In orthodontics, retention (the post-treatment phase) is crucial to maintaining the alignment achieved through active treatment. Fixed lingual retainers are widely used in this period, but there is no universally accepted standard regarding their design, material selection, and testing methods. This article examines the major issues and the need for standardization in fixed retention, discussing applicable test methods and their advantages.

Most practitioners seem to adopt a more “liberal” approach, believing they have the right to design appliances in any way they wish. However, this is not ideal. We should address the concept of physiological tooth movement and the restraining effect of fixed retainers.

Additionally, it is crucial to determine which retainer designs and materials perform best at restricting physiological tooth movements. In this regard, clinicians and technicians should not create custom-made appliances (e.g., via 3D printing) without first assessing the degree of tooth mobility and how the chosen design or material may restrict it—primarily due to tooth root resorption.

1. Restriction of Tooth Mobility

After orthodontic treatment, different designs of fixed retainers are employed to keep teeth in place. However, it remains unclear how each design impacts the physiological movement of teeth—especially within the first 24 months when some level of tooth mobility persists. Key points include:

• Physiological Tooth Movement: Post-treatment teeth require a minimal degree of natural movement. Overly rigid, non-flexible retainer wires can create unwanted stress on the tooth roots and surrounding tissues.
• Design and Material Selection: Each material (e.g., stainless steel, FRC) has unique properties regarding stiffness and flexibility, influencing how much it restricts or permits physiological tooth movement.
• Custom Manufacturing (3D Printing, etc.): With the advent of customized retainers, it is essential to assess how these newly designed or 3D-printed appliances affect natural tooth mobility.

The lack of formal standards results in a wide variety of clinical practices. Many studies and expert opinions point to designs that allow a slight degree of flexibility, thus preserving a certain amount of physiological movement.

We know from the literature that Ni-Ti retainers allow more tooth mobility than other materials, and this is beneficial for patients who want to avoid tooth loss due to root resorption in the long run.

2. Material Fatigue and Testing Environment

Fixed retainers are subject to numerous factors in the oral environment, including chewing forces and moisture, which can accelerate material fatigue. Yet, current conventional test methods do not fully simulate the real stresses and movements that occur in the mouth. Also, we do not observe metal fatigue or wire breakage during well-known test applications.

• Tooth Mobility Simulation Gap: Most chewing simulators do not accurately replicate the dynamic tooth mobility seen in the critical first 24 months after orthodontic treatment.
• Laboratory Models: Various in-vitro models (e.g., 3D-printed designs, Bicon/Trinia–FRC) have been introduced, but a fully realistic tooth mobility simulation remains elusive.
• Recommendation of ISO 7801: To better understand and quantify the fatigue behavior and bendability of lingual retainer wires, testing according to ISO 7801 is suggested, offering more standardized and reproducible outcomes. (The Permatter LLC team has started building ISO 7801 test equipment, and we will publish the results by summer 2025.)

3. Testing Methods: Pull-Out vs. Shear Bond Tests

Pull-out tests and shear bond tests are common for assessing the mechanical performance of fixed retainers and their bonding interfaces. Both, however, come with a range of challenges and debates:

3.1 Pull-Out Test Considerations

• Gripper Type (Hydraulic vs. Pneumatic): Hydraulic grippers provide a tighter, non-slipping grip, generally yielding higher measured forces (~1200 MPa), whereas pneumatic grippers may allow partial slipping and thus lower readings (~900 MPa). (According our test applications)
• Effects of Bonding Resins: Different bonding resins can permit partial slippage during testing, complicating pull-out results.
• Lack of Standardization: While some studies propose revising ISO 6892 for orthodontic pull-out tests (notably advocated in some Korean academic circles), global acceptance has not yet been achieved.

(Pneumatic grippers allow the retainer to slip, meaning the measured force is generally lower. We measured 900 MPa for this test.)

(Hydraulic grippers, on the other hand, create a stronger grip with no slip, producing higher force values. We measured 1200 MPa for this test.)

From our pneumatic gripper testing, we know that this method provides poor-quality results because it lacks hydraulic grippers and allows the wire to slip.

Similarly, certain bonding resins may allow partial slippage, further complicating the pull-out method. Because of these discrepancies, we need a standardized protocol that ensures consistent gripping and loading conditions.

3.2.Shear Bond Test Considerations

Widespread Use: Despite being very common in orthodontics, there is no universally accepted standard for shear bond testing.

Need for Standardization: Variations in bonding area and tooth surface hardness significantly affect outcomes, highlighting the importance of a unified protocol.

4. Proposals for Standardization

Proposed Standardization Criteria
To establish a reliable standard for retention treatments, the following points should be considered:
1.Hardness Measurement
Standard methods to measure the hardness of the toe is necessary.
2.Physical Dimensions
A specified width/length/depth (or diameter/thickness) of the toe or bonding pad material should be measured and reported in each study or clinical case.
3.Uniform Bonding Surface Area
Using standardized bonding jigs or “minimolds” can help ensure the same surface area is used for bonding tests, improving test consistency and comparability.

Conclusion

Fixed retention is vital for maintaining orthodontic treatment results. However, issues like tooth mobility, material fatigue, and the lack of standardized testing methods remain unresolved. Standardizing pull-out and shear bond tests would allow for more reliable comparisons across studies and improved clinical application. Integrating standards such as ISO 7801 can also help measure wire performance and material longevity more accurately. In short, standardizing fixed retainer design and testing protocols will not only increase clinical success rates but also yield more consistent and meaningful results in the scientific literature.

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