Assessing viscoelastic properties of clear aligners using a geometry-independent indentation method

Clear aligners are widely used in digital orthodontics, but their clinical force delivery is governed by the time-dependent viscoelastic behavior of thermoplastic materials. Due to the thin-walled, curved, and geometrically irregular characteristics of clinical aligners, conventional tensile or bending tests and standard indentation analyses cannot reliably recover their intrinsic viscoelastic properties. Thus, the present study aimed to introduce and validate a geometry-independent indentation framework for characterizing the intrinsic viscoelastic response of clear aligners. In the present context, intrinsic viscoelastic properties refer to the reduced relaxation function and its characteristic relaxation spectrum after normalization removes time-independent geometric factors, rather than to the absolute instantaneous modulus.

The proposed method was derived from dimensional analysis and the elastic-viscoelastic correspondence principle. Its validity was examined by finite element simulations of a clinically realistic aligner geometry. Indentation experiments were further conducted on raw thermoplastic films, thermoformed films, as-fabricated aligners, and intraorally used aligners to track the viscoelastic evolution of materials during manufacturing and clinical service.

Numerical simulations showed that the normalized creep and relaxation functions obtained from different aligner regions and indenter profiles agreed closely with the prescribed reference functions. Experimental measurements revealed pronounced differences between flat films and shaped aligners, together with position-dependent differences between two regions of the same appliance. These results show that the suggested method can suppress time-independent geometric contributions while retaining spatial variations in the local viscoelastic response.

The suggested indentation method enables minimally destructive, direct characterization of the reduced viscoelastic behavior of intact clear aligners without geometry-specific inverse analysis. By separating time-independent geometric effects from the normalized time-dependent response, the method can also reveal position-dependent viscoelastic characteristics associated with local forming history and clinical wear.