ABSTRACT
The aim of this in vitro study was to determine the duration of heat retention (sD) at the root surface as the canal was enlarged with a Ho:YAG laser. Thirty single rooted human teeth were randomly assigned to one of three groups according to laser power settings: 0.50 W, 0.75 W and 1.00 W. Each tooth was subjected to lasing using fiberoptic sizes of 140, 200, 245, 300, 355, and 410 microns. Thermocouples gave the duration of heat retention (sD) for each coronal and apical measurement. The dependent variables included: (a) change in temperature (sT), (b) duration of heat retention (sD), (c) depth of the tooth during lasing, (d) depth of the tooth using conventional files, and (e) tooth measurements. The means for sD ranged from 10 to 60 seconds. Both Coronal and Apical sD means differed statistically significantly according to fiber size and power setting. Correlation coefficients (r) showed that for each fiber size both for coronal and apical there was a moderate and moderate to strong significant correlation between temperature change sT and Duration sD.
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If a laser is going to be used to debride and prepare the canal for the endodontic filling material, the accumulation and dissipation of heat is of great concern in this procedure. Too great an increase in heat for too long a time period may necrotize the periodontal ligament, alveolar bone or both.
Cohen et al,1 demonstrated that when using a single 245 micron fiber optic tip and a maximum power of 1.0 watt, using a Ho:YAG laser in vitro, the external temperature of the cementum was not increased by more than 5 degrees Celsius (mean apical temperature 2.21C and mean coronal temperature 1.16C).1 In another in vitro study by Cohen et al.,2 enlarging the canal using three different power settings (0.50,0.75 and 1.00 W) and four different fiber optic diameters all temperature differences observed apically and coronally were between 0 to 10C, with the majority (>98%) being between 0 to 5C.2 These studies seem to indicate that multiple fiber optic tips are necessary to enlarge the canal to an appropriate size for Gutta Percha obturation.
Therefore, when using multiple fiber optic tips, it is not only prudent to know to what temperature the tissue around the root will be heated to but also to know for how long the bone and ligament will be heated.
The aim of this experiment was to measure the duration of heat retention in the cementum after lasing with various fiber optic sizes at various power settings.
MATERIALS AND METHODS
Thirty freshly extracted single rooted human teeth (maxillary incisors and canines) were randomly assigned to one of three groups according to laser power setting: 0.50, 0.75 and 1.00 Watts. Teeth were sectioned at the CEJ and instrumented 10mm from the coronal portion of the tooth with a number 15 K file (to 150 microns). Each tooth root was then measured in several dimensions with the use of a digital caliper (Fowler & NSK, Tokyo, Japan). These dimensions were a) apical to coronal distance b) mesial/distal coronal c) mesial/distal apical d) buccal/lingual coronal e) buccal/lingual apical f) average coronal g) average apical. Microprocessor thermometers (Model HH23, Omega, Stanford CT) with T-type thermocouplers (Omega) were applied to the cementum of each root at approximately 2mm from the coronal and 2mm from the apical end, (Fig.1) as described previously by Cohen et al.1-2 The thermometer and thermocouple measured the change in temperature (sT) at the surface of the root. In addition these thermocouples were connected to a programmable chart recorder (RD 102, Omega), which also gave the duration of heat retention (sD) for each coronal and apical measurement. sD difference was calculated by a ratio of the known chart speed (12000mm per hour) and distance for the temperature delta difference (duration of heat retention sD) for apical and coronal measurements.
Each tooth in each power group was subjected to lasing using fiber sizes of 140, 200, 245,300, 355, and 410 microns. The fibers were placed into the tooth toward the apical end. The fiber optic guide was then energized with Holmium YAG (Ho:YAG) laser energy at 2.09 microns and withdrawn slowly at approximately 4mm per second. The active cutting laser energy was directed away from the long axis of the fiber optic guide in the shape of an annular ring. In this energy configuration there was no cutting energy in the center of the ring. The cutting energy was in the shape of a donut, with no energy in the hole center. In affect the lateral walls of the canal were lased and not the apical area (Fig. 2.). This lasing configuration was accomplished using a proprietary optical setup in the laser itself. Please note that the maximum length for lasing was 10mm as per the initial instrumentation with the 15 K file. Temperature (sT) and duration of heat retention (sD) were recorded and tabulated, for each fiber from 140, 200, 245, 300, 355, to 410 micron.
After debridement with the laser was completed, endodontic files were inserted into the canal as far as they would go. The depth of penetration of that file into the canal was measured and recorded. This was done to determine the maximum size of the file that would now fit into the canal and consequently the amount that each canal diameter was enlarged by the laser energy used in that group.
STATISTICAL METHODS
The dependent variables analyzed included: (a) change in temperature (sT), (b) duration of heat retention (sD, i.e., time from lasing until return to initial temperature), (c) diameter of the tooth during lasing, (d) depth of the tooth using conventional files, and (e) tooth measurements. For (e), each tooth had three replicate measurements; the mean of the three measurements was used in the analysis. For parameters involving both apical and coronal measurements [(a), (b) and (e)], these two sites were analyzed separately.
Each of the five types of tooth measurements (mesiodistal coronal [(MDC) and apical (MDA), buccolingual coronal (BLC) and apical (BLA), and total length] were made in triplicate. The triplicate measures were averaged to yield a summary measure of that particular tooth dimension. In addition, to simplify the use of covariates as described below, MDC and BLC were averaged to yield a summary coronal measurement (AVG Coronal); the same was done for the MDA and BLA (AVG Apical). Standard one-way analysis of variance (ANOVA) was used to compare the various tooth measurements across the three power setting groups. Repeated measures analysis of covariance (RMANACOVA) was used to analyze (a) and (b). The “within subjects” factor was fiber size and the “between subjects” factor was power setting. For (a) and (b), Average Coronal, Average Apical, and total tooth length were considered as covariates. The actual covariates(s) used in the final statistical model depended on the results of the RMANACOVA. The use of tooth size as a covariate was essential in drawing proper inferences. Although, tooth sizes were equally distributed across the three power settings, the actual tooth size was clearly related to changes in temperature and duration of that temperature change in any specific tooth. Therefore, adjustment for tooth size was important.
Repeated measures analysis of variance (RMANOVA) was used to analyze (c), tooth diameter, with fiber size and power setting as the factors. RMANOVA was also used to compare depths using files (d), where file size was the “within” factor, power setting the “between” factor.
Upon finding a significant fiber effect, within-subjects pairwise contrasts were computed in order to determine which fibers differed from one another. A significant power setting effect was followed up with a Student-Newman-Keuls (SNK) multiple comparisons test. For the analyses of temperature change and duration of heat retention (sD), the logarithmic transformation was considered; however, residuals seemed reasonably consistent with a normal distribution in both the transformed and untransformed cases. Therefore, for simplicity, the results were reported using
the original, untransformed data.
Results were considered statistically significant if P<0.05. However, when performing the multiple pairwise contrasts for fiber and file analyses, a result was considered significant only if P<0.003, which corresponds to a Bonferroni correction for 15 pairwise comparisons.
The Pearson correlation coefficient was used to estimate the correlation between change in temperature sT and duration of heat retention sD.
RESULTS
Tooth Measurements- Summary statistics (mean and standard deviation) for the seven tooth measurements (buccolingual coronal and apical, mesiodistal coronal and apical, total length, average coronal and average apical), using ANOVA showed no significant differences between power settings for any of the seven tooth measurements (P=0.9110). This result demonstrates that there was no bias with respect to the way that teeth were assigned to treatment condition.
TEMPERATURE CHANGE
Coronal–Preliminary RMANACOVA showed that average coronal measurement was the only tooth size measure that was associated with change in coronal temperature (P<0.0001). RMANACOVA for coronal showed that, after adjustment for average, mean temperature change differed significantly according to fiber size (P<0.026). There was no significant difference in temperature changes between power settings. No interaction between fibers and power settings was observed.
Despite the significant fiber effect in the RMANACOVA, pairwise multiple contrasts for fibers could not show which fiber sizes differed from one another. This inability to discriminate is likely due to low power, which is a result of large variation and/or too small a sample size.
Apical–Preliminary RMANACOVA showed that total tooth measurement was the only tooth size measure that was associated with change in apical temperature (P<0.0001). RMANACOVA for apical showed that, after adjustment for mean temperature change differed significantly according to fiber size (P<0.003). There was no significant difference in temperature changes between power settings. No interaction between fibers and power settings was observed. Pairwise contrasts revealed that fiber size 410 yielded temperature changes that were significantly smaller than those obtained for sizes 140, 200, 245, and 300, but not from 355. No other differences between fibers were observed. Once again, the inability of these tests to discriminate further may be due to either: lack of any further differences or, low power, the latter a result of large variation and/or too small a sample size.
DURATION OF HEAT RETENTION (sD)
Table 1 demonstrates mean and standard deviations for results for duration of heat retention (sD) apical and coronal. Overall results range from 10 seconds to 60 seconds per root. Table 2 illustrates summary statistics for duration of Heat Retention (sD) according to power setting (Watts), fiber size and site (Apical or Coronal).
Coronal–Preliminary RMANACOVA showed that average coronal measurement was the only tooth size measure that was associated with change in coronal temperature (P<0.0001). RMANACOVA for coronal showed that, after adjustment for average, mean duration differed significantly according to fiber size (P<.014) and power setting (P<0.0001). No interaction between fibers and power settings was observed.
Pairwise multiple contrasts for fibers showed that the duration of temperature change for fiber size 355 was significantly larger than for sizes 140, 200, and 245, but not for 300 or 410. The inability of these tests to discriminate further may be due to either: lack of any further differences or, low power, the latter a result of large variation and/or too small a sample size.
Apical–Preliminary RMANACOVA showed that total tooth measurement was the only tooth size measure that was associated with change in apical temperature (P<0.0001). RMANACOVA for apical showed mean duration differed statistically significantly according to fiber size (P<.014) and power setting (P<0.0001). No interaction between fibers and power settings was observed. Despite the significant fiber effect in the RMANACOVA, pairwise multiple contrasts for fibers could not show which fiber sizes differed from one another. This inability to discriminate is likely due to, low power, which is a result of large variation and/or too small a sample size.
CORRELATION BETWEEN TEMPERATURE CHANGE (sT) AND DURATION OF HEAT RETENTION (sD)
In this part of the analysis, the degree of correlation between temperature change and its duration was investigated using standard correlation analysis.
Coronal–The correlation coefficients (r) show that, for each fiber size, there was a moderate, but significant, correlation between temperature change (sT) and duration (sD) (r ranging from r=0.37 to r=0.66, P-values ranging from P<0.045 to P<0.0001).
Apical–The correlation coefficients (r) show that, for each fiber size, there was a moderate to strong correlation between temperature change and duration (r ranging from r=0.60 to r=0.76, P-values ranging from P<0.030 to P<0.0001).
DEPTH OF FIBERS
In this section, fiber sizes refer to testing fiber, not lasing fiber. Some fibers had depths with standard deviation of zero; this was because all of the depths were 10mm. RMANOVA showed a significant difference between fibers (P<0.0001), none between power settings, and no interaction between fibers and power setting. Pairwise contrasts showed that mean depths for 200 and 245 microns were not different from each other, but were different from all other fibers; furthermore, all other fiber sizes (300 through 410) differed from one another.
DEPTH USING FILES
RMANOVA showed a significant difference between file sizes (P<0.0004), but not between power settings. No interaction between file size and power settings was observed. Pairwise contrasts revealed that results for file sizes 40 and 45 were not different from each other, but were different from file sizes 50, 55, 60 and 70, which were also different from one another. In fact, the downward trend in mean depth (Fig. 3) starting at file size 45 is statistically significant.
DISCUSSION
All temperature data (sT) obtained in this study (ranged from 0 to 11 degrees Celsius, with the majority being (greater than 92%) between 0 to 7.5 degrees Celsius) was similar to the data obtained from Deutsch et al.3 in which a thermocouple was used. In this study a more precise instrument was used (a chart recorder with thermocouples incorporated within it). On average temperature readings were slightly higher in this experiment than that measured in Deutsch et al.3 The duration (sD) of heat retention for all fibers, and wattages was between 10 to 60 seconds. This was a worst case scenario test where heat dissipation was carried out by only the surrounding ambient air. In a clinical situation heat dissipation would be carried out by the root, bone, periodontal ligament and flowing blood. As discussed in Cohen et al2 and Zach and Cohen,4 a sT subsequent to lasing of 5 degrees Celsius lasting one minute or less would most probably not cause necrosis in any of the vital tissues surrounding the root. In this experiment sT and sD fell within these parameters.
In order to ascertain how much the 140, 200, 245, 300, 355, and 410 micron fiber optics lased the canal, measurements of the depth of penetration using various sized files were recorded. Initially a depth of 10mm was instrumented with a number 15 file and then lased with a 140 micron fiber optic. A statistically significant downward trend in mean depth starting at file size 45 was noted (Fig. 3). As the file size increased the depth of penetration into the canal decreased. This means that the Ho: YAG laser did not widen the canal sufficiently enough for the larger sized files to go down to the original 10mm depth. This phenomena is not associated with debris from a charred fiber optic tip blocking the canal. This was also demonstrated by Deutsch et al in a recent experiment.5
This data warrants further attention and research in using a laser to debride the ro
ot canal.
Dr. Allan S. Deutsch is Co-Director of Dental Research; Dr. Brett I. Cohen is Vice-President of Dental Research; and Dr. Barry Lee Musikant is Co-Director of Dental Research, all at Essential Dental Laboratories, S. Hackensack, NJ.
Oral Health welcomes this original article.
REFERENCES
1.Cohen BI, Deutsch AS, Musikant BL. Effect of Power Settings on Temperature Change at the Root Surface when Using a Holmium YAG Laser in Enlarging the Root Canal. J Endodon 1996;22:596-599.
2.Cohen BI, Deutsch AS, Musikant BL, Pagnillo MK. Effect of Power Settings versus Temperature Change at the Root Surface when Using Multiple Fiber Sizes with a Holmium YAG Laser while Enlarging a Root Canal. J Endodon 1998;242:802-806.
3.Deutsch AS, Cohen BI Musikant BL, Temperature Change at the Root Surface, When Enlarging a Root Canal with a Holmium YAG (Ho:YAG) Laser, Using Six Different Fiber Optic Sizes. Oral Health Submitted 2003.
4.Zach L, Cohen G. Pulp response to externally applied heat. Oral Surg Oral Med Oral Path 1965;19:515-530.
5.Deutsch A, Cohen BI, Musikant BL, Inductively Coupled Plasma-Emission Spectroscopy and Atomic Absorption for the use of Elemental Analysis of a Root Canal after Lasing with a Holmium:YAG Laser, J Endodon 2003;29 (6):404-406.
