Table 1. The characteristics of the included studies .

Characteristic	Study
	Ji et al (28)	Stappert et al (27)	Rasperini et al (16)	Boke et al (8)	Yared et al (25)	Szarmach et al (30)	Melsen, Allias (29)	Ngan et al (26)
Study Design	Retrospective	Prospective	Prospective	Retrospective	Retrospective	Cross-sectional	Retrospective	Retrospective
Participants	403 (285 F, 118 M)	29	16 (6 F, 10 M)	251 (177 F, 74 M)	34	18 (11 F, 7 M)	150 (114 F, 36 M)	20 (12 F, 8 M)
Age	11–43	≤13 14–18 ≥19	Mean age: 21±8.20	8-17.8 (mean 13.37 ± 2.06)	18-33	12-39	F: 22-65 M: 23-50	11-16
Biotype determination	Probing transparency	Transgingival probing	Biotype probe (Hu-Friedy)	Intra-oral photographs, Visual inspection of the gingival texture and capillary transparency	Scaled digital caliper,	Periodontal probe	Intra-oral photographs, Visual inspection of the gingival texture and capillary transparency	Visual inspection
Biotype classification	Thin Thick	Thin (≤2.5mm) Thick (>2.5mm)	Thin Medium Thick Very thick	Thin Thick	<0.5 mm ≥0.5 mm	Thin gingiva and the root bone layer	Thin Thick	Thin Moderate Thick
Intervention	NM	Fixed appliance with premolar extraction	Fixed appliance	58 Fixed appliance with extraction 173 Fixed appliance without extraction 20 Functional appliance	Fixed appliance	Fixed appliance	Fixed appliance without extraction	NM
Periodontal Outcome	GI, PI, GR, and plaque index	Gingival cleft	PPD, GR, CAL, KTW, FMBS	GR, Plaque, Inflammation	Plaque index, GI, PPD, CAL, GR	GR	GR, Plaque, Inflammation	GR, Inflammation
Quality assessment*	***	****	******	***	***	***	***	****

F = Female, M = Male, GI = Gingival Index, PI = Periodontal Index, GR = Gingival Recession, PPD = Probing Pocket Depth, CAL = Clinical Attachment Level, KTW = Keratinized Tissue Width, FMBS = Full-mouth Bleeding Score

NM = not mentioned

*Number of the stars represents quality of the study based on the Newcastle-Ottawa Quality Assessment Scale