Table 5. Summary of reports on isthmus prevalence and locations (all reported values are percentages).

Author	Year	Country	Ref.	Method	Sample size	Mesial root	Most common location	Distal root	Most common location
de Pablo et al.	2010	Spain	[20]	Review of literature of 41 articles	18,781	54.8% (range 23%–77%)	Middle third	20.2% (range, 8%–55%)
Demirbuga et al.	2013	Turkey	[27]	Micro-CT (in vitro)	1,748	30%	-	12%	-
Chourasia et al.	2012	India	[29]	In vitro (clearing technique)	150	30%	-	10%	-
Al-Qudah and Awawdeh	2009	Jordan	[7]	In vitro (clearing technique)	330	37.3%	Middle and apical thirds	13%	Middle third
Ahmed et al.	2007	Sudan	[8]	In vitro (clearing technique)	100	62%	-	8%	-
Villas-Bôas et al.	2011	Brazil	[25]	In vitro (micro-CT)	60 first/second molars	61%	3–4 mm from apex	-	-
Gulabivala et al.	2001	Myanmar	[9]	In vitro (clearing technique)	139	69.2%	-	9.2%	-
Mohammadzadeh Akhlaghi et al.	2017	Iran	[2]	In vitro (clearing technique)	150	44.6%	22.5% (2 mm from apex), 41.7% (4 mm), 35.8% (6 mm)	27.3%	34.3% (2 mm), 36.5% (4 mm), 29.3% (6 mm)
Faraz et al.	2015	-	[28]	In vitro (clearing technique)	123	10%	-	5%	-
Peiris et al.	2008	Japan	[10]	In vitro (clearing technique)	177 first/second molars	77.4%	Middle third	11.3%	Middle third
Mehrvarzfar et al.	2012	Iran	[4]	In vitro (clearing technique)	60	83%	92%, 88%, and 70% respectively in ranges 6, 4, and 2 mm from apex	-	-
Gu et al.	2009	China	[24]	Micro-CT (in vivo)	120	81%	At 6 mm from apex of mesial root: 20–40 years old (80%), 40–60 (67%), and over 60% (30%)
von Arx et al.	2005	-	[6]	Endoscope (in vivo)	56	88.5%	3–4 mm from apex	-	-
Fan et al.	2010	-	[26]	Micro-CT (in vivo)	126	85%	5 mm from apex	-	-

CT, computed tomography.