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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
² Department of Biometry and Medical Statistics, University of Freiburg i.Br., Hugstetter Str. 55, D-79106 Freiburg, Germany;

View larger version (76K): [in a new window]   Figure 1. Transducers for lip-pressure measurements in the incisal and cervical areas of both maxillary central incisor crowns. (A) Four pressure sensors were attached to a carrier halfway in the mesio-distal dimension of the crowns so that the lowest edges of the 2 incisal sensors were level with the incisal edges of the maxillary centrals. (B) Dimensions and attachment levels of sensors and pressure-receiving surfaces.

View larger version (51K): [in a new window]   Figure 2. Results of lip-line measurements and model cast analysis, including an illustration of the method for determining the inclination of the maxillary central incisors. (A) Measurement of left upper central's inclination in relation to the occlusal plane on the prepared dental cast. (B/C) Relationships between lip line, upper central incisors, and pressure transducers in the Class I group (B) and the Class II, Division 2 group (C). (Table) Mean values and standard deviations of the upper central inclination, the level of the lip line, the frontal overbite, and buccal occlusion in the Class I group (n = 21) and the Class II, Division 2 group (n = 21). *Standard deviations. op1, occlusal plane (defined by the tip of the cuspid and the most prominent cusp of the upper first molar); op2, model base (trimmed parallel to the occlusal plane); gs1, gingival sulcus at the lingual side of the left upper central incisor; gs2, gingival sulcus at the labial side of the left upper central incisor; ca, crown axis of the left upper central incisor; cr, center of resistance of the upper central incisor; and dc/i, distance from the cervical sensor location (dc) and the incisal sensor location (di) to the center of resistance of the upper central incisor. In our experiment, the ratio of di/dc was 1.6 (according to Burstone and Pryputniewicz, 1980). ll1/2 = level of the lip line (defined as the distance between the lip line and the incisal edge) in the Class I group (ll1) and the Class II, Division 2 group (ll2).

View larger version (29K): [in a new window]   Figure 3. Lip pressure on the upper central incisors and peri-oral muscle activity (both recorded in the resting position of the lips) in the Class I group (n = 21) and the Class II, Division 2 group (n = 21). (A) Resting lip pressure in the incisal and cervical areas, as well as the weighted average of incisal and cervical pressures. The latter parameter represents the lingual tipping effect on the upper centrals. All values are mean values of the right and left upper central incisors. (B) Resting activity of the orbicularis oris superior muscle (OOS), the orbicularis oris inferior muscle (OOI), the depressor labii inferioris muscle (DLI), and the mentalis muscle (MEN). *Differences to the corresponding values of the Class I group were statistically significant (Mann-Whitney U test, p < 0.01).

View larger version (25K): [in a new window]   Figure 4. Individual resting lip-pressure values in relation to the level of the lip line (n = 42). (A) Pressure in the incisal area of the upper centrals, (B) pressure in the cervical area of the upper centrals, and (C) weighted average of incisal and cervical pressures. Different symbols indicate to which study group the subjects belonged. In each diagram, linear regression curves were separately calculated for the Class I group (n = 21, dashed lines) and the Class II, Division 2 group (n = 21, solid lines). The formulae of the curves are included in the diagrams. In the statistical evaluation, no significant inter-group differences in the slope and position of the regression curves could be found (ANCOVA, p > 0.19).