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Analysis of the forces developed during obturation: Warm vertical compaction
0099-2399/97/2302-0091 $03.00/0 JOURNALOF ENDODONTICS Copyright © 1997 by The American Association of Endodontists
Printed in U.S.A. VOL. 23, No. 2, FEBRUARY1997
Analysis of the Forces Developed During Obturation: Warm Vertical Compaction Jean-Yves Blum, DDS, Eric Parahy, DSO, and Jean-Paul Micailef, PhD
heat the gutta-percha to decrease the pressure of compaction. But to prevent destruction of its properties, too-high temperatures must be avoided (4). Excessive compaction pressures may cause root fracture. Using strain gauges, Saw and Messer (5) demonstrated that root canal obturation is the major cause of vertical root fracture because of the wedging effect generated in the canal by the compaction strain. Ricks-Williamson, et al. (6) studied the stress induced by obturations with finite-element analysis. They reported that vertical condensation produces more detrimental stresses throughout the root than lateral condensation. Other studies have analyzed the stresses induced by obturations by means of different experimental methods, such as the Instron testing machine (7) and photoelastics (8). To date, no investigation has studied the pressure applied during warm vertical compaction obturation. The aim of this study was to determine and display the vertical and lateral forces developed during an obturation using the warm vertical compaction technique.
The aim of this work was to measure and analyze the forces applied by endodontists during an obturation. This was achieved by devising a system of force transducers linked to acquisition software. The software allowed us to study the obturation forces in real time or to store them. In this initial study, the forces developed by endodontists and students during a warm vertical compaction were analyzed. The vertical and frontware backware horizontal direction forces were first stored. Graphs of the compaction forces were then generated, permitting the analysis of the obturation method, indeed, two cases of obturation failure were analyzed from these graphs. The mean values for the vertical forces applied by the endodontists and students were, respectively, 2.5 + 0.4 kg and 1.9 +- 0.9 kg; the mean values for the lateral forces were, respectively, 0.85 - 0.2 kg and 1.4 +- 0.6 kg. This device permits the analysis of compaction forces and may thus be highly useful in obtaining improvements in obturation techniques.
M A T E R I A L S AND M E T H O D S A computerized recording system was developed to record the forces applied during obturations. The system was composed of two force transducers (Captels, St. Mathieu de Treviers, France), electronic amplifiers, an analog-to-digital converter, a PC-compatible computer, and acquisition software. The sensitive component of the force transducers was a Wheatstone bridge of strain gauges. All strain variations were transformed by this bridge into electrical voltage variations in application of Poisson's hypothesis, i.e. a linear relation existed between a given pressure and the level of the corresponding electrical signal. The two transducers for the force measurement (Fig. 1 and 2) were disposed perpendicularly, with the end of one fixed on a frame and the other connected to a cupule in which the tooth was embedded (Fig. 3). The forces were measured in the vertical and frontward and backward horizontal directions. The sensors were connected with data acquisition software (LAPS) developed by INSERM (Unit 103, Montpellier, France) (Fig. 4). The electrical gain of amplification was first set at 1000 to obtain a voltage range between - 2 . 5 V to +2.5 V. The signals were then digitilized by an analogic-digital converter connected to the RS232 port of the PC. The sample frequency of the inputs was 800 Hz. The software gave access to the following functions:
The new endodontic techniques often use gutta-percha, the most acceptable and widely used filling material. Warm vertical compaction, as described by Schilder (1), is a technique that produces an homogeneous and dimensionally stable mass of gutta-percha. By a succession of heating and compaction, a wave of softened gutta-percha is gradually pushed down into the root canal system. Because of its overall efficiency, this technique has never been greatly modified. Indeed, only the heating procedure has been improved by new technology. The original heat carrier, heated by an open flame, has now been replaced by the Touch 'n Heat 5002 model instrument (2). In contrast, compaction is still performed by manual pluggers. The use of these instruments, in association with a suitable preparation, produces a tridimensional obturation. Schilder et al. (3) reported that pressure is necessary to adapt gutta-percha to the walls of the canal. Their research further showed that gutta-percha is not compressible, but that dimensional variations result from transitions in the gutta-percha's phases. These findings were later confirmed by Marciano and Michailesco (4). To perform warm vertical condensation, the endodontist has to 91
92
Blum, et al.
Journal of Endodontics
FiG. 3. Tooth embedded into the cupule (with light silicone).
F v = T v + Rv F u - Tn - R~
FIG. 1. Frontal photograph of the device.
FiG. 4. Description of the recorded forces (Fv, Fh). The vertical forces are transmitted (Fv = Tv + Rv). Only the resultants of the horizontal forces are transmitted (Fh = Th-Tv). (1) Plugger; (2) Gutta-Percha; and (3) Tooth.
Acquisition: three modes were available: storage and visualization, storage without visualization, and numerical storage. Disk access: for save and loading. Visualization: shows a graph in function of time. Oscilloscope: on-line visualization of data without storage. Transfer to Excel: transfer of the LAPS files to Excel's format. Protocol
FrG. 2. Frontal view of the device: (1) transducers forming a right angle; (2) cupule; (3) support frame; (4) to the Wheatstone bridge; (5) gutta-percha; (6) tooth; and (7) plugger.
Configuration: selection of port (COM 1 or COM 2) on the computer and the band rate of transmission (9600 to 110 K bands). Identification: identification of the channels to acquire numbers, names, frequency of the sampling, and duration of the acquisition.
Fifty freshly extracted human central maxillary incisors were selected for similarity in size, shape, and root canal anatomy. Teeth were stored in an environment maintained at 37°C and 100% moisture. A standardized preparation was performed (1). A #45 gutta-percha cone was fitted as master cone and adjusted to give a tug back and to be, radiographically, 1 mm shorter than the canal length. Warm vertical compaction was performed using four finger pluggers (Hu Friedy, Chicago, IL). The first was set to the entry of the canal, the second at the halfway point, and the third and fourth at, respectively, 9 and 7 mm short of the root length. A Touch 'n Heat device was used to heat the gutta-percha cone. The power setting was 8 and the device was used in discontinuous mode. The heating time was 8 s. The compaction was considered to be finished when the last plugger reached 7 mm from the apex. The remainder of the canal was completely filled using a MacSpadden compactor (10). Ten practitioners participated in this study. Five
Vol. 23, No. 2, February 1997
3.5 I A 3
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Warm Vertical Compaction Forces
93
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0,5
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-0.2
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F~G. 5. Typical graphs generated by the warm vertical compaction technique. Line represents the tracing from gauge over time. (5A) Vertical forces. (5B) Horizontal forces. (1) First pushing phase; (2) Last compaction; and (3) Mac-Spadden compaction. were endodontists, and five were dental school students. The teeth were embedded in the cupule with Night silicone (Coltex Fine, Pierre Rolland, France) (Fig. 3). The operators performed five sessions of four successive warm vertical compactions. During the obturations, the screen of the PC was occluded, but forces were stored. Off-line, the stored files were loaded and the graphs of the forces exerted during the obturations were analyzed as a function of time (Fig. 5A and 5B). The choice of teeth and operators was randomized. After the first session, the practitioners viewed their graphs. The plots of force versus time generated by the endodontists and students were analyzed and a new session of obturations was then performed. Between all sessions, graphs were analyzed in the same manner.
RESULTS An analysis of variance was performed for all results. When the A N O V A F ratio was significant, a contrast test was used. The
1
2
3
4
5
6
7
8
number of compaction
FIG, 8. Comparison of mean values of vertical and lateral forces during successive sessions.
mean number ( + sem) of compactions for one obturation was 8 -+ 1 for the endodontists (Fig. 6). The mean value of the vertical forces was 2.5 _+ 0.4 kgF (Fig. 7). No significant difference was found for the forces exerted over the course of the obturation. The mean value of the lateral forces was 0.85 +- 0.2 kgF (Fig. 8). The mean duration of compaction was, respectively, 2.03 +_ 0.13 min for the apical packing and 5 + I s for filling of the remainder of the canal. Duration between two compactions was I l + 5 s (Fig. 9). The mean duration of one compaction was 6 -+ 3 s except for the last, which lasted 15 + 3 s. During the last compaction, the exerted force was never constant but instead showed a decrease from beginning to end (i.e. 2.8 _+ 0.2 kgF to 2.0 -+ 0.1 kgF) (F = 14.6, p < 0.05). For the students, the number of compactions was 11 -+ 5 for the three first sessions and 8 + 2 for the last two (Fig.
94
Blum, et al.
~o]
35 A 3t
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Journal of Endodontics
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,
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. . . . . -05 Time (Inn)
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6). T h e difference between these results was significant (F = 121.3, p < 0.01). During the three first sessions the mean values of the fc,rces exerted by the students were significantly lower than those ,.ff the endodontists (F = 13.5, p < 0.05). In contrast, the last two sessions showed no difference in either number or force of compaction between the two groups (Fig. 7). The lateral forces for the students were always greater (1.4 ± 0.6 kgF) than those of the endodontists (F = 13., p < 0.05). Moreover, the time between two compactions was also always greater (F = 14.6, p < 0.001) (Fig. 9).
DISCUSSION
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2
0.4 0,3
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The measurement device allowed the storage of the forces developed during warm vertical compactions performed by endodontists and students. Details of compaction were revealed on graphs (Figs. 5A and 5B). In the horizontal plane, the recorded forces were not true intracanal forces but only the resultant forces (Fig. 4). Recording revealed an imbalance in the intracanal forces, which may produce highly detrimental lateral forces. Our results were in agreement with those of Ricks-Williamson et al. (6) who reported that warm vertical compaction produced more detrimental stresses than other techniques such as lateral condensation. The imbalance of the students' forces was initially very great. In contrast, that of the endodontists was always very low. This indicates the difficulty of producing a balanced vertical force during obturation. The recorded forces did not agree with the results of Telli et al. (11) who used a 2-D-study design, nor with those of Gimlin (9) who used a 2-D finite-element method. They reported that guttapercha needed to be vertically condensed with a load of 4.5 kg. In our study, the recorded forces never exceeded 3 kg. This difference may be due to differences in the heating and pushing sequences. However, as this study is the first to our knowledge to present visual information on the sequence durations, it is impossible to analyze our results in comparison with those of other studies. Nevertheless, our comparatively low value is in agreement with the concept of the softened wave developed by Machton (12). The choice of pluggers, which ensured that there were no interferences between the walls of the root canal and the instruments, prevented the wedging effect described by Ricks-Williamson et al. (6). Indeed, the depth of penetration by the pluggers was controlled by a stop that bad been fixed on the plugger.
Time Iron)
FIG. 10. Typical graphs of failure in compaction due to underestimated master cone. (10,4)Vertical forces; and (10B) Lateral forces. (1) lack of vertical forces; and (2) lack of last compaction.
Schilder (1) described the importance of the last compaction, which is performed to reduce the final cooling dimensional variations. This last compaction should last 15 _+ 4 s (14). The graphs show that the endodontists always performed this compaction. They were unable, however, to exert a constant force. At the beginning of the study (Fig. 10A and 10B) the students did not produce this last compaction, whereas Figures 1 IA and 11B) show the improvement in the last two sessions. From its inception, warm vertical compaction has been considered to be a very difficult technique (15) because of the inability to monitor forces and heating time. The only accessible observation has been the radiograph of the tooth after treatment. One of the advantages of the device developed for this study is the potential for on-line monitoring of forces and heating and compaction sequences during obturation. Evaluation and correction zre possible in real time, offering, for the first time to our knowledge, a bio-feedback method for improving technique. Two kinds of evaluation are possible. Sequences between heating and compaction can be studied, with corrections made on-line or off-line of the operating act. The applied forces may also be studied by direct analysis during the obturation itself (on-line) or by comparison to mean force values (off-line), i.e. corrections can be made in reference to stored graphs. Perhaps the greatest advantage of the graphs is for the detection
Vol. 23, No. 2, February 1997
3
Warm Vertical Compaction Forces
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The graph shows a lack of sufficient vertical force; with no reaction force opposed to the compaction forces, sliding of the cone resulted. The increase in the diameter again blocked the cone and thus the forces again appeared normal. The graph indicates on-line the failure of the obturation. During this study, the students involuntarily produced these failures but were able to analyze their errors following a brief explanation of the graphs. This monitoring of their obturations permitted the students to progress (Figs. 6 and 7). Indeed, perhaps the most interesting analysis was the comparison between the working techniques of the students and the endodontists. The result of this comparison was a modification in the applied forces and the sequences of heating and compaction. Instead of assessing the quality of their work by radiograph of the treated tooth, students were able to focus their analysis on the individual compactions forces and sequences. It also appeared that each endodontist had his own work protocol for the heating and compaction phases. The force values were not exactly the same between the different endodontists but seemed to be uniform for each individual endodontist. Thus, each graph could be considered as a signature. In conclusion, the device developed for this study appears to have potential for research and teaching. As our results show, it allows the analysis of the forces and sequences of warm vertical compaction. Other obturation techniques may also be studied. The authors thank INSERM, Unit 103, for the use of their LAPS acquisition software and Catherine Scott Carmeni for technical assistance. Address requests for reprints to Dr. Jean-Yves Blum, 12 rue de I'Aubier, 34130 St. Aunes France.
-0.1 -0.2
References
-0,3
Time (mn) FiG. 1 1. Typical graphs of failure in c o m p a c t i o n due to underestimated apical diameter of master cone. (11A) Vertical forces; (11B) Lateral forces. (1) Normal forces; (2) Lack of sufficient forces (sliding of master cone); and (3) last c o m p a c t i o n .
and analysis of failure, as the following examples show. Figures 10A and 10B show the specific obturation graph of an underestimated cone. This failure appeared as a lack of sufficient vertical force in the beginning of the obturation. The pluggers pushed into the canal to a length of approximately 6 mm from the entry without any opposition, and obturation proceeded without reaction forces. As a result, the root canal system was filled but not three-dimensionally obturated. Moreover, because the coronal plug was not performed, accidental withdrawals of the master cone occurred during the student obturations, indicating the importance of the choice of master cone, as described by Schilder (1) and Machtou (12). The procedure may also fail because the master cone is not adapted to the apical constriction. The tug back is thus obtained, not at the apical constriction level but on the walls of the canal. As Figures 11A and 11B show, initially the obturation appeared as normal, but when the coronal plug was eliminated, the compaction forces pushed the cone through the apex into the periodontium.
1. Schilder H. Filling the root canal in three dimensions. Dent Clin North Am 1967;11:723-44. 2. Instruction Guidelines for the Touch 'n Heat model 5002. Analytic Technology, Redmond, WA. 3. Schilder H, Goodman A, Aldrich W. The thermomechanical properties of gutta-percha. Part II1: determination of phase transition temperatures for gutta-percha. Oral Surg Oral Med Oral Pathol 1974;38:109-14. 4. Marciano J, Michailesco P. Dental gutta-percha: chemical composition, X-ray identification, enthalpic studies and clinical implications. J Endodon 1989; 15:149 -53. 5. Saw L-H, Messer HH. Root strain associated with different obturation techniques. J Endodon 1995;21:314-20. 6. Ricks-Williamson LJ, Fotos PG, Goel VK, Spivey JD, Rivera EM, Khera SC. A three-dimensional finite-element stress analysis of an endodontically prepared maxillary central incisor. J Endodon 1995;21:362-67. 7. Dang DA, Walton RE. Vertical root fracture and root distortion: effect of spreader design. J Endodon 1989;15:294-301. 8. Pitts DL, Matheny HE, Nicholls JI. An in vitro study of spreader loads required to cause vertical root fracture during lateral condensation. J Endodon 1983;9:544-50. 9. Gimlin DR, Parr CH, Aguirre-Ramirez G. A comparison of stresses produced during lateral and vertical condensation using engineering models. J Endodon 1986;12:235=41. 10. MacSpadden J. Self-study course of the thermal condensation of gutta percha. Form N°8 337 V.S; 1980:10-80. 11. Telli C, Gulkan P, Gunel H. A critical reevaluation of stresses generated during vertical and lateral condensation of gutta percha in the root canal. Endod Dent Traumatol 1994;10:1-10. 12. Machtou P. Endodontie, 1st ed. CDP:Paris 1993:143-98. 13. 8lum J-Y, Parahy E, Machtou P. Thermomechanical analysis of gutta percha during warm vertical compaction. (in press.) 14. Nguyen NT. Obturation of the root canal system. Pathways of the pulp. St. Louis: CV Mosby, 1984;205-99.
Printed in U.S.A. VOL. 23, No. 2, FEBRUARY1997
Analysis of the Forces Developed During Obturation: Warm Vertical Compaction Jean-Yves Blum, DDS, Eric Parahy, DSO, and Jean-Paul Micailef, PhD
heat the gutta-percha to decrease the pressure of compaction. But to prevent destruction of its properties, too-high temperatures must be avoided (4). Excessive compaction pressures may cause root fracture. Using strain gauges, Saw and Messer (5) demonstrated that root canal obturation is the major cause of vertical root fracture because of the wedging effect generated in the canal by the compaction strain. Ricks-Williamson, et al. (6) studied the stress induced by obturations with finite-element analysis. They reported that vertical condensation produces more detrimental stresses throughout the root than lateral condensation. Other studies have analyzed the stresses induced by obturations by means of different experimental methods, such as the Instron testing machine (7) and photoelastics (8). To date, no investigation has studied the pressure applied during warm vertical compaction obturation. The aim of this study was to determine and display the vertical and lateral forces developed during an obturation using the warm vertical compaction technique.
The aim of this work was to measure and analyze the forces applied by endodontists during an obturation. This was achieved by devising a system of force transducers linked to acquisition software. The software allowed us to study the obturation forces in real time or to store them. In this initial study, the forces developed by endodontists and students during a warm vertical compaction were analyzed. The vertical and frontware backware horizontal direction forces were first stored. Graphs of the compaction forces were then generated, permitting the analysis of the obturation method, indeed, two cases of obturation failure were analyzed from these graphs. The mean values for the vertical forces applied by the endodontists and students were, respectively, 2.5 + 0.4 kg and 1.9 +- 0.9 kg; the mean values for the lateral forces were, respectively, 0.85 - 0.2 kg and 1.4 +- 0.6 kg. This device permits the analysis of compaction forces and may thus be highly useful in obtaining improvements in obturation techniques.
M A T E R I A L S AND M E T H O D S A computerized recording system was developed to record the forces applied during obturations. The system was composed of two force transducers (Captels, St. Mathieu de Treviers, France), electronic amplifiers, an analog-to-digital converter, a PC-compatible computer, and acquisition software. The sensitive component of the force transducers was a Wheatstone bridge of strain gauges. All strain variations were transformed by this bridge into electrical voltage variations in application of Poisson's hypothesis, i.e. a linear relation existed between a given pressure and the level of the corresponding electrical signal. The two transducers for the force measurement (Fig. 1 and 2) were disposed perpendicularly, with the end of one fixed on a frame and the other connected to a cupule in which the tooth was embedded (Fig. 3). The forces were measured in the vertical and frontward and backward horizontal directions. The sensors were connected with data acquisition software (LAPS) developed by INSERM (Unit 103, Montpellier, France) (Fig. 4). The electrical gain of amplification was first set at 1000 to obtain a voltage range between - 2 . 5 V to +2.5 V. The signals were then digitilized by an analogic-digital converter connected to the RS232 port of the PC. The sample frequency of the inputs was 800 Hz. The software gave access to the following functions:
The new endodontic techniques often use gutta-percha, the most acceptable and widely used filling material. Warm vertical compaction, as described by Schilder (1), is a technique that produces an homogeneous and dimensionally stable mass of gutta-percha. By a succession of heating and compaction, a wave of softened gutta-percha is gradually pushed down into the root canal system. Because of its overall efficiency, this technique has never been greatly modified. Indeed, only the heating procedure has been improved by new technology. The original heat carrier, heated by an open flame, has now been replaced by the Touch 'n Heat 5002 model instrument (2). In contrast, compaction is still performed by manual pluggers. The use of these instruments, in association with a suitable preparation, produces a tridimensional obturation. Schilder et al. (3) reported that pressure is necessary to adapt gutta-percha to the walls of the canal. Their research further showed that gutta-percha is not compressible, but that dimensional variations result from transitions in the gutta-percha's phases. These findings were later confirmed by Marciano and Michailesco (4). To perform warm vertical condensation, the endodontist has to 91
92
Blum, et al.
Journal of Endodontics
FiG. 3. Tooth embedded into the cupule (with light silicone).
F v = T v + Rv F u - Tn - R~
FIG. 1. Frontal photograph of the device.
FiG. 4. Description of the recorded forces (Fv, Fh). The vertical forces are transmitted (Fv = Tv + Rv). Only the resultants of the horizontal forces are transmitted (Fh = Th-Tv). (1) Plugger; (2) Gutta-Percha; and (3) Tooth.
Acquisition: three modes were available: storage and visualization, storage without visualization, and numerical storage. Disk access: for save and loading. Visualization: shows a graph in function of time. Oscilloscope: on-line visualization of data without storage. Transfer to Excel: transfer of the LAPS files to Excel's format. Protocol
FrG. 2. Frontal view of the device: (1) transducers forming a right angle; (2) cupule; (3) support frame; (4) to the Wheatstone bridge; (5) gutta-percha; (6) tooth; and (7) plugger.
Configuration: selection of port (COM 1 or COM 2) on the computer and the band rate of transmission (9600 to 110 K bands). Identification: identification of the channels to acquire numbers, names, frequency of the sampling, and duration of the acquisition.
Fifty freshly extracted human central maxillary incisors were selected for similarity in size, shape, and root canal anatomy. Teeth were stored in an environment maintained at 37°C and 100% moisture. A standardized preparation was performed (1). A #45 gutta-percha cone was fitted as master cone and adjusted to give a tug back and to be, radiographically, 1 mm shorter than the canal length. Warm vertical compaction was performed using four finger pluggers (Hu Friedy, Chicago, IL). The first was set to the entry of the canal, the second at the halfway point, and the third and fourth at, respectively, 9 and 7 mm short of the root length. A Touch 'n Heat device was used to heat the gutta-percha cone. The power setting was 8 and the device was used in discontinuous mode. The heating time was 8 s. The compaction was considered to be finished when the last plugger reached 7 mm from the apex. The remainder of the canal was completely filled using a MacSpadden compactor (10). Ten practitioners participated in this study. Five
Vol. 23, No. 2, February 1997
3.5 I A 3
I
Warm Vertical Compaction Forces
93
peaksof compaction]
2.5
A
== "
2 1.5
ito
1
number of session FiG. 6, Mean values of number of compactions during successive sessions,
0,5
-05
3
Time (ran) 0.8 -B
-~ 2
I,..do~oo,~=I
w
~ students
l
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4
5
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!0
FIG. 7. Mean values of vertical forces during successive sessions.
-0.2
3~
43,4 2
B verticalforces Illatera forces J
43.6
Time (ran) 1
F~G. 5. Typical graphs generated by the warm vertical compaction technique. Line represents the tracing from gauge over time. (5A) Vertical forces. (5B) Horizontal forces. (1) First pushing phase; (2) Last compaction; and (3) Mac-Spadden compaction. were endodontists, and five were dental school students. The teeth were embedded in the cupule with Night silicone (Coltex Fine, Pierre Rolland, France) (Fig. 3). The operators performed five sessions of four successive warm vertical compactions. During the obturations, the screen of the PC was occluded, but forces were stored. Off-line, the stored files were loaded and the graphs of the forces exerted during the obturations were analyzed as a function of time (Fig. 5A and 5B). The choice of teeth and operators was randomized. After the first session, the practitioners viewed their graphs. The plots of force versus time generated by the endodontists and students were analyzed and a new session of obturations was then performed. Between all sessions, graphs were analyzed in the same manner.
RESULTS An analysis of variance was performed for all results. When the A N O V A F ratio was significant, a contrast test was used. The
1
2
3
4
5
6
7
8
number of compaction
FIG, 8. Comparison of mean values of vertical and lateral forces during successive sessions.
mean number ( + sem) of compactions for one obturation was 8 -+ 1 for the endodontists (Fig. 6). The mean value of the vertical forces was 2.5 _+ 0.4 kgF (Fig. 7). No significant difference was found for the forces exerted over the course of the obturation. The mean value of the lateral forces was 0.85 +- 0.2 kgF (Fig. 8). The mean duration of compaction was, respectively, 2.03 +_ 0.13 min for the apical packing and 5 + I s for filling of the remainder of the canal. Duration between two compactions was I l + 5 s (Fig. 9). The mean duration of one compaction was 6 -+ 3 s except for the last, which lasted 15 + 3 s. During the last compaction, the exerted force was never constant but instead showed a decrease from beginning to end (i.e. 2.8 _+ 0.2 kgF to 2.0 -+ 0.1 kgF) (F = 14.6, p < 0.05). For the students, the number of compactions was 11 -+ 5 for the three first sessions and 8 + 2 for the last two (Fig.
94
Blum, et al.
~o]
35 A 3t
" .
Journal of Endodontics
!! ,
_
oo,.o,,od ,, II.,
I ill ,
,
i- 5o'-
lj,,,tllll I
number of session FiG. 9, Mean values of time between two compactions. Variations over the successive sessions.
. . . . . -05 Time (Inn)
0.6-
6). T h e difference between these results was significant (F = 121.3, p < 0.01). During the three first sessions the mean values of the fc,rces exerted by the students were significantly lower than those ,.ff the endodontists (F = 13.5, p < 0.05). In contrast, the last two sessions showed no difference in either number or force of compaction between the two groups (Fig. 7). The lateral forces for the students were always greater (1.4 ± 0.6 kgF) than those of the endodontists (F = 13., p < 0.05). Moreover, the time between two compactions was also always greater (F = 14.6, p < 0.001) (Fig. 9).
DISCUSSION
0.5
B
[
2
0.4 0,3
i 0.2 0.1 0 -0.1 -0.2 43.3
The measurement device allowed the storage of the forces developed during warm vertical compactions performed by endodontists and students. Details of compaction were revealed on graphs (Figs. 5A and 5B). In the horizontal plane, the recorded forces were not true intracanal forces but only the resultant forces (Fig. 4). Recording revealed an imbalance in the intracanal forces, which may produce highly detrimental lateral forces. Our results were in agreement with those of Ricks-Williamson et al. (6) who reported that warm vertical compaction produced more detrimental stresses than other techniques such as lateral condensation. The imbalance of the students' forces was initially very great. In contrast, that of the endodontists was always very low. This indicates the difficulty of producing a balanced vertical force during obturation. The recorded forces did not agree with the results of Telli et al. (11) who used a 2-D-study design, nor with those of Gimlin (9) who used a 2-D finite-element method. They reported that guttapercha needed to be vertically condensed with a load of 4.5 kg. In our study, the recorded forces never exceeded 3 kg. This difference may be due to differences in the heating and pushing sequences. However, as this study is the first to our knowledge to present visual information on the sequence durations, it is impossible to analyze our results in comparison with those of other studies. Nevertheless, our comparatively low value is in agreement with the concept of the softened wave developed by Machton (12). The choice of pluggers, which ensured that there were no interferences between the walls of the root canal and the instruments, prevented the wedging effect described by Ricks-Williamson et al. (6). Indeed, the depth of penetration by the pluggers was controlled by a stop that bad been fixed on the plugger.
Time Iron)
FIG. 10. Typical graphs of failure in compaction due to underestimated master cone. (10,4)Vertical forces; and (10B) Lateral forces. (1) lack of vertical forces; and (2) lack of last compaction.
Schilder (1) described the importance of the last compaction, which is performed to reduce the final cooling dimensional variations. This last compaction should last 15 _+ 4 s (14). The graphs show that the endodontists always performed this compaction. They were unable, however, to exert a constant force. At the beginning of the study (Fig. 10A and 10B) the students did not produce this last compaction, whereas Figures 1 IA and 11B) show the improvement in the last two sessions. From its inception, warm vertical compaction has been considered to be a very difficult technique (15) because of the inability to monitor forces and heating time. The only accessible observation has been the radiograph of the tooth after treatment. One of the advantages of the device developed for this study is the potential for on-line monitoring of forces and heating and compaction sequences during obturation. Evaluation and correction zre possible in real time, offering, for the first time to our knowledge, a bio-feedback method for improving technique. Two kinds of evaluation are possible. Sequences between heating and compaction can be studied, with corrections made on-line or off-line of the operating act. The applied forces may also be studied by direct analysis during the obturation itself (on-line) or by comparison to mean force values (off-line), i.e. corrections can be made in reference to stored graphs. Perhaps the greatest advantage of the graphs is for the detection
Vol. 23, No. 2, February 1997
3
Warm Vertical Compaction Forces
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The graph shows a lack of sufficient vertical force; with no reaction force opposed to the compaction forces, sliding of the cone resulted. The increase in the diameter again blocked the cone and thus the forces again appeared normal. The graph indicates on-line the failure of the obturation. During this study, the students involuntarily produced these failures but were able to analyze their errors following a brief explanation of the graphs. This monitoring of their obturations permitted the students to progress (Figs. 6 and 7). Indeed, perhaps the most interesting analysis was the comparison between the working techniques of the students and the endodontists. The result of this comparison was a modification in the applied forces and the sequences of heating and compaction. Instead of assessing the quality of their work by radiograph of the treated tooth, students were able to focus their analysis on the individual compactions forces and sequences. It also appeared that each endodontist had his own work protocol for the heating and compaction phases. The force values were not exactly the same between the different endodontists but seemed to be uniform for each individual endodontist. Thus, each graph could be considered as a signature. In conclusion, the device developed for this study appears to have potential for research and teaching. As our results show, it allows the analysis of the forces and sequences of warm vertical compaction. Other obturation techniques may also be studied. The authors thank INSERM, Unit 103, for the use of their LAPS acquisition software and Catherine Scott Carmeni for technical assistance. Address requests for reprints to Dr. Jean-Yves Blum, 12 rue de I'Aubier, 34130 St. Aunes France.
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References
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Time (mn) FiG. 1 1. Typical graphs of failure in c o m p a c t i o n due to underestimated apical diameter of master cone. (11A) Vertical forces; (11B) Lateral forces. (1) Normal forces; (2) Lack of sufficient forces (sliding of master cone); and (3) last c o m p a c t i o n .
and analysis of failure, as the following examples show. Figures 10A and 10B show the specific obturation graph of an underestimated cone. This failure appeared as a lack of sufficient vertical force in the beginning of the obturation. The pluggers pushed into the canal to a length of approximately 6 mm from the entry without any opposition, and obturation proceeded without reaction forces. As a result, the root canal system was filled but not three-dimensionally obturated. Moreover, because the coronal plug was not performed, accidental withdrawals of the master cone occurred during the student obturations, indicating the importance of the choice of master cone, as described by Schilder (1) and Machtou (12). The procedure may also fail because the master cone is not adapted to the apical constriction. The tug back is thus obtained, not at the apical constriction level but on the walls of the canal. As Figures 11A and 11B show, initially the obturation appeared as normal, but when the coronal plug was eliminated, the compaction forces pushed the cone through the apex into the periodontium.
1. Schilder H. Filling the root canal in three dimensions. Dent Clin North Am 1967;11:723-44. 2. Instruction Guidelines for the Touch 'n Heat model 5002. Analytic Technology, Redmond, WA. 3. Schilder H, Goodman A, Aldrich W. The thermomechanical properties of gutta-percha. Part II1: determination of phase transition temperatures for gutta-percha. Oral Surg Oral Med Oral Pathol 1974;38:109-14. 4. Marciano J, Michailesco P. Dental gutta-percha: chemical composition, X-ray identification, enthalpic studies and clinical implications. J Endodon 1989; 15:149 -53. 5. Saw L-H, Messer HH. Root strain associated with different obturation techniques. J Endodon 1995;21:314-20. 6. Ricks-Williamson LJ, Fotos PG, Goel VK, Spivey JD, Rivera EM, Khera SC. A three-dimensional finite-element stress analysis of an endodontically prepared maxillary central incisor. J Endodon 1995;21:362-67. 7. Dang DA, Walton RE. Vertical root fracture and root distortion: effect of spreader design. J Endodon 1989;15:294-301. 8. Pitts DL, Matheny HE, Nicholls JI. An in vitro study of spreader loads required to cause vertical root fracture during lateral condensation. J Endodon 1983;9:544-50. 9. Gimlin DR, Parr CH, Aguirre-Ramirez G. A comparison of stresses produced during lateral and vertical condensation using engineering models. J Endodon 1986;12:235=41. 10. MacSpadden J. Self-study course of the thermal condensation of gutta percha. Form N°8 337 V.S; 1980:10-80. 11. Telli C, Gulkan P, Gunel H. A critical reevaluation of stresses generated during vertical and lateral condensation of gutta percha in the root canal. Endod Dent Traumatol 1994;10:1-10. 12. Machtou P. Endodontie, 1st ed. CDP:Paris 1993:143-98. 13. 8lum J-Y, Parahy E, Machtou P. Thermomechanical analysis of gutta percha during warm vertical compaction. (in press.) 14. Nguyen NT. Obturation of the root canal system. Pathways of the pulp. St. Louis: CV Mosby, 1984;205-99.