Today, increasing number of orthodontic patients, especially adults demand for a more comfortable, less conspicuous orthodontic appliance.¹ Clear aligners are removable transparent appliances which first introduced by Kesling in the early 1940s. They have been making progress ever since and become more popular as computer software design and material properties evolve dramatically.²,³

Several advantages have been advocated for clear aligner treatment. The most prominent one is that they are invisible appliances which let us avoid placing conventional braces on teeth and has less negative impression on patients' social activity.³ Fixed orthodontic appliances can increase the accumulation of dental plaque and cause whitespot lesions, caries, and gingivitis during and after the orthodontic therapy;⁴,⁵ however, removable appliances such as clear aligners can be removed for eating, drinking, brushing, and flossing and thus improve patient's oral hygiene.⁶,⁷

On the other hand, there are some disadvantages including higher cost of treatment, challenges in treatment of some types of malocclusion, unpredictable appliance efficacy and effectiveness, and patients compliance.⁸,⁹

The duration of the treatment is of paramount importance in adult orthodontic patients. Various surgical and nonsurgical methods have been suggested for reducing the treatment duration. Surgical methods have some limitation such as cost, patients' discomfort, and dissatisfaction. On the other hand, some have claimed that various nonsurgical methods (including direct electric current, vibrations, and low level laser therapy) can accelerate tooth movement.¹⁰,¹¹ Among these conservative methods, vibration has gained a lot of attention since it can be administered by the patient at home.¹² Furthermore, vibration devices claimed to have relieving effect on general and biting pain, therefore reducing discomfort in patients undergoing orthodontic treatment.¹³,¹⁴

Vibration would seem to be an appealing adjunct to clear aligner therapy, because of its potential to accelerate tooth movement. However, evidence on vibration effectiveness was equivocal; some studies concluded that vibration can improve tooth movement rate while inducing higher level of inflammatory cytokines which are the indicators of orthodontic tooth movement.¹⁵,¹⁶ In contrast, some others revealed no significant alteration of tooth movement rate.¹⁷ The purpose of this literature review is to investigate the effect of varying vibration protocols on clinical outcomes in patients treated with clear aligner.

Protocol and registration

The current systematic review followed the PRISMA guidelines¹⁸ and the Cochrane Handbook for the Systematic Review of Interventions (version 5.1.0)¹⁹ and was registered in the PROSPERO database (registration number: CRD42020223245).

Eligibility criteria

According to the PICOS format, selection of papers was performed based on the following criteria:

Population: Patients which treated with clear aligner

Primary outcomes: Rate of tooth movement, clear aligner exchange intervals, and treatment duration

Secondary outcomes: Other clinical effects such as pain experience, root length and…

Exclusion criteria

Animal study

Studies involving orthognathic surgery

Review articles, case reports, case series, and experimental animal studies

Studies in a non-English language.

Information sources and search strategy

A comprehensive electronic search with no limitation regarding the language or the publication year was carried out in PubMed, Scopus, Cochrane library, and Embase databases, until March 2021. Search strategy for PubMed was developed as follows: (Vibratory Orthodontic Device) OR (vibration)) OR (accelerated)) OR (acceleration)) OR (AcceleDent)) OR (low frequency vibration)) OR (high frequency vibration)) OR (VPro)) OR (Propel orthodontics)) AND (((((clear aligner)) OR (aligner appliance)) OR (invisalign)) OR (Clear Aligner Appliances) OR [Propel orthodontics]). The same strategy was applied for other databases (the search strategy for each database is mentioned in Table 1). We also performed manual searching of the references list of all identified trials for further studies.{Table 1}

Deduplication and management of all citations were done using Endnote X7.

Study selection and data extraction

Two investigators independently performed the screening process of titles and abstracts considering the inclusion and exclusion criteria. Disagreement between reviewers resolved by consensus or by the decision of a third independent reviewer. Full text of relevant papers was reviewed thoroughly. The details about the study design, publication year, type of appliance used, the items reviewed, the diagnostic method, the number of participants, the different groups, and outcomes are provided in Table 2.{Table 2}

Assessment of risk of bias

Two review authors independently assessed the risk of bias of the included studies. Cochrane's risk of bias tool was used for the assessment of randomized controlled trials (RCTs).¹⁹ The study was considered as low risk of bias if all the domains were low. If only one domain assessed as high risk, the study was considered as high risk. A moderate risk of bias was considered when at least one domain was judged as unclear.

The Newcastle-Ottawa Quality Assessment Scale was utilized for retrospective studies,²⁰ considering the number of stars in each category.

Study selection and characteristics

An overall of 83 papers were found out through the electronic search. One article was found in manual search. After duplicate papers were removed, 53 papers were left. Forty-one studies excluded based on title or abstract. Full-text was reviewed for studies that could not be excluded definitively based on the titles and abstracts. Figure 1 illustrates the flow diagram of electronic search procedure, inclusion, and exclusion of studies.Figure 1

Flow diagram of the review.

Figure 1

Seven studies were ultimately picked out for the systematic review. Four studies were randomized clinical studies,¹²,²¹,²²,²³ and three were retrospective studies.²⁴,²⁵,²⁶

Six studies evaluated the tooth movement rate in different manner (accuracy of tooth movement, exchange interval of aligner, rate of alignment.,),¹²,²¹,²²,²³,²⁴,²⁶ three the pain and discomfort,²¹,²²,²³ one the cytokine level,²¹ one bone density,²⁴ one oral health-related quality of life and compliance,²² and one tooth length.²⁵

Assessment of risk of bias

The quality of included studies in this systematic review was comprehensively appraised to determine the potential risk of bias. Among four RCT, one was assessed to be low risk, and three were considered to be high risk Table 3. All three retrospective studies have scored as low risk of bias Table 4.{Table 3}{Table 4}

Vibration is low-level mechanical oscillatory signals which enhance bone metabolism, increase the remodeling rate, prevent osteoporosis, improve bone density, and diminish bone loss in postmenopausal women.²⁷,²⁸,²⁹ Stimulation of cell differentiation and maturation, and therefore, bone remodeling required for tooth movement, is the basis of application of vibration in orthodontics.³⁰

Mechanical vibration implication in orthodontics is controversial. The recent systematic review concluded that evidences regarding the positive effects of mechanical vibration on different aspects of treatment (e.g., alignment of the anterior teeth, pain relief, space closure, etc.) are insufficient.³¹ However, many clinicians recommend the use of vibration as an adjunct to clear aligner treatment and claimed that the duration of treatment can be decrease 50% or more. Although the underlying mechanism is not clear, the tight contact of the aligner with the entire tooth surface and more efficient transmission of vibration to the root and surrounding bone is a rationale hypothesis.³²

Different type of vibrations used in orthodontics such as LFV and HFV.³³,³⁴ Alikhani demonstrated in an animal study that 30 Hz frequency application causes 1.45-fold increase in the rate of tooth movement. Increasing the frequency to 60 Hz and 120 Hz caused a 2.1-and 2.4-fold increase in the rate of tooth movement, respectively.³⁵

Among included studies in this systematic review, three used LFV¹²,²²,²³ and four used HFV.²¹,²⁴,²⁵,²⁶

A major determinative factor for clear aligner treatment duration is the frequency of aligner exchange. This interval is usually 2 weeks. Decreased interval is prone to failure because planned tooth movement will not succeed, which is known as “nontracking.”³⁶

Six studies included in this review, evaluate the effects of vibration on orthodontics tooth movement in different terms such as: exchange interval, accuracy of movement, rate of tooth movement, and incisor irregularity index. Three of them, used LFV,¹²,²²,²³ and three HFV.²¹,²⁴,²⁶ Shipley et al.²⁴,²⁶ and Alansari et al.²¹ showed that HFV reduce intervals between aligners, and the planned movement was more closely tracked by teeth. Therefore, they have claimed HFV as an adjunctive to clear aligner treatment can reduced length of treatment. Furthermore, Shipley et al.²⁶ showed that in HFV group, number of aligners reduced, because no refinements were required, though in control group, six patients (75%) required 1 or more refinements. In contrast, Bragassa²³ concluded LFV has no effect on tooth movement efficiency and accuracy and also shorter aligner exchange interval (4 days vs. 14 days) did not reduce total treatment time due to the increased requirement for midcourse correction and refinement. Lombardo et al.¹² conducted a RCT and found no difference between the accuracy of tooth movement at the 14-day replacement interval and the 7-day interval using LFV. Although this accuracy was not seen in group with 7 days' interval and no application of LFV, they concluded that LFV seems to improve the accuracy of conventional protocol. Another study done by Katchooi et al.²² placed all the participants on the 7 days' exchange interval and finally found no differences in completion rate of the initial set of aligner and irregularity index of incisors between the LFV group and control group.

In one of the studies, the level of inflammatory and bone remodeling markers in the gingival crevicular fluid were evaluated before the start of the aligner treatment and at the end of the second aligner. Statistically, significant higher levels of markers (interleukin [IL]-10, IL-8, IL-13, IL-1ra, and IL-4, granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, SCD40 L and receptor activator of nuclear factor Kappa-B ligand, epidermal growth factor, platelet-derived growth factor AA/BB, and platelet-derived growth factor AA) measured in HFV groups.²¹

As Alikhani showed,³⁵ the effect on tooth movement rate is more pronounced with higher frequency vibration. Judex and Pongkitwitoon³⁷ compared the effect of different vibratory devices (HFV and LFV). They showed that both devices increase cell proliferation and gene expression in osteoblasts and fibroblasts, but greater response was seen with HFV compared to LFV. Type I collagen (COLA1), alkaline phosphatase (ALPL), Runt-related transcription factor 2 (RUNX2), fibroblast growth factor 2 (FGF2), and connective tissue growth factor (CTGF) were measured as the indicators of osteoblast activity, osteoblast differentiation, osteoblast differentiation level, human periodontal ligament fibroblasts activity, respectively. There was an upregulation of COLA1, ALPL, FGF2, and CTGF levels by both devices, but their levels were greater in HFV. RUNX2 upregulated with HFV but not with LFV.

Two studies did not show the positive effect of vibration in composition with clear aligner treatment,²²,²³ used LFV (30 Hz). All studies using HFV (120 Hz) found out significant effect of vibration in the rate of tooth movement and more accurate tracking of planned movement.²¹,²⁴,²⁶ It seems that employing HFV combined with clear aligner treatment is more effective than LFV.

Three included studies, evaluated the patients' pain and discomfort. Two of them used LFV and one used HFV. Alansari et al.²¹ used the (Numeric Rating Scale-10) for patient's pain assessment. They concluded that HFA significantly reduces orthodontic force-related pain. On the other hand, two studies used LFV in conjunction with clear aligner showed no pain relief in patients.²²,²³ Previously, two RCTs have shown that LFV was not effective in reducing pain in fixed orthodontic treatment.³⁸,³⁹ It seems that HFV can be effective in reducing the pain of patients treated with clear aligner and is a practical option for patients due to its short duration of use (5 min/day).

Farouk et al.²⁵ measured the maxillary incisor's teeth lengths, before and after clear aligner treatment, on the cone-beam computed tomography, and compared the results, in two groups, control and HFV. They investigated no significant change of tooth lengths in HFV group, while this was not the case in the control group. Previous studies about the role of vibration on root resorption are controversial. A study demonstrated that HFV in the presence of orthodontic force was catabolic and decreased the bone density. Therefore, roots moved through a nondense bone and this may prevent root resorption.³⁵ On the other hand, Shipley et al. compared the bone density of patients who treated with clear aligner and HFV with control group. They found that bone density in the HFV group was higher than control at the end of treatment and initiation of retention phase.²⁴ This higher density is important for stability and avoiding relapse.⁴⁰ Another study found that 113 Hz vibration for four weeks seems to prevent or reduce orthodontic root resorption.⁴¹ Further studies are needed to determine with certainty the effect of vibration with clear aligner on root resorption and bone density.

According to the studies discussed in this review (RCTs with high risk of bias and retrospective studies), applying vibration could produce the same tooth movement in a shorter period of time and more accurate tracking of the planned movements in clear aligner treatment. It also could reduce the interval of clear aligner exchange. However, further properly-designed RCTs are necessary to prove this claim.

The effect of vibration on patients' pain, root resorption, and bone density is ambiguous. Some data have proposed that HFV can reduce discomfort, root resorption, and increase the bone density, but more studies required to prove it.

In addition, it seems that combining clear aligner with HFV is more effective than LFV. HFV has shown pleasant results, whereas this is not the case for LFV.

Financial support and sponsorship

Nil.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.

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