Influence of Low-Level Laser Therapy on the Rate of Orthodontic Movement: A Literature Review

Abstract

Objective: The purpose of this study was to review low-level laser therapy (LLLT) protocols that have been used to date, and to indicate which parameters appear to be most effective to guide future research. Background data: Studies assessing the influence of LLLT on the rate of orthodontic tooth movement have produced controversial results as a result of methodological differences. Methods: The MEDLINE^® database (1975–2012) and the Cochrane library (subject 8) were reviewed. Clinical studies and animal experiments written in English and focusing on the effects of LLLT on the rate of orthodontic movement were browsed. Article selection was conducted by one reviewer and checked by a second investigator. Results: A total of 109 articles were identified, of which 14 were selected for detailed analysis. Diode laser was used in all studies with different energies, frequencies, and doses. In animal studies, the most common and effective energy input was 54 J per session daily; in humans, it was 2 J per session on the first days of each month, with 72–96 h intervals. Orthodontic force also influenced orthodontic movement. A force of 10 g/force seems to be indicated for moving molars in rats, versus 150 g for canines in humans. Conclusions: Most authors report positive effects of the use of LLLT on speed increase of orthodontic tooth movement when compared with control or placebo groups. Diode laser, especially gallium aluminum arsenide, used continuously and in direct contact with the irradiated areas, was the most frequent protocol. Further studies are warranted to determine the best protocols with regard to energy, dose, and intervention schedule.

Introduction

L aser has been used in dentistry for over a decade now, and the phenomenon continues to grow. Orthodontic treatment is based on the principle of tooth movement resulting from the application of prolonged forces on a tooth, creating areas of tension and pressure in the periodontal ligament. The process is characterized by acute followed by chronic inflammation, once again followed by acute inflammation (after reactivation of orthodontic forces). These changes to periodontal tissues cause bone remodeling, essential for the promotion of orthodontic tooth movement.¹

Studies conducted in animal models and human beings have shown that low-level laser therapy (LLLT) can improve orthodontic tooth movement by influencing bone repair and analgesia.^2

–9 Specifically, some studies have been designed to assess the influence of LLLT on the rate of orthodontic tooth movement. However, differences in laser application protocols, such as type of laser used, wavelength, output power, dose, and treatment time, have produced controversial results.^{2,3,5,7,9

–12}

To the authors' knowledge, no literature review has been conducted to investigate the influence of LLLT on the rate of orthodontic movement. Therefore, the objective of the present article was to review the literature for LLLT protocols that have been used to date, and to indicate which parameters appear to be most effective to guide future research.

Materials and Methods

A computerized literature review was performed using the MEDLINE^® database (1975–2012) and the Cochrane library (subject 8). The following keywords were used: orthodontic, movement, laser, and LLLT.

The following selection criteria were taken into consideration: articles written in English, disclosing the wavelength employed, clearly describing LLLT application protocols, measuring the rate or speed of orthodontic movement, and including control and/or placebo groups. Specifically for clinical studies, patients should not have presented any systemic disease, should not have taken any medication likely to influence orthodontic movement, and should have permanent dentition; animal studies should describe adequate animal maintenance conditions. Figure 1 shows the article selection process.

FIG. 1.

Article selection flowchart.

Clinical studies and animal experiments studying the effects of LLLT on the rate of orthodontic tooth movement were browsed. The selection of articles was performed by one reviewer and checked by a second investigator. The titles and abstracts of potentially relevant articles were analyzed before full-text analysis.

Results

The computerized literature review yielded a total of 109 articles. The abstracts of these articles were read and screened. One article was found to be a duplicate, resulting in a total of 14 articles selected for a more detailed analysis, with full-text reading.

Of the 14 articles read in full,^{2

–6,8,10,12

–18} three were excluded either for not reporting all information necessary for study reproduction or for containing inconsistencies.^11
–13 As a result, the present literature review included a total of 11 articles, namely 3 clinical studies and 8 animal studies.

Because of the biological differences between animals and humans, and also because of the impossibility to affirm that the doses applied to animal models are appropriate for humans, the results of the present review are divided into two major sections, one devoted to the analysis of animal studies and the other to the analysis of human studies.

Following the separate analysis of these two groups (animals and humans), the data found for both types of studies will be compared and discussed.

Animal studies

Tables 1 and 2 summarize the results of the animal studies assessed.

Table 1.

Studies Assessing the Effect of LLLT on the Rate of Orthodontic Movement in Rats

Article	Laser type	Sample	Method	Results
Kawasaki and Shimizu⁶	Ga-Al-As 830 nm Continuous, direct contact at each point 100 mW Ø 0.6 mm 0.0028 cm² (tip) 6000 J/cm²/point 18,000 J/cm²/session 234,000 J/cm²/13 days 18 J/point 54 J/session 702 J/13 days	24 male Wistar rats 6 weeks old 180 g 12 molars irradiated	Control group LLLT group: left maxillary first molar Three points at the gingiva: mesial, buccal, and palatal Time/point=3 min Time/session=9 min Once a day Total of 13 days Force of 10g Measurement in plaster models Distance between the top of the mesiobuccal cusp of the first and second maxillary left molars	LLLT stimulates tooth movement, accelerates bone remodeling by increasing the number of osteoclasts and stimulating cell proliferation in the periodontal ligament and mineralization of the newly formed bone A higher rate of movement on days 2, 4, and 12
Fujita et al.¹⁰	GaAlAs 810 nm Continuous, direct contact at each point Ø 0.6mm 0.0028 cm² (tip) 18 J/point 54 J/session 432 J/8 days 6000 J/cm²/point 18,000 J/cm²/session 144,000 J/cm²/ 8 days LLLT group 810 nm 100 mW LED group 850 nm 75 mW	75 male Wistar rats 6 weeks old 180 g 50 first molars irradiated	3 groups with 25 animals each Control group LLLT group 1 mesial, 1 mesiobuccal, and 1 mesiopalatal point Time/point=3 min Time/session=9 min Once a day. Total of 8 days LED group 1 mesial, 1 mesiobuccal, and 1 mesiopalatal point Tempo/point=4 min Once a day Total of 8 days Force of 10g Measurement in models Distance between the central fossa of the right maxillary first molar and the mesial surface of the right maxillary second molar	LLLT accelerates orthodontic tooth movement via induction of RANK/RANKL The expression of RANK was detected in osteoclast precursor cells at an early stage (days 2 and 3) in the irradiated group These findings suggest that LLLT accelerates bone remodeling, shortening the duration of orthodontic treatment
Yoshida et al.¹⁷	GaAlAs 810 nm Continuous, direct contact at each point 100 mW Ø 0.6 mm 0.0028 cm² (tip) 4 intraoral points 4500 J/cm²/point 18,000 J/cm²/session 162,000 J/cm²/9 days 13.5 J/point 54 J/session 486 J/9 days	60 male Wistar rats 6 weeks old 180 g 30 first molars irradiated	2 groups Control group LLLT group 1 mesial, 1 buccal, 1 palatal, and 1 distal point Time/point=2 min 15 sec Time/session=9 min Once daily from day 0 to 6 One session on day 13 One session on day 20 Force of 10g Measurement on tomographic images Distance between the point of contact on the right maxillary first and second molars	This LLLT protocol accelerates orthodontic tooth movement, stimulating bone remodeling. The rate of tooth movement was significantly higher in the LLLT group on days 3 (1.4-fold), 7 (1.19-fold), 14 (1.26-fold), and 21 (1.34-fold). Bone density in the LLLT group was higher on days 7 (1.08-fold), 14 (1.09-fold), and 21 (1.14-fold).
Gama et al.¹⁴	Diode 790 nm Continuous, direct contact at each point 40 mW Ø 2 mm=0.03 cm² (tip) 2 intraoral points 0.135 J/point 4.5 J/cm²/point 9 J/cm²/session Extraoral point 0.33 J/point 11 J/cm²/point/session 0.6 J/session 20 J/cm²/session	30 male Wistar rats 3 months old 250–300 g 15 irradiated molars	2 groups (random division): Group I – orthodontic treatment (control) Group II – orthodontic treatment+LLLT 1 mesial, 1 distal, and 1 buccal point (extraoral application) 48 h intervals between applications Total of 19 days Force of 40g Intraoral clinical measurement Distance between the mesial surface of the first molar and a perforation made in the resin of incisors	In this protocol, the use of LLLT did not significantly interfere with orthodontic tooth movement. A lower rate of movement was observed in the LLLT group up to day 7 when compared with the control group.
Marquezan et al.¹⁶	GaAlAs 830 nm Continuous, direct contact at each point 100nW 0.0028 ncm² (tip) 6000 nJ/cm²/point 18,000 J/cm²/session 18 J/point 54 J/session	36 male Wistar rats 12 weeks old 250 g 18 irradiated molars	2 groups (random division): Control group: CG1 - no orthodontic treatment, death day 0 CG2 - orthodontic treatment, death day 2 CG3 - orthodontic treatment, death day 7 LLLT group: IrG1 - orthodontic treatment+2 laser, death day 2 (108 J) IrG2 - orthodontic treatment+2 laser, death day 7 (108 J) IrG3 - orthodontic treatment+7 laser, death day 7 (378 J) Laser: 1 mesial, 1 buccal, and 1 palatal point Time/point=3 min Total of 7 days Force of 40.78g Intraoral clinical measurement. Distance between the mesial surface of the first molar and a perforation made in the resin of incisors	The two protocols did not have significant effects on the rate of orthodontic tooth movement when compared with the control group Laser applications at late stages can have a role in maintaining the stimulatory effect of LLLT. The absence of laser can decrease stimulus Daily laser application caused an increase in the number of osteoclasts after 7 days, but inhibited the expression of immature collagen on the tension side
Altan et al.¹⁸	GaAlAs 820 nm Continuous, direct contact at each point 100 W Ø 2 mm=0.03 cm² (tip) Group II 10.8 J/point 54 J/session 343.9 J/cm²/point 1717.2 J/cm²/session Group III 3 J/point 15 J/session 95.5 J/cm²/point 477 J/cm²/session	38 male Wistar rats 10 weeks old 175 g 22 irradiated incisors	4 groups (random division): Group I – orthodontic treatment Group II – orthodontic treatment+laser Time/point=108 sec Group III – orthodontic treatment+laser Time/point=30 sec Group IV – control Laser: 5 points on the right incisor: 2 distobuccal, 1 distal, and 2 distopalatal point Days 0, 1, 2 Total of 9 days Force of 20g Spring between maxillary incisors Intraoral clinical measurement Distance between the incisors and the level of the gingival papilla	No statistically significant differences were observed among the groups in the rate of orthodontic movement, even though group II (54 J) showed a higher rate of movement During orthodontic tooth movement, LLLT accelerates the bone remodeling process by stimulating osteoblast and osteoclast cell proliferation and their functions

LLLT, low-level laser therapy.

Table 2.

Studies Assessing the Effect of LLLT on the Rate of Orthodontic Movement in Dogs

Article	Laser type	Sample	Method	Results
Goulart et al.³	Ga-As-Al 780 nm Continuous, direct contact at each point 70 mW 0.04 cm² LLLT group 5.25 J/cm²/point/session 0.21 J/point/session LED group 35 J/cm²/point/session 1.4 J/point/session	18 adult dogs (male and female) 4650–9600 g Maxillary third molars extracted 18 first molars irradiated	2 groups with 9 first molars each: Placebo group Contralateral LED application Time/point=20 sec LLLT group Second premolar 1 point in the middle third of distal root Time/point=3 sec Application every 7 days Total of 9 weeks Force of 85g Intraoral clinical measurement Distance between perforations made on molars and premolars	Irradiation at 5 J/cm² stimulates orthodontic tooth movement in the early stage (0–21 days) Lower doses are indicated for anti-inflammatory effects Higher doses are indicated for anchorage results with increased bone formation
Kim et al.¹⁵	GaAlAs 808 nm Pulsed, no direct contact with each point 763 mW Ø 0.4 mm (fiber) Ø 1.75 mm (focal spot) 10 Hz 75 mJ per pulse 41.7 J/cm²/ point 333.6 J/cm²/session	12 Beagles 12 maxillary second premolars irradiated	4 groups with 6 dogs each: Group A – orthodontic treatment (control) Group B – orthodontic treatment+corticotomy Group C – orthodontic treatment+LLLT Group D – orthodontic treatment+corticotomy+LLLT Laser: 4 buccal points and 4 palatal points Time: 20 sec each point (9 sec of laser application) Every 3 days Total of 8 weeks Force of 150g Measurement in models Distance between cervicodistal point on the second premolar and third molar	Orthodontic tooth movement increased with LLLT in this protocol The use of LLLT had late effects (from 5th to 8th week) Effect of LLLT on healthy alveolar bone differs from application on injured bone

LLLT, low-level laser therapy.

Sample characteristics

Of the eight studies assessing animal models, six used male Wistar rats 6–12 weeks or age,^{6,10,14,16
–18} and the other two used dogs.^3,15

The tooth chosen for orthodontic movement in rats was the maxillary first molar, except for the study of Altan et al.,¹⁸ in which maxillary incisors were used. In the studies with dogs, maxillary first molars¹⁵ and second premolars³ were selected for treatment.

Orthodontic movement

Nickel-titanium closed coil springs were used in most experiments for orthodontic movement. In only one study, a steel wire pendulum appliance was used to move the maxillary incisors of rats.¹⁸

Kawasaki and Shimizu,⁶ Fujita et al.,¹⁰ and Yoshida et al.¹⁷ applied a force of 10g on the maxillary molars of rats. Gama et al.,¹⁴ Marquezan et al.,¹⁶ and Altan et al.,¹⁸ in turn, applied higher forces, of 20–40.78g. In the studies conducted with dogs, higher forces were used, namely of 85 and 150g.

Laser type and wavelength

All animal experiments included in the review used diode laser, most often gallium aluminum arsenide (GaAlAs).^{3,6,10,15

–18} Infrared was the wavelength most frequently used, ranging from 780 to 830 nm.

Laser application power

Laser output power ranged from 40 to 100 mW. Five of the studies used an output power of 100 mW.^{6,10,16
–18} Among the authors working with rat models, only Gama et al.¹⁴ reported an output power different from 100 mW, namely, 40 mW. Goulart et al.³ and Kim et al.,¹⁵ both working with dogs, used 70 and 76.3 mW, respectively.

Application protocol, irradiation points, and energy input

Kawasaki and Shimizu,⁶ Fujita et al.,¹⁰ and Marquezan et al.¹⁶ applied laser to three distinct points (mesial, buccal, and palatal) around the tooth subjected to orthodontic movement. Yoshida et al.¹⁷ used four laser application points (mesial, distal, buccal, and palatal). All four studies used a total energy input per session of 54 J, and an energy density per session of 18,000 J/cm². Gama et al.¹⁴ used LLLT at three points, one extraoral (buccal surface). Total energy per session was 0.6 J, and the energy density per session was 20 J/cm².

Incisors were the teeth selected for analysis by Altan et al.¹⁸ LLLT was applied to five distinct points: two distobuccal, two distopalatal, and one distal. Total energy per session was 54 J in group II and 15 J in group III, with energy densities per session of 1717.2 and 477 J/cm², respectively.

With dogs, Goulart et al.³ used only a palatal point for irradiation, at a total energy input of 0.21 J per session and an energy density of 5.25 J/cm² per session. Kim et al.,¹⁵ in turn, applied laser to eight different points, four buccal and four palatal. Total energy density per session was 333.6 J/cm². All studies applied laser continuously, in direct contact with the points irradiated, except Kim et al.,¹⁵ who used pulsed laser.

Laser intervention schedule

Of the eight animal studies selected, five used daily laser applications,^{6,10,16
–18} but not necessarily throughout the study period.^17,18 One study applied laser every 48 h,¹⁴ another every 72 h,¹⁵ and one study applied laser every 7 days only.³

Influence of LLLT on orthodontic movement

The results of our animal models show that application of LLLT during orthodontic treatment increases the rate of tooth movement when compared with nonirradiated control groups.^3,6,10,15,17

Altan et al.¹⁸ did not observe statistically significant differences with regard to the rate of orthodontic tooth movement between control and study groups, but they reported that LLLT accelerated the bone remodeling process, stimulating osteoblast and osteoclast cell proliferation and their functions. Those authors also suggested that their nonsignificant results might have been because of the small size of their sample.

Gama et al.¹⁴ and Marquezan et al.¹⁶ also failed to find significant results associated with LLLT.

Clinical studies

Table 3 describes the results found in the three clinical studies reviewed.

Table 3.

Studies Assessing the Effect of LLLT on the Rate of Orthodontic Movement in Humans

Article	Laser type	Sample	Method	Results
Cruz et al.²	Ga-Al-As 780 nm Continuous, direct contact at each point 20 mW 0.04 cm² (tip) 5 J/cm²/point 50 J/cm²/session 200 J/cm²/month 0.2 J/point 2 J/session 8 J/month	11 patients (male and female) 12–18 years old Maxillary first premolars extracted 11 maxillary canines irradiated	Placebo group: contralateral canine LLLT group: irradiated maxillary canine 5 buccal and 5 palatal points Time/point=10 sec Time/session=100 sec Days 0, 3, 7, 14, 33, 37, and 44 (no reactivation) Total of 60 days Force of 150g Intraoral clinical measurement. Distance from the distal bracket slot of the canine to the mesial slot of the first molar	LLLT application accelerates orthodontic tooth movement
Limpanichkul et al.⁵	Ga-Al-As 860 nm Continuous, direct contact at each point 100 mW 0.09 cm² (tip) 25 J/cm²/point 204 J/cm²/session 612 J/cm²/month 2.3 J/point 18.4 J/session 55.2 J/month	12 young adults (male and female) Mean age of 22.11 years Maxillary first premolars extracted 12 maxillary canines irradiated	Placebo group: contralateral canine LLLT group: irradiated maxillary canine 3 buccal, 3 palatal, and 2 distal points Time/point=23 sec Time/session=184 sec Days 0, 1, 2, 28, 29, 30, 58, 59, 60, 88, 89, and 90 Total of 90 days Force of 150g Measurement in models Distance from the most mesial point of each retracted canine to the incisive papilla	LLLT applied with these parameters do not affect the speed of orthodontic tooth movement Possible error resulting from small sample An energy input of 25 J/cm²/session is probably too low to cause stimulatory or inhibitory effects
Sousa et al.⁸	Ga-Al-As 780 nm Continuous, direct contact at each point 20 mW 0.04 cm² (tip) 5 J/cm²/point 50 J/cm²/session 150 J/cm²/month 0.2 J/point 2 J/session 6 J/month	10 patients (male and female) 10.5–22.2 years old Maxillary and/or mandibular first premolars extracted 13 maxillary and mandibular canines irradiated	Placebo group: contralateral canine LLLT group: canine randomly selected 5 buccal and 5 palatal/lingual points Time/point=10 sec Time/session=100 sec Days 0, 3, 7, 30, 33, 37, 60, 63, and 67 (after reactivation) Total of 90 days Force of 150g Measurement in models Distance from the distal bracket slot of the canine to the mesial slot of the first molar	Statistically significant differences were observed between the two groups The irradiated group showed an almost double increase in the rate of orthodontic tooth movement when compared with the placebo group

LLLT, low-level laser therapy.

Sample characteristics

The samples of the clinical studies selected for review included both male and female patients, ranging from 10 to 22 years of age, and requiring orthodontic treatment with extraction of first premolars. Sousa et al.⁸ irradiated maxillary and mandibular canines; the other two groups of authors irradiated maxillary canines only.

Orthodontic movement

Cruz et al.² and Limpanichkul et al.⁵ used straight-wire brackets with Roth Prescription and continuous arch wires. Sousa et al.,⁸ however, used Andrews Prescription and segmented arch wires, all with 0.22×0.25 slots.

Cruz et al.² used a modified Nance holding arch cemented to the second premolars, and a transpalatal bar attached to the first premolars for anchorage during retraction of the upper canine, which was tied to the stainless steel rectangular arch wire (0.17×0.25) with a 0.10 mm stainless steel ligature wire.

Limpanichkul et al.⁵ used for anchorage a 3 mm vertical loop with stops mesial to the first premolar tubes tied to the hook of the device, and the upper incisors tied together to the 0.45 mm, stainless steel arch wire, which served as a guide for the retraction of the upper canines. Retracted canine teeth were bracketed with a self-ligating bracket to standardize the effects of friction during movement.

Sousa et al.⁸ did not describe the anchorage system used, only the segmented arch wire from the first molar to the canine, with a 0.016 stainless steel wire used as a guide for retraction.

In all three studies, nickel-titanium closed coil springs were used for the retraction of canines, with a force of 150g for canine retraction.

Laser type and wavelength

All clinical studies used GaAlAs diode laser with an infrared wavelength ranging from 780 to 860 nm.^2,5,8

Laser application power

Cruz et al.² and Sousa et al.⁸ used a laser application output power of 20mW, compared to 100mW in Limpanichkul et al.⁵

Application protocol, irradiation points, and energy input

The three clinical studies applied laser continuously, in direct contact with the areas to be irradiated. Cruz et al.² and Sousa et al.⁸ used the same points of irradiation and the same energy input at each point and session. These authors used five buccal points and five palatal or lingual points. Energy and energy density per session were 2 J and 50 J/cm², respectively. Limpanichkul et al.,⁵ used three buccal, three palatal, and three distal points in relation to the irradiated canine.

Laser intervention schedule

Laser application frequencies were different in each study. Cruz et al.,² for example, irradiated teeth on days 0, 3, 7, and 14 in the first month, and on days 33, 37, and 44 in the second month, always with the same intervals. Springs were reactivated on days 0 and 30 in the control and irradiated experimental groups after the measurement of distances.

Limpanichkul et al.⁵ used daily applications from the 1st to the 3rd day of the study. At the end of the 1st month, laser applications were performed daily once again, as well as in the end of the 2nd and 3rd months. In that study, the authors reactivated springs once a month.

Finally, Sousa et al.⁸ adopted a similar protocol to that of Cruz et al.,² with irradiation sessions on days 0, 3, and 7, and in the beginning of the 2nd and 3rd months, always maintaining the same intervals. Canine retraction springs were reactivated at the beginning of each month.

Influence of LLLT on orthodontic movement

Cruz et al.² and Sousa et al.⁸ observed positive results, that is, a higher rate of orthodontic tooth movement in the irradiated group when compared with the placebo group, at a statistically significant difference. Conversely, Limpanichkul et al.⁵ did not find any effect of LLLT on the rate of orthodontic tooth movement.

Discussion

Animal studies

Most of the animal studies included in this review found that LLLT increases the rate of orthodontic tooth movement, stimulating bone remodeling by increasing the number of osteoclast and osteoblast cells and reinforcing their functions.^{3,6,10,15,17,18}

With regard to the type of laser employed, the use of diode laser predominated, especially GaAlAs, as did infrared wavelengths. Infrared lasers are known to penetrate biological tissues more deeply than red lasers, stimulating deeper tissues such as bone tissue, which is heavily implicated in orthodontic tooth movement. Fujita et al.¹⁰ found a higher number of multinucleated osteoclast cells in the irradiated group, as well as an increased expression of RANK in osteoclast precursor cells at early stages.¹⁰

Continuous laser emission, in direct contact with irradiated tissues and limited to each point, was the most frequent and effective method for producing positive effects on orthodontic tooth movement.^3,6,10,17 When laser is applied directly to an irradiation point and is in direct contact with tissue, the chances of energy absorption by the irradiated tissue increase, avoiding laser reflection. The only study reporting the use of pulsed laser, not in direct contact with tissues,¹⁵ found significant results later in the course of LLLT when compared with the other studies.^6,10,15,17

Among the animal studies that reported positive results, three used rats and applied an energy of 54 J per session distributed over different points around the orthodontically moved tooth, on a daily basis, at a total dose of 18,000 J/cm² per session.^6,10,17 Goulart et al.³ and Kim et al.¹⁵ used an energy input of 0.21 J and 75 mJ per pulse in dogs, with doses of 35 and 333.6 J/cm² per session, respectively. Not only did these two latter studies use different energy inputs, doses, and application frequencies, they were also applied differently, as previously mentioned. Even though the number of studies conducted with dogs is too small to allow comparisons, we hypothesize that different energy inputs and doses may be most adequate to different animals to produce an increase in the rate of tooth movement.

The studies conducted by Gama et al.,¹⁴ Marquezan et al.,¹⁶ and Altan et al.¹⁸ failed to observe increased tooth movement associated with LLLT. Those authors used older Wistar rats, 70–120 days old, and also employed higher forces, at least double when compared with those used in the studies reporting positive associations.

Marquezan et al.¹⁶ did not find statistically significant differences between the irradiated and control groups with regard to the rate of orthodontic tooth movement. Those authors used the same parameters described in studies with positive results; used the same teeth (maxillary molars); and used GaAlAs laser applied at 54 J and 18,000 J/cm² per session, continuously, and in direct contact with the irradiation point. The only difference was the age of rats, which was double the age of rats from other studies,^6,10,17 and the orthodontic force employed, which was four times higher. The age of animal models can be an important variable, as a result of the effects of aging on periodontal tissues, which determine different responses to forces when compared with young tissues (e.g., injury and consequently a decreased rate of orthodontic movement). According to the histological findings described in the studies, the daily use of LLLT caused an increase in the number of osteoclasts after 7 days, but inhibited the expression of immature collagen on the tension side.

Altan et al. also applied the laser continuously, in direct contact with the irradiation point, using energy inputs of 54 J and 15 J and doses of 1717.2 and 477 J/cm² per session. However, those authors failed to observe a statistically significant effect of LLLT on tooth movement. This finding may be the result of their small sample size or because incisors were the teeth selected for orthodontic treatment in their study rather than molars, as in Kawasaki and Shimizu,⁶ Fujita et al.,¹⁰ and Yoshida et al.¹⁷ Another possible explanation for the nonsignificant results observed is the lower total dose used per session by the latter authors, namely 18,000 J/cm², versus only 1717.2 J/cm² in Altan et al.¹⁸ Despite these differences, Altan et al.¹⁸ observed a trend toward an increased rate of orthodontic tooth movement in the group irradiated with 54 J per session when compared with the one irradiated with 15 J. Histologically, an accelerated proliferation of osteoclast cells was observed, corroborating the idea that LLLT interferes with bone remodeling during orthodontic tooth movement.

With the use of an older sample (12 weeks) and a higher orthodontic force, Gama et al.¹⁴ showed that LLLT application may decrease induced tooth movement in comparison with controls when specific energy inputs and doses are applied. Another difference in that study was the use of an extraoral irradiation point. Even though the authors tried to address the loss of energy in the course of penetration until reaching the desired tissue (by increasing the energy applied), it remains to be known how much energy was actually absorbed.

Clinical studies

All three clinical studies included in the review used GaAlAs diode laser with infrared wavelengths (as also observed for animal studies), applied continuously and in direct contact with irradiation points.^2,5,8 Sample size was very similar across the studies, including both male and female patients; however, this aspect is worthy of further consideration to determine whether the sample size was actually reliable. Also, with respect to study samples, patient age varied greatly, including different age groups, such as adolescents and adults, which may directly have affected the results, as skeletal age and bone maturity are determining factors in orthodontic tooth movement rates. The teeth chosen for orthodontic retraction and LLLT were the maxillary canine in the studies by Cruz et al.² and Limpanichkul,⁵ versus the maxillary and mandibular canines in Sousa et al.⁸

The type of orthodontic mechanics used in the three studies varied with respect to bracket prescription, continuous or segmented arch wire for retraction, and reactivation of orthodontic force.^2,10,14 All these factors are of great importance to orthodontic tooth movement and may directly interfere with the results of the experiment.

Regarding laser application, Cruz et al.² and Sousa et al.⁸ used 2 J of energy at a dose of 50 J/cm² per session, and found statistically significant effects of LLLT during orthodontic tooth movement. Laser intervention schedule was a major difference between those two studies: Cruz et al.² used LLLT on days 0, 3, 7, 14, 33, 37, and 44, whereas Sousa et al.⁸ skipped day 14, included day 30, and repeated the same application sequence adopted in the 1st month. Moreover, Sousa et al.⁸ extended applications up to 67 days, whereas Cruz et al.² terminated the experiment on day 44. These findings suggest that even a lower number of applications at a lower intervention schedule may produce positive effects on the rate of tooth movement.

Limpanichkul et al.⁵ used 18.4 J at a dose of 204 J/cm² per session, applied daily during the first 3 days and again in the last 3 days of the 1st, 2nd, and 3rd months of treatment. Results were negative, showing no influence of these LLLT parameters on the rate of orthodontic tooth movement. The authors hypothesized that their sample was too small and that the dose of 25 J/cm² was too low to produce any stimulatory or inhibitory effect. If we compare it with the other two studies reporting positive effects, we can observe that the sample size was adequate, and that the source of a possible failure may been in the dose used per session and the intervention schedule of laser application. Perhaps, in humans, higher doses cause a decrease in, or even no effect on, the speed of orthodontic movement, whereas lower doses increase the speed of orthodontic movement, unlike what occurs in animals.

Despite the small number of studies, failures in patient selection and differences in the type of orthodontic mechanics employed, we can learn from these mistakes and not repeat them in the future, thereby producing more reliable results.

Conclusions

In this review of the literature, we observed that most authors reported positive effects of the use of LLLT on speed increase of orthodontic tooth movement when compared with control or placebo groups. GaAlAs diode laser, applied continuously, in direct contact with irradiation points, seemed have been the most frequently indicated to produce such effects. Also, the energies and doses that produced the desired effect were different for animals and humans, leading us to believe that these parameters are different between these two groups. Further studies are warranted to determine the best protocols with regard to energy, dose, and intervention schedule. Sample standardization as to size and patient age, as well as to the type of orthodontic mechanics used, should be rigorously studied, especially in clinical trials, so that the results of such studies can be compared and validated.

Footnotes

Acknowledgments

This study was funded by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).

Author Disclosure Statement

No competing financial interests exist.

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