Bone Bending Piezoelectric And Magnetic Forces

There was considerable interest in piezoelectricity as a stimulus for bone remodelling during the 1960s. This arose because it was noted that distortion of crystalline structures generated small electrical charges, which potentially may have been responsible for signalling bone changes associated with mechanical forces. The interest therefore in 'electricity' and bone was considerable.

Magnets have been used to provide the force needed for orthodontic tooth movement. Classically an unerupted tooth has a magnet attached to it and a second magnet is placed on an orthodontic appliance with the poles orientated to provide an attractive force. It is unlikely that the magnetic forces alone have any actions on tissues. If magnetic fields are broken (as in pulsed electromagnetic fields) then there is some evidence that tissues will respond. It is worth making the following points about the effects of magnetic and electric fields on tooth movement:

• The periodontal ligament is unlikely to transfer forces to bone. If the periodontal ligament is disrupted, orthodontic tooth movement still occurs

• Magnetic fields alone have little, if any, effect on tissues

• Pulsed magnetic fields (which induce electric fields) can increase the rate and amount of tooth movement

• When an orthodontic force is applied, the tooth is displaced many times more than the periodontal ligament width. Bone bending must therefore occur in order to account for the tooth movement over and above the width of the periodontal ligament

• Physically distorting dry bone produces piezoelectric forces which have been implicated in tooth movement. Piezoelectric forces are those charges which develop as a consequence of distorting any crystalline structure. The magnitude of the charges is very small and there is some doubt whether they are sufficient to induce cellular change.

• It must also be remembered that in hydrated tissues, streaming potential and nerve impulses produce larger electrical fields and thus it is unlikely that piezoelectric forces alone are responsible for tooth movement.2

A wider application of the phenomenon of mechanically induced bone remodelling is seen where sutures are stretched. In young orthodontic patients the midline palatal suture can be split using rapid maxillary expansion techniques. The resulting tension generates new bone which fills in between the distracted maxillary shelves. A similar technique is also used to lengthen limbs. This method, known as distraction osteogenesis, can be used in any situation where it is hoped that new bone will be generated. Originally this was described in Russia where many soldiers returning from war faced the problem of non-union limb fractures. Initially attempts were made to induce new bone formation by compressing bone ends. It was only when a patient inadvertently turned the screw for compression of bone ends in the wrong direction that it was noted excessive new bone formation was seen where bone ends were distracted rather than compressed.

This may also have application in patients whose sutures fuse prematurely (craniosynos-toses such as Crouzon's or Aperts Syndrome). In this situation continued growth of the brain results in a characteristic appearance of the cranium but more importantly the eyes become protuberant with possible damage to the optic nerve. Treatment involves surgically opening the prematurely fused sutures and burring out to enable normal brain growth. If distraction forces are applied prior to this early fusion then bony infill could occur at a controlled rate. The phenomenon of pressure resulting in bone loss is also seen in pathological lesions. Much work was done to examine pressures within cystic lesions and to equate this with the rate of bone destruction. It is now recognised that cytokines and bone resorbing factors produced by cystic and malignant lesions are more likely to be responsible for the associated bone resorption.

Undermining Resorption

Fig. 3 This is an area of excessive pressure where the periodontal ligament has been crushed or 'hylanized' and the periodontal ligament has lost its structure. There is a large cell lying in a lacunae behind the frontal edge which is probably an area of undermining resorption

Fig. 4 Area of root resorption associated with orthodontic tooth movement. The apex of the tooth has a large excavation of the root surface and this is typical of excessive tipping forces that are placed on the apices of the teeth

Fig. 3 This is an area of excessive pressure where the periodontal ligament has been crushed or 'hylanized' and the periodontal ligament has lost its structure. There is a large cell lying in a lacunae behind the frontal edge which is probably an area of undermining resorption

Tension results in bone formation, this can be used to generate new bone for digit lengthening or suture distraction

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