Canal-centering ability: An endodontic challenge.

During instrumentation of the root canal, it is important to develop a continuously tapered form and to maintain the original shape and position of the apical foramen. However, the presence of curvatures may cause difficulty in root canal instrumentation. The ability to keep the instruments centered is essential to provide a correct enlargement, without excessive weakening of the root structure. Several studies have shown that Ni-Ti instruments remain significantly more centered and demonstrated less canal transportation than stainless steel files. Considerable research has been undertaken to understand the several factors related to an instrument's canal-centering ability. In this article, we have discussed the influence of various parameters such as alloys used in the manufacture of instruments, instrument cross-section, taper, and have given tips on canal-centering ability.

INTRODUCTION

Canal-shaping is a critical aspect of endodontic treatment because it influences the outcome of the subsequent phases of canal irrigation, filling, and the overall success of the treatment itself. Root canal therapy involves the use of instruments and irrigants to shape and chemomechanically prepare the root canal system to receive a three-dimensional filling of the entire root canal space.[12] The goal of instrumentation is to produce a continuously tapered preparation that maintains the canal anatomy, keeping the foramen as small as possible[13] without any deviation from the original canal curvature.[12]

Deviation from the original canal curvature can lead to:

Excessive and inappropriate dentin removal[4]

Straightening of the canal and creation of a ledge in the dentinal wall[5]

A biochemical defect known as an elbow which forms the coronal to the elliptical-shaped apical seal[6]

Canals with hourglass appearance in cross-section that requires stripping[4]

Overpreparation that weakens the tooth, resulting in fracture of the root[4]

Various parameters that affect canal-centering ability:

Alloys used in manufacturing instruments

Instrument design

Cross-section

Taper

Tip

Alloys used in manufacturing instruments:

The most commonly used materials are:

Stainless steel

Nickel-Titanium alloy

Historically, root canal instrumentation has involved the use of stainless steel hand files. Numerous investigations have shown that the preparation of curved root canals with stainless steel instruments frequently results in undesirable aberrations such as elbows, zips, and danger zones.[7]

Civijan[9] was one of the first investigators to propose a nickel-titanium (Ni-Ti) alloy for its use in endodontics in 1975. In 1988, Walia et al.[10] suggested a greater modification in endodontic instruments—replacing stainless steel with a Ni-Ti alloy.[2] Ni-Ti endodontic instruments were introduced to facilitate instrumentation of curved canals. Ni-Ti instruments are superelastic and could flex far more than stainless steel instruments before exceeding their elastic limits.[10-13]

If the instruments are not precurved in a curved canal, the amount of stress acting on the instrument to negotiate the curve is more for stainless steel and less for Ni-Ti instruments. The amount of force required or acting on the instrument to bend is the amount of force the instrument will exert and lead to more cutting on the outer curvature of the canal, which results in eccentricity. As Ni-Ti instruments require less stress to bend, they exert less force and being nonaggressive by nature, do not lead to excess cutting on either side. Stainless steel, however, tends tendency to cut more in one wall than the other.

This phenomenon occurs because the bending of the instrument within the canal occurs within its elastic limit, and this tendency to recoil is the cause for eccentricity in cutting. Precurving of stainless steel instruments is done within the plastic phase.

Ni-Ti alloys have been found to be 2–3 times more elastic than similarly manufactured stainless steel files. This property may allow Ni-Ti files to negotiate curved canals with less lateral stress but do not allow the precurving of Ni-Ti files. Whether the physical tendency of Ni-Ti files to remain straight, prevents ideal instrumentation or whether their high flexibility allows a better negotiation of curved canals despite the inability to precurve, still remains questionable.[2]

Parameswaran et al.,[14] Al omarii et al.,[15] Coleman et al.,[16] and Miglani et al.[17] reported transportation, zipping, and straightening of canals using stainless steel instruments. Several studies have confirmed that rotary Ni-Ti files maintain the original canal curvature better than do stainless steel files.[12718-25] The stainless steel files produce a larger extent of movement because of their hardness, which was shown to be 3–4 times harder than Ni-Ti alloys.[17] Carvalho reported that even after precurving and anticurvature filing, a small amount of transportation could be expected from stainless steel instruments.[2]

CROSS - SECTION

A study by Dina Al-Sudani[26] compared the canal-centering ability of a U- shaped instrument (Profile) with other asymmetric cross-section (K3) and convex, triangular cross-section (RaCe) instruments. The results of the study showed that the Profile system produced significantly less transportation and remained centered around the original canal to a greater degree than did other systems. This differential performance could be attributed to the different designs of these instruments. The Profile instrument uses the U-shaped cross section with radial land areas having a negative rake angle that cuts equally over 360°C with a planing action and is considered to be self-centering. The K3 instrument also has a U-shaped file design but has a positive 45°C rake angle. As dentin is a resilient material, the K3 instrument's positive rake angle makes it work like a shaver on the dentin surface; thus, maintaining canal-centering will be difficult. Studies done by Short et al.[27] and Versumer et al.[28] compared the canalcentering ability of ProFile & Lightspeed rotary Ni-Ti instruments in mandibular molars with curvatures between 20 and 40°C. The results showed that both systems had a U-shaped cross-section and produced significantly less transportation and were well centered.

Furthermore, in a study by Miglani et al.[17] that compared the canal-centering ability of ProFile, HERO 642 & SS K files in canal curvatures ranging from 20 to 40° C, the Profile series instruments showed superior canal-centering ability and performed better than both Hero 642 and SS Kfiles. The trihelical Hedstrom design of the Hero system having a thicker inner core is less flexible and resistant to bending. Hence, it could have caused the Hero 642 to show more transportation than did the triple helical configuration of Profile.

Previous studies by Iqbal et al.[29] compared the apical transportation between the ProFile and ProTaper instruments and showed that the ability of the file to remain centered may not entirely depend on U-file design or the presence of radial lands. The variable taper design of Protaper dampens the screw-in effect. Thus, a simpler convex triangular design, as seen in the case of Protaper, is capable of performing equally well or slightly better than ProFile. A comparative study[30] of six rotary Ni-Ti systems (Flexmaster, System Gt, HERO 642, K3, ProTaper, and Race) in extracted mandibular molars with curvatures up to 70° and embedded in a muffle system, showed that ProTaper instruments created more regular canal diameters.

Conversely, Yoshmine et al.[31] compared the shaping effects of three Ni-ti rotary instruments: ProTaper, K3, & RaCe, in simulated S-shaped canals. They showed that the ProTaper group caused significantly greater widening of canals, especially on the inner sides of the curved region, tending towards straightening of the canal, whereas the K3 and RaCe groups showed no indication of deviation. Schafer et al.[32] have ndicated that correlations between the bending properties and cross-sectional surface areas of different Ni-Ti rotary instruments are highly significant. According to their results, the cross-sectional area of 0.04-tapered K3 files was nearly twice the size of the RaCe files with the same tip size and taper, indicating that the former files are less flexible than latter. In conclusion, Ni-ti systems having less cross-sectional area as well as more flexible instruments like K3 and RaCe should be used for canals with more complex curvature.

Musikant et al. compared the shaping ability of conventional reamers and files with a newly introduced, noninterrupted, flat-sided design (EZ-Fill SafeSider) reamer and file system. Conventional files produce the greatest engagement of the instrument with the walls of the canal and consequently, the greatest amount of resistance as they negotiate to the apex. This is not the case for noninterrupted, flat-sided instruments (EZ-FILL safe siders). The flat side reduces the cross-sectional diameter, making it thinner and more flexible, allowing it to negotiate the highly curved canals more easily and have better centering ability.[33]

TAPER

Yang et al. studied the shaping ability of progressive vs constant taper instruments in curved root canals with curvatures ranging from 20 to 40°C.[34] Better compliance was obtained with the original canal shape using the constant taper (Heroshaper). The constant taper produced good centering ability in the apical section compared to instruments with progressive tapers along the cutting surface (ProFile). The final file of the Protaper F3 has an apical taper of 0.09 which is much larger than the Heroshaper which has a 0.04 taper. As the large taper of the F3 instrument increases the stiffness of the tip, the use of larger and greater taper instruments in moderately to severely curved canals should be considered carefully.

Schafer & Vlassis[35] and Paque et al.[36] compared the shaping ability of ProTaper and RaCe in simulated curved canals. Studies showed that both instruments were relatively safe although Race respected the original curvature better than ProTaper. The reason could be attributed to the variable tapers along the cutting surface of ProTaper files. The decreasing taper sequence of the finishing files enhances the strength of the file, but increases the stiffness of their tips. For example, the taper at the tip of ProTaper size 30 is 9%, whereas the taper of a size 20 is only 7%. And it is also due to increased taper of ProTaper shaping files up to 19%,whereas RaCe instruments are available only with tapers of maximum 10%.

TIP [TABLES 1 AND 2]

Ponce de Leon Del Bello et al.[8] studied the shaping effects of three types of stainless steel files that differ only in tip shape by using curved canals in acrylic blocks. The tip shapes were: (a) pyramidal (Flex-O)with sharp transition angles and a forward-cutting ridge on the face, (b) conical (Mor-Flex) with sharp transition angles and a smooth face, and (c) biconical (Flex-R) with reduced transition angles and dual-guiding faces. The study suggested that during crown-down rotational instrumentation, the original canal curvature is maintained better by biconical file tips. Cutting edges increased shaping to the inside because the canal deflects the instrument′s tip through a curvature. This transportation is increased when a file design prevents removal of the outer wall as it progresses through a curved region. Thus, the biconical design generates diameters greater than the file tip at levels that are more coronal than 1 mm. The diameters are consistently less for the biconical design than for the pyramidal and conical designs. Removal of the transition angle and formation of lands allows the canal to reorient the tip through curvatures. As the transition angles are reduced, the file stays centered within the original canal and cuts all sides more evenly. Ponce de Leon Del Bello et al. agree that the instrumentation of curved canals is more successful with "modified" tip instruments, i.e ., biconical-shaped tips.

Veltri et al.[37] compared the shaping abilities of Protaper and GT rotary files to shape curved canals. Dentin removal and mean asymmetry showed no significant differences between the two systems. The canal-centering ability of ProFile, Hero 642, & SS K files was compared using the kuttler cube method in a study done by Miglani et al.[17] Both the Ni-Ti systems showed superior canal-centering ability compared to the SS hand instruments. The standard cutting tip can be too aggressive because the first flute makes the initial cut in canal transportation, whereas the rotary system has a modified noncutting tip. The noncutting tip that guides the blades of the instrument in the canal lumen could be the reason for Ni-Ti systems like the ProFile and Hero 642 remaining more centered than the standard K-files.

Hulsmann et al.[7] compared several parameters of root canal preparation using two different Ni-Ti instruments, the Hero 642 and the Quantec SC systems, with canal curvatures between 20 and 40° C. They found that Quantec instruments produced more severe straightening probably because they have a rather aggressive, four-faceted cutting tip instead of the noncutting tip seen in the Hero 642 instruments.

Song et al. compared the centering ability for three instrumentation techniques using two Ni-Ti (GT and Nitiflex files) and one SS K-type file in teeth with curvatures between 15 and 45°C. Results showed that both Ni-Ti instruments had a blunt transition angle in the tip which allowed the instrument to plane the canal walls rather than engaging and screwing into them. This may contribute to the even cutting of dentin along the canal wall and making these instruments self-centered in comparison to the SS K-type files.[38]

CONCLUSION

From a review of the available literature, we conclude that:

Ni-Ti instruments show better canal-centering ability than stainless steel instruments.

Instruments with less cross-sectional area & taper will show better canal-centering ability.

Instruments with noncutting tips show better canal-centering ability.[59]

Table 1

Design of rotary instruments

Instruments	Cross-section	Rake angle	Taper	Tip
Profile	U-shaped	Negative	0.02–0.06	Noncutting
Light speed LS1	U-shape	Negative	Taperless	Noncutting
Quantec	Double helical	Negative	0.02–0.12	Cutting, noncutting
Hero 642	Trihelical hedstrom	Positive	0.02–0.06	Guiding
RaCe	Triangular	Negative	0.02–0.10	Safe-cutting
Protaper	Convex triangle	Negative	Increase/decrease	Guiding
K3	Modified K file	Positive	0.02–0.10	Safe-cutting
Endowave	Triangular	Negative	2, 4, 6, 8, 12	Rounded safety
M two	Italic S-shaped	Positive	4,6	Noncutting
Lightspeed LSX	Spade-shaped	Negative	Taperless	Noncutting
V taper	Parabolic	Neutral	6, 8, 10	Noncutting
Liberator	Triangular	Negative	2, 4, 6	Noncutting
EZ-fill safe sider	D-shaped	Negative	2, 4, 6, 8, 10, 12	Noncutting

Table 2

Various studies comparing canal-centering ability

Author	Instruments assessed	Best in canal centering abil
Gambill et al.[9]	K flex SS files-quarter turn/pull tech	Ni-Ti hand files (Mity files)-reaming tech
	Ni-Ti hand files (Mity files)-quarter turn/pull tech
	Ni-Ti hand files (Mity files)-reaming tech
M.A.O.Al-omari et al.[15]	Mani K-files	Mani K-files
	Micromega K-files
Short et al.[27]	Profile	No significant difference
	Lightspeed
	McXIM
	Flex-R
Kosa et al.[56]	Profile series 29	Profile series 29
	Flex R files
	Quantec 2000 rotary system
	Shaping Hedstrom files
Carvalho et al.[2]	SS Flexofile K-files	Nitiflex
	NiTi Mity K-files
	NiTi NitiflexK-files
Hansoo Park[1]	GT files	Profiles 6%
	Profiles 6%	K–flexofiles
Peters et al.[39]	GT rotary
	Niti K-files	Light speed
	Profile .04
Hulsmann et al.[7]	HERO 642	HERO 642
	Quantec SC
Gluskin et al.[40]	Ni-Ti GT rotary files	Ni-Ti GT rotary files
Deplazes et al.[41]	SS Flexofiles and Gates Glidden burs	Ni-Ti GT rotary files
	Light speed	No significant
	Light speed	No significant difference
	Ni-Ti K files
Ponti et al.[42]	Profile .06 taper series 29	No significant difference
	Profile GT series
Hata et al.[43]	Profile
	GT rotary	No significant difference
	Flex-R
Versumer et al.[28]	Profile .04	No significant difference
	Light speed
Schafer & Schlingemann[57]	K3 Ni-Ti rotary	K3 Ni-Ti rotary
	SS hand KFlexofile
Iqbal et al.[44]	Profile .06 series 29-crown down	No significant difference
	Profile files-step back
	GT files followed by Profile .06-crown down
	GT files-crown down, Profile- step back
Yun and Kim[45]	ProFile	Pro Taper
	GT rotary
	Quantec
	Pro Taper
Bergmans et al.[46]	Pro Taper	Pro Taper
	K3 files
Veltri et al.[37]	Pro Taper	No significant difference
	GT rotary
Song et al.[38]	GT hand files	NiTi flex
	NiTi flex
	Stainless steel K type files
Miglani et al.[17]	HERO 642	ProFile.04 & .06 series
	ProFile.04 & .06 series
	SS K files
Iqbal et al.[29]	ProFile	No significant difference
	Pro Taper
Musikant et al.[33]	Conventional file	EZ-Fill safesider reamer
	EZ-Fill safesider file
	Conventional reamer
	EZ-Fill safesider reamer
Tasdemir et al.[47]	HERO 642	HERO 642
	SS K files
	Pro Taper
	K3	K3 & RaCe
	RaCe
Perez et al.[58]	SS ENDOflash	Ni-Ti HERO shaper
	Ni-Ti HERO shaper
Guelzow et al.[30]	FlexMaster	ProTaper
	System GT
	Hero 642
	K3
	ProTaper
	RaCe
Paque et al.[36]	Hand instrumentation (reamer & H files)	No significant difference
	RaCe
	Pro Taper
Dina al-sudani et al.[26]	ProFile	ProFile
	K3
	RaCe
Uyanik et al.[48]	Hero shaper	No significant difference
	Pro Taper
	RaCe
Merrett et al.[49]	RaCe	RaCe
	Flexmaster
Loizides et al.[50]	Niti rotary Profile	ProFile
	Stainless steel K flexofiles
Rodig et al.[51]	ProFile .04	No significant difference
	GT rotary
Loizides et al.[52]	Hero group	Hero group
	Protaper NiTi rotary
Yang et al.[34]	Pro Taper	Heroshaper
	Heroshaper
Iqbal et al.[53]	Light speed LS1	No significant difference
	Light speed LSX Ni-Ti
Uzun et al.[54]	Hero 642	No significant difference
	Heroshaper
	Profile
	Protaper
Hartmann et al.[55]	K-files-hand instrumentation	Manual technique
	K-file-oscillatory system
	Protaper Ni-Ti rotary system
Matwychuk et al.[3]	Sequence	No significant difference
	Liberator
	Flex-R
Javaheri & Javaheri[24]	Hero 642	Protaper
	Race
	Protaper
Miglani et al.[59]	ProFile	RaCe
	RaCe
	Pro Taper