Invisalign can close extraction gaps but in my experience, this requires either additional treatment with fixed braces to ensure the teeth on either side of the gap end up in correct alignment or additional tools (such as power arms) during space closure.
What tends to happen when aligners are used to close spaces, is that the crowns of the teeth tend to tip towards the gap instead of moving towards each other in an upright alignment (bodily movement). Even with fixed braces, this can be difficult to achieve. This tipping is accentuated further when patients do not wear their aligners enough.
Power arms are either metal/plastic pieces that are glued to the teeth on either side of the gap and elastics are used to squeeze the arms together as the space closes. This tries to counteract the tipping of the crowns towards the extraction space. If the aligners are not worn as prescribed, the opposite problem may happen: the roots of the teeth come closer together than the crowns, instead of both being parallel.
Performing extraction space closure with aligners is challenging, and a lot depends on patient compliance. Despite ideal patient compliance, finishing with fixed braces (or several sets of refinement aligners) so that a proper bite and alignment is achieved at the end of treatment, is almost mandatory.
