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ASK AN EXPERT THINGS YOU WANT TO KNOW Q BJÖRN U. ZACHRISSON OSLO, NORWAY A Mndiulr third molrs nd lte lower rch crowding the evidence se The issue of the influence of unerupted third molrs on the dentition hs long een controversil one. There is not much dete tht symptomtic or pthologiclly involved third molrs necessitte removl. On the other hnd, wisdom teeth hve een ccused of cusing tooth displcement nd orthodontic relpse. Studies hve shown tht the force exerted y erupting third molrs is not significnt enough to influence the nterior dentition. Yet, there re still mny prctitioners who recommend the extrction of symptomtic unerupted third molrs. Wht re the current recommendtions concerning the extrction of symptomtic third molrs? Is there consensus in the dentl profession? Wht is your position on tht topic? Jules E. Lemy III, Sherrooke, Quéec, Cnd Etiology of lte mndiulr incisor crowding The etiology of postpuertl mndiulr crowding in oth orthodonticlly treted nd untreted persons is multifctoril. The cuses of reduced rch dimensions my vry from one suject to nother, nd severl fctors cting together or t different stges of development my contriute to lte mndiulr crowding. In n excellent literture overview, Richrdson 1 concluded tht, during the teenge yers, pressure from the ck of the rch is n importnt cuse of lte mndiulr incisor crowding. Such pressure my cuse forwrd movement of the uccl teeth with shortening of the dentl rch nd n increse in crowding. 2 7 The pressure my e due to vrious fctors including (1) physiologic mesil drift, (2) the nterior component of the force of occlusion on mesilly inclined teeth, (3) the mesil vectors of musculr contrction, or (4) the presence of developing third molr. Other possile etiologic fctors include (5) lte mndiulr growth, (6) skeletl morphology nd complex growth ptterns, (7) soft tissue mturtion, (8) periodontl forces, (9) tooth morphology, (10) occlusl fctors, nd (11) connective tissue chnges. 8 In persons who hve received orthodontic tretment, relpse fter lterl rch expnsion nd/or incisor proclintion is yet nother reson for posttretment mndiulr incisor crowding. 9 180

Fig 1 Averge eruption pth of the mndiulr first molr from childhood to dulthood () ccording to implnt mrker superimpositions y Björk nd Skieller 15 (reprinted y permission), nd () rcil growth nlysis y Ricketts et l. 14 Note tht the molrs erupt in mesil nd upwrd direction (compre with solid rrows in Fig 4). Wekness of presently ville clinicl third molr reserch Due to the mny possile vriles involved in lte mndiulr rch crowding, direct evidence for significnt cuse-nd-effect reltionships of ny single fctor is extremely difficult to estlish in reserch project. This is the min reson for the controversy tht presently exists in our profession. Filure to isolte the third molrs from ll other etiologic fctors in crefully controlled clinicl study is the min reson for the confusion. Furthermore, mechnicl tooth movement in nonextrction cses or premolr extrction with or without mechnicl tretment introduces so mny new vriles into the eqution tht the effect of third molrs on crowding is likely to e msked. Consequently, severl recent studies were unle to demonstrte tht third molrs exert pressure on the teeth mesil to them. 10 13 For exmple, two well-known studies sed on the Settle mteril 10,11 concluded tht the presence of third molrs does not produce more mndiulr nterior crowding thn in ptients with third molr genesis. However, the illustrtions in oth reports showed cses in which considerle expnsion of the mndiulr intercnine width hd een performed during the orthodontic tretment. Such cses re likely to relpse fter tretment nd retention, with resultnt nterior crowding. In my opinion, more pproprite conclusion from these studies would e tht lterl expnsion of norml mndiulr 3-3 distnce during orthodontic tretment will led to postretention crowding, whether third molrs re present or not. Effect of erupting third molrs on eruption pttern of the first molrs A more dissective pproch to the controversy my e to look t the effect of erupting second nd third molrs on the continued eruption pttern of the mndiulr first molrs. Bsed on severl reports, there is evidence tht the norml verge eruption pth of the mndiulr first molrs is in n upwrd nd mesil direction 14,15 (Fig 1). This eruption continues during the time when the third molrs re emerging into the orl cvity. In longitudinl study of girls from 9 to 25 yers of ge, Iseri nd Solow 16 demonstrted tht the eruption of first molrs nd incisors continued until the end of the oservtion period t ge 25, nd showed no evidence of hving terminted t tht time. Since the mndiulr first molrs continue their eruption in persons etween 20 nd 30 yers of ge, the potentil influence of the eruptive pressure from third molrs on the tendency to forwrd movement of the uccl segments nd nterior crowding cn e studied in clinicl experiments in which the mndiulr second or third molrs re extrcted. 3,4,6 Such studies hve provided evidence tht the presence of developing third molr my indeed e one of the cuses of incresed mndiulr rch crowding. Furthermore, the incidence of such crowding my e reduced y extrction of the second or third molr. 181

c Fig 2 Cephlometric method () nd superimposition trcing () used y Richrdson nd Mills 17 to nlyze how extrction of mndiulr second molrs will ffect the eruption pttern of the first molrs, nd (c) photo documenttion method used y Schwrze 3,4 for computer nlysis of the 3-dimensionl positionl chnges. Men forwrd movement of the mndiulr first molrs is reduced fter second or third molr extrction (see text). Fig 3 Typicl effect on tooth positions severl yers fter erly extrction (t out 8 yers of ge) of crious mndiulr first molrs in nonorthodontic cse. Note distl drift of the second premolrs, which now contct the second molrs. Fig 4 Digrmmtic representtion of the sptil reltionship etween third molrs () nd the continued forwrd nd upwrd eruption of the mndiulr first nd second molrs (). When third molrs re extrcted, the first nd second molrs erupt in more distl direction (ex) thn normlly (solid rrow). Conversely, when third molrs re llowed to erupt, the first nd second molrs will erupt in more mesil direction (er) thn norml. The molr eruption continues into the second nd third decdes. 16 Using mesurements from 60-degree ngled cephlogrms, which give consistently cler picture of the molr re on ech side nd provide more relile structurl superimpositions thn lterl cephlogrms (Fig 2), Richrdson nd Mills 17 showed tht when the second molrs were extrcted, the mesil movement of the first molrs ws significntly reduced (Fig 2). More recently, Richrdson 8 demonstrted tht in ptients etween 12.5 nd 15.5 yers of ge who demonstrted incresed mndiulr incisor crowding, there ws significntly more forwrd movement of the second molr, nd greter increse in the interincisl ngle, thn in sujects who hd no increse in mndiulr incisor crowding during the sme period. This evidence supports tht mesilly directed force is n importnt cuse of incresed mndiulr incisor crowding in the erly teenge yers. Similrly, Schwrze, 3 using photo documenttion nd computer nlysis of 3-dimensionl tooth movement in reltion to the third pltinl ruge (Fig 2c), showed less mesil movement of the first molrs in group of 56 ptients in whom the third molrs were extrcted etween 12 nd 22 yers of ge thn in group of 49 nonextrction cses. How cn third molrs ffect the forwrd movement of the uccl segments? We recll from the times when cries were rmpnt tht fter erly extrction of crious mndiulr first molrs, the mndiulr second premolrs could sometimes occur in contct with the second molrs some yers lter (Fig 4). When spce ecomes ville, mndiulr posterior teeth my spontneously drift in distl direction. A similr mechnism is likely when impcted developing third molrs re extrcted. Figure 4 indictes how mndiulr third molr eruption (pointed rrow) or extrction (dshed rrow) my ffect the continued eruption pth of the mndiulr second nd first molrs. If pressure from n erupting third molr chnges the pth of first molr eruption to 182

c Fig 5 Prolonged retention with onded lingul retiners in Clss II, Division 1 premolr extrction cse, while witing for full eruption of third molr in close proximity to the mndiulr second molr. Pnormic overviews t 13 (), 19 (), nd 22 yers of ge (c). Fig 6 Typicl ge chnge in reltionship etween mndiulr third nd second molrs. At ge 9 (), third molr germs (rrows) re ove the second molrs in this ptient; fter tretment, t ge 17 (), they re elow the second molrs (rrows). The sptil reltionships reflect the mgnitude of mndiulr molr eruption during this 8-yer time period, since the third molrs re sttionry in the jw. c Fig 7 The sptil reltionships etween the mndiulr third nd second molrs etween 8 to 10 yers of ge () re optiml for third molr germectomy (,c). The third molr cn generlly e oserved s white ring (solid rrow in ) when the second molr ifurction re clcifies (dshed rrow in ). more mesil upwrd direction, nd if the incisors do not come forwrd to the sme extent, mndiulr incisor crowding will result. It my e recommended, therefore, to keep onded mndiulr retiner in plce until the third molrs hve erupted properly (Fig 5). Fctors other thn relief of pressure from n erupting third molr cn ply role. Spontneous chnges in molr eruption ptterns my e expected when more distl spce ecomes ville in the rches. 17 Mngement strtegy for third molr removl For prcticl resons, there re only two lterntives for third molr removl ssocited with orthodontic therpy: either (1) etween 8 to 10 yers of ge when these teeth re germs nd locted ove the second molrs (Figs 6, 7, 8, 9d) or (2) etween 20 to 25 yers of ge, or older, with the third molrs re in prtilly erupted position. In the time period etween 10 nd 20 yers of ge, the third molrs re locted elow the second molrs (Figs 5, 6) nd their removl my e unnecessrily trumtic for the ptient. Impcted disese-free third molrs my sfely e left in situ, indefinitely, in dults. 18,19 However, erly extrction of impcted third molrs is indicted in cses with root resorption in second molrs djcent to nonerupted third molrs. This is especilly true in cses with mesiongultion of 60 degrees or more nd n picl loction in proximity to the distl root of the second molr. 20 183

Fig 8 Spontneous uprighting of the mndiulr second molrs (,c) fter ilterl third molr germectomy t 10 yers of ge. Pnormic overviews efore germectomy () nd t follow-ups t 10 yers 8 months () nd 11 yers 2 months of ge (c), when orthodontic tretment ws strted. (d) Dentition fter orthodontic tretment t 14 yers 3 months of ge. Note tht the spontneous chnge in xil inclintion of the second molrs ( to c) ws mintined during the orthodontic tretment (d). c d Effect of germectomy of third molrs The mndiulr third molr germs occupy spce. If they re extrcted, this spce cn e used for spontneous (Fig 8) or ctive uprighting of the mndiulr second molrs nd posterior uccl segments (Fig 9). Figures 9 nd 10 show girl, 10 yers of ge, with full Clss II, Division 2 mlocclusion nd imxillry crowding. The cse ws treted with highpull hedger nd custom-mde trnspltl rch fter ilterl mndiulr third molr germectomy, nd prevention of the norml mesil migrtion of the first molrs with holding rch, using the ville leewy spce to resolve the nterior crowding. The chnge in xil inclintion of the mndiulr molrs with tretment is evident (Figs 9d to 9i). Conclusions The scientific evidence ville t present supports the following conclusions regrding the role of mndiulr third molrs in spce conditions in orthodontics: A mesilly directed force is n importnt cuse of the incresed mndiulr rch crowding in teenge nd young dult persons. The presence of developing mndiulr third molr with insufficient spce cn e one cuse of lte mndiulr rch crowding. The key to understnding the effect of n erupting mndiulr third molr on the occlusion lies in 3-dimensionl nlysis of the forwrd nd upwrd eruptive movement of the mndiulr uccl segments, which continues into the second nd third decdes of life. Erly extrction of mndiulr third molrs my mke the teeth in the mndiulr posterior segment erupt in less mesil direction. Mndiulr third molrs occupy spce. If these teeth re extrcted, the spce cn e used for distl uprighting of the mndiulr uccl segments in nonextrction therpy. Impcted disese-free third molrs my sfely e left in situ, indefinitely, in dults. None of the presently ville studies on third molrs nd crowding is optimlly designed to isolte the effect of the molr eruption from the multitude of other contriuting fctors, nd therefore the conclusions in these studies pprently do not estlish the true reltionship etween third molr eruption nd incresed mndiulr crowding. 184

c d e f g h i Fig 9 Young girl with Clss II, Division 2 mlocclusion nd imxillry crowding ( to c) treted without premolr extrctions y using leewy spce fter ilterl germectomy of mndiulr third molrs. Pnormic overview efore germectomy t 9 yers 6 months of ge (d). A 2 x 4 holding rch ws used in the mndile, llowing distl eruption of the second premolrs (e,f). The distl molr reltionship ws treted y mens of high-pull hedger nd customized trnspltl rch. (g to i) Pnormic overviews t 11 yers 8 months of ge, end of tretment t 13 yers 6 months of ge, nd t follow-up t 15 yers 6 months of ge, respectively. Note upright xil inclintion of mndiulr second nd first molrs during nd fter tretment (d to i). c d e f Fig 10 Sme cse s in Fig 9, t the end of orthodontic tretment. Gold-coted lingul retiner wires in oth dentl rches (d,e), plus removle plte, were used for retention. 185

REFERENCES 1. Richrdson ME. The etiology of lte lower rch crowding lterntive to mesilly directed forces. Am J Orthod Dentofcil Orthop 1994;105:592 597. 2. Bergström K, Jensen R. The significnce of third molrs in the etiology of crowding. Trns Eur Orthod Soc 1966:84 96. 3. Schwrze CW. The influence of third molr germectomy comprtive long-term study. Trns 3rd Int Orthod Congress. London: Crosy Lockwood Stples, 1975:551 562. 4. Schwrze CW. Nchuntersuchungsefunde ei Ptienten mit Extrktion zweiter Molren. Fortschr Kiefer orthop 1980;41:105 128. 5. Lindqvist B, Thilnder B. Extrction of third molrs in cses of nticipted crowding in the lower jw. Am J Orthod 1982;81:130 139. 6. Richrdson ME. The effect of lower second molr extrction on lte lower rch crowding. Angle Orthod 1983;53:25 28. 7. Richrdson ME. The role of the third molr in the cuse of lte lower rch crowding: A review. Am J Orthod Dentofcil Orthop 1989;95:79 83. 8. Richrdson ME. Lte lower rch crowding in reltion to skeletl nd dentl morphology nd growth chnges. Br J Orthod 1996;23:249 254. 9. Little RM, Riedel RA, Årtun J. An evlution of chnges in mndiulr nterior lignment from 10 to 20 yers postretention. Am J Orthod 1988;93:423 428. 10. Kpln RG. Mndiulr third molrs nd postretention crowding. Am J Orthod 1976;70:147 153. 11. Ades AG, Joondeph DR, Little RM, Chpko MK. A long-term study of the reltionship of third molrs to chnges in the mndiulr dentl rch. Am J Orthod Dentofcil Orthop 1990;97:323 335. 12. Hrrdine NWT, Person MH, Toth B. The effect of extrction of third molrs on lte lower incisor crowding: A rndomized controlled tril. Br J Orthod 1998;25:117 122. 13. vn der Schoot EAM, Kuitert RB, vn Ginkel FC, Prhl-Andersen B. Clinicl relevnce of third permnent molrs in reltion to crowding fter orthodontic tretment. J Dent 1997;25:167 169. 14. Ricketts RM, Bench RW, Gugino CF, Hilgers JJ, Schulhof RJ. Bioprogressive Therpy. Denver: Rocky Mountin Orthodontics, 1979. 15. Björk A, Skieller V. Norml nd norml growth of the mndile: A synthesis of longitudinl cephlometric implnt studies over period of 25 yers. Eur J Orthod 1983;5:1 46. 16. Iseri H, Solow B. Continued eruption of mxillry incisors nd first molrs in girls from 9 to 25 yers, studied y the implnt method. Eur J Orthod 1996;18:245 256. 17. Richrdson ME, Mills K. Lte lower rch crowding: The effect of second molr extrction. Am J Orthod Dentofcil Orthop 1990;98:242 246. 18. Brickley MR, Shepherd JP. An investigtion of the rtionlity of lower third molr removl, sed on USA Ntionl Institutes of Helth criteri. Br Dent J 1996;180:249 254. 19. Ahlqwist M, Gröndhl H. Prevlence of impcted teeth nd ssocited pthology in middle ged nd older Swedish women. Community Dent Orl Epidemiol 1991;19:116 119. 20. Nemcovsky CE, Lifeld H, Zuery Y. Effect of non-erupted 3rd molrs on distl roots nd supporting structures of pproximl teeth. A rdiogrphic survey of 202 cses. J Clin Periodontol 1996;23:810 815. Hve question you would like to see fetured in this column? Send it to: T. M. Grer, Editor-in-Chief University of Illinois t Chicgo, College of Dentistry 801 South Pulin, M/C 842 Chicgo, Illinois 60612, USA or E-mil to: tgrer@uic.edu 186