Influence of Different Preactivation Patterns and Aligner Materials on the Capability of Aligners to Induce Palatal Root Torque of Upper Incisors: An In Vitro Biomechanical Study

Abstract

Objectives: Previous studies have demonstrated that aligners with labial-cervical pressure points can induce root movement, but with initial unwanted tipping. This study assessed the impact of palatal-incisal pressure points on improving root movement and reducing initial offset. Additionally, the influence of aligner materials on force and moment generation was evaluated.

Material and methods: The experimental setup consisted of an acrylic upper jaw model with teeth 11 and 21 separated and secured to a Hexapod using a 3D force-moment sensor, allowing for the simulation of various malpositions of the measurement teeth. In addition to labial pressure points set close to the cervical margins at a depth of 1.5 mm, we investigated palatal pressure points positioned close to the incisal edge at depths ranging from 0.1 to 0.9 mm. We evaluated the force/moment (F/M) systems generated by both mono- and multi-layered aligner materials during the simulated correction of 2° retroinclination of the measurement teeth. Five aligners were tested for each configuration. The relevant palatal torque range (palTR) was identified when the aligners simultaneously induced a negative palatal force (-Fy) and a negative palatal torque moment (-Mx).

Results: PET-G aligners without pressure points showed no effective torque range. In contrast, aligners with pressure points generated an effective torque range of an average of 1.02° ± 0.03° following initial tooth tipping. The palatal-incisal pressure points showed a significant reduction or elimination of the initial offset. Our findings revealed a general correlation between palTR-start displacement (initial offset range) and palatal pressure point depth (linear mixed-effects models, p < 0.05). In this manner, the initial offset for the 0.6 mm pressure points was reduced by 81.1% compared to that of the unmodified aligners (from 1.57° to 0.3°).

Conclusion: The addition of palatal-incisal pressure points alongside labial-cervical pressure points demonstrated a promising reduction in the initial offset range in an in vitro setting, potentially enhancing the efficiency of torque movement with aligners. However, further biomechanical and clinical studies are necessary for the clinical translation of these results.

Keywords: aligner; bodily movement; force; modification; moment; root torque.

FIGURE 1

Detailed view of the test setup revealing measurement teeth 11 and 21 fixed to a 3D force‐moment sensor. A horseshoe‐shaped opposite jaw component with a silicone underlay (blue) secured the aligners during testing.

FIGURE 2

Screenshots of the digital model with the capsular pressure points inserted into both the labial‐cervical (a) and the palatal‐incisal tooth surfaces (b) of the measuring teeth 11, 21. These geometries were implemented at a distance of 2 mm from the gingival margin or 2 mm below the incisal edge. While the labial depth was uniformly kept at 1.5 mm for all test models, the palatal depth varied from 0.1 to 0.9 mm.

FIGURE 3

Labio‐palatal forces (Fy, red curve) and moments (Mx, black curve) measured during the simulated palatal torque movement of teeth 11 and 21 with the 0.75 mm Duran aligners, including 1.5 mm deep labial and 0.6 mm deep palatal capsular modifications. The black arrow marks the start of the effective palatal torque range (palTR‐start).

FIGURE 4

Effective palatal displacements [°] at which the palatal torque range started (palTR‐start). PalTR‐start values for 0.75 or 0.76 mm thick aligners with 1.5 mm deep labial and palatal capsular modifications in various depths (range 0–0.9 mm) integrated in three different foil types (CA Pro, Zendura FLX, and Duran) are shown.

FIGURE 5

Schematic representation of the force‐moment (F/M) system generated by the aligners, highlighting the labio‐cervical pressure region and the initial aligner play. (a) The dominant labio‐cervical pressure point and the associated labial contact force (Fy‐lab) create palatal moments (Mx), resulting in initial tipping of the tooth. (b) Introduction of an additional contact force (Fpal) as a pressure point reduces or eliminates the initial aligner play, thereby mitigating tipping moments and potentially initiating the palatal torque range earlier.