If it's been a while since you took the impressions or scans, you’ll need to evaluate whether they’re still accurate. Only approve a setup if you’re confident it accurately represents the current position of the patient’s teeth. Otherwise, you’re likely to have fitting problems, starting with step 1.
Here's what our Chief Technology Officer, Dr. James Mah, has to say on the subject:
It is best practice to promptly revise and approve the setup as the patient's dentition may change over time. The amount of change and the time frame depends on the individual patient. The dentition is not immune to the effects of wear and aging and is more dynamic than most people appreciate.
Practically, patients may be classified in the stable group where any changes are undetectable or subtle enough that there is no effective difference over many months. These dentitions typically have all teeth present in an adult, healthy periodontium, no habits, neuromuscular problems or parafunction and no unusual amounts of dental wear/attrition. A good rule of thumb for these patients is to take new impressions/scans after three months have elapsed to avoid the possibility that changes have occurred.
On the other hand, patients may have unstable dentitions where the teeth may change sufficiently to impair fit of the aligner. A common situation is following extraction of a tooth. Adjacent teeth often shift and the opposing tooth may super-erupt. For this reason, extraction treatments with aligners almost always have the extractions performed only after the aligners are available. In situations involving extractions, there is great variation and in some patients changes over a short period of time such as 2 weeks is enough to cause the aligners to not fit.
Another group that experiences tooth shifting are patients with ongoing periodontal disease. Active periodontal disease is often associated with unintended tooth movement. It is best to manage the periodontal disease and re-evaluate to determine whether orthodontic treatment is even a possibility.
Patients that clench and grind or other situations where there may be loss of tooth structure such as bulimia may also end up with poor fitting aligners depending on the severity of the problem and the amount of resultant enamel and dentin loss.
Unfortunately, in these situations of instability, there are no general rules to follow as each patient is different with different etiologies and severity. Certainly, much is up to the discretion of the doctor but change of some degree is anticipated. For this reason, these treatments have additional complexity and the patient should be informed of the issue. If treatment is elected, the best practice of promptly revising and approving the setup is recommended, rather than allowing the teeth to potentially shift for months before approving.