INTRODUCTION

Rapid maxillary expansion (RME) applies forces exceeding the orthodontic tooth movement limits,¹ which may accumulate potential energy in the appliance.² Long-term stability of the configuration continues to be questionable as authors have reported a reduction in arch width^3–5 and varied success with different lengths of retention to avoid relapse up to 4 years.⁶ However, Ekström et al.⁷ reported that mineral content rose rapidly in the first month, and all measured zones had similar mineral content within 3 months after the expansion. The acceleration of bone formation in the expanded suture area may be helpful for preventing relapse and therefore permitting a shortened retention period.⁸

Low-level laser therapy (LLLT), or photobiomodulation, promotes tissue healing and reduces edema, inflammation, and pain.⁹ Orthodontists are interested in accelerating bone remodeling using LLLT in procedures that lead to tooth and/or jaw movement.¹⁰

The effects of LLLT are still controversial. One of the reasons is that there are too many parameters to adjust or test. For example, the wavelength, fluence, power density, pulse structure, and treatment timing may all be varied.¹¹ The most important issue is the amount of energy required to attain more favorable results and whether it is suitable for clinical application.¹²

Photobiomodulation using various diode lasers has been previously reported.^9,10–12 However, the effects of the Indium Gallium Arsenide Phosphide (InGaAsP) (940 nm) diode laser on bone formation has not yet been studied. Therefore, the purpose of this research was to evaluate the effects of this wavelength on orthopedically expanded rat sutures at varying energy levels.

MATERIALS AND METHODS

This research was approved by the Bezmialem Foundation University Laboratory Animals Local Ethics Committee on August 6, 2015, and registered as 2015/195.

Eighty 11- to 12-week-old male Wistar rats weighing 180–220 g and supplied by the Bezmialem Foundation University were utilized, The experimental aspect of the study was carried out at the Experimental Application and Research Center. The histologic studies were performed at the University of Erciyes Faculty of Medicine Histology-Embryology Laboratory.

Groups

The study design included four groups (n = 20 each) defined by the amount of total energy provided through the laser: control (C), low (L), moderate (M), and high (H) dosage. The groups were each subdivided into two groups (n = 10 each) that were scheduled for sacrifice on either the 7th or 21st day (Table 1).

Table 1 Description of Groups

View Fullscreen

Expansion

A spring that applied 70 g of force was used. The appliances were fixed to the teeth by steel ligature ties (Figure 1A) and covered with composite resin (Transbond LR, 3M Unitek, Monrovia, Calif) (Figure 1B). Expansion was completed in 1 week, and passive retention appliances were placed (Figure 1C).

View Figure

• Download as Powerpoint
• Download Figure

RESULTS

All animals survived to the end of the study, and all sutures were expanded successfully. The histologic images in low and high magnification, representative of all groups, are shown in Figures 2 and 3.

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

Osteoblasts

On the 7th day, the number of osteoblasts was higher in the low-level laser group (LLLG) than in the control group (CG) (P < .001), the moderate-level laser group (MLLG) (P = .01), and the high-level laser group (HLLG) (P < .01), but no significant difference was found between the other groups (Table 2). These results were similar on the 21st day (with different P values) between the LLLG and other groups: CG (P < .01), MLLG (P < .05), and HLLG (P < .001) (Table 3).

Table 2 Comparisons Between Control Group and Low-, Moderate-, and High-level Laser Groups at the 7th Day

View Fullscreen

Table 2 Extended

View Fullscreen

Table 3 Comparisons of Control Group and Low, Moderate and High Level Laser Groups in 21st Day

View Fullscreen

Table 3 Extended

View Fullscreen

Inflammation Areas

There was no difference between scores of the groups on the 7th day. However, the area of inflammation was significantly higher in the HLLG compared with all other groups (CG: P < .01; MLLG: P < .01; LLLG: P < .05) on the 21st day (Table 4).

Table 4 Distribution of Animals According to Inflammation Scores, Inter- and Intragroup Comparisons^a

View Fullscreen

Intragroup Comparison

In the MLLG, the number of vessels was higher on the 21st day (P < .05), and the area of connective tissue was larger in the HLLG on the 21st day (P < .05) (Tables 4 and 5).

Table 5 Intragroup Comparisons for 7th and 21st Days

View Fullscreen

DISCUSSION

While too low of a dose in energy may not have any effect, an excessive dose can have an inhibitory effect on the same cell. There is a therapeutic window within a certain dose range, which is referred to as the Arndt-Schultz law.^9,14 In this study, the effects of different doses of energy on expanded rat sutures were compared to determine the optimal dosage.

Wistar rats as young as 4 weeks old and weighing at least 300 g have been used in previous studies.^12,15 In this study, 11- to 12-week-old Wistar rats weighing 180–220 g were used in accordance with previous studies.^8,13,16–19

A helical spring adapted to the upper incisor was used to induce midpalatal separation in the rats as previously reported.^13,16–22 Sutural expansion in rats is achievable with only 50 to 70 g of force.²³ In this study, all sutures were expanded with 70 g of force.

The diode laser (980 nm) has been reported to accelerate the wound-healing process in rats.²⁴ However, David et al.²⁵ evaluated the effects of helium-neon laser radiation of 0 J, 2 J, or 4 J every other day for 2 to 6 weeks on rat tibial healing and reported no positive effects. On the other hand, studies have reported increased bone formation through the application of using 10 J/cm² at intervals of 48 hours²⁶ to 178 J/cm² in a single application.¹² Ekizer et al.²² applied the laser once a day for 10 days. The intermittent application of the laser in the early stages of expansion was found to be effective compared with a single episode of irradiation.⁸ In this study, the effects of twice-weekly application was tested, which is a frequency that can be well tolerated by both patients and clinicians. Therefore, the results of this study can be used as a reference for further clinical studies. Three different doses were tested (18, 42, and 60 J/cm²) as there was no consensus on dosages in the previous literature.

The number of osteocytes in the LLLG was found to be higher than in the CG on the 7th and 21st days and only the HLLG on the 21st day, but there were no differences compared with the other groups, and no difference was observed among the CG, MLLG, and HLLG. Compared with the other groups, the LLLG showed a higher number of osteoblasts, area of newly formed bone, and ratio of newly formed bone to the total bone area, while the connective tissue area was less. However, no difference was found among the remaining three groups. These findings confirmed the positive effect of the application of a low-dose laser in bone formation of expanded sutures, similar to results of previous studies that tested different wavelengths.^8,22

Pretel et al.¹² reported a moderate presence of inflammatory cells found in only one rat of the laser group while an intense presence of inflammatory cells was detected in three of five animals in the CG on the 15th day. In this study, when the inflammation scores were compared, no difference was found between the LLLG and the other groups on the 7th day. However, on the 21st day, the HLLG had significantly higher scores compared with the other groups. This finding was an indication of the high-level laser's negative effect on healing when applied over a prolonged period.

It was previously reported that LLLT increased the number of blood vessels on the 7th day in rat tibial healing, but this increase was not conspicuous²⁷ as a decrease was detected up to the 14th day.²⁸ Ekizer et al.²² found an increased number of vessels on the 10th day of irradiation. In this study, the number of vessels was not different among groups within the first 7 days. However, the LLLG had significantly more vessels than the MLLG and HLLG on the 21st day. The MLLG was found to have fewer vessels than the CG. while no difference was indicated between the CG and HLLG. There were fewer vessels in the HLLG compared with all other groups. These findings showed that moderate- and high-level settings over a prolonged period of time played a negative role in vascularization.

Dörtbudak et al.²⁹ reported more bone deposition in cultures that were irradiated by a diode soft-tissue laser after a period of 8 to 16 days. In another study, mesenchymal stem cells were irradiated with LLLT and examined at 1 to 7, 10, 14, 21, and 28 days. Enhanced tissue formation and more ossification were found for the irradiated group.³⁰ In this study, two different time points were evaluated, and differences were found for the number of vessels in the MLLG and the connective tissue area in the HLLG when findings of the 7th and 21st days were compared. The moderate-level laser induced the development of a significantly lower number of blood vessels, and the high-level laser caused significantly more connective tissue growth at the 21st day. These differences were possibly a result of differences in the level of energy or the reaction of the target structure.

A gallium aluminum arsenide laser increased collagen fibers in the early stage of healing and caused the formation of well-organized bone trabecules,³¹ stimulated cellular proliferation, alkaline phosphatase activity, and osteocalcin gene expression at early periods of culture.¹⁰ Also, application of the laser in the first stages of expansion was found to be effective.⁸ Similarly, in this study, the LLLG showed a favorable effect on the tested variables in the first aspects of healing, except for the number of vessels present, and no favorable change was found with the continuing application of the laser until the 21st day. Therefore, low-level InGaAsP laser application in the early phase of RME retention offers promising results for the acceleration of new bone formation and shortening of the retention period. However, further studies with different settings would be of value prior to performing clinical studies.

CONCLUSIONS

• At a low dosage, the InGaAsP laser caused biostimulation in bone healing, which may play a role in shortening the retention period of an RME device. This finding was supported by measurements of the following variables: the number of osteocytes and osteoblasts, area of newly formed bone, area of connective tissue, and ratio of newly formed bone to the total bone area from the beginning of the 7th day.

• The low-level laser did not affect the number of vessels. However, the high- and moderate-level dosages had a comparatively negative effect on the number of vessels on the 21st day.