Hopefully, you’ll only need one set of Invisalign trays to achieve the final result that you desire in terms of straight teeth and correct bite alignment. However, if your first set of Invisalign trays do not produce a result that satisfies both you and your orthodontist, it is often possible to get further refinement trays created.
These refinements are additional set(s) of aligners that work in the same way as your first set of aligners – i.e. you wear each pair of trays continuously for a fixed time (e.g. 2 weeks) before changing to the next pair in the series. Before getting refinement trays, you’ll usually need to get new impressions or 3D scans taken as a starting point.
The number of refinement sets that are possible (and the cost, if any) depends on the type of Invisalign treatment – for example, the MCO Orthodontics website (see source reference below) explains that a mid-course correction and up to 3 refinements are available with both Invisalign and Invisalign Teen treatments, but that only one set of refinements is possible for Invisalign Express treatment cases and only if an additional fee is paid.
Note that even for Invisalign and Invisalign Teen treatments, the cost of refinements may not be included in your original fee if they are required due to dental work (e.g. crowns or fillings), non-compliance or a change in treatment plan. It’s best to discuss the possibility of refinements with your orthodontist prior to starting Invisalign so you know what your individual obligations and options are.
Source: http://www.torontobraces.ca/whatisinvisalign, accessed June 2012