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IntroductIon

The primary objective o aesthetic dental treatment is to generate a natural, healthy appearance or an otherwise dam-aged dentition.1 The word aesthetic implies beauty, naturalness and a youthul appearance relative to ones age, andaesthetic dentistry has been called the art o the imperceptible by McLaren and Rikin.2 A pleasing dental appearanceis the subjective appreciation o the shade, shape and arrangement o the teeth and their relationship to the gingiva,

lips and acial eatures.3

Symmetry, the property o being symmetrical, with a correspondence in size, shape and rela-tive position o parts on opposite sides o a dividing line or median plane or about a center or axis,4 plays a large partin the perception o dental aesthetics (Fig. 1).

In a study by Dunn, when evaluating photographs o male and emale smiles, 24 out o the 25 demographic groupspicked the same attractive emale smile, which was characterized by natural teeth having a light shade, a high lip line, alarge display o teeth and radiating symmetry.5 Multiple studies show that society places a great amount o importanceon appearance, with attractive people having more success, higher paying and more prestigious jobs, better luck inobtaining dates, more avorable jury verdicts and more positive responses, even rom inants.6 The ability o the dentistto communicate the location and orientation o the patients acial landmarks to the dental technician will dictate the

success o the esthetic outcome.

ARTICLE and CLINICAL PHOTOS

byLeendert Boksman, DDS, FADI, FICD

The Dental Midline Position,

Incisal Cant and Incisal Horizontal Plane

Simplifying Laboratory Communication:

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Figure 2: The smile arc shows the anteriors in harmony with thecurvature o the lower lip.

Figure 1: The acial midline, interpupillary line and incisal line

should be in symmetrical harmony (note minor nasal asymmetry).

This article will look at the aesthetic parameters o dentalmidline position, incisal cant and incisal horizontal plane,and provide a simple methodology to relate these parametersto the dental technician when multiple anterior restorationsare prepared. O course, details o the smile arc,7 the curve

ormed by the incisal edges o the maxillary anterior teethin relationship to the lower lip, need to be communicatedto the laboratory technician. The maxillary incisal curve andthe lower lip curve should be roughly parallel to one anotherand perpendicular to the vertical midline drawn between themaxillary central incisors (Fig. 2).8,9 For complicated cases, itis critical to give an accurate relationship o the casts in a sag-ittal or lateral axis when designing the curvature and angleo the smile line (bicuspids to molars),10 as the potential ormisalignment o the casts increases with the number o resto-

rations involved.11

the dental MIdlIne

The median plane is a line passing longitudinally throughthe middle o the body rom ront to back, dividing it intothe right and let halves.12 The acial midline is a critical re-erence position or determining multiple design criteria13with the maxillary midline position relative to the acial mid-

line stressed in orthodontic treatment planning,

14

as it is animportant unctional component o occlusion.15 In a totallysymmetrical ace, the dental midline and the acial midlineshould coincide, but this is oten not the case (Fig. 3).A study by Miller showed that the midline is situated in the exactmiddle o the mouth in approximately 70 percent o people,and the maxillary and mandibular midlines ail to coincide inalmost three-ourths o the population16(Fig. 4, 5). However,in a study looking at dental students, Soares ound that thecoincidence o acial midline with the arch midline occurred

in only hal o the students.17 Among orthodontic patients, themost common asymmetry trait is mandibular midline devia-tion rom the acial midline.18 Thus, the mandibular midlinecannot be used as a reerence point by the dental technicianin deciding where to put the maxillary midline (Fig. 6).

However, there is conicting data as to how important themaxillary and/or mandibular midline position is to patientsand their perception o esthetics and the ability o den-tal proessionals and laypeople to perceive dental midlineshits. It seems midline abnormalities are the least noticed.2

In a study by Dunn, when evaluating photographs of male and female

smiles, 24 out of the 25 demographic groups picked the same attractive

female smile, which was characterized by natural teeth having a light shade,

a high lip line, a large display of teeth and radiating symmetry.

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Johnston, when evaluating the dental attractiveness o acialphotographs with orthodontists and laypeople, ound that asthe size o the dental to acial midline discrepancy increas-es, attractiveness ratings decrease. There was a 56 percentprobability or a layperson to record a less avorable attrac-

tiveness when there was a 2 mm discrepancy between thedental and acial midline, while a discrepancy o 2 mm ormore was noticed by 83 percent o orthodontists.19 A studyby Chan quantiying a laypersons ideal and maximum de-viation o the midline ound the smile became unattractive when the maxillary midline deviated 2.9 mm, or once themaxillary-mandibular midlines deviated 2.1 mm.20 Cardash,in his study o midlines, states that nearly hal o the ob-servers were unable to detect midline deviations o less than2 mm; however, some detected midline deviations o less than

1 mm.21 Kockich claims that general dentists and laypeopleare unable to detect even a 4 mm midline deviation.22 Irre-spective o these studies, many authors state that when re-storing multiple anterior teeth, the ideal choice is always tomaximize the esthetic result by placing the maxillary dentalmidline in harmony with the acial midline.1,23,24

MIdlIne cant or oblIque MIdlIne

In the ideal ace, the midline o the teeth should be centeredin the ace and be completely vertical.23 Even when the mid-lines o one or both arches are not centered, it is more impor-tant to ensure the anterior teeth are vertically oriented in theace and perpendicular to the incisal plane.25 Attractivenessscores and acceptability ratings decline consistently as axialmidline angulation increases (Fig. 7).26 Studies have shownthat midline deviations o up to 3 mm or 4 mm are not noticedby laypeople i the long axes o the teeth are parallel with thelong axis o the ace.22,26 Spear states that perhaps the most

important relationship to evaluate is the mediolateral inclina-tion o the maxillary incisors.27 I the incisors are inclinedby 2 mm right or let, laypeople regard this as unesthetic.26Thus, this type o midline deviation (the oblique midline) isnoticeable and should be corrected with interproximal prepa-ration.27

the IncIsal horIzontal Plane

The interpupilary line is a reerence plane used to deter-mine the incisal horizontal plane, gingival plane and occlusalplane.1 An incisal plane cant o 1 mm is rated as signifcantlyless aesthetic.22 Kockich ound that an occlusal plane cant isa very displeasing smile characteristic to health proessionalsand laypeople.22An incisal occlusal cant is a orm o asym-metry that is apparent when a person smiles but is not per-ceived on intraoral images or study casts.28 Even when usinga acebow transer, the condylar determinants do not takeinto account the aesthetic orientation requirements because

the anterior and posterior occlusal determinants are evalu-ated and transerred to the articulator rom a unctional stand-point with the assumption that the aesthetic orientation o the

Figure 3: The acial and dental midline should coincide or opti-

mum esthetics.

Figure 5: The patient in Figure 4 with temporaries in place.

Figure 4: This patients midlines do not coincide, thus the man-

dibular midline cannot be used as a reerence point or the nal

restorations.

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anterior teeth is correct.11 In addition, when reerencing re-maining unprepared teeth, their positions may not accuratelyrepresent the incisal horizontal plane o the incisors. Caninepositions are generally asymmetric and are in dierent verti-cal positions as well as being angled dierently(Fig. 8, 9).2,29,30

Thus, mounting casts reerenced to the vertical position othe canines will result in the restorations having an incorrectincisal plane (Fig. 10).

laboratory coMMunIcatIon

I the clinician is to transer these important parameters o themaxillary dental midline, the lack o midline cant, and the in-cisal horizontal plane to the dental technician, which shouldall be reerenced to the acial midline and interpupillary line,

how is this reliably accomplished? In the past, classic stickbites, cotton swabs, pencils, plastic stir sticks and symmetrybites have been used to capture these dimensional relation-ships.10 The limitations o these systems are many. All havelimitations due to the short working time o many bite reg-istration materials, so the clinician is orced to work quicklyto center and place these beore the material sets. I the stickbite or symmetry bite is slightly o, the whole process needsto be repeated. With fxed symmetry bites, the vertical andhorizontal are fxed at 90 degrees to each other, assuming thehorizontal incisal plane matches exactly to the interpupillaryline and no correction is indicated or anticipated. A stick biteto the horizontal assumes the patient can keep his or her headperectly still and upright.

A simple solution to the transer o the required data to thedental technician is the Onebite (Precision Dental Products;Draper, Utah) acial plane relator. There are a number o dis-tinct advantages to the Onebite over other available systems.

The bite ork portion is separate rom the adjustable horizon-tal and vertical components, so that i the bite ork is placedslightly o center when placed into the bite material, the abil-ity to move the components laterally eliminates the need orrepeating the procedure. Figure 1113 show the placement othe bite registration material Afnity Quick Bite (CliniciansChoice; New Milord, Conn.) onto the anterior teeth, onto thebite ork and the intraoral placement o the bite ork. Ater thevertical and horizontal components were placed frmly intothe bite ork slot, it can be seen (Fig. 14) that the bite ork wasplaced slightly o laterally to the patients let side. The biteork placement does not have to be redone; Figure 15 showsthe horizontal adjustment is easy to accomplish by looseningthe screw, sliding the component laterally until centered tothe patient acial midline and then securely tightening thelocking screw. Another beneft o Onebite is that the verticaland horizontal component can be let in a locked 90-degreerelationship to each other i the patient demonstrates sym-metry o the midline, horizontal and interpupillary line, or

the components can be unlocked by rotating the horizontalbar so the locking pins are acing the clinician, i there is adiscrepancy with the interpupillary line. Figure 16 shows theFigure 9: The two lines show the disparity between the incisal hori-

zontal plane and the position o the cusp tips o the canines.

Figure 8: Using the incisal edges o the cuspids when the anteriors

are prepared could lead to an erroneous horizontal cant on the res-

torations because the cast is not mounted to the interpupillary line.

Figure 6: The dental technician cannot use the mandibular midline

to decide where to place the maxillary midline when multiple teeth

are prepared.

Figure 7:A midline cant is consistently rated as unaesthetic.

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patients right side horizontal portion o the Onebite is slight-ly lower than the interpupillary line. This can easily be ad-justed by unlocking the components, rotating the horizontalbar until it is in harmony with the interpupillary line, securelytightening the locking screw and then fxing the componentstogether by injecting temporary C&B material into the lateralslot and screw. For illustrative purposes, the rotation has beenexaggerated in Figure 17 to show the wide range o adjust-ments that are easily managed by the Onebite. The compo-nents are then taken apart by placing lateral orce on thelocking screw, which acilitates transport to the laboratory.Another advantage o the Onebite is that in the laboratory,the vertical component can be reduced in length at the plasticcross supports to ft easily onto a semi-adjustable articulator.

conclusIon

The rationale or the need o accurate communication bythe dental clinician to the laboratory technician o the den-tal midline, incisal midline cant and incisal horizontal planehave been discussed. A simple technique that acilitates thiscommunication has been presented and should minimize theneed or expensive remakes or esthetically driven restora-tions. CM

Dr. Len Boksman is adjunct clinical proessor at the Schulich School o Medicine andDentistry and maintains a private practice in London, Ontario, Canada. He is also a paid

part-time consultant to Clinical Research Dental Inc. and Clinicians Choice. Contacthim at lboksman@clinicalresearchdental.com or 519-641-3066, ext. 292.

Figure 10: This photograph shows the obvious error that would

result i the casts were mounted in a horizontal incisal relationship

based on cuspid position.

Figure 11: Quick Bite is injected onto the anterior teeth.

Figure 12: Quick Bite is injected onto the bite ork o the Onebite,

and the bite ork is centered on the ace when the patient closes.

Figure 13: Even i the bite ork is slightly o center, as shown, the

adjustment o the other components will allow or centering o the

vertical bar to the center o the ace.

A study by Chan quantifying a

laypersons ideal and maximum

deviation of the midline found thesmile became unattractive when the

maxillary midline deviated 2.9 mm,

or once the maxillary-mandibular

midlines deviated 2.1 mm.

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references

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3. Nohl FSA, Steele JG, Wassell RW. Crowns and other extra-coronal restorations:Aesthetic Control BDJ 2002;192(8):44350.

4. Bryan M, Calhoun K. All about chin augmentation acial proportions and analysis.Dept o Otolaryngology, UTMB, Grand rounds, Chin and Malar Implants Sept 6,1995. http://www.chinaugmentation.com/acial_ormula.htm.

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6. Patnaik VVG, Singula RK, Bala S. Anatomy o a beautiul ace & smile. J Ant. Soc.India 2003;52(1):7480.

7. Ackerman MB, Ackerman JL. Smile analysis and design in the digital era. JCO April2002;26(4):22136.

8.Ruenacht CR. Principles o esthetic setup. In: Ruenacht CR, editor. Principles oesthetic integration. Quintessence Publishing 2000. P 205041.

9. Sarver DM. The importance o incisor positioning in the esthetic smile: the smile arc.American Journal o Orthodontics and Dentoacial Orthopedics 2001;120(2):98111.

10. Chan CA. Architecting the occlusal plane. Las Vegas Institute or Advanced DentalStudies.

11. Chiche GJ, Aoshima H. Functional Versus Aesthetic Articulation o Maxillary Ante- rior Restorations. PP&A 1997;9(3):33542.

12. http://medical-dictionary.thereedictionary.com/ Frankort+horizontal+plane

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14. Beyer JW, Lindauer SJ. Evaluation o dental midline position. Seminars in Ortho-dontics Sept 1998;4(3):14652.

15. Thomas JL, Hayes C, Zawaideh S. The eect o axial midline angulation on dentalesthetics. Angle Orthod. 2003;73:35964.

16. Miller EL, Bodden WR Jr, Jamison HC. A study o the relationship o the dentalmidline to the acial median line. J. Prosthet Dent. Jun 1979;41(6):65760.

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19. Johnston CD, Burden DJ, Stevenson MR. The infuence o dental to acial midlinediscrepancies on dental attractiveness ratings. European Journal o Orthodontics1999;21:51722.

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21. Cardash HS, Ormanier Z, Ben-Zion L. Observable deviation o the acial and ante-rior tooth midlines. JPD March 2003;89(3):28285.

22. Kokich Vo, Kikak A, Shapiro PA. Comparing the perception o dentists and lay-people to altered dental esthetics. J o Esth and Rest Dent 1999;11(6):31124.

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Reprinted with permission o Oral Health Journal, 2010 Oral Health Journal.

Figure 14: When the components are mated, it can be seen that

the bite ork is o center.

Figure 15: By loosening the screw and sliding the components tothe patients right, the vertical bar now corresponds to the patients

acial midline.

Figure 16: The horizontal bar does not line up with the interpupil-

lary line and can be adjusted parallel to it by unlocking the bars,

aligning the horizontal and tightening the locking screw.

Figure 17: For illustrative purposes, the angulation o the horizontal

bar has been exaggerated and xation is achieved with Temptation

acrylic temporary material (Clinicians Choice).

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