Originl Article Agenesis of Third Molr Germs Depends on Sgittl illry Jw Dimensions in Orthodontic Ptients in Jpn Tkshi S. Kjii, DDS, PhD ; Yoshiki Sto, DDS, PhD b ; Sori Kjii, DDS c ; Yuki Sugwr, DDS, PhD ; Junichiro Iid DDS, PhD d Abstrct: The im of this study ws to determine the correltion between congenitlly missing third molr tooth germs nd sgittl mxillomndibulr jw dimensions in orthodontic ptients in Jpn. The subjects were 391 ptients from the orthodontic clinic of the Hokkido University Dentl Hospitl who were less thn 15 yers of ge. Assessments were mde from pnormic rdiogrphs nd lterl cephlogrms. The subjects were divided into mxillry/mndibulr third molr bsent nd n existent group. The ngle nd the sgittl dimensions of the nsl floor (), mxillry bsl bone (), mndibulr corpus (), nd mndibulr bsl bone () were mesured. Logistic regression nlysis ws used to estimte ssocitions between third molr genesis nd these mesures. The following results were obtined: (1) The frequency of the mxillry third molr genesis significntly incresed with decresing (odds rtio 0.559, 95% confidence intervl 0.377 0.829). The frequency of the mndibulr third molr genesis lso incresed with decresing (odds rtio 0.532, 95% confidence intervl 0.330 0.856). (2) There were no significnt correltions between nd mndibulr third molr genesis. These results suggest tht genesis of third molr germs does not depend on nteroposterior dimensions of the mndible but depends on nteroposterior dimensions of the mxill in Jpnese orthodontic ptients. (Angle Orthod 2004;74:337 342.) Key Words: Third molr germs; Congenitl bsence; ill; dible; Posterior discrepncy INTRODUCTION There hve been mny reports tht describe the congenitl bsence of third molrs in Europen Americn, 1 7 nd Asin 8 11 ptients. In Jpn, mny investigtors nd clinicins, especilly orthodontists, believe tht n increse in genesis of permnent teeth is relted to degenertion of dentofcil development over the pst 5000 yers. 9 Is there tendency for higher incidence of genesis of third molrs? Unfortuntely, there hve been few reports on chronologicl chnges in third molr genesis. 12,13 Therefore, we previously investigted 14 the congenitl Assistnt Professor, Section of Orthodontics, Grdute School of Dentl Medicine, Hokkido University, Spporo, Jpn. b Associte Professor, Section of Orthodontics, Grdute School of Dentl Medicine, Hokkido University, Spporo, Jpn. c Privte Prctice, Mito, Ibrgi, Jpn. d Professor nd Chir, Section of Orthodontics, Grdute School of Dentl Medicine, Hokkido University, Spporo, Jpn. Corresponding uthor: Tkshi S. Kjii, Section of Orthodontics, Deprtment of Orl Functionl Science, Division of Orl Medicl Science, Grdute School of Dentl Medicine, Hokkido University, Kit 13 Nishi 7 Kit-ku, Spporo 060-8586, Hokkido, Jpn (e-mil: kjii@den.hokudi.c.jp). Accepted: July 2003. Submitted: April 2003. 2004 by The EH Angle Eduction nd Reserch Foundtion, Inc. bsence of third molr germs in Jpnese orthodontic ptients, nd we exmined the reltionships between the bsence of third molrs nd sgittl mxillomndibulr jw reltionships. The following results were obtined: (1) the percentge of Jpnese individuls who hve congenitlly missing third molrs seems to hve decresed slightly, (2) the frequency of the bsence of mndibulr third molr germs is lower thn tht of mxillry third molr germs in Jpnese individuls, nd (3) in Jpnese orthodontic ptients, the percentge of skeletl Clss II ptients with one or more third molr geneses is lower thn tht of skeletl Clss III ptients. On the other hnd, the reltionship between third molrs nd crowding hs been debted for mny yers. 15 18 Merrifield 19 dvocted posterior discrepncy nd suggested tht orthodontists should consider the entire dentition. The reltionship between posterior discrepncy nd relpse fter retention hs been debted 20 22 for more thn 50 yers. A posterior discrepncy is thought to hve n inhibitory effect on the eruption of second nd third molrs nd my cuse relpse fter retention regrdless of whether premolrs hve been extrcted. Spce deficiency for the eruption of not only third molrs but lso second molrs hs recently been reported in Clss II ptients. 23,24 Skeletl Clss II ptients generlly hve lrge mxill 337
338 KAJII, SATO, KAJII, SUGAWARA, IIDA FIGURE 1. Distributions of ges of subjects in this study. () ll subjects; (b) mle; nd (c) femle. TABLE 1. Subjects Number Dte of Birth Age t Initil Exmintion 391 (145 mles, 246 femles) October 28, 1966 July 20, 1987 5 y 7 mo 14 y 11 mo nd/or smll mndible, 25 wheres skeletl Clss III ptients generlly hve smll mxill nd/or lrge mndible. The percentge of Jpnese orthodontic ptients with one or more third molr geneses is lower in skeletl Clss II ptients thn in skeletl Clss III ptients. 14 In ddition, some reports speculte tht the sme genes my regulte both crniofcil nd tooth morphogenesis. 8 On the bsis of these fcts nd specultions, we hypothesize tht the genesis of mxillry third molr germs depends on nteroposterior dimensions of the mxill when third molr formtion begins, lthough genesis of mndibulr third molr germs does not depend on nteroposterior dimensions of the mndible. To prove this hypothesis, we investigted the correltions between genesis of third molr germs nd sgittl mxillomndibulr jw dimensions in orthodontic ptients in Jpn. MATERIALS AND METHODS Subjects Three hundred ninety-one ptients (145 mles nd 246 femles) were selected for this study from the orthodontic clinic of the Hokkido University Dentl Hospitl (Figure 1; Tble 1). All the subjects were younger thn 15 yers old when they were exmined initilly. Subjects with congenitl deformities, such s cleft plte, were excluded from the study. Mssler et l 26 reported tht third molr crypt formtion
AGENESIS OF THIRD MOLARS DEPENDS ON MAXILLARY MASS 339 begins t three to four yers of ge. Clcifiction strts t 7 to 10 yers of ge, nd clcifiction of the crown is completed t 12 to16 yers of ge nd eruption begins t 17 to 21 yers of ge. This mens tht few people younger thn 15 yers old would hve hd third molr extrcted becuse of dentl disese such s pericoronitis. This ws the reson for the selection of subjects younger thn 15 yers old for our study. Investigtions by Grn et l 27 nd Grvely 4 suggested tht the upper ge limit for third molr genesis is 13 yers. There re some reports, 1,5,28,29 however, of third molr development s lte s 14 or 15 yers of ge. We, therefore, exmined ptients up to 14 yers old. Mterils Pnormic rdiogrphs nd lterl cephlogrms tken t the initil exmintion were used to determine the presence of third molr germs nd to mesure ngles nd dimensions of the jw (Figure 2). In cses where it ws impossible to judge the presence of third molr germs from the pnormic rdiogrphs tken t the initil exmintion, subsequent pnormic rdiogrphs tken before the ge of 14 yers were used. Third molrs or third molr germs refer to both impcted germs nd erupted teeth. The subjects were divided into right nd/or left mxillry third molr bsent group (cse n 64) nd bothexistent group (control n 327). In the sme wy, the subjects were lso divided into right nd/or left mndibulr third molr bsent group (n 38) nd both-existent group (n 353). Cephlometric nlysis The ngle nd the nteroposterior lengths of the nsl floor (), the mxillry bsl bone (A-Ptm ), the mndibulr corpus (), nd mndibulr bsl bone (ABR-B ) were mesured on lterl cephlogrms of ech subject exposed t the initil exmintion (Figure 2). ABR is the point where the occlusl plne crosses the nterior edge of the rmus. Sttisticl nlysis The vlues of these mesurements depend on the ge of the subjects. Therefore, these vlues were stndrdized using verge vlues nd stndrd devitions selected from seril records of Jpnese subjects included in the files of longitudinl crniofcil growth study t the Hokkido University 30 or t the Osk University Dentl School. 31 Nondjusted nd djusted logistic regression nlyses were used to estimte the ssocitions between third molr genesis nd these cephlometric vlues. These nlyses were crried out with the sttisticl pckge SPSS Ver. 8.0 (SPSS Inc, Chicgo, Ill), with probbility level of.05 considered sttisticlly significnt. Hosmer Lemeshow tests were used for ssessment of overll model goodness-of-fit. FIGURE 2. Liner cephlometric mesurements relting to sgittl jw dimensions. () (mm), nteroposterior length of the nsl floor; (b) A-Ptm (, mm), nteroposterior length of the mxillry bsl bone; (c) (mm), nteroposterior length of the corpus; (d) ABR-B (, mm), nteroposterior length of the mndibulr bsl bone (ABR: cross point between occlusl plne nd nterior edge of the rmus). TABLE 2. Crude Odds Rtios: illry Third Molr Absent Group vs Existent Group Vribles Odds Rtio 1.485 0.803 0.826 0.896 0.649 1.135 Intervl RESULTS 0.831 2.653 0.686 0.941 0.656 1.041 0.747 1.075 0.508 0.828 0.938 1.372 P Vlue.182.007**.105.238.005**.193 Tble 2 shows the results of the nondjusted logistic regression nlysis tht estimtes the ssocitions between mxillry third molr genesis nd cephlometric mesure-
340 KAJII, SATO, KAJII, SUGAWARA, IIDA TABLE 3. Adjusted Odds Rtios: illry Third Molr Absent Group vs Existent Group Vribles Odds Rtio 1.821 0.910 1.260 0.847 0.559 1.227 Intervl 0.963 3.443 0.717 1.154 0.896 1.773 0.672 1.068 0.377 0.829 0.947 1.588 P Vlue.065.434.184.160.004**.121 TABLE 5. Adjusted Odds Rtios: dibulr Third Molr Absent Group vs Existent Group Vribles Odds Rtio 0.998 0.938 1.378 0.977 0.532 0.968 Intervl 0.482 2.065 0.701 1.254 0.898 2.115 0.740 1.289 0.330 0.856 0.703 1.335 P Vlue.996.665.143.867.009**.844 TABLE 4. Crude Odds Rtios: dibulr Third Molr Absent Group vs Existent Group Vribles Odds Rtio 0.792 0.848 0.871 0.968 0.628 0.970 Intervl 0.401 1.562 0.698 1.031 0.654 1.160 0.773 1.211 0.464 0.849 0.760 1.237 P Vlue.501.098.344.776.003**.805 ments. The numbers of subjects were 64 in the mxillry third molr bsent group nd 327 in the existent group. The frequency of mxillry third molr genesis significntly incresed with decresing (odds rtio 0.803, 95% confidence intervl 0.686 0.941) nd with decresing (odds rtio 0.649, 95% confidence intervl 0.508 0.828). After djustment for sex,,,,, nd, the frequency of mxillry third molr genesis incresed significntly further with decrese in (odds rtio 0.559, 95% confidence intervl 0.377 0.829) (Tble 3). Tble 4 shows the results of the nondjusted logistic regression nlyses tht estimte the ssocitions between mndibulr third molr genesis nd cephlometric mesurements. The numbers of subjects were 38 in the mndibulr third molr bsent group nd 353 in the existent group. The frequency of mndibulr third molr genesis lso incresed with decresing (odds rtio 0.628, 95% confidence intervl 0.464 0.849). Tble 5 shows the results of the logistic regression nlyses tht estimte the ssocitions between mndibulr third molr genesis nd cephlometric mesurements fter djustment for sex,,,,, nd. The frequency of mndibulr third molr genesis incresed further with decresing (odds rtio 0.532, 95% confidence intervl 0.330 0.856). There were no significnt ssocitions between mndibulr third molr genesis nd or. Hosmer Lemeshow tests were used for ssessment of overll model goodness-of-fit. Probbility vlues were.465 (mxillry third molr model) nd.665 (mndibulr third molr model). Thus, these models were fitted well. DISCUSSION The frequency of mxillry third molr genesis significntly incresed with decresing (Tble 3). The frequency of mndibulr third molr genesis lso incresed with decresing (Tble 5). On the other hnd, there were no significnt correltions between nd mndibulr third molr genesis (Tble 5). These results suggest tht genesis of third molr germs is not relted to nteroposterior dimensions of the mndible but is relted to those of the mxill in Jpnese orthodontic ptients. Only few reports 32,33 support our suggestion. Becuse skeletl Clss II ptients generlly hve lrge mxill nd/or smll mndible 25 nd skeletl Clss III ptients generlly hve smll mxill nd/or lrge mndible, these results lso explin why the percentge of skeletl Clss II ptients missing one or more third molrs is lower thn tht of skeletl Clss III ptients. 14 Therefore, spce deficiency for eruption of not only mndibulr third molrs but lso mndibulr second molrs is often found in Clss II ptients. 23,24 There hve been some reports compring the genesis of third molrs in different rces. Brothwell et l 34 nd Stewrt 35 reported tht third molr genesis in the Mongolin popultion, including the Jpnese popultion, is higher thn tht in the Europen Americn popultion. They lso reported tht the highest frequency of third molr germs existent is found in blck subjects. We speculte tht one of resons for these rcil differences is tht the Mongolin popultion my hve more skeletl Clss III ptients who hve smll mxill thn the Europen Americn popultion. There seems to be difference in third molr genesis in the upper nd lower rches between Asins nd Europen Americns. Specificlly, mndibulr third molr genesis is lower thn mxillry third molr genesis in Asins 8 11,14 but not in Europen Americns. 1 7 This suggestion is supported by results reported by Hillson. 36 However, the reson why there my be difference in third molr genesis in the upper nd lower rches between Asins nd Europen Americns is lso not cler.
AGENESIS OF THIRD MOLARS DEPENDS ON MAXILLARY MASS The reson why smll mxill is ssocited with not only mxillry third molr geneses but lso mndibulr third molr genesis is not cler. On the other hnd, some reports hve suggested tht homeobox genes nd growth fctor regulte crniofcil nd tooth morphogenesis. A missense muttion of the MSX1 gene t chromosome 4p16.1 cuses genesis of second premolrs nd third molrs in humns. 37,38 PAX9 t chromosome 14q12-q13 is lso ssocited with tooth genesis, 39 especilly molr genesis. 40 Thus, some polygenetic inheritnce controlling mxillry dimensions my be relted to genes on formtion of third molr germs. In future study, we will investigte the reltionship between genesis of third molr germs nd some congenitl deformities using cephlometric nlyses. Moleculr genetics of tooth morphogenesis nd of crniofcil mturtion should lso be studied. Some polygenetic inheritnce of congenitl deformities my lso be relted to genes controlling formtion of third molr germs. CONCLUSIONS The frequency of mxillry third molr genesis significntly incresed with decresing sgittl dimensions of the mxillry bsl bone. The frequency of mndibulr third molr genesis lso incresed with decresing sgittl dimensions of the mxillry bsl bone. On the other hnd, there were no significnt ssocitions between sgittl dimensions of the mndibulr bsl bone nd mndibulr third molr genesis. These results suggest tht genesis of third molr germs does not depend on nteroposterior dimensions of the mndible but depends on nteroposterior dimensions of the mxill in Jpnese orthodontic ptients. ACKNOWLEDGMENT The uthors would like to thnk Dr Hideyuki Imi (Grdute School of Engineering, Hokkido University) for his contributions to this study. This study ws supported by the grnt-in-id for Scientific Reserch (No. 14771171) from the Ministry of Eduction, Culture, Sports, Science nd Technology of Jpn. REFERENCES 1. Bnks HV. Incidence of third molr development. Angle Orthod. 1934;4:223 233. 2. Nnd RS. 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