The main occluding area in normal occlusion and mandibular prognathism

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1 Originl Article The min occluding re in norml occlusion nd mndibulr prognthism Mkoto Kurokw ; Hiroyuki Knzki b ; Hjime Tokiw c ; Hideho Hnd d ; Kzutoshi Nkok e ; Yoshiki Hmd f ; Hitoshi Kto g ; Yoshiki Nkmur h ABSTRACT Objective: To clrify whether the concept of min occluding re, where hrd food is initilly crushed, exists in ptients who hve jw deformity. Mterils nd Methods: Nineteen subjects with norml occlusion, 18 ptients with mndibulr prognthism, nd 11 ptients with mndibulr prognthism who hd undergone orthognthic surgery prticipted in this study. The min occluding re ws identified by clenching Temporry Stopping. The coincidence, loction of the min occluding re, nd distnce from the first molrs to min occluding re were exmined. Results: High coincidence of the min occluding re ws obtined in ll groups, signifying tht the min occluding re exists even in these ptients. Mndibulr min occluding re ws locted on the first molr in ll groups. Mxillry min occluding re in subjects with norml occlusion ws locted on the first molr. However, it ws locted on the second premolr nd first molr in ptients with mndibulr prognthism, nd on the first nd second molrs in ptients with mndibulr prognthism who hd undergone orthognthic surgery. There ws sttisticlly significnt difference in distnce from the mxillry first molr to the min occluding re mong groups, but there ws no difference in the distnce from the mndibulr first molr mong groups. Conclusion: The min occluding re is more stble on the mndibulr first molr thn the mxill in ll groups. (Angle Orthod. 2016;86:87 93.) KEY WORDS: Min occluding re; Mndibulr prognthism; Orthognthic surgery; Occlusion; Mstiction Reserch Associte, Deprtment of Orthodontics, School of Dentl Medicine, Tsurumi University, Yokohm, Jpn. b Lecturer, Deprtment of Orthodontics, School of Dentl Medicine, Tsurumi University, Yokohm, Jpn. c Clinicl Professor, Deprtment of Orthodontics, School of Dentl Medicine, Tsurumi University, Yokohm, Jpn. d Director, Hnd Orthodontic Office, Ymto, Jpn. e Lecturer, Deprtment of Orl nd Mxillofcil Surgery, School of Dentl Medicine, Tsurumi University, Yokohm, Jpn. f Professor, Deprtment of Orl nd Mxillofcil Surgery, School of Dentl Medicine, Tsurumi University, Yokohm, Jpn. g Director, Dentl Clinic of Tokyo Securities Industry Helth Insurnce Society, Tokyo, Jpn. h Professor, Deprtment of Orthodontics, School of Dentl Medicine, Tsurumi University, Yokohm, Jpn. Corresponding uthor: Professor Y. Nkmur, Deprtment of Orthodontics, Tsurumi University, Tsurumi, Tsurumi-ku, Yokohm, Kngw , Jpn (e-mil: nkmur-ys@tsurumi-u.c.jp) Accepted: Jnury Submitted: November Published Online: April 15, 2015 G 2016 by The EH Angle Eduction nd Reserch Foundtion, Inc. INTRODUCTION Mstiction is n importnt function of the stomtognthic system. 1 Mstictory function hs been evluted in terms of mstictory efficiency, 2 electromyogrphic nlysis, 3 mstictory movement, 4 nd occlusl force. 5 However, the process of hrd food crushing by molrs during mstiction hs not been clrified. Kto et l. 6 first described n occlusl re where hrd foods re initilly crushed in the erly step of mstiction nd nmed the re, the min occluding re. He found tht the min occluding re plys n importnt role in the initil step of mstiction, nd is usully locted on the mxillry nd mndibulr first molrs in most of the subjects. This concept of the min occluding re hs been bsed on reserch in subjects with norml occlusion. However, no study hs yet confirmed the existence of such n re or loction in ptients with mndibulr prognthism who hve lrge nteroposterior discrepncy between mxillry nd mndibulr molrs s compred to tht of subjects DOI: /

2 88 KUROKAWA, KANZAKI, TOKIWA, HANDA, NAKAOKA, HAMADA, KATO, NAKAMURA with norml occlusion. In ddition, there is no report bout the detiled process of mstiction in such ptients. Despite the lrge discrepncy in molrs, these ptients cn crush hrd food, signifying these ptients would hve min occluding re too. Therefore, we hypothesized tht ptients with mndibulr prognthism lso hve stble min occluding re. Reserch on the min occluding re in ptients with mndibulr prognthism will help to elucidte the mstictory mechnism. The objective of this study, therefore, ws to exmine the min occluding re in ptients with mndibulr prognthism before nd fter surgicl orthodontic tretment. MATERIALS AND METHODS The uthors hve no conflicts of interest. The present study ws pproved by the Ethics Committee of Tsurumi University Hospitl nd conformed to the principles of the Declrtion of Helsinki. All subjects provided informed consent, nd written consent forms were obtined before this study. Subjects with norml occlusion (NORM), ptients with mndibulr prognthism (MP), nd ptients with mndibulr prognthism who hd undergone orthognthic surgery (MPOS) were included in the present study. The NORM group (n 5 19) met the following inclusion criteri: N no missing teeth other thn the third molr, N no mjor restortion covering the cusp, N pproprite overjet nd overbite, N midline devition of 1.0 mm or less, N no functionl symptoms such s temporomndibulr joint disorder, N no history of orthodontic tretment, N Angle Clss I molr reltionship (Figure 1), nd N norml intermxillry reltionship identified or clssified by ANB ngle nd Wits nlysis (Figure 1b). The MP group comprising 18 ptients with mndibulr prognthism who required orthognthic surgery t the Deprtment of Orthodontics, School of Dentl Medicine, Tsurumi University, met the following inclusion criteri: N no congenitl bnormlities, N no missing teeth other thn the third molr, N no mjor restortion covering the cusp, N overbite of 1.0 mm or more, N overjet of less thn 0 mm, N no notble mndibulr symmetry, N severe Angle Clss III molr reltionship with the occlusion between the mxillry second premolr nd mndibulr first molr (Figure 1), nd N skeletl Clss III intermxillry reltionship in ANB ngle nd Wits nlysis (Figure 1b). The MPOS group, consisting of 11 ptients with mndibulr prognthism who hd undergone orthognthic surgery, met the following inclusion criteri: N no missing teeth other thn the third molr t the first visit, N no mjor restortion covering the cusp, N pproprite overjet nd overbite, N midline devition of 1.0 mm or less, N preopertive occlusion between the mxillry second premolr nd mndibulr first molr, N t lest 1 yer since orthognthic surgery, N Angle Clss II molr reltionship due to the extrction of the mxillry bilterl first premolrs in orthodontic tretment nd mndibulr setbck (Figure 1), nd N norml intermxillry reltionship identified or clssified by ANB ngle nd Wits nlysis (Figure 1b). Tble 1 shows the morphologic chrcteristics of the subjects in the NORM, MP, nd MPOS groups in terms of ANB ngle nd Wits pprisl on the lterl cephlometric rdiogrph. Test Mteril Dentl stopping (Temporry Stopping, GC, Tokyo, Jpn; 3.4 mm in dimeter), which is commonly used for dentl temporry filling in Jpn, ws used s the test mteril in this study. The mteril ws cut to 4.0 mm in length t room temperture, nd one piece ws used for ech exmintion. Identifiction of the Min Occluding Are The min occluding re ws identified in ccordnce with the method of Kto et l. 6 Briefly, one piece of dentl stopping ws plced on the center of the dorsl surfce of the tongue in ech subject s test food, nd the subject ws instructed to clench it once on the prticulr occluding re tht ws preferbly used during mstiction (Figure 2). The subjects were trined to clench dentl stopping severl times on the left nd right side respectively. Then the subjects were instructed to clench it on their preferble side. After tht, the dentl stopping ws crefully tken out from the orl cvity nd subsequently trnsferred onto prepred dentl stone model for nlysis. The loction of the dentl stopping on the dentl rch ws defined s the min occluding re. The sme procedure ws repeted three times for ech subject. Evlution Method Coincidence of the min occluding re. We exmined the loction of the min occluding re in three trils to confirm the reproducibility of the method.

3 MAIN OCCLUDING AREA IN MANDIBULAR PROGNATHISM 89 Figure 1. Representtive photogrphs of the dentl stone models nd cephlometric digrms of ech group. () Representtive photogrphs of dentl stone models of subjects in the NORM, MP, nd MPOS groups re shown. Photogrphs re of dentl stone models of the subject closest to the chrcteristics of ech group. (b) Representtive cephlometric digrms of subjects in the NORM, MP, nd MPOS groups re shown. Figure trcing of lterl cephlometric rdiogrphs of the subject closest to the verge of the skeletl chrcteristics of ech group. The dentl stopping ws exmined from the point of whether the site includes the sme cusp or interdentl region. Tble 1. Cephlometric Chrcteristics of the Subjects Groupsb Prmeter NORM ANB, (degrees) ; NS (Wits mm) * MP MPOS * ; NS * * NORM indictes subjects with norml occlusion; MP, ptients with mndibulr prognthism; nd MPOS, ptients with mndibulr prognthism who hd undergone orthognthic surgery. * The Mnn-Whitney U-test (comprison mong groups); P,.05. NS indictes no significnt difference. Sttisticlly significnt difference ws observed mong groups, except for the ANB between NORM nd MPOS. Loction of the min occluding re. The loction of the mxillry nd mndibulr min occluding res in subjects of the NORM, MP, nd MPOS groups ws compred. Distnce from the first molrs to the midpoint of the min occluding re. The distnces from the mesiobuccl cusp tip of the mxillry first molr, or the mesiobuccl groove of the mndibulr first molr to the mesiodistl midpoint of the dentl stopping were mesured by clipers. The mesiobuccl cusp tip of the mxillry first molr or the mesiobuccl groove of the mndibulr first molr served s the reference point. Mesil distnces were regrded s positive vlue, nd distl distnces s negtive vlue (Figure 2). These mesurements were performed by single investigtor.

4 90 KUROKAWA, KANZAKI, TOKIWA, HANDA, NAKAOKA, HAMADA, KATO, NAKAMURA Figure 2. Identifiction of the min occluding re using piece of dentl stopping. () Plcement of the stopping on the tongue. (b) Chewing the stopping (rrow). (c) Imge of dentl stone model with the stopping indicting the min occluding re (circle). (d) Distnce from the first molrs to the midpoint of the min occluding re. Mesil distnces from molr were regrded s positive vlue, nd distl distnces s negtive vlue. Sttisticl Anlysis The coincidence of loctions within ech group ws nlyzed using the chi-squre test. The Fisher exct test ws used to compre the percentge of the coincidence mong the three groups. The Mnn- Whitney U-test for between-group comprisons with Bonferroni correction ws used for multiple comprisons of the men distnce from the midpoint of the min occluding re to the first molrs. A P vlue less thn.05 ws considered sttisticlly significnt. Sttisticl nlyses mentioned bove were performed with SPSS for Windows (Chicgo, Ill). RESULTS Coincidence of the Min Occluding Are Most of the subjects showed high coincidence of theminoccludingreinthethreetrils(tble2). IntheNORMgroup,chewingwsseentthesme loction in ll three trils in 16/19 subjects (84.2%), nd two trils in 3/19 subjects (15.8%). In the MP group, chewing ws seen t the sme loction in ll three trils in 17/18 subjects (94.4%), nd two trils in 1/18 subjects (5.6%). Interestingly, 11/11 subjects (100%) in the MPOS group chewed in the sme loction in ll three trils. The percentge of subjects who chewed in the sme loction in ll three trils ws significntly higher thn those who chewed in the sme loction twice within ll groups. There ws no sttisticlly significnt difference in thepercentgeofthesubjectswhochewedinthe sme loction in ll three trils mong three groups. These results suggest tht the min occluding re in ll three groups seems reproducible, even in the MP group. Tble 2. The Coincidence of the Min Occluding Are Groups b Three of Three Times (%) (A) Two of Three Times (%) (B) Significnce Between (A) nd (B) NORM (n 5 19) 16 (84.2); NS 3 (15.8) * MP (n 5 18) 17 (94.4); NS 1 (5.6) * MPOS (n 5 11) 11 (100); NS 0 (0) * Chi-squre test (comprison within group). Fisher exct test (comprison mong groups). b NORM indictes subjects with norml occlusion; MP, ptients with mndibulr prognthism; nd MPOS, ptients with mndibulr prognthism who hd undergone orthognthic surgery. * P,.05. NS indictes no significnt difference in three of the three times (%) between groups.

5 MAIN OCCLUDING AREA IN MANDIBULAR PROGNATHISM 91 Tble 3. Loction of the Min Occluding Are (n 5 48) Loction of the Min Occluding Are Groups Second Premolr (%) First Molr (%) Second Molr (%) NORM (n 5 19) Mxillry 19 (100) Mndibulr 19 (100) MP (n 5 18) Mxillry 13 (72.2) 5 (27.8) Mndibulr 13 (72.2) 5 (27.8) MPOS (n 5 11) Mxillry 8 (72.7) 3 (27.3) Mndibulr 11 (100) NORM indictes subjects with norml occlusion; MP, ptients with mndibulr prognthism; nd MPOS, ptients with mndibulr prognthism who hd undergone orthognthic surgery. Loction of the Min Occluding Are We then nlyzed the loction of the min occluding re in ech group (Tble 3). The min occluding re ws locted on the mxillry nd mndibulr first molrs in the NORM group. However, the min occluding res in the MP nd MPOS groups were somewht different from tht in the NORM group. The mxillry min occluding re ws locted on the second premolr in 13/18 subjects (72.2%) nd first molr in 5/18 subjects (27.8%) in the MP group. The mndibulr min occluding re ws locted on the first molr in 13/18 subjects (72.2%) nd on the second molr in 5/18 subjects (27.8%) in the MP group. Interestingly, in the MPOS group, the mxillry min occluding re ws locted on the first molr in 8/ 11 subjects (72.7%) nd second molr in 3/11 subjects (27.3%); but in the mndible, it ws locted on the first molr in 11/11 subjects (100%) (ie, the min occluding re ws converged to the first molr). The loction of the min occluding re ws more stble on the mndibulr first molr thn mxillry first molr in the MP group. Mesiodistl Distnce From the Reference Points to the Min Occluding Are Finlly, we mesured the distnce from the reference points to the midpoint of the min occluding re in both the mxill nd mndible (Figure 3). The men distnce from the reference points in mxillry first molr ws 23.3, +2.1, nd 25.9 mm in the NORM, MP, nd MPOS groups, respectively (Figure 3). There were sttisticlly significnt differences mong the groups. On the other hnd, the distnce from the reference points in mndibulr first molr ws 23.2, 24.1, nd 21.2 mm, respectively (Figure 3b), nd no significnt difference ws observed. Compred to the men distnce bsed on the mxillry molr s reference, the men distnce bsed on the mndibulr molr s reference ws more stble mong groups, signifying tht the min occluding re of the mndible seems more stble thn tht of the mxill. DISCUSSION The present study using the cross-sectionl smples demonstrted tht the concept of the min occluding re is pplicble to ptients with mndibulr prognthism. Furthermore, by compring mndibulr prognthism with norml occlusion, we reveled tht the mndibulr first molr plys more importnt role s the min occluding re thn the mxillry first molr, irrespective of the nteroposterior jw reltionship. This study ws the first to exmine the presence of min occluding re in both the mxill nd mndible in ptients with mndibulr prognthism. Kto et l. 6 hd reported tht the min occluding re in norml occlusion ws locted on the mxillry nd mndibulr first molrs in most subjects, nd concluded tht the mximl biting force might be exerted in the first molr region. Nkt et l. 7 reported the chnge of the min occluding re with dentl ge. In the deciduous dentition, the min occluding re is locted on the second deciduous molrs nd then it shifts to the permnent first molr with eruption of the first molr nd exfolition of the second deciduous molr. Furthermore, Tsuchiy et l. 8 reported tht the min occluding re in dults locted on the permnent first molr tends to be more stble thn on the second deciduous molr. These results, however, were ll derived from the dt of the subjects with norml occlusion. No previous study hs exmined the ptients with nteroposterior discrepncy in the mxillomndibulr jw reltionship, such s mndibulr prognthism, or those who hve experienced remrkble chnge in the mxillomndibulr jw reltionship by orthognthic surgery. Therefore, we exmined the min occluding re in the ptients with mndibulr prognthism nd mndibulr prognthism who hd undergone orthognthic surgery. As for the coincidence of the min occluding re, Kto et l. 6 reported tht the percentge of the subjects who showed the coincidence in five trils ws 67%. In the present study, the coincidence in ll three trils ws over 80% in ll groups nd higher thn tht in the report by Kto et l. 6 This high coincidence

6 92 KUROKAWA, KANZAKI, TOKIWA, HANDA, NAKAOKA, HAMADA, KATO, NAKAMURA Figure 3. Distnce from the first molrs to the midpoint of the min occluding re. () Distnce from the mesiobuccl cusp tip of the mxillry first molr to the midpoint of the min occluding re re shown. *P,.05 between groups. (b) Distnce from the mesiobuccl groove of the mndibulr first molr to the midpoint of the min occluding re. NS indictes no significnt difference between groups. suggests tht our experimentl method is relible nd reproducible even though we exmined just three trils. In ddition, the high coincidence in the ptients with mndibulr prognthism s compred to the subjects with norml occlusion indictes tht the concept of the min occluding re exists in ptients with mndibulr prognthism. The percentge of the loction of the min occluding re on the first molrs in both the mxill nd mndible in our study is somewht different from the others. In our study, the first molr ws the min occluding re in both the mxill nd mndible in ll subjects with norml occlusion, with very high frequency, s compred with the dt of other studies where the min occluding re ws locted on the mxillry nd mndibulr first molr in 55.0% 6 nd 55.6% of cses. 9 This might be due to the strict selection criteri of the subjects in this study. Our inclusion criteri for norml occlusion included not only Angle Clss I molr reltionship, but lso intermxillry reltionship judged by lterl cephlometric rdiogrphs. In most of the ptients with mndibulr prognthism nd mndibulr prognthism who hd undergone orthognthic surgery, the min occluding re ws lso locted on the mndibulr first molr, which ws the sme s tht in the subjects with norml occlusion. However, the percentge of ptients with mxillry occluding re on the first molr ws only 27.8%, which is considerbly different from tht in the subjects with norml occlusion. This is due to the difference in molr reltionship between norml occlusion nd mndibulr prognthism (Angle Clss I nd severe Angle Clss III). These results indicte tht the min occluding re on the mndibulr first molr is stble, irrespective of molr reltionship. The min occluding re on the mndibulr first molr remined unchnged during the tretment consisting of extrction of first premolrs nd mndibulr setbck. It ws locted on the mndibulr first molr in ll ptients with mndibulr prognthism who hd undergone orthognthic surgery. However, the mxillry min occluding re chnged from the second premolr nd first molr (severe Angle Clss III molr reltionship) to first nd second molrs (Angle Clss II molr reltionship), both with which the mndibulr first molr occludes. These results strongly indicte tht teeth with the min occluding re in the mxill re the ntgonists to mndibulr first molr. Becuse these results were obtined from ptients with extrction of the mxillry bilterl first molrs in orthodontic tretment nd mndibulr setbck in orthognthic surgery, it is necessry to exmine the min occluding re in ptients with only extrction of the mxillry bilterl first molr in future study. The results in this study indicte the importnce of the mndibulr first molr in occlusion nd mstiction. In food bolus trnsport, food bolus on the occlusl surfce of the mndibulr molrs is very stble nd esily crushed by occlusion with the ntgonists. The mndibulr first molr hs the lrgest coronl dimeter mong teeth, both mesiodistlly nd buccolingully, which mkes it esier for the tongue to crry food bolus on the occlusl surfce s compred with those of the incisors nd premolrs. 15 The mndibulr first molr is locted nterior to the second molr, which mkes the tongue smoothly crry food bolus onto the surfce. In summry, we found tht the concept of the min occluding re is pplicble to ptients with mndib-

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