Low Level Laser Therapy has many biostimulative effects such as acceleration of mesiodistal orthodontic tooth movement. However, its effects on extent and short-term stability of rotational tooth movement have not been researched.
Objective: The purpose of this study was to investigate the effect of low level laser irradiation during rotational tooth movement, on rate of movement and amount of relapse in dogs.
Materials and Methods: In this interventional study, fixed orthodontic appliance was used to rotate both mandibular lateral incisors in nine dogs. Eighteen teeth were divided into two groups: Experimental group with LLLT employed during orthodontic force application and a control group with orthodontic couple force application only. In the first group, the Gallium-aluminum-arsenide (Ga-Al-As) diode laser (810nm, 200mw, 10s, 2 J/session, 32 J/cm2/point) was emitted on 2 points at buccal side of tooth root 9 times scheduled during 4 weeks of movement and the amount of relapse was then observed for 3 months. The extent of rotational movement was measured on dental casts and the statistical analysis was carried out using "Wilcoxon signed ranks test" (SPSS 11.5). ±
Results: There were no significant difference between LLLT group and control groups on the amount of rotational tooth movement (p=0.48). The mean percentage relapse after 1week, 1month and 3months were %33.02±4.21, %53.44±4.86, %60.64±6.33 in LLLT group and %54.22±4.04, %68.74±3.02, %73.92±3.85 in control group respectively which demonstrates significant differences between these two groups in all reflected times (p=0.012). This difference was greatest in first week and then, decreased until the end of third month. The greatest percentage of relapse in both groups was registered in the first week after tooth movement (54.67% in lased group and 73.52% in control group).
Conclusion: The energy density of the laser that was used in this study could not accelerate rotational tooth movement, whereas it could effectively reduce relapse tendency in teeth rotated by orthodontic movements.
Key words: low level laser therapy, rotational movement, relapse, orthodontic
Introduction
Orthodontic treatments are performed to improve patients' smile, facial appearance and consequently promoting their quality of life. (1).A tendency of teeth to return to their early position is called relapse which is an inevitable reality in orthodontic treatment. It is an inconvenient situation which may lead to patients' frustrations and consequent problems for orthodontist. Moreover, any prediction is impossible since definite causes of orthodontic relapse are still unknown. (5). Long-term usage of retainers is one of the most widespread methods used for preventing the incidence of relapse (5, 6).However; they cannot stop the relapse completely, particularly after rotational movements of teeth 3, 7-9). Currently, supracrestal fiberotomy is an effective method to decrease the rate of the relapse followed by tooth rotational movements, since this procedure stimulates the remodeling of periodontal fibers in its new position while it can be accompanied by several complications, as well. (6, 10-12).
Recently, the effect of low level laser therapy (LLLT) on remodeling of both the soft and the hard tissues surrounding the teeth has been confirmed) 13, 14, 19, 20, 24). Tooth movement mechanisms, both during and after orthodontic treatments, are based on selective stimulation of bone remodeling. Regarding the effect of LLLT on releasing of inflammatory mediators and on the activity of osteoblasts and osteoclasts which are present in the bone around the tooth root; dental movements are supposed to be influenced by LLLT both during and after the orthodontic treatments (13)
Up to now, many studies have been conducted on exploring the effect(s) of LLLT on orthodontic tooth movements. In most of these researches, only one type of dental movements; i.e., mesiodistal movement of the teeth inside the alveolar bone, has been investigated and other dental movements, such as the rotational movements or root torque which might be requisite in orthodontic treatments, have not been taken into account (14-18). Moreover, due to the various bone apposition-resorption (remodeling) patterns in different dental movements, the results of the previously mentioned studies could not be generalized to all orthodontic movements without further investigations.
LLLT can reduce the relapse of orthodontic treatments by increasing the tissue remodeling as well as the bone density (19). This theory, in fact, suggests a non invasive physiological method to prevent relapse .Preserving the results after the active phase of the orthodontic treatment and reducing the incidence of the relapse are considered as the maintenance of the treatment process. Only a limited number of studies have investigated the effect of LLLT on the relapse of dental movements after the orthodontic treatments19, 23. An important reason for the relapse is considered to be the gingival and transseptal fibres of the periodontium, which are stretched and warped in rotational movements and are assumed to be the main factor in returning of teeth to their early position (2). Low-level laser therapy (LLLT) can biostimulate collagen synthesis, and speed up bone and dental fibre remodelling during tooth movement. (1-3 check the reference in my comments). This can in turn; increase the stability of these rotated teeth and decrease the rate of relapse and also pain and inflammation induced by orthodontic treatments. Therefore, this study was managed to evaluate the effectiveness of LLLT on orthodontically rotated tooth and its potential to increase stability of the treatment (20).
Materials and Methods
The present study was conducted based on the principles of animal rights defined by Vice-Chancellery Shiraz University of Medical Sciences, Shiraz, Iran. The study was performed on 9 healthy male beagles each aged between 12-18 months old and weighed about 18 - 24 kg Intraoral examination confirmed that all the permanent teeth were present in the mouth, with no caries or mobility. There were neither localized nor generalized gingivitis and absence of any ankylosis was assured by percussion test.
To eliminate the effect of intrinsic factors on the speed and degree of tooth movements and decreasing the relapse tendency after orthodontic movements, both mandibular lateral incisors were studied. Eighteen mandibular incisors were divided in two groups: left teeth were irradiated during the orthodontic movements and considered as the experimental group while the right side teeth were considered as the control group. All the research processes were performed under general anesthesia by Intramuscular injection of a mixture of Xylazine (10mg/kg b.wt.) and Ketamine (87 mg/kg b.wt.)
Orthodontic Force Application
In order to induce the rotational movement, couple forces were applied to mandibular lateral incisors of both sides. Orthodontic buttons (American- U.S.A.) were bonded to the labial and lingual surfaces of the lateral incisors, the labial surface of canines, and the lingual surface of the central incisors. Then, two elastic chains (American- U.S.A.), which exerted a force of 50 gram, were engaged between the buttons of the buccal sides of the lateral incisors and the canines. A single elastic chain was used to create forces since the distance between the lingual side of the lateral incisors and the central incisors were very short to create rotational couple forces. This single-ring elastic chain was stretched before being placed between the buttons (prestretched) to prevent any force application more than 50 gr. Force application was accomplished within 4 weeks and the elastic chains were changed once a week. After 4 weeks of applying rotational force, all orthodontic appliances were removed and the teeth were monitored for 3 months in order to investigate the stability of the movements.
Laser Irradiation
In this study, Ga-Al-As diode laser (THOR: DD2 - England) with power output of 200 mw and a continuous wavelength of 820nm (within the infrared rage) was emitted from a probe (2 mm in diameter). Irradiation was performed on 2 points at the buccal side of the left mandibular lateral incisor: 1- between cervical third and middle third of the root (5 sec) and 2- between middle third and apical third of the root (5 sec). During the irradiation, probe was placed perpendicular the tissue with a gentle contact without applying any pressure on it. Therefore, the tooth received 2 J energy (200 mwÃ-10 s) (31.8 J/cm²/point) during each irradiation session. Laser irradiation was performed in days 0, 1, 2, 3, 4, 7, 14, 21, and 28 after the beginning of force application (9 sessions).
Time and Method of Measuring the Dental Movements
In order to record the teeth status, impression was performed using silicon materials (Speedex - Iran) and measurements were performed on dental casts made of Stone. Moreover, the teeth position was recorded in T0 (before orthodontic force application), T1 (at the end of the 4th week, end of the phase of active dental movement), T2 (at the end of the 5th week, 1 week after stopping the orthodontic force), T3 (at the end of the 2nd month, 1 month after stopping the orthodontic force), and T4 (at the end of the 4th month, 3 months after stopping the orthodontic force).
Dental casts were trimmed in a way that the occlusal surface of the posterior teeth was parallel to the horizon. Then, in a background with proper contrast, photography was prepared from the occlusal view of the dental models. Afterwards, the digital images were converted into PSD format through Adobe Photoshop (version 8) and related measurements were performed using SolidWorks Premium 2010 software. In doing so, images were opened in sketch mode and three lines were drawn: 1-a line crossing the most prominent cusp tip of the second premolars of both sides, 2- a line along the incisal edge of the right lateral incisor, and 3- a line crossing the incisal edge of the left lateral incisor. Then, by using the Extend Entities item from Trim Entities menu, the 2nd and the 3rd lines were continued until they crossed the 1st line. By using the Smart Dimension item, the angel between the 1st and the 2nd line as well as the angel between the 1st and the 3rd line were measured up to 7 decimal places. The obtained numbers were then rounded to 2 decimal places. At the end of the study, in order to determine the accuracy of the images, 20 dental casts were randomly selected by using chart of random numbers and were photographed under similar conditions. Moreover, in order to determine the accuracy of the measurement method, all the measurement processes were repeated for 30 images.
Results
Investigation of the Accuracy and Repeatability of the Measurement Method
Evaluation of the distribution of the data of repeated measurement of 30 photographs, the data were collected from the repeated photography of the 20 dental casts, and drawing their regression model led to drawing lines which crossed the intercept and had slope 1, which in fact confirmed the consistency of the numbers obtained from the repeated measurements. Furthermore, investigation of the error of measurement after repeated measurement of 30 dental casts through Dahlberg's formula revealed 0.7 degree of error.
Comparison of the Rate of Tooth Movement in T1
Four weeks after the beginning of couple force application, the degree of rotation of the experimental mandibular lateral incisor was more than their counterpart in the control group; however this difference was not statistically significant.
Comparison of Rotational Movement Relapse after 1 Week (T2)
Investigation of rate of relapse of the rotational movement in the active phase of dental movement in both groups demonstrated that, less relapse had occurred in the experimental group after one week and the difference between the two groups was statistically significant.
Comparison of Rotational Movement Relapse after 1 Month (T3)
Relapse had occurred less significantly in the experimental group compared to the control group when the active orthodontic force was discarded at the end of the 1st month. On the other hand, no significant difference was observed between the two groups regarding the percentage of relapse occurring from the end of the 1st week up to the end of the 1st month (T2-T3).
Comparison of Rotational Movement Relapse after 3 Months (T4)
Three months after finishing the orthodontic force, the percentage of relapse in the experimental group was significantly less than the control group. Nevertheless, no significant difference was observed between the two groups regarding the percentage of relapse occurring from the end of the 1st month up to the end of the 3rd month (T3-T4).
Percentage of Relapse after 1 Week, 1 Month, and 3 Months
In both study groups, the highest percentage of relapse had occurred during the 1st week, while the lowest percentage of relapse had taken place between the end of the 1st month and third month.
Discussion
According to the results of the present study, although the dose of LLLT used in this study (32 J/cm²/point) did not accelerate the rotational movement of lateral incisors in the samples, it considerably reduced the relapse following the rotational movements. Although this effect is reduced in time, a considerable decrease in the rate of relapse can be observed, particularly in the 1st week after the end of active dental movement. Moreover, the total percentage of relapse in the LLLT group remained significantly lower than that of the control group (P=0.012) at the end of the 3rd month of the study.
The analgesic and anti-inflammatory effects of LLLT have been well known for years. Also, several studies have emphasized its biological stimulation (15, 21-23). In fact, laser is able to increase the proliferation as well as the activity of osteoblasts, osteoclasts, and fibroblasts and, consequently, accelerate bone formation or deformation and, at the same time, increase the bone density in the irradiated part (15, 19, 24-27). Therefore, researchers can utilize laser therapy as an ideal method in accelerating the dental movements as well as reducing the pain and inflammation resulting from orthodontic treatments. Of course, contradictory results have sometimes been attained in this regard, which might be due to the variants in the type of the laser used (wavelength, dose, number of irradiations, and time of irradiation) and different animals used in the studies. It should be noted that all the studies on the effect of laser on orthodontic movements have investigated mesiodistal dental movements and other dental movements, such as dental rotations have not been contemplated (14-17, 22, 28). Nevertheless, mesiodistal and rotational dental movements are quite different regarding the bone apposition-resorption pattern during force application, the way which orthodontic force affects the dental supporting fibers, and the amount of hard tissue which should be removed when it is necessary to enhance orthodontic movement. Furthermore, no studies have been conducted on the stability of the orthodontic movements by LLLT. In fact, the present study is the first one which has investigated all the above-mentioned aspects.
Type of laser is selected based on the features of the target tissue as well as the intended effects on that tissue. Moreover, 550 nm - 950 nm has been considered as the best wavelength for tissue stimulation (13). Generally, irradiation in the infrared range penetrates more than the visible light in the soft tissue. Similarly, In fact, lasers with their wavelength in the infrared range are less absorbed by hemoglobin or water and, as a result, deeply penetrate through the irradiated tissues (26). Since the present study aimed to stimulate the bone cells which were located deep under the soft tissue (gingiva and oral mucosa) and in the PDL space, infrared laser with the wavelength of 810 nm was employed.
Determining the effective dose is considered to be the most difficult issue regarding LLLT. Several studies which investigated the effect of laser on orthodontic movements have employed doses of 2-54 J and conveyed their prompt effects on accelerating the dental movements (14, 15, 18, 29). On the other hand, some studies have used doses of 8.1-25 J and reported negative results (16, 17, 22). Therefore, the range of non-effective doses overlaps with the range of the effective ones and, as a result, no ideal dose can be determined.
The time and the number of irradiations are also very important. In fact, the effects of frequent irradiations are collected in the tissue and the final result depends on the total irradiation dose. Therefore, the moment of commencement and times of repeating the laser irradiation is of great importance. Investigation of the effect of laser irradiation on bone regeneration in mid-palatal expansion during rapid palatal expansion in rats revealed that either single or late (after 4th - 6th days) laser irradiation had no effects. On the other hand, the greatest effect was occurred when irradiation started in days 0-2(30). In fact, primary laser irradiations stimulated bone formation, while the subsequent ones were preserving the process of bone formation (31). Considering all the above-mentioned issues, energy equal to 32 J/cm² was used in each irradiation point in the present study. Moreover, since the greatest effects of laser on dental movements are observed during the initial days (28), laser irradiation was begun from the beginning of force application in day 0 and repeated everyday during the primary days. Furthermore, due to the high output power of the laser utilized in the present study (200 mw) and in order to avoid the range of inhibitory doses during the rotational dental movement, irradiation was only performed from the buccal side of the tooth and on 2 points. Moreover, elastics were changed regularly accompanied by force application and laser irradiation which was repeated and continued up to the last day of force application (14, 18, 28). In order to estimate the final laser dose, ray dispersion in the tissue must also be taken into account. Luger et al. believe that ray dispersion reduces the level of energy to 3-6% of its initial density (32). Besides, Kawasaki claims that only 50% of laser penetrates from the mandibular cortex to the depth of 1 mm (18). Evidently, to enhance bone formation, higher doses of laser have been recommended. (28).
The rate and the method of couple force application in the present study are consistent with those of the study conducted by Kim et al.; however, in order to apply couple force to the central incisors, they placed the elastic chain between the two central incisors from the lingual side (23). In that way, although the number of the orthodontic appliances bonded to the teeth was reduced and their control was enhanced, the rate of incisor movement in one side could affect the incisor movement of the other side. Therefore, in order to apply couple force to the lateral incisors in the present study, central incisors were employed for bonding the elastic chain from the lingual side.
In line with other studies investigating dental rotations, the measurement method of the present study also evaluated the status of the teeth related to the constant reference line (23, 33-36). In addition, the reference line of the present study crossed the most prominent cusp tip of mandibular second premolars. Moreover, we used the aforementioned software for the first time to measure the degree of tooth rotations. In comparison to similar studies this method has reduced the error of measurement to 0.7 degrees. In this method, photography from the study sample and its dental cast (34, 35) or tracing the dental casts was used instead of radiography (6, 36, 37), which reduces the dose of received rays as well as the probability of error in tracing the teeth status. Nevertheless, if laser scanners were employed and direct digital images were prepared from the intraoral status, photography could be omitted and, consequently, the measurement method is improved.
In general, the studies which have investigated the effect of LLLT on dental movements can be classified into 3 categories:
1. The studies which have shown LLLT to accelerate dental movements (14, 15, 24, 26, 28, 29, 38).
2. The studies which have revealed LLLT to have no effects on dental movements (13, 17, 22, 39, 40).
3. The studies which have shown LLLT to reduce the speed of dental movements (16, 28).
These studies are quite different regarding the study samples, method of force application, and the utilized laser dose; so that the ideal dose affecting the speed of dental movements could not be determined. The findings of the present study revealed similar dental movements in the control and the LLLT group, which was mostly consistent with the results of the 2nd group studies. Nevertheless, all the studies conducted on the issue have investigated mesiodistal dental movements and the effect of laser irradiation on orthodontic rotational movements could not be detected in the published articles. Therefore, the findings of the present study might be interpreted in two ways. One possibility would be that the laser dose utilized in the present study (32 J/cm²/point) could not accelerate the rotational dental movements; concerning the fact that both lower (15, 28, 29) and higher (14, 24, 38) doses used in other studies had accelerated the orthodontic movements. Yet, another possibility could be that these findings resulted from the difference between the mechanism of rotational dental movements and that of the mesiodistal movement, which required the removal of a considerable amount of bone from the direction of tooth movement. Therefore, more studies using different laser doses are requisite to be conducted on the issue. Furthermore, increasing the number of impressions and also investigating the teeth with different movements might also lead to different outcomes. In fact, laser is believed to be most effective in initial dental movements and, as the study continues longer, the effect of laser is reduced when finally, similar dental movements can be observed between the control and the lased groups (28).
The turning point of the present study is investigating the stability of rotational dental movements following LLLT. All the previous studies have shown that not only LLLT affected the speed of dental movements, but it also influenced the remodeling in the tissues surrounding the teeth and, consequently, resulted in formation of new bone of a higher quality(13,19,22,24-26,28,39), which in turn could affect the short-term stability of the dental movements. The findings of the present study are also consistent with this theory.
In this research, the relapse of the rotational movements had considerably been decreased during the 1st week after ending the force application and laser irradiation. The amount of relapse increased in the LLLT group by time and the decrease in the rate of relapse at the beginning of force termination was so considerable that even after 3 months from the end of force application and laser irradiation, the total amount of relapse in the LLLT group was significantly lower than the control group.
The rate of relapse was measured based on the percentage of relapse from the total rotational movement so that our results could be comparable with similar studies. In fact, studies conducted on orthodontic movements are different since many variables are involved .These include amount of force , time of inserting force ,, degree of tooth rotation through the active phase, and period of time which relapse is studied , hence we could not compare the results by amount of relapse in degrees(6,11,12,34-37). Besides, this could not also show the extent of relapse relative to the total tooth movement. In the present study, the total amount of relapse was reported as 60.64% and 73.92% of the total tooth movement in the LLLT and the control group, respectively. Other studies which studied the relapse of rotational movements have reported its range to be from 24% (39) to 58% (58). The findings of the present study are higher than this range which might be due to the retention period in those studies. Of course, this amount of relapse cannot be compared to the rate of relapse following circumferential supracrestal fiberotomy (CSF) in rotational movements (23) and LLLT is not comparable to this surgical method. Besides, simultaneous use of the two methods can reduce the rate of relapse to a greater extent. Frequent high-dose irradiation of laser during the beginning months after treatment decreases the relapse.
In the present study, the highest range of relapse in both study groups was detected in the 1st week after force stopped (54.67% in the LLLT group vs. 73.52% in the control group); and also showed, that the highest rate of relapse occurred through the early days after force ending (35). This issue can be considered as a great advantage for laser irradiation during the dental movement. When relapse is at its highest rate, laser can be highly effective in eliminating its occurrence. Regarding the various reasons reported for the relapse of rotational movements (2, 10, 12, 34, 41), the mechanism of laser cannot be accurately explained; however, it might result from remodeling stimulation as well as reorganization of dental retainer bone and fiber. Even if LLLT cannot accelerate the rotational movements, it reduces the pain as well as the inflammation during the dental movements (20) and, at the same time, reduces the Reduces the short time relapse
In orthodontic treatments, morphologically malpositioned teeth are moved toward their ideal positions. In animal orthodontic studies, however, the teeth are moved from their natural positions toward an inappropriate, unstable status. Therefore, one may pose the question: "Don't these different conditions lead to the incidence of different tissue responses as well as the relapse tendencies". Kusters et al. revealed that the status of dental supporting fibers did not depend on tooth morphology but on the position of the tooth in the dental arch. Therefore, the status of periodontal fibers in a tooth erupted in a rotated status is quite similar to that of a naturally erupted tooth (42).
Finally, biological differences between human beings and animal samples, would definitely limit the outcome of this study. Moreover, dose of energy used in animal studies must be changed when used for human samples.
Conclusion
Irradiation of low-level laser with the energy dose of 32 J/cm²/point on the dogs' lateral incisors had no effects on the rate of rotational movements; however, it significantly reduced the relapse up to 3 months after termination of force application and laser irradiation. Furthermore, rate of the relapse was significantly decreased in the 1st week after teeth movements were stopped but in longer times it was nondescript.
Acknowledgements
The present study was financially supported by Orthodontic Research Center of Shiraz University of Medical Sciences, Shiraz, Iran. The authors would like to thank Mr. Mojtaba Heidary for his assistance in selecting and utilizing the software employed in the present research.
