19: Retention and stability following aligner therapy

Josef Kučera, Ivo Marek

Retention and stability in orthodontic treatment

Introduction

Orthodontic treatment is an area of medicine and dentistry that has to address not just health and function but also aesthetics. It is usually the aesthetic considerations that make patients seek out orthodontic treatment in the first place. Achieving an excellent aesthetic and functional result can be lengthy and expensive, therefore it is in the interests of both the patient and the clinician that the result of orthodontic treatment remains stable in the long term. Unfortunately the importance of the retention phase is often underestimated, cbd school when in reality it is as important to the patients as the active orthodontic treatment itself.

The period after the completion of active treatment can be divided into a retention period and a postretention period. The purpose of the retention phase following active orthodontic treatment is to prevent relapse defined as the natural tendency of the teeth to migrate into their original position in the dental arch and to eliminate the influence of other factors that might destabilize the result. It is very difficult to say how long the retention phase should last. The literature offers many recommendations, although they vary considerably and are often vague. Some authors suggest that, following orthodontic treatment, teeth should be held in the position achieved by treatment for as long as it is necessary to sustain the result,¹ or that the retention phase should be as long as needed and as short as possible.² Others suggest that retainers should be used until the patient’s growth is complete or the third molars erupt,³ or for a period of 10 years⁴ or even 20 years,⁵ or simply as long as the patient wishes to keep the teeth aligned.⁶

It is generally recommended that nongrowing patients wear retainers for at least 1 year and is biologically defined as the completion of the reorganization of bone and periodontal ligaments around the teeth.⁷ Collagen fibers are reorganized within the first 3 to 4 months.⁶ This period is critical, and the wearing of retention appliances is essential because relapse is very likely at this stage; after this critical period the risk decreases substantially.⁸ However, the reorganization of elastic supracrestal fibers may take more than 1 year, which makes the retention of severely rotated teeth particularly difficult; some authors recommend adjunctive surgical procedures such as fiberotomy to decrease the amount of relapse.⁹^,¹⁰ In growing patients retainers should be worn until the growth is complete.⁶ At the time patients stop wearing the retention appliances, the postretention period begins, and it is only then that we get a true picture of the stability of the original result that had been achieved by the orthodontic treatment. During the postretention period numerous factors and the complexity of their interactions may ultimately destabilize treatment results.

Factors influencing long-term stability

With regard to stability there are some general guidelines and recommendations for orthodontic treatment, and so long as these are respected when making and carrying out the treatment plan they tend to produce stable results with relatively little risk of relapse. In such cases, long-term changes in dental arches of treated patients are then similar to those occurring in untreated subjects.

Before starting treatment, orthodontists need to keep in mind that the position of the teeth and the shape of the dental arches are the balanced result of many factors, especially the influence of the forces exerted by the surrounding soft tissues (i.e., pressure from the cheeks, lips, and tongue) that create a “neutral zone” or “zone of stability.” Orthodontic movement of the teeth outside of this neutral zone pushes them into an unbalanced zone, with consequent relapse.⁶^,¹¹ The shape of the dental arch, particularly the mandibular arch, should therefore be respected in the planning and implementation of treatment because changes in arch shape tend to relapse into the original shape in the long term.¹²^,¹³ The upper dental arch may be expanded more than the lower arch in indicated cases (rapid maxillary expansion); however, even in these cases, the long-term stability appears to be quite problematic.¹⁴ Any changes in the lower intercanine distance are also very prone to relapse,¹⁵^,¹⁶ partly because decrease in the lower intercanine distance is due to the natural changes that occur in the dental arch as a result of aging.¹⁷^,¹⁸ The quality of articulation and intercuspation can also be very important for the long-term stability.¹⁶^,¹⁹^,²⁰ The correct intercuspation of the teeth in lateral segments with high cusps itself provides the best retention, both in sagittal and transverse dimensions.² It is also important to achieve the correction in the vertical direction, and especially for sufficient correction of the deep bite, as its deepening reduces the space for the lower incisors.²⁰ Incisor shape can also be a source of posttreatment instability. In triangular-shaped incisors, recontouring of the approximal surfaces (i.e., interproximal enamel reduction, stripping) provides more stable contact between the incisors. According to some studies, this stabilizing effect of lower incisor stripping is comparable to the efficiency of bonded retainers.²¹^,²² Similarly, the adjustment of large proximal enamel ridges on the palatal surfaces of the upper incisors is also important for the stability of the incisor region.²³

Continuing growth is a separate issue and needs to be addressed with particular attention in more pronounced skeletal malocclusions, especially in the sagittal and vertical dimensions, which continue to grow over a longer period than in the transverse dimension. Unfavorable growth of the jaws has a negative impact on the occlusal relationship and on the position of incisors due to the dentoalveolar compensation process.²⁴ This is one of the reasons why it is recommended to plan comprehensive treatment of severe skeletal malocclusions after the patient’s growth is complete. However, even after growth completion, the dental arches are also subject to changes related to the patient’s aging, and these processes are in fact lifelong and may result in the development of irregularities in the incisor segment¹⁷^,¹⁸^,²⁵^,²⁶ that often bring patients back for retreatment.

Retention protocols and the choice of retention appliance

Retention protocols

To date, there is no universal retention protocol, and there is insufficient high-quality scientific literature to reliably establish such a protocol in terms of the length of the retention phase, the wearing regime, and the choice of type of retention device.²⁷^,²⁸ This is because we cannot generalize a single procedure for patients who differ in diagnosis, severity of the malocclusion, age, type of growth, treatment type, and quality of treatment result. Thus the choice of retention device should always be individualized, with consideration of all the potential factors of instability mentioned earlier. This approach is called “differential retention,” meaning that for every patient, orthodontists must focus and aim the retention on those points that pose the greatest threat and risk of relapse in the individual patient (Fig. 19.1).⁴

Fig. 19.1Examples of relapse after orthodontic treatment, where either the patient failed to wear the retention appliances after rapid maxillary expansion (A-C) or the retention regime selected was insufficient for a noncompliant patient; the rotational relapse of lateral incisors (D-F) and palatal movement of upper left canine (G-I) shown could have been prevented by bonding a fixed retainer and including problematic teeth.

According to surveys on retention protocols, the most common retention devices are the Hawley retainers and clear thermoplastic retainers. For the mandible, a fixed retainer is often indicated, either on its own or in combination with a removable appliance.²⁹^,³⁰ An increasing trend has been observed in the use of thermoplastic retainers, which patients prefer because of their good aesthetics and inconspicuousness.³¹ A similar trend can also be observed with fixed retainers in both jaws. In terms of the frequency of use of the various retention devices, an indefinite use of fixed retainers is recommended by many clinicians.²⁹^,³²^,³³ If the decision is made to use a retention appliance long term, a fixed retainer seems to be the best option mainly because it prevents relapse of the aesthetically important anterior teeth very efficiently and without any need for patient cooperation.³⁴^,³⁵ Bonded retainers have also been described in the literature as safe, predictable, and posing no health risks to the patient.⁴^,⁵^,³⁶ Some studies, however, have indicated that there is a tendency toward increased buildup of plaque and calculus around bonded retainers (Fig. 19.2), having negative consequences on the periodontium³⁷; however, this can be minimized with regular care, exercised by the patient and a dental hygienist.

Fig. 19.2Calculus accumulation and gingival inflammation around the lower bonded retainer (A and B).

The biggest disadvantage of bonded retainers that impacts their long-term or lifelong use is failure rate. According to the literature, the failure rate varies widely, from 0.1% to 53%.³⁸^,³⁹ However, we believe that the occurrence of common failures, such as abrasion of the layer of adhesive resin caused by food attrition or occlusal contacts, is only a matter of time (Fig. 19.3). Other considerable risks associated with prolonged use of bonded retainers are the so-called unexpected complications, where unexpected tooth movement occurs, even when the integrity of the bonded retainer has not been compromised in any way. The incidence of these complications is quite small, occurring in approximately 1% to 5% of cases,⁴⁰^,⁴¹ but their clinical consequences can be very severe. In addition, it is estimated that up to 50% of such cases require retreatment.⁴¹ There are two distinct types (Fig. 19.4), characterized by a torque difference between two adjacent incisors (X effect) or opposite inclination of contralateral canines (Twist effect).⁴⁰^,⁴² These complications are surprising because they may appear after a relatively long period of problem-free retention, often occurring after several years.⁴⁰^,⁴¹^,⁴³ The unwanted tooth movement can be so pronounced that the root is moved outside of the alveolar bone (Fig. 19.5), which is in many cases accompanied by the occurrence of gingival recession. In such severe cases, orthodontic retreatment is necessary, and often a surgical periodontal intervention may also be needed (Fig. 19.6).⁴³^,⁴⁴