Page 21 - REMOVABLE ORTHODONTIC APPLIANCES

Biomechanics of tooth movement 13
Individual variation magnitude of the local pressure changes. There
is undoubtedly some threshold of force below
There is considerable variation between indi- which tooth movement will not occur, but at
viduals in bone density. In the majority of cases least in the case of a continuously applied force,
there are large cancellous spaces within the this must be quite low. Where possible, the
alveolar bone, whereas in a few individuals the force applied to the tooth should be low enough
bone is dense with very sparse cancellous to avoid areas of hyahnization in the periodon-
spaces. Very dense bone of this type will be tal ligament. Reitan (1967) has shown that with
resorbed only slowly. This is of particular bodily movement, because the pressure is more
importance if areas of hyalinization are uniformly distributed over the root surface, it is
produced in the periodontal ligament. In cases possible to avoid hyalinization: but with tipping,
with a dense bone, there will be little undermin- some hyalinization is usually produced at the
ing resorption and if the area of hyalinization is alveolar crest where pressure is maximal.
extensive, it may be a considerable time before However, with light forces, this area is small and
the bone is resorbed from the periphery. Indi- once it has been removed, in 2 or 3 weeks, direct
viduals with this type of alveolar bone are quite surface resorption will occur. If larger forces are
rare, but if. on a radiograph, the bone structure used, the area of hyalinization will be greater,
appears to be very dense, active treatment with resorption will take longer and tooth movement
removable appliances should be minimized and will be delayed. When tooth movement eventu-
very light forces should be used to move the ally does occur, the tooth will become slightly
teeth. A similar problem arises where a tooth, loose. If the applied force continues to be exces-
often an upper permanent canine, is buccally sive, further hyalinization and delay will follow.
displaced and surrounded by a dense cortical Other problems, such as loss of anchorage will
plate. In these circumstances, priority should be arise if heavy forces are used.
given to moving the tooth into the line of the
arch where it can be retracted more readily. On empirical grounds, it has been estabhshed
that for tipping movements of single-rooted
teeth, with minimal hyalinization, the force
should fall in the range between 25 g and 40 g
Age (approximately 1-1.5oz), the lower figure
being appropriate for teeth with short roots,
Teeth can be moved orthodontically at any age. such as lateral incisors. However, for the first 2
In the adult, the periodontal ligament is less or 3 weeks of tooth movement, even lighter
cellular than in the growing child, and so the forces (about half the above values) should be
tissue changes may take longer to get under used. Where a number of teeth with a large root
way. In addition, the alveolar bone may be area are to be moved, for example in the retrac-
rather more dense and tooth movement will be tion of upper buccal segments, greater forces
a little slower. However, in general, variations may be appropriate.
between individuals in the rate of tooth move-
ment are greater than the changes with age. The
types of treatment which are slower in the adult
are those which depend to some extent upon The rate of tooth movement
facial growth. For example, overbite reduction
is more difficult in the adult and spontaneous The rate of tooth movement varies between
movement of teeth following relief of crowding patients but, in general, a rate of at least 1 mm
is much more limited after facial growth and a month is considered to be satisfactory.
occlusal development are complete. However, if tooth movement is less than this, it
is probable that something is wrong with the
adjustment of the appliance, or that it is not
being worn as instructed.
The forces used in producing tooth
movement
The variations in the physical properties of the Retention
periodontal ligament and in the pressure distri-
bution within it, makes it impossible to relate Remodelling of the supporting tissues contin-
directly the force applied to the crown with ues for some months after tooth movement has