An Occlusal and Cephalometric Analysis of Maxillary First and Second Premolar Extraction Effects

Transcription

1 Originl Article An Occlusl nd Cephlometric Anlysis of Mxillry First nd Second Premolr Extrction Effects Hoe Boon Ong, BDS (Sing), MDSc (Melb) ; Michel G. Woods, DDSc, FRACDS, FRACDS (Orth), DOrthRCS (Eng) b Abstrct: The purpose of this study ws to exmine dimensionl chnges in the mxillry rch following the extrctions of mxillry first or second premolrs. Pre- nd posttretment records of 71 ptients treted by one experienced orthodontist were rndomly selected from completed premolr extrction cses. Fortyfive ptients involved the extrction of mxillry first premolrs; of these, 15 lso hd extrctions of mndibulr first premolrs nd 30 hd extrctions of mndibulr second premolrs. Twenty-six ptients involved the extrction of mxillry second premolrs, nd ll of these lso hd extrctions of mndibulr second premolrs. Pretretment fctors tht seemed to suggest bsis for the extrction choice in this smple included incisl overjet, molr reltionship, nd mxillry incisor protrusion. reductions with tretment in the nteroposterior rch dimension were similr within ll premolr extrction groups. There ws evidence of greter men mxillry intermolr-width reduction following the extrctions of mxillry second premolrs thn following extrctions of mxillry first premolrs. Greter men mxillry incisor retrction ws found in the mxillry first premolr extrction group thn in the mxillry second premolr group. A wide rnge of individul vrition in incisor nd molr chnges did, however, ccompny tretment involving both mxillry premolr extrction sequences. (Angle Orthod 2001;71: ) Key Words: Premolr extrctions; Arch dimensions; Incisor retrction INTRODUCTION The role of extrctions in orthodontic tretment hs been historiclly controversil. 1 5 This controversy continues tody nd, with the exception of whether or not to ctully undertke tretment t ll, the extrction decision is still the most criticl decision mde by orthodontists when plnning tretment. 6 This is further complicted by the fct tht the reltive efficcy of extrction or nonextrction strtegies, in either the short or long term, hs yet to be fully estblished. 7 A number of previous workers hve documented tht premolrs re the most commonly extrcted teeth for orthodontic purposes Conveniently locted between the nterior nd posterior segments, premolr extrctions would seem to llow for the most strightforwrd relief of crowding or the correction of n uncceptble interincisor reltionship. 11 It hs been suggested tht improvements in tech Privte prctice of orthodontics, Singpore. b Associte Professor nd Hed of Orthodontics, School of Dentl Science, The University of Melbourne, Victori, Austrli. Corresponding uthor: Associte Professor Michel Woods, Orthodontic Unit, School of Dentl Science, The University of Melbourne, 711 Elizbeth Street, Melbourne, Victori 3000, Austrli. (e-mil: m.woods@dent.unimelb.edu.u). Accepted: October Submitted: June by The EH Angle Eduction nd Reserch Foundtion, Inc. niques for controlling movements of teeth in 3 dimensions nd improvements in the correltion of these movements with nticipted fcil growth chnges hve both incresed the number of extrction options. 12 In fct, different uthors, for vrious resons, hve recommended vritions in extrction sequences, including mxillry nd mndibulr first nd/or second premolrs It is well ccepted tht, during orthodontic tretment involving the extrction of teeth, rch dimensionl chnges occur nd tht these dimensions continue to chnge following ctive tretment Quntifiction of these chnges in the mxillry rch, however, hs only recently been provided. 22 Furthermore, the bility for mxillry extrction spces to be used in predictble fshion hs not yet been widely presented in the literture In one such study, Willims nd Hosil 24 found tht, in cses involving the extrction of 4 first premolrs, pproximtely 66.5% of the vilble extrction spce ws tken up by the retrction of the nterior segment. In cses involving extrctions of mxillry first nd mndibulr second premolrs, 56.3% of the vilble extrction spce ws tken up by the retrction of the nterior segment. Since it is generlly ccepted tht strong reltionship exists between root surfce re nd nchorge potentil, the choice of teeth to be extrcted should hve direct influence on the mount of nterior segment retrction. 15,16,26 For instnce, Creekmore 16 stted, s rule-of-thumb, tht 90

2 MAXILLARY FIRST OR SECOND PREMOLAR EXTRACTION EFFECTS 91 when first premolrs re extrcted, one cn expect the posterior teeth to move forwrd pproximtely one-third of the spce, leving two-thirds of the spce for the relief of crowding nd incisor retrction. When second premolrs re extrcted, one cn expect the posterior teeth to move forwrd pproximtely hlf the extrction spce, leving the remining hlf for the relief of crowding nd the retrction of nterior teeth. 16 Some uthors hve reported definite correltions between incisor movements nd chnges in the overlying soft tissue profile Others hve shown tht chnges in tooth position re not necessrily followed by proportionl chnges in tht soft tissue profile. 25,32 37 Vritions in fctors such s lip morphology, the type of tretment, gender, nd ge hve ll been held responsible for individul differences in soft tissue response. While it hs been climed by some tht extrction tretment is likely to hve detrimentl effect on the fcil profile, it hs now lso been shown tht the decision to extrct teeth in orthodontic tretment does not hve to compromise posttretment esthetics if the decision is bsed on sound dignostic criteri. 17,18,25,38 43 Stbility following orthodontic tretment continues to chllenge ll orthodontists The bility to mintin long-term lignment following orthodontic tretment involving the extrction of premolrs hs unfortuntely lso been unpredictble. 20,21,47 50 Controversy still surrounds the question of whether better long-term results re chieved with extrction or nonextrction tretment, with different studies producing conflicting results. 17,21,51 53 With ll this in mind, it is obvious tht there re some differences in the dentl nd fcil effects of extrction nd nonextrction tretment. The influence, however, of vrious extrction sequences in these res hs been lrgely derived from necdotl clinicl observtions, nd there still seems to be little scientific evidence to support the choice of one sequence over nother. It ws, therefore, the purpose of the present study to investigte the differences in mxillry rch dimensionl nd positionl chnges following orthodontic tretment involving the extrction of either mxillry first or second premolrs. MATERIALS AND METHODS Experimentl smple Pre- nd posttretment records of 71 premolr extrction cses treted by n experienced orthodontist with prengulted Edgewise pplinces ( inches) were obtined for this study. The cses were selected ccording to the following criteri: 1. All ptients hd 4 premolr extrctions s prt of their comprehensive orthodontic tretment pln. Ptients with symmetric premolr extrctions within the dentl rches were excluded. 2. None of the ptients hd ny djunctive pplinces such TABLE 1. Experimentl Smple Group Totl Mles Femles Exo mxillry 4s 4/4 4/5 Exo mxillry 5s, 5/5 n Age t Commencement (Months) Durtion of Active Tretment (Months) /4, mxillry nd mndibulr first premolrs; 4/5, mxillry first nd mndibulr second premolrs; 5/5, mxillry nd mndibulr second premolrs. s hedgers, trnspltl rches, qud helices, functionl pplinces, or rpid mxillry expnders, s prt of their orthodontic tretment. Interrch elstics were used s necessry. 3. All cses included minimum of pre- nd posttretment lterl cephlogrms, study csts, nd tretment history records. The ge t commencement, the durtion of ctive tretment, nd the numbers of subjects in the vrious extrction subgroups re shown in Tble 1. The verge durtion of fixed-pplince tretment ws 26.4 months, with rnge of 14 to 44 months. Severl pretretment vribles were evluted so tht chnges due to ny initil group differences could be distinguished from ctul tretment effects (Tble 2). The 3 extrction subgroups were compred sttisticlly nd the differences quntified using one-wy nlysis of vrince (ANOVA). Four mesurements were identified s significntly different mong the groups t the 95% confidence level. These vribles included 2 study cst mesurements (incisl overjet nd Clss II molr reltionship) nd 2 cephlometric mesurements (incisor ngultion nd position in reltion to the APog reference line). The 4/4 group required tretment of moderte mounts of overjet nd Clss II molr correction. The 4/5 group hd the lrgest men overjet (6.1 mm) nd Clss II molr reltionship. The 5/5 group hd men overjet identicl to the 4/4 group but men Clss I molr reltionship. Reltive to the APog line, the men mxillry incisor position ws further forwrd in ll 3 groups thn the verge reported by Ricketts. 54 Among the subgroups, however, the 5/5 group hd the lest men incisor protrusion. Cephlometric nd occlusl nlysis The cephlometric mesurements used in this study re described in Tble 3 nd re illustrted in Figures 1 through 3. All lterl cephlogrms hd been tken using the sme

3 92 ONG, WOODS TABLE 2. Pretretment Vribles Vrible Age (months) Tretment time (months) ANB ( ) Overjet Overbite Molr reltionship FA ( ) SNMP ( ) 11,21-APog ( ) 11,21-APog 11,21-ANS, PNS ( ) Crowding 4/4 (n 15) *** * ** 8.8* /5 (n 30) *** * ** 7.9* ANOVA: *, P.05; **, P.01; ***, P.005. See Tbles 3 nd 4 for definitions. 5/5 (n 26) *** * ** 6.7* Post Hoc Comprisons (P Vlues) 4/4 vs 4/5 4/4 vs 5/5 4/5 vs 5/ TABLE 3. Cephlometric Mesurements No. Mesurement Definition 1 ANB ( ) Angle formed by the intersection of nsion point A nd nsion point B lines. 2 SNMP ( ) Angle formed by the intersection of sell-nsion line nd the gonion-menton line. 3 FA ( ) Angle formed by the intersection of the bsion-nsion line nd the fcil xis. 4 IIA ( ) Angle formed by the intersection of the long xes of the mxillry nd mndibulr centrl incisors. 5 11,21-ANS, PNS ( ) Angle formed by the intersection of the long xis of the mxillry incisor nd the pltl plne. 6 11,21-APog Horizontl distnce from the mxillry incisor tip to the point A pogonion line. 7 11,21-APog ( ) Angle formed by the intersection of the long xis of the mxillry incisor nd the point A pogonion line. 8 ML Distnce between rticulre nd pogonion. 9 Incisl tip chnge From superimposition on the pltl plne t ANS the horizontl distnce between mxillry incisor tip initil nd finl, mesured perpendiculr to the pterygomxillry verticl line. FIGURE 1. Cephlometric mesurements (numbers 1 4, Tble 3). clibrted cephlostt nd were trced under the sme viewing conditions, ie, in drkened room using light box with extrneous light blocked out. Mesurements were mde using the Westcef progrm ( customized reserch cephlometric nlysis progrm written for The University of Melbourne by Mr Geoffrey West), which utomticlly rottes the digitized lndmrks so tht the pterygomxillry (PM) verticl line through sphenoethmoidle is in fct verticl (Figure 2). The use of the PM line s verticl reference plne for nteroposterior chnges hs previously been suggested. 55 Absolute horizontl nd verticl distnces between lndmrks were mesured reltive to the X nd Y coordintes of those lndmrks. To evlute mxillry incisl chnges, trcings were superimposed on the pltl plne registered t nterior nsl spine (ANS), s described by Ricketts, 56 method tht hs previously been shown to be cceptble. 57 The PM line ws trnsferred from the pretretment trcing to the posttretment trcing to provide consistent reference plne for evluting chnges. Incisl chnges were then mesured perpendiculr to the PM verticl reference plne, with forwrd movement of the incisl tip ssigned positive vlue. Liner cephlometric me-

4 MAXILLARY FIRST OR SECOND PREMOLAR EXTRACTION EFFECTS 93 FIGURE 3. Mxillry incisor tip chnge (superimposition on pltl plne t ANS; number 9, Tble 3). FIGURE 2. Cephlometric mesurements (numbers 5 8, Tble 3). surements were multiplied by fctor of 0.92 to tke into ccount the 9% enlrgement fctor. The study cst mesurements used in this study re described in Tble 4. Where pproprite, they re further illustrted in Figures 4 nd 5. An electronic digitl sliding cliper (Mitutoyo Corportion, Tokyo, Jpn) ws used to mesure the distnces between occlusl lndmrks to the nerest 0.1 mm. The mount of crowding ws determined using Proffit nd Fields 58 segmentl method, ie, by subtrcting the pretretment segmentl totl from the posttretment segmentl totl nd then dding bck in the ctul mesiodistl widths of the 2 extrcted premolrs. The residul spce ws clculted by subtrcting the mount of crowding from the sum of the mesiodistl widths of the extrcted mxillry premolrs. chnges for the cephlometric nd study cst vr- TABLE 4. Study Cst Mesurements No. Mesurement Definition 10 Overbite Verticl overlp of the mxillry nd mndibulr incisors mesured perpendiculr to the occlusl plne. 11 Overjet Horizontl distnce between the mxillry nd mndibulr incisors mesured prllel to the occlusl plne. 12 Crowding Spce required for crowding relief nd leveling, clculted using Proffit nd Field s segmentl method, 58 ie, by subtrcting the pretretment segmentl totl from the posttretment segmentl totl, then dding bck in the mesiodistl widths of the 2 extrcted premolrs. 13 Chordl rch length Distnce from the mesil contct points of the mxillry first molrs to the contct point of the mxillry centrl incisors. 14 Arch depth Perpendiculr distnce from the line joining the mesil contct points of the mxillry first molrs to the contct point of the mxillry centrl incisors. 15 Interpremolr width Horizontl distnce between the pltl cusp tips of the most nterior mxillry premolrs. 16 Intermolr width Horizontl distnce between the mesiopltl cusp tips of the mxillry first molrs. 17 Arch segments Distnce between the lines perpendiculr to the contct points of segment of teeth; between the first molr nd the distl surfce of the lterl incisor nd between tht distl surfce nd the mesil surfce of the centrl incisor. 18 Molr reltionship Distnce between the mesiobuccl cusp tip of the mxillry first molr nd the buccl groove of the mndibulr first molr mesured prllel to the occlusl plne.

5 94 ONG, WOODS TABLE 5. Posttretment Vribles Vrible ANB ( ) Overjet Overbite Molr reltionship FA ( ) SNMP ( ) 11,21-APog ( ) 11,21-APog 11,21-ANS, PNS ( ) 4/4 (n 15) 4/5 (n 30) 5/5 (n 26) No significnt differences mong groups. See Tbles 3 nd 4 for definitions. ibles were clculted, nd nlysis of vrince ws used to identify ny sttisticlly significnt differences in the chnges observed within the 3 subgroups. Person s product moment correltion coefficients (r) were lso clculted to determine whether ny ssocition existed between mxillry incisor chnges nd ny other vribles. Error study To evlute trcing nd mesurement error, the records of 10 ptients (20 sets of study csts nd 20 cephlogrms) were selected t rndom nd the experimentl procedure repeted. Results of the pired-smples t-test showed no significnt differences between the first nd second sets of mesurements t the 95% confidence intervl. RESULTS FIGURE 4. Arch dimensionl mesurements (numbers 13 17, Tble 4). At the end of ctive tretment, ll 3 extrction subgroups showed men Clss I incisor nd molr reltionships. In reltion to the APog line, the men posttretment mxillry incisor position nd ngultion were lso similr mong the 3 extrction subgroups (Tble 5). Arch dimensionl chnges FIGURE 5. Molr reltionship mesurement (number 18, Tble 4). The mxillry rch dimensionl chnges for ll groups re summrized in Tble 6. Since there ws no sttisticl evidence of sexul dimorphism, ech of the extrction subgroups ws not further divided into mle nd femle subgroups. As expected, men reductions in rch depth nd chordl rch length were noted in ll groups. The men reductions were, however, similr in ll groups. In ll groups, there ws men increse in mxillry rch width cross the most nterior premolrs. The only sttisticlly significnt difference mong the groups ws for the reduction in intermolr width. reductions of 1.5 mm ( 1.7), 2.6 mm ( 2.3) nd 3.3 mm ( 2.2) were observed in the 4/4, 4/5, nd 5/5 groups, respectively.

6 MAXILLARY FIRST OR SECOND PREMOLAR EXTRACTION EFFECTS 95 TABLE 6. Mxillry Arch Dimensionl Chnges With Tretment (men ) Group n Arch Depth Chordl Arch Length Interpremolr Width Intermolr Width Totl Mles Femles Exo 4s 4/4 4/5 Exo 5s, 5/ * * * ANOVA: *, P.05. TABLE 7. Mxillry Incisor Position nd Angultion Chnges With Tretment (men ) Group n 11,21-APog 11,21-APog ( ) 11,21-ANS, PNS ( ) IIA ( ) Incisl Tip Chnge (Superimposition) Totl Mles Femles Exo 4s 4/4 4/5 Exo 5s * * * * * * Student s t test;, P.05;, P.005. ANOVA: *, P.05. Mxillry incisor position nd ngultion chnges Chnges in mxillry incisor position nd ngultion with tretment re presented in Tble 7. A men reduction in mxillry incisor protrusion nd proclintion ws noted in ll mxillry premolr extrction groups. Sttisticlly significnt differences for ll 5 vribles were found mong the different extrction groups. There ws significntly lrger men retrction of the mxillry incisors in reltion to the APog line in both the mxillry first premolr extrction subgroups thn in the second premolr group. The 4/4 nd 4/5 subgroups hd men incisor retrctions of 4.2 mm nd 3.7 mm, respectively, wheres the 5/5 group hd men retrction of 2.3 mm. Similr results were noted for the chnges in mxillry incisor ngultion to the APog line, with the overll mxillry first premolr extrction group showing significntly lrger men reduction in ngultion (8.2 ) thn the second premolr group (3.3 ). The men incisor ngultion chnges in reltion to the pltl plne lso were significntly different mong the groups, with men reductions of 5.0 nd 1.6, respectively, in the mxillry first premolr nd mxillry second premolr extrction groups. As with ll vribles, however, there were lrge rnges of individul vrition. Interincisl ngultion incresed on verge s result of chnges in the ngultions of both mxillry nd mndibulr incisors. Lrge stndrd devitions were lso noted for this mesurement. Mxillry cephlometric superimposition Anteroposterior chnges recorded t the mxillry incisor tip when pre- nd posttretment trcings were superimposed on the pltl plne t ANS re shown in Tble 7 nd re illustrted in Figure 6. There ws sttisticlly significnt difference in the men incisor retrction in the mxillry first premolr group (2.5 mm) compred with the mxillry second premolr group (1.6 mm). Incisors were retrcted from their pretretment positions in ll subjects in the 4/4 nd 4/5 groups, wheres only 85% of the subjects within the 5/5 group showed ny incisor retrction. A wide rnge of individul chnges ws evident within ech group. Molr versus incisor chnges The nteroposterior chnge in mxillry first molr position ws estimted by clculting the difference between the rch-depth chnge nd incisor position chnge mesured from superimposition on the pltl plne t ANS. This ws mde possible by the multipliction of the liner cephlometric mesurements by the 0.92 fctor. Anteroposterior chnges in the mxillry first molr position re shown in Tble 8 nd re illustrted in Figure 7. chnges in the estimted molr movement were not found to be significntly different mong the groups. Clculted in this wy, men forwrd movements of the molrs for the groups rnged from 3.7 mm to 4.7 mm. When compring reltive mounts of movement, the incisors hd un-

7 96 ONG, WOODS TABLE 8. Estimted Molr Movement Forwrd Molr Movement Group Totl Mles Femles Exo 4s 4/4 4/5 Exo 5s, 5/5 n No significnt differences mong groups dergone greter movement thn the first molrs in 20% of the cses in ech of the mxillry first premolr extrction subgroups nd in 19% of cses in the mxillry second premolr extrction group. Little, if ny, incisor retrction ws found in 13% of the subjects in both the 4/4 nd 4/5 groups, while the sme ws true in 19% of subjects in the 5/5 group. Individul vrition Becuse of the wide rnges of individul vrition within ech of the groups, it ws decided to look for ny similrities tht might exist between the individuls who hd shown extremes of incisor movement within ech group. Two individuls were chosen from ech extrction subgroup, one in whom there ws the gretest incisor retrction nd nother in whom there ws only miniml incisor chnge. A third individul, in whom there ws ctul protrusion of the incisors, ws chosen in the 5/5 group (Tble 9, Figures 8 10). When viewing these cses, there ppered to be tendency towrd greter retrction of the incisors in cses with less crowding or greter mounts of residul spce. It ws lso interesting to note tht chnges in the position of the mxillry incisors on the underlying bone were not necessrily consistent with mxillry incisor chnges in reltion to the APog line. For exmple, in individul B (Tble 9), reduction in the prominence of the incisors in reltion to the APog line ws ccompnied by little, if ny, chnge on the underlying bone. It seems therefore tht chnges in the nteroposterior nd verticl positions of point A nd pogonion were occurring t the sme time. Further correltions FIGURE 6. Frequency of mxillry incisor nteroposterior movement following different premolr extrction sequences. When Person s coefficients were clculted, chnges in interincisl ngultion nd the initil incisor position in reltion to the APog line were both found to be significntly correlted with mxillry incisor movement (Tble 10). These findings would not be unexpected since these mesurements re ll somewht dependent on ech other. No other significnt correltions were found.

8 MAXILLARY FIRST OR SECOND PREMOLAR EXTRACTION EFFECTS 97 DISCUSSION FIGURE 7. Frequency of estimted molr movement following different premolr extrction sequences. The presence of significnt differences in men pretretment overjet, Clss II molr reltionship, nd incisor protrusion mong the different extrction subgroups suggests tht these fctors hd somehow influenced the extrction sequence decision. Crowding hs consistently been considered the mjor fctor to be tken into ccount when deciding whether to extrct teeth s prt of orthodontic tretment, 19,59 61 nd the choice of prticulr extrction sequences seems to hve been bsed lrgely on necdotl clinicl opinions. 15,16,26 Recently, Selens nd De Smit 25 evluted pretretment vribles involved in extrction decisions nd reported tht the mount of crowding hd significnt influence on the decision to extrct 4 first premolrs. In cses with mild crowding nd mild dentl protrusion, however, the decision ws often mde to extrct 4 second premolrs insted. It is interesting to note tht, in n erlier study on mndibulr premolr extrction effects, it ws found tht the underlying verticl fcil pttern lso seemed to significntly influence the mndibulr extrction-sequence decision. 62 This ws not found to be the cse in this present mxillry rch study. Tht my be due to the fct tht the finl positions of the mndibulr incisors re more likely to be chosen on the bsis of the verticl fcil pttern nd the consequentil lterl profile effects. It is lso interesting to note tht, in this rndomly selected premolr extrction smple, the 4/4 group ccounted for only 21% of the cses, while the 4/5 nd 5/5 groups ccounted for 37 nd 42% of the cses, respectively. This is in contrst with the findings of previous studies 8 10 tht hve reported tht 4 first premolrs re the most likely teeth to be extrcted s prt of orthodontic tretment. One limittion of the present study hs been this smller smple size for the 4/4 group. One should relize, however, tht the min focus of this study hs been on the different mxillry premolr extrction choice. In the present study, mxillry rch dimensionl chnges in ech group involved, in generl, some contrction of the nteroposterior dimension. The fct tht the men reductions in both rch depth nd chordl rch length were similr in ll groups might hve been expected nywy since there were similr men mounts of crowding in ech group, resulting in similr mounts of overll residul spce. The men overll chordl rch-length reduction of 11.3 mm is consistent with those reductions reported by Pquette et l 17 nd De L Cruz et l 20 but is somewht greter thn tht reported by Luppnpornlrp nd Johnston. 18 These ltter uthors reported men chordl rchlength reduction of 8.3 mm during tretment in first premolr extrction cses. The difference is likely to be due to the greter men crowding found in the Luppnpornlrp nd Johnston smple (5.8 mm) compred with the present smple (3.5 mm). Becuse significnt number of the cses included in

9 98 ONG, WOODS FIGURE 8. Individul mxillry cephlometric nd occlusl superimpositions (4/4 extrction sequence). FIGURE 9. Individul mxillry cephlometric nd occlusl superimpositions (4/5 extrction sequence).

10 MAXILLARY FIRST OR SECOND PREMOLAR EXTRACTION EFFECTS 99 FIGURE 10. Individul mxillry cephlometric nd occlusl superimpositions (5/5 extrction sequence). this experimentl smple involved unerupted, impcted, or bucclly displced mxillry cnines, the width cross the nterior segment of the rch ws mesured cross the most nterior premolrs on both pre- nd posttretment csts. A men increse in rch width in this region ws noted in ll groups. This mens tht the rch form ws, on verge, rounded out somewht cross the premolrs regrdless of the extrction sequence. In contrst with this increse in rch width cross the premolrs, there ws significntly greter men reduction in intermolr width in the 5/5 group. This would suggest tht, in clinicl prctice, it might be esier to mintin the initil intermolr width if the second premolrs hve not been extrcted. The men chnges in nteroposterior position of the mxillry incisors found in this study re consistent with those reported in previous studies. 18,25 In the present smple, there ws men mxillry incisor retrction of 2.5 mm ( 1.9) nd 1.6 mm ( 1.6) in the mxillry first premolr nd mxillry second premolr extrction groups, respectively. Selens nd De Smit 25 reported n verge 2.1 mm ( 2.5) nd 1.9 mm ( 2.4) retrction in their 4 first premolr nd 4 second premolr extrction groups, respectively. Luppnpornlrp nd Johnston 18 found n overll men 2 mm to 3 mm retrusive effect with first premolr extrctions. It should be noted tht there were wide rnges of individul vrition in mxillry incisor chnges in both the present study nd the study of Selens nd De Smit. 25 It would, therefore, seem to be unresonble to estimte likely incisl position chnges in n individul ptient by simply using men vlues, s hs been suggested by some uthors. 15,18 The men chnges in mxillry incisor position in reltion to the APog line re consistent with the incisor chnges on the underlying bone in tht they do seem to vry ccording to the chosen extrction sequence. It is importnt to relize, however, tht, s ws seen in individul B (Tble 9), the chnges in the 2 mesurements in individul ptients my not lwys be consistent. It hs been generlly ccepted tht greter forwrd move-

11 100 ONG, WOODS TABLE 9. Individul Vrition in Incisl Behvior Extrction Sequence Incisl Tip Chnge (Superimposition) Crowding Residul Spce Initil Molr Reltionship Initil 11,21-APog Finl 11,21-APog 4/4 Individul A Individul B 4/5 Individul C Individul D 5/5 Individul E Individul F Individul G II II II II I II I TABLE 10. Correltions With Mxillry Incisor Movement (superimposition) Vrible Age (months) Initil 11,21-APog Initil 11,21-APog ( ) Initil molr reltionship Crowding Residul spce ML chnge SNMP chnge ( ) IIA chnge ( ) 11,21-ANS, PNS chnge ( ) Overjet chnge Overbite chnge Molr reltionship chnge Estimted molr movement Interpremolr width chnge Intermolr width chnge Person s Correltion (r) * * *, Significnt correltion. See Tbles 3 nd 4 for definitions. ment of the molrs should be expected following the extrctions of mxillry second premolrs thn first premolrs. 16,23,26,63 This ws, however, not necessrily found to be the cse in the ptients within this experimentl smple. When reltive mxillry incisor nd molr movements were compred, greter molr movements occurred in 73%, 80%, nd 81% of cses in the 4/4, 4/5 nd 5/5 groups, respectively. If this were true for ny mxillry premolr extrction smple, it might suggest tht differentil extrction lone my not provide sufficient mxillry nchorge control in ll cses. Other methods of nchorge control would need to be considered. Individul vrition ws lso evident when extremes of incisor movement were evluted (Tble 9). It does seem possible for vriety of mxillry incisor chnges to ccompny ech of these premolr extrction sequences, lthough there do pper to be some definite trends in incisl behvior. For instnce, those ptients in ech of the groups in whom mximum incisor retrction hd occurred ppered to hve consistently less crowding nd, in turn, greter residul spce thn other individuls in the sme groups. A similr tendency hs previously been documented for the mndibulr rch. 62 In both mxillry nd mndibulr rches, therefore, it seems tht both crowding nd the residul spce following leveling nd relief of crowding ply significnt roles in determining the finl incisl positions. Becuse such wide individul vrition hs been found in response to orthodontic tretment with ny of the investigted premolr extrction sequences, it is importnt to ssess ech cse on n individul bsis when mking detiled tretment pln rther thn simply choosing prticulr extrction sequence bsed on published men incisl chnges for different extrction sequences. CONCLUSIONS From the study results, the conclusions re s follows: 1. Pretretment chrcteristics, which my influence the mxillry premolr extrction-sequence decision, include the mount of incisl overjet, the first permnent molr reltionship, nd the initil mount of incisor protrusion. 2. There re likely to be similr rnges of decreses in mxillry nteroposterior rch dimension regrdless of the chosen premolr extrction sequence. Greter reduction in intermolr width is likely to occur following the extrctions of mxillry second premolrs thn first premolrs. 3. Although there is some evidence tht greter incisor retrction ccompnies mxillry first premolr extrctions, considerble individul vrition in incisor nd molr movements is likely to be seen with ny premolr extrction sequence. A specific extrction sequence does not necessrily seem to gurntee tht certin mounts of incisor retrction or molr protrction will occur. 4. Individul vrition in response to growth nd tretment is likely to be result of different tretment mechnics nd fcil nd occlusl objectives nd is likely to depend s much on pretretment chrcteristics s on the extrction sequence itself.

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