A certain amount of wear and tear can be expected with clear aligners.

Following are some examples of wear and tear you might see during treatment:

• Normal wear of the aligners can cause them to lose their elasticity over time. Which can result in aligners that do not fit as tight as they did when first worn.
• The patient's body temperature can also soften the aligner. 
• Saliva has been known to break down material, so a small amount of change to the aligner is due to saliva. 
• Discoloration of aligners can happen when they come into contact with colored drinks, or heavily dyed foods and this can cause some wear to the aligner.
• Not removing aligners before eating, and not practicing good dental hygiene can also cause wear.
• Teeth grinding and clenching can cause aligners to wear down faster, deform, and/or possibly crack in some scenarios.
• Missing teeth can cause faster wear and breaks in the aligners as well.

If an aligner breaks, it's not a problem, all you need to do is order a replacement. Otherwise, the patient will be getting a new set of aligners in a few weeks, so any wear and tear should not impede treatment.