Page 19 - REMOVABLE ORTHODONTIC APPLIANCES

Biomechanics of tooth movement 11
lion of a tooth can only be undertaken with a osteoclasts are recruited to cause bone resorp-
couple. With an upper central incisor, it may be tion in the area of pressure. The stimulus for
possible to correct a rotation with a couple the recruitment and activation of osteoclasts is
between a labial bow and a palatal spring at the thought to arise from the mechanical perturba-
baseplate, but this requires careful manage- tion of the osteoblast which then signals to the
ment. osteoclast to increase its activity. The vascular-
ity is an important aspect of these changes since
osteoclasts are thought to form by fusion of
Tissue changes during tooth circulating monocytes. A patent and intact
movement blood vessel is therefore needed adjacent to the
site of resorption since this is where the bone
As a result of the initial tooth movement resorbing cells are recruited. Within a few days
described above, areas of compression and active resorption progresses so that the osteo-
tension are set up within the periodontal liga- clasts come to lie in shallow depressions
ment. The distribution of these areas depends (Howship's lacunae) on the bone surface.
on the nature of the initial tooth movement. If The integrity of the periodontal ligament is
the tooth lips, the pressure varies along the maintained by the turnover of periodontal
root, being greatest at the alveolar crest and fibres. Fibroblasts are responsible for the
apex. It should be remembered that the pres- formation of collagen and the ligament
sure at any level will vary with the width of the comprises 80% type I collagen and a significant
periodontal ligament and around the circum- level (15-20%) of type III collagen. The break-
ference of the root. With bodily tooth move- down of these coUagens is through a family of
ment, which is not possible with simple enzymes called the matrix metalloproteinases
removable appliances, the distribution of pres- (MMPs) and their activity is balanced by the
sure is more uniform along the root length, but natural inhibitors of MMPs, the so-called tissue
still varies around the circumference. The more inhibitors of metalloproteinases (TIMPS).
uniform distribution of pressure with bodily Eventually, direct surface resorption gradually
tooth movements means that for a given force remodels the socket wall and allows the tooth
applied to the crown of a tooth, the maximal to move.
pressure (and tension), within the periodontal The tissue changes are not confined to the
ligament will be less than with tipping move- periodontal ligament and socket wall: within
ments. the canellous spaces of the bone, remodelling
In response to these changes in pressure changes occur and on the external alveolar
induced in the periodontal ligament, tissue surface periosteal appositon of bone maintains
changes take place. The nature of the changes the thickness of the alveolar process (Figure
depends on whether the area in question is one 2.7).
of compression or tension and whether capil-
lary blood pressure is exceeded locally.
Where capillary blood pressure is locally
exceeded
Areas of compression The capillaries are occluded, cells in the peri-
odontal ligament die and on a histological level,
the area becomes structureless or 'hyalinized'.
Where capillary blood pressure is not The hyaline material is merely compressed
exceeded
collagenous tissue. Bone resorption depends
In this case there may be local changes in blood both on a blood supply and on a cellular
flow, but the capillaries remain patent. An response. Neither of these is present in hyalin-
increase in cellular proliferation takes place, ized areas and so direct surface resorption of
both in the fibroblasts of the periodontal liga- bone is not possible. The areas of hyalinization
ment and among the osteonrngenitor cells, are often quite localized. Deep to these areas
which line the socket. The stimulus for prolifer- (within the cancellous spaces of the alveolar
ation of the osteoprogenitor cells, which even- bone) and peripherally (in surrounding areas of
tually results in increased bone formation, is the periodontal ligament which are
seen on the tension side. On the pressure side, compressed, but not hyalinized), osteoclastic