Effects of a low level laser on periodontal tissue in hypofunctional teeth

Abstract

Malocclusions, such as an open bite and high canines, are often encountered in orthodontic practice. Teeth without occlusal stimuli are known as hypofunctional teeth, and numerous atrophic changes have been reported in the periodontal tissue, including reductions in blood vessels in the periodontal ligament (PDL), heavy root resorption, and reduced bone mineral density (BMD) in the alveolar bone. Low Level Laser (LLL) has been shown to have a positive effect on bone formation and the vasculature. Although the recovery of hypofunctional teeth remains unclear, LLL is expected to have a positive influence on periodontal tissue in occlusal hypofunction. The aim of the present study was to elucidate the relationship between LLL and periodontal tissue in occlusal hypofunction. Twenty-four male rats aged 5 weeks were randomly divided into control and hypofunctional groups. An anterior metal cap and bite plate were attached to the maxillary and mandibular incisors in the hypofunctional group to simulate occlusal hypofunction in the molars. LLL irradiation was applied to the maxillary first molar through the gingival sulcus in half of the rats. Rats were divided into four groups; control, control+LLL, hypofunctional, and hypofunctional+LLL. Exposure to LLL irradiation was performed for 3 minutes every other day for 2 weeks. Animals were examined by Micro-CT at 5 and 7 weeks and were subsequently sacrificed. Heads were resected and examined histologically and immunohistologically. The hypofunctional group had obvious stricture of the PDL. However, no significant differences were observed in the PDL and alveolar bone between the hypofunctional+LLL and the control groups. In addition, the expression of basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF)-positive cells were higher in the hypofunctional + LLL group than in the hypofunctional group. These results indicated that LLL enhanced the production of bFGF and VEGF in the periodontal tissue of hypofunctional teeth.

Figure 1. Experimental model and itinerary.

(A) In order to eliminate occlusal force at the molar region, an anterior bite plate and metal cap were attached to the mandibular and maxillary incisors, respectively. (B) X-ray images were obtained to confirm the hypofunctional condition (○). (C) Itinerary.

Figure 2. Measurement items in detail.

(A) Micro-CT images of rat periodontal tissue. (B) Measurement items in detail. PDL thickness of the distal palatal root. BMD in the alveolar bone on the palatal side (500 µm³). Counting bFGF- and VEGF-positive cells. The middle 1/3 of the buccal aspect of the distal palatal root was selected for observations. A rectangular area (300×400 µm) including PDL cells and alveolar bone lining cells was used for measurements. B: buccal, P: palatal.

Figure 3. PDL thickness and BMD of in the alveolar bone.

(A) Micro-CT images of periodontal tissue at 7 weeks of age. (B) Comparison of PDL thicknesses between the four groups. (C) Comparison of BMDs between the four groups.

Figure 4. H-E staining and Immunohistochemical staining for bFGF and VEGF.

(A) H-E staining. H-E stained sections of rat periodontal tissue. (a) control group, (b) control+LLL group, (c) hypofunctional group, (d) hypofunctional+LLL group. R: root, B: alveolar bone, Bar = 100 µm. (B) Immunohistochemical staining for bFGF and VEGF. One of the bFGF- and VEGF-positive cells in the control group (arrow head) was selected to determine the density for cell counting. (a) control group, (b) control+LLL group, (c) hypofunctional group, (d) hypofunctional+LLL group. R: root, B: alveolar bone, PDL: periodontal ligament, Bar = 100 µm. (C) Comparison of the number of bFGF- and VEGF-positive cells between the four groups.