Retention is an important part of any orthodontic treatment and is the process by which orthodontists try to minimize relapse following treatment. This chapter will review the causes of relapse and the best contemporary evidence behind attempts to reduce this relapse using retainers and other adjunctive techniques. The section on contemporary evidence is based on the findings of a Cochrane review entitled “Retention Procedures for Stabilizing Tooth Position After Treatment with Orthodontic Braces” (Littlewood et al. 2011), and the reader is directed to the Cochrane Library for the most up-to-date version of this review.
What Is Relapse and What Are the Causes?
Relapse is any change from the final tooth position at the end of treatment. This is often, but not always, movement back toward the original malocclusion. There are broadly four main causes of relapse (Melrose & Millett 1998):
- Periodontal and gingival factors
- Occlusal factors
- Soft tissue pressures
Periodontal and Gingival Factors
After tooth movement, the periodontium around teeth needs to remodel. The majority of collagen fibers of the periodontal ligament reorganize on average after 3–4 months. However, elastic fibers around the neck of the teeth, known as the dentogingival and interdental fibers, can take 8 months or longer to reorganize (Reitan 1959). Clinically, this means that teeth have to be held in the corrected position until these fibers have remodeled. This is particularly important when these fibers are stressed, for example after rotational correction. An approach in those cases is to surgically sever the supracrestal fibers using a technique called pericision (see the section titled Pericision, below).
The way the teeth occlude at the end of treatment may also affect the stability of the teeth. Positioning the teeth in the correct occlusal relationship can reduce the relapse in deep bite cases (Houston 1989). It has also been shown retrospectively that the better the quality of the occlusal finish, the less relapse occurs (de Freitas et al. 2007).
Soft Tissue Pressures
The teeth lie in an area of balance between the tongue on one side and the cheeks and lips on the other side (Proffit 1978). This area of balance is sometimes referred to as the neutral zone. Although the forces from the tongue are stronger, the activity of a healthy periodontium will resist proclination of the teeth. However, the further the teeth are moved out of this zone of stability, the more unstable they are likely to be.
The problem for the clinician is that it is not possible to visualize the neutral zone, so as a guide it is acknowledged that the bigger the change in the arch form from the start of treatment, the more unstable the treatment is likely to be. This would then need to be taken into account when planning the retention stage of treatment.
Soft tissue pressures are affected by muscle and general soft tissue tone. As these may change with age, the neutral zone and therefore forces on the teeth may alter as the patient gets older.
While the vast majority of growth is complete by the end of the second decade, it has been suggested that growth may continue, unpredictably, throughout life (Behrents et al. 1989). Subtle changes in the relationship between the maxilla and mandible may exert forces on the teeth later in life, causing changes in tooth position.
Can the Orthodontist Prevent Relapse?
In cases with a healthy periodontium, the orthodontist has the ability to influence the periodontal factors, either by maintaining the teeth long enough for the fibers to remodel or by cutting the supracrestal fibers. The orthodontist can also reduce relapse potential by positioning the teeth in the correct occlusal relationship. Positioning the teeth in the correct relationship with the soft tissues is less predictable, but generally maintenance of the original lower arch form is advisable. The orthodontist may decide to deliberately alter the original lower arch form for improved aesthetics and so increase the likelihood of instability of the final result.
The orthodontist cannot control age changes—this includes soft tissues changes and further growth. At the present time, we are unable to predict the nature of these age changes, so there is the potential for unpredictable relapse throughout life. These late changes may well have little or nothing to do with the orthodontic treatment, but the patient will often attribute the unwanted relapse to it.
Contemporary Approach to Long-Term Relapse
Although some patients will be able to stop wearing retainers without suffering any relapse, it is not possible to identify these patients, and therefore every patient needs to be treated as if he or she has the potential for relapse (Little, Wallen & Riedel 1981). This means there is a need for informed consent before treatment begins, letting patients know about the unpredictable nature of relapse, and the need for some sort of life-long retention to reduce the risk of the relapse. Whether patients choose to wear retainers forever is up to them, but it is important that they are informed of the risk of unpredictable relapse if they choose to discontinue retention.
Retainers can be removable or fixed. Removable retainers make oral hygiene easier for patients, and they can be worn part-time if required. Clearly, their success is related to patient compliance. The use of removable retainers means the responsibility for retention is with the patient.
Fixed retainers are bonded in place, usually, but not exclusively, on the lingual surface. They have the advantage that the patient does not need to remember to put them in, and they are particularly useful when the final result is so unstable that anything less than full-time wear would be unacceptable. Examples of situations when bonded retainers are indicated include
1. teeth with periodontally compromised support,
2. malocclusions with initial spacing,
3. following correction of severely rotated teeth, and
4. where the lower labial segment has been significantly moved during treatment
The original removable retainer was the Hawley retainer (Figure 12.1). This was originally an active removable appliance that was adapted as a retaining appliance. There are nu/>