Twin-block appliance promotes condylar remodeling in patients with Class II malocclusion: a systematic review

Background: To evaluate the changes of condylar in with Class II with Embase, Cochrane library, Chinese Biomedical Literature Database, China National Knowledge Infrastructure and VIP Database were searched until October 15, 2019 without language restrictions. Randomized controlled trials (RCTs) and clinical controlled trials (CCTs) reporting condylar changes produced by Twin-block appliance in Class II malocclusion patients were included. Literature filtering, data extraction, and methodological quality assessment were finished by two reviewers independently. Meta-analysis was performed with Review Manager (version5. 3; Nordic Cochrane Centre, Cochrane Collaboration, Copenhagen, Denmark). For continuous data, the mean difference and their 95% confidence intervals were used for statistical pooling. Results: Three studies with a total of 100 patients were included (1 RCT and 2 CCTs). The meta-analysis indicated a significant increase of condylar dimensions after Twin-block treatment in Class II malocclusion patients, including 0.91mm in height, 1.03mm in width, and 1.08mm in length. There were statistically significant increments in the superior and posterior joint spaces and no significant change in the anterior joint space. Conclusions: Twin-block appliance promotes the remodeling of condylar and stimulates forward and downward movement of condylar. Further high-quality studies with large samples and long-term evidence are still required to obtain more reliable conclusions.

make the best of growth potential and generate pressure on the skeletal and dental structures as well as muscles and soft tissues to promote skeletal growth modification and tooth movement.
[3] The Twin-block appliance is one kind of removable Class II functional appliances, invented by professor Clark in 1982.
[4] Composed of two separate removable acrylic bite blocks and occlusal inclined planes trimmed to an angle of 70 degrees, Twin-block appliance is small and tolerable for patients to wear all day, promising a relatively satisfying treatment outcome.
To date, the changes of dentofacial structures and soft tissues produced by Twin-block appliance have been clarified by sufficient studies.[5-10] However, reliable scientific evidence about the effects on the temporomandibular joint (TMJ) of Twin-block appliance is insufficient, with a few articles reporting conflicting results. Some studies reported that Twin-block appliance enables forward movement of the condylar and promotes new bone deposition on the condylar. [5,[11][12][13][14] However, some studies found that the treatment effects of Twin-block appliance on TMJ, such as changes of joint spaces, were small. [15] Therefore, it is of great significance to critically evaluate the corresponding studies by systematic evidence-based review, to provide more accurate and reliable conclusions about treatment effects of Twin-block appliance on TMJ in Class II malocclusion patients, especially in terms of condylar position and dimensions.

Methods
The PRISMA statement was used as the basis for reporting the systematic review findings. unclear risk. And each study was assigned an overall risk of bias rating: low risk (low for all aspects), high risk (high for ≥1 aspect), or unclear risk (unclear for ≥1 aspect). Meanwhile, a quality score was calculated by a modified version of the method described by Jadad [17].
The Newcastle-Ottawa Scale was used for quality assessment of CCTs. For each study, 3 domains were examined: (1) the selection of the study groups; (2) the comparability of the groups; (3) the ascertainment of the outcome of interest. By this "star system", each study was evaluated as high quality (its score was ≥6) or low quality (its score was <6). The risk of bias and quality assessment was performed by two reviewers. Any disagreement was resolved after consulting another author

Data extraction
The data extraction form was developed by two reviewers independently, including the following items: the first author's name and year of publication; study type; sample size; age and gender of participants; duration of treatment; measurement method; mean observation period; and outcomes.
Any obscure or missing data were obtained by contacting authors.
To evaluate the changes of condylar dimensions, we collected data on its height, width and length after treatment of Twin-block appliance. Data on the anterior, superior and posterior space were

Description of included studies
A total of 254 articles were remained after removing duplicates from 263 initial identified citations.
After screening the titles and abstracts, full texts of 13 studies were retrieved for final consideration, among which 10 studies were eliminated for specific reasons. Finally, 3 studies [5,11,15] were included in the meta-analysis (Fig 1).

Quality of included studies
The RCT was of low quality with high risk of bias. And with the scores ranging from 6 to 8 respectively, the 2 CCTs were assessed as high quality according to the Newcastle-Ottawa scale. The detailed results of the quality assessment are shown in the Table 3 and Table4 (Table 3, Table 4).

Discussion
In this meta-analysis, only 3 studies were included (1 RCT and 2 CCTs). There are some limitations in this review, although normalized process of meta-analysis was conducted carefully. First, only 2 studies were included in the meta-analysis thus limiting the reliability of the results. Second, we failed to carry out a sensitivity analysis between RCT and CCT due to the lack of studies met inclusion criteria. Third, the quality assessments of included studies are not satisfying because of the selection bias and unsuccessful blinding of patients and orthodontists.
Fourth, the statistical power was also limited by the small simple size of each study. Moreover, longterm follow-up of patients is greatly required to obtain more reliable conclusions.
Therefore, the results of the present review should be interpreted and applied with caution and more high-quality studies are required to obtain more reliable conclusions.

Conclusions
This systematic review showed that Twin-block appliance promoted the remodeling of condylar with the increase of height, length as well as width. Forward and downward movement of condylar was observed after treatment of Twin-block appliance. More high-quality studies with large sample and long-term effect of Twin-block appliance on TMJ remains unknown due to the lack of long-term followup results.

Ethics approval and consent to participate
Ethical approval and consent to participate are not applicable because the results are based on previously published papers.

Consent for publication
Not applicable.

Availability of data and materials
The datasets used and/or analyzed during the current study are included in the study, and additional datasets are available from the corresponding author on reasonable request.

Competing interests
The authors declare that they have no competing interests.

Funding
Not applicable.

Authors' contributions
JCX and LX were involved in the design of the work. LX developed and conducted the search strategy.
JCX and LX were involved in the screening the retrieved articles for their eligibility and data analysis.  ("Angle Class II malocclusion" OR "Class II malocclusion" OR " mandibular retraction" OR "mandibular deficiency") AND ("Orthodontic Appliances, Functional" OR "Functional Appliance" OR "Twin-block appliance) AND ("Temporomandibular Joint" OR "TMJ" OR "Jaw Joint" OR "Condylar") Embase ("Angle Class II malocclusion" OR "Class II malocclusion" OR " mandibular retraction" OR "mandibular deficiency") AND ("Orthodontic Appliances, Functional" OR "Functional Appliance" OR "Twin-block appliance) AND ("Temporomandibular Joint" OR "TMJ" OR "Jaw Joint" OR "Condylar") Cochrane library ("Angle Class II malocclusion" OR "Class II malocclusion" OR " mandibular retraction" OR "mandibular deficiency") AND ("Orthodontic Appliances, Functional" OR "Functional Appliance" OR "Twin-block appliance) AND ("Temporomandibular Joint" OR "TMJ" OR "Jaw Joint" OR "Condylar") CBM ("Angle Class II malocclusion" OR "mandibular retraction" OR "mandibular deficiency") AND ("Functional Appliance" OR "Twin-block appliance) AND ("Temporomandibular Joint" OR "Condylar") CNKI ("Angle Class II malocclusion" OR "mandibular retraction" OR "mandibular deficiency") AND ("Functional Appliance" OR "Twin-block appliance) AND ("Temporomandibular Joint" OR "Condylar") VIP ("Angle Class II malocclusion" OR "mandibular retraction" OR "mandibular deficiency") AND ("Functional Appliance" OR "Twin-block appliance) AND ("Temporomandibular Joint" OR "Condylar")   Representativeness of the Twin-block group * * Selection of the control group * * Ascertainment of Twin-block group * * Demonstration that outcome of interest was not present at start of study * * Comparability of participants on the basis of the design or analysis * ** Assessment of outcome with independent blinding Adequacy of follow-up Lost to follow-up acceptable(<10% and reported) * * Total quality(score) High(6) High (7) This table shows the quality assessment of nonrandomized studies. Each item received 1 star(*), except for comparability, which can receive 2 stars. The total number of stars represents the score. Meta-analysis of the changes of joint spaces