Korean J Orthod.  2018 Sep;48(5):339-345. 10.4041/kjod.2018.48.5.339.

Effects of continuous force application for extrusive tipping movement on periapical root resorption in the rat mandibular first molar

• Matsumoto
• Sringkarnboriboon S
• Ono T

• KMID: 2421319
• DOI: http://doi.org/10.4041/kjod.2018.48.5.339

Abstract

OBJECTIVE
The purpose of this study was to clarify the effects of continuous force application for extrusive tipping movement and occlusal interference on periapical root resorption in the rat mandibular first molar.
METHODS
We constructed an appliance comprising a titanium screw implant with a cobalt-chromium post as the anchorage unit and a nickel-titanium closed coil spring (50 cN) as the active unit. Force was applied on the mandibular left first molar of rats for 8 (n = 10) and 15 days (n = 10; experimental groups), with the tooth in occlusion. Five rats were included as a non-treated control group to examine the body effect of the appliance. Active root resorption lacunae, identified using tartrate-resistant acid phosphatase, were evaluated in terms of the length, depth, and area.
RESULTS
The rat mandibular first molars were mesially tipped and extruded in the occlusal direction. This mesio-occlusal tipping movement and occlusion resulted in the formation of a compression zone and active root resorption lacunae in the distoapical third of the distal roots. However, there was no significant difference in the amount of root resorption between the two experimental groups. The control group did not exhibit any active root resorption lacunae.
CONCLUSIONS
Periapical root resorption was induced by continuous extrusive tipping force and occlusal interference in rat mandibular molars. These data suggest that we orthodontists had better take care not to induce occlusal interference during our orthodontic treatment.

Keyword

Apical root resorption; Tooth movement; Mechanical stimuli; Rat mandibular molar

MeSH Terms

Acid Phosphatase
Animals
Molar*
Orthodontists
Rats*
Root Resorption*
Titanium
Tooth
Tooth Movement
Acid Phosphatase
Titanium

Figure

• 

  Figure 1 A and B, The orthodontic appliance used for extrusive tipping movement. The mandibular left first molar is mesio-occlusally tipped by an orthodontic appliance comprising a 3.5-mm-long titanium screw implant (I) with a 1.0-mm diameter; a cobalt-chromium wire (P) with a 1.2-mm diameter; and a 2-mm-long, 50-cN nickel-titanium (Ni-Ti) closed coil spring (CS). The attachment of the appliance to the body of the mandible and the mandibular first molar is also illustrated (A). B shows the appliance and screw implant fixed in the oral cavity of a rat. C, The load-displacement curve. A part of the load-displacement curve for the 50-cN Ni-Ti closed coil spring shows the unloading (reverse martensitic transformation) part of the force, when the spring returns to the original length. The spring was activated to 130–180% (arrowheads), which resulted in approximately 50 cN of continuous force. D, The areas assessed for root resorption. The horizontal distance between the first and second molars (the space between arrows) was recorded using a noncontact digital microscopic gauge. The distoapical compression zone (small rectangular area) in the distal root of the rat mandibular first molar (dotted line) is evaluated by averaging the area in the buccal, central, and lingual portions of the middle third of the root canal. The cross-section of the distal root is shown in the figure in the right panel. E, The measurement of root resorption lacunae. a, b, and c show how the length (RL, black line), depth (RD, dotted line), and area (RA, black region) of the active root resorption lacunae were measured on a histological section. Hematoxylin and eosin stain; bar, 250 µm. L, Lower incisor; M, mesial; D, distal; T, tooth; B, alveolar bone; PDL, periodontal ligament.

• 

  Figure 2 Changes in the body weight of rats in the control (solid line) and experimental (dotted line; extrusive tipping movement via continuous force application on the mandibular left first molar for 8 or 15 days) groups during the study period. Values are expressed as means ± standard deviations for 5 to 10 rats. All animals showed normal growth during the experiment, with no significant differences. This indicates few systemic problems caused by the implants and appliances.

• 

  Figure 3 Horizontal tooth movement in the two experimental groups of rats (continuous extrusive tipping force application on the mandibular left first molar for 8 or 15 days). Horizontal tooth movement was observed in both the 8-day movement group (8d; 0.15 ± 0.09 mm, n = 10) and 15-day movement group (15d; 0.26 ± 0.13 mm, n = 10) groups, but not in the control (C) group. The amount of tooth movement was not significantly different between the two experimental groups. The bar indicates the standard deviation. **p < 0.01 compared with the control group.

• 

  Figure 4 Tartrate-resistant acid phosphatase (TRAP) and hematoxylin and eosin (H–E)-stained sections from the distoapical third of the distal roots of rat mandibular left first molars. Active root resorption lacunae in the control (A, D), 8-day (B, E; continuous extrusive tipping force application for 8 days), and 15-day (C, F; continuous extrusive tipping force application for 15 days) specimens are stained with TRAP (A, B, C) and H–E (D, E, F). The solid line indicates the active root resorption sites. TRAP-positive multinucleated cells, i.e., odontoclasts (red-stained cells, arrowheads), are observed in the active lacunae (B, C). Bar, 250 µm. B, Alveolar bone; T, tooth; PDL, periodontal ligament; M, mesial; D, distal.

• 

  Figure 5 Amount of root resorption measured in terms of the length (RL), depth (RD), and area (RA) of resorption lacunae in 8- and 15-day specimens obtained from rat mandibular left first molars subjected to continuous extrusive tipping force for 8 and 15 days, respectively The amount and severity of root resorption exhibited no significant differences between the 8-day group (8d) and 15-day group (15d). **p < 0.01 compared with the control (C) group. The bar indicates the standard deviation.

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