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Development and sequelae of canaltransportationEDGAR SCHAFER & TILL DAMMASCHKE
Instrumentation of curved root canals is still a challenge even for skilled and experienced operators. During shaping
of these canals, canal transportation, straightening, or canal deviation may occur. This paper describes different
preparation outcomes as possible results of canal transportation, and the action of different root canal instruments
when used in curved canals, and provides an overview of current in vitro and in vivo studies assessing canal
transportation. In the second part of this paper, the clinical consequences of canal transportation such as insufficient
cleaning and over-reduction of radicular dentin are addressed. Finally, based on currently available evidence, the
impact of canal transportation on the clinical outcome of root canal treatment is discussed.
Received 28 January 2008; accepted 19 May 2008.
Introduction
The primary aim of any preparation of the root canal
system is to enlarge the root canal space to facilitate
either disinfection by antibacterial agents or to prevent
(re-)infection through the placement of a fluid-tight
root canal filling in combination with a sufficient
coronal restoration. Despite recent advances in the field
of endodontic instruments and devices, the mechanical
preparation of a curved root canal is still a challenge
even for very skilled and experienced clinicians.
Different, well-described preparation errors may result
during the shaping of these curved root canals, such as
‘canal transportation,’ ‘straightening,’ or ‘deviation’
(1). As most root canals are curved (2), a high pre-
valence of preparation errors or canal aberrations has
been reported (3, 4).
What is canal transportation?
According to the Glossary of Endodontic Terms of the
American Association of Endodontists, canal transpor-
tation is defined as follows (5): ‘Removal of canal wall
structure on the outside curve in the apical half of the
canal due to the tendency of files to restore themselves
to their original linear shape during canal preparation;
may lead to ledge formation and possible perforation.’
As a result of this asymmetrical material removal during
shaping, the long axis of the curved root canal will be
displaced and the angle of curvature will decrease,
resulting in straightening of the original curvature of
the root canal (Fig. 1).
Independent of the alloy used, any root canal
instrument tends to straighten itself inside the root
canal (3, 6–8). Owing to the restoring forces, an
uneven force distribution of the cutting edges of the
instrument in certain contact areas along the root canal
wall results, leading to an asymmetrical dentin removal
(3, 8). In particular, the cutting edges are pressed
against the outer side of the curved canal (convexity) in
the apical third and against the inner side at the middle
or coronal thirds (concavity). As a result, apical canal
areas tend to be overprepared toward the convexity of
the canal, whereas more coronally greater amounts of
dentin will be removed at the concavity, leading to
canal transportation or straightening of varying de-
grees (3, 8) (Fig. 2).
The following undesirable apical preparation out-
comes have been described as possible results of canal
transportation (1, 4, 9–12):
� Damage to the apical foramen – Deviation from the
original curvature of the root canal or displacement
of the original long axis of the canal may result in a
loss of an apical stop (4). As a consequence, the
periapical tissues may be irritated by extruded
debris, irrigants, or filling materials.
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Endodontic Topics 2009, 15, 75–90All rights reserved
2009 r John Wiley & Sons A/S
ENDODONTIC TOPICS 20091601-1538
� Zip formation – A transported root canal with its
overpreparation along the outer aspect of the
curvature adopts an elliptical shape at the apical
endpoint. Similar terms describing the shape of a
zipped apical part of the root canal are an ‘hour-
glass shape,’ a ‘teardrop,’ or a ‘foraminal rip’ (1, 4,
8). Zipping of a root canal influenced the apical seal
negatively when these transported canals were
obturated in vitro by lateral compaction (11).
� Elbow formation – Narrow portion of the root
canal between the excessive material removal along
the outer aspect in the apical and the overenlarge-
ment of the inner aspect of the curvature more
coronally, which is usually localized at the point of
maximum curvature (Fig. 2). This preparation
outcome results in insufficient taper and flow and
may endanger proper cleaning and obturation of
the apical part of the root canal (1, 4).
� Perforation – A perforation of the apical part of the
root canal may result when instruments with sharp
cutting tips are used in a rotary working motion
(3, 4) and represent a communication between
the root canal space and the external root sur-
face, causing irritation of the periradicular tissues
(Fig. 3).
� Strip perforation – Whereas all the aforementioned
procedural errors are located at the apical part of the
curved root canal, a strip perforation may result
from overpreparation along the inner side of the
curvature in the middle or the coronal third of the
canal (4, 13). Strip perforations signify a commu-
nication between the root canal system and the
periodontal ligament, mainly found in mesial roots
of mandibular molars at the furcal aspect, which is
therefore termed the ‘danger zone.’
� Ledging – A ledge is located at the beginning or the
outer side of the curvature as a platform (4) when
the working length can no longer be reached and
the original path of the root canal has deviated.
Hence, ledge formations can be localized in the
middle or the apical part of root canals (Figs. 4 and
5). More details concerning ledge formations can
be found in an excellent review published recently
(12).
The following aspects are associated with an in-
creased risk of canal transportation:
� Insufficiently designed access cavities, as this leads
to an improper guiding of the instruments by the
walls of the cavity with a loss of control of the
instruments during root canal preparation (12). An
Fig. 1. Extracted human molar with a severely curved root canal. (a) Initial radiograph with a size 15 instrumentinserted; curvature 351. (b) Final radiograph after instrumentation with a size 40 file inserted; curvature 131. As a resultof instrumentation, the root canal was markedly transported, a loss of working length occurred, and the canal wasstraightened.
Schafer & Dammaschke
76
unrestricted access of the instruments to the apical
foramen minimizes the risk of canal transportation.
� Alloy (stainless-steel versus nickel–titanium) and
design features (cross-sectional design, number
of flutes, and rake angle) of root canal instruments
(6, 8, 14, 15).
� Use of instruments with sharp cutting tips (1, 10,
12, 16–19).
� Use of inflexible instruments in sizes above #20 in
severely curved root canals (7, 20).
� Forcing the instrument into the root canal (that is,
to avoid the use of a crown-down approach in
curved canals) (12).
� Instrumentation technique (step-down approaches
or balanced-force technique versus a step-back or a
standardized technique) (4, 7, 8, 10, 21–23).
� Insufficient irrigation during mechanical enlarge-
ment (12).
� Operator-related factors (e.g. experience) (3,
24–26).
� Degree of canal curvature and radius of the
curvature (Fig. 6) – In general, it can be stated
that the greater the degree of curvature and the
smaller the radius of curvature, the greater the risk
of canal transportation (3, 27–30). There is
evidence that root canals with a large angle and a
small radius of curvature can hardly be enlarged
without any transportation, independent of
whether rotary nickel–titanium or stainless-steel
hand instruments were used (10).
Fig. 2. Simulated canal after preparation with stainless-steel K-files showing the characteristic patterns of canaltransportation. Notice the remaining red color, indicat-ing insufficient material removal.
Fig. 3. Severe canal straightening resulting in apicalperforations.
Fig. 4. Ledge formation at the beginning of the curvatureassociated with a marked loss of working length.
Canal transportation: development & sequelae
77
� Radiographically unseen canal curvatures (31) –
These unseen curvatures can play a significant role
in the cleaning and shaping process because they
may account for loss of working length during
instrumentation. Also, the enlargement of a canal
with a proximal curvature can result in severe canal
transportation or even in strip perforation (9, 25).
Instrument action and developmentof transportation
Numerous studies have been performed on the
preparation of curved root canals, focusing on canal
transportation, straightening, or maintenance of the
original root canal curvature (4). The vast majority of
them used either simulated curved canals in plastic
blocks or extracted human teeth with defined canal
curvatures (4) (Table 1). Unfortunately, clinical studies
assessing the effects of different instruments or
instrumentation techniques are very scarce (Table 2).
Until now, only one report has described the impact of
canal transportation on the outcome of endodontic
therapy.
In vitro studies
Stainless-steel reamers, which should be used in a
reaming working motion, cause marked transportation
or straightening of the canal, especially in canals with
ovoid cross-sections (29, 53–55). Distinct transporta-
tion of curved canals was also a consistent finding for
stainless-steel K-files with a rotational cutting action in
combination with a longitudinal filing motion (20, 55–
58) (Fig. 7). Hedstroem files used in a linear filing
motion (7, 15, 29, 59) are associated with severe
straightening of the inner canal wall as well as excessive
material removal on the outer side of the curvature,
resulting in distinct transportation or even apical
perforations (20, 29, 60, 61) (Fig. 8). These undesir-
able effects can be explained by the metallic memory
Fig. 5. Ledge formation in the apical portion of themesial root canals.
Fig. 6. Characteristics of a curved root canal. a is theangle of curvature; r the radius of curvature; and M thecenter of the circle describing the curved part of the rootcanal.
Schafer & Dammaschke
78
Table 1. Root canal straightening after preparation of curved canals in vitro (extracted human molars)
Authors
Mean original
curvatures (1) Instruments
Straightening
degree (1)
Yang et al. (32) 27.8 � 8.6 ProTaper 1.20 � 0.74
29.2 � 9.4 Hero Shaper 0.74 � 0.56
Rodig et al. (33) 26.8 ProFile .04 0.7
27.0 GT Rotary 0.3
Burklein & Schafer (34) 30.71 � 3.36 Mtwo (torque-limited handpiece) 2.24 � 2.35
30.66 � 3.27 Mtwo (torque-control motor) 1.24 � 2.36
Schafer et al. (35) 30.20 � 3.26 Mtwo 1.22 � 1.32
30.21 � 3.33 K3 2.84 � 2.40
30.15 � 3.54 RaCe 2.59 � 2.32
Jodway & Hulsmann (36) 27.8 NiTi–TEE 0.2
27.7 K3 0.4
Guelzow et al. (37) 20.5 � 15.9 FlexMaster 0.9 � 0.9
20.2 � 16.1 System GT 0.5 � 0.7
20.4 � 17.1 HERO 642 0.6 � 0.7
21.7 � 19.3 K3 0.9 � 1.0
21.2 � 18.7 ProTaper 1.2 � 0.6
20.7 � 20.1 RaCe 0.7 � 0.9
16.6 � 13.7 Hand stainless-steel Reamer1Hedstroem files 0.7 � 0.9
Schafer et al. (38) 29.70 � 3.41 FlexMaster (torque-limited handpiece) 3.08 � 1.87
29.65 � 3.18 FlexMaster (torque-limited handpiece) 3.88 � 2.82
29.60 � 3.20 FlexMaster (torque-control motor) 2.17 � 1.59
Paque et al. (39) 28.0 RaCe 0.9
28.5 ProTaper 0.8
Atlas et al. (40) 24.50 � 8.24 Flex-R stainless-steel hand files 7.00 � 7.22
21.00 � 9.51 Onyx-R NiTi hand instruments 4.50 � 6.60
23.50 � 8.71 Nitiflex NiTi hand instruments 4.50 � 6.82
Schafer & Vlassis (41) 29.71 � 3.23 ProTaper 3.22 � 2.64
30.04 � 3.46 RaCe 1.72 � 1.74
Hulsmann et al. (42) 26.6 Lightspeed 1.8
26.7 Quantec SC 1.7
Canal transportation: development & sequelae
79
Table 1. Continued
Authors
Mean original
curvatures (1) Instruments
Straightening
degree (1)
Karagoz-Kucukay et al. (43) 28.15 � 5.36 Hero 642 (300 r.p.m.) 3.70
30.30 � 8.62 Hero 642 (400 r.p.m.) 2.95
29.55 � 5.05 Hero 642 (600 r.p.m.) 2.00
Hulsmann et al. (44) 27.1 FlexMaster 0.6
26.3 Hero 642 0.5
Schafer & Schlingemann (45) 29.57 � 2.84 K3 1.36 � 1.55
29.85 � 3.08 Stainless-steel K-Flexofile hand instruments 6.91 � 3.34
Schafer & Lohmann (46) 30.70 � 2.87 FlexMaster 2.14 � 2.42
30.60 � 2.24 Stainless-steel K-Flexofile hand instruments 7.31 � 2.90
Versumer et al. (47) 27.8 ProFile .04 0.2
28.4 Lightspeed 0.4
Hulsmann et al. (48) 26.7 Hero 642 1.6
25.6 Quantec SC 2.3
Hulsmann et al. (49) 28.4 Excalibur: Flexofiles 10.6
31.0 Stainless-steel hand instruments 8.5
‘Mean original curvatures’ are the canal curvatures assessed before instrumentation according to the method described bySchneider (50); ‘straightening degree’ was defined as the difference between canal curvature before and afterinstrumentation. Extremes are marked in bold.
Table 2. Clinical studies assessing root canal straightening
Authors
Mean original
curvatures (1) Instruments Straightening degree (1)
Hu et al. (51) 23.30 � 5.80 ProTaper 4.02 � 2.80
21.60 � 4.67 Hero 642 1.72
Schafer et al. (26) 23.31 � 8.70 FlexMaster 1.12 � 1.87
22.75 � 7.17 Stainless-steel hand instruments 3.36 � 4.34
Pettiette et al. (52) 28.62 Stainless-steel K-files hand instruments 14.44 � 10.33
30.09 NiTi hand instruments 4.39 � 4.53
‘Mean original curvatures’ are the canal curvatures assessed before instrumentation according to the method described bySchneider (50); ‘straightening degree’ was defined as the difference between canal curvature before and afterinstrumentation. Extremes are marked in bold.
Schafer & Dammaschke
80
and inherent inability of all but the smallest stainless-
steel instruments to maintain the original canal
curvature (7, 12, 55). Instruments, especially above
size 20, become inflexible, resulting in a noticeable
tendency to straighten themselves inside the curved
canals (Fig. 9). If the first instrument starts to overcut
dentin along the outer aspect of the curved canal, this
effect will be intensified with each subsequently used
larger instrument (20, 59).
Recently, several experiments assessed the effects of
tip modifications on canal preparation. Instruments
with a non-cutting tip, that is a reduction of the tip or
the transition angle and incorporation of a guiding
plane (12, 17–19) (Fig. 10), exert less canal transpor-
tation and lead to more material removal at the inner
curvature of curved root canals compared with similar
instruments having conventional tips (10, 14, 17–19,
62–64). There is clear evidence that instruments with
Fig. 7. Moderate canal straightening resulting in a loss ofworking length.
Fig. 8. Canal transportation with ledge formation andapical perforation; both aberrations are localized at theouter aspect of the relatively moderately curved root canals.
Fig. 9. Severely curved root canal. While the smallinstrument follows the original curvature well,enlargement with subsequent instruments with greaterdiameters might be associated with the risk of canaltransportation.
Fig. 10. Representative example of a non-cutting instru-ment tip.
Canal transportation: development & sequelae
81
modified tips are superior in maintaining the original
canal curvature, independent of whether stainless-steel
or nickel–titanium hand instruments or rotary nickel–
titanium instruments were used (3, 4, 12, 65). Never-
theless, even flexible stainless-steel instruments with
non-cutting tips do not produce entirely satisfactory
results in terms of adequately centered enlargement of
severely curved canals (13, 22, 57).
The introduction of the superelastic alloy nickel–
titanium has resulted in instruments that exert much
lower forces on the root canal walls compared with
stainless-steel instruments (4, 6). There is evidence that
nickel–titanium hand instruments cause less deviation
from the original curvature than their stainless-steel
counterparts when used in curved root canals (20, 66–
71). Moreover, the substantially increased flexibility of
nickel–titanium instruments compared with stainless-
steel instruments has made it possible to produce
instruments with tapers greater than the ISO standard,
which is a 2% taper (3, 4, 6, 72). These nickel–titanium
rotary instruments, nearly all of which have non-
cutting tips (6, 72), are particularly suitable for
preparing curved root canals with fewer procedural
aberrations (Fig. 11) than stainless-steel hand instru-
ments. In fact, the magnitude of absolute canal
transportation caused by these instruments is very
small (6, 41, 37) (Fig. 12). Differences in the centering
ability of rotary nickel–titanium versus stainless-steelFig. 11. Instrumented curved root canals with noradiographically visible canal transportation.
Fig. 12. Extracted human molar with a severely curved root canal. (a) Initial radiograph with a size 15 instrumentinserted; curvature 421. (b) Final radiograph after instrumentation with a size 40 nickel–titanium instrument inserted;curvature 351. As a result of instrumentation, the root canal was only slightly transported toward the outer aspect of thecurvature.
Schafer & Dammaschke
82
hand instruments are more pronounced when the final
apical preparation is larger than size 30 (6).
Even those rotary instruments with active cutting
edges do not negatively affect the maintenance of the
original curvature of the root canals (3, 8, 41).
Nevertheless, these instruments should be used with
some caution and should be withdrawn as soon as the
apical endpoint of instrumentation is reached. Other-
wise, an overpreparation might result with the risk of
apical canal transportation (8, 72). Also, apical prepara-
tion with rotary instruments having tapers 40.04
should be avoided as canal transportation toward the
outer aspect of the curvature might result (73).
Clinical studies
On a qualifying note, evidence from clinical studies is
currently scarce (Table 2) and therefore conclusions
should be drawn with some caution.
A total of 520 root canals with various degrees of
curvature were treated by supervised dental students
using either a reaming or a filing technique with
stainless-steel Reamers and Hedstroem files, respec-
tively (74). Canal straightening was more pronounced
in the root canals prepared according to the reaming
technique, whereas perforations in the apical part of
the root canals were more often found with the filing
technique (74).
More recently, senior students prepared a total of 221
root canals in patients either by hand with stainless-
steel K-files using the step-back technique or by rotary
instrumentation with nickel–titanium files. While the
incidence of canal transportation was low for the rotary
nickel–titanium instruments (3%), 20% of the canals
were transported when using stainless-steel instru-
ments with the step-back technique (25).
In a further clinical study, 60 molar teeth in patients
were treated by inexperienced dental students using
nickel–titanium 0.02 taper and stainless-steel K-files
hand instruments. All root canals were enlarged
according to the step-back technique, and canal
straightening was defined as the difference between
pre-operative and post-operative canal curvature (52).
Significantly less canal straightening (Po0.001) was
caused by nickel–titanium than by stainless-steel
instruments.
Subsequently, the same group of researchers pub-
lished a further investigation (75) comparing the 1-
year success rates of the same teeth used in the
aforementioned study. The comparison of success rate
was based primarily on radiographic interpretation of
the periapical status of the teeth. A significantly better
success rate for teeth prepared with nickel–titanium
than with stainless-steel K-files was observed
(Po0.03). The authors concluded that maintaining
the original canal shape leads to a better prognosis for
the endodontic treatment (75).
A recently published study compared the canal
straightening of teeth prepared by dentists experienced
in endodontics using different hand instruments and
the rotary nickel–titanium FlexMaster system (VDW,
Munich, Germany) (26). Hand instruments produced
a more pronounced straightening of the curved canals
than the FlexMaster files. Overall, the original curva-
ture of the root canals was significantly better main-
tained with automated FlexMaster files than with hand
instruments. The authors hypothesized that the hand
instrumentation left the possibility of the canal space
being inadequately debrided of vital or necrotic pulp
tissue, which might result in an inadequate obturation
of the root canal space (26).
Clinical consequences of canaltransportation: insufficient cleaning
A consecutive clinical problem of canal transportation
is that a part of the original root canal will remain
unprepared and thereby insufficiently cleaned (4) (Fig.
13). Post-operative canal cleanliness in extracted
human teeth has been investigated histologically and
under the SEM using longitudinal and horizontal
sections (4). More recently, three-dimensional analyses
using micro-computed tomography were used for more
accurate evaluation of post-operative canal shapes (3).
Analyses of cross-sections indicate that usually 60–
80% of the root canal outlines were touched by root
canal instruments during the instrumentation proce-
dure (76, 77). This is in agreement with results
obtained from longitudinal sections showing that
neither instrumentation technique was totally effective
in cleaning the entire root canal space (33, 35, 39, 42,
44–49, 78, 79). After instrumentation of curved root
canals in extracted teeth, both rotary nickel–titanium
instruments and stainless-steel hand instruments left
uninstrumented canal areas with debris remaining
behind, especially in the apical third of the root canal
(33, 35, 39, 44–46, 48). These observations were
Canal transportation: development & sequelae
83
supported by micro-CT data. Both stainless-steel K-
files and modern rotary nickel–titanium instruments
left about 35% of the canal wall surface untouched
(46). In general, the amount of prepared root canal
surface seemed to be independent of the instrument
type used but was significantly affected by pre-
operative canal anatomy (80–82).
Keeping these data in mind, it becomes evident that,
when treating an infected root canal, transportation
may further increase the amount of residual infected
debris in the uninstrumented portion of the root canal
(Fig. 14). Canal transportation will jeopardize the
ability of the subsequent instruments with greater
diameters to clean the inner aspect of the apical
curvature and the outer aspect of the middle part of
the canal, especially when already established during
the initial enlargement of the root canal when the first
instruments are inserted to the full working length.
Thus, canal transportation may result in insufficient
cleaning because of the inability to eliminate intra-
radicular micro-organisms from the infected root canal
systems. In particular, canal sections opposite to the
regions, which in the case of canal transportation are
characterized by typically overreduction of dentin, may
harbor great amounts of residual debris.
In summary, it remains questionable whether canal
transportation might be the direct cause of treatment
failure. Rather, in infected root canals, tissue remnants
and remaining micro-organisms might impede the
outcome.
Clinical consequences of canaltransportation: overreduction ofradicular dentin
Canal transportation is defined as any undesirable
deviation from the natural canal path (3). In curved
canals, the outside (convex) wall of the apical portion
may be overinstrumented and healthy dentin may be
unnecessarily removed. The inside (concave) wall may
be untouched and infected dentin may remain (83).
This can result in inadequately cleaned root canals,
with the possible outcome of persistent apical lesions,
but also in strip perforations of the lateral root canal
Fig. 14. Scanning electron microscopic photographs of instrumented root canals showing insufficient cleaning oftransported root canals. Although one side of the root canals seems to be adequately cleaned with only minimal debrisadjacent to the walls, the opposite sides are totally covered with huge amounts of debris. Owing to canal transportation,only the cleaned canal walls were touched by the root canal instruments whereas the opposite sides were not, resulting ininsufficient canal cleanliness.
Fig. 13. Owing to canal transportation, the apical part ofthe root canal remains unprepared, harboring hugeamounts of tissue remnants. From Hulsmann M, SchaferE. Probleme in der Endodontie. Berlin: Quintessenz, 2007.Reprinted with permission.
Schafer & Dammaschke
84
wall and, due to the unnecessary removal of healthy
dentin, in weakening of the entire root (3, 24).
Fracture resistance
It is generally accepted that the strength of an
endodontically treated tooth is directly related to the
amount of remaining sound tooth structure (84–88).
With regard to fracture resistance, intact roots are
significantly stronger than instrumented roots. Instru-
mentation alone can significantly weaken the roots
(89). Also, roots enlarged using instruments with a
0.12 taper are weaker than those prepared with hand
instruments (taper 0.02). Zandbiglari et al. (90) found
an in vitro loss of fracture resistance of 25% after hand
instrumentation (taper 0.02), 22% after enlargement
with FlexMaster (taper 0.02–0.06), and 43.7% after
preparation with GT files (taper 0.04–0.12) compared
with intact control teeth. Thus, roots enlarged with
instruments with greater tapers were more prone to
fracture than those prepared with instruments with
smaller tapers. The reason for the lower fracture
resistance of roots prepared with a greater taper is
probably that more dentin is removed. The amount of
remaining dentin thickness significantly affects the
resistance to fracture of prepared root canals (86).
Root canals unquestionably need to be opened
coronally to a certain extent and the entire root canal
should be adequately tapered (conical shape) in order to
allow effective irrigation and to facilitate obturation, but
overenlargement of the root canal must be avoided to
prevent unnecessary weakening of the root (91).
Residual dentin width should be preserved as much as
possible so as not to compromise tooth integrity (88).
Although scientific evidence is lacking, common
sense suggests that severe canal transportation asso-
ciated with an overreduction of sound dentin along the
inner aspect in the middle part and along the outer
aspect in the apical part of the root canal may result in a
marked reduction of the fracture resistance of enlarged
root canals.
Strip perforation
Overpreparation of curved root canals along the inner
side of the curvature can lead to strip perforation in the
middle or the coronal part of the root canal (13, 92,
93). As long as straight root canals are treated, the
incidence of perforation seems rather low. But as soon
as curved canals are instrumented, the incidence of
straightening and other procedural mishaps increases,
especially if stainless-steel files are used (52). The
authors compared the effects of stainless-steel and
nickel–titanium hand instruments by students on the
extent of straightening and on other endodontic
procedural errors in vivo. In 10% of the root canals
enlarged with stainless-steel K-files, strip perforations
occurred, whereas none were observed with nickel–
titanium instruments (52).
In a retrospective study, Eleftheriadis and Lambria-
nidis examined the treatment outcome after in vivo
root canal preparation by students and found root
perforations in 2.7% of the cases. The root canal curva-
ture was found to be the only statistically significant
factor associated with root perforations (24).
Ouzounian and Schilder sectioned the distal roots of
20 endodontically treated mandibular molars at 3, 5,
and 7 mm from the apex and found the average furcal
side dentin thickness to be o0.8 mm at all three levels
(94). Kuttler et al. examined the residual dentin
thickness in distal roots of in vitro mandibular molars
after endodontic treatment. In 82% of the cases, the
canal wall on the furcal side was thinner than 1 mm and
in 17.5%, thinner than 0.5 mm, with an average of
0.708 mm (95). It was recommended that the residual
dentin thickness after post-space preparation should be
a minimum of 1 mm (96). Ouzounian and Schilder
(94) as well as Kuttler et al. (95) showed that after sole
root canal preparation (and before post-space prepara-
tion), residual dentin thickness can already be con-
siderably less than 1 mm. Moreover, Berutti and Fedon
compared the thickness of dentin–cementum on
sections and radiographs and found that the amount
of hard tissue was effectively about one-fifth less than
that appearing on the radiograph (97). This again
emphasizes the risk of root weakening and strip
perforation during mechanical root canal preparation.
Clinical outcome
Although it is widely assumed that several aspects of
canal preparation (such as the final preparation size,
maintenance of the original canal curvature, and
incidence of procedural errors) have a huge impact
on the outcome of the endodontic therapy (3, 98),
especially when treating infected root canals (94),
little evidence is currently available supporting this
Canal transportation: development & sequelae
85
assumption. Of note, all laboratory and most clinical
studies investigated the technical quality of root canal
preparation and this might not necessarily result in a
better prognosis for the teeth prepared without canal
transportation.
Concerning the final apical preparation size, conflict-
ing results have been reported (4, 95, 96). Recently,
Ørstavik et al. (99) performed a multivariate analysis on
treatment variables that may influence the outcome.
Only a few variables affected the outcome, but apical
extension showed no impact on treatment outcome in
any group. The variables of significant impact were the
age of the patient, periapical status, marginal support,
overinstrumentation, apex-to-filling distance, and root
filling density (99). Friedman (100) has pointed out
that the inability to demonstrate differences in the
prognosis with regard to apical preparation size may be
due to the fact that both alternatives (extensive versus
minimal enlargement of the apical part of the root
canal) are associated with certain problems. Wider
apical preparation might result in some canal straigh-
tening and undesirable weakening of the tooth
structure, whereas minimal enlargement may leave
tissue remnants and infected dentin behind (3, 100).
Thus, both alternatives have a certain risk of compro-
mising the outcome to some extent. Nevertheless, it
should be noted that most rotary nickel–titanium
instruments allow wider apical preparations (even in
severely curved root canals) than stainless-steel hand
instruments without major preparation errors or over-
reduction of radicular dentin (3, 4, 26). However,
whether these properties automatically result in
improved disinfection and thereby an enhanced prog-
nosis currently remains an open question.
In a retrospective clinical study, 268 teeth with 661
root canals were instrumented with three different
rotary nickel–titanium instruments, obturated, and
radiographically assessed after a mean observation time
of 27.1 months. No differences regarding the treat-
ment outcome between the different instruments were
observed (101). A radiographic and histopathologic
evaluation of the effect of rotary nickel–titanium and
manual stainless-steel K-files in dogs failed to show any
significant impact of the instrumentation technique on
the healing of the experimentally induced periapical
lesions after an observation period of up to 12 days
(102).
In contrast, two clinical studies indicate that prog-
nosis may depend on the instrumentation technique
(75, 103). In the first one, using stainless-steel
instruments, treatment outcome was superior using
the standardized compared with the serial technique
(103). The second study used a prospective cross-over
design to evaluate root canal treatment performed by
undergraduate students. While the first part of this
investigation (52) showed that canal curvatures were
better maintained with nickel–titanium hand instru-
ments than with stainless-steel ones, the second part
(75) demonstrated a significantly better success rate for
teeth prepared with nickel–titanium instruments at the
1-year recall. Hence, better maintenance of the original
canal curvature was associated with a better prognosis.
Moreover, there is some evidence gained from
laboratory studies that canal transportation is corre-
lated with apical leakage. To some extent, obturated
root canals showing transportation were associated
with leakage, whereas canals with only minimal or no
transportation were not (11, 104).
In summary, it can be concluded that canal trans-
portation seems not to be the direct cause of reduced
treatment outcome or even failure (98). Rather, this
procedural error increases the risk of leaving behind
infected tissue in the uninstrumented canal areas (12,
98). It seems reasonable that canal transportation is
correlated with a huge amount of debris and micro-
organisms left in the apical portion of the root canal
(12), especially when formed early in the shaping
procedure. For instance, the first instruments of the
preparation sequence create an overpreparation of the
outer aspect of the curved root canal, leaving unin-
strumented areas behind at the opposite side of the
root canal (Fig. 14).
Conclusion
Canal transportation is an inherent problem when
enlarging curved root canals, irrespective of the
instrumentation technique or the type of instrument.
In principle, canal transportation may result in (i)
inadequately cleaned root canals, harboring debris and
residual micro-organisms, (ii) overreduction of sound
dentin with the possible outcome of reduced fracture
resistance, and (iii) destruction of the integrity of the
root, that is, an apical or strip perforation. Currently,
evidence on the impact of these consequences of canal
transportation on the clinical outcome is lacking. Also,
at present, no evidence links reduced incidence of canal
Schafer & Dammaschke
86
transportation through nickel–titanium instruments
with superior clinical success rates.
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