Background: While polyethylene terephthalate glycol (PETG) is widely used in orthodontic appliances such as clear aligners and retainers, there is limited experimental data assessing its performance under functional stresses, such as those encountered during dental movements and palatal expansion.

Objective: This study aims to evaluate the ability of PETG thermoplastic material to withstand deformation under functional and expansion forces, specifically within the context of orthodontic applications.

Subjects and Methods: To estimate the firmness of the screw within the appliance, a universal Instron testing machine was used to record the forces released by each activation of the expander within the upper part of 10 clear modified twin blocks (MTBs) made from PETG and compare it with that released by 10 conventional twin blocks (CTBs). On the other hand, to determine the ability of the thermoplastic appliance to withstand the deformation during functional forces, a three‐point bending test was used to investigate the response of both appliances under static loading. Independent samples t ‐test was used to compare the differences between groups.

Results: Both CTB and MTB groups follow the same pattern of increase and decrease in the amount of mean load with the CTB group line showing a considerably higher amount of mean load reaching the peak (334.5 N) at turn 25 of screw activation while the peak of mean load for MTB group was equal to 252.6 N at turn 23. There was a statistically significant difference between the CTB and MTB groups in the three‐point bending test ( p = 0.001). However, both appliances did not deform at the required force.

Conclusions: The MTB can withstand both required expansion and functional load without deformation.

Trial Registration: ClinicalTrials.gov identifier: NCT06116500 .