Journal of Dental Medicine

Background and Aims: Ensuring the long-term stability of orthodontic treatments and preventing tooth relapse remains a significant challenge in orthodontics, often addressed through the extended use of retainers. Given the pivotal role of retainers' physical and mechanical properties in this regard, this study aimed to investigate the tensile strength, flexural strength, and deflection of three distinct types of fixed orthodontic retainers.
Materials and Methods: In this laboratory study, each group comprised 12 samples of twisted, flat, and coaxial retainer wires, each measuring 8 mm in length. These samples were affixed onto the polyethylene blocks using cyanoacrylate adhesive and subsequently subjected to compressive loads using a universal testing machine. The maximum bending strength and deflection rate were recorded from stress-strain diagrams. To evaluate the tensile strength, the identical samples underwent tensile loading. Statistical analysis was performed utilizing SPSS software, employing analysis of variance tests to compare data between groups.
Results: Significant differences were observed in the bending strength (P=0.004), deflection (P<0.001), and tensile strength (P<0.001) among the investigated retainer wires. The flat retainer wire exhibited the highest bending strength, averaging 822.876±136.07 MPa, while the coaxial retainer wire showed the lowest, with an average of 673.65±73.35 MPa. Regarding deflection, the flat retainer wire displayed the highest average at 4.04±0.6 mm, surpassing the other two types of retainers. However, the tensile strength of the twisted retainer wires, averaging 699.7±83.46 MPa, exceeded that of both the flat and coaxial retainer wires.
Conclusion: The flat retainer wires demonstrated the highest bending and deflection strength, while the twisted retainer wires exhibited the highest tensile strength. Overall, the bending strength, deflection, and tensile strength of the 3-strand twisted and flat retainer wires outperformed those of the 5-strand coaxial retainer wires, increased flexural and tensile strengths imply a higher resistance of the wires to the forces induced by harder and more viscous foodstuffs.

Background and Aims: Post-treatment relapse is one of the common challenges in maintaining orthodontic results, and the selection of the type and characteristics of retainers plays a crucial role in treatment stability. Vacuum-formed retainers (Essix) are widely used due to their ease of fabrication, esthetics, and high patient acceptance. However, the effect of different thicknesses of these retainers on the preservation of occlusal characteristics has not been fully elucidated. The aim of this study was to evaluate the impact of Essix retainer thickness on maintaining occlusal characteristics.
Materials and Methods: This prospective cohort study was conducted in 2025 on 30 patients (23 females and 7 males) who had completed fixed orthodontic treatment at the Department of Orthodontics, Babol University of Medical Sciences, Iran. Patients were randomly assigned to two groups receiving Essix retainers of different thicknesses (1.5 mm and 1 mm) under similar retention protocols. The variables of overjet, overbite, inter-canine width, inter-molar width, upper arch length, and the dental irregularity index were measured immediately after debonding at 6 and 9 months of the retention phase. Data were analyzed using repeated measures ANOVA, and comparisons were made between the two groups.
Results: The results demonstrated that the use of 1.5 mm thickness of Essix retainers led to a significant increase in overjet over time (p=0.04), whereas the 1 mm thickness showed no significant change (p=0.22). Changes in overbite were not significant in either group (p=0.60) and the difference between the two groups at 9 months was also not significant (p=0.23). Inter-canine width (p=0.38) and inter-molar width (p=0.13) did not change significantly over time and the difference between groups at 9 months was not significant (p>0.05). Regarding the arch length, the 1.5 mm thickness showed significant changes (p=0.04), whereas the 1 mm  thickness  did not (p=0.16). However, the difference between groups at 9 months was not significant (p=0.44). The dental irregularity index decreased significantly in both groups (p≤0.001), but the degree of change between groups was not significant (p=0.35).
Conclusion: Based on the findings of this study, clinically, the thinner Essix retainers did not result in greater relapse compared to the thicker Essix retainers. Therefore, the selection of Essix retainer thickness can be based on the patient preference and clinician judgment, without a need to recommend a specific thickness definitively. However, further studies with longer follow-up periods are suggested to evaluate the long-term stability of treatment outcomes.