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RESEARCH REPORT |
1 Department of Orthodontics, School of Dental Medicine, and
2 Department of Biometry and Medical Statistics, University of Freiburg i.Br., Hugstetter Str. 55, D-79106 Freiburg, Germany;
*corresponding author, lapatkib{at}zmk2.ukl.uni-freiburg.de
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: Class II • Division 2 • lip line • lip pressure • peri-oral muscle • peri-oral electromyography
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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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.
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Sensors for Lip-pressure Measurements
To determine the resting lip pressure against the upper central incisors, we attached 4 miniature pressure sensors (Fig. 1A
) to the teeth with help of a thin plastic stent (Copyplast®, Scheu Dental, Iserlohn, Germany). The capacitive transducers (GISMA GmbH, Buggingen, Germany) were 4.5 by 5 mm (Fig. 1B) and protruded 1.3 mm from the underlying teeth. They were able to measure both positive and negative pressures with a resolution of 0.012 cN/cm2 and a thermal drift of 0.03 cN/cm2/°K. Sensor leads were positioned to exit the oral vestibule in the lip line toward the corners of the mouth so as to cause minimal interference with lip posture.
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Experimental Procedure
Lip pressure and peri-oral muscle activity were simultaneously recorded while the lips were in a resting position. This position was established by the use of three exercises: (1) by command, (2) by having the subject hum an "m", and (3) by having the subject swallow 2 mL of water. Between exercises, the subjects pouted their lips. This series of exercises was repeated 7 times.
Recording and Evaluation of the Lip-pressure Signals and the EMG Signals
Pressure and EMG signals were recorded and sampled with a frequency of 2000 Hz (Biovision, Wehrheim, Germany). The resting lip pressure was calculated as the difference between the lip-on pressure (recorded in the resting lip position) and the lip-off pressure (recorded while the lips were pouted), determined in intervals of 50 msec just prior to and after subjects pouted their lips. The amplitude of the myoelectric signals was characterized by calculation of the root mean square (RMS) value in an interval of 500 msec prior to the lip-pouting.
Clinical Measurements
To determine the level of the lip line, we positioned a toothpick between the subject's resting lips. After the stick was adjusted parallel to the occlusal plane, it was held in position by the investigator. The subject then pouted the lips, and measurements of the distance from the top of the stick to the incisal edge were taken. This procedure was carried out on both upper central incisors and was repeated 3 times.
Model Cast Analysis
To measure the inclination of the upper central incisors, we trimmed the bases of the subject's upper dental casts parallel to the occlusal plane. The casts were then divided at the midline. Both halves were further ground off at their median side so that exactly half the crown of the upper central incisor was removed in the mesio-distal dimension. The crown axis was then drawn through the incisal edge and through half the distance between the lingual and labial gingival sulcus (Fig. 2A). The crown inclination relative to the occlusal plane was measured 3 times on each side. Model cast analysis included determination of buccal occlusion and frontal overbite.
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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
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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).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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). 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) show that, in most of the Class II, Division 2 subjects, the pressure in the incisal area of the maxillary centrals was positive and the pressure in the cervical area was negative. In the control group, the reverse pressure distribution was found. Negative pressure in both samples was almost of the same magnitude (-1.24 cN/cm2 and -1.25 cN/cm2, respectively), whereas positive incisal pressure in the malocclusion group (+3.05 cN/cm2) was more than twice as high as positive cervical pressure in the control group (+1.34 cN/cm2). The weighted average of incisal and cervical pressures, representing the lingual tipping effect on the upper centrals, was significantly higher in the Class II, Division 2 sample than in the Class I sample (Mann-Whitney U test, p < 0.01).
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). 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).
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). 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. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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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.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Received September 10, 2001; Last revision February 18, 2002; Accepted February 25, 2002
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REFERENCES |
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