Literature Review

Retention management

decisions: A review of current evidence and emerging trends

Wellington J. Rody Jr, and Timothy T. Wheeler

& 2017 1073-87 http://d

Departm Florida, Ga

Address Department Center, D7-1 ufl.edu

Retention protocols remain controversial despite the fact that long-term stability is rarely observed in orthodontics. In this review article, we examine current evidence and discuss emerging trends to managing retention which may ultimately improve patient care. Our goal is to provide information that may help overcome the gap between scientific evidence and clinical practice. (Semin Orthod 2017; 23:221–228.) & 2017 Elsevier Inc. All rights reserved.

Introduction

A major concern driving orthodontic patientdissatisfaction during retention is rooted in the instability, which can be broadly defined as an excessive post-orthodontic displacement of tooth position accompanied by esthetics and/or functional concerns. It is well known in the orthodontic community that the periodontal and gingival fibers need additional time to stabilize around the teeth after active therapy; never- theless, the etiology of orthodontic relapse still remains unclear and controversial. Alignment instability varies widely within the population and may occur through various mechanisms, some of which are beyond the clinician’s control. Arch expansion, rebound of periodontal fibers, alveolar bone remodeling, skeletal growth pat- tern, occlusal settling, mesial drift of posterior teeth, parafunctional habits, quality of final occlusion, and lack of patient’s compliance are but a few of the potential factors that may play a role in post-orthodontic relapse.1,2 Indeed, clas- sic studies done by Little3–5 showed that long- term stability of the lower teeth alignment is rarely observed; thus, management decisions regarding retention and the patient’s care after active orthodontic therapy must not be

Elsevier Inc. All rights reserved. 46/17/1801-$30.00/0 x.doi.org/10.1053/j.sodo.2016.12.009

ent of Orthodontics, College of Dentistry, University of inesville, FL. correspondence to Wellington Rody Jr, DDS, MS, of Orthodontics, University of Florida, Health Science 9, Gainesville, FL 32610-0444. E-mail: wrody@dental.

Seminars in Orthodontics, Vol 2

overlooked by clinicians and should be consid- ered when writing the patient’s treatment plan.

The recommended length of time for wearing a retainer varies from orthodontist to ortho- dontist and recent systematic reviews state that there is insufficient evidence to make recom- mendations on retention procedures.6–8 In addition, there is no consensus in the literature regarding the retention protocol that will ensure more long-term stability and predictability.9 It is beyond the scope of this article to describe in detail all the retention devices available in orthodontics and readers are referred to Johnston and Littlewood10 or classic orthodontic textbooks for further descriptions about individual appliances. Instead, the goal of this article is to elaborate on retention protocols, examine current evidence, and discuss emerging trends to managing retention which ultimately may improve patient care. Some of the topics described below may seem obvious for many orthodontists; however, for a variety of reasons they are often ignored as we tend to practice based on our individual training and experience, along with all the inherited bias that comes with it.

Current evidence about orthodontic retention

There is an extensive orthodontic literature describing a variety of retention regimens and appliances; however, most of the studies lack a high level of scientific precision. Randomized controlled trials (RCTs) remain the gold stand- ard of research and while the number of RCTs in dentistry has increased dramatically in recent

2213, No 2, 2017: pp 221–228

Rody Jr and Wheeler222

years, very few RCTs have been done on ortho- dontic retention,11–14 which results in less reli- able research guiding clinical practice. This leaves the evidence open to criticism and pro- vides challenges to clinicians in delivering reli- able information to post-orthodontic patients. These challenges will become increasingly apparent as patients can now turn to the web to obtain health information instead of consulting with their own orthodontists. This raises con- cerns, given that Doğramacı and Rossi-Fedele15

recently noted that the quality of the content about retention on the internet is generally questionable and a large discrepancy exists for retention protocols between practices that offer web resources. Thus, this section of the paper provides a critical review of selected studies with moderate to low risk of bias. A small selection that highlights key points and important clinical questions in retention will be reviewed here. This approach will allow us to understand what questions have already been answered by current evidence and what conclusions are still influenced by anecdotal opinion.

(1)

Hawley-type versus clear retainers: Currently there is a trend toward the use of clear retainers in orthodontics,16–18 which are vacuum-formed appliances made of a thin sheet of thermoplastic material that adapts closely to the teeth including the occlusal surfaces. Although there is a plethora of reasons for this trend, the most significant reasons lie in the excellent esthetic charac- teristics, low cost and most importantly, easy fabrication.19 In the quest for evidence-based interventions important questions still remain though: Is there enough evidence to substantiate a transition from traditional Hawleys to clear retainers? Are clear retainers reliable over the long term? Is reliability of clear retainers dependent on the thermoplastic used? The controversy in question was initially tested in 1998 by Lindauer and Shroff20 using a non- randomized prospective approach and the authors could not detect any differences between Hawley and clear retainers. More recently, a systematic review by Mai et al.21

has reached the same conclusion and suggested that further high-quality RCTs are necessary to determine which retainer

is a better selection for orthodontists. Quite a few RCTs were published in this area and in spite of their limitations, have resulted in important progress in the field of orthodon- tic retention. By using a large sample size of 397 individuals in a single-center RCT done in Britain, Rowland et al.22 could not detect a significantly better clinical performance by clear retainers during the first 6 months after debond. The clinical variables analyzed in this RCT included tooth rotations, irregularity index, inter-molar and inter- canine widths. Subsequently, two other RCTs added to this evidence and indicated that clear retainers are as effective as Hawley retainers in maintaining post-orthodontic alignment and dental arch dimensions.23,24

Even though Hichens et al.19 specifically reported some advantages of clear retainers over Hawley retainers including cost- effectiveness, improved patient satisfaction and less breakage, the latter was not further supported by another RCT of 120 adolescent patients where Sun et al.25 reported equal brakeage rates between the two during a 1-year period. Reduced occlusal settling after debond has also been reported as a potential disadvantage of clear overlay retainers; how- ever, the basis for this statement is largely anecdotal and supported primarily from a case control study done by Sauget et al.26 in which 30 patients were followed up clinically for 3 months after debond. The authors concluded that the Hawley retainer allowed for a significant increase in the number of posterior occlusal contacts whereas the clear retainer did not allow the posterior teeth to settle down as efficiently. A similar result was reported by Tsai27 in a randomized trial at St. Louis University that evaluated short-term occlusal contact changes in 40 post- orthodontic patients who received either clear or Hawley retainers. After 3 months, a significant difference in the quantity of posterior contacts and near contact was found between groups; thus, he concluded that the Hawley retainer allowed for slightly better settling. In summary, the literature indicates that there is no strong evidence to support the use of one retainer over another; nevertheless, cost-effectiveness and patient acceptability seem to be the driving forces

Retention management decisions 223

behind the increased popularity of clear retainers. The ability of Hawley retainers to provide better occlusal settling warrants further investigation in more controlled RCTs that pay attention to a large sample size and duration of follow-up.

(2)

Part-time versus full-time wear: How long should removable retainers be worn full time? Perhaps this is the clinical question which most commonly occurs to the orthodontist from the patient at the end of active orthodontic therapy. The good news is that this question may have already been answered by two RCTs. The study by Gill et al.11 merits further discussion, as it was the first high level of evidence study to compare part-time and full-time removable retainer wear regimens. The results of their study showed that part-time wear of clear retainers is a successful retention regimen to prevent relapse of the final occlusion outcome. In addition to measuring the Little’s irregularity index, they also analyzed three other impor- tant aspects of the occlusion (arch width, overjet, and overbite) and none of the patients in the part-time group showed significant changes within 6 months after debond. These results are similar to those in Jäderberg et al’s study,13 who compared the alignment stability of dental arches receiving full-time clear retainers for 3 months with those wearing retainers full time only during the first week after debond. Their results showed that both regimens are equally effective to prevent unwanted relapse; thus, the authors concluded that the clear retainer is sufficient for maintaining the results after orthodontic treatment and that night-time wear is adequate. To the best of our knowl- edge, no similar RCTs exist to date for other types of removable retainers, leaving the question open as to the attainable effectiveness of part-time wear of Hawley retainers for the first six months after treatment is completed.

(3)

Figure 1. Hybrid retention in the lower arch with a clear overlay retainer on top of a fixed retainer.

Removable versus fixed retainers: It is said that a lot can happen in 7 days when it comes to orthodontic retention. Unfortunately, this is a fact and we know it is true. Perhaps that is why a recent survey clearly shows a preference towards the use of fixed retention in modern orthodontics.18 If a clinician wants to be on the safe side and rule out the danger of relapse,

fixed retention may be the obvious choice. Nevertheless, this approach may bring long- term harm if the appliance is left unsupervised. Unwanted side-effects due to distorted fixed retainers are common and range from minor rotations to bone fenestrations and excessive displacement of the anchoring teeth.28,29

Moreover, the ability of fixed retainers to compromise oral hygiene practices cannot be neglected. The literature is full of ambiguous and contradictory findings regarding the long- term periodontal outcomes of fixed retainers. Some evidence suggests that fixed retainers are clinically safe to the periodontium as long as they remain intact30–32; however, increased plaque accumulation, soft tissue changes and detrimental effects in the alveolar bone have been reported by other groups.33–36 Thus, the next question that comes to mind is: Are fixed retainers really necessary for long-term stabil- ity? Although the obvious answer may be yes, this is not in agreement with the results of a study carried out by Edman Tynelius et al.,37

who demonstrated that retention protocols without fixed retention were equally efficient in preventing post-orthodontic relapse for a period of five years. Certainly, this decision making process between fixed versus remov- able retainer may be challenging in different clinical scenarios, including the one where the patient is unable to keep regular follow-up retention appointments. At the University of Florida, hybrid retention is a common practice in these cases. By hybrid retention, we mean that a clear overlay retainer is fabricated on top of the bonded retainer and is worn nightly (Fig. 1). In our anecdotal opinion the hybrid approach has many advantages: (1) it reduces

Tab

Categ

Oste

Pro-i

Enzy degr

Chem

Rody Jr and Wheeler224

the likelihood of relapse if the composite gets detached from the wire or teeth, (2) it will prevent unwanted tooth movement and its deleterious effects on the periodontium if the fixed retainer gets distorted, and (3) the patient does not need to rush to his/her emergency appointment given that the clear overlay works as a backup retainer and thus will give the patient some extra-time to seek orthodontic care.

Figure 2. Gingival crevicular fluid collection with a paper strip (Periopaper, Oraflow Inc.).

Emerging trends to managing retention

The relationship between orthodontic proce- dures and periodontal health is considered challenging because some appliances can alter the oral microbiota and induce periodontal disease.38 To date, radiographs and cone beam computed tomography (CBCT) scans are the best imaging methods to monitor periodontal conditions; nevertheless, radiation exposure and expense preclude routine use. In addition, these methods do not indicate if the process of periodontal destruction is ongoing or historical. For more than a decade, dental researchers have been exploring alternative options to identify individuals at increased risk for periodontal disease, such as the emerging field of biomarker discovery in oral fluids. The analysis of specific constituents in saliva and/or gingival crevicular fluid (GCF) may provide quantitative biochemical indicators for evaluation of the level of inflammation and the status of bone turnover in the presence of periodontal disease39,40; thus, growth in this field of research tends to be steep. GCF has been found to contain large amounts of serum-derived proteins, inflammatory mediators, host-response modifiers and products of tissue breakdown. Because GCF is more site-specific than saliva,

le. Categories of biomarkers in gingival crevicular flu

ory Biomarker

oclastogenesis-related factors Receptor activator of n kappa-B ligand (RANK Osteoprotegerin (OPG

nflammatory cytokines Interleukin 1 beta (IL- Interleukin 8 (IL-8)

mes of extracellular matrix adation

Matrix metalloproteina

Matrix metalloproteina

oattractant for monocytes Monocyte chemoattrac (MCP-1)

research in the area has been heavily geared towards this body fluid and a lot of data has been published. GCF collection with paper strips is by far the most popular method and it is considered a non-invasive procedure since the filter paper is inserted only 1–2 mm into the gingival sulcus (Fig. 2). The natural sulcus in a healthy individual has a depth of 1–3 mm; thus, the paper strip would go as deep as a regular dental floss during oral hygiene procedures.

There are only few published studies that use GCF samples to compare the periodontal effects of different retainer types34,41; thus, pioneering this type of research can lead to clinical improvement and biochemical control of reten- tion regimens. More recently, our group made use of cutting-edge platforms to investigate expression of protein biomarkers related to periodontal disease in the GCF of patients wearing ortho- dontic retainers and interest patterns were observed.39 We have grouped the biomarkers in four categories based on their underlying biological role in periodontal disease (Table). In our initial investigation,34 we compared biomarker profiles in the GCF samples of

id (GCF) and their relevance in periodontal disease.

Biological relevance

uclear factor L)

Monitoring of bone remodeling and osteoclast activity

)

1β) Recruitment of cells to infection sites Promotion of bone resorption

se 9 (MMP-9) Destruction of periodontal tissues

se 3 (MMP-3)

tant protein 1 Recruitment of inflammatory and immune cells to periodontal sites

Figure 3. Microarray images showing a higher intensity of the fluorescent signal that comes from the line (arrow) that represents the biomarker matrix metalloproteinase 9 (MMP-9) in a patient wearing fixed retainer. Notice that each antibody droplet, together with the positive (pos) and negative (neg) control, is printed in quadruplicate. GCF samples were collected from the lingual surfaces of the lower incisors. (Reprinted from Rody et al.39 with permission from Dr. James McNamara Jr., editor of of the Craniofacial Growth Series, University of Michigan).

Retention management decisions 225

patients wearing fixed retainers bonded only to canines and Hawley-type removable retainers using microarray chips. In this technique, small droplets of antibodies are fixed in an orderly pattern onto a glass surface and are used as capture molecules to detect biomarkers in oral fluids. Our results showed a distinct pattern in patients with fixed retainers made of a large diameter round stainless steel wire bonded only to canines. In particular, the biomarker matrix metalloproteinase 9 (MMP-9) was detected in higher levels in patients with fixed retainers (Fig. 3), who also showed higher plaque levels in the incisor region. MMP-9 is a key enzyme for degradation of extracellular matrix that can be generated by different cell types in response to pro-inflammatory cytokines stimulation such as intereleukin 1beta (IL-1β). Higher GCF levels of this biomarker had been previously correlated with gingivitis, bleeding on probing, and attach- ment loss by other groups.42–46

The result of this initial study compelled us to further investigate the impact of different fixed retainer designs on the periodontal status of post- orthodontic patients. We have recently finished a second study47 that aimed to analyze GCF biomarker levels and clinical periodontal parameters in patients wearing retainers constructed of braided wires bonded to all six lower anterior teeth. This design of fixed retainer was initially introduced by Zachrisson48 and it is becoming popular worldwide as the braided wire can be easily delivered at debond appointment without the need of alginate impressions or extra visits to the office. Nevertheless, the detrimental effect on periodontal health of this type of retainer is clinically obvious in patients with poor oral

hygiene. Our study47 results confirmed those of the initial study, that higher loads of MMP-9 is found in GCF of patients wearing fixed retainers compared with the other groups. Thus, further research is necessary to establish whether GCF MMP-9 may be used as a prognostic marker of periodontal disease in the presence of fixed retention. In addition, we found positive correlations between plaque build- up and GCF levels of MMP-9 and IL-1β, as well as a negative correlation between interleukin-6 (IL-6) and probing depths. While no individual bio- marker candidate is already successful for early detection of periodontal disease, we hypothesize that a combination of GCF biomarkers could increase diagnosis or prognosis efficiency and might predict underlying periodontal changes in the presence of fixed retention. Future longi- tudinal studies are needed to determine the use- fulness of these markers as monitors for periodontal disease.

Another trend that has recently appeared in the orthodontic literature is that vibration may enhance alignment stability following ortho- dontic treatment. This hypothesis was put for- ward by Zhang et al.49,50 based on the premise that low-magnitude high-frequency mechanical vibration stimulates osteoblast proliferation and thus have an anabolic effect on bone. More recently, the work of Yadav et al.51 gives this hypothesis further ground by showing that vibration applied at 30 Hz for 15 min on a daily basis seem to increase tissue density and collagen integrity in the mouse molar periodontium. Moreover, the authors report a decrease in osteoclast number and osteoclast surface area in the vibration group, which is also a strong indication of the bone anabolic effect of

Rody Jr and Wheeler226

vibration in the periodontium of rodents. If this also proves to be true in humans, we may see a shift in the clinical application of vibration devices in our specialty. In the past decade, vibration therapy has been introduced in \orthodontics as a means of accelerating tooth movement and alveolar bone resorption. This is indeed quite surprising due to the fact that vibration therapy in the medical field has always been used in an opposite fashion as a means of improving bone formation in patients with osteoporosis. The uniqueness of the alveolar bone may help to explain the discrepancy between medical and dental clinical applications. In a comprehensive review, Sodek and McKee52

highlights that the alveolar bone undergoes what they call “asynchronous” remodeling in response to functional demands imposed by the dentition. By and large, research results have been mixed in relation to whether or not vibration therapy is capable of accelerating tooth movement and increase osteoclast activity. While the works of Nishimura et al.53 and Pavlin et al.54 reassert the usefulness of vibration devices in accelerating tooth movement, the literature is still growing in this field and a randomized trial published by Woodhouse et al.55 seems to contradict previous findings showing that vibrational force did not significantly increase the rate of initial tooth movement when fixed appliances were used. Our group at the University of Florida has recently finished a single-center randomized controlled crossover trial to test the efficiency of vibration therapy in accelerating tooth movement with clear aligners56,57 and neither the rate of tooth move- ment nor the expression of biomarkers in GCF was found to be influenced by vibratory forces. Our biomarker panel in this study encompassed molecules related to bone resorption and osteo- clast activity and none of them were differentially expressed in GCF due to vibration stress. As a future direction, research may consider focusing on GCF markers of osteoblast activity to explore the potential anabolic effect of vibration devices in the alveolar bone which may be promising as a clinical tool to reduce relapse according to this new trend.

Conclusion

The etiology of alignment instability following orthodontic treatment is still enigmatic and

largely influenced by individual factors and dif- ferent retention protocols. Given the high inci- dence of orthodontic relapse, clinicians should be familiar with relevant studies and develop- ments that can impact retention management decisions. In this article, we have described some of the key areas where gaps of knowledge exist and where further research is warranted. Insights into new areas of research that may impact retention protocols in the future were also discussed.

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