Effects of variable attachment shapes and aligner material on aligner retention

Abstract

Objective: To evaluate the retention of four types of aligners on a dental arch with various attachments.

Materials and methods: For this study, three casts were manufactured, two of which contained attachments (ellipsoid and beveled), and one without any attachments to serve as a control. Four types of aligners were thermoformed: Clear-Aligner (CA)-soft, CA-medium, and CA-hard, with various thicknesses, and Essix ACE. Measurements of vertical displacement force during aligner removal were performed with the Gabo Qualimeter Eplexor. Means and standard deviations were next compared between different aligner thicknesses and attachment shapes.

Results: CA-soft, CA-medium, and CA-hard did not present a significant increase in retention, except when used in the presence of attachments. Additionally, CA-medium and CA-hard required significantly more force for removal. Essix ACE demonstrated a significant decrease in retention when used with ellipsoid attachments. The force value for Essix ACE removal from the cast with beveled attachments was comparable to that of CA-medium. Forces for aligner removal from the model without attachments showed a linear trend. Essix ACE did not show a continuous increase in retention for each model. Overall, ellipsoid attachments did not present a significant change in retention. In contrast, beveled attachments improved retention.

Conclusions: Ellipsoid attachments had no significant influence on the force required for aligner removal and hence on aligner retention. Essix ACE showed significantly less retention than CA-hard on the models with attachments. Furthermore, beveled attachments were observed to increase retention significantly, compared with ellipsoid attachments and when using no attachments.

Keywords: Aligner therapy; Attachment shapes; Retention; Thermoplastic appliances.

Figure 1.

Stone model with ellipsoid attachments, which were added to both premolars. Attachment shapes inspired by Invisalign attachments.

Figure 2.

Attachment dimensions. Ellipsoid attachments were oriented vertically and measured 3 mm in height, 2 mm in width, and 1mm in depth. Rectangular beveled attachments were oriented horizontally, and were 2 mm high, 3 mm wide, and 0.25 mm deep toward the incisal edge and 1.25 mm deep toward the gingival margin.

Figure 3.

Duplicated model of cold-curing dental resin (Palapressvario, Heraeus Kulzer, Hanau, Germany) in a Biostar Thermoforming Machine (Scheu Dental).

Figure 4.

Reproducible locations of holes on the model base (6 mm in diameter) and the occlusal surfaces (3 mm in diameter and 5 mm in depth) of the first molars. The central mounting hole was used for fixation of the model in the Gabo Qualimeter Eplexor (GQE). The location was determined with one median line through the location of the palatal raphe intersecting with a line through the mesiolingual cusps of the first molars. The molar holes provided room for the stops of two steel ropes that passed through the aligner into the bolting apparatus on the GQE.

Figure 5.

Model inside the GABO Qualimeter Eplexor (GQE). Aligner attached via steel ropes running straight from the first molars through the aligner to the bolting apparatus. The upward displacement force was measured in 10-second intervals until the aligner was removed from the model.

Figure 6.

Quantification of variable aligner material on retention. (A) Vertical displacement forces (Newtons) of each aligner material during removal from the control cast containing no attachments. (B) Vertical displacement forces of each aligner material during removal from the cast containing ellipsoid attachments. (C) Vertical displacement forces of each aligner material during removal from the cast containing beveled attachments. Each column represents one of the tested aligner materials (CA soft, medium, hard; Essix ACE).

Figure 7.

Quantification of variable attachments on retention. (A) CA soft aligner removal from three different casts containing either no attachments, ellipsoid, or beveled attachments. (B) CA medium aligner removal from three different casts containing either no attachments, ellipsoid, or beveled attachments (C) CA hard aligner removal from three different casts containing either no attachments, ellipsoid, or beveled attachments (D) Essix ACE aligner removal from three different casts containing either no attachments, ellipsoid, or beveled attachments.