Clinical Research
The Influence of an Isthmus on the Outcomes of
Surgically Treated Molars: A Retrospective Study
Sunil Kim, DDS, MSD,* Hoiin Jung, DDS, PhD,† Sooyun Kim, DDS, MSD,*
Su-Jung Shin, DDS, MSD, PhD,* and Euiseong Kim, DDS, MSD, PhD‡
Abstract
Introduction: The purpose of this study was to investigate the effects of an isthmus on the success rate of surgically treated molars. Methods: The study included
106 maxillary and mandibular first molars with endodontic lesions limited to the periapical area. Endodontic microsurgical procedures were performed
according to the Yonsei protocol reported in a previous
study. When an isthmus was observed after a highmagnification inspection, it was included in the retrograde preparation design. When an isthmus was not
observed, only the main root canal space was prepared.
The patients were followed up periodically every year
after treatment. Success was defined as the absence
of clinical signs and symptoms and radiographic evidence of complete or incomplete healing. Results: Of
the 106 teeth included in the study, 72 teeth had an
isthmus, and 34 did not. Kaplan-Meier analysis revealed
that the cumulative survival rate after surgery was
61.5% and 87.4% for 4 years when an isthmus was present and absent, respectively. A multivariate Cox proportional hazards regression analysis showed that the
adjusted hazard ratio for failure was 6.01 times higher
for the isthmus-present teeth than for the isthmusabsent teeth (P < .05). Conclusions: In this study, the
success rate for endodontic microsurgery on isthmusabsent teeth was higher than that for isthmus-present
teeth. Considering the success rate and potential risk
of weakening the root after preparation, the techniques
of isthmus preparation need to be improved. (J Endod
2016;42:1029–1034)
Key Words
Clinical outcome, endodontic microsurgery, isthmus,
root canal anatomy, success rate
O
ne of the reasons for
Significance
the difficulty of root
In this study, the success rate for endodontic
canal treatment is the
microsurgery on isthmus-absent teeth was higher
complex structure of its
than that for isthmus-present teeth. Considering
anatomy. Many different elthe success rate and potential risk of a weakened
ements compose anatomic
root after preparation, the technique of isthmus
complexity, including the
retrograde-preparation needs to be improved and
fin and lateral canal, and
carried out carefully, and no additional preparation
isthmuses, in particular,
should be necessary for a type I isthmus with no
are challenging for endobserved communication.
odontists. An isthmus is a
narrow, ribbon-shaped
communication between 2 root canals that contains pulp tissue (1). It is also known as
a corridor (2) or a transverse anastomosis (3). There have been many studies on the incidence of isthmus, with the reported incidence in the mesiobuccal root of maxillary first
molars ranging from 76%–100% and that in the mesial root of mandibular first molars
being approximately 83% (1, 4–6). Because the structure of an isthmus resembles a
thin net, it is difficult to perform direct mechanical preparation and chemical
disinfection. Although various preparation and irrigation methods have been
introduced to overcome the anatomic complexities (7, 8), no perfect method for
cleaning and shaping of an isthmus has yet been devised. An isthmus, which contains
necrotic debris and tissue remnants, can be a shelter for bacteria to grow and multiply,
ultimately causing failure of the nonsurgical root canal treatment (9).
Surgical endodontic treatment is an option when nonsurgical treatment or
retreatment fails to resolve the periapical infection because of anatomic complexities. The introduction of the surgical operating microscope and ultrasonic devices
has changed many of the procedures of endodontic surgery and increased the
success rate accordingly (10). The magnification and illumination provided by
the surgical operating microscope allow for a more precise procedure that
inspects the resected root surface and can identify and treat the isthmus (10).
Small ultrasonic tips enable the clinician to prepare ideal retropreparations in
nearly all clinical situations (11). The identification and treatment of an isthmus
may be important factors in the improvement of success rates of endodontic
microsurgery for posterior teeth (12). However, when overlooked, an isthmus
can lead to failure.
The use of the surgical operating microscope and ultrasonic devices has
made it possible to identify and prepare an isthmus after root resection, but
the retrograde preparation of a long and narrow isthmus is still one of the
most difficult procedures in apical surgery. Isthmuses are of many types based
From the *Department of Conservative Dentistry, Gangnam Severance Dental Hospital, †BK21 PLUS Project, and ‡Microscope Center, Department of Conservative
Dentistry and Oral Science Research Center, College of Dentistry, Yonsei University, Seoul, South Korea.
Address requests for reprints to Dr Euiseong Kim, Microscope Center, Department of Conservative Dentistry and Oral Science Research Center, College of Dentistry,
Yonsei University 50 Yonsei-Ro, Seodaemun-Gu, Seoul, 120-752, South Korea. E-mail address: [email protected]
0099-2399/$ - see front matter
Copyright ª 2016 American Association of Endodontists.
http://dx.doi.org/10.1016/j.joen.2016.04.013
JOE — Volume 42, Number 7, July 2016
Isthmus and Surgically Treated Molars
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Clinical Research
on their structures. According to Hsu and Kim (13), a type I
isthmus was defined as having either 2 or 3 canals with no noticeable communication. Types II and III contain 2 and 3 canals,
respectively, each having a definite communication between the
main canals. A type IV isthmus has canals that extend into the
isthmus area. Type V was recognized as a connection throughout
the section. In types II to V, in which an isthmus is clearly
observed, they should be prepared and retrograde filled in a
manner consistent with surgical principles, which is to remove
bacteria and block the source of reinfection. In types IV and V,
clinicians can complete the root-end preparation along the longitudinal root axis with passive movement of an ultrasonic device
because of the clear structure of the isthmus. However, in types
II and III, where the structure of the isthmus is less clear, the
route needs to be actively established. It can be difficult to accurately determine the proper path and angulation of the ultrasonic
tip.
Previous studies on the prognostic factors of endodontic
microsurgery have reported lower success rates for molars than
for anterior teeth (14, 15). The low success rate was attributed to
the limited accessibility of the site and the root canal anatomy,
particularly the presence of an isthmus (10). Despite the many clinical studies on the treatment outcomes of endodontic microsurgery,
studies on the effects of an isthmus on surgical outcomes have been
rare. Therefore, the purpose of this study was to investigate the effects of an isthmus on the outcomes of endodontic microsurgery on
molars.
Materials and Methods
Case Selection and Surgical Procedure
The clinical database of the Department of Conservative Dentistry
at the Yonsei University College of Dentistry, Seoul, South Korea, was
searched for patients with a history of endodontic microsurgery
between July 2001 and May 2014.
The same surgeon performed all surgical procedures using a
surgical operating microscope (OPMI PICO; Carl Zeiss, Gottingen,
Germany), except for the incisions, flap elevation, and suturing. The
surgical technique was presented in a previous article (16). Briefly,
after deep anesthesia, flap elevation, and osteotomy, a 3-mm root tip
with a 0 to 10 bevel angle was amputated with a #170 tapered fissure
bur under copious water irrigation. After homeostasis, the resected
root-end surfaces were stained with methylene blue and inspected
with a micromirror (Obtura Spartan, Fenton, MO) under a high magnification of 20 to 26 to identify anatomic details. When an isthmus
was observed after a high-magnification inspection, it was included in
the retrograde preparation design to form a cavity. When an isthmus
was not observed, only the root canal space stained with methylene
Figure 1. Example of each type of isthmus according to the Hsu and Kim classification (13). (A) Type I isthmus. (A-1) Resected root surface showing 2 canals
without communication (arrow). (A-2) Root surface after retrofilling. Only the root canal space stained with methylene blue was prepared and retrofilled (*). (B)
Type II isthmus. (B-1) Two filled root canals were connected by a fine line (arrow) that is stained with methylene blue. (B-2) An isthmus was included in the
preparation design and retrofilled with root canal space (*). (C) Type III isthmus. (C-1) Two filled root canals and 1 unfilled missing canal were connected by an
isthmus (arrows) that is stained with methylene blue. (C-2) Space for 3 canals and isthmus were included in the preparation design and retrofilled (*). (D) Type IV
isthmus. (D-1) Two canals extending (arrows) into the isthmus. (D-2) The isthmus was included in the preparation design and retrofilled (*). (E) Type V isthmus.
(E-1) Two filled root canals were connected by a large corridor (arrow). (E-2) Space for 2 canals and an isthmus were included in the preparation design and
retrofilled (*).
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Kim et al.
JOE — Volume 42, Number 7, July 2016
Clinical Research
blue was prepared (Fig. 1A-1–E-2). The root-end cavities were prepared with KIS ultrasonic tips (Obtura Spartan) driven by a piezoelectric
ultrasonic unit (Spartan MTS, Obtura Spartan) and filled with either
ProRoot MTA (Dentsply, Tulsa, OK) or Super EBA (Harry J. Bosworth,
Skokie, IL). The wound site was sutured with a 5-0 monofilament. After
surgery, the surgeon documented the procedure, including the preoperative and intraoperative factors. The surgical record contained preoperative and postoperative periapical radiographs, high-magnification
(20) photos of the resected root surface, and the root surface after
retrograde preparation and retrograde filling. The surgical record
was periodically updated whenever the patients were followed to assess
the clinical and radiographic signs of healing.
Inclusion and Exclusion Criteria
After a review of their surgical records, patients were selected on
the basis of the following inclusion criteria:
1. Cases involving surgery of the maxillary first molar or the mandibular first molar
2. Two or more root canals within 1 root
3. Documented follow-up of at least 1 year (failure cases within 1 year
were included)
Patients were excluded from the study if there were intraoperative findings of an apical lesion and complete denudement of the
buccal plate. During the follow-up period, teeth that were extracted
with no direct link to surgical failure, such as strategic extraction for
prosthodontics treatment or periodontal problems, were excluded
from this study.
Clinical and Radiographic Evaluation
The patients were followed up once annually subsequent to treatment. To evaluate treatment outcomes, clinical and radiographic examinations were conducted. At each follow-up visit, any sign and/or
symptoms of tenderness related to percussion, mobility, periodontal
pocket formation, sinus tract formation, or subjective discomfort
were evaluated, and periapical radiographs were taken from 3 different
horizontal angles (straight and 20 mesial and distal).
Outcome Assessment
An assessment of treatment outcomes was made through clinical
and radiographic examination during each patient’s final visit. Two
blinded examiners evaluated the periapical radiographs using the
criteria established by Molven et al (Fig. 2A-1–D-3) (17). The healing
classifications consisted of complete healing, incomplete healing (scar
tissue), uncertain healing, and unsatisfactory healing. Success was
defined as the absence of clinical signs and symptoms along with radiographic evidence of complete or incomplete healing. The following
results were considered to indicate failure: any clinical sign and/or
symptom or radiographic evidence of uncertain healing or unsatisfactory healing. The 2 examiners standardized the evaluation criteria
before the case analyses so that their results were based on the same
evaluation methods and conditions. Cohen kappa statistical analysis
was used to measure interexaminer variability.
Statistical Methods
A Kaplan-Meier survival analysis and the log-rank test were conducted to compare the cumulative success rates as a function of the
isthmus preparation. Multivariate Cox proportional hazards models
were used to estimate the effect of isthmus preparation on the risk of
surgery failure. The proportional hazards assumption was assessed
by a log minus log survival function and found to hold. All data were
JOE — Volume 42, Number 7, July 2016
analyzed using IBM SPSS statistics v21.0 software (IBM Corp, Somers,
NY), and the significance level was established at 0.05.
Results
A total of 106 teeth were included in the study from 2001 to 2014
(mean observed time = 42.8 months). The demographics of the inception cohort and failure cases are summarized in Table 1. There were 72
teeth with an isthmus and 34 without. Of the 106 teeth, 27 teeth were
categorized as failures, and 79 teeth were successfully treated. Of the
27 failures, 24 cases underwent an isthmus preparation, whereas 3
did not. In addition, 10 teeth among the failures were impossible to
maintain because of clinical symptoms, and they were either extracted
or the root was amputated. The most frequent cause of failure identified
during extraction or root amputation was a vertical root fracture, which
occurred in 9 cases. The other cause of failure was a periodontal problem. The Cohen kappa value was 0.89, which is indicative of very good
agreement between the 2 examiners.
The Kaplan-Meier analysis revealed that the cumulative survival
rate after surgery was 61.5% for 4 years when an isthmus was present
and prepared. The survival rate after 4 years was 87.4% when an
isthmus was absent and unprepared (Fig. 3). The survival rate of
isthmus-absent teeth was significantly higher than that for isthmuspresent teeth (P < .05. log-rank test). The multivariate Cox proportional
hazards regression analysis showed that the adjusted hazard ratio for
failure was 6.01 times higher for the isthmus-present teeth (95% confidence interval, 1.78–20.25; P < .05; Table 2) than for the isthmusabsent teeth. Although age and sex influenced outcomes, the jaw and
root-end filling materials exerted no influence.
Discussion
The present study aimed to determine the effects of an isthmus
on the success rate of endodontic microsurgery for molars. This study
only included cases from a single operator, which can be both a
strength and a weakness. By using cases from only 1 operator, we
can minimize intraoperative variations related to the surgical procedure. However, the study population would be relatively small and
might not represent the generalized outcomes of an endodontic
microsurgery. The study also conducted survival analysis and reported on the cumulative success rates over time, thus providing
more information than comparison of success rates at a single arbitrary point in time. According to the Kaplan-Meier survival statistics
and log-rank test results, the teeth with a prepared isthmus recorded
a lower cumulative survival rate than the teeth that had no isthmus, a
statically significant difference. Kaplan-Meier statistics allow researchers to estimate the cumulative survival rates of patients over
time, even when the follow-up periods are inconsistent among the patients; accordingly, it is an appropriate method for analyzing the results of the present study. However, the method has its limitations
because it can determine the effects of only 1 variable at a time.
Thus, we conducted a multivariate Cox proportional hazards regression analysis as well. This analytic method can determine the effects of
an isthmus on the success rate of surgery even in the presence of other
variables that could influence the treatment outcome. Previous studies
on the prognostic factors of endodontic microsurgery identified factors that could affect surgical success rates. Those potential confounding factors, including age, sex, and jaw and root-end filling
material, were adjusted for in the Cox regression analyses (14, 15).
The results of the multivariate Cox proportional hazards regression
analysis show that the adjusted hazard ratio of isthmus-present teeth
was 6.01 times higher than that of isthmus-absent ones, another statistically significant difference.
Isthmus and Surgically Treated Molars
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Clinical Research
Figure 2. (A) A mandibular first molar radiographically assessed as ‘‘complete healing’’ 7 years after endodontic microsurgery with MTA root-end filling. (A-1)
Preoperative, (A-2) postoperative, and (A-3) 7-year follow-up. (B) A mandibular first molar radiographically assessed as ‘‘incomplete healing’’ 3 years after endodontic microsurgery. (B-1) Preoperative and (B-2) postoperative: the resected root face of the mesial root was filled with MTA including the isthmus. (B-3) The 3year follow-up. (C) A mandibular first molar radiographically assessed as ‘‘uncertain healing’’ 3 years after endodontic microsurgery. (C-1) Preoperative and (C-2)
postoperative: the resected root face of the mesial root was filled with MTA including the isthmus. (C-3) The 3-year follow-up. (D) A mandibular first molar radiographically assessed as ‘‘unsatisfactory healing’’ 6 years after endodontic microsurgery. (D-1) Preoperative and (D-2) postoperative: the resected root face of the
mesial root was filled with MTA including the isthmus. (D-3) The 6-year follow-up.
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Kim et al.
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Clinical Research
TABLE 1. Demographics of the Inception Cohort (n = 106) and Failure Cases
(n = 27)
Inception
cohort
Censored
Failure
Variable
n
n (%)
n (%)
Total
Sex
Male
Female
Isthmus*
Absent
Type I
Present
Type II
Type III
Type IV
Type V
Jaw
Maxilla
Mandible
Root-end filling material
MTA
Super EBA
106
79 (74.5)
27 (25.5)
39
67
25 (64.1)
54 (80.6)
14 (35.9)
13 (19.4)
34
72
22
2
18
30
31 (91.2)
48 (66.7)
11 (50)
2 (100)
12 (66.7)
23 (76.7)
3 (8.8)
24 (33.3)
11 (50)
0 (0)
6 (33.3)
7 (23.3)
41
65
32 (78.0)
47 (72.3)
9 (22.0)
18 (27.7)
74
32
53 (71.6)
26 (81.3)
21 (28.4)
6 (18.7)
MTA, mineral trioxide aggregate.
*Classification scheme from Hsu and Kim (10).
One of the reasons behind the relatively lower success rate in
isthmus-present teeth is the difficulty associated with the isthmus preparation procedure itself. Mechanical cleaning ultrasonic units and tips,
which were introduced to endodontic surgery in the 1990s, have been
a great help for root-end preparation (11). Compared with the conventional method of using a microhandpiece and microbur, the
long and thin design of the ultrasonic tips is useful for managing a
thin isthmus. Although it has become possible to identify an isthmus
with the surgical operating microscope and to achieve better retropreparations than before with the use of an ultrasonic device, isthmus
management is still one of the most difficult steps in endodontic microsurgery. There are some guidelines for isthmus preparation techniques. Hsu and Kim (13) reported that an isthmus could be
prepared through a pendulum movement without imposing pressure.
However, that approach is applicable only when there is a guide
through which the ultrasonic tip can pass, such as for a type IV and
type V isthmus. A technique designed to supplement this procedure
was also reported, which was applicable when a preparation was difficult, such as for a type II and type III isthmus (11, 18). After an
appropriate cavity design is planned, several dots are placed along
an imaginary line connecting the 2 canals with an ultrasonic tip.
Then, the dots are connected to one another in a featherlike,
passive movement to deepen the preparation to its full length.
However, the technique has its limitations because the isthmus
location is assumed arbitrarily. In this study, the failure rate for a
type II isthmus was higher than for a type IV and V isthmus (Table 1).
In the case of an isthmus preparation, it is also important to
consider the possibilities of the remaining root becoming weak after
surgery. Of the 10 cases in which the cause of failure was identified
through tooth extraction or root amputation in the present study, 9
failed because of a vertical root fracture. Eight of the 9 had an isthmus
preparation. Ultrasonic root-end preparations produce more conservative and less perforated cavities than those produced with conventional microhandpiece bur preparations (19). However, despite the
procedural advances, there is no avoiding root weakening because
of the reduction in the thickness of the remaining dentin after an
isthmus preparation. In this study, we used tips with the smallest
JOE — Volume 42, Number 7, July 2016
Figure 3. Kaplan-Meier survival curves according to the presence or absence
of an isthmus. The cumulative survival rate after surgery was 61.5% for 4 years
when an isthmus was present. The cumulative survival rate after 4 years was
87.4% when an isthmus was absent. Although the isthmus-absent teeth maintained a stable survival rate 4 years after surgery, the survival rate in the
isthmus-present teeth continued to drop 4 years after surgery, reaching a
rate of approximately 42% at 100 months. The survival rate for isthmusabsent teeth was significantly higher than for isthmus-present teeth
(P < .05. log-rank test).
diameter possible, from 0.5-mm and 0.7-mm KIS ultrasonic tips, to
minimize the weakening of the remaining root. The power setting
for the ultrasonic unit was also set to the lowest level possible. In
fact, the thickness of the remaining dentin is the most important of
several factors influencing fracture susceptibility. Sathorn et al (20)
reported that fracture susceptibility increased as the remaining root
dentin thickness after canal preparation decreased. They also found
that a reduction of dentin thickness was the only factor that increased
fracture susceptibility. Although the reduction in dentin thickness
should be minimized to prevent such root weakening, it is not easy
to meet both the objectives:
1. Proper bacteria elimination and prevention of reinfection through
mechanical preparation and retrofilling
2. Prevention of root weakening through the preservation of root
dentin
Degerness and Bowles (21) reported that an optimal root resection level is 3–4 mm, considering an accessory canal incidence, isthmus
detection, and canal wall thickness. They also reported an average of
1 mm of remaining dentin thickness in the mesial and distal roots
around an isthmus in the case of a 3-mm root resection at the maxillary
first molar (22). Considering that the average diameter of ultrasonic tips
used in root-end preparation is 0.6 mm, isthmus preparation, however
minimal it is, will leave only a thin dentin layer of under 0.7 mm in the
mesial and distal roots around an isthmus in the case of a 3-mm root
resection at the maxillary first molar. Isthmus-present teeth require
the reduction of an additional tooth structure as well as root canal
space, so further weakening is unavoidable compared with isthmusabsent ones, which will then reduce the success rate of surgery.
Isthmus and Surgically Treated Molars
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Clinical Research
TABLE 2. Multivariate Cox Proportional Hazards Regression Analysis of Time
until Failure
Multivariate
Variables
Hazard ratio
95% CI
P value
Age
Sex
Male
Female
Isthmus
Absent
Present
Jaw
Maxilla
Mandible
Filling material
MTA
Super EBA
1.04
1.02–1.07
.002
1
0.3
Reference
0.13–0.68
.004
1
6.01
Reference
1.78–20.25
.004
1
1.01
Reference
0.43–2.37
.967
1
0.83
Reference
0.31–2.19
.714
There is an argument that there should be a preparation between
canals when there are 2 or more canals in a single root, even without
the confirmation of communication, as in a type I isthmus and types II
to V, where an isthmus is generally or partially observed. Weller et al
(1) and Teixeira et al (23) suggested that when 2 canals are present
in the same root without the determination of the presence of an isthmus
via staining, the presence of an isthmus should be expected, and the area
between the 2 canals should always be prepared and sealed with a rootend filling material. Stropko (18) suggested that even when no isthmus
was observed on the resected surface, there was a good possibility that
one would be present immediately underneath it and raised the need
to always prepare an isthmus when there are 2 or more canals in
1 root because one could be exposed in the remodeling process of a
resected root once the healing process begins after surgery. However,
the present study found that the 4-year success rate was relatively high
(87.4% in the teeth with no isthmuses) and thus were unprepared. It
was also as high as 87.4% for the anterior teeth reported in previous
studies (14, 15). These findings suggest that no additional preparation
should be necessary for a type I isthmus with no observed
communication.
Within the limitations of this study, the success rate of isthmusabsent teeth was higher than that of the teeth that had an isthmus that
was prepared. Considering the success rate and potential risk of a
weakened root after preparation, the technique of isthmus retrograde
preparation needs to be improved. More attention and care are
required to prepare these entities, which occur in some form in
75% of the posterior teeth presenting for surgical treatment. The
development of an ultrasonic tip that is capable of minimizing the
weakening of the remaining dentin or a new method of root-end preparation will help to increase the success rate of endodontic microsurgery. Research on the anatomy of an isthmus for each tooth will make
it possible to examine the effects of an isthmus on surgical outcomes
in greater depth.
Kim et al.
The authors thank Ms Heekyung Lee at the Yonsei University
for her help with manuscript preparation.
Supported by the Basic Science Research Program through
the National Research Foundation of Korea (NRF) funded by the
Ministry of Education (grant no. 2015R1D1A1A09057552).
The authors deny any conflicts of interest related to this study.
References
CI, confidence interval; MTA, mineral trioxide aggregate.
1034
Acknowledgments
1. Weller RN, Niemczyk SP, Kim S. Incidence and position of the canal isthmus. Part 1.
Mesiobuccal root of the maxillary first molar. J Endod 1995;21:380–3.
2. Green D. Double canals in single roots. Oral Surg Oral Med Oral Pathol 1973;35:
689–96.
3. Vertucci FJ. Root canal anatomy of the human permanent teeth. Oral Surg Oral Med
Oral Pathol 1984;58:589–99.
4. Cleghorn BM, Christie WH, Dong CC. Root and root canal morphology of the human
permanent maxillary first molar: a literature review. J Endod 2006;32:813–21.
V, Estevez R, Peix Sanchez M, et al. Root anatomy and canal configuration
5. de Pablo O
of the permanent mandibular first molar: a systematic review. J Endod 2010;36:
1919–31.
6. von Arx T. Frequency and type of canal isthmuses in first molars detected by endoscopic inspection during periradicular surgery. Int Endod J 2005;38:160–8.
7. Johnson M, Sidow SJ, Looney SW, et al. Canal and isthmus debridement efficacy using a sonic irrigation technique in a closed-canal system. J Endod 2012;38:1265–8.
8. Lloyd A, Uhles JP, Clement DJ, Garcia-Godoy F. Elimination of intracanal tissue and
debris through a novel laser-activated system assessed using high-resolution microcomputed tomography: a pilot study. J Endod 2014;40:584–7.
9. Nair PN, Henry S, Cano V, Vera J. Microbial status of apical root canal system of human
mandibular first molars with primary apical periodontitis after ‘‘one-visit’’ endodontic
treatment. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005;99:231–52.
10. Kim S, Kratchman S. Modern endodontic surgery concepts and practice: a review.
J Endod 2006;32:601–23.
11. Carr GB. Ultrasonic root end preparation. Dent Clin North Am 1997;41:541–54.
12. Rubinstein RA, Kim S. Long-term follow-up of cases considered healed one year
after apical microsurgery. J Endod 2002;28:378–83.
13. Hsu YY, Kim S. The resected root surface. The issue of canal isthmuses. Dent Clin
North Am 1997;41:529–40.
14. Song M, Jung IY, Lee SJ, et al. Prognostic factors for clinical outcomes in endodontic
microsurgery: a retrospective study. J Endod 2011;37:927–33.
15. von Arx T, Jensen SS, Hanni S, Friedman S. Five-year longitudinal assessment of the
prognosis of apical microsurgery. J Endod 2012;38:570–9.
16. Kim E, Song JS, Jung IY, et al. Prospective clinical study evaluating endodontic
microsurgery outcomes for cases with lesions of endodontic origin compared
with cases with lesions of combined periodontal-endodontic origin. J Endod
2008;34:546–51.
17. Molven O, Halse A, Grung B. Observer strategy and the radiographic classification of
healing after endodontic surgery. Int J Oral Maxillofac Surg 1987;16:432–9.
18. Stropko J. Predictable apical microsurgery (part II). Roots 2012;2:8–18.
19. Lin CP, Chou HG, Kuo JC, Lan WH. The quality of ultrasonic root-end preparation: a
quantitative study. J Endod 1998;24:666–70.
20. Sathorn C, Palamara JE, Palamara D, Messer HH. Effect of root canal size and
external root surface morphology on fracture susceptibility and pattern: a finite
element analysis. J Endod 2005;31:288–92.
21. Degerness R, Bowles W. Anatomic determination of the mesiobuccal root resection
level in maxillary molars. J Endod 2008;34:1182–6.
22. Degerness RA, Bowles WR. Dimension, anatomy and morphology of the mesiobuccal root canal system in maxillary molars. J Endod 2010;36:985–9.
23. Teixeira FB, Sano CL, Gomes BP, et al. A preliminary in vitro study of the incidence
and position of th e root canal isthmus in maxillary and mandibular first molars. Int
Endod J 2003;36:276–80.
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