CLEAR ALIGNER BIOMECHANICS AND PREDICTABILITY: INHERENT MOVEMENT LIMITATIONS AND THE UNMODELLED ROLE OF MATERIAL FORCE DECAY

Clear aligner therapy (CAT) has become a widely used alternative to fixed appliances, yet clinical predictability continues to lag behind the outcomes simulated in digital treatment-planning software. This narrative review examines two interrelated determinants of aligner biomechanical performance: inherent limitations in achieving complex tooth movements, and force decay arising from intraoral degradation of thermoplastic aligner materials. A structured search of PubMed, Scopus and Google Scholar was conducted for the period 2005 to 2026 using combinations of the terms “clear aligner,” “Invisalign,” “predictability,” “biomechanics,” “stress relaxation,” “torque,” “rotation,” and “refinement.” Movement-specific accuracy data indicate that pure bodily translation, rotation of round-shaped teeth, root torque expression, and vertical movements remain the least predictable outcomes, with reported accuracies for canine and premolar derotation as low as 36 to 55 percent, against overall movement predictability generally cited between 55 and 72 percent. Refinement is required in a substantial proportion of cases treated to completion. In parallel, in vitro studies confirm that thermoplastic aligner materials undergo rapid stress relaxation within the first hours of loading and continue to lose force-delivery capacity across the standard 7- to 14-day wear cycle, a time-dependent behaviour that current digital planning software does not incorporate into its staging algorithms. The combination of biomechanically unfavourable force systems for specific movement types and unmodelled material fatigue helps explain the persistent discrepancy between virtual treatment plans and clinically achieved outcomes. Narrowing this gap will likely require incorporation of time-dependent material behaviour into simulation software, more selective use of auxiliaries such as attachments and power ridges, shortened wear-cycle protocols, and calibrated clinician and patient expectations regarding refinement.