INTRODUCTION

Class II division 2 malocclusion is characterized by the permanent mandibular incisors occluding posterior to the cingulum plateau of retroclined permanent maxillary incisors.1 This gives rise to a reduced overjet and frequently an increased overbite. Classically, the permanent maxillary central incisors are retroclined and the maxillary lateral incisors are proclined and mesiolabially rotated. However, all four permanent maxillary incisors may be retroclined with or without facially displaced and proclined permanent maxillary canines.

The relationship of the labial soft tissues to the permanent maxillary incisors has been implicated as the principal etiological factor in the development and relapse of Class II division 2 malocclusion.2–7 Specifically, the maxillary incisor retroclination is thought to result from increased resting lip pressure as a consequence of a combination of hyperactivity of the labial musculature and a higher resting lip line.578 Despite cephalometric studies confirming the higher resting lip line, electromyographic studies have not confirmed unanimously increased labial musculature activity in subjects with Class II division 2 malocclusions when compared with those with Class I malocclusion.9–11

Furthermore, no previous study has investigated simultaneously lip shape and position as a factor in the development of Class II division 2 malocclusion. This is because shape data cannot be derived from linear and angular measurements.12 Morphometric techniques overcome many of the limitations of conventional cephalometric measurements because they integrate geometric location and biologic homology allowing shape comparisons to be made.1314

The objective of this study is to determine whether differences exist in the shape and position of the lips between Class II division 2 and Class I malocclusions. The null hypothesis tested is that there are no differences in the shape and position of the lips between Class II division 2 and Class 1 malocclusions.

MATERIALS AND METHODS

Of the 499 lateral cephalograms recorded at a university orthodontic clinic during 2001, 135 were excluded from this study for various reasons affecting the cephalometric visualization of the permanent maxillary central incisor form. The lateral cephalometric radiographs of three other subjects could not be located. The remaining 361 lateral cephalometric radiographs were stratified according to incisor relationship as defined by the British Standards Institute1 and recorded in the clinical records by experienced orthodontists:

• Class I: The lower incisor edges lie on or below the cingulum plateau on the palatal surface of the upper incisors with a normal overjet.

• Class II division 1: The lower incisor edges lie palatal to the cingulum plateau of the upper incisors and the upper incisors are proclined or of normal inclination with an increased overjet.

• Class II division 2: The lower incisor edges lie palatal to the cingulum plateau of the upper incisors with the upper incisors being retroclined, the overjet is usually minimal but may be increased.

• Class III: The lower incisor edges lie labial to the cingulum plateau of the upper incisors.

The resultant groupings were as follows: Class I (n = 71); Class II division 1 (n = 139); Class II division 2 (n = 60); and Class III (n = 91). Full details regarding the selection of these groups are described elsewhere.15

Because this study evaluated lip shape and position in Class II division 2 malocclusion compared with Class 1 malocclusion, cephalograms of subjects with Class II division 1 and Class III malocclusions were excluded along with those from the study groups that showed poor definition of the labial soft tissues. Thirty Class II division 2 and 29 Class I cephalograms were excluded to leave 30 Class II division 2 and 42 Class I images. The 30 images of the Class II division 2 group and the 30 cephalograms which were randomly selected from the Class I group were scanned at 300 dpi to produce monitor-displayed digital cephalograms using a Heidelberg NewColor 4000 flatbed scanner (Heidelberg CPS GmbH, Bad Homburg, Germany, http://www.hdcps.com/) attached to a DELL Dimension XPS T550 computer and DELL D1626HT (UltraScan 1600HS Series) 21-inch Color Graphics Display monitor (DELL, Round Rock, Tex, http://www.dell.com).

Using tpsDIG32 (http://life.bio.sunysb.edu/morph/), each digital cephalogram was digitized on-screen in random order under identical conditions with all sources of extraneous background lighting eliminated. Where any uncertainty existed as to incisor orientation or soft tissue contour, images were enlarged. The subject's study models were also consulted where necessary. Magnification was standardized during the recording, scanning, and digitizing of the radiographs.

The x-y coordinates of 21 landmarks (Figure 1; Table 1) representing the form of the most labially positioned maxillary and mandibular central incisors and the profile of the lips were digitized and exported for subsequent analysis. Twenty-five of the images were redigitized 1 month later to evaluate intraoperator reproducibility for individual landmarks.16

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Table 1.  Landmarks

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RESULTS

Landmark Reproducibility

Random error values for all landmarks were above 0.95. None was associated with systematic error (P < .1). These have been suggested as acceptable thresholds.1617

There were highly significant differences in lip morphology and lip relationship to the permanent central incisors between the groups (P < .001; Figure 2; Table 2). The upper and lower lips were relatively thicker with more apical contact of the lower lip with the labial surface of the permanent maxillary central incisor (higher lip line) in the Class II division 2 group compared with the Class I group. The permanent mandibular central incisors were observed to be more upright with respect to the skeletal base in the former group than in the latter (Figure 2). The retroclined permanent maxillary central incisors, which had longer clinical crowns with a reduced labiopalatal thickness, were relatively more extruded in the Class II division 2 group compared with the Class I group (Figure 2).

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Table 2.  Principal Components of Shape

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When the principal components (PC) were examined, PC1 and PC2 accounted for 57% of the total variance in the Class II division 2 group. PC1 (46% of the variance) involved thicker upper and lower lips (Figure 3). PC2 (11% of the variance) was characterized by relatively more apical contact of the upper and lower lips with the labial surface of the permanent maxillary central incisor and greater coverage of the labial surface of the permanent maxillary central incisors by the lower lip (Figure 4).

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DISCUSSION

Compared with Class I malocclusion, we identified that subjects with Class II division 2 malocclusion have a different lip shape and position. Shape is the information that remains when size, location, and orientation are removed from the morphological data.14 Although previous studies have sought to investigate shape changes occurring with growth,18 orthodontic treatment,19 and orthognathic surgery,20 labiodental shape has not been evaluated previously in Class II division 2 malocclusion. In this study, thicker lips were identified in the Class II division 2 group. A greater proportion of the labial surface of the permanent maxillary central incisor was also found to be in contact with the lower lip. This confirms the findings of previous cephalometric studies with respect to lower lip line height in this malocclusion group.237821

Lateral cephalograms with the lips at rest were used in this investigation to provide valid labial and related incisor assessments. Although residual perioral musculature activity exists at rest, this rather than intermittent activity determines incisor positions.22 Lip position at rest is also highly reproducible.23 The most significant sources of bias in a cephalometric investigation result from sample selection and digitization, but every effort was made to minimize bias in our study by excluding images where the incisor inclination and related soft tissue profile were not clearly visible.

The lip shape and position factors, which we identified, in conjunction with the greater lower than upper lip pressure on the permanent maxillary incisors9 and the unfavorable crown-root ratio of the permanent maxillary central incisors,1524 favor their retroclination as they erupt. However, because two-thirds of the root of the permanent maxillary incisor is mineralized before eruption begins,25 the increased lip pressure acting on the labial surface is unlikely to be the main cause of the crown-root deviation identified in previous studies of Class II division 2 subjects.1526–32 The dentoalveolar effects of the distinctive perioral soft tissues associated with Class II division 2 malocclusion not only involve the permanent maxillary incisors but also the permanent mandibular incisors, which were found to be retroclined. This finding is in accordance with that of Mills.5

Procrustes superimposition followed by the discriminant analysis of the PCS was used to investigate lip shape and position. Using this method, the findings of thicker upper and lower lips and a higher lip line accounted for 57% of the Class II division 2 variance. Factor analysis is another statistical method that would have reduced the quantity of shape data.33 Although that statistical technique and the one adopted in our study operate differently, similar results are likely to have been found if factor analysis had been used. APS software using the discriminant analysis of the PCS, however, was used because it allowed visual demonstration of the differences between the test groups.

The shape and position of the lips in Class II division 2 malocclusion not only affects incisor inclinations but can also complicate orthodontic mechanotherapy and posttreatment stability. The latter is more complex than correcting the centroid relationship by controlled incisor proclination.34 The proportion of the labial surface of the permanent maxillary central incisor covered by the lower lip must also be reduced.734 Because the findings of this study and those reported previously15 indicate relative extrusion and a longer clinical crown of the permanent maxillary incisors in Class II division 2 malocclusion compared with Class 1 malocclusion, intrusion of the permanent maxillary incisors during fixed appliance treatment is a necessity during orthodontic mechanotherapy for individuals with Class II division 2 malocclusion.

A prediction tracing to simulate the effect of planned maxillary incisor torque and determine whether this would result in fenestration of the palatal cortex can be useful.1535 Incorporating an outline of the lips and their change in position with tooth movement could also aid retention planning.

CONCLUSION

• The shape and position of the upper and lower lips differs between Class II division 2 and Class I malocclusions.