Evaluation of Physical Changes in Single-Layer and Multi-Layer Clear Aligners

aim of the study: To evaluate and compare the surface roughness (Ra, Rq, Rz), Vickers microhardness (VHN), and thickness of single-layer and multi-layer clear aligner sheets at three stages: before thermoforming, after thermoforming, and following intraoral wear.

material and methods: This in vivo study will be conducted on clear aligners fabricated from two commercial thermoplastic materials: single-layer (0.76 mm, ) and multi-layer (0.76 mm, ). Each material will be evaluated at three stages: before thermoforming (raw sheet), after thermoforming on 3D-printed casts, and after 2 weeks of intraoral wear. At each stage, specimens will be retrieved and tested for changes in mechanical properties, specifically surface roughness and hardness.

Study Design Type: A prospective, randomized, controlled, two-arm parallel clinical study

Design Details: participants will be randomly assigned to one of two groups:

Group A:

Upper arch: Multi-layer clear aligners Lower arch: Multi-layer clear aligners

Group B:

Upper arch: Single-layer clear aligners Lower arch: Single-layer clear aligners Baseline control: Unused aligner sheets of both types to serve as reference for measurements.

Sample Size Calculation: Sample size calculation was calculated using G*Power v3.1.9.7, a paired t-test with an expected effect size dz = 0.65 (based on Bucci et al.), α error = 0.05, and power = 80% indicated a minimum of 19 subjects. To simplify group allocation and ensure adequate power, the sample size was rounded up to 20 patients (10 patients in each group: single-layer vs. multi-layer).

Ethical Considerations:

Informed Consent: All participants (or their legal guardians) will sign an informed.

Appliance Fabrication and Treatment Protocol

• Digital impressions will be taken for all patients.
• Treatment setup will be performed with standardized staging: maximum 0.25-0.3 mm linear movement or 2-3° rotation per stage.
• Aligners will be fabricated by thermoforming using single and multilayer sheets (0.76 mm), cut and finished identically.

Specimen Preparation For each group, specimens will be prepared at three stages: before thermoforming (T0), after thermoforming (T1), and after intraoral wear (T2). At T0, square-shaped specimens (approximately 5 × 5 mm) will be cut directly from the unused raw sheets. At T1 and T2, specimens of the same dimensions will be obtained from the mid-labial surface of the central incisor region of the thermoformed aligners, both immediately after thermoforming and after two weeks of intraoral wear. The full thickness of the material will be preserved in all specimens to ensure consistency across groups.

Outcome Measures:

Primary Outcome:

Surface Roughness (Ra):

Measured at three stages:

T0: Raw, unused sheet specimens (baseline) T1: After thermoforming on dental casts T2: After 2 weeks of intraoral use Assessed using contact profilometry. Roughness defined as the arithmetic average surface deviation (Ra, in µm).

Secondary Outcomes:

Hardness:

Measured at T0, T1, and T2. Tested using Vickers hardness method (ISO 6507 compliant). Higher VHN values indicate greater resistance to indentation and deformation.

Thickness Change:

Measured at T0, T1, and T2. Evaluated using a digital micrometer or 3D scanning system. Calculated as the difference in thickness between the original sheet, thermoformed aligners, and post-use aligner

Inclusion criteria

• Class I malocclusion with mild-moderate crowding (≤5 mm per arch).
• Full permanent dentition (excluding third molars).
• No restorations, caries, or attachments planned on the labial surface of upper/lower central incisors.

Exclusion criteria

• Systemic disease affecting saliva.
• Parafunctional habits (bruxism, nail-biting).
• Smoking.
• Pregnancy.
• Previous orthodontic treatment within 1 year.