I came across two old studies about thermoplastic materials properties. One by Gracco et al. in 2009 and the second one by Schuster et al. in 2004.
These studies showed that aligner materials at the buccal segments suffers from a structural change called masticatory-induced cold work (work hardening) which results in an increase in the hardness of the aligner materials and subsequently higher elastic modulus and force on the teeth.
Cold work hardening might make aligner tracking of the posterior teeth problematic especially with the weekly change of the aligners. Having said that, one of the study is in vitro.
This reminded me of some recent studies that showed that buccal movements are not tracking well with the weekly protocol.
Shall we exclude the weekly-activation from cases that require buccal movements? What do you think?
List of the papers: Gracco A, Mazzoli A, Favoni O, et al. Short-term chemical and physical changes in invisalign appliances. Aust Orthod J. 2009;25(1):34–40
Schuster S, Eliades G, Zinelis S, Eliades T, Bradley TG. Structural conformation and leaching from in vitro aged and retrieved zInvisalign appliances. Am J Orthod Dentofacial Orthop. 2004;126(6):725–728
PS: I have no financial interest.