Biomechnics IN Orthodontics PRINCIPLES AND PRACTICE Rm S. Nnd, BDS, DDS, MS, PhD Professor Emeritus Deprtment of Orthodontics College of Dentistry University of Oklhom Oklhom City, Oklhom Yhy S. Tosun, DDS, PhD Privte Prctice Dui, United Ar Emirtes Former Professor Deprtment of Orthodontics University of Aege İzmir, Turkey Quintessence Pulishing Co, Inc Chicgo, Berlin, Tokyo, London, Pris, Miln, Brcelon, Istnul, Moscow, New Delhi, Prgue, São Pulo, nd Wrsw
Contents 1 2 3 4 5 6 7 8 9 Prefce vii Physicl Principles 1 Appliction of Orthodontic Force 17 Anlysis of Two-Tooth Mechnics 55 Frictionl nd Frictionless Systems 71 Anchorge Control 83 Correction of Verticl Discrepncies 99 Correction of Trnsverse Discrepncies 125 Correction of Anteroposterior Discrepncies 133 Spce Closure 145 Glossry 154 Index 156
Prefce Once comprehensive dignosis nd tretment plnning hve set the stge for inititing tretment procedures, pplince design nd systems hve to e developed to chieve tretment gols. Correct ppliction of the principles of iomechnics ssists in the selection of efficient nd expedient pplince systems. Over the lst three decdes, there hs een n explosion in the development of technology relted to orthodontics. New mterils nd designs for rckets, onding, nd wires hve comined to crete nerly infinite numer of possiilities in orthodontic pplince design. As these new mterils re rought together in the configurtion of orthodontic pplinces, it is necessry to understnd nd pply the principles of iomechnics for successful nd efficient tretment outcome. Lck of proper understnding my not only set up inefficient force systems ut lso cuse collterl dmge to the tissues. The pth to successful tretment is through good knowledge of iomechnics. This ook is written with the purpose of introducing student of orthodontics to the evolving technology, mteril properties, nd mechnicl principles involved in designing orthodontic pplinces. vii
3 Anlysis of Two-Tooth Mechnics Fig 3-20 Four exmples of mechnics used to extrude cnine. () An open coil spring etween the lterl incisor nd premolr on 0.016- inch stinless steel wire mintins the spce while preventing the djcent teeth from tipping. () A cntilever with V-end cn e used to move the cnine down. The cntilever should e ttched to the cnine with ligture t only one point to void unwnted moment. (c) Reciprocl nchorge to level mxillry nd mndiulr cnines with n up-nd-down elstic. (d) An uxiliry 0.014 or 0.016 NiTi wire cn e used long with rectngulr SS min rchwire to ring the high cnine down. c d Fig 3-21 Lceck prevents the crown from tipping nd helps correct the inclintion of n upright cnine. When the wire is engged in the incisor rckets, three possile effects cn e oserved. First is the rowot effect, which is cused y counterclockwise moment on the cnine tht strins the nchorge. 10 12 This moment tends to push the crown forwrd, resulting in incisor protrusion, which cn e prevented only y mens of lceck. In Clss II, division 1 extrction cses, the rowot effect is n undesirle side effect ecuse of the round tripping or jiggling effect, 13 15 which my occur during retrction of nterior teeth nd result in root resorption. Lceck cn prevent the cnine crown from tipping forwrd. In Clss II, division 2 nonextrction cses, incisor protrusion my e desirle; therefore, use of stright wire will help induce nterior protrusion s well s quick lignment. 68
Conclusion Fig 3-22 () To void extrusion, if stright wire pssing through verticlly positioned cnine rcket psses elow the incisors, one should not plce the wire in the incisor rckets. ( nd c) In this cse, insted of using nickel titnium (NiTi) wires, thin stinless steel wires with step-up ends ypssing the incisor rckets my e used. This is desirle pproch for treting nterior open ites needing correction y incisor extrusion. c Fig 3-23 () The owing effect my cuse ite deepening ecuse of the cnine position. () If endle wire is used, step-up cn e ent to ypss the incisors gingivlly. If the stepped rch is engged in the nterior rckets, it will cuse the cnine crown to tip distlly ecuse of the clockwise moment. The second effect is deepening of the ite (owing effect; Fig 3-23). Deepening of the ite during tretment is usully not desired effect unless it is indicted in n nterior open ite tht needs to e corrected y mxillry incisor extrusion. To dignose or predetermine this effect, plce the rchwire in the cnine rcket slot efore ligting it. If the nterior prt of the wire runs elow the incisor rckets, it should not e tied to the rckets to void incisor extrusion. If endle wire is used, step-up cn e ent to ypss the incisors gingivlly. Another method is to plce continuous intrusion rch long with the stright wire. The extrusive effect of the stright wire would therefore e compensted for y the intrusion rch. If the stepped, ypss rchwire is not left pssive nd is engged in the incisors to intrude them or prevent them from extruding, it will cuse the cnine crown to tip more distlly owing to the clockwise moment occurring on its rcket (Fig 3-23). In the explntions ove, the min resons for the dverse effects re the positions or xil inclintions of the teeth or the rckets. If the prolem is cused y the xil inclintions of the cnines, it is importnt to upright them with lceck efore inserting continuous wire. Conclusion In the nlysis of the reltionship etween two teeth, the slot sizes nd rcket widths re ssumed to e equl in ll the exmples given here. Nturlly, s the slot sizes nd widths chnge, the mgnitudes of the lncing forces nd the moments lso chnge. In clinicl 69
6 Correction of Verticl Discrepncies Tongue Fig 6-35 In open ite cses, control of verticl movement of the molrs cn e chieved effectively with high-pull hedger trnspltl rch comintion. If the trnspltl rch crosses the plte 2 to 3 mm wy from the mucos, the molrs will e intruded y verticl tongue forces during swllowing. Fig 6-36 ( nd ) In open ite cses, erupting second molrs cn e controlled using 0.016 0.022 inch SS segmented rch tht psses through the uxiliry tue of the first molr. 28 Fig 6-37 Using reverse-curved rchwires to close n nterior open ite. The strong nterior ox elstics prevent the premolrs from erupting, while the molrs intrude nd tip ck nd the incisors extrude. These mechnics work quite effectively in very short time, ut they re hevily dependent on ptient coopertion. Elstics must e worn ll dy, otherwise the ite my open with quick extrusion of the premolrs. trol their eruption efore they rech the occlusl plne. For this purpose, 0.016 0.022 inch SS segmented rch cn e used s n occlusl stop 28 (Fig 6-36). Arches with reverse curve of Spee Anterior open ite cn e closed with comintion of reverse-curved rchwire nd nterior ox elstics 31,32 (Fig 6-37). The rchwire tends to extrude the mxillry nd mndiulr premolrs, opening the ite, while strong nterior ox elstics prevent eruption of the premolrs nd extrude the nteriors. Becuse the premolrs cnnot erupt, the molrs intrude nd tip ck with reciprocl forces. These mechnics effectively close the ite in 1 or 2 months, ut they re hevily dependent on ptient coopertion. If the ptient fils to wer the elstics, the premolrs will extrude nd cuse the ite to open more. Even though this pproch is very effective in closing the ite, the elstics should not e worn longer thn 2 months ecuse of the possiility of gingivl recession nd gummy smile from overeruption of the incisors. Molr intrusion with microimplnt nchorge Molr intrusion my e required to control the verticl discrepncy in skeletl open ite. However, using conventionl techniques, this movement is one of the most chllenging procedures in orthodontics depending on strong nchorge ut introrl nchorge is usully not 122
Tretment of High-Angle Cses nd Correction of Open Bite Fig 6-38 Molr intrusion using two TADs. Fig 6-39 Molr intrusion with one TAD nd trnspltl rch. Fig 6-40 Molr protrction in comintion with intrusion my result in counterclockwise rottion of the mndile, thus helping to correct the skeletl open ite y reducing lower fcil height. enough without extrusion of djcent teeth. High-pull hedger with long rms in conjunction with trnspltl rch is usully needed to chieve effective intrusion of posterior teeth (see Figs 5-7 nd 6-35). Microimplnt nchorge lso is very effective wy to intrude molrs. There re two sic methods to intrude molrs with microimplnt nchorge: Two TADs cn e inserted, oth ucclly nd pltlly, nd elstic trction pplied to the hooks (Fig 6-38). If two or more posterior teeth need intrusion, the force cn e pplied to the rchwire. One TAD cn e inserted ucclly, while trnspltl rch controls uccolingul tipping of the molr (Fig 6-39). Tongue forces during swllowing will ssist this intrusion (see Fig 6-35). In either method, TADs cn e plced etween the mxillry first molr nd second premolr roots or etween the first nd second molr roots. Molr intrusion in the mndiulr rch is usully more difficult thn in the mxill. Becuse microimplnt insertion is not recommended on the lingul of the mndiulr dentl rch, lingul r cn e used to control uccolingul molr inclintion. Molr protrction in conjunction with intrusion cuses the mndile to rotte counterclockwise nd helps close the ite 28 (Fig 6-40). A microimplnt on the zygomtic corticl one uttress is lso recommended for intruding molrs more effectively. 33 Even though it is stronger, zygomtic microimplnt insertion requires flp surgery, which my cuse soft tissue irrittion. 123