Page 20 - REMOVABLE ORTHODONTIC APPLIANCES

12 Removable Orthodontic Appliances
magnitude of the tension has only minor
effects on the pattern of tissue activity.
However, where the tension is excessive, peri-
odontal fibres may be torn and capillaries
ruptured so that there is haemorrhage into the
periodontal ligament.

Biochemical
The biochemical aspects of tooth movement
are complex and are beyond the scope of this
book. The reader is referred to papers bv Hill
Figure 2.7 (a) Areas of bone resorption (stipple) and (1998), Sandy et al. (1993) and McDonald
apposition (horizontal shading) associated with
orthodontic tipping movement, (b) Pressure in the (1993) for further details.
periodontal ligament varies around the
circumference of the root.
The supra-alveolar connective tissues
Unlike the principal fibres of the periodontal
action takes place. This gradually removes the ligament which pass between tooth and bone,
bone associated with the hyalinized area. When the trans-septal and free gingival fibres do not
this has been done, the tooth is free to move. If rapidly re-adapt to the new tooth position. This
the force applied is still excessive, a further is well illustrated by the rotation of teeth.
hyalinized area will be set up against the newly Reitan (1967) has shown that the principal
exposed bone surface: but if the pressure is now fibres become realigned within a few months,
below capillary blood pressure, the hyalinized but the free gingival and other supra-alveolar
area will be invaded by blood vessels and cells connective tissue fibres remain under tension
and direct surface resorption will then take for considerable periods of lime (Figure 2.8).
place. The residual tension in these fibre systems may
contribute to relapse following orthodontic
rotation.
Areas of tension
The width of the periodontal ligament is
increased by the initial tooth movement.
Within a short period of time there is a prolif-
eration of the fibroblasts of the periodontal
ligament and of the osteoprogenitor cells in the
socket wall. In areas of tension, these osteo-
progenitor cells differentiate to become
osteoblasts which lay down bone matrix
(osteoid tissue). This osteoid tissue rapidly
becomes calcified to form loose, vascular,
woven bone. Over a period of months, the
woven bone is remodelled to form mature
trabeculae. Within the cancellous spaces
remodelling of bone takes place and on the
external surface of the alveolar process,
periosteal resorption of bone is found. Thus
the alveolar process drifts in the direction in
which the tooth is moving. The fibres of the
periodontal ligament are lengthened or Figure 2.8 When a tooth is rotated about its long
axis, the supra-alveolar connective tissues remain
reformed. Unlike the areas of compression, the under tension.