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

Evaluation of Physical Changes in Single-Layer and Multi-Layer Clear Aligners After Thermoforming and Intraoral Wear

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.

material and methods: the 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, after thermoforming on 3D-printed casts, and after 2 weeks of intraoral wear.

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.

Outcomes:

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

Secondary Outcomes:

Hardness:

Measured at T0, T1, and T2. Tested using Vickers hardness method (ISO 6507 compliant).

Study Overview

• Status: Active, not recruiting
• Conditions: Surface Roughness and Thickness Clear Aligner Sheat
• Intervention / Treatment: Device: clear aligner

Detailed Description

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

• Study Type: Interventional
• Enrollment (Actual): 20
• Phase: Not Applicable

Contacts and Locations

Study Locations

• Egypt
  • Al Mansurah, Egypt
    • Mansoura University

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Child; Adult
• Accepts Healthy Volunteers: No

Description

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.

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Treatment
• Allocation: Randomized
• Interventional Model: Parallel Assignment
• Masking: None (Open Label)

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: Multi-layer clear aligners; clear aligners fabricated from thermoplastic materials: multi-layer (0.76 mm). the 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. Baseline control: Unused aligner sheets of both types to serve as reference for measurements.	Device: clear aligner; 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.
Experimental: Single-layer clear aligners; clear aligners fabricated from thermoplastic materials: single-layer (0.76 mm) . the 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. Baseline control: Unused aligner sheets of both types to serve as reference for measurements.	Device: clear aligner; 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.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
surface roughness of clear aligner	evaluate and compare the surface roughness of single-layer and multi-layer clear aligner sheets at three stages: before thermoforming, after thermoforming, and following intraoral wear. Quantitative evaluation of the arithmetic mean surface roughness (Ra) using profilometer	2 weeks

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
microhardness of clear aligner	evaluate and compare the microhardness of single-layer and multi-layer clear aligner sheets at three stages: before thermoforming, after thermoforming, and following intraoral wear. Quantitative evaluation of surface microhardness using the Vickers hardness test.	2 weeks
thickness change of clear aligner	evaluate and compare the thickness of single-layer and multi-layer clear aligner sheets at three stages: before thermoforming, after thermoforming, and following intraoral wear. Thickness will be measured using a calibrated digital caliper with an accuracy of ±0.01 mm	2 weeks

Collaborators and Investigators

• Sponsor: Mansoura University

Study record dates

Study Major Dates

• Study Start (Actual): January 1, 2026
• Primary Completion (Estimated): January 1, 2027
• Study Completion (Estimated): February 1, 2027

Study Registration Dates

• First Submitted: February 15, 2026
• First Submitted That Met QC Criteria: March 3, 2026
• First Posted (Actual): March 4, 2026

Study Record Updates

• Last Update Posted (Actual): March 4, 2026
• Last Update Submitted That Met QC Criteria: March 3, 2026
• Last Verified: December 1, 2025

More Information

Terms related to this study

• Additional Relevant MeSH Terms: Dentistry; Orthodontic Appliances; Orthodontics; Orthodontic Appliances, Removable
• Other Study ID Numbers: MS.25.11.85

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No