Movement of lower incisors in the mandibular bone

The range of orthodontic tooth movement for lower incisors within the bony housing of the mandible is limited. Mesial bodily movement of lower incisors is normally not possible, due to the anatomy of the bone in the lower incisor area (Fig. 7.31). Therefore, any mesial movement of the lower incisor tips is mainly as a result of a change in torque (Fig. 7.32). A rule of thumb limit for this proclination is 100° to mandibular plane as set in the Class II mandibular triangle (p. 176). As these arbitrary limits are exceeded, there comes a perceived risk of instability, poor esthetics, or gingival problems.

Proclination of lower incisors from the starting position is normally necessary in Class 11/2 cases, and many thumb-sucking Class 11/1 cases, where the lower incisors are typically retroclined. It is accepted practice to move the lower incisors mesially in such cases. In Class ll/l cases with Class II skeletal bases, it is becoming more acceptable to procline lower incisors beyond the traditional 95° to mandibular plane and +2 mm to APo. Conventional orthodontic thinking was against this, because of the risk of gingival recession and relapse. I lowever, gingival recession or periodontal disadvantage has been shown not to occur1213 and relapse can be controlled with bonded retainers.

Therefore, modest proclination may be used to bring lower incisors into an acceptable relationship with PIP for upper incisors. Slight proclination of lower incisors may be acceptable for some Class 11/1 cases with a mild Class II skeletal pattern. In this way, over-retraction of upper incisors, with consequent loss of upper lip concavity (leading to poor facial profile) may be avoided.

In Class II treatment the reasonable limit to lower incisor proclination is 100° to the mandibular plane, in most cases. Accordingly, in many Class II cases the lower incisors can be proclined forwards.

Geometrically, ever)' 2.5° of proclination moves the lower incisor incisal edges forward by 1 mm (resulting in space gains of 2 mm for every 2.5° of proclination). Consequently, because of this space gain in Class II treatment, lower premolar extractions are not normally needed.

The -6° torque feature in the MB'FIM lower incisor bracket is helpful in preventing excessive lower incisor proclination. A well-aligned lower arch with a .019/.025 steel rectangular wire tied in place can therefore often be used to support Class II elastics for upper incisor retraction, if the case needs this.

Fig. 7.31 The range of movement of lower incisors within the Fig. 7.32 Mesial movement of lower incisors is mainly due to bony housing is limited. proclination, with a change of torque.

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