Root resorption

Some degree of external root resorption is inevitably associated with fixed appliance treatment, although the extent is unpre-dictable.21 Resorption may occur on the apical and lateral surface of the roots, but radiographs only show apical resorption to a certain degree. Many cases will not show any clinically significant resorption but, microscopic changes are likely to have occurred on surfaces which are not visualised with routine radiographs. Resorption however rarely compromises the longevity of the teeth.22 Vertical loss of bone through periodontal disease creates a far greater loss of attachment and support than its equivalent loss around the apex of a tooth.

The mechanism of tooth resorption is unclear. Theories include excessive force and hyalinisa-tion of the periodontal ligament resulting in excessive cementoclast and osteoclast activity. What is clear are the risk factors which are associated with cases with severe resorption. These can be summarised as:

• Blunt and pipette shaped roots show a greater amount of resorption than other root forms.

• Short roots are more at risk of resorption than average length roots.

• Teeth previously traumatised, have an increased risk of further resorption.

• Non vital teeth and root treated teeth have an increased risk of resorption.

• Heavy forces are associated with resorption, as well as the use of rectangular wires, Class II traction, the distance a tooth is moved and the type of tooth movement undertaken.

• Combined orthodontic and orthognathic procedures.

Treatment of ectopic canines may induce resorption of the adjacent teeth because of the length of treatment time and the distance the canine is moved. Tooth intrusion is also associated with increased risk as well as movement of root apices against cortical bone. Above the age of 11 years the risk of resorption with treatment seems to increase. Adults have shorter roots at the outset and the potential for resorption is increased.

Opinion is divided on whether treatment length is associated with increased resorption. Some find no correlation with treatment time, whereas others find that there is increased resorption with increased treatment time. In a few patients systemic causes may contribute, for example hyperthyroidism, but for the most part no underlying cause is isolated other than individual susceptibility. Familial risk is also known.

A wide range in the degree of resorption is seen, highlighting the role of individual susceptibility over and above the risk factors identified. Research is still required in this area to identify the mechanisms of resportion, trigger factors and reparative mechanisms if treatment modalities are to be modified in the future to minimise root damage. Currently, no case is immune from the risk of root resorption, to some degree, and patients should be warned at the outset of treatment that such a risk exists. Recognition of specific risk factors, accurate radiographs and interpretation of radiographs at the outset of treatment are important if root resorption is to be minimised. Once resorption is recognised clinically during treatment, light forces must be used, root length monitored six monthly with radiographs and treatment aims reconsidered to maximise the longevity of the dentition. The use of thyroxine to minimise root resorption has been advocated by some authors, but this is not routinely used.23, 24

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