Basic Principles of Orthodontic Treatment

BASIC PRINCIPLES OF ORTHODONTIC TREATMENT

The teeth and their supporting tissues show life-long ability to reposition themselves and adapt to functional demands. It is ilustrated by the fenomenon of physiological migration. It is well known that the teeth of the side segments tend to migrate in a mesial direction. There is also a tendency for continued eruption if a balance is not established with the antagonistic tooth, or if the balance is lost. By these means , eruptiom and migration, throughout life the teeth will seek to establish the best possible relationship between the jaws. These continuous physiological processes are affected by the growth of the craniofacial skeleton and are sensitive to any type of presure ( pressure from muscles, soft tissues, oclusal and functional factors or direct external forces ). The great potential for dentoalveolar modification is due:

- firstly to an extraordinary ability of the periodontal membrane to remodel itself and - secondly to an adaptability on the part of supporting alveolar structures in response to movement of the teeth

What is more, the basal parts of the jaws show adaptive reactions to stimuli directed at growth zones. This potential for modification is the basis for orthodontic changes.

Orthodontic treatment may involve the control of forces physiologicaly acting upon the teeth and associated structures or producing and use external forces. The goal for orthodontic treatment may be limited to preventing or eliminating unwanted impulses ( i.e. dysplastic muscle function ) by restraining such forces from acting on the teeth or adjacent supporting structures. Such a change in the equilibrium of forces may lead to considerable positional changes if continued over prolonged period of time. A tooth can be guided into position during eruption by being subjected to occasional contact with an inclined plane or a lightly activated element, while more extensive tooth movement may be obtained by subjecting the teeth, and eventualy also the alveolr process, to direct external forces.

During the physiological tooth migration as the orthodontic therapeutic movement the characteristic tissue changes take place. The main evidence for the qualitative histological responces comes from experimental studies on animals. The bone in direction which tooth is moving is resorbed while on the bone wall which the tooth is moving away from an bone apposition occurs. Among the fundamental problems that require elucidation are following:

- Why is the alveolar bone resorbed during tooth movement whilst the cementum remains intact? - What protects the root surface?

It is known that turnover rate of the bone tissue is high. The bone system acts as a mineral reservoir for the whole organism and there is permanent circulation of minerals between the bone system and inside environment of organism. The bone tissue shows high ability to remodel itself following the functional pressure on it. On the other hand the cementum is fully maturated tissue, built up as a permanent depository of mineral salts. But slow apposition continues on the cementum surface throughout life. This fact is of great importance for the resorptive mechanism. The unmineralized precementum layer has been considered to be a resorption-resistant coating layer. It protects the root surface and permit physiological tooth migration and orthodontic tooth movement. The periodontal ligament, the conective tissue which attaches the teeth to the alveolar bone, has also ability to remodel itself. However, the turnover rate is not uniform throughout the ligament. The cells are more active on the bone side than near the cementum, so that major remodelling take place near the alveolar bone.

Physiological tooth migration During the physiological migration the resorbing cells, called osteoclasts, are seen in the scattered lacunae associated with the resorptive surface. Resorptive surface is the alveolar bone wall towards which the tooth is moving. Unlike the osteoclastic resorption of bone to provide the space for tooth movements, the corresponding remodeling processes of the fibrous attachement is not clearly understood. There is a meshwork of collagen fibres of small diameter present, which explaines this rapid reorganisation process. The alveolar bone wall which the tooth is moving away from is characterized by osteoblasts depositing non-mineralized osteoid whichlater mineralizes in the deeper layer. The older fibres of the periodontal membrane are surrounded by newly deposited bone matrix and become embedded in bone. Simultaneously, new collagen fibrils are produced by the cells on the bone surface. The sites of active lengthening and rebuilding of the fibrous apparatus lie in the middle of the ligament and near the alveolar bone side. How this comes about is unknown.

Orthodontic tooth movement Orthodontic forces are usually more powerful than normal functional forces so response elicited in the periodontal ligament is more marked and extensive, although it is the same in principles as than seen during physiological migration.

Pressure side: Application of a continuous force on the crown of a tooth will lead to a tooth movement within the alveolous that is marked initially by narrowing of the periodontal membrane, particularly in the marginal area. This compresion will impede the vascular circulation and cell differentiation. After a few hours a certain reduction in the number of cells may be observed, indicating a temporary slowing down of cell renewal. After a certain period of time, when conditions are favourable, the cells will increase in number and differentiate into osteoclasts and fibroblasts. The width of the membrane is increased by direct osteoclastic removal of bone and orientation of the fibres in the periodontal membrane will change. During the critical stage of the initial application of force, high compression in some areas may cause degradation of the cells and vascular structures. The tissue reveals a glass-like appearance in light microscopy, which is termed hyalinization. It represents a sterile necrotic area. In a hyalinized zone the cells cannot differentiate into osteoclasts and no bone resorption can take place from the periodontal membrane. Tooth movement will stop until the hyalinized structures has been removed and the area repopulated by cells. The process displays three main stages : degeneration, elimination of destroyed tissue and establishment of the new tooth attachment. The adjacent alveolar bone is removed by indirect resorption by cells which have differentiated into osteoclasts on the surface of adjacent marrow spaces. The hyalinization may be limited to parts of the membrane or may extend from the root surface to the alveolar bone. Limited hyalinization is almost unavoidable in the initial period of tooth movement in clinical orthodontics. However, extended hyalinisation areas may later cause root resorptions which may lead to permanent root shortening. If the active force on the tooth persists, or if gentle reactivation of the force is undertaken, direct resorption of the alveolar bone is likely to occur. When the application of force is favourable, a large number of ostoclasts will be seen along the bone surface and tooth movement will be rapid. The fibrous attachment apparatus will to some extent be reorganized by the production of new periodontal fibrils, These are attached to the root surface and to those part of the alveolar bone wall where direct resorption is not occurring by deposition of new tissue, in which the fibrils become embedded.

Tension side: The main feature is the deposition of new bone on the alveolar surface which the tooth is moving away from, although degradative changes, with reduction in the number of cells, may be observed. Cell proliferation is usually seen after 30-40 hours in young humans. The original periodontal fibres become embedded in the new layers of pre-bone, or osteoid, which mineralizes in the deeper parts. New bone is deposited until the width of the membrane has returned to normal limits, and the fibrous system is remodelled.

In order to maintain the dimension of the supporting bone tissue, concomitantly with bone apposition on the periodontal surface on the tension side, an accompanying resorption process occurs on the spongiosa surface of the alveolar bone. Correspondingly, during the resorption of the alveolar bone on a pressure side, maintenance of the alveolar lamina thickness is ensured by apposition on the spongiosa surface. Tese processes are mediated by the cells of endosteum, which cover all the internal bone surfaces, marrow spaces, Haversion canals and dental alveoli. Extensive remodelling, a reaction which tends to restore the thickness of supporting bone, takes place in periosteum, in deeper cell-rich layers. As regards control of tissue reactions many mechanisms have been considered responsible for the differentiation of cells incident upon the application of an orthodontic force. Orthodontic tooth movement shows local traits of a damage/repair process with inflammation- like reactions: high vascular activity, many leucocytes and macrophages, involvement of the nervous and immune systems.

The forces in orthodontics should be very precisely controled not to damage periodontal ligament tissue, pulp of the teeth or cementum of the roots. The devitalization of teeth or root resorption may occur as a response to high presure and very rapid tooth movement. Since we wish our terapeutic movements to stay within physiological limits, knowledge of orthodontic forces needed in terms of magnitude and duration is very important. The critical question regarding orthodontic tooth movement is whether direct resorption without hyalinization areas take place on the alveolar surface. It has been observed that a ligth force acting over a certain distance moves a tooth more rapidly than a powerful one, because there is no need to eliminate necrotic hyaline tissue. What is considered a light or powerful force depends on:

- type and anatomy of the tooth to be moved - architecture of the periodontal ligament and the supporting bone - type of the tooth movement and mode of force application

The size, form, number and characteristic of the roots will influence the mechanical resistance to an external force. Thus cuspids or molars require stronger force to move than incisors or premolars. As regards the architecture of the periodontal ligament and alveolar bone, it is closely related to age. The number of cementoblasts, fibroblasts and osteoblasts is much higher in young patients than in adults, indicating higher activity. The necessary increase in cell numbers during the initial phase of the application of force in adults occurs more slowly and is more critical than in young individuals, and the deposition of the osteoid is similarly slower and less extensive. In addition the type of bone through which the tooth is displaced must be considered in the treatment plan. The alveolar process consists of the dense outer cortical bone plates and varying amounts of spongious or cancellous bone between them. The thickness of the cortical laminae varies in different locations, but is considerable in the mandibular side segments in particular. The movement of the tooth is more difficult and slower in the cortical dense bone than in spongious bone. ( i.e. distalisation of an upper canine situated in a labial position in cortical bone is difficult and timeconsuming, whereas if the tooth is moved lingually into the spongious bone of the central part of the alveolar process before distalisation the desired movement will occur more readily.) In general the bone is more dense in side segments than anteriorly, and in the mandible than in maxilla. When a tooth is moved into the reorganizing alveolus of a newly extracted tooth, remodeling is very rapid, due to the many differentiating cells present and to the limited amount of bone to be resorbed. Despite these facts, individual variations in alveolar bone architecture are considerable. The magnitude of the force needed depend also on type of the tooth movement wanted. ( i.e. intrusion or extrusion requires very light forces while bodily movement of a tooth requires stronger force). The mode of application and the mechanical arrangement of the recipient tooth units are also of importance. A local force intended to move an individual tooth should be only a small fraction of a force which is applied against full dental arch, where all teeth are united into a block and where it is intended not only to move the teeth but even influence basal parts of the jaws. The magnitude of a force depends also on its duration. We distinguish:

- continuous forces - continuous, but interrupted after a limited period ( forces working over a short distance, typicaly exemplified by a tooth ligated to a labial arch wire) - intermittent forces, mainly induced by removable plates - intermittent forms of a functional type, induced by functional appliances, transmitting muscular activity into impulses directed at the teeth and alveolar processes

In case of intermittent application , frequent discontinuation provokes increased vascular circulation and cell proliferation. Also interupted continuous forces create favourable conditions for further tissue changes. Since the force decreases rapidly, despite inicial hyalinisation, the tissue will readily be reorganized. The strong continuous force is unwanted because it may lead to considerable injury. In addition to any applied force, the normal chewing forces will always be present.

So to sum up, it is apparent in modern orthodontics that a complicated interaction exist between many combinationns of forces. What is more, also individual variation exist. This partly explains the different patterns of tissue reaction to orthodontic treatment.