The behaviour of the periodontal ligament is influencing the use of new treatment tools.

Orthodontic treatment can cause resorptions on teeth. Factors causing resorptions are forces and moments. Other factors that cause resorptions are less influenced by an orthodontic treatment. By carrying out experiments and finite element calculations, it was possible to demonstrate that forces and moments will produce stress in certain parts of the periodontal ligament and the root. This stress reaches or exceeds the blood pressure in some areas. To avoid high pressure in those areas, new treatment devices were constructed consisting of NiTi wires or a combination of NiTi and stainless steel. In the levelling phase, the acting forces remained below 0.5 N using NiTi wire with a diameter of 0.012' (using elastic ligatures). With a NiTi stainless steel spring molars can be uprighted. The orthodontist can easily choose between an intrusive force, a very low intrusive force and an extrusive force. Because of the use of NiTi elements, there was acting an almost constant moment.