Morphologic evaluation of the incisive canal and its proximity to the maxillary central incisors using computed tomography images

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1 Originl Article Morphologic evlution of the incisive cnl nd its proximity to the mxillry centrl incisors using computed tomogrphy imges Eun-Ae Cho ; Sung-Jin Kim ; Yoon Jeong Choi b ; Kyung-Ho Kim c ; Chooryung J. Chung d ABSTRACT Objective: To evlute the morphologic fetures nd the reltive position of the incisive cnl with regrd to the mxillry incisor roots using computed tomogrphy (CT). Mterils nd Methods: Morphologic evlution of the incisive cnl nd its proximity to the mxillry centrl incisors were mesured using CT imges of 38 dults with skeletl nd dentl clss I norml occlusion. Liner mesurements were performed on the xil cross-sectionl imges corresponding to three verticl levels, the pltl opening of the incisive cnl (L1), midlevel between the opening level nd the root pex of the mxillry centrl incisors (L2), nd the root pex of the mxillry centrl incisors (L3). Results: The percentge of subjects with n incisive cnl width greter thn the interroot distnce of the centrl incisors ws 86.8% nd 63.2% t levels L1 nd L2, respectively. The nteroposterior distnce between the mxillry incisor roots nd the border of the incisive cnl ws pproximtely 5 6 mm t levels L1 nd L2. Conclusion: The nteroposterior distnce between the mxillry centrl incisor roots nd the incisive cnl ws pproximtely 5 6 mm. More thn 60% of the subjects hd n incisive cnl width greter thn the interroot distnce. Evlution of the proximity of the incisive cnl to the mxillry incisors, long with its dimensionl chrcteristics, my be helpful when considerble mount of mxillry retrction is plnned. (Angle Orthod. 2016;86: ) KEY WORDS: Incisive cnl; Tooth movement; Envelope of discrepncy; Mximum retrction; Compliction INTRODUCTION The mxillry nterior teeth re importnt not only in chieving plesing dentl nd fcil esthetics but lso Grdute Student, Deprtment of Orthodontics, College of Dentistry, Yonsei University, Seoul, Kore. b Assistnt Professor, Deprtment of Orthodontics, Institute of Crniofcil Deformity, College of Dentistry, Yonsei University, Seoul, Kore. c Professor nd Chir, Deprtment of Orthodontics, Gngnm Severnce Hospitl, Institute of Crniofcil Deformity, College of Dentistry, Yonsei University, Seoul, Kore. d Associte Professor, Deprtment of Orthodontics, Gngnm Severnce Hospitl, Institute of Crniofcil Deformity, College of Dentistry, Yonsei University, Seoul, Kore. Corresponding uthor: Dr Chooryung J. Chung, Associte Professor, Deprtment of Orthodontics, Gngnm Severnce Hospitl, Institute of Crniofcil Deformity, College of Dentistry, Yonsei University, 211 Eonjuro, Gngnm-gu, Seoul , Kore (e-mil: crchung@yuhs.c) Accepted: October Submitted: June Published Online: November 27, 2015 G 2016 by The EH Angle Eduction nd Reserch Foundtion, Inc. in physiologicl functions including pronuncition nd mstiction. 1 5 Therefore, determintion of the threedimensionl (3D) position of the mxillry incisors is n integrl prt of orthodontic dignosis nd tretment plnning, nd vrious biomechnicl tretment modlities re employed to chieve the idel incisor position. The extent of orthodontic tooth movement is constrined by the periodontl ttchment pprtus; djcent ntomicl structures, such s the lveolr bone, tongue, nd lips; nd the biomechnicl limits of nchorge during orthodontic mechnotherpy Ackermn nd Proffit introduced the concept of the envelope of discrepncy, which grphiclly shows the extent of chnges possible with orthodontic tooth movement lone, with orthopedic or functionl pplince therpy for growth modifiction, nd with orthognthic surgery in combintion with orthodontic tretment. 7,11 Accordingly, it is trditionlly thought tht the mount of chnges possible for the mxillry incisors with orthodontic tretment lone re pproximtely 7, 2, 4, nd 2 mm for retrction, protrction, extrusion, nd intrusion, respectively. Interestingly, the rnge of tooth movement during retrction of the DOI: /

2 572 CHO, KIM, CHOI, KIM, CHUNG mxillry incisors fr exceeds the rnge of movement possible in other directions nd/or for other teeth. The ntomicl limit of mxillry incisor retrction is reportedly the pltl corticl plte However, nother ntomicl structure, the incisive cnl, runs more closely to the mxillry incisor roots between the centrl incisor roots in the medin plne thn does the pltl corticl plte. The incisive cnl connects the floor of the nsl cvity with the plte nd opens into the orl cvity s the incisive formen posterior to the mxillry centrl incisors. It runs prllel to the mxillry centrl incisors nd trnsmits the nsopltine vessels nd nerves, brnches of the mxillry rtery, nd the trigeminl nerve nd is surrounded by thick lyer of corticl bone Becuse of its proximity to the mxillry incisors, the possibility of surgicl invsion of the incisive cnl during dentl procedures hs been reported, nd this cn result in nonosseointegrtion of dentl implnts or sensory dysfunction. 23,24 Although the overll ntomy of the incisive cnl is well defined, its precise loction in reltion to the mxillry incisors is not well documented in the orthodontic literture. This my be becuse of the difficulties in detecting incisive cnl morphology using conventionl orthodontic rdiogrphs. However, with recent dvncements in 3D imging, the pproximtion of the mxillry incisor roots to the incisive cnl cn be frequently detected fter nterior retrction following orthodontic tretment (Figure 1). Interestingly, the contct of mxillry centrl incisor root with the incisive cnl fter mximum retrction ws ssocited with pprent root resorption rising potent complictions. 25 Therefore, the objective of this study ws to evlute the morphologic fetures nd the reltive position of the incisive cnl with regrd to the mxillry incisor roots using computed tomogrphy (CT). MATERIALS AND METHODS Mterils The mterils used for the study were retrieved from preexisting rchive of cross-sectionl evlutions of dults (ged. 18) with skeletl nd dentl clss I norml occlusion from the Institute of Crniofcil Deformity, Yonsei University (Seoul, Republic of Kore) 26,27 with the pprovl of the institutionl review bord. The retrospectively retrieved mterils were selected bsed on the following inclusion criteri: (1) the presence of clinicl, lterl cephlogrm, nd CT imges; (2) norml nteroposterior skeletl reltionship (ANB of 0u to 4u nd Wits pprisl of 4.0 mm to 0 mm); (3) norml overjet nd overbite with clss I molr nd cnine reltionship; (4) no history of Figure 1. Cone bem computed tomogrphy (CBCT) imges of the incisive cnl nd superimposition before nd fter nterior retrction. Axil sections of the mxillry nterior region representing the picl one third of the mxillry incisors before (A) nd fter (B) nterior retrction. Notice the contct of the mxillry right centrl incisor nd the pproximtion of the mxillry left centrl incisor roots to the incisive cnl (rrow) fter tretment. (C) CBCT superim position on the crnil bse before (in white) nd fter tretment (in blck). Notice the chnges in tooth position nd the lterl profile following orthodontic tretment. No distinct chnges were noted in the position of the incisive cnl (rrow). orthodontic or prosthetic tretment; nd (5) no missing teeth except the third molrs. Of 38 sets of mterils (men ge, yers), 21 were from men nd 17 were from women. The verge upper incisl ngle to the SN plne (U1 to SN) of the subjects ws 104.7u , indicting tht the upper incisor inclintion ws within the norml rnge. Morphologic Evlution of the Incisive Cnl nd Its Proximity to the Mxillry Centrl Incisors The CT imges were obtined using HiSpeed Advntge (GE Medicl Systems, Milwukee, Wis) with the high-resolution bone lgorithm (200 ma, 120 kv, scnning time of 1 second) using slice thickness of 1 mm. The CT imges sved in Digitl Imging nd Communiction in Medicine formt were nlyzed using InVivoDentl imging softwre (version 5.2, Antomge, Sn Jose, Clif). The xil plne ws oriented with the Frnkfort-horizontl (FH) plne s horizontl reference plne. It ws determined tht the sgittl plne ws perpendiculr to the xil plne nd prllel to the plne pssing through nterior nsl spine nd posterior nsl spine. Liner mesurements were performed on the xil cross-sectionl imges corresponding to three verticl levels tht were determined to exist in the sgittl plne: (1) the pltl opening of the incisive cnl (opening level, L1), (2) midlevel between the opening level nd the root pex of the mxillry centrl incisors (midlevel, L2), nd (3)

3 INCISIVE CANAL PROXIMITY TO THE MAXILLARY INCISORS 573 were not divided ccording to gender. One-wy nlysis of vrince followed by Tukey test ws used to investigte the influence of the differences in verticl levels. The sttisticl nlyses were performed using SPSS 18.0 (IBM, Armonk, N.Y.), with significnce level of P,.05. Figure 2. Lndmrks nd liner mesurements. (A) Three verticl levels of the incisive cnl: pltl opening level (L1), midlevel (L2), nd root pex level (L3). (B) Lndmrks for trnsverse mesurements: Rm indictes the most medil point of the mxillry centrl incisor roots; Rp, the most posterior point of the mxillry centrl incisor roots; Cl, the most lterl point of the incisive cnl; Rm-Rm, interroot distnce; Rp-Rp, posterior interroot distnce; Cl-Cl, cnl width. (C) Lndmrks for nteroposterior mesurements: C indictes the most nterior point of the incisive cnl; Ct, the tngent line through C; Rm-Ct, the distnce from Rm to Ct; Rm- Cnl, the distnce from Rm to the nterior border of the incisive cnl; Cl-Root, the distnce from Cl to the posterior border of the mxillry centrl incisor root. the root pex of the mxillry centrl incisors (root pex level, L3) (Figure 2A). Lndmrks nd mesurements re defined s follows: Rm, the most medil point of the mxillry centrl incisor roots; Rp, the most posterior point of the mxillry centrl incisor roots; Cl, the most lterl point of the incisive cnl; Rm-Rm, interroot distnce; Rp-Rp, posterior interroot distnce; Cl-Cl, cnl width (Figure 2B); C, the most nterior point of the incisive cnl; Ct, the tngent line through C; Rm-Ct, the distnce from Rm to Ct; Rm-Cnl, the distnce from Rm to the nterior border of the incisive cnl; Cl-Root, the distnce from Cl to the posterior border of the mxillry centrl incisor root (Figure 2C). With regrd to nteroposterior distnces, the smller vlue from the bilterl mesurements ws dopted s representtive vlue. Sttisticl Evlution All mesurements were mde by single exminer, who repeted them fter 2-week intervl. The Dhlberg formul ws used to clculte method errors: pp Se~ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d 2 =2n), where d 5 the difference between two mesurements nd n 5 the number of mesurement pirs. 28 The method errors obtined rnged from 0.28 to 0.64 mm. Becuse the two-smple t-test showed no significnt differences between men nd women for ny of the mesurements, mesurements RESULTS Incisive Cnl Width nd Interroot Distnce of the Mxillry Centrl Incisors The incisive cnl width (Cl-Cl) ws , , nd mm t levels L1, L2, nd L3, respectively. The incisive cnl width t L1 ws significntly smller thn tht t L3 (P,.05). Interroot distnce (Rm-Rm) ws , , nd mm t levels L1, L2, nd L3, respectively. Interroot distnce ws significntly incresed from L1 to L3 (P,.05). Posterior interroot distnce (Rp-Rp) ws , , nd mm t levels L1, L2, nd L3, respectively. At L3, Rm nd Rp represented the sme point, nd Rm-Rm ws consequently equivlent to Rp-Rp. In contrst to Rm-Rm, Rp-Rp t L3 ws significntly smller thn t L1 (P,.05) (Tble 1). The percentge of subjects with n incisive cnl width greter thn the interroot distnce (Rm-Rm) ws 86.8% (33/38), 63.2% (24/38), nd 0% (0/38) t levels L1, L2, nd L3, respectively. On the other hnd, the incisive cnl width ws smller thn Rp-Rp in ll of the subjects t ech verticl level (Tble 2). Proximity of the Anterior Border of the Incisive Cnl to the Mxillry Centrl Incisor Root Rm-Ct ws , , nd mm t L1, L2, nd L3, respectively. The mesurements of Rm-Cnl nd Cl-Root were nd mm t L1 nd nd mm t L2. Rm-Cnl nd Cl-Root were not mesurble t L3 becuse the root pex ws frther wy from the medin plne thn ws the most lterl border of the incisive cnl in ll subjects. Rm-Ct, Rm-Cnl, nd Cl-Root mesurements did not show significnt differences ccording to the verticl levels (Tble 3). DISCUSSION Bsed on the envelope of discrepncy, the mximum mount of mxillry nterior retrction is 7 mm, 11 nd the recent development of skeletl nchorge hs lso brodened the limits of orthodontic tooth movement. 29 Our results indicte tht the biologicl nteroposterior distnces between the mxillry centrl incisor roots nd the incisive cnl were pproximtely 5 6 mm, slightly less thn our conventionl

4 574 CHO, KIM, CHOI, KIM, CHUNG Tble 1. The Incisive Cnl Width nd Interroot Distnce of the Mxillry Centrl Incisors t Three Verticl Levels Verticl Level Opening (L1) Midlevel (L2) Root Apex (L3) Mesurement Men 6 SD Men 6 SD Men 6 SD Cnl width (mm) B AB A Rm-Rm (mm) A B C Rp-Rp (mm) B AB A Men vlues with the sme superscript letters within row re not significntly different from ech other. SD indictes stndrd devition; Rm, the most medil point of the mxillry centrl incisor roots; Rp, the most posterior point of the mxillry centrl incisor roots; Rm-Rm (interroot distnce), the trnsverse distnce between the bilterl Rms; Rp-Rp (posterior interroot distnce), the trnsverse distnce between the bilterl Rps. Cnl width is the trnsverse distnce between the most lterl point of the incisive cnl. Tble 3. Proximity of the Anterior Border of the Incisive Cnl to the Mxillry Centrl Incisor Roots t Three Verticl Levels Verticl Level Opening (L1) Midlevel (L2) Root Apex (L3) Mesurement Men 6 SD Men 6 SD Men 6 SD Rm-Ct (mm) Rm-Cnl (mm) N/A Cl-Root (mm) N/A SD indictes stndrd devition; Rm, the most medil point of the mxillry centrl incisor roots; Ct, the tngent line through the most nterior point of the incisive cnl; Cl, the most lterl point of the incisive cnl; Rm-Ct, the nteroposterior distnce from Rm to Ct; Rm-Cnl, the nteroposterior distnce from Rm to the nterior border of the incisive cnl tht meets the tngent line through Rm; Cl-Root, the nteroposterior distnce from Cl to the posterior border of the mxillry centrl incisor root tht meets the tngent line through Cl; N/A, not pplicble. guidelines. This 5- to 6-mm distnce does not necessrily imply the sfety zone for retrction becuse individuls with reltively lrge interroot distnces re not t risk of cnl invsion or contct even following mximum retrction. However, more thn 60% of our subject pool hd n incisive cnl width lrger thn the interroot distnce, nd individul vritions of cnl dimension, especilly enlrgement nd symmetry of the cnl morphology, re frequently reported with 3D evlution. 20,21,30,31 Antomicl fetures of the incisive cnl hve been studied in connection with rehbilittion of the mxillry nterior region nd the plcement of orthodontic mini-implnts. 30,32 34 However, the consequences or the possibilities of incisor roots being in contct with the incisive cnl following tooth movement re poorly documented in the orthodontic literture. Similr to the exmple presented in Figure 1, it ws recently noted through 3D evlution tht the mxillry centrl incisor root in direct contct with the incisive cnl following nterior retrction ws ssocited with severe root resorption, but without ny neurologic symptoms. 25 Given tht the incisive cnl is surrounded by thick corticl bone, similr rections my be induced when the roots re in contct with the lingul corticl plte. Contct of tooth roots with the Tble 2. The Percentge of Ptients With n Incisive Cnl Width Greter Thn the Interroot Distnces Verticl Level Cnl Width Opening (L1), % Midlevel (L2), % Root Apex (L3), %. Rm-Rm Rp-Rp Rm-Rm (interroot distnce) indictes the trnsverse distnce between the bilterl Rms; Rp-Rp (posterior interroot distnce), the trnsverse distnce between the bilterl Rps. corticl plte hs been ddressed s contributor to root resorption, 12 14,16 result in delyed tooth movement, 12 nd my lso cuse perfortion nd dehiscence of the corticl plte. 6,9,10,12,15 In this context, it is interesting to note tht the incidence of orthodontic root resorption is by fr more frequent in the mxillry centrl incisors, even with their lrger tooth dimensions, thn in the lterl incisors. 14,35 Considering the morphologic dimensions of the centrl incisor roots nd the incisive cnl, the posterior-medin spect of the picl third (L1 or L2 levels) of the roots rther thn the root pex per se is most likely to pproximte with the cnl following mxillry nterior retrction nd root movement. However, in mny clinicl situtions nterior intrusion lso occurs during the retrction process, rising the possibility of pproximtion long the pth of tooth movement. It is lso noteworthy tht in the mndibulr cnl, temporry presthesi of the lower lip ws reported in ssocition with tooth root when in contct with the cnl wll following orthodontic tooth movement. 36,37 Thus, 3D imges my be helpful in estimting the proximity of the incisive cnl long with the dimensionl chrcteristics when considerble mount of mxillry incisor retrction with verticl control is plnned. Tooth movement induces remodeling of the surrounding bone. Although remodeling of the incisive cnl following orthodontic tooth movement hs not, to our knowledge, been reported, 25 reltive chnges in the position of the incisive cnl becuse of chnges in the surrounding lveolr bone following tooth loss hve been noted in the edentulous dentition. 22,38 Therefore, to determine the remodeling potentil of the incisive cnl wll following orthodontic tooth movement, further evlutions using comprisons of pre- nd posttretment mterils re necessry.

5 INCISIVE CANAL PROXIMITY TO THE MAXILLARY INCISORS 575 CONCLUSIONS N The nteroposterior distnce between the mxillry centrl incisor roots nd the incisive cnl ws pproximtely 5 6 mm. N More thn 60% of subjects hd n incisive cnl width greter thn the interroot distnce. N Evlution of the proximity of the incisive cnl to the mxillry incisors, in ddition to its dimensionl chrcteristics, my be helpful when considerble mount of mxillry retrction is plnned. ACKNOWLEDGMENT This reserch ws supported by the Bsic Science Reserch Progrm through the Ntionl Reserch Foundtion of Kore (NRF) funded by the Ministry of Science, ICT & Future Plnning (NRF-2013R1A1A ). The uthors declre no conflicts of interest relted to this study. REFERENCES 1. Riedel RA. Esthetics nd its reltion to orthodontic therpy. Angle Orthod. 1950;20: Rins MD, Nnd R. Soft-tissue chnges ssocited with mxillry incisor retrction. Am J Orthod. 1982;81: Arnett GW, Bergmn RT. Fcil keys to orthodontic dignosis nd tretment plnning. Prt I. 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