Lithium reduces orthodontically induced root resorption by suppressing cell death, hyalinization, and odontoclast formation in rats

Abstract

Objectives: To examine whether lithium suppresses orthodontically induced root resorption (OIRR) via two mechanisms (prevention of hyalinization in periodontal tissue and suppression of odontoclasts) and to investigate the changes in the periodontal tissue and alveolar bone, focusing on the appearance of cell death, hyalinization, and odontoclasts.

Materials and methods: The maxillary first molars of 10-week-old male Wistar rats were moved mesially by a closed-coil spring for 14 days. Lithium chloride (LiCl; 0.64 mM/kg) or saline (control) was administered intraperitoneally daily. Tooth movements were measured using micro-computed tomography. Appearances of cell death, hyalinization, and odontoclasts were evaluated by histological analysis.

Results: OIRR observed on day 14 in the control group was suppressed strongly by LiCl administration. Apoptotic cells observed on day 1 in the compression area were gradually diminished on days 2 and 3 and transformed to hyalinization tissue in the control group. LiCl administration remarkably suppressed this cell death and subsequent hyalinization. Also, the appearance of odontoclasts in the compression area observed on day 7 was significantly suppressed by LiCl administration. Accordingly, these degenerative processes to OIRR were suppressed substantially by LiCl treatment.

Conclusions: Lithium reduces OIRR through the suppression of periodontal ligament cell death, hyalinization, and odontoclast formation.

Keywords: Apoptosis; Hyalinization; Lithium; Odontoclast; Orthodontic tooth movement; Root resorption.

Figure 1.

Orthodontic appliance, experimental design, measurements of tooth movement, and position of histological analysis. (A) Sagittal and axial views of the appliance; arrows indicate the direction of orthodontic force. Gray boxes indicate the light-cured composite resins. (B) Experimental time schedule. (C–E) A method to measure tooth movement: (C) ShD, (D) CPD, and (E) TIA. (F) Sagittal micro-CT image of the upper right molars (M1, M2, and M3). The dotted red line indicates the position of the sliced tissue section, which is located at the cervical one-third of the distal root of M1. A double circle indicates the center of rotation. (G) An axial micro-CT image of the upper right molars sliced at the level of the dotted line in Figure 1F. (H) HE staining of a rat corresponding to the image shown in Figure 1G. The dashed white box indicates the measured area of the distobuccal root of M1. Appl indicates orthodontic appliance.

Figure 2.

Measurements of tooth movement. *P < .05 and **P < .01 compared with the control group.

Figure 3.

HE staining (magnification 20 × 20). Hyalinization of (A) control and (B) LiCl-administered rats on day 3. The hyalinization area is surrounded by a dotted line. (C) The measured area of hyalinization in the PDL. Root resorption in (D) control and (E) LiCl-administered rats on day 14. (F) Measurement values of the area of root resorption craters. Dotted line indicates root resorption lacuna. *P < .05 compared with the control group. Scale bars = 50 μm. AB indicates alveolar bone; D, dentine; F, direction of orthodontic force; P, pulp; and R, root resorption lacuna.

Figure 4.

TRAP staining of the compression side by the orthodontic force of distobuccal root (magnification 20 × 20) of control (A, B, C) and LiCl (E, F, G). Days 3 (A, E), 7 (B, F), and 14 (C, G) are shown. (D) The number of TRAP-positive osteoclasts in the control and LiCl-administered rats. (H) The number of TRAP-positive odontoclasts in the control and LiCl-administered rats. White arrows indicate TRAP-positive multinucleate odontoclasts on the dentin and cementum, and black arrows indicate osteoclasts on the alveolar bone. *P < .05 and **P < .01 compared with the control group. Scale bars = 50 μm.

Figure 5.

TUNEL-positive cells of the control group on days 1 (A), 2 (B), and 3 (C) and of the LiCl group on days 1 (D), 2 (E), and 3 (F) stained brown (magnification 20 × 20). (G) The number of TUNEL-positive cells of the control and LiCl groups. *P < .05 and **P < .01 compared with the control group. Scale bars = 50 μm.