Korean J Orthod.  2023 May;53(3):205-216. 10.4041/kjod22.209.

Micro-computed tomography evaluation of the effects of orthodontic force on immature maxillary first molars and alveolar bone mineral density of Sprague–Dawley rats

• Wang J
• Zhang R
• Zhang Z
• Geng C
• Zhang Y

• KMID: 2542815
• DOI: http://doi.org/10.4041/kjod22.209

Abstract

Objective
To investigate changes in the immature teeth of Sprague–Dawley rats during orthodontic treatment and to explore the changes in the peri-radicular alveolar bone through micro-computed tomography (CT).
Methods
Twenty-five 26-day-old male Sprague–Dawley rats were included. The maxillary left first molar was moved mesially under a continuous force of 30 cN, and the right first molar served as the control. After orthodontic treatment for 7, 14, 21, 28, and 42 days, the root length, tooth volume, and alveolar bone mineral density (BMD) around the mesial root were measured through micro-CT.
Results
The immature teeth continued to elongate after application of orthodontic force. The root length on the force side was significantly smaller than that on the control side, whereas the differences in the volume change between both sides were not statistically significant. Alveolar bone in the coronal part of the compression and tension sides showed no difference in BMD between the experimental and control groups. The BMD of the experimental group decreased from day 14 to day 42 in the apical part of the compression side and increased from day 7 to day 42 in the apical part of the tension side. The BMD of the experimental group decreased in the root apex part on day 7.
Conclusions
The root length and volume of immature teeth showed continued development under orthodontic forces. Alveolar bone resorption was observed on the compression side, and bone formation was observed on the tension side.

Keyword

Root development; Alveolar bone; Tooth movement

Figure

• 

  Figure 1 A, Rat model of orthodontic tooth movement; B, sagittal view. The yellow line represents the horizontal plane passing through the mesial and distal cemento-enamel junction of the maxillary second molar; C, horizontal view. The red line represents the sagittal plane passing through the center of the mesial root of the maxillary first molar and the mesiobuccal root of the maxillary second molar; the green dotted line indicates the division between the upper first and second molars, and the upper second and third molars. D, Measurement of movement at the crown (a), neck (b), apical foramen (c), and inclination of the root (d). m1, represents the maxillary first molar; m2, represents the maxillary second molar; m3, represents the maxillary third molar.

• 

  Figure 2 The mesial root length of the maxillary first molar. A–E, Micro-computed tomography (CT) reconstruction of the maxillary first molars in the control group at 7, 14, 21, 28, and 42 days after applying orthodontic force. F–J, Micro-CT reconstruction of the maxillary first molars of the orthodontic force group at 7, 14, 21, 28, and 42 days after applying orthodontic force. The blue and green arrows represent the cemento-enamel junction and mesial root length, respectively. H–J, The root apex shape was altered at 21, 28, and 42 days after application of orthodontic force.

• 

  Figure 3 Selection of regions of interest (ROI). 210 × 210 × 210-µm cubes, 200 µm mesial and distal to the mesial root. In the apical region, the ROI was defined as a 210 × 210 × 100-µm cube below the apical foramen. A, Sagittal view; B, horizontal view; C, coronal view. Coronal part of the compression side (M1), coronal part of the tension side (D1), apical part of the compression side (M2), apical part of the tension side (D2), and root apex part (A).

• 

  Figure 4 Graphical representation of the root length (mm), tooth volume (mm3), and tooth movement at different time points. A, Root length; B, tooth volume; C, crown distance; D, neck distance; E, apical foramen distance; F, root inclination. *p < 0.05.

• 

  Figure 5 Changes in the bone mineral density (BMD) of the alveolar bone at different time points. A, Coronal part of the compression side (M1). B, Apical part of the compression side (M2). C, Coronal part of the tension side (D1). D, Apical part of the tension side (D2). E, Root apex part (A). Box edges represent the upper and lower quantiles, the middle lines in the boxes represent the medians, and the whiskers represent the maxima and minima. *p < 0.05.

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