Immunohistochemical evaluation of osteoclast recruitment during experimental tooth movement in young and adult rats

OBJECTIVE: Orthodontic tooth movement starts slower in adults than in juveniles, but the rate of tooth movement in later phases is the same in both age groups. The hypotheses to be tested are that these phenomena are related to slower osteoclast recruitment in adults than in juveniles, but that in later phases the osteoclast numbers are the same in both age groups.

DESIGN: Standardized orthodontic tooth movement was performed in two groups of 30 rats, aged 6 weeks and 9-12 months, respectively. All maxillary molars at one side were together moved mesially by a continuous force of 10 cN. The other side served as a control. After 1, 2, 4, 8 and 12 weeks, groups of animals were killed. After ED1 staining osteoclast numbers at the mesial and distal sides of selected roots were counted.

RESULTS: At the compression sides osteoclast numbers increased in both age groups. In young rats, a maximum was reached at 2 weeks, in adults at 4 weeks. In later phases of tooth movement, the number of osteoclasts in the adults was approximately twice as high as in the juveniles, while the rate of tooth movement was the same. A positive correlation between the rate of tooth movement and osteoclast numbers was found only in young rats.

CONCLUSIONS: Orthodontic forces induce faster osteoclast recruitment in young than in adult rats and more osteoclasts are needed to achieve a certain rate of tooth movement in adult than in young rats.