Continuing Education
Volume 32 No. 12 Page 82
Isthmus Gifts
Authored by Lisa Germain, DDS
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Continuing Education
Isthmus Gifts
Effective Date: 12/1/2013
BACKGROUND: THE ISTHMUS
Derived from the Greek word isthmos, the isthmus as
defined by Weller et al2 is a narrow connection between 2
root canals that contains pulp tissue. Others have called it
a transverse anastomosis,3 a corridor,4 and a lateral
interconnection.5
Isthmuses are prevalent in all types of roots in which 2
canals are normally found. These include mandibular incisors,
the maxillary and mandibular first and second premolars, the
mesial roots of maxillary and mandibular molars, maxillary
second molars, and the distal root of mandibular molars.6 Not
all anomalies occur within the isthmus, but it is necessary to
thoroughly explore this area and be aware of the anatomic
variations that arise from this connection.
The embryonic origin of the isthmus in the root canal
system is through Hertwig’s Epithelial Root Sheath. In single
roots, gaps occur in dentin formation that in turn prevent
cementum deposition in these areas. This leads to the
formation of lateral and accessory canals most commonly
observed in the apical third of the root. In multiple rooted
teeth with multiple canals, these defects occur with a higher
incidence, and an isthmus is formed when an individual root
projection is unable to close itself off. Partial fusion of the
root projections result in the formation of 2 root canals with
an isthmus formed in between, such as the distal root of the
mandibular first molar. If no fusion occurs, a large, ribbonshaped canal forms an isthmus throughout the entire root.7
During endodontic treatment, the tissue trapped in the
ramifications between the main canals is not readily removed
with instruments. Isthmus cleaning has been historically
relegated to copious irrigation with appropriate solutions.8 In
earlier years, intracanal medicaments such as formocresol,
cresatin, and camphorated p-monochlorophenol were relied
upon to render the unreachable areas sterile. These agents
were responsible for that classic “dental office smell,” and did
more in the way of fumigating the canals than disinfecting
them. Sodium hypochlorite (bleach) is still considered the
gold standard for canal cleaning due to its outstanding
disinfectant properties, easy availability, and cost
effectiveness. In addition, lubricants, chelating agents,
calcium hydroxide, alcohol, peroxide, and assorted
proprietary formulas provide adjunctive methods for cleaning
the complete root canal system. In recent years, various
Expiration Date: 12/1/2016
ABOUT THE AUTHOR
Dr. Germain graduated from Boston
University School of Graduate Dentistry
with a specialty degree in endodontics in
1981. She is a Diplomate of the American
Board of Endodontics, on the Faculty
of the American Academy of Facial
Esthetics, and a Fellow of the International Congress of Oral
Implantologists. Dr. Germain maintains a private practice
in New Orleans, La. She can be reached via e-mail at
[email protected].
Disclosure: Dr. Germain reports no disclosures.
INTRODUCTION
The construction of the Panama Canal, which created a
water passage from the Atlantic to the Pacific Ocean, was
a difficult and lengthy process. Early explorers of the
Americas and European colonists of central America
recognized the potential, but it was not attempted until
technological advances allowed first France, and then
ultimately the United States, to carry out the deep
excavations necessary to complete the project. Opened in
1914, and still in operation today, it cuts across the Isthmus
of Panama, serving as an important conduit of international
maritime trade. In a parallel world in the early 20th century,
dentists were exploring a different type of canal system.
In 1921, Walter Hess published a study1 detailing the
“purely anatomical conditions in the root canals of human
teeth.” He cleared and sectioned 2,800 specimens
obtained from people aged 5 to 55 years. These teeth did
not have prior pulp extirpation. His study confirmed
previous theories that root canals are complex, and he
wrote: “a thorough knowledge of them is important.”1 The
images he showed brought to the forefront in endodontics
an awareness of the complex interconnections of the root
canal system, as well as the challenges of not having the
necessary technological advances to negotiate them.
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Isthmus Gifts
energy sources (ie, ultrasonic devices and lasers) have
shown promise as a more complete solution to this clinical
dilemma and will most likely continue to be developed.
Microsurgical techniques have stressed the importance of
including the isthmus in the ultrasonic retropreparation for
root end filling.9
Root canal anatomy can vary between the genders and
is not static throughout a person’s lifetime. With age, the
formation of secondary dentin results in an increase in the
development of cross-linking within the canals. In addition,
ethnicity plays a significant role in anatomical development
of the root canal space, with some races having a greater
number of canals than previously reported in the
literature.10-12
For decades, Vertucci’s classification3 for root canal
anatomy has been used to categorize anomalous root canal
systems. Vertucci3 noted 8 different canal system types based
on the number of canals within each root and whether they
had anastomosis, a common or separate orifice, and a
common or separate portal of exit. In 2004, Sert and Bayiril13
studied teeth by gender in the Turkish population and reported
14 additional types not included in the Vertucci classification.
a
b
Figure 1a. Pre-op
No. 26.
Figure 1b. Post-op
No. 26.
a
Figure 2a. Pre-op
No. 4.
b
MANDIBULAR ANTERIORS
Using radiographs, Benjamin and Dowson14 studied 364
mandibular incisors and reported 41.4% of their sample had
2 separate canals, with separate foramina occurring in only
1.3% of the samples. In contrast, Miyashita et al15 studied
1085 specimens and reported that only 3.1% had separate
canals and foramina. This wide variation in reported data
makes endodontic treatment planning difficult.16
Rather than relying on the statistics, the clinician should
look at each tooth as having the potential for multiple
interconnected canals. The mandibular anterior teeth are
particularly challenging due to the small coronal tooth
structure and position in the mouth, particularly if they are
inclined lingually. Adequate radiographic angles should be
taken if a cone beam computed tomography (CBCT) is not
available. In addition, illumination and magnification are
imperative when searching for delicate anatomy. An
important clinical observation is that if a root is wide buccallingually, it is likely to have more than one canal.
Figure 1a shows a preoperative radiograph of tooth No.
Figure 2b. Post-op
No. 4.
26 with 2 canals in a 65-year-old white male. After access
was created, an isthmus was visualized in a buccal-lingual
direction. The main canal was located, cleaned, and
shaped. The second canal was located within the isthmus
under the cingulum of the tooth. Figure 1b shows a
postoperative radiograph with 2 canals cleaned, shaped,
and obturated in 3 dimensions.
PREMOLARS
Premolar teeth vary widely in their anatomy as well.
Vertucci3 described a varying number of roots with
intercommunications. In maxillary first premolars, 2 or more
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Isthmus Gifts
canals were found in one root 30% of the time, while 41%
of the maxillary second premolars had 2 or more canals. In
addition, he reported that mandibular first premolars
presented with 2 or 3 canals 27% of the time, and second
premolars presented with an additional canal 2% of the
time. Green4 reported 2 orifices on the floor of the pulp
chamber joined by an isthmus in 92% of maxillary first
premolars, and in 28% of maxillary second premolars. In
mandibular first premolars he reported 2 orifices in 14% of
the cases and 8% in mandibular second premolars.
Figure 2a shows a preoperative radiograph of tooth No.
4 in a 48-year-old Asian female. Note the disappearance of
a distinct canal system, which is often a clue when
predicting difficult anatomy. Figure 2b shows a postoperative
radiograph revealing 3 separate canals with separate
terminus.
Figure 3a shows a preoperative radiograph of teeth
Nos. 20 and 21 in a 52-year-old African American male. In
the previous root canal treatment of tooth No. 20, 2 canals
were located, but pain persisted and was reproduced on
both teeth. The radiograph of tooth No. 21 reveals a canal
system that disappears, becoming almost amorphous in
nature, and is indicative of the potential for a multirooted
tooth. The final retreatment of this case can be observed in
Figure 3b. Tooth No. 21 shows a rare 3-canal system. Even
more notable is the filled isthmus in the retreatment of tooth
No. 20 and an apical interconnection. By cleaning out the
isthmus and negotiating the previously untreated canals,
the symptoms resolved.
a
Figure 3a. Pre-op
Nos. 20 and 21.
b
Figure 3b. Post-op
Nos. 20 and 21.
a
Figure 4a. Pre-op
No. 2.
b
MOLARS
In 1990, Kulild and Peters17 reported up to 95% of the
mesiobuccal roots of maxillary molars have 2 canal
systems. Many clinical articles have been written, discussing
cases whereby these canals were found to have an isthmus
that connected them.18 Figures 4a and 4b illustrate another
common anomaly. Clinically, this maxillary second molar
presents with one main ribbon-shaped canal divided by a
narrow fin. Unlike most maxillary second molars whereby
access to the area makes canal negotiation challenging, this
type of anatomy has the benefit of being treated like a
central incisor in the back of the mouth. (Talk about an
isthmus gift!)
Figure 4b. Post-op
No. 2.
Many methods have been used to study root canal
systems in teeth. Dr. Sergio Kuttler has done extensive
work with laboratory micro-CT imaging, allowing 3dimensional (3-D) microscopic viewing of an extracted
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Continuing Education
Isthmus Gifts
tooth, with a voxel size of about 7 µm. Note
a
b
that a clinical CBCT device will have a voxel
size of about 80 µm. Figure 5a shows 3
distinct canals in the mesial root of a
mandibular molar. Clinically, this phenomenon
can be quite elusive; exploration of the
isthmus in all mandibular molars is highly
recommended. Figure 5b is a preoperative
radiograph of tooth No. 19 in a 28-year-old
Figure 5a. Microcomputed tomography
Figure 5b. Pre-op No. 19.
male of Honduran descent. Of note is the (micro CT) mesial root of mandibular
amorphous appearance of the mesial root. molar (courtesy of Dr. Sergio Kuttler,
Fort Lauderdale, Fla).
Figure 5c shows the isthmus between the
c
d
mesial roots. Upon exploration with a No. 15
hand file, a middle mesial canal was located
(Figure 5d). The postoperative radiograph
(Figure 5e) shows the cleaned, shaped, and
3-D obturation of 3 canals in the mesial root
and 2 canals in the distal root.
A discussion about isthmuses would not
be complete without mention of the
c-shaped canal configuration. Figure 6a shows Figure 5c. Isthmus between mesial roots, Figure 5d. Middle mesial canal with file,
No. 19.
an example of this type of isthmus when seen No. 19.
clinically. These canals present either as a single ribbonlike
the interior of the
e
canal from the orifice to the apex (Figure 6b) or, more
tooth with magnicommonly, a c-shaped isthmus located on the floor of the
fication and illumipulp chamber that divides into 2 or 3 canals with a thin
nation, and a
thorough underfinlike connection (Figure 6c).19 Recognition of these
configurations facilitates cleaning, shaping, and 3-D
standing of anaobturation, and will maximize the prognosis of the
tomic variations in
endodontic treatment.
root canal morphFigures 7a and 7b show the pre- and postoperative
ology are necesFigure 5e. Post-op No. 19.
radiographs of tooth No. 18 with a single ribbonlike canal
sary to perform sucfrom the orifice to the apex. Figures 8a and 8b are pre- and
cessful endodontic treatment.
postoperative radiographs of the more commonly seen
The clinician should pay special attention to the isthmus
c-shaped configuration on tooth No. 31 whereby there are
connecting canals, as its presence makes thorough
separate canals that radiate from within the isthmus.
debridement of the root canal system difficult. Additional
canals may originate within this area below the level of the
IN SUMMARY
pulp chamber floor. Ultrasonic instruments may be used to
Root canal system anatomy has been studied and
trough several millimeters along the isthmus to obtain better
documented for an entire century. While these studies vary
access to the tissues associated with the isthmus or any
in their findings, the presence of a communication between
potential hidden canals; however, special care must be
the canal systems within the individual root is a common
taken to avoid a perforation in the furcation.
observation. Adequate access preparation, exploration of
As long as clinical endodontics is performed, the
4
Continuing Education
Isthmus Gifts
a
Figure 6a. Occlusal view of c-shaped canal
configuration.
c
b
Figure 6b. Micro CT of c-shaped canal type 1 Figure 6c. Micro CT of c-shaped canal type 2
(courtesy of Dr. Sergio Kuttler).
(courtesy of Dr. Sergio Kuttler).
mysteries that lie within the isthmus will be
explored both clinically and academically.
One thing that is certain: the isthmus is the
gift that keeps on giving.
a
b
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
Hess W. Formation of root-canals in
human teeth. The Journal of the
National Dental Association.
1921;8:704-734.
Figure 7a. Pre-op No. 18.
Figure 7b. Post-op No. 18.
Weller RN, Niemczyk SP, Kim S.
Incidence and position of the canal
a
b
isthmus. Part 1. Mesiobuccal root of the
maxillary first molar. J Endod.
1995;21:380-383.
Vertucci FJ. Root canal anatomy of the
human permanent teeth. Oral Surg Oral
Med Oral Pathol. 1984;58:589-599.
Green D. Double canals in single roots.
Oral Surg Oral Med Oral Pathol.
1973;35:689-696.
Pineda F. Roentgenographic investigation of
Figure 8a. Pre-op No. 31.
Figure 8b. Post-op No. 31.
the mesiobuccal root of the maxillary first
10. Pineda F, Kuttler Y. Mesiodistal and buccolingual
molar. Oral Surg Oral Med Oral Pathol. 1973;36:253-260.
roentgenographic investigation of 7,275 root canals. Oral
Hsu YY, Kim S. The resected root surface. The issue of
Surg Oral Med Oral Pathol. 1972;33:101-110.
canal isthmuses. Dent Clin North Am. 1997;41:529-540.
11.
Kartal N, Yaniko lu F. The incidence of mandibular
Orban BJ, Bhaskar SN. Orban’s Oral Histology and
premolars with more than one root canal in a Turkish
Embryology. 11th ed. St. Louis, MO: Mosby Year Book;
population. J Marmara Univ Dent Fac. 1992;1:203-210.
1991:86-87.
12. Tinelli ME. Ethnic variations in the topography of root
Schilder H. Cleaning and shaping the root canal. Dent
canals. Electronic Journal of Endodontics Rosario.
Clin North Am. 1974;18:269-296.
2011;2:558-562.
Cleghorn BM, Goodacre CJ, Christie WH. Morphology of
13. Sert S, Bayirli GS. Evaluation of the root canal
teeth and their root canal systems. In: Ingle JI, Bakland
configurations of the mandibular and maxillary
LK, Baumgartner JC. Ingle’s Endodontics. 6th ed.
permanent teeth by gender in the Turkish population.
Hamilton, Ontario, Canada: BC Decker; 2008:151-220.
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J Endod. 2004;30:391-398.
14. Benjamin KA, Dowson J. Incidence of two root canals in
human mandibular incisor teeth. Oral Surg Oral Med
Oral Pathol. 1974;38:122-126.
15. Miyashita M, Kasahara E, Yasuda E, et al. Root canal
system of the mandibular incisor. J Endod.
1997;23:479-484.
16. Mauger MJ, Schindler WG, Walker WA III. An evaluation
of canal morphology at different levels of root resection in
mandibular incisors. J Endod. 1998;24:607-609.
17. Kulild JC, Peters DD. Incidence and configuration of
canal systems in the mesiobuccal root of maxillary first
and second molars. J Endod. 1990;16:311-317.
18. Teixeira FB, Sano CL, Gomes BP, et al. A preliminary in
vitro study of the incidence and position of the root canal
isthmus in maxillary and mandibular first molars. Int
Endod J. 2003;36:276-280.
19. Jafarzadeh H, Wu YN. The C-shaped root canal
configuration: a review. J Endod. 2007;33:517-523.
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Continuing Education
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1. A study by Walter Hess confirmed previous theories
that root canals are not complex systems
a. True
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b. False
2. Derived from the Greek word isthmos, the isthmus as
defined by Weller et al, is a narrow connection
between 2 root canals that contains pulp tissue.
a. True
b. False
3. Isthmuses are prevalent in all types of roots in which
one canal is normally found.
a. True
b. False
4. The embryonic origin of the isthmus in the root canal
system is rooted in the activities of the ameloblastoma.
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a. True
b. False
5. During endodontic treatment, the tissue trapped in
the ramifications between the main canals is not
readily removed with instruments. Isthmus cleaning
has been historically relegated to copious irrigation
with appropriate solutions.
a. True
b. False
6. In recent years, various energy sources (ie,
ultrasonic devices and lasers) have shown promise
as a more complete solution to this clinical dilemma
and will most likely continue to be developed.
a. True
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b. False
7. In 1990, Kulid and Peters reported that only 23% of
the mesiobuccal roots of maxillary molars have 2
canal systems.
a. True
b. False
8. The clinician should pay special attention to the
isthmus connecting canals, as its presence makes
thorough debridement of the root canal system
difficult.
This CE activity was not developed in accordance with
AGD PACE or ADA CERP standards.
CEUs for this activity will not be accepted by the AGD
for MAGD/FAGD credit.
a. True
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b. False
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