (Ormco), Alastik Chain CK Blue Met (3M/Uni-tek), Elasto Force Closed (Dentaurum).

Four chains were in the group of chains with a short connecting link: Dyna-Link Short (G&H) - the connec-ting link length 0.2 mm; Chain Elastic Short (Highland Metals) - the connecting link length 0.2 mm; Plastic Chain Short (AO) - the connecting link length 0.3 mm; Power Chain Open Space (Ormco) - the connecting link length 0.5 mm.

Seven chains were in the group of chains with a long connecting link: Dyna-Link Long (G&H) - the connec-ting link length 0.9 mm; Plastic Chain Long (AO) - the connecting link length 0.9 mm; Elasto Force Connec-tor (Dentaurum) - the connecting link length 1 mm; Dura Chain Medium Link (Ortho-Care) - the connecting link length 1.2 mm; Power Chain Wide Space (Ormco) -the connecting link length 1.7 mm; Memory Chain Long (AO) - the connecting link length 1.8 mm; Dura Chain Wide Link (Ortho-Care) - the connecting link length 1.8 mm.

Individual chains may differ in a unit shape; there-fore the shapes of units of all the chains included in the experiment were analyzed. In all chains the units were circular, and the crossection oblong.

Further effects of prestretching on the amount of the initial force and its subsequent decrease over a period of time were evaluated. For this experiment three chains by the same manufacturer were used: Power Chain Closed Space, Power Chain Open Space, and Power Chain Wide Space by Ormco.

Methods

For the first experiment there were 20 samples of each chain, of 11.5 units. Samples were cut with a lancet. On one end of the sample a half of a unit was left in order that a lancet would not damage an outside unit that was atta-ched to the steel pin for stretching. On the other side a whole unit was left, that was used for measurements -a dyn-amometer w-as inserted into it. S-amples were divi-ded into two groups of 10. The samples in the first group were stretched by 50% of their initial length, in the second group by 100%. First, in each chain we measured the length of ten units before elongation. The length of each chain after stretching by 50%, and 100% was calculated. To stretch all samples, measurement plastic (PHS) plates were produced width of 3 mm. At the calculated distan-ces corresponding to agiven stretching, holes were pred-Soubor 18 rÏetõÂzkuÊ byl rozdeÏlen podle toho,

zdarÏetõÂ-zek meÏl cÏi nemeÏl mezicÏlaÂnek adaÂle podle deÂlky mezi-cÏlaÂnku. ProtozÏe deÂlkamezicÏlaÂnkuÊ aslovnõÂ oznacÏenõÂ rÏe-tõÂzkuÊ jednotlivyÂch vyÂrobcuÊ se lisÏõÂ avzaÂjemneÏ si neod-povõÂdaÂ, byla stanovena hranice 0,9 mm, kterou se rÏõÂdilo zarÏazenõÂ rÏetõÂzku bud'do skupiny rÏetõÂzkuÊ s kraÂtkyÂm nebo s dlouhyÂm mezicÏlaÂnkem.

Sedm rÏetõÂzkuÊ bylo zarÏazeno do skupiny rÏetõÂzkuÊ bez mezicÏlaÂnku: Dyna-Link Continuous (G&H), Plastic Chain Closed (AO), Memory Chain Closed (AO), Dura Chain Close Link (Ortho-Care), Power Chain Closed Space (Ormco), Alastik Chain CK Blue Met (3M/Uni-tek), Elasto Force Closed (Dentaurum).

CÏtyrÏi rÏetõÂzky byly zarÏazeny do skupiny rÏetõÂzkuÊ s kraÂt-kyÂm mezicÏlaÂnkem: Dyna-Link Short (G&H) - deÂlka mezicÏlaÂnku 0,2 mm, Chain Elastic Short (Highland Me-tals) - deÂlkamezicÏlaÂnku 0,2 mm, Plastic Chain Short (AO) - deÂlkamezicÏlaÂnku 0,3 mm, Power Chain Open Space (Ormco) - deÂlkamezicÏlaÂnku 0,5 mm.

Sedm rÏetõÂzkuÊ bylo zarÏazeno do skupiny rÏetõÂzkuÊ s dlou-hyÂm mezicÏlaÂnkem: Dyna-Link Long (G&H) - cÏlaÂnku 0,9 mm, Plastic Chain Long (AO) - deÂlkamezicÏlaÂnku 0,9 mm, Elasto Force Connector (Dentaurum) -deÂlkamezicÏlaÂnku 1 mm, DuraChain Medium Link (Ortho-Care) - deÂlkamezicÏlaÂnku 1,2 mm, Power Chain Wide Space (Ormco) - deÂlkamezicÏlaÂnku 1,7 mm, Memory Chain Long (AO) - deÂlkamezicÏlaÂnku 1,8 mm, DuraChain Wide Link (Ortho-Care) - deÂlkamezicÏlaÂnku 1,8 mm.

DaÂle byla provedena analyÂzatvaru ocÏek u vsÏech rÏe-tõÂzkuÊ zahrnutyÂch do experimentu, protozÏe jednotlive rÏetõÂzky se mohou lisÏit i tvarem ocÏek. Bylo zjisÏteÏno, zÏe vsÏechnaocÏka u vsÏech rÏetõÂzkuÊ majõ kruhovy tvar. Take byl porovnaÂn tvar pruÊrÏezu rÏetõÂzku. VsÏechny zkoumane rÏetõÂzky majõ obdeÂlnõÂkovy tvar pruÊrÏezu.

DalsÏõÂm uÂkolem bylo zhodnotit vliv prestretchingu elastickeÂho rÏetõÂzku nahodnotu vyÂchozõ sõÂly ajejõ na -sledny pokles v cÏase. V tomto experimentu byly pou-zÏity trÏi rÏetõÂzky od jednoho vyÂrobce: Power Chain Clo-sed Space, Power Chain Open Space a Power Chain Wide Space, vsÏechny od firmy Ormco.

Metodika

K prvnõÂmu experimentu bylo od kazÏdeÂho rÏetõÂzku zhotoveno 20 vzorkuÊ v deÂlce 11 apuÊl ocÏek. Vzorky byly oddeÏleny bez natazÏenõ pomocõ skalpelu. Na jedne straneÏ vzorku bylaponecha napolovinaocÏka, aby prÏi oddeÏlovaÂnõ skalpelem nedosÏlo k posÏkozenõ krajnõÂho ocÏka, ktere se v experimentu navleÂkalo na ocelovy trn urcÏeny k natazÏenõ rÏetõÂzku. Nadruhe straneÏ vzorku bylo ponechaÂno ocÏko celeÂ. To pak slouzÏilo k meÏrÏenõ ado neÏj byl zasouvaÂn hrot silomeÏru. Vzorky byly rozdeÏ-leny do dvou skupin po 10 kusech v kazÏde skupineÏ. Bylo rozhodnuto, zÏe v prvnõ skupineÏ budou vzorky jed-noraÂzoveÏ natazÏeny o 50 % jejich puÊvodnõ deÂlky ave

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rilled into which steel pins, the diameter of 1.1 mm, were inserted (they served for a sample attachment). The first unit of a chain was attached to one steel pin, the chain was stretched to the set distance with the help of a clamp, and the tenth unit of the chain was attached to the other steel pin. Measurements were performed at the given time intervals: 0, 2, 8, 24 hours, 7 days, and 21 days. There were 2160 measurements performed.

The experiment was carried out in vitro under speci-fic conditions. After stretching on measurement pla-tes, the chains were put into polyfunctional incubator CulturaM, fy ALMEDICA AG, Guglera1, 1735 Giffers, Switzerland. During the experiment there was the con-stant temperature 37°C, the accuracy of a thermome-ter±1°C. The experiment was performed in a humid environment. The humidity was ensured with a band of gauze that was moistened every 12 hours with 20 ml of water of room temperature.

To measure forces induced by stretched chains at individual intervals a manual analogue force gauge SHITO DGD-8 was used, the range of measurement 0 - 600 g, the accuracy of 2%, fy Huatest Electronic Co., Ltd., Shenzhen, China. The point of the force gauge was inserted into the last (eleventh) chain unit. The force was read at the moment when the tenth unit did not touch the steel pin, and the pin was in the mid-dle of the unit (Fig. 1).

For the second experiment 20 samples were prepa-red, the length of 11.5 units, of all the three chains, and they were divided into two groups of 10 samples each. Again, the chains were stretched on the measuring plate - elongated by 50% and by 100% of the initial length. Prior to that, ¹prestretchª to 180% of the initial length was carried out 5 times in a row. On the measure plate the line representing 180% of the original length of ten units was drawn. An outside unit was attached to the steel pin. The end of the chain (the eleventh unit) was held with a clamp, and the chain was 5 times stret-ched according to the line drawn. This prestretching druhe skupineÏ o 100 % puÊvodnõ deÂlky. Nejprve byla

u ka zÏdeÂho rÏetõÂzku zmeÏrÏenadeÂlkadeseti ocÏek v klidu prÏed natazÏenõÂm. BylaspocÏõÂtaÂna deÂlkakazÏdeÂho rÏetõÂzku prÏi protazÏenõ o 50 % jejich puÊvodnõÂdeÂlky aprÏi protazÏenõ o 100 % puÊvodnõ deÂlky. Pro standardnõ natazÏenõ vsÏech vzorkuÊ byly vyrobeny meÏrÏõÂcõ plastove desticÏky (mate-riaÂl PSH) tlousÏt'ky 3 mm. Ve vypocÏtenyÂch vzdaÂleno-stech odpovõÂdajõÂcõÂch zvoleneÂmu natazÏenõ byly prÏed-vrtaÂny otvory ado nich byly zafixova ny ocelove trny o pruÊmeÏru 1,1 mm pro uchycenõ vzorkuÊ. Prvnõ ocÏko rÏe-tõÂzku bylo navlecÏeno najeden ocelovy trn, pomocõ peaÂnu byl rÏetõÂzek jednoraÂzoveÏ protazÏen do nastavene vzdaÂlenosti adesa te ocÏko rÏetõÂzku bylo navlecÏeno na druhy ocelovy trn. MeÏrÏenõ u vsÏech rÏetõÂzkuÊ probeÏhlo ve zvolenyÂch cÏasovyÂch intervalech: 0, 2, 8, 24 hodin, 7 dnuÊ a21 dnuÊ. CelkoveÏ probeÏhlo 2160 meÏrÏenõÂ.

Cely experiment probõÂhal in vitro za prÏesneÏ specifi-kovanyÂch podmõÂnek. RÏetõÂzky, po natazÏenõ nameÏrÏõÂcõÂch desticÏkaÂch, byly vlozÏeny do polyfunkcÏnõÂho inkubaÂtoru CulturaM, vyÂrobce ALMEDICA AG, Guglera1, 1735 Giffers, Switzerland. Po celou dobu experimentu byla zajisÏteÏnakonstantnõ teplota37°C, teplotnõ prÏesnost te-plomeÏru je±1°C. Cely experiment probõÂhal za vlhka. Vlhkost prostrÏedõ zajisÏt'oval pruh gaÂzy vlhcÏeny ka zÏdyÂch 12 hodin 20 ml vody pokojove teploty.

K meÏrÏenõ sõÂly, kterou produkovaly natazÏene rÏetõÂzky v jednotlivyÂch cÏasovyÂch intervalech, byl pouzÏit rucÏnõ analogovy silomeÏr SHITO DGD-8 s meÏrÏõÂcõÂm rozsahem 0-600 gramuÊ s prÏesnostõ 2 %, vyÂrobce Huatest Elec-tronic Co, Ltd, Shenzhen, China. MeÏrÏenõ bylo prove-deno tak, zÏe hrot silomeÏru byl zasunut do poslednõÂho jedenaÂcteÂho ocÏka rÏetõÂzku. HodnotasõÂly bylaodecÏtena v okamzÏiku, kdy se desaÂte ocÏko rÏetõÂzku nedotyÂkalo oceloveÂho trnu atrn byl v okamzÏiku meÏrÏenõ uprostrÏed ocÏka(Obr. 1).

Pro druhy experiment bylo zhotoveno 20 vzorkuÊ v deÂlce 11 apuÊl ocÏkaze vsÏech trÏõ rÏetõÂzkuÊ abyly rozdeÏ-leny do dvou skupin po 10 kusech v kazÏde skupineÏ. RÏetõÂzky byly opeÏt natazÏeny nameÏrÏõÂcõ desticÏku navzdaÂ-lenost odpovõÂdajõÂcõ protazÏenõ o 50 ao 100 % puÊvodnõ deÂlky. PrÏed tõÂmto natazÏenõÂm bylo nejprve provedeno ªprÏednatazÏenõª rÏetõÂzkuÊ na180 % puÊvodnõ deÂlky 5x po sobeÏ. NameÏrÏõÂcõ desticÏku bylazakreslenalinie odpovõÂ-dajõÂcõ 180 % vyÂchozõ deÂlky deseti ocÏek rÏetõÂzku. Krajnõ ocÏko bylo navlecÏeno naocelovy trn. PeaÂnem byl za-chycen konec rÏetõÂzku (jedenaÂcte ocÏko) a5x po sobeÏ byl rÏetõÂzek natazÏen k vyznacÏene linii. Toto 5x opako-vane ªprÏednatazÏenõª trvalo celkem 5 vterÏin. Pote byly rÏetõÂzky definitivneÏ umõÂsteÏny do pozic avzdaÂlenostõ od-povõÂdajõÂcõÂch stanovenyÂm podmõÂnkaÂm experimentu. MeÏrÏenõ probõÂhalo podle stejneÂho protokolu jako u prvnõÂho experimentu. V druheÂm experimentu bylo

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took 5 seconds. Then the chains were placed into po-sitions and distances according to the set conditions of the experiment. The measurement followed the same protocol as in the first experiment. There were 360 measurements performed.

All measurements were statistically processed. First, the data obtained were described by means of descriptive statistics (arithmetic mean, median, mini-mum and maximini-mum values, standard deviation). Fur-ther the individual sets of measured values were tested for normal distribution. In case the values showed nor-mal distribution, they were subjected to parametric tests (Student's t-tests, pair t-tests with Bonferroni's correction), otherwise they were subject of non-para-metric tests (Friedman's test, Wilcoxon tests with Bonferroni's correction, Mann-Whitney U-tests). The results were plotted in tables, statistically significant values on the level of statistical significance of 0.05 were highlighted.

Results

The results of all measurements of chain induced forces at given intervals in both experiments were plot-ted in tables and graphs, and processed with Microsoft Excel software. Arithmetic means for each force mea-surement and percentage of force degeneration were calculated.

After stretching of chains by 50% of their initial length, the initial values of forces at time 0 fluctuated between 2.5 N (Dura Chain Wide Link, Ortho-Care) and 5.8 N (Dyna Link Continunous, G&H). After 21 days the range was between 1.5 N (Dura Chain Wide Link, Ortho-Care) and 2.4 N (Dyna Link Continuous, G&H; Power Chain Closed Space, Ormco; Plastic Chain, AO; Elasto Force with connector, Dentaurum). The VsÏechnameÏrÏenõÂ byla statisticky zhodnocena. Nejprve

byly hodnoty popsaÂny pomocõ popisne statistiky (arit-meticky pruÊmeÏr, mediaÂn, minimaÂlnõ amaxima lnõ hod-noty, smeÏrodatna odchylka). NaÂsledneÏ byly jednotlive soubory nameÏrÏenyÂch hodnot testovaÂny nanormaÂlnõ roz-lozÏenõÂ. Pokud meÏly hodnoty normaÂlnõ rozlozÏenõÂ, byly podrobeny parametrickyÂm testuÊm (Studentovy t-testy, paÂrove t-testy s Bonferroniho korekcõÂ), v opacÏneÂm prÏõÂ-padeÏ byly podrobeny testuÊm neparametrickyÂm (Fried-manuÊv test, Wilcoxonovy testy s Bonferroniho korekcõÂ, Mann-Whitney U-testy). VyÂsledky byly zpracovaÂny do tabulek s vyznacÏenõÂm statisticky vyÂznamnyÂch hodnot nahladineÏ statisticke vyÂznamnosti 0,05.

VyÂsledky

VyÂsledky vsÏech provedenyÂch meÏrÏenõ sõÂly produko-vane rÏetõÂzky ve zvolenyÂch cÏasovyÂch intervalech v prvnõÂm i druheÂm experimentu byly zaznamenaÂny do tabulek a grafuÊ azpracova ny v programu Microsoft Excel. Byly vypocÏteny aritmeticke pruÊmeÏry pro kazÏde meÏrÏenõ sõÂly aprocentuaÂlnõ poklesy sil.

PrÏi protazÏenõÂ rÏetõÂzkuÊ o 50 % puÊvodnõÂ deÂlky se pocÏaÂ-tecÏnõÂ hodnoty sil v cÏase 0 pohybovaly v rozmezõÂ od 2,5 N (Dura Chain Wide Link, Ortho-Care) do 5,8 N (Dyna Link Continuous, G&H). Po 21 dnech bylo rozmezõÂ sil produkovanyÂch rÏetõÂzky od 1,5 N (DuraChain Wide Link, Ortho-Care) do 2,4 N (Dyna Link Continuous, G&H; Power Chain closed space, Ormco; Plastic chain, AO; Elasto Force with connector, Dentaurum). ProcentuaÂlnõÂ pokles sil se pohyboval od 24 % do 62 % po 24 hodinaÂch aod 31 % do 70 % po 21 dnech. NejmensÏõÂ procentuaÂlnõÂ pokles sil s ohledem navyÂchozõÂ poskytovanou uÂrovenÏ sõÂly meÏly rÏetõÂzky Memory chain closed, AO; Elasto Force with connector, Dentaurum a Dura Chain close link, Orthocare.

Tab. 1.PruÊmeÏrne hodnoty sil aprocentuaÂlnõ poklesy sil pro protazÏenõ o 50 % puÊvodnõ deÂlky ve zvolenyÂch cÏasovyÂch intervalech Tab. 1.Mean values of forces and forces degeneration in per cents after stretching by 50% of the initial length at given time intervals

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percentage of force degeneration oscillated between 24 and 62 per cents after 24 hours and between 31 and 70 per cents after 21 days. The least force degene-ration in per cents was recorded for Memory Chain Closed, AO; Elasto Force with connector, Dentaurum; and Dura Chain close link, Orthocare.

In Figure 2 we can observe an obvious distribution of initial values of forces induced by elastic chains. Shapes of curves for all chains as well as their decline are very similar. The steepest shape is observed within the first 24 hours.

After stretching of chains by 100% of their initial length, the initial values of forces induced by chains at time 0 fluctuated between 3.2 N (Dura Chain Wide Link, Ortho-Care) and 6N (Dyna Link Continuous, G&H). After 21 days the range was between 1.6 N (Dura Chain Wide Link, Ortho-Care) and 3.2 N (Dyna Link Continuous, G&H). The percentage of force degeneration oscillated between 28 and 60 per cents after 24 hours and bet-Naobr. 2 je patrny rozptyl vyÂchozõÂch hodnot sil

pro-dukovanyÂch elastickyÂmi rÏetõÂzky. PruÊbeÏh krÏivek pro vsÏechny rÏetõÂzky i charakter jejich poklesu je si velmi podobnyÂ. NejstrmeÏjsÏõÂpruÊbeÏh je v prvnõÂch 24 hodinaÂch. PrÏi protazÏenõÂ o 100 % puÊvodnõÂ deÂlky se pocÏaÂtecÏnõÂ hodnoty sil produkovanyÂch v cÏase 0 pohybovaly v roz-mezõÂ od 3,2 N (Dura Chain Wide Link, Ortho-Care) do 6 N (DynaLink Continuous, G&H). Po 21 dnech bylo roz-mezõÂ sil produkovanyÂch rÏetõÂzky od 1,6 N (DuraChain Wide Link, Ortho-Care) do 3,2 N (Dyna Link Conti-nuous, G&H). ProcentuaÂlnõÂ pokles sil se pohyboval od 28 % do 60 % po 24 hodinaÂch aod 39 % do 68 % po 21 dnech. NejmensÏõÂ procentuaÂlnõÂ ztraÂtu sil oproti vyÂchozõÂ sõÂle meÏly rÏetõÂzky Power Chain open space, Ormco; Memory chain closed, AO a Elasto Force with connector, Dentaurum.

Naobr. 3 je videÏt podobny charakter poklesu sõÂly apruÊbeÏhu krÏivek jako prÏi protazÏenõ o 50 %, opeÏt s nej-strmeÏjsÏõÂm poklesem v prvnõÂch 24 hodinaÂch.

KrÏivkapo-Obr. 2.Pokles pruÊmeÏrnyÂch sil v cÏase prÏi protazÏenõÂ o 50 % vyÂchozõÂ deÂlky rÏetõÂzku

Fig. 2.Mean forces degeneration over a period of time after stret-ching by 50% of the initial length

Obr. 3.Poklesu pruÊmeÏrnyÂch sil v cÏase prÏi protazÏenõÂ o 100 % vyÂchozõÂ deÂlky rÏetõÂzku

Fig. 3.Mean forces degeneration over a period of time after stret-ching by 100% of the initial length

Tab. 2.PruÊmeÏrne hodnoty sil aprocentuaÂlnõ poklesy sil pro protazÏenõ o 100 % puÊvodnõ deÂlky ve zvolenyÂch cÏasovyÂch intervalech Tab. 2.Mean values of forces and forces degeneration in per cents after stretching by 100% of the initial length at given time intervals

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ween 39 and 68 per cents after 21 days. The least force decrease in per cents was recorded for Power Chain Open Space, Ormco; Memory Chain Closed, AO; and Elasto Force with connector, Dentaurum).

In Figure 3 we can observe similar force degeneration as in stretching by 50%; the steepest shape is observed within the first 24 hours. However, the shape of curve re-presenting force degeneration of one chain is different. Af-ter stretching by 100%, forces of this chain decline slowly, and at the end of the experiment they are still at 3.2 N.

In the experiment on prestretching, mean forces va-lues and percentage of forces degeneration after stret-ching by 50 and 100% of the initial length of chains with prestretching were plotted in Table 3; there were plotted also values for the same brands of chains that did not undergo prestretching.

klesu sõÂly jednoho rÏetõÂzku se ale svyÂm pruÊbeÏhem vy-mykaÂ. PrÏi 100 % protazÏenõÂ hodnoty sil tohoto rÏetõÂzku klesajõÂ pomalu a i v zaÂveÏru experimentu se pohybujõÂ nahodnoteÏ 3,2 N.

V experimentu zkoumajõÂcõÂm vliv prestretchingu byly do tabulky 3 zapsaÂny pruÊmeÏrne hodnoty sil aprocen-tuaÂlnõ poklesy sil prÏi protazÏenõ o 50 % a100 % vyÂchozõ deÂlky rÏetõÂzkuÊ s prestretchingem ahodnoty pro tyteÂzÏ rÏe-tõÂzky, ktere prestretchingem neprosÏly.

Z pruÊbeÏhu krÏivek (obr. 4) lze usuzovat, zÏe provedeny prestretching, prÏi naÂsledneÂm protazÏenõ o 50 % vyÂchozõ deÂlky, ovlivnil produkci sil rÏetõÂzkuÊ.

Dle pruÊbeÏhu krÏivek (obr. 5) muÊzÏeme v tomto prÏõÂpadeÏ usuzovat, zÏe provedeny prestretching, prÏi naÂsledneÂm protazÏenõ o 100 % vyÂchozõ deÂlky sõÂly produkovane rÏe-tõÂzky neovlivnil.

Tab. 3.PruÊmeÏrne hodnoty sil aprocentuaÂlnõ poklesy sil pro protazÏenõ o 50 % a100 % puÊvodnõ deÂlky ve zvolenyÂch cÏasovyÂch intervalech u rÏetõÂzkuÊ s prestretchingem abez neÏho (rÏetõÂzky s prestretchingem oznacÏeny barevneÏ av oznacÏenõ rÏetõÂzku jsou oznacÏeny P ( CP, MP, WP) ) Tab. 3.Mean values of forces and forces degeneration in per cents after stretching by 50% and 100% of the initial length at given time in-tervals for the chains with prestretching, and without prestretching. Chains with prestretching have the letter P in the name/acronym of their type (CP, MP, WP).

Obr. 4.RÏetõÂzky s prestretchingem abez neÏho prÏi vyÂchozõÂm protazÏenõÂ o 50 % vyÂchozõÂ deÂlky. RÏetõÂzky s prestretchingem jsou oznacÏeny prÏi-daÂnõÂm P v typu ( CP, MP, WP).

Fig. 4.Chains with and without prestretching at stretching by 50% of the initial length. Chains with prestretching have the letter P in the name/acronym of their type (CP, MP, WP).

Obr. 5.RÏetõÂzky s prestretchingem abez neÏho prÏi vyÂchozõÂm protazÏenõÂ o 100% vyÂchozõÂ deÂlky. RÏetõÂzky s prestretchingem jsou oznacÏeny prÏi-daÂnõÂm P v typu ( CP, MP, WP).

Fig. 5.Chains with and without prestretching at stretching by 100% of the initial length. Chains with prestretching have the letter P in the name/acronym of their type (CP, MP, WP).

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Shapes of curves (Fig.4) suggest that prestretching affected forces induced by chains.

Shapes of curves (Fig.5) suggest that prestretching did not affect forces produced by chains.

Summary of statistical evaluation

For both elongations (by 50 and 100 per cents) it was proved that the mean force decreases over a pe-riod of time, the value (in per cents) of force degenera-tion increases, and the rate of force degeneradegenera-tion dec-reases with time, and that the force degeneration is the greatest within the first 24 hours. Comparison of mean forces stretched by 50% and mean forces stretched by 100% showed that mean forces value was always statistically higher in case of stretching by 100%. In case of 100% stretching the rate of force degeneration was significantly higher only at the interval 0-2 hours. The effect of connecting link length on the delivery of force by elastic chains was not proved. There is only a tendency suggesting that mean force value for both stretchings is significantly lower in chains with a long connecting link (unlike in chains with a short connec-ting link or chains without a connecconnec-ting link). In case of stretching by 100% there was recorded a tendency to more rapid force degeneration in chains without aconnecting link.

Comparison of forces in chains with or without pres-tretching proved significant percentage of force dege-neration in chains without prestretching (they were stret-ched by 50% at all intervals). Mean forces values and the rate of force degeneration were significantly higher only at time 0 in chains without prestretching. In case of stret-ching by 100% there was not proved any impact of pres-tretching with regard either to mean forces value or per-centage of forces decline at all intervals. The rate of force degeneration was significantly higher only at inter-val 2-8 hours in chains with prestretching.

Discussion

Elastic chains bring a number of advantages for orthodontists - they are easy to use, the cooperation of a patient is not necessary, and they are relatively cheap. However, their application may also have weak points: after they are inserted into mouth cavity they ab-sorb water and saliva, they change in colour, and their internal bonds may be damaged which results in their permanent deformation. Moreover, on the surface of chains there occurs ion precipitation and protein film is formed. That leads to rapid force degeneration resulting in a gradual decrease of effectiveness [2, 15, 16, 17]. Due to this loss, it is difficult for an orthodontist to deter-mine actual force delivered on teeth. In addition, it is ne-cessary to judge the force individually according to indi-vidual patients' mouth cavity conditions. Despite the Souhrn statistickeÂho hodnocenõÂ

Pro obeÏ vyÂchozõ protazÏenõ (50 i 100 %), bylo signifi-kantneÏ prokaÂzaÂno, zÏe pruÊmeÏrna sõÂla v cÏase klesaÂ, stoupa hodnotaprocentuaÂlnõÂho poklesu sõÂly arychlost poklesu sõÂly v cÏase klesaÂ, prÏicÏemzÏ v prvnõÂch 24 hodi-naÂch je pokles sõÂly nejveÏtsÏõÂ. PrÏi vzaÂjemneÂm porovnaÂnõ pruÊmeÏrnyÂch sil prÏi protazÏenõ o 50 % se silami prÏi prota-zÏenõ o 100 % bylave vsÏech cÏasech statisticky vyÂ-znamneÏ vysÏsÏõ hodnotapruÊmeÏrnyÂch sil prÏi protazÏenõ o 100 % vyÂchozõ deÂlky rÏetõÂzku. Rychlost poklesu sõÂly bylasignifikantneÏ vysÏsÏõ prÏi protazÏenõ o 100% pouze v cÏasoveÂm intervalu 0-2 hodiny.

PrÏi posouzenõ vlivu deÂlky mezicÏlaÂnku naprodukci sil elastickyÂmi rÏetõÂzky se nepodarÏilo jednoznacÏneÏ prokaÂ-zat, zÏe deÂlkamezicÏlaÂnku tuto produkci ovlivnõÂ. Byl po-uze zachycen trend, ktery naznacÏuje, zÏe hodnotapruÊ-meÏrnyÂch sil pro obeÏ protazÏenõ je statisticky vyÂznamneÏ nizÏsÏõ u rÏetõÂzkuÊ s dlouhyÂm mezicÏlaÂnkem oproti rÏetõÂzkuÊm s kraÂtkyÂm mezicÏlaÂnkem abez mezicÏlaÂnku. PrÏi prota-zÏenõ o 100 % vyÂchozõ deÂlky rÏetõÂzku pak byl zachycen i trend vysÏsÏõ rychlosti poklesu sõÂly u rÏetõÂzku bez mezi-cÏlaÂnku.

PrÏi srovnaÂnõ sil rÏetõÂzkuÊ, u kteryÂch byl proveden pres-tretching s teÏmi, u kteryÂch prespres-tretching proveden ne-byl, byl prokaÂzaÂn prÏi protazÏenõÂo 50 % vyÂchozõ deÂlky rÏe-tõÂzku ve vsÏech cÏasech statisticky vyÂznamny procen-tuaÂlnõ pokles sõÂly u rÏetõÂzkuÊ, ktere prestretching nepodstoupily. HodnotapruÊmeÏrnyÂch sil arychlost po-klesu sõÂly byly signifikantneÏ vysÏsÏõ pouze v cÏase 0 u rÏe-tõÂzkuÊ bez prestretchingu. PrÏi protazÏenõÂo 100 % vyÂchozõ deÂlky nebyl prokaÂzaÂn vliv prestretchingu ani na pruÊ-meÏrnou hodnotu sil ani na procentuaÂlnõ pokles sil ve vsÏech cÏasech. Rychlost poklesu sõÂly byla signifikantneÏ vysÏsÏõ pouze v intervalu 2-8 hodin u rÏetõÂzkuÊ s prestret-chingem.

Diskuse

PouzÏitõÂelastickyÂch rÏetõÂzkuÊ skyÂta ortodontistuÊm rÏadu vyÂhod. Pro jejich pouzÏitõ hovorÏõ jednoznacÏneÏ jejich jednoducha aplikovatelnost, nenõ nutna spolupraÂce pa -cientaajsou relativneÏ levneÂ. Jejich uzÏitõ ma i nevyÂhody. Po vlozÏenõ do dutiny uÂstnõ absorbujõ vodu a slinu, zbar-vujõ se adochaÂzõ k posÏkozenõ vnitrÏnõÂch vazeb vedoucõ k trvale deformaci aplikovanyÂch rÏetõÂzkuÊ. NavõÂc na po-vrchu rÏetõÂzkuÊ dochaÂzõ k precipitaci iontuÊ atvorbeÏ pro-teinoveÂho filmu. Z toho pak pramenõ rychla ztraÂtasõÂly vedoucõ k postupneÂmu snizÏovaÂnõ efektivity [2, 15, 16, 17]. KvuÊli teÂto ztraÂteÏ je pro ortodontistu obtõÂzÏne urcÏit aktuaÂlnõ sõÂlu prÏenaÂsÏenou nazuby. NavõÂc je sõÂlu nutno posuzovat individuaÂlneÏ dle prostrÏedõ dutiny uÂstnõ jed-notlivyÂch pacientuÊ.

Navzdory probõÂhajõÂcõÂm debataÂm, zda lze aplikovat vyÂsledky experimentuÊ provedenyÂch in vitro napod-mõÂnky dutiny uÂstnõÂ, kde musõÂme vzõÂt v uÂvahu dalsÏõÂ vlivy

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current debate on application of results of the experi-ments carried out in vitro in the conditions of mouth ca-vity, where we have to consider other impacts, e.g. mic-robial flora, enzymes, nutrition, etc., the experiment car-ried out in vitro has many advantages with regard to materials characteristics - this is possible thanks to standardized conditions. The study comparing results of the experiment performed in vitro in the open air and the experiment carried out in vivo showed relatively great differences [8, 10, 12]. On the contrary, compari-son of results of the experiment performed in vitro in a damp environment and the experiment done in vivo showed only slight differences [8].

Results of the experimental part of the presented study proved a great variability in the value of the initial force induced by elastic chains - it is due to the type and manufacturer of a given chain. When stretched by 50% of the initial length, the force values were between 2.5 N and 5.8 N; when stretched by 100% of the initial length, the values oscillated between 3.2 N and 6 N. This agrees with the results of many studies published [3, 4, 5, 6, 7, 8, 10, 11, 12, 13]. We also proved and specified the force degeneration over a period of time; the rate of the force degeneration is higher within the first 24 hours, and then it gradually decreases during the whole period of 3 weeks. This conclusion agrees with the lite-rature available [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]. Accor-ding to the published works alonger testing period sho-wed only aminute decline in the amount of the force in-duced by chains; the longest testing period took 100 days [14]. For our clinical practice this means that the force delivered by elastic chain measured at the interval of 4-6 weeks during apatient's check-up should be very close to the value obtained after 3 weeks.

The views on the impact of a connecting link length on the force induced by elastic chains are not uniform. E.g. De Genova, Stevenson, and Kusy state that chains with a short connecting link produce bigger force than those with a long connecting link [9, 14]. Our experiment did not prove the relationship between force delivery and the connecting link length. However, there is atendency suggesting that the forces induced by chains without aconnecting link or with ashort con-necting link are bigger than those induced by chains with a long connecting link. Eliades et al. conclude that chains with a short connecting link or without a con-necting link lose the force more quickly because a resi-dual strain in the module concentrates into a chain unit which makes the module weaker, and additionally, there is also a higher risk of damage to the unit during the process of its attachment to a bracket. This is ano-ther risk factor that may affect delivery of forces [2, 16]. The views on the effect of prestretching on the pro-duction of forces by elastic chains are even more con-jako je mikrobiaÂlnõ flora, enzymy, vyÂzÏivove faktory atd.,

ma experiment provedeny in vitro mnoho vyÂhod, po-kud jde o charakteristiky materiaÂluÊ dõÂky mozÏnosti stan-dardizace prostrÏedõÂ. Studie srovnaÂvajõÂcõ vyÂsledky experimentu provedeneÂho in vitro navzduchu aprove-deneÂho in vivo ukaÂzaly pomeÏrneÏ velke rozdõÂly [8, 10, 12]; naopak srovnaÂnõ vyÂsledkuÊ experimentu provede-neÂho in vitro ve vlhkeÂm prostrÏedõ aexperimentu prove-deneÂho in vivo ukaÂzalo rozdõÂly nepatrne [8].

VyÂsledky experimentaÂlnõ cÏaÂsti teÂto praÂce potvrdily velkou variabilitu uÂrovneÏ vyÂchozõ sõÂly produkovane ela-stickyÂmi rÏetõÂzky danou typem rÏetõÂzku avyÂrobcem. PrÏi natazÏenõ o 50 % vyÂchozõ deÂlky se hodnoty sõÂly pohybo-valy v rozsahu 2,5 N do 5,8 N a prÏi natazÏenõ o 100 % vyÂ-chozõ deÂlky v rozsahu 3,2 N do 6 N.Toto zjisÏteÏnõ je v souladu s vyÂsledky mnohapublikovanyÂch studiõ [3, 4, 5, 6, 7, 8, 10, 11, 12, 13]. RovneÏzÏ se naÂm podarÏilo prokaÂzat a uprÏesnit uÂbytek produkovane sõÂly v cÏase, prÏicÏemzÏ rychlost poklesu sõÂly produkovane rÏetõÂzky je nejveÏtsÏõ v prvnõÂch 24 hodinaÂch a daÂle se v cÏase snizÏuje po celou dobu testovacõÂho obdobõ 3 tyÂdnuÊ. Take tento poznatek se shoduje s dostupnou literaturou [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]. Dle dostupne literatury delsÏõ te-stovacõ obdobõ ukaÂzalo jen nepatrny pokles uÂrovneÏ produkovane sõÂly, nejdelsÏõ sledovana testovacõ pe-riodabyla100 dnuÊ [14]. Pro nasÏõ klinickou praxi to zna-menaÂ, zÏe uÂrovenÏ sõÂly poskytovane elastickyÂm rÏetõÂzkem prÏi kontrole pacienta v intervalu 4-6 tyÂdnuÊ, by meÏla byÂt pravdeÏpodobneÏ velmi podobna sõÂle nameÏrÏene po 3 tyÂdnech.

Pokud se zabyÂvaÂme posouzenõÂm vlivu deÂlky mezi-cÏlaÂnku nasõÂlu produkovanou elastickyÂmi rÏetõÂzky, zde jizÏ naÂzory jednotne nejsou. NaprÏõÂklad De Genova, Ste-venson aKusy ve svyÂch studiõÂch potvrdili, zÏe rÏetõÂzky s kraÂtkyÂm mezicÏlaÂnkem produkujõ vysÏsÏõ uÂrovenÏ sil nezÏ rÏetõÂzky s dlouhyÂm mezicÏlaÂnkem [9, 14]. V nasÏem experimentu se naÂm jednoznacÏneÏ nepodarÏilo prokaÂza t ovlivneÏnõ produkce sil deÂlkou mezicÏlaÂnku. Pouze jsme zachytily trend, ktery naznacÏil, zÏe sõÂly produkovane rÏe-tõÂzky bez mezicÏlaÂnku as kraÂtkyÂm mezicÏlaÂnkem jsou vy-sÏsÏõ nezÏ ty, ktere produkujõ rÏetõÂzky s dlouhyÂm kem. Eliades a kol. tvrdõÂ, zÏe rÏetõÂzky s kraÂtkyÂm mezicÏlaÂn-kem abez mezicÏlaÂnku budou ztraÂcet sõÂlu rychleji, protozÏe zbytkove napeÏtõ v modulu se po jeho natazÏenõ koncentruje do ocÏka rÏetõÂzku, cozÏ ho oslabuje, a navõÂc je zde veÏtsÏõÂriziko posÏkozenõ ocÏka prÏi navleÂkaÂnõÂnazaÂmek. To je dalsÏõ rizikovy faktor, ktery muÊzÏe ovlivnit produkci sil [2, 16].

NaÂzory naefekt prestretchingu jakozÏto mozÏneÂho zpuÊsobu ovlivneÏnõ produkce sil elastickyÂmi rÏetõÂzky jsou jesÏteÏ võÂce kontroverznõÂ. VeÏtsÏinastudiõ dospeÏlake zji-sÏteÏnõÂ, zÏe prestretching vyÂslednou hodnotu sil produko-vanyÂch elastickyÂmi rÏetõÂzky neovlivnõÂ[18, 19, 20]. Pokud neÏktere z nich prestretching doporucÏily, nedokaÂzaly

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troversial. The majority of studies conclude that pres-tretching does not influence the resulting amount of force induced by elastic chains [18, 19, 20]. The works recommending prestretching did not give the extent of elongation [6]. Our experiment suggests that prestret-ching can influence resulting force delivery by chains only in case that the distance to which the chain is stretched is significantly shorter than the length of prestretching. At the same time, we can also alleviate the initial discomfort of a patient. Stevenson and Kusy maintain the same position [14].

With respect to the results of our experiment, it is inte-resting to assess the problem of chain induced forces transfer on a tooth as the means producing continual for-ces or, as Proffit states, interrupted forfor-ces [21]. It turned out that after stretching by 100% of the initial length, the force delivery by some chains reached values excee-ding ªsafeªchains application. In such cases the unwan-ted and undesirable effects of elastic chain might prevail. According to these results we would classify such forces induced by elastic chains as unsuitable.

Conclusion

1. The study proved that forces induced by chains stretched by 50 and 100% of their initial length are not identical; they differ considerably according to the chain type and brand. The values oscillate between 2.5 N and 5.8 N after 50% stretch and between 3.2 N and 6 N after 100% stretch.

2. The study proved that within the first 24 hours the forces induced by chains degenerate rapidly, and the rate of force degeneration slows down over a period of time. Comparison of the force degeneration after 50% stretching and 100% stretching showed significantly more rapid decline only at the interval 0-2 hours. The mean value of forces induced by chains is higher at all intervals after stretching by 100% of the initial length.

3. In chains stretched by 50 and 100% we could not prove the effect of aconnecting link length on the amount of the force induced or on the rate of its subse-quent decline.

4. Testing of prestretching effects on the amount of initial force and its subsequent decline over a period of time proved that the initial force in prestretching and subsequent stretching by 50% is lower than the initial force in chains that did not undergo prestretching. The following decline of force in time (given in per cents) is also lower in chains that underwent prestretching.

5. In chains without prestretching that were stretched by 100% of their initial length, there were found no signi-ficant differences in the initial amount of force or in force decline (given in per cents) over aperiod of time.

The above given findings may be reflected in the fol-lowing clinical recommendations:

stanovit mõÂru prestretchingu [6]. Dle nasÏeho experi-mentu se zdaÂ, zÏe prestretching ovlivnõ vyÂslednou pro-dukci sõÂly rÏetõÂzkuÊ, pokud vzdaÂlenost, nakterou rÏetõÂzek nataÂhneme, bude vyÂrazneÏ mensÏõ nezÏ vzdaÂlenost, na kterou jsme provedli prestretching. ZaÂrovenÏ tõÂm muÊ-zÏeme ovlivnit pocÏaÂtecÏnõ neprÏõÂjemne pocity vnõÂmane pacientem. PodobneÏ se k teÂto problematice ve sve studii stavõ Stevenson a Kusy [14].

S ohledem navyÂsledky experimentu je pomeÏrneÏ za -jõÂmave zhodnotit problematiku prÏenosu sil rÏetõÂzku na zub jako prostrÏedku produkujõÂcõÂho sõÂly kontinuaÂlnõ nebo, jak tvrdõ Proffit, sõÂly prÏerusÏovane [21]. UkaÂzalo se, zÏe prÏi protazÏenõ o 100 % vyÂchozõ deÂlky se produkce sil neÏkteryÂch rÏetõÂzkuÊ pohybuje v hodnotaÂch prÏesahujõÂ-cõÂch ªbezpecÏneª uzÏitõ rÏetõÂzkuÊ. PrÏi tomto protazÏenõ by mohly prÏevaÂzÏit nechteÏne anezÏaÂdoucõ uÂcÏinky elastic-keÂho rÏetõÂzku. Dle teÏchto vyÂsledkuÊ bychom tyto sõÂly pro-dukovane elastickyÂmi rÏetõÂzky zarÏadili do kategorie sil nevhodnyÂch.

ZaÂveÏr

1. PodarÏilo se prokaÂzat, zÏe sõÂly produkovane rÏetõÂzky protazÏenyÂmi o 50 a100 % jejich vyÂchozõ deÂlky nejsou stejne apodleÂhajõÂsÏiroke variabiliteÏ dle typu rÏetõÂzku avyÂ-robce. Pohybujõ se v rozmezõ 2,5 N - 5,8 N pro prota-zÏenõ o 50 % av rozmezõ 3,2 N - 6 N pro protaprota-zÏenõ o 100 % vyÂchozõ deÂlky rÏetõÂzkuÊ.

2. Bylo potvrzeno, zÏe beÏhem prvnõÂch 24 hodin do-chaÂzõÂk nejveÏtsÏõÂm ztraÂtaÂm sõÂly produkovane rÏetõÂzky ary-chlost poklesu sil v cÏase se daÂle snizÏuje. SrovnaÂme-li rychlosti uÂbytku sõÂly prÏi protazÏenõÂo 50 % aprÏi protazÏenõ o 100 %, statisticky vyÂznamneÏ vysÏsÏõ rychlost poklesu najdeme pouze v intervalu 0 - 2 hodiny. Velikost pruÊmeÏrneÏ puÊsobõÂcõÂch sil vyvinutyÂch rÏetõÂzky je ve vsÏech cÏa -sovyÂch intervalech vysÏsÏõ prÏi protazÏenõ o 100 % vyÂchozõ deÂlky.

3. PrÏi protazÏenõ rÏetõÂzkuÊ o 50 i 100 % jejich vyÂchozõ deÂlky se nepodarÏilo jednoznacÏneÏ prokaÂzat vliv deÂlky mezicÏlaÂnku nauÂrovenÏ produkovane sõÂly ani na rychlost jejõÂho naÂsledneÂho poklesu.

4. PrÏi testovaÂnõ vlivu prestretchingu nauÂrovenÏ vyÂ-chozõ sõÂly ajejõ naÂsledny pokles v cÏase se podarÏilo jed-noznacÏneÏ prokaÂzat, zÏe vyÂchozõ sõÂlaprÏi prestretchingu anaÂsledneÂm protazÏenõ o 50 % jejich vyÂchozõ deÂlky je nizÏsÏõ nezÏ vyÂchozõ sõÂlau rÏetõÂzkuÊ, ktere prestretching ne-podstoupily. NaÂsledny procentuaÂlnõ pokles sõÂly v cÏa se je pak rovneÏzÏ nizÏsÏõ u rÏetõÂzkuÊ, ktere prosÏly prestretchin-gem ve srovnaÂnõ s rÏetõÂzky, u kteryÂch jsme prestretching neprovedly.

5. U rÏetõÂzkuÊ, kde prestretching nebyl proveden anaÂ-sledneÏ byly protazÏeny o 100 % jejich vyÂchozõÂdeÂlky, ne-byly vyÂrazne rozdõÂly ani ve vyÂchozõ uÂrovni sõÂly ani v naÂ-sledneÂm procentuaÂlnõÂm poklesu sõÂly v cÏase.

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1. With regard to different amounts of the initial force induced by chains of different brands, the appro-priate chain should be chosen according to the effect required. The problem is not only an eventual overload of teeth that should alter their positions due to chain force delivery, but also the potential negative effect on the anchorage. Our aim is to move a tooth with as light force applied as possible, in order to eliminate po-tential risks of unwanted and undesirable effects ari-sing from the use of elastic chains.

2. It turned out that during excessive stretching some chains would produce forces that raise the risk significantly. Therefore, we should always be very cau-tious when treating our patients even if we use as com-mon a tool as an elastic chain.

3. In case we want to better control forces induced by elastic chains, we should set the initial amount of force with a force gauge, and then - depending on the data obtained - plan further check-ups of a patient. 4. Prestretching may positively affect the initial amount of the force applied, and thus can alleviate subjective discomfort of our patients, and the subse-quent force decline over aperiod of time will be less no-ticeable.

Acknowledgment: The authors want to express their gratitude to Mgr.KaterÏina LangovaÂ, Ph.D., for her help in statistical processing of the data.

The authors have no commercial, proprietary or financial interest in products or companies mentioned in the article.

Uvedena zjisÏteÏnõ mohou veÂst k teÏmto klinickyÂm do-porucÏenõÂm:

1. Vzhledem k rozdõÂlnosti uÂrovneÏ iniciaÂlnõÂsõÂly produ-kovane rÏetõÂzky ruÊznyÂch vyÂrobcuÊ zvaÂzÏit vyÂbeÏr odpovõÂ-dajõÂcõÂho rÏetõÂzku podle pozÏadovaneÂho uÂcÏinku. Nejde to-tizÏ pouze o prÏõÂpadne prÏetõÂzÏenõ zubuÊ, u nichzÏ ocÏekaÂ-vaÂme, zÏe puÊsobenõÂm rÏetõÂzku zmeÏnõ sve postavenõÂ, ale take o prÏõÂpadne negativnõ ovlivneÏnõ kotevnõ jednotky. NasÏõÂm cõÂlem je dosaÂhnout pohybu zubu co nejmensÏõÂ, avsÏa k uÂcÏinnou aplikovanou silou, abychom eliminovali v nejvysÏsÏõ mozÏne mõÂrÏe riziko nechteÏnyÂch anezÏaÂdou-cõÂch uÂcÏinkuÊ spojenyÂch s pouzÏitõÂm elastickyÂch rÏetõÂzkuÊ.

2. UkaÂzalo se, zÏe neÏktere rÏetõÂzky prÏi nadmeÏrneÂm na-tazÏenõ budou produkovat sõÂly, ktere toto riziko vyÂrazneÏ zvysÏujõÂ. Proto je namõÂsteÏ urcÏita obezrÏetnost prÏi leÂcÏbeÏ nasÏich pacientuÊ i prÏi pouzÏitõ tak beÏzÏne pomuÊcky jako je elasticky rÏetõÂzek.

3. Pokud budeme chtõÂt leÂpe kontrolovat sõÂly produ-kovane elastickyÂmi rÏetõÂzky, je namõÂsteÏ vyÂchozõ kon-trolasõÂly nasilomeÏru adle tohoto uÂdaje naplaÂnovat dalsÏõ kontroly pacienta v ordinaci.

4. PouzÏitõÂm prestretchingu muÊzÏeme pozitivneÏ ovliv-nit vyÂchozõ hodnotu aplikovane sõÂly, tõÂm zmõÂrnõÂme ne-prÏõÂjemne subjektivnõ pocity nasÏich pacientuÊ a na sledny pokles sõÂly v cÏase bude meÂneÏ vyÂraznyÂ.

PodeÏkovaÂnõÂ: Autorky deÏkujõ Mgr. KaterÏineÏ LangoveÂ, Ph. D. za pomoc prÏi zpracovaÂnõ statistickyÂch vyÂsledkuÊ. Autorky nemajõ komercÏnõÂ, vlastnicke nebo financÏnõ zaÂjmy na pro-duktech nebo spolecÏnostech popsanyÂch v tomto cÏlaÂnku.

Literatura/ References

1. Mleziva, J.: Polymery-vyÂroba, struktura, vlastnosti a pou-zÏitõÂ, Praha: SobotaÂles, 1993.

2. Brantley, W. A.; Eliades, T.: Orthodontics Materials, Scientific and Clinical Aspects, 1st ed. New York: Thieme Stuttgart, 2001.

3. Baty, D. L.; Storie, D. J.; von Fraunhofner, J. A.: Synthetic elastomeric chains: a literature review. Amer. J. Ortho-dont. dentofacial Orthop. 1994, 105, s. 536-542. 4. Bishara, S. E.; Andreasen, G. F.: A comparison of

time-related forces between plastic alastics and latex elastics. Angle Orthodont. 1970, 40, s. 319-328.

5. Hershey, G.; Reynolds, W.: The plastic module as an orthodontic tooth moving mechanism. Amer. J. Ortho-dont. 1975, 67, s. 554-662.

6. Wong, A. K.: Orthodontic elastic materials. Angle Ortho-dont. 1976, 46, s. 196-205.

7. Kovatch, J.; Lautenschlager, D.; Keller, J.: Load exten-sion-time behavior of orthodontic alastiks. J. dent. Res. 1976, 55, s. 783-786.

8. Ash, J.; Nikolai, R.: Relaxation of orthodontic elastic chains and modules in vitro and in vivo. J. dent. Res. 1978, 57, s. 685-690.

9. De Genova, D. C.; McInnes-Ledoux, P.; Weinberg, R.; Shaye, R.: Force degradation of orthodontic elastomeric

chains-a product comparison study. Amer. J. Orthodont. 1985, 87, s. 377-384.

10. Rock, W.; Wilson, H.; Fisher, S.: A laboratory investiga-tion of orthodontic elastomeric chain. Brit. J. Orthodont. 1985, 12, s. 202-207.

11. Killiany, D.; Duplessis, J.: Relaxation of elastomeric chains. J. clin. Orthodont. 1985, 19, s. 592-593. 12. Kuster, R.; Ingervall, B.; Burgin, W.: Laboratory and

in-traoral test of the degradation of elastic chains. Eur. J. Orthodont. 1986, 8, s. 202-208.

13. Buchmann, N.; Senn, Ch.; Ball, J.; Brauchli, L.: Influence of initial strain on the force decay of currently available elastic chains over time. Angle Orthodont. 2012, 82, s. 529-535.

14. Stevenson, J. S.; Kusy, R. P.: Force application and de-cay characteristics of untreated and treated polyure-thane elastomeric chains. Angle Orthodont. 1994, 64, s. 455-464.

15. Huget, E. F.; Patrick, K. S.; Nunez, L. J.: Observations on the elastic behavior of a synthetic orthodontic elastomer. J. dent. Res. 1990, 69, s. 496-501.

16. Eliades, T.; Eliades, G.; Watts, D. S.: Structural confor-mation of in vitro and in vivo aged orthodontic elastome-ric modules. Eur. J. Orthodont. 1999, 21, s. 649-658.

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17. Ferriter, J.; Meyers, C.; Lorton, L.: The effect of hydrogen ion concentration on the force degradation rate of ortho-dontic polyurethane chain elastics. Amer. J. Orthodont. dentofacial Orthop. 1990, 98, s. 404-410.

18. Hershey, H.; Brooks, D.: Effect of heat and time on stret-ched plastic orthodontic modules. J. Dent. Res. 1976, 55B, s. 363. [Cit. in: Baty, D. L.; Storie, D. J.; von Fraun-hofner, J. A.: Synthetic elastomeric chains: a literature review. Amer. J. Orthodont. dentofacial Orthop. 1994, 105, s. 536-542.]

19. Young, J.; Sandrik, J. L.: The influence of preloading on stress relaxation of orthodontic elastic polymers. Angle Orthodont. 1979, 49, s. 104-109.

20. von Fraunhofner, J. A.; Coffelt, M.-T. P.; Orbell, G. M.: The effects of artificial saliva and topical fluoride treat-ments on the degradation of the elastic properties of orthodontic chains. Angle Orthodont. 1992, 62, s. 265-274.

21. Proffit, W. R.; Fields, H. W.: Contemporary orthodontics. 5th ed., St. Louis: Mosby, 2013.

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