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A Comparative Study of Three Different Root Canal Curvature Measurement Techniques and Measuring the Canal Access Angle in Curved Canals
Clinical Research
A Comparative Study of Three Different Root Canal Curvature Measurement Techniques and Measuring the Canal Access Angle in Curved Canals Mahir Günday, DDS, PhD, Hesna Sazak, DDS, PhD, and Yy´ldy´z Garip, DDS, PhD, Abstract In the first part of this study the Schneider (S), Weine (W), and Long-Axis (LA) techniques are used for comparing the measurement of canal curvature. One hundred mandibular first and second molar teeth were selected. Radiographs were taken after inserting size 10 K-files into the mesiobuccal root canals. The radiographic findings were digitized on a computer, and the three different curvature angles were measured from drawings of the same root canal and compared statistically. ANOVA showed that there were significant differences between the curvature angle values determined using each technique (p ⬍ 0.001). In the second part of this study the term “canal access angle” (CAA) was introduced and it was defined by examining the morphology of canal curvature. Canal length, curvature distance (y), curvature height (x), Schneider angle, and the newly defined CAA were evaluated statistically. Using a multiple regression analysis, the CAA was significantly related to x (p ⬍ 0.001) and y (p ⬍ 0.005). There was a positive correlation (r ⫽ 0.74) between the CAA and curvature height (x). The results indicated that the CAA is a more effective way of evaluating the root canal curvature.
From the Department of Endodontics, Faculty of Dentistry, Marmara University, Istanbul, Turkey. Address requests for reprint to Dr. Yy´ldy´z Garip, Department of Endodontics, Faculty of Dentistry, Marmara University, Istanbul, Turkey. E-mail address: [email protected]. Copyright © 2005 by the American Association of Endodontists
796
Gunday et al.
T
he biomechanical preparation of curved root canals is an important consideration in endodontic treatment. In addition to the canal instruments and preparation techniques, root canal morphology and the degree of curvature are determining factors in endodontic root canal preparation. Difficulties in the preparation of curved root canals have prompted the development of new preparation methods and investigations of root canal geometry (1– 6). Weine (7) reported that canal curvatures exceeding 30° lead to complications in root canal preparation and cases are more complex. Lim and Webber (8) described some complications resulting from the preparation of curved root canals. The deformation of canal instruments placed in a curved canal places stress on the instrument. Tensile stresses form on the noncurved parts, and compressive stresses occur on the curved parts of the canal instrument (3). When the curvature of canal increases distorted part of the file becomes greater and the risk of breakage increases. The morphology of curved root canal is of great importance to the outcome of root canal instrumentation, with several studies being conducted to describe the curvature. In 1971, Schneider (10) performed pioneering work on measuring canal angulation. Subsequently, Weine (7) developed an alternative method for determining canal angulation. A third method for determining canal angulation, known as the long-axis (LA) technique, was first described by Hankins et al. (1). In contrast, Kyomen et al. (2) introduced a linear parameter described as the maximum curvature height, which differs from the angular measurement techniques. Likewise, Pruett et al. (3) introduced a new parameter described as the “curvature radius” for measuring root canal curvature. Radius of curvature with its resultant increased stress on endodontic instruments may also be a significant factor clinically contributing instrument breakage and canal transportation (11). The aim of this study was to compare and evaluate three different methods determining curvature angles and to introduce a new parameter the “canal access angle” (CAA) that is compared with Schneider angle.
Materials and Methods One hundred human mandibular first and second molars were used in this study. Teeth with incompletely formed apices, external resorption, and very narrow canals, or with obstructed canals that would make identification impossible, were eliminated. After extraction, all the molars were placed in a 10% formalin solution, and artifacts on the root surfaces were removed by storing them in distilled water. After endodontic access, a size 10 K-file was placed in the mesiobuccal canal extending to the apical foramen and radiographs were taken. The teeth were attached to Kodak Ultra-speed film (Kodak, Stuttgart, Germany) with soft wax and were aligned so that the long axis of the root was parallel and as close as possible to the surface of the X-ray film. Radiographs of each root canal were taken in buccolingual direction and long axis of the root was perpendicular to the central X-ray beam. Exposure time was the same for all radiographs with a constant distance about 40 cm between the film and X-ray source. The films were developed, fixed, washed, and dried. After that the radiographs were scanned with a computer (Scanner: Agfa–Duascan, Germany). The Schneider method involves first drawing a line parallel to the long axis of the canal, in the coronal third; a second line is then drawn from the apical foramen to intersect the point where the first line left the long axis of the canal. The Schneider angle is the intersection of these lines. In the
JOE — Volume 31, Number 11, November 2005
Clinical Research TABLE 1 o
CAA ( ) Canal length (mm) x (mm) y (mm) Schneider angle (o)
X ⴞ SD
Minimum–Maximum
15.45 ⫾ 4.99 12.68 ⫾ 2.15 1.01 ⫾ 0.35 3.76 ⫾ 0.88 22.42 ⫾ 6.31
4.42–26.86 10.26–17.85 0.30–2.08 1.91–6.41 7.98–35.45
TABLE 2
Canal length (mm) x (mm) y (mm) Schneider angle (o)
(CAA) r
p
0.31 0.74 ⫺0.38 0.93
0.001 0.001 0.001 0.001
the curvature height (x), and the distance from A to point D is the curvature distance (AD ⫽ y). The angular and linear values used in this study were plotted in a PC environment using the program Free Hand (Macromedia, Inc., San Francisco, CA), and the pertinent measurements were made using the program AutoCAD R12 (Autodesk, Inc., San Rafael, CA). The resultant values were evaluated statistically using Pearson correlation and multiple regression analyses.
Results
Figure 1. (a) Curvature angle measurement from the same root canal of a representative molar using three different techniques. (b) CAA, the angle between the line from the canal entrance (A) to apex (B) and a line parallel to the long axis of the canal extending from the coronal part of canal. S: Schneider angle, AC, The distance between points A and C; CD(x), curvature height; AD(y): curvature distance. (c, d) The canal access angles of two canals with different canal geometry may differ, even if they have the same canal curvature when measured using the Schneider technique.
Weine technique, a straight line is drawn from the orifice through the coronal portion of the curve, and a second line is drawn from the apex through the apical portion of the curve. The Weine angle is the intersection of these lines. The LA technique involves drawing a line passing through the apical one-third of the canal; the angle formed by the intersection of that line with the long axis of the tooth is known as the LA angle (Fig. 1a). In the second part, CAA was described and compared with Schneider Angle technique. The canal orifice (A) and apex (B) points were connected with a line. The angle formed by the intersection between this line (AB) and one drawn parallel to the long axis of the canal from the coronal part (AC) (used in the Schneider method), is defined as the CAA (Fig. 1b). At the point (C) where the parallel line described in the Schneider method leaves the root canal a perpendicular line was drawn to AB. The point that the perpendicular line intersects AB is D. CD gives
JOE — Volume 31, Number 11, November 2005
In the first part of our study, the mean curvature angle values measured using Schneider, Weine and LA methods are 22.42° (⫾6.31), 29.28° (⫾9.78), and 16.79° (⫾10.04), respectively. The largest and smallest average curvature angles measured using Schneider, Weine and LA methods are 7.98° to 35.45°, 11.70° to 56.79°, and 0.35° to 46.25°, respectively. ANOVA showed that there were significant differences between the curvature angles measured using each technique (p ⬍ 0.001). The Pearson correlation analysis found significant positive correlation between angles S and W (r ⫽ 0.83) and angles W and LA (r ⫽ 0.89), and a moderate correlation between angles S and LA (r ⫽ 0.67). The results of the second part of the investigation are summarized in Table 1. The curvature starting distance corresponded to the coronal third in 67% of the roots and to the medium third in the remaining 33%. Furthermore, the CAA was significantly smaller than the Schneider curvature angle (p ⬍ 0.001). The Pearson correlation analysis revealed the following (Table 2): 1. A positive correlation (r ⫽ 0.31) between the CAA and canal length (p ⬍ 0.001). 2. A positive correlation (r ⫽ 0.74) between the CAA and curvature height (x) (p ⬍ 0.001), and a negative correlation (r ⫽ – 0.38) between the CAA and curvature distance (y) (p ⬍ 0.001). 3. A positive correlation (r ⫽ 0.93) between the CAA and Schneider angle (p ⬍ 0.001). The multiple regression analysis indicated that the values of x (p ⬍ 0.001) and y (p ⬍ 0.005) influence the CAA, i.e. change in the CAA depends on the values of x and y.
Discussion In the studies of root canal curvature Schneider angle is usually used (8, 10, 12). Whereas the Schneider technique mainly emphasizes the canal curvature in the coronal region, the Weine technique also considers the apical region. In contrast, the LA technique considers only
Comparing the Measurement of Canal Curvature
797
Clinical Research the apical curvature of the canal and does not evaluate the overall root canal curvature (7, 10, 13). Hankins et al. (1) investigated widening techniques used for curved canals using the Schneider and LA angles and reported that the LA technique revealed the changes in the apical curvature of the root canal better than the Schneider technique. The angular values obtained using the curvature radius method introduced by Pruett et al. (3) were geometrically equivalent to the curvature angle measured using the Weine technique in the same canal. In our study, the largest and smallest average curvature angles were those measured using the Weine (29.28° ⫾ 9.78) and LA (16.79° ⫾ 10.04) techniques, respectively. The maximum curvature angle (56.79°) was measured using the Weine technique, and the minimum curvature angle (0.35°) was obtained using the LA technique. The deformation of canal instruments and instrument breakage in root canals are serious problems that are encountered in endodontics. An increase in canal curvature can result in preparation errors (7, 8, 14). Bending during use in curved canals causes endodontic instruments to exert a force on the wall of the curved zone. Consequently, an equivalent force acts on the canal instrument in the dentine. The stress acting on a canal instrument is highest in the curvature zone. The contact between the file and the surface of the canal may cause enough stress to break the file. Despite their superelasticity, recently developed Ni-Ti canal instruments can suffer from cyclic fatigue effects in curved canals, as determined by Pruett et al. (3) who reported that a sharp canal curvature increases the stress on canal instruments. Sattapan et al. (15) demonstrated that torque delivered to the endodontic instrument was dependent on tip size, taper, and canal size. Clinically the fatigue of an instrument may be related to the degree of flexure when placed in a curved canal. Zelada et al. (9) concluded that both the speed of rotation and the curvature of the root canals contribute to an increased risk of breakage of endodontic rotary instrument. The curvature, however, would seem to be by far the most important factor. This study introduced two new curvature parameters pertaining to the coronal zone of curved root canals: the curvature starting distance (y) and the curvature height (x). Our results show that the canal access angles of two canals with different canal geometry can differ, even if they have the same canal curvature as measured using the Schneider technique. The curvature starting distance (y) recedes from the root canal entrance as the curvature increases. As seen in Fig. 1, the CAA in Fig. 1c is smaller for the canal with the larger curvature starting distance (Fig. 1d) among canals with identical Schneider angles. In this case, the deformation and stress on the canal instrument would intensify at the tip of the instrument, which would in turn affect the rest of the instrument. In the situation where you get the same degree of canal curvature, because of access limitation, it is impossible that an instrument would remain straight during instrumentation. As a result, not only torsion but also flexural fatigue of an instrument will predispose an instrument to fracture. To reduce the risk of torsional fracture in the root canal, apical force should be moderate during instrumentation. Previous studies (9, 16) reported that all breakage occurred in the apical portion of the canal (15), which might be explained by the fact that a curvature with a small radius usually occurs in the apical foramen. A new parameter, the
798
Gunday et al.
CAA, was introduced to take into account the stress on instrumentation during canal preparation. An increase in the curvature distance (y) results in displacement of the curvature point away from the canal entrance. In such a case, deformation and stress on the canal instrument intensify toward the tip. Most studies have used the Schneider method to determine root canal curvature (5, 12, 17–19). However, our results show that the CAA is as effective as the Schneider angle in evaluating root canal curvature with respect to its influence on the operation of root canal instruments. In addition, it is a better method to measure effectiveness of new root canal instruments.
Acknowledgments The authors would like to thank Eng. Faruk Berker for providing technical assistance.
References 1. Hankins PJ, ElDeeb ME. An evaluation of the canal master, balanced-force, and step-back techniques. J Endod 1996;22:123–30. 2. Kyomen SM, Caputo AA, White SN. Critical analysis of the balanced force technique in endodontics. J Endod 1994;20:332–7. 3. Pruett JP, Clement DJ, Carnes DL. Cyclic fatigue testing of nickel-titanium endodontic instruments. J Endod 1997;23:77– 85. 4. Esposito PI, Cunningham CJ. A comparison of curved canal preparations with nickeltitanium and stainless steel instruments. J Endod 1995;21:171–3. 5. Harlan AL, Nicholls JI, Steiner JC. A comparison of curved canal instrumentation using nickel-titanium or stainless steel files with the balanced-force technique. J Endod 1996;22:410 –3. 6. Roane IB, Sabala CL, Duncanson MG. The balanced force concept for instrumentation of curved canals. J Endod 1985;11:203–11. 7. Weine FS. Endodontic therapy, 3rd ed. St. Louis: CV Mosby, 1982:288 –306. 8. Lim KC, Webber J. The effect of canal preparation on the shape of the curved root canal. Int Endod J 1985;18:233– 6. 9. Zelada G, Varela P, Martin B, Bahillo JG, Maga´n F, Ahn S. The effect of rotational speed and the curvature of root canals on the breakage of rotary endodontic instruments J Endod 2002;28:540 –2. 10. Schneider SW. A comparison of canal preparations in straight and curved root canals. Oral Surg 1971;32:271–5. 11. Haı¨kel Y, Serfaty R, Bateman G, Senger B, Allemann C. Dynamic and cyclic fatigue of engine-driven rotary nickel-titanium endodontic instrument. J Endod 1999;25:434 – 40. 12. Cunningham CJ, Senia ES. A three-dimensional study of canal curvatures in the mesial roots of mandibular molars. J Endod 1992;18:294 –9. 13. Wildey WL, Senia ES, Montgomery S. Another look at root canal instrumentation. Oral Surg Oral Med Oral Pathol 1992;74:499 –507. 14. Booth JR, Scheetz JP, Lemons JE, Eleazer PD. A comparison of torque required to fracture three different nickel-titanium rotary instruments around curves of the same angle but of different radius when bound at the tip. J Endod 2003;29:55–7. 15. Sattapan B, Palamara J, Messer H. Torque during canal instrumentation using rotary nickel-titanium files. J Endod 2000;26:156 – 60. 16. Ankrum MT, Hartwell GR, Truitt JE. K3 Endo, Pro Taper, and Pro File Systems: breakage and distortion in severely curved roots of molars. J Endod 2004;30:234 –7. 17. Scha¨fer E, Diez Ch, Hoppe W, Tepel J. Roentgenographic investigation of frequency and degree of canal curvatures in human permanent teeth. J Endod 2002;28:211– 6. 18. Ponti TM, McDonald NJ, Kuttler S, Strassler HE, Dumsha TC. Canal-centering ability of two rotary file systems. J Endod 2002;28:283– 6. 19. Davis RD, Marshall JG, Baumgartner JC. Effect of early coronal flaring on working length change in curved canals using rotary nickel-titanium versus stainless steel instruments. J Endod 2002;28:438 – 42.
JOE — Volume 31, Number 11, November 2005
A Comparative Study of Three Different Root Canal Curvature Measurement Techniques and Measuring the Canal Access Angle in Curved Canals Mahir Günday, DDS, PhD, Hesna Sazak, DDS, PhD, and Yy´ldy´z Garip, DDS, PhD, Abstract In the first part of this study the Schneider (S), Weine (W), and Long-Axis (LA) techniques are used for comparing the measurement of canal curvature. One hundred mandibular first and second molar teeth were selected. Radiographs were taken after inserting size 10 K-files into the mesiobuccal root canals. The radiographic findings were digitized on a computer, and the three different curvature angles were measured from drawings of the same root canal and compared statistically. ANOVA showed that there were significant differences between the curvature angle values determined using each technique (p ⬍ 0.001). In the second part of this study the term “canal access angle” (CAA) was introduced and it was defined by examining the morphology of canal curvature. Canal length, curvature distance (y), curvature height (x), Schneider angle, and the newly defined CAA were evaluated statistically. Using a multiple regression analysis, the CAA was significantly related to x (p ⬍ 0.001) and y (p ⬍ 0.005). There was a positive correlation (r ⫽ 0.74) between the CAA and curvature height (x). The results indicated that the CAA is a more effective way of evaluating the root canal curvature.
From the Department of Endodontics, Faculty of Dentistry, Marmara University, Istanbul, Turkey. Address requests for reprint to Dr. Yy´ldy´z Garip, Department of Endodontics, Faculty of Dentistry, Marmara University, Istanbul, Turkey. E-mail address: [email protected]. Copyright © 2005 by the American Association of Endodontists
796
Gunday et al.
T
he biomechanical preparation of curved root canals is an important consideration in endodontic treatment. In addition to the canal instruments and preparation techniques, root canal morphology and the degree of curvature are determining factors in endodontic root canal preparation. Difficulties in the preparation of curved root canals have prompted the development of new preparation methods and investigations of root canal geometry (1– 6). Weine (7) reported that canal curvatures exceeding 30° lead to complications in root canal preparation and cases are more complex. Lim and Webber (8) described some complications resulting from the preparation of curved root canals. The deformation of canal instruments placed in a curved canal places stress on the instrument. Tensile stresses form on the noncurved parts, and compressive stresses occur on the curved parts of the canal instrument (3). When the curvature of canal increases distorted part of the file becomes greater and the risk of breakage increases. The morphology of curved root canal is of great importance to the outcome of root canal instrumentation, with several studies being conducted to describe the curvature. In 1971, Schneider (10) performed pioneering work on measuring canal angulation. Subsequently, Weine (7) developed an alternative method for determining canal angulation. A third method for determining canal angulation, known as the long-axis (LA) technique, was first described by Hankins et al. (1). In contrast, Kyomen et al. (2) introduced a linear parameter described as the maximum curvature height, which differs from the angular measurement techniques. Likewise, Pruett et al. (3) introduced a new parameter described as the “curvature radius” for measuring root canal curvature. Radius of curvature with its resultant increased stress on endodontic instruments may also be a significant factor clinically contributing instrument breakage and canal transportation (11). The aim of this study was to compare and evaluate three different methods determining curvature angles and to introduce a new parameter the “canal access angle” (CAA) that is compared with Schneider angle.
Materials and Methods One hundred human mandibular first and second molars were used in this study. Teeth with incompletely formed apices, external resorption, and very narrow canals, or with obstructed canals that would make identification impossible, were eliminated. After extraction, all the molars were placed in a 10% formalin solution, and artifacts on the root surfaces were removed by storing them in distilled water. After endodontic access, a size 10 K-file was placed in the mesiobuccal canal extending to the apical foramen and radiographs were taken. The teeth were attached to Kodak Ultra-speed film (Kodak, Stuttgart, Germany) with soft wax and were aligned so that the long axis of the root was parallel and as close as possible to the surface of the X-ray film. Radiographs of each root canal were taken in buccolingual direction and long axis of the root was perpendicular to the central X-ray beam. Exposure time was the same for all radiographs with a constant distance about 40 cm between the film and X-ray source. The films were developed, fixed, washed, and dried. After that the radiographs were scanned with a computer (Scanner: Agfa–Duascan, Germany). The Schneider method involves first drawing a line parallel to the long axis of the canal, in the coronal third; a second line is then drawn from the apical foramen to intersect the point where the first line left the long axis of the canal. The Schneider angle is the intersection of these lines. In the
JOE — Volume 31, Number 11, November 2005
Clinical Research TABLE 1 o
CAA ( ) Canal length (mm) x (mm) y (mm) Schneider angle (o)
X ⴞ SD
Minimum–Maximum
15.45 ⫾ 4.99 12.68 ⫾ 2.15 1.01 ⫾ 0.35 3.76 ⫾ 0.88 22.42 ⫾ 6.31
4.42–26.86 10.26–17.85 0.30–2.08 1.91–6.41 7.98–35.45
TABLE 2
Canal length (mm) x (mm) y (mm) Schneider angle (o)
(CAA) r
p
0.31 0.74 ⫺0.38 0.93
0.001 0.001 0.001 0.001
the curvature height (x), and the distance from A to point D is the curvature distance (AD ⫽ y). The angular and linear values used in this study were plotted in a PC environment using the program Free Hand (Macromedia, Inc., San Francisco, CA), and the pertinent measurements were made using the program AutoCAD R12 (Autodesk, Inc., San Rafael, CA). The resultant values were evaluated statistically using Pearson correlation and multiple regression analyses.
Results
Figure 1. (a) Curvature angle measurement from the same root canal of a representative molar using three different techniques. (b) CAA, the angle between the line from the canal entrance (A) to apex (B) and a line parallel to the long axis of the canal extending from the coronal part of canal. S: Schneider angle, AC, The distance between points A and C; CD(x), curvature height; AD(y): curvature distance. (c, d) The canal access angles of two canals with different canal geometry may differ, even if they have the same canal curvature when measured using the Schneider technique.
Weine technique, a straight line is drawn from the orifice through the coronal portion of the curve, and a second line is drawn from the apex through the apical portion of the curve. The Weine angle is the intersection of these lines. The LA technique involves drawing a line passing through the apical one-third of the canal; the angle formed by the intersection of that line with the long axis of the tooth is known as the LA angle (Fig. 1a). In the second part, CAA was described and compared with Schneider Angle technique. The canal orifice (A) and apex (B) points were connected with a line. The angle formed by the intersection between this line (AB) and one drawn parallel to the long axis of the canal from the coronal part (AC) (used in the Schneider method), is defined as the CAA (Fig. 1b). At the point (C) where the parallel line described in the Schneider method leaves the root canal a perpendicular line was drawn to AB. The point that the perpendicular line intersects AB is D. CD gives
JOE — Volume 31, Number 11, November 2005
In the first part of our study, the mean curvature angle values measured using Schneider, Weine and LA methods are 22.42° (⫾6.31), 29.28° (⫾9.78), and 16.79° (⫾10.04), respectively. The largest and smallest average curvature angles measured using Schneider, Weine and LA methods are 7.98° to 35.45°, 11.70° to 56.79°, and 0.35° to 46.25°, respectively. ANOVA showed that there were significant differences between the curvature angles measured using each technique (p ⬍ 0.001). The Pearson correlation analysis found significant positive correlation between angles S and W (r ⫽ 0.83) and angles W and LA (r ⫽ 0.89), and a moderate correlation between angles S and LA (r ⫽ 0.67). The results of the second part of the investigation are summarized in Table 1. The curvature starting distance corresponded to the coronal third in 67% of the roots and to the medium third in the remaining 33%. Furthermore, the CAA was significantly smaller than the Schneider curvature angle (p ⬍ 0.001). The Pearson correlation analysis revealed the following (Table 2): 1. A positive correlation (r ⫽ 0.31) between the CAA and canal length (p ⬍ 0.001). 2. A positive correlation (r ⫽ 0.74) between the CAA and curvature height (x) (p ⬍ 0.001), and a negative correlation (r ⫽ – 0.38) between the CAA and curvature distance (y) (p ⬍ 0.001). 3. A positive correlation (r ⫽ 0.93) between the CAA and Schneider angle (p ⬍ 0.001). The multiple regression analysis indicated that the values of x (p ⬍ 0.001) and y (p ⬍ 0.005) influence the CAA, i.e. change in the CAA depends on the values of x and y.
Discussion In the studies of root canal curvature Schneider angle is usually used (8, 10, 12). Whereas the Schneider technique mainly emphasizes the canal curvature in the coronal region, the Weine technique also considers the apical region. In contrast, the LA technique considers only
Comparing the Measurement of Canal Curvature
797
Clinical Research the apical curvature of the canal and does not evaluate the overall root canal curvature (7, 10, 13). Hankins et al. (1) investigated widening techniques used for curved canals using the Schneider and LA angles and reported that the LA technique revealed the changes in the apical curvature of the root canal better than the Schneider technique. The angular values obtained using the curvature radius method introduced by Pruett et al. (3) were geometrically equivalent to the curvature angle measured using the Weine technique in the same canal. In our study, the largest and smallest average curvature angles were those measured using the Weine (29.28° ⫾ 9.78) and LA (16.79° ⫾ 10.04) techniques, respectively. The maximum curvature angle (56.79°) was measured using the Weine technique, and the minimum curvature angle (0.35°) was obtained using the LA technique. The deformation of canal instruments and instrument breakage in root canals are serious problems that are encountered in endodontics. An increase in canal curvature can result in preparation errors (7, 8, 14). Bending during use in curved canals causes endodontic instruments to exert a force on the wall of the curved zone. Consequently, an equivalent force acts on the canal instrument in the dentine. The stress acting on a canal instrument is highest in the curvature zone. The contact between the file and the surface of the canal may cause enough stress to break the file. Despite their superelasticity, recently developed Ni-Ti canal instruments can suffer from cyclic fatigue effects in curved canals, as determined by Pruett et al. (3) who reported that a sharp canal curvature increases the stress on canal instruments. Sattapan et al. (15) demonstrated that torque delivered to the endodontic instrument was dependent on tip size, taper, and canal size. Clinically the fatigue of an instrument may be related to the degree of flexure when placed in a curved canal. Zelada et al. (9) concluded that both the speed of rotation and the curvature of the root canals contribute to an increased risk of breakage of endodontic rotary instrument. The curvature, however, would seem to be by far the most important factor. This study introduced two new curvature parameters pertaining to the coronal zone of curved root canals: the curvature starting distance (y) and the curvature height (x). Our results show that the canal access angles of two canals with different canal geometry can differ, even if they have the same canal curvature as measured using the Schneider technique. The curvature starting distance (y) recedes from the root canal entrance as the curvature increases. As seen in Fig. 1, the CAA in Fig. 1c is smaller for the canal with the larger curvature starting distance (Fig. 1d) among canals with identical Schneider angles. In this case, the deformation and stress on the canal instrument would intensify at the tip of the instrument, which would in turn affect the rest of the instrument. In the situation where you get the same degree of canal curvature, because of access limitation, it is impossible that an instrument would remain straight during instrumentation. As a result, not only torsion but also flexural fatigue of an instrument will predispose an instrument to fracture. To reduce the risk of torsional fracture in the root canal, apical force should be moderate during instrumentation. Previous studies (9, 16) reported that all breakage occurred in the apical portion of the canal (15), which might be explained by the fact that a curvature with a small radius usually occurs in the apical foramen. A new parameter, the
798
Gunday et al.
CAA, was introduced to take into account the stress on instrumentation during canal preparation. An increase in the curvature distance (y) results in displacement of the curvature point away from the canal entrance. In such a case, deformation and stress on the canal instrument intensify toward the tip. Most studies have used the Schneider method to determine root canal curvature (5, 12, 17–19). However, our results show that the CAA is as effective as the Schneider angle in evaluating root canal curvature with respect to its influence on the operation of root canal instruments. In addition, it is a better method to measure effectiveness of new root canal instruments.
Acknowledgments The authors would like to thank Eng. Faruk Berker for providing technical assistance.
References 1. Hankins PJ, ElDeeb ME. An evaluation of the canal master, balanced-force, and step-back techniques. J Endod 1996;22:123–30. 2. Kyomen SM, Caputo AA, White SN. Critical analysis of the balanced force technique in endodontics. J Endod 1994;20:332–7. 3. Pruett JP, Clement DJ, Carnes DL. Cyclic fatigue testing of nickel-titanium endodontic instruments. J Endod 1997;23:77– 85. 4. Esposito PI, Cunningham CJ. A comparison of curved canal preparations with nickeltitanium and stainless steel instruments. J Endod 1995;21:171–3. 5. Harlan AL, Nicholls JI, Steiner JC. A comparison of curved canal instrumentation using nickel-titanium or stainless steel files with the balanced-force technique. J Endod 1996;22:410 –3. 6. Roane IB, Sabala CL, Duncanson MG. The balanced force concept for instrumentation of curved canals. J Endod 1985;11:203–11. 7. Weine FS. Endodontic therapy, 3rd ed. St. Louis: CV Mosby, 1982:288 –306. 8. Lim KC, Webber J. The effect of canal preparation on the shape of the curved root canal. Int Endod J 1985;18:233– 6. 9. Zelada G, Varela P, Martin B, Bahillo JG, Maga´n F, Ahn S. The effect of rotational speed and the curvature of root canals on the breakage of rotary endodontic instruments J Endod 2002;28:540 –2. 10. Schneider SW. A comparison of canal preparations in straight and curved root canals. Oral Surg 1971;32:271–5. 11. Haı¨kel Y, Serfaty R, Bateman G, Senger B, Allemann C. Dynamic and cyclic fatigue of engine-driven rotary nickel-titanium endodontic instrument. J Endod 1999;25:434 – 40. 12. Cunningham CJ, Senia ES. A three-dimensional study of canal curvatures in the mesial roots of mandibular molars. J Endod 1992;18:294 –9. 13. Wildey WL, Senia ES, Montgomery S. Another look at root canal instrumentation. Oral Surg Oral Med Oral Pathol 1992;74:499 –507. 14. Booth JR, Scheetz JP, Lemons JE, Eleazer PD. A comparison of torque required to fracture three different nickel-titanium rotary instruments around curves of the same angle but of different radius when bound at the tip. J Endod 2003;29:55–7. 15. Sattapan B, Palamara J, Messer H. Torque during canal instrumentation using rotary nickel-titanium files. J Endod 2000;26:156 – 60. 16. Ankrum MT, Hartwell GR, Truitt JE. K3 Endo, Pro Taper, and Pro File Systems: breakage and distortion in severely curved roots of molars. J Endod 2004;30:234 –7. 17. Scha¨fer E, Diez Ch, Hoppe W, Tepel J. Roentgenographic investigation of frequency and degree of canal curvatures in human permanent teeth. J Endod 2002;28:211– 6. 18. Ponti TM, McDonald NJ, Kuttler S, Strassler HE, Dumsha TC. Canal-centering ability of two rotary file systems. J Endod 2002;28:283– 6. 19. Davis RD, Marshall JG, Baumgartner JC. Effect of early coronal flaring on working length change in curved canals using rotary nickel-titanium versus stainless steel instruments. J Endod 2002;28:438 – 42.
JOE — Volume 31, Number 11, November 2005