title: Age-related effects on osteoclastic activities after orthodontic tooth movement.
abstract: OBJECTIVE
  To elucidate the effects of age on the expression levels of the receptor activator of the nuclear factor-κB ligand (RANKL) and osteoclasts in the periodontal ligament during orthodontic mechanical loading and post-orthodontic retention.
  
  
  METHODS
  The study included 20 male Sprague-Dawley rats, ten in the young group (aged four to five weeks) and ten in the adult group (aged 18 to 20 weeks). In each rat, the upper-left first molar was subjected to a seven-day orthodontic force loading followed by a seven-day retention period. The upper-right first molar served as a control. The amount of orthodontic tooth movement was measured after seven-day force application and seven-day post-orthodontic retention. The expression levels of RANKL and the tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts were evaluated on day 7 (end of mechanical force loading) and day 14 (after seven days of post-orthodontic retention). Statistical analysis was performed using the t-test, and significance was set at p < 0.05.
  
  
  RESULTS
  There was no significant difference between the amount of tooth movement in the young group (0.96, standard deviation (sd) 0.30mm) and that in the adult group (0.80mm, sd 0.28) (p > 0.05) after the seven-day force application. On the compression side, the expression of RANKL and TRAP-positive osteoclasts in both the young and the adult groups increased after the application of force for seven days, and then decreased at the end of the seven-day retention period. However, by the end of the period, the expression of RANKL on the compression side dropped to the control level in the young group (p > 0.05), while it was still higher than that on the control side in the adult group (p < 0.05). The expression of RANKL on the compression side did not show significant difference between the young and the adult groups after seven-day force application (p > 0.05), but it was significantly higher in the adult group than that in the young group after seven-day post-orthodontic retention (p < 0.05). Similarly, the decreasing trend of TRAP-positive osteoclasts during the retention period in the adult group was less obvious than that in the young group.
  
  
  CONCLUSIONS
  The bone-resorptive activity in the young rats was more dynamic than that in the adult rats. The expression of RANKL and the number of osteoclasts in adult rats did not drop to the control level during the post-orthodontic retention period while RANKL expression and the number of osteoclasts in young rats had returned to the baseline.Cite this article: X. Li, M. Li, J. Lu, Y. Hu, L. Cui, D. Zhang, Y. Yang. Age-related effects on osteoclastic activities after orthodontic tooth movement. Bone Joint Res 2016;5:492-499. DOI: 10.1302/2046-3758.510.BJR-2016-0004.R2.
authors: Department of Orthodontics, Peking University School and Hospital of Stomatology, Beijing, China.
authors: X
authors: X
authors: Li
authors: Orthodontics, Faculty of Dentistry, The University of Hong Kong, Hong Kong SAR, China.
authors: M
authors: M
authors: Li
authors: Taizhou Polytechnic College, Jiangsu, China.
authors: J
authors: J
authors: Lu
authors: Department of Cariology and Endodontology, Peking University School and Hospital of Stomatology, Beijing, China.
authors: Y
authors: Y
authors: Hu
authors: Department of Stomatology, Peking Union Medical College Hospital, Chinese Academy of Medical Science, Beijing, China.
authors: L
authors: L
authors: Cui
authors: Department of Stomatology, Peking Union Medical College Hospital, Chinese Academy of Medical Science, Beijing, China.
authors: D
authors: D
authors: Zhang
authors: Orthodontics, Faculty of Dentistry, The University of Hong Kong, Hong Kong SAR, China yangyanq@hku.hk.
authors: Y
authors: Y
authors: Yang
keywords: Age-related effect; Osteoclastic activity; Post-orthodontic retention