2 × 4 appliance in the mixed dentition stage: a scoping review of the evidence

2.1 Design

The present scoping review was designed and developed following the methodology stated by Arksey & O’Malley [18], Levac and co-workers [19], and the Preferred Reporting Item for Systematic Reviews and Meta-analysis extension for scoping reviews guidelines (PRISMA-ScR) [20]. A ‘scoping’ study encompasses a further type of literature review, whose main purpose is to map relevant literature and the key concepts underpinning a clinical topic of interest. Furthermore, the main sources and types of evidence available, especially when this topic has not been reviewed comprehensively before. In general, a scoping review consists of five steps: (i) constructing the research question, (ii) identifying relevant studies, (iii) study selection, (iv) charting the data, and (v) collating, summarizing, and reporting the results. Nowadays, scoping reviews represent an increasingly popular approach to reviewing dental research evidence. Scoping studies differ from systematic reviews because authors do not assess the quality of included studies; scoping studies also differ from narrative or literature reviews in that the scoping process requires analytical reinterpretation of the literature [19].

The present scoping review was conducted following the Preferred Reporting Items for Scoping Reviews 2020 methodology guidelines (https://prisma-statement.org/Extensions/ScopingReviews). The study protocol was approved a priori by all authors and later registered in the Open Science Framework (OSF) database, under the Registration DOI 10.17605/OSF.IO/BWK8N.

2.2 Question research

A scoping review intended to answer the following PICO (population, intervention, comparison, outcome) question research: In pediatric patients at mixed dentition stage (P), what are the early clinical benefits of using the 2 × 4 appliance (I), in comparison with other orthodontic appliances (C), in the correction of diverse common malocclusions at this age (O)?

2.3 Eligibility criteria

References from publications focused on the 2 × 4 appliance, in which the authors explored and described the different clinical and technical procedures for the resolution of different malocclusions in the mixed dentition stage were screened. Inclusion criteria: only available (1) randomized controlled clinical trials, (2) observational studies (e.g., cohort, case-control, cross-sectional designs), (3) pilot studies, and (4) clinical case reports/series were eligible. These articles should have been written in English languages only and published between 2000 and 2022. The reasons why the authors limited the inclusion of only English studies were the restricted language skills for other languages translation (except for Spanish articles). Exclusion criteria:studies performed in permanent dentition (adolescents or adults; e.g., treatment of ectopic or impacted upper permanent canines), narrative reviews, and letters to the editor were excluded. Reasons for exclusion after full-text reading were recorded.

2.4 Study screening selection

A structured electronic and manual search was conducted on four electronic databases: PubMed, Cochrane Library, Google Scholar, and Dentistry & Oral Sciences Source (EBSCO); two grey literature databases were also consulted (Greynet and Grey Literature Report). The following MeSH or free-text terms, keywords, and Boolean operators were used, alone or in combination: “early orthodontic treatment”, “interceptive orthodontics”, “mixed dentition”, “fixed orthodontic appliances”, and “2 × 4 appliance”. This search strategy was appropriately adapted for each database. In PubMed, the next search algorithm was employed:

((“early” (All Fields) AND (“orthodontal” (All Fields) OR “orthodontic” (All Fields) OR “orthodontical” (All Fields) OR “orthodontically” (All Fields) OR “orthodontics” (MeSH Terms) OR “orthodontics” (All Fields)) AND (“therapeutics” (MeSH Terms) OR “therapeutics” (All Fields) OR “treatments” (All Fields) OR “therapy” (MeSH Subheading) OR “therapy” (All Fields) OR “treatment” (All Fields) OR “treatment s” (All Fields))) OR (“orthodontics, interceptive” (MeSH Terms) OR (“orthodontics” (All Fields) AND “interceptive” (All Fields)) OR “interceptive orthodontics” (All Fields) OR (“interceptive” (All Fields) AND “orthodontics” (All Fields)))) AND ((“2 × 4” (All Fields) AND (“appliance” (All Fields) OR “appliances” (All Fields) OR “instrumentation” (MeSH Subheading) OR “instrumentation” (All Fields) OR “appliances” (All Fields))) OR (“two” (All Fields) AND “four” (All Fields) AND (“appliance” (All Fields) OR “appliances” (All Fields) OR “instrumentation” (MeSH Subheading) OR “instrumentation” (All Fields) OR “appliances” (All Fields))))

Titles and abstracts derived from the electronic and hand searches were carefully reviewed and screened by two independent, trained, and pre-calibrated authors (GHG and EYG), according to the inclusion and exclusion criteria, for selecting the most relevant records. Any title/abstract classified as potentially eligible or relevant by either reviewer was retrieved in full text and independently evaluated. The reference lists of the reviewed articles were also hand-searched for additional studies that may have been missed in the initial search. In cases in which significant data were missing from the article, the reviewers made an effort to contact the authors to obtain this extra information. The levels of inter-and intra-reviewer of these authors were determined using Cohen’s kappa coefficient [21]. Any discrepancy or conflict of opinion was resolved through discussion and consensus by consulting the other two authors (SRR and MRB).

2.5 Data charting and result synthesis

A predesigned/standardized tabular form for data charting was constructed and piloted with relevant information entries, and approved by all authors’ consensus. From each selected article, the following items were summarized and recorded: publication first author and year, country, study methodological design, clinical interventions and related outcomes, and main findings. In cases of observational (cross-sectional or longitudinal) studies, target population, sample size, and follow-up period (e.g., cohort designs) were also taken. All items were collected by two experienced reviewers (SRR and MRB) independently; again, any disagreement or discrepancy was resolved by discussion and consensus with the other authors.

2.6 Bias risk

The methodological quality of included randomized controlled clinical trials was evaluated through the Cochrane tool, which is based on random sequence generation, allocation concealment, blinding of participants/evaluators, blinding of outcome assessment, incomplete outcome data, selective reporting, and other sources of bias [22]. The bias risk of observational studies was assessed through the Newcastle-Ottawa tool based on selection, comparability, and exposure [23]. For clinical case reports/case series, the tool developed by Murad et al. [24] was employed. Only those studies with moderate to high quality were included in the final list of the present scoping review.

3.1 Study selection

A total of 161 references were identified in the different electronic databases, after which 115 titles remained after removing duplicates. After the article title and abstract screening, 18 potential full-text articles were carefully reviewed. Finally, 16 relevant and most informative studies, according to the performed critical appraisal, were included in the present scoping review. Even though articles written in a language other than English were not eligible, one relevant article written in Spanish was retrieved and finally included. The whole selection process was according to the PRISMA-ScR statement flowchart, as can be seen in Fig. 1. On the other hand, good to very good intra- and inter-reviewer agreement levels were found for the search strategy and screening processes, according to the kappa coefficients (0.87 and 0.91, respectively).

Regarding the study methodology, fourteen studies were case reports/case series with a brief narrative review and clinical treatment descriptions, and two were randomized controlled clinical trials; no descriptive or comparative cross-sectional designs, case-control studies, or cohorts were detected. Regarding the authors’ country, most articles were conducted in India and Europe (mainly in the UK). Two studies presented any type of descriptive or inferential statistical analysis (with p values). The publication dates ranged from 2000 to 2020. The main characteristics, numerical data, findings, and conclusions from these articles were summarized in a Supplementary Table 1 [1, 4, 9, 10, 12, 13, 14, 15, 16, 17, 25, 26, 27, 28, 29, 30].

Fig. 1.Flow chart for literature search.

3.2 Critical review

Qualitative analysis of case reports/case series and risk of bias assessment of clinical trials are depicted in Tables 1 [1, 4, 9, 10, 12, 13, 14, 15, 16, 17, 26, 28, 29, 30] and 2 [25, 27], respectively.

4.1 Anterior and posterior crossbites

Anterior crossbite is usually a major functional and esthetic issue during the early stages of oral development in children [9]. There is an old orthodontic maxim: “the best time to treat a crossbite is the first time it is seen” [30, 34]. Early management of anterior crossbites has been strongly recommended for avoiding the possible progress of the condition to a true class III malocclusion in the future [27, 28]. The 2 × 4 appliance is frequently employed for the correction of crossbites in the mixed dentition because it allows the protrusion of upper incisors and the stimulation of bone development in the root apex area [28]. It is mandatory the previous elimination of those premature contacts between opposite teeth, in cases of functional crossbites [9, 15].

The appliance has a great power of action and can provide lighter continuous forces for resolving anterior crossbites when compared to conventional removable devices [11, 30]; therefore, it can achieve excellent results in a short time of therapy [1, 26, 27]. Gu and colleagues [25] reported an average time of treatment of 8.5 months in 17 consecutive patients (mean age, 9.7 years) with pseudo-class III malocclusion and anterior crossbite using a 2 × 4 appliance together with reverse headgear, and no relapse was detected after 1 year of follow-up; no retainers were placed.

In cases of posterior crossbites, when maxillary expansion is indicated, it can be carried out by adding a quad-helix to the 2 × 4 appliance [6, 13]. Further, the use of interarch elastics attached to lingual and buccal buttons has been suggested, as a complementary procedure to restore the functional contacts between malpositioned teeth [26].

4.2 Ectopic/impacted upper permanent central incisors

Another indication of the 2 × 4 appliance is at an early stage is the correction of ectopic or impacted permanent incisors. Once the etiological factor has been identified, it may be that the necessary space to be created to allow the adequate reposition of the affected tooth and bring it into its correct site in the dental arch [13]. 2 × 4 appliances can easily perform a careful space opening and also control the traction force magnitude and vector, with much more precision than with removable appliances [6]. In this respect, Das and co-workers [17] have established the determining factors to take on account for the successful alignment of an impacted incisor with the appliance: (i) the position and direction of the impacted tooth; (ii) the degree of root dilacerations; (iii) the degree of root development; and (iv) the presence of sufficient space for the impacted incisor.

4.3 Midline diastemas, alignment of rotated/spaced upper incisors, and maxillary anterior crowding

Once eliminated the local etiological factor for the diastema (e.g., mesiodens or abnormally implanted labial frenum), a sectional 2 × 4 appliance can be placed for achieving a better-controlled space closure and the alignment and leveling of the upper incisors. Sometimes, double-helical wire loops, inter-brackets elastomeric chains/elastics, and micromagnetic devices are added to facilitate the space closure [35]. The orthodontic midline diastema closure in mixed dentition has been classified into four categories [29]: (i) treatment involving mesial tipping of incisors, for the approximation of spaced tooth crowns only; (ii) treatment involving mesial bodily movements, for the approximation of both central incisors; (iii) treatment involving a decrease of enlarged overbite and intrusion of the upper incisors; and (iv) closure of the space as part of a more comprehensive orthodontic treatment.

4.4 Clinical recommendations and precautions

The orthodontic wires employed in the 2 × 4 technique must be thin in diameter to appropriately provide light, continuous and well-controlled forces over the incisors. These light orthodontic forces are applied for incisor derotations, alignment, and leveling, and are considered clinically safe and effective; according to the identified literature, initial archwires can be “0.012” to “0.016” NiTi (Nickel-Titanium alloy or Nitinol) (Titanium-Molybdenum-Titanium Alloy (TMA) wires can also be used); in a second phase, “0.016” stainless steel wires are employed; and finally, rectangular wires (“0.017” or “0.018” × “0.025”) are placed. These wire arches are changed every two to four weeks. Dowsing et al. [13] and Naidu et al. [11] suggest in some cases the placement of a long supporting well-shaped stainless-steel tube on the archwire, filling the entire space between the first molar and the incisors. This tube strengthens the long unsupported span of the wire from distortions, due to occlusal forces. If additional space is required for incisor proclination, a compressed nickel-titanium open coil spring can be inserted. On the other hand, these same authors also recommend annealing the extra wire distal to the molar soldered tubes to allow the wire segment to be turned down against the tube; this procedure prevents the archwire from sliding forward, thus avoiding a possible arch length increase, and thus, the subsequent possible damage to cheek soft tissues, and potential undesirable movements of the first permanent molar.

4.5 Limitations

The present scoping review carefully followed the recommendations stated by Arksey and O’Malley for screening specific papers and extracting relevant data from them [18]. However, and after an exhaustive searching process, only relevant clinical case reports/series—which are highly prone to bias- and two clinical controlled trials were found and included in the review. This concern represented a considerable methodological heterogeneity, making it impossible to conduct a systematic review/meta-analysis; given this, we intended to improve the quality of the study by performing a critical evaluation of the risk bias of the selected articles, using two well-validated and widely recognized specific scales. In addition, we also limited our searching to published articles written only in English (and one additional in Spanish); so, other language-relevant studies could be missed. Despite these limitations, and the fact that no cross-sectional and longitudinal observational were found, we are confident that sufficient reliable and useful information could be collected and synthesized, to aid clinicians in better understanding the clinical performance of the 2 × 4 appliance in mixed dentition children.