The Effects of Different Storage Conditions on Removable Retainers

Orthodontic removable retainers are appliances that hold the position of the upper and lower teeth after fixed braces treatment and they can be removed from a patient's mouth. Basically, there are 2 types of removable retainers; Hawley retainer and vacuum-formed retainer.

Hawley retainer is made of an acrylic resin plate that covers the palate for the upper jaw and the back of the arch facing the tongue for the lower jaw. It has thin stainless-steel wires incorporated to fit the first molars and the six front teeth as retention.

Vacuum-formed retainer (VFR) is a clear or transparent plastic retainer. It covers the teeth completely and a small part of the gum.

The investigators' interest is to determine whether the storage conditions will affect the amount of bacteria, appearance and strength of removable retainers. Primarily, the amount of bacteria that sticks to the retainers may cause overall dental and oral health effects such as mouth odour, dental caries and gum problems. The investigators would also like to assess the oral health-related quality of life (OHRQoL) of participants after storing retainers in different storage conditions by answering a questionnaire.

RATIONALE OF THE STUDY

A non-bias clinical study is required to facilitate the best recommendation to care for the orthodontic retainers. This may prevent the risk of further harms to the oral condition and increase the longevity of the retainers. In turn, this would improve the OHRQoL of orthodontic patients and potentially save the cost of retainer replacements. Therefore, the findings may help to provide evidence-based guidelines for the storage method of removable retainers.

Primary Objective

1. To compare the microbial colony counts of removable retainers in wet and dry storage conditions.

   Secondary Objectives
2. To compare the surface roughness of removable retainer materials in wet and dry storage conditions.
3. To compare the colour stability of removable retainer materials in wet and dry storage conditions.
4. To compare the compressive strength of removable retainer materials in wet and dry storage conditions.
5. To evaluate OHRQoL of participants storing their removable retainers in dry and wet conditions.

SAMPLE SIZE CALCULATION

Sample size calculation was done using G\* Power 3.1. The total sample size is 18, for an alpha error equivalent to 0.05, a study power equivalent to 80%, a two-sided test type, and an effect size of 1.316 (Farhadian et al., 2016). Considering a potential dropout of 30%, therefore 24 participants will be recruited.

METHODOLOGY

Participants who fit the inclusion and exclusion criteria will be invited to enrol at the Orthodontic Postgraduate Clinic, Faculty of Dentistry, Universiti Malaya. Once the participants consent to take part in this study, an explanation will be given in detail of the procedure of the research with the aid of the flowchart diagram. The participants will also be given a participant information sheet. An appointment for the removal of fixed braces will be scheduled.

All participants will be required to draw lots from the sealed envelope which has been randomised by the consultant orthodontist to know their first retainer storage condition. The investigator will have to check the group allocation with the consultant orthodontist.

On the first appointment day, the removal of braces will be done according to the standard protocol. Then, four sets of impressions will be taken to construct the models and retainers. The participants will be issued with their regular upper and lower retainers, a new toothbrush and a retainer box. The participants will be instructed to store their retainers in a dry retainer box or in a glass of clean water the entire time when not in use. Other home care instructions include: 1) wear the retainers part-time (8-12 hours daily), 2) clean the retainers with a soft-bristled toothbrush and rinse thoroughly, and 3) no additional cleaning solutions and tablets are allowed. The participants will need to answer a self-administered questionnaire 1 week later to assess their personal judgment on the removable retainers.

The participants will be called 3 months later for review (T1). During this visit, the standard retainers will be taken by the investigator for analysis. The interventional retainers will be issued to the participants with an instruction to exchange the storage method from dry to wet conditions and from wet to dry conditions. The same home care instructions will be advised. The participants will receive a new toothbrush to clean their appliance and a new retainer box to keep their retainers. The participants will need to answer a questionnaire sheet again.

The participants will be called 6 months later for review (T2). The first interventional retainers will be taken by the investigator for the final analysis. The second interventional retainers will be issued to the participants and they will be instructed to continue caring for their retainers. All participants will be required to answer a questionnaire sheet for the final time.

Three different sets of the newly constructed Hawley and vacuum-formed retainers will be tested for microbial analysis, physical and mechanical properties as baseline characteristics for comparisons.

STATISTICAL ANALYSIS

Results obtained will be computed using SPSS Statistics for Windows, Version 26.0. Statistical significance will be set at p \< 0.05.

The manipulating variables are the types of storage conditions, which are dry and wet with α = 5% = 0.05 and confidence interval (CI) of 95%. Test of normality will be done using the Shapiro-Wilk test for each of the dependent variables (outcomes).

i. Mann-Whitney U-Test will be applied to compare the dependent variables of the two groups (Hawley retainers and VFRs) at baseline, T1 and T2.

ii. Wilcoxon Signed Rank test will be used to compare the storage conditions of the two groups (Hawley retainers and VFRs) at baseline, T1 and T2.

iii. Kruskal Wallis will be carried out to investigate the interaction between the two samples and the storage conditions.

iv. If the data is normally distributed, then the equivalent parametric analyses (paired t-test and independent t-test) will be performed.

Eligibility Criteria:
Inclusion Criteria:

* Patients aged 18 years and above
* Fixed appliance treatment on both the upper and lower arches
* Ready for debond
* Planned for either Hawley retainers or VFRs for retention
* Fit and healthy patients without systemic disease
* Non-smoking patients

Exclusion Criteria:

* Sectional fixed appliance treatment or fixed appliance treatment on single arch only
* Planned for double retention with bonded retainers
* Systemic disease that may affect salivary flow
* Smoking patients