A Systematic Review of Skeletal , Dental and Soft Tissue Treatment Effects of Twin Block Appliance

This systematic review aims to assess skeletal, dental and soft tissue treatment effects of the conventional Twin-block appliance compared to control in Class II malocclusion till date. The electronic databases of Medline, PubMed and Cochrane library were systematically searched until September 2019. The human studies that had used the conventional Twin block appliance with a control group evaluated by a Lateral Cephalogram were included. A total of 321 articles were examined. 10 articles were selected. Majority studies reported a significant improvement in the mandible and in the vertical height, maxillary restrictive effect was less consistently reported. A restrictive maxillary molar effect and an advancement in the mandibular molar has been suggested. Retroclination of the maxillary incisors and proclination of mandibular incisor inclination has been reported. There is some evidence to suggest a retrusive upper lip effect and a protrusive lower lip effect. Increase in Nasolabial angle and some decrease in the mentolabial sulcus to be expected. In conclusion the conventional Twin Block appliance have shown desirable skeletal, dental and soft tissue effects. However certain inevitable side effects are also seen mainly its effect on lower anterior proclination.


INTRODUCTION
The Twin block was developed by Dr William J. Clark in 1977. 1 Even though it's a removable appliance requiring patient compliance, it's still one of the most widely used functional correctors.
Twin-block appliance consists of upper and lower acrylic bite blocks with interlocking occlusal inclined planes at 70 o angle, which functions to bring the mandible in its desired forward and downward position. 2 It's mainly indicated in actively growing Class II division I malocclusions. 1,3 Multiple authors have reported variable results with variable intensities till-date. [4][5] A recent systematic review reporting all the treatment effects of Twin Block was lacking.
The aim of this review is to assess the possible skeletal, dental and soft tissue treatment effects of the conventional Twin-block appliance compared to controls in Class II malocclusion individuals till date.

Study selection
For each database search, first the titles and abstracts were evaluated. The articles that did not match the inclusion criteria, case reports, literature reviews, systematic reviews, editorial articles were all excluded.
Articles including animal studies, modified twin block, non-cephalometric analysis were excluded.
Selected abstracts were further subjected to scrutiny of the complete text. Also, when the abstracts were found to be unclear, the full text was obtained.
The 10 articles (FLOW CHART 1) which met all the inclusion criteria were finally included in the systematic review. Google Scholar did not reveal any different results than those from Pubmed/ Medline or Cochrane library. (FLOW CHART 2).

Data items and collection
Skeletal cephalometric findings denoting anteroposterior maxillary changes, mandibular changes, vertical changes, dental findings denoting effects on maxillary and mandibular molars and incisors, overjet and soft tissue findings such as the nasolabial angle, mentolabial sulcus and the position of upper and lower lip were collected.

Risk of bias in individual studies
Risk of bias was assessed through the evaluation of methodological quality study characteristics (Table  2). 5,17 Intra-rater reliability, interrater reliability, and blinding of examiner and/or statistician were considered. The studies were labeled as low risk, medium risk and high risk for bias. (Table 3).

Summary measures
Basic study characteristics for each of the 10 studies are mentioned in Table 4.

RESULT Selection and study characteristics
A total of 10 articles meeting the inclusion criteria were selected as shown in Flow Chart 1 & 2.

Risk of Bias within Studies
Out of the 10 studies 3 studies 7,10,11 have a low risk of bias, 5 studies 12-16 have a high risk of bias and remaining 8,9 have a medium risk of bias.
Antero-posterior effects on maxilla (Table 5) Illing et al, 7 Dauvravu et al, 12 Baysal et al 15 have reported a significant headgear effect. Whereas, Jena et al, 8 Lund and Sandler, 9 Khoja et al, 13 Baysal et al 14 and Tümer et al 16  Antero-posterior effects on mandible (Table 6) All authors have reported statistically significant increase in mandible.

CONCLUSION
This systematic review has aimed to find out the dental, skeletal and soft tissue effects of the conventional Twin Block appliance as compared with the control. From this systematic review it can be safely concluded that the conventional Twin Block appliance is indicated for individuals diagnosed with a Class II malocclusion with a horizontal growth pattern and a frank mandibular retrognathism. The use of this appliance is contraindicated in the cases of maxillary prognathism.
Effective and favourable soft tissue changes are seen.
However, some inevitable dental changes mainly lower anterior proclination are strongly reported which need to be timely addressed. OJN