endo mishaps 1

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.

75

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.


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


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.


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.


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.


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


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.


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


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


86

transportation through nickel–titanium instruments

with superior clinical success rates.

References

1. Weine F, Kelly R, Lio P. The effect of preparationprocedures on original canal shape and on apicalforamen shape. J Endod 1975: 1: 262–266.

2. Schafer E, Diez C, Hoppe W, Tepel J. Roentgeno-graphic investigation of frequency and degree of canalcurvatures in human permanent teeth. J Endod 2002:28: 211–216.

3. Peters OA. Current challenges and concepts in thepreparation of root canal systems: a review. J Endod2004: 30: 559–567.

4. Hulsmann M, Peters OA, Dummer PMH. Mechanicalpreparation of root canals: shaping goals, techniquesand means. Endod Topics 2005: 10: 30–76.

5. American Association of Endodontists. Glossary ofEndodontic Terms, 7th edn. Chicago: AAE, 2003.

6. Bergmans L, Van Cleynenbreugel J, Wevers M,Lambrechts P. Mechanical root canal preparation withNiTi rotary instruments: rationale, performance andsafety. Status report for the American Journal ofDentistry. Am J Dent 2001: 14: 324–333.

7. Saunders EM. Hand instrumentation in root canalpreparation. Endod Topics 2005: 10: 163–167.

8. Young GR, Parashos P, Messer HH. The principles oftechniques for cleaning root canals. Aust Dent J Endod2007: 52(Suppl): S52–S63.

9. Thompson SA, Dummer PMH. Shaping ability ofQuantec Series 2000 rotary nickel–titanium instru-ments in simulated root canals. Parts 1 and 2. IntEndod J 1998: 31: 259–274.

10. Dummer PMH, Al-Omari MAO, Bryant S. Com-parison of the performance of four files with roundedtips during shaping of simulated root canals. J Endod1998: 24: 364–371.

11. Wu MK, Fan B, Wesselink PR. Leakage along apicalroot fillings in curved root canals. Part 1: effects ofapical transportation on seal of root fillings. J Endod2000: 26: 210–216.

12. Jafarzadeh H, Abbott PV. Ledge formation: review ofa great challenge in endodontics. J Endod 2007: 33:1155–1162.

13. Abou-Rass M, Frank AL, Glick DH. The anti-curvature filing method to prepare the curved rootcanal. J Am Dent Assoc 1980: 101: 792–794.

14. Schafer E. Relationship between design features ofendodontic instruments and their properties. Part 2:instrumentation of curved canals. J Endod 1999: 25:56–59.

15. Schafer E. Root canal instruments for manual use: areview. Endod Dent Traumatol 1997: 13: 51–64.

16. Miserendino LJ, Moser JB, Heuer MA, Osetek EM.Cutting efficiency of endodontic instruments. Part 1:a quantitative comparison of the tip and fluted regions.J Endod 1985: 11: 435–441.

17. Sabala CL, Roane JB, Southard LZ. Instrumentation

of curved canals using a modified tipped instrument: acomparison study. J Endod 1988: 14: 59–64.

18. Powell SE, Simon JHS, Maze BB. A comparison of the

effect of modified and non-modified instrument tipson apical canal configuration. Part I. J Endod 1986:12: 293–300.

19. Powell SE, Wong PD, Simon JHS. A comparison ofthe effect of modified and non-modified instrumenttips on apical canal configuration. Part II. J Endod1988: 14: 224–228.

20. Lam TV, Lewis DJ, Atkins DR, Macfarlane RH,

Clarkson RM, Whitehead MG, Brockhurst PJ, MouleAJ. Changes in root canal morphology in simulatedcurved canals over-instrumented with a variety of

stainless steel and nickel titanium files. Aust Dent J1999: 44: 12–19.

21. Al-Omari MA, Dummer PM. Canal blockage and

debris extrusion with eight preparation techniques. JEndod 1995: 21: 154–158.

22. Schafer E. Effect of four instrumentation techniques

on curved canals: a comparison study. J Endod 1996:22: 685–690.

23. Goerig AC, Michelich RJ, Schultz HH. Instrumenta-tion of root canals in molar using the step-downtechnique. J Endod 1982: 8: 550–554.

24. Eleftheriadis GI, Lambrianidis TP. Technical quality ofroot canal treatment and detection of iatrogenic errorsin an undergraduate dental clinic. Int Endod J 2005:

38: 725–734.25. Kfir A, Rosenberg E, Zuckerman O, Tamse A, Fuss Z.

Comparison of procedural errors resulting during root

canal preparations completed by senior dental stu-dents in patients using an ‘8-step method’ versus

‘serial step-back technique’. Oral Surg Oral Med OralPathol Oral Radiol Endod 2004: 97: 745–748.

26. Schafer E, Schulz-Bongart U, Tulus G. Comparison

of hand stainless steel and nickel titanium rotaryinstrumentation: a clinical study. J Endod 2004: 30:432–435.

27. Ruddle C. Cleaning and shaping the root canalsystem. In: Cohen S, Burns RC, eds. Pathways of thePulp, 8th edn. St Louis: Mosby, 2002: 231–292.

28. Alodeh MHA, Doller R, Dummer PMH. Shaping ofsimulated root canals in resin blocks using the step-back

technique with K-files manipulated in a simple in/outfilling motion. Int Endod J 1989: 22: 107–117.

29. Alodeh MHA, Dummer PMH. A comparison of the

ability of K-files and Hedstrom files to shape simulatedroot canals in resin blocks. Int Endod J 1989: 22: 226–235.

30. Greene KJ, Krell KV. Clinical factors associated withledged canals in maxillary and mandibular molars.

Oral Surg Oral Med Oral Pathol 1990: 70: 490–497.31. Cunningham CJ, Senia ES. A three-dimensional study

of canal curvatures in the mesial roots of mandibular

molars. J Endod 1992: 18: 294–300.32. Yang GB, Zhou XD, Zheng YL, Zhang H, Shu Y, Wu

HK. Shaping ability of progressive versus constant


87

taper instruments in curved root canals of extracted

teeth. Int Endod J 2007: 40: 707–714.33. Rodig T, Hulsmann M, Kahlmeier C. Comparison of

root canal preparation with two rotary NiTi instru-ments: ProFile 0.04 and GT Rotary. Int Endod J 2007:

40: 553–562.34. Burklein S, Schafer E. The influence of various

automated devices on the shaping ability of Mtwo

rotary nickel–titanium instruments. Int Endod J 2006:39: 945–951.

35. Schafer E, Erler M, Dammaschke T. Comparativestudy on the shaping ability and cleaning efficiency

of rotary Mtwo instruments. Part 2: cleaning effec-tiveness and shaping ability in severely curved root

canals of extracted teeth. Int Endod J 2006: 39:203–212.

36. Jodway B, Hulsmann M. A comparative study of rootcanal preparation with NiTi–TEE and K3 rotary NiTi

instruments. Int Endod J 2006: 39: 71–80.37. Guelzow A, Stamm O, Martus P, Kielbassa AM.

Comparative study of six rotary nickel–titanium

systems and hand instrumentation for root canalpreparation. Int Endod J 2005: 38: 743–752.

38. Schafer E, Erler M, Dammaschke T. Influence ofdifferent types of automated devises on the shaping

ability of rotary nickel–titanium FlexMaster instru-ments. Int Endod J 2005: 38: 627–636.

39. Paque F, Musch U, Hulsmann M. Comparison of root

canal preparation using RaCe and ProTaper rotary Ni–Ti instruments. Int Endod J 2005: 38: 8–16.

40. Atlas DM, Leonardi L, Raiden G. Shaping of curvedroot canals instrumented with stainless steel and

nickel titanium files. Acta Odontol Latinoam 2005:18: 23–30.

41. Schafer E, Vlassis M. Comparative investigation of tworotary nickel–titanium instruments: ProTaper versusRaCe. Part 2: cleaning effectiveness and shaping

ability in severely curved root canals of extractedteeth. Int Endod J 2004: 37: 239–248.

42. Hulsmann M, Herbst U, Schafers F. Comparativestudy of root-canal preparation using Lightspeed and

Quantec SC rotary NiTi instruments. Int Endod J2003: 36: 748–756.

43. Karagoz-Kucukay I, Ersev H, Engin-Akkoca E,Kucukay S, Gursoy T. Effect of rotational speed on

root canal preparation with Hero 642 rotary Ni–Tiinstruments. J Endod 2003: 29: 447–449.

44. Hulsmann M, Gressmann G, Schafers F. A compara-

tive study of root canal preparation using FlexMasterand HERO 642 rotary Ni–Ti instruments. Int Endod J2003: 36: 358–366.

45. Schafer E, Schlingemann R. Efficiency of rotary

nickel–titanium K3 instruments compared with stain-less steel hand K-Flexofile. Part 2: cleaning effective-

ness and shaping ability in severely curved root canalsof extracted teeth. Int Endod J 2003: 36: 208–217.

46. Schafer E, Lohmann D. Efficiency of rotary nickel–

titanium FlexMaster instruments compared withstainless steel hand K-Flexofile. Part 2: cleaning

effectiveness and instrumentation results in severely

curved root canals of extracted teeth. Int Endod J2002: 35: 514–521.

47. Versumer J, Hulsmann M, Schafers E. A comparative

study of root canal preparation using Profile 0.04 andLightspeed rotary Ni–Ti instruments. Int Endod J2002: 35: 37–46.

48. Hulsmann M, Schade M, Schafers F. A comparativestudy of root canal preparation with HERO 642 andQuantec SC rotary Ni–Ti instruments. Int Endod J2001: 34: 538–546.

49. Hulsmann M, Gambal A, Bahr R. An evaluation of

root canal preparation with the automated Excaliburendodontic handpiece. Clin Oral Investig 1999: 3:70–78.

50. Schneider SW. A comparison of canal preparations instraight and curved root canals. Oral Surg Oral MedOral Pathol 1971: 32: 271–275.

51. Hu XL, Ling JQ, Chen H, Gu HJ. Clinical evaluationof two types of NiTi rotary instruments in preparationof root canals. Zhonghua Kou Qiang Yi Xue Za Zhi2005: 40: 30–33.

52. Pettiette MT, Metzger Z, Phillips C, Trope M.

Endodontic complications of root canal therapyperformed by dental students with stainless-steel K-files and nickel–titanium hand files. J Endod 1999: 25:230–234.

53. Schilder H. Cleaning and shaping the root canal. DentClin North Am 1974: 18: 269–296.

54. Gutierrez JH, Garcia J. Microscopic and macroscopicinvestigation on results of mechanical preparation ofroot canals. Oral Surg Oral Med Oral Pathol 1968:

25: 108–116.55. Schafer E, Tepel J, Hoppe W. Properties of endodon-

tic hand instruments used in rotary motion. Part 2:instrumentation of curved canals. J Endod 1995: 21:493–497.

56. Briseno B, Sonnabend E, Detzer F. Effect of differenthand instruments on the shape of the root canal. DtschZahnarztl Z 1989: 44: 446–448.

57. Briseno BM, Sonnabend E. The influence of differentroot canal instruments on root canal preparation: an invitro study. Int Endod J 1991: 24: 15–23.

58. Briseno BM. Einflu� verschiedener Wurzelkanalin-strumente bzw. Aufbereitungstechniken auf die Pra-

paration gekrummter Wurzelkanale. Endodontie1992: 1: 279–290.

59. Szep S, Gerhardt T, Leitzbach C, Luder W, Heide-

mann D. Preparation of severely curved simulatedroot canals using engine-driven rotary and conven-tional hand instruments. Clin Oral Investig 2001: 5:17–25.

60. Schafer E, Zapke K. A comparative scanning electronmicroscopic investigation of the efficacy of manual and

automated instrumentation of root canals. J Endod2000: 26: 660–664.

61. Schafer E, Tepel J. Cutting efficiency of Hedstrom, Sand U files made of various alloys in filing motion. IntEndod J 1996: 29: 302–308.


88

62. Ponce de Leon Del Bello T, Wang N, Roane JB.

Crown-down tip design and shaping. J Endod 2003:29: 513–518.

63. Zmener O, Marrero G. Effectiveness of different

endodontic files for preparing curved root canals: ascanning electron microscopic study. Endod DentTraumatol 1992: 8: 99–103.

64. Saunders WP, Saunders EM. Comparison of threeinstruments in the preparation of the curved root canalusing the modified double-flared technique. J Endod1994: 20: 440–444.

65. Kuhn WG, Carnes DL, Clement DJ, Walker WA. Effect

of tip design of nickel–titanium and stainless steel fileson root canal preparation. J Endod 1997: 23: 735–738.

66. Garip Y, Gunday M. The use of computed tomogra-

phy when comparing nickel–titanium and stainlesssteel files during preparation of simulated curvedcanals. Int Endod J 2001: 34: 452–457.

67. Coleman CL, Svec TA. Analysis of Ni–Ti versusstainless steel instrumentation in resin simulatedcanals. J Endod 1997: 23: 232–235.

68. Bishop K, Dummer PM. A comparison of stainlesssteel Flexofiles and nickel–titanium NiTiFlex files

during the shaping of simulated canals. Int Endod J1997: 30: 25–34.

69. Himel VT, Ahmed KM, Wood DM, Alhadainy HA.

An evaluation of nitinol and stainless steel files used bydental students during a laboratory proficiency exam.Oral Surg Oral Med Oral Pathol Oral Radiol Endod1995: 79: 232–237.

70. Carvalho LA, Bonetti I, Borges MA. A comparisonof molar root canal preparation using stainless-steel

and nickel–titanium instruments. J Endod 1999: 25:807–810.

71. Song YL, Bian Z, Fan B, Fan MW, Gutmann JL, PengB. A comparison of instrument-centering ability withinthe root canal for three contemporary instrumentation

techniques. Int Endod J 2004: 37: 265–271.72. Vaudt J, Bitter K, Kielbassa AM. Evaluation of rotary

root canal instruments in vitro: a review. ENDO(London) 2007: 1: 189–203.

73. Thompson SA, Dummer PM. Shaping ability of Hero642 rotary nickel–titanium instruments in simulated

root canals: Part 2. Int Endod J 2000: 33: 255–261.74. Stadler LE, Wennberg A, Olgart L. Instrumentation

of the curved root canal using filing or reamingtechnique – a clinical study of technical complications.Swed Dent J 1986: 10: 37–43.

75. Pettiette MT, Delano EO, Trope M. Evaluation ofsuccess rate of endodontic treatment performed bystudents with stainless-steel K-files and nickel–tita-

nium hand files. J Endod 2001: 27: 124–127.76. Tan BT, Messer HH. The quality of apical canal

preparation using hand and rotary instruments with

specific criteria for enlargement based on initial apicalfile size. J Endod 2002: 28: 658–664.

77. Weiger R, El Ayouti A, Lost C. Efficiency of hand androtary instruments in shaping oval root canals. J Endod2002: 28: 580–583.

78. Hulsmann M, Gambal A, Bahr R. An improved

technique for the evaluation of root canal preparation.J Endod 1999: 25: 599–602.

79. Hulsmann M, Stryga F. Comparison of root canal pre-

paration using different automated devices and handinstrumentation. J Endod 1993: 19: 141–145.

80. Peters OA, Schonenberger K, Laib A. Effects of fourNi–Ti preparation techniques on root canal geometry

assessed by micro computed tomography. Int Endod J2001: 34: 221–230.

81. Peters OA, Peters CI, Schonenberger K, Barbakow F.

ProTaper rotary root canal preparation: assessment oftorque and force in relation to canal anatomy. IntEndod J 2003: 36: 93–99.

82. Hubscher W, Barbakow F, Peters OA. Root canal

preparation with FlexMaster: canal shapes analysed bymicro-computed tomography. Int Endod J 2003: 36:740–747.

83. Zuolo ML, Walton RE, Imura N. Histologic evaluationof three endodontic instrument/preparation tech-

niques. Endod Dent Traumatol 1992: 8: 125–129.84. Trabert KC, Caput AA, Abou-Rass M. Tooth fracture:

a comparison of endodontic and restorative treat-ments. J Endod 1978: 4: 341–345.

85. Gutmann JL. The dentin–root complex: anatomic andbiologic considerations in restoring endodontically

treated teeth. J Prosthet Dent 1992: 67: 458–467.86. Abuzeid ST, Ezzat KM, Seef RE, Mohsen MM.

Comparative study of two filling techniques using

glass-ionomer (Ketac-endo) root canal cement onfracture resistance of endodontically treated roots (invitro). Egypt Dent J 1995: 41: 1367–1372.

87. Sornkul E, Stannard JG. Strength of roots before and

after endodontic treatment and restoration. J Endod1992: 18: 440–443.

88. Pilo R, Corcino G, Tamse A. Residual dentin thickness

in mandibular premolars prepared with hand androtatory instruments. J Endod 1998: 24: 401–404.

89. Trope M, Ray HL. Resistance to fracture of endo-dontically treated roots. Oral Surg Oral Med OralPathol 1992: 73: 99–102.

90. Zandbiglari T, Davids H, Schafer E. Influence ofinstrument taper on the resistance to fracture en-

dodontically treated roots. Oral Surg Oral Med OralPathol 2006: 101: 126–131.

91. Wu MK, van der Sluis LWM, Wesselink PR. Com-parison of mandibular premolars and canines with

respect to their resistance to vertical root fracture.J Dent 2004: 32: 265–268.

92. Kessler JR, Peters DD, Lorton L. Comparison of the

relative risk of molar root perforation using variousendodontic instrumentation techniques. J Endod1983: 9: 439–447.

93. Lim SS, Stock CJR. The risk of perforation in the

curved canal: anticurvature filling compared with thestepback technique. Int Endod J 1987: 20: 33–39.

94. Ouzouian ZS, Schilder H. Remaining dentin thick-

ness after endodontic cleaning and shaping beforepost space preparation. Oral Health 1991: 81: 13–15.


89

95. Kuttler S, McLean A, Dorn S, Fischzang A. The impact

of post space preparation with Gates-Glidden drills on

residual dentin thickness in distal roots of mandibularmolars. J Am Dent Assoc 2004: 135: 903–909.

96. Caputo AA, Standlee JP. Pins and posts: why, whenand how. Dent Clin North Am 1976: 20: 299–311.

97. Berutti E, Fedon G. Thickness of cementum/dentinin mesial roots of mandibular first molars. J Endod1992: 18: 545–548.

98. Lin LM, Rosenberg PA, Lin J. Do procedural errors

cause endodontic treatment failure? J Am Dent Assoc2005: 136: 187–193.

99. Ørstavik D, Qvist V, Stoltze K. A multivariate analysis

of the outcome of endodontic treatment. Eur J OralSci 2004: 112: 224–230.

100. Friedman S. Prognosis of initial endodontic therapy.Endod Topics 2002: 2: 59–88.

101. Peters OA, Barbakow F, Peters CI. An analysis ofendodontic treatment with three nickel–titaniumrotary root canal preparation techniques. Int EndodJ 2004: 37: 849–859.

102. De Rossi A, Silva LAB, Leonardo MR, Rocha LB,Rossi MA. Effect of rotary or manual instrumentation,with or without calcium hydroxide/1% chlorhexidineintracanal dressing, on the healing of experimentallyinduced chronic periapical lesions. Oral Surg OralMed Oral Pathol Oral Radiol Endod 2005: 99:628–636.

103. Kerekes K, Tronstad L. Long-term results of endo-dontic treatment performed with a standardizedtechnique. J Endod 1979: 5: 83–90.

104. Fan B, Wu MK, Wesselink P. Leakage along warmgutta-percha fillings in the apical canals of curvedroots. Endod Dent Traumatol 2000: 16: 29–33.


90

endo mishaps 1

Documents