Increment of Activation

During tooth movement there is an initial mechanical compression of the PDL (Periodontal Ligament) and minor deformation of the alveolar bone resulting in initial tooth movement of the order of 0.3-0.9 mm in magnitude.

-Lindauer, 2000

Staying within this range is critical.

Mechanical compression produces hydraulic movement of fluids of the ECM (Extracellular Matrix). The bony wall of the socket contains fenestrations allowing fluid to be expressed into marrow spaces.

-Norton, 2000

Changes in Cell Shape: The hydraulic forces produce changes in cell shape, and blood vessel formation and the ECM trigger mechanical and chemical signalling pathways which will recruit the formation of osteoclasts. Progenitor cells come mainly from within the periodontal ligament space; therefore, it is crucial to avoid collapsing and occluding this space because it is the source of cells.

Being too aggressive is not compatible with biology. In fact, the biological response will be the opposite of what is desired.

If the compression is too great, the blood supply is occluded and an avascular, necrotic area, called the hyalinization zone appears. The tooth STOPS MOVING until the zone is resorbed by osteoclasts from the marrow side (indirect resorption). The resulting lag in tooth movement lasts 14-25 days.

Lindauer, 2000

Dental anatomy provides that the maximum movement be less than 0.3-0.9 mm to avoid hyalinization. Since this is an average number for all teeth, it is preferable to stay within the lower part of this range to reduce the chances that this value may be exceeded for some teeth.

Indeed, most commercially available aligner systems offer increments of 0.25-0.5mm.

Although there are limited studies on this specific topic, a study by Chisari et. al. shows that the smaller increment of 0.25mm may be slightly more effective than 0.5mm as tooth movement accuracy was 62% and 54% respectively.