3D-Printed Vs Thermoformed Retainers: Comparison of Post-Treatment Stability, Changes in Mechanical Properties and Patients' OHRQoL

Following completion of orthodontic treatment, prolonged retention with either part-time or full-time wear of retainers is crucial in preventing relapse. Clear thermoformed retainers (TFR) are easy to fabricate and popular among orthodontic patients. With the advent of digital orthodontics and the development of biocompatible photopolymerizable resin, it is now possible to fabricate direct 3D-printed retainers.

The aim of this study is to determine and compare the post-treatment stability of dentition, changes in thickness and mechanical properties of the retainers, and oral health-related quality of life (OHRQoL) of patients wearing direct 3D-printed retainers and conventional thermoformed retainers over a retention period of 6 months.

Primary Objective:

1. To determine and compare the post-treatment stability for direct 3D printed retainers (3DPR) and conventional TFR (TFR) at baseline (T0), 3 months (T1) and 6 months (T2).

Secondary Objectives:

1. To determine and compare the thickness and mechanical properties for 3DPR and TFR after 6 months of usage.
2. To determine and compare the OHRQoL of patients wearing 3DPR and TFR at T0, T1 and T2.

Sample size calculation:

Sample size calculation is based on the study done by Kumar \& Bansal (2011) which compared the effectiveness of two different removable retainers on stability. With an effect size of 1.127, α = 0.05, and power = 0.80. Using G\*Power version 3.1.9.7 software, the total number of samples is n = 22 (11 subject per group). This number is increased by 30% to account for the dropout rate to produce the final sample size, n = 30 (15 subjects per group).

Methodology:

Subjects who have completed their orthodontic treatment (fulfil the inclusion and exclusion criteria) and are ready for debond, will be invited to participate in the research and consecutively recruited, between April 2023 until September 2023, in the Postgraduate Orthodontic Clinic, Dental Specialists and Research Tower, Faculty of Dentistry, Universiti Malaya, Malaysia. Patient information sheets and consent forms will be issued to the subjects that fulfil the criteria. Only consented respondents will be recruited.

All 30 subjects will be randomly allocated (simple randomization with 1:1 allocation) to either the TFR group or the 3DPR group by using number generated list provided by www.random.org . The randomization will be performed by the main supervisor, who is not involve in the treatment, data collection and data analysis of the research. The operator will be concealed from the group allocation. The allocation will be revealed to the operator on the day of debond appointment with a opaque white envelope.

All the subjects will undergo the same standard debond process. and receive a post debond thermoformed retainers within 24 hours which they are required to part-time for every night (8-12 hours).

The subjects will be reviewed after 1 week (T0) for scanning of teeth with Trios 3 intraoral scanner. The interventional retainers (TFR or 3DPR) will be issued after 2 weeks. The subjects are instructed to wear the retainers part-time (8-12 hours) for every nights. A pamphlet that contains detailed information on the care of retainers will also be provided to every subject. All the subjects will be reviewed after 3 months (T1) and at 6 months (T2) after wearing the interventional retainers.

The primary outcome is to assess the post-treatment stability after part time wear of the retainers for a period of 6 months. The subject's upper and lower dentition will be scanned by using intraoral scanner at T0, T1, and T2. The measurements will be performed digitally using 3Shape orthodontic analyzer software. Overjet, overbite, intermolar width, and little irregularity index will be measured.

OHIP-14 questionnaire will be given to the participants to answer during T0, T1 and T2. All the retainers will be retrieved at T2 to undergo mechanical test.

Statistical Analysis:

The data collected will be entered into SPSS software (version 26.0). i. Chi-square tests will be used to compare baseline characteristics between the 2 groups.

ii. Repeated measures ANOVA or paired t-test will be used to determine the mean differences in post treatment stability, thickness of retainers, mechanical properties of retainers and OHIP-14 scores at different time intervals. The p-value will be set to less than 0.05.

iii. Two samples independent t test will be used to compare the post-treatment stability, changes in thickness, mechanical properties, and OHIP-14 scores between TFR and 3DPR group. The p-value will be set to less than 0.05.

iv. Shapiro-Wilk tests will be referred, and non-parametric tests will be performed for skewed data.

Data will be analyzed following the intention-to-treat analysis.

Eligibility Criteria:
Inclusion Criteria:

* Fixed appliance treatment in both arches and indicated for VFR in the retention phase as part of their original treatment plan.
* No intention to relocate within the study period; able to attend the three-monthly review appointments for half a year.

Exclusion Criteria:

* Single-arch or sectional fixed appliances.
* Space dentition.
* Hypodontia requiring tooth replacement on the retainer as a temporary measure.
* Previous treatment with maxillary expansion.
* Indicated for fixed retainer or double retention regime (such as VFRs fitted over fixed retainers).
* Premature debond from the original fixed appliances course.
* Cleft lip and/or palate; or orthognathic cases.
* Learning difficulties and inability to read written instructions/ questionnaire in English or Malay