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The Importance of the Level of the Lip Line and Resting Lip Pressure in Class II, Division 2 Malocclusion
1 Department of Orthodontics, School of Dental Medicine, and Correspondence: *corresponding author, lapatkib{at}zmk2.ukl.uni-freiburg.de
Many clinicians hypothesize that retroclination of the maxillary central incisors in Class II, Division 2 malocclusion is caused by increased resting lip pressure against these teeth. The purpose of this study was (1) to verify this assumption by means of simultaneous lip-pressure measurements at two different levels on the maxillary central incisor crowns, and (2) to examine factors that could possibly contribute to the increased resting lip pressure. This is the first study to prove that individuals with Class II, Division 2 malocclusion (n = 21) have the upper central incisors exposed to significantly higher lip pressure than those with Class I malocclusion (n = 21). Our statistical evaluation revealed that this is primarily attributed to a high lip line and not to a hypertonic peri-oral musculature. We concluded that orthodontic treatment of Class II, Division 2 cases should include intrusion of the maxillary incisors, to eliminate the non-physiologically high pressure exerted by the lower lip on these teeth and, consequently, to reduce the high risk of a post-orthodontic relapse.
Key Words: Class II • Division 2 • lip line • lip pressure • peri-oral muscle • peri-oral electromyography
The influence of the forces exerted by the lips, cheeks, and tongue on the positions of the teeth has been the subject of scientific debate. Most authors accept, as a basis, the equilibrium theory of tooth position (Weinstein et al., 1963; Proffit, 1978). Previous work on the relationship between tongue-lip pressures and tooth position has shown that the lips and cheeks, rather than the tongue, are the most important environmental determinants of tooth position; a second finding was that resting pressure and not functional pressure is the dominant factor (Lear et al., 1974; Proffit et al., 1975; Thüer et al., 1999). The discussion on the equilibrium of tooth position is closely related to the etiology of certain malocclusions. Class II, Division 2 malocclusion, characterized by distocclusion of the buccal teeth and retroclination of some or all of the upper incisors, is predominantly determined by hereditary factors (Christiansen-Koch, 1981; Schulze, 1993). Many clinicians have hypothesized that the upper incisor retroclination results from non-physiologically high lip pressure against these teeth. This suggests that the lips act as a local genetic factor in Class II, Division 2 malocclusion. The finding in longitudinal cephalometric studies—that the retroclination occurs progressively during the intra-oral eruption period—may support this view (Fränkel and Falck, 1967; Fletcher, 1975). However, up to now, no experimental study has proven the impact of increased lip pressure on the upper central incisors in Class II, Division 2 malocclusion. The most likely reason for this is that, in previous investigations (Gould and Picton, 1968; Luffingham, 1969; Thüer and Ingervall, 1986), pressure measurements on the upper incisors were carried out at a single location only, and therefore, uneven pressure distribution on the crown had not been taken into account. Another question which still needs to be resolved concerns the causes of increased resting lip pressure. These causes may include a high lip line relative to the upper incisors and/or hyperactivity of the peri-oral musculature, particularly the mentalis muscle (Brodie, 1953; Jarabak and Fizzell, 1972; Mills, 1973; Van der Linden, 1983). The objectives of the present study were (1) to compare a Class II, Division 2 group and a control group regarding the magnitude of resting pressure against the maxillary central incisors in the incisal and cervical area of the crowns, and (2) to assess the relevance of the level of the lip line and the peri-oral muscle activity as causative factors for the increased resting lip pressure.
Subjects Forty-two subjects (males, n = 18; females, n = 24) with a completely developed dentition participated in this study after providing informed consent. The Medical Research Ethics Commission of the University of Freiburg i.Br. raised neither ethical nor legal objections to this study. Twenty-one of the subjects showed the characteristic dental morphology of Class II, Division 2 malocclusion. The control group consisted of 21 subjects with natural dentition, Class I occlusion, and physiologic frontal relationships. The mean age was 24.9 yrs in the malocclusion sample and 26.8 yrs in the control sample.
Sensors for Lip-pressure Measurements
Electrodes for Peri-oral Electromyography (EMG) We used miniature surface electrodes (M&S Mühl, Freiburg, Germany) to measure the resting activity of the upper and lower orbicularis oris muscles, the depressor labii inferioris muscle, and the mentalis muscle on the left side of the face. The bipolar electrodes were specially designed for the peri-oral area (Lapatki, 1998) and consisted of 2 Ag/AgCl disks 4 mm in diameter, with an inter-electrode distance (center to center) of 8 mm.
Experimental Procedure
Recording and Evaluation of the Lip-pressure Signals and the EMG Signals
Clinical Measurements
Model Cast Analysis
Statistical Evaluation The lingual tipping effect on the upper central incisors caused by the incisal and cervical lip pressure together was evaluated according to the formula:
Incisal pressure was weighted more in this calculation, because the amount of tooth tipping resulting from the application of pressure is proportional not only to the magnitude of the pressure, but also to the distance of its application from the tooth's center of resistance. In our experiment, the distance from the incisal sensor location to the center of resistance was 1.6 times greater than the corresponding distance from the cervical sensor location (Figs. 2B, 2C Median pressure and RMS values from the 7 repetitions per exercise were used for further evaluation and statistical analysis. Differences in the results from the 3 exercises, carried out to establish the resting lip position, were analyzed with the Friedman test and the Wilcoxon signed-rank test. Since these differences were small and statistically insignificant (Wilcoxon signed-rank test, p > 0.05), all the results were averaged for further statistical evaluation and illustration. Values of the right and left sides, as well as the values obtained from male and female subjects, were also combined and averaged for the same reasons. Inter-group differences in the pressure values and the muscle activity were evaluated by means of the Mann-Whitney U test. We quantified correlations between the variables by calculating the Spearman's rank correlation coefficients. The interrelation between the lip-pressure values and the level of the lip line was evaluated by means of an Analysis of Variance and Covariance (ANCOVA). We analyzed the reproducibility of the new methods for determining the lip-line level and the inclination of the maxillary central incisors by calculating the upper and lower tolerance intervals for the differences between the repeated measurements (according to Bland and Altman, 1999).
In the statistical evaluation, the lip-line and incisor inclination measurements proved to be reproducible (Appendix, www.dentalresearch.org). In the Class II, Division 2 group, the level of the lip line was 5.1 mm above the incisal edge of the upper central incisors (Fig. 2  ). The corresponding mean value in the control group was only 2.7 mm. The inter-group difference in the inclination of the upper centrals was 15.8°. The buccal occlusion was approximately half a cusp to the distal in the Class II, Division 2 sample and neutral in the control sample.
Box-plots of resting lip-pressure values (Fig. 3
The Spearman Correlation Test revealed a relatively high and statistically significant correlation between the lip-pressure values and the inclination of the upper centrals (the coefficients were 0.66 for the incisal pressure and -0.61 for the cervical pressure, p < 0.0001). The correlations between the lip-pressure values and the levels of the lip line were even higher; the corresponding Spearman Correlation Coefficients were 0.86 for the incisal and -0.80 for the cervical pressure (p < 0.0001). The close interrelationship between pressure values and the lip-line level is also demonstrated by the individual results (Fig. 4  ). The higher the lip line, the higher the incisal pressure (Fig. 4A), the lower the cervical pressure (Fig. 4B), and the higher the lingual tipping effect on the upper centrals (Fig. 4C). In the statistical analysis, these interrelations proved to be significant (ANCOVA, p < 0.01), and they did not differ between the two study groups (ANCOVA, p > 0.19). The slopes of the linear regression curves in these diagrams illustrate that the increase in the lingual tipping effect with rising lip-line level (Fig. 4C) was due mainly to the higher increase in the incisal pressure (Fig. 4A) when compared with the decrease in the cervical pressure (Fig. 4B).
In evaluating the EMG signals, we could not verify any significant inter-group differences in the resting activity of the peri-oral musculature (Mann-Whitney U test; p > 0.05). Subjects with a hypertonic mentalis muscle were included with the same frequency in the Class II, Division 2 group as in the control group (Fig. 3B ). The correlations between peri-oral muscle activity and other parameters turned out to be relatively low (Spearman's Correlation Coefficients < 0.17) and not statistically significant.
In previous studies, the techniques used to quantify lip pressure have varied widely. Although some types of transducers are able to register only positive pressure values, the capacitive pressure sensors we used in this report measure both positive and negative pressures. The finding of negative resting lip pressure in the present study, as well as in former investigations (Bookhold and Hensel, 1989; Shellhart et al., 1997; Thüer et al., 1999), proves the great importance of measuring negative pressures as well. Since resting lip-pressure values are very small and temperature fluctuations are large in intra-oral pressure measurements, accurate measurements require highly sensitive transducers with a small thermal drift. The technical data prove that our transducers fulfill these requirements. Peri-oral soft-tissue pressure measurements are generally biased due to the displacement of tissue by the pressure-sensing device (Lear et al., 1965). Investigations on this effect proved that pressure exponentially increases when tissue displacement is greater than 1.5 mm (Ho et al., 1984). Therefore, the thickness of the measuring device plays an extremely important role. Since our sensors were relatively thin (1.2 mm) compared with those used in previous lip-pressure studies, this effect was minimized. This study is the first to provide evidence that individuals with a Class II, Division 2 malocclusion have maxillary central incisors exposed to significantly higher resting lip pressure than those with a Class I malocclusion. This indicates that, in Class II, Division 2 malocclusion, the balance of forces on the retroclined upper central incisors is established on a relatively high level, with an area of equilibrium in a relatively lingual position. By means of simultaneous pressure measurements at 2 different levels on the crowns, we could prove that the significantly increased resting lip pressure in Class II, Division 2 subjects is closely related to the high lip line. This interrelationship is based on the fact that the lower-lip resting pressure is generally higher than the upper-lip resting pressure; this is widely acknowledged in lip-pressure studies (Proffit et al., 1975; Thüer and Ingervall, 1986; Bookhold and Hensel, 1989). Consequently, the total amount of pressure on the upper centrals must increase, if the contact area between the lower lip and the upper centrals enlarges to the disadvantage of the contact area with the upper lip. A high lip line results in not only an increase in the total magnitude of pressure, but also in a redistribution of the pressure from the cervical area to the incisal area of the crown; this further increases the potential of the pressure to tip the upper centrals lingually. A high lip line should theoretically cause retroclination of all the upper anterior teeth. However, there are many Class II, Division 2 cases with labially malposed upper lateral incisors and canines, respectively. The eruption of the maxillary lateral incisors in a position labial to the central incisors is explained by the early, unimpeded retroclination of the central incisors, together with the later eruption of the lateral incisors in a relatively proclined path (Baume, 1955; Jarabak and Fizzell, 1972; Schulze, 1993). The persistence of this labial malposition is most often attributed to a space discrepancy in the upper anterior segment, which then prevents the lateral incisors or, in a later stage, the canines from becoming retroclined by the lower lip (Fletcher, 1975; Van der Linden, 1983). This space discrepancy may either develop or increase owing to the retroclination of the upper centrals. A second explanation for the persistent labial malposition is that in Class II, Division 2 cases, lateral incisors and canines are, almost without exception, less elongated than the central incisors (Jarabak and Fizzell, 1972). Therefore, these teeth are also less, if at all, covered by the lower lip and are consequently subjected to less pressure than are the central incisors. In Class I cases with a high lip line (as in one of our control subjects), upper incisor retroclination is often only mildly pronounced or even completely lacking. This is probably due to the impediment to upper incisor retroclination provided by the support of the lower anterior teeth (Fletcher, 1975; Schulze, 1993) and again suggests that a high lip line alone (without other co-factors) will produce little, if any, effect. The results of this study emphatically support the theory that local genetic factors play an important role in the etiology of Class II, Division 2 malocclusion. The local influence is mainly based on an imbalance in the vertical relation between the lips and the upper anterior dento-alveolar structure, and not on an increased resting tonus of the peri-oral musculature. The latter conflicts with the opinions of several authors, but concurs with the results of a previous EMG study (Marx, 1965). Cephalometric studies (Smeets, 1962; Fletcher, 1975) revealed that excessive eruption of the upper incisors is not a feature characteristic of Class II, Division 2, and therefore must be excluded as the main cause for the high lip line. In contrast, infra-occlusion of the buccal segments, most often found in this type of malocclusion, may play a more important role; the resulting counter-clockwise rotation of the mandible may lead to an excess of soft tissue in the lower face (Burstone, 1967; Jonas, 2000). The deep mentolabial sulcus frequently observed in Class II, Division 2 cases supports this view. This rotation of the mandible may result in a cranially directed compression of the soft tissues in the lower face, and consequently, in a cranially directed shift of the lip line as well. The question of the significance of soft-tissue pressures is not only important for our understanding of the development of dental malocclusion, it is also of clinical significance for better evaluation of treatment limitations and possibilities. Orthodontic treatment of Class II, Division 2 malocclusion has always been considered of dubious prognosis, being very prone to relapse (Selwyn-Barnett, 1991). Our results indicate that, with regard to stability, the best therapeutic approach is certainly the intrusion and torque of the upper incisors. This is the only means of eliminating the high pressure exerted by the lower lip on these teeth. If this issue is not given first priority in the orthodontic treatment of malocclusion Class II, Division 2, the clinician must be aware that continued pressure of the lower lip on the upper centrals will probably result in a post-orthodontic relapse.
The authors gratefully acknowledge the technical support provided by Mr. Gerhard Mühl and Mr. Thomas Bilger, so important in the development of the methods. All financial support of this project was provided by the University of Freiburg i.Br. (Germany). A preliminary report was presented at the 76th Congress of the European Orthodontic Society (2000), Hersonissos, Crete, Greece.
A supplemental appendix to this article is published electronically only at http://www.dentalresearch.org. Received for publication September 10, 2001. Revision received February 18, 2002. Accepted for publication February 25, 2002.
Journal of Dental Research, Vol. 81, No. 5,
323-328 (2002) This article has been cited by other articles:
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