20: Tooth movement and related problems

20 Tooth movement and related problems

Case 1

Summary

Darren, a 13-year-old boy, has been undergoing upper removable appliance therapy for 6 months to retract and align image, following extraction of image. Tooth movement has been very slow with no movement recorded at the last two visits.

image What are the possible reasons for a slow rate of tooth movement?

These can be divided broadly into patient, appliance and operator factors.

Patient factors

The patient is not complying with appliance wear as instructed.
Incorrect positioning of the spring on appliance insertion or distortion of the spring.
Contact of the tooth with the buccal cortical plate or with a retained root part of image.
The occlusion with the opposing arch may prevent tooth movement.

Appliance factors

Acrylic and/or wire may be interfering with tooth movement.

Operator factors

Design flaws, underactivation, overactivation or activation of the spring such that image is directly buccally into contact with the cortical plate rather than through the cancellous corridor of bone.

image What force range is optimal for retraction of image by tipping movement?

The optimal force range is 30–50 g or 0.3–0.5 N.

image What cellular response is there following activation of the spring to retract image by tipping movement?

The consequence of the spring activation is the setting up of pressure and tension zones within the periodontal ligament. Half of the periodontal ligament is stressed with maximum pressure created at the alveolar crest in the direction of movement and at the diagonally opposite apical area (Fig. 20.1).

image

Fig. 20.1 Diagram illustrating zones of pressure (A) and tension (B) in the periodontal ligament induced by a tipping force.

Key point

Tipping movement, typically, requires forces of 30–50 g or 0.3–0.5 N.

Pressure zones

The cellular response depends on whether a light or heavy force is applied. With a light sustained force, tooth movement occurs within a few seconds as periodontal ligament fluid is squeezed out and the vascular supply is compressed, setting off a complex biochemical response. Within 2 days, osteoclast invasion occurs and frontal resorption follows.

When a heavy sustained force is applied, the periodontal ligament is compressed to such a degree that the blood supply is cut off completely, producing an area of sterile necrosis (hyalinization). Small areas of hyalinization are inevitable even with light forces, but the area of hyalinization is extended with forces of greater magnitude. Osteoclast differentiation is impossible within the necrotic periodontal ligament space, but after several days osteoclasts appear adjacent to and within the adjacent cancellous spaces. From there they invade the bone adjacent to the hyalinized area and tooth movement eventually occurs by undermining resorption.

Tension zones

Following initial application of a light force, the blood vessels vasodilate and the periodontal ligament fibres are stretched, while fibroblast and preosteoblast proliferation occurs. The stretched fibres become embedded in osteoid, which later mineralizes. The normal periodontal ligament width is eventually regained by simultaneous collagen fibre remodelling.

With heavy forces, rupture of blood vessels and severing of the periodontal ligament fibres are likely but both are restored with the remodelling processes.

Key point

Application of a sustained force to a tooth creates areas of pressure and tension within the periodontal ligament, which ultimately lead to bone resorption and deposition, respectively.

image What is the mechanism for tooth movement?

Although the histological response to an applied orthodontic force has been investigated extensively, the mechanism by which a mechanical stimulus is transferred to a cellular response is complex and is at present unresolved. It is likely that vascular changes in the periodontal ligament in areas of pressure and tension, electrical signals in response to alveolar bone flexing following force application, as well as prostaglandins and cytokine release interact in the process.

image How would you manage the problem in this case?

Management is specific to each cause. The action taken may be one or more of the following:

The need for full-time wear of the appliance must be emphasized to the patient/parent. They must be made aware that treatment will be ceased unless full cooperation is forthcoming.
Show the patient how to insert the appliance with the spring positioned correctly and explain the action of the spring. Ensure that the appliance is not being removed by the spring.
Check the activation of the spring and adjust it appropriately.
Remove any acrylic or wire obstruction to tooth movement. This may involve remaking the appliance to an improved design.
Add to a flat anterior biteplane or buccal capping to disengage the occlusion, if this is hampering tooth movement.
Ask the patient and check their case records regarding any noted problem with the extraction of image. If indicated, take a periapical radiograph of image extraction site to check for a retained root fragment. If this is detected, seek an oral surgical opinion regarding its removal or whether it could be left in situ and its position monitored radiographically.
Check image is not ankylosed. This is unlikely in this case as some movement of image has occurred since image was extracted.
Darren admitted to intermittent wear of the appliance and, in particular, to leaving it out at meals. He was advised accordingly.

Key point

Full-time wear of an upper removable appliance is required for an optimal rate of tooth movement.

image What do you notice on the periapical radiograph of another case (Fig. 20.2)?

There is a small retained apical root fragment of image.

image

Fig. 20.2 Periapical radiograph.

image What treatment would you advise?

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Jan 2, 2015 | Posted by in Orthodontics | Comments Off on 20: Tooth movement and related problems

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