These are cases where there is only mild crowding or protrusion, but where extraction of premolars is needed to achieve proper treatment goals. It is necessary to close remaining spaces mainly by mesial movement of molars, thereby increasing available space for third molars and protecting facial profile. Second premolars may be chosen for extraction in this type of case, and second molars are normally not banded or bracketed. The thinking behind this approach is based on the mathematical balance of two molar teeth against eight anterior teeth during sliding mechanics. Logically, mainly mesial molar movement could be expected (Figs 9.31 & 9.32).
In this type of case, it is correct to commence treatment soon after extraction of second premolars, to avoid the possibility of the alveolar bone becoming narrow. Light Class II elastics (lOOgm), worn just at night, can support mesial movement of the lower molars during sliding mechanics. The torque of + 17° for upper central incisors and +10° for upper lateral incisors seems to be clinically helpful in maintaining the A/P position of the upper labial segment, against the 10-to 12-hour Class II elastic force. The molars receive a 24-hour mesializing force, which is greater in the lower arch, due to the light Class II elastics.
If careful treatment mechanics are followed after extraction of second premolars, it is possible to close space mainly by mesial movement of first and second molars, increasing available space for third molars, and maintaining the position of the lower incisors in the facial profile.
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