Accessory mesial canal=Middle Mesial Canal

Middle Mesial Canal is a canal in-between the mesial buccal and mesial lingual canals of a lower molar. The occurrence rate ranges from .3-46% (Kim v Azim) depending on your source but the number with own POE is possibly under 3% or as high as 46%.

Great photo of a middle mesial canal

What is the middle mesial canal?

One can think of the Middle Mesial Canal or Accessory mesial canal as a component of the isthmus space between the mesiobuccal (MB) and mesiolingual (ML) canals that is both cleansable and shapable. There is no consensus of a definition between canal and isthmus.

Middle mesial canal has been known about for a long time.  It has been proposed that the mesial roots of mandibular molars develop as 1 RCS and compress with age in the middle to form two. Hess 1925

• Mesial roots of first and second molars mostly had one large canal until 11 and 15 years of age, respectively. Pieris 2008
• Aging could explain lower incidence of MMC in first versus second molars
• Age is a large factor in likelihood of middle mesial canal existing. Nosrat JOE 2015

We can hypothesize that the space between the mesiobuccal (MB) and mesiolingual (ML) canals is an isthmus that can we clean and shape.

Root canal by Bryan Bauer DDS. All images of teeth and x-rays are the work of Dr. Bauer

What size should you open a middle mesial canal up to?

The canal itself is often very small and often connects to one of the other canals through fins or directly joins. If it joins another canal then smaller is probably better. If the canal is independent then larger is better from a bacterial standpoint. However, anything greater than #25 4% taper significantly decreases fracture strength. Kılıç JOE 2021. Therefore, it seems no larger than #25 4% taper is likely ideal.

Research on percentages of teeth with mid mesial canal

Lit Review 13% MMC Navarro Med Oral Path Oral Cir Buccal 2007

de Toubes JOE April 2012 1-18% Accessory mesial canal or is it just an isthmus (83% have isthmus von Arx 2005 Int Endo J)?  Study also finds Dental Operating Microscope and CBCT better at finding

36% had three canals at some point along root and 100% had isthmus Harris JOE 2013

12% but only 2% with separate canal but unsure if second or first molar 1981 Pomeranz JOE

8% increase in number canals found with DOM 8-13x from naked eye Carvalho 2000 JOE Sept

micro-CT study of middle mesial canals

As ususall the micor Ct study shows why root canals are impossible to treat 100% successfully. Versiani Arch Oral Biol 2016 shows 54% have 3 orifices and 44% have 3 foramina!

CBCT research on middle mesial canals

16% true mid mesials canals however only 2.4% had own POE.  25% and 8% of 1st and 2nd respectively and 65% and isthmus BUT excluded “calcified canals” whatever that means. Tahmasbi JOE 2017

Research on aspects of the mid mesial such as fins and isthmus

Azim JOE Feb 2015  38% had MMC

• Magnification from 6-11x
• To be included canal had to be negotiable to within 5mm of apex
• Age was a big factor
• 10% had own apical foramina unknown if first or second
• 12% were fin unknown if first or second

MMC highly correlated to age Nosrat 2015 JOE

de Pablo JOE 2010  Systemic review 41 studies

• Isthmus communications = 55% on mesial and 20% on distal root.  What is an isthmus and what is a canal?  Depends on age likely.  In the young it is a canal in the elderly an isthmus.

Isthmus are common (50% in 20-39) but decrease with age 24% in 60 and over (Gu 2009 JOE Chinese micro CT study)

Tend to be close to ML canal JOE Aug 2010 Kazandag also shows from 4.5x to DOM percentage found went from 16%-18%  Pomeranz 81 and Nosrat 15 showed same

X-rays of middle mesial canal

Often the middle mesial canal ends up looking like this on the x-ray. All 3 of these canals connected in some way through fins or isthmus.

Mesial roots of first and second molars mostly had one large canal until 11 and 15 years of age, respectively. Pieris 2008

Tend to be close to ML canal JOE Aug 2010 Kazandag also shows from 4.5x to DOM percentage found went from 16%-22%

1% separate canal but unsure if second or first molar 1981 Pomeranz JOE

Faint small mid mesial joining the mesial lingual canal. We fill the mid mesial with sealer rather than gutta percha when they join like this.

Another case