Page "Human tooth" Paragraph 54

A technique for altering the natural progression of eruption is employed by orthodontists who wish to delay or speed up the eruption of certain teeth for reasons of space maintenance or otherwise preventing crowding and / or spacing.

If a primary tooth is extracted before its succeeding permanent tooth's root reaches ⅓ of its total growth, the eruption of the permanent tooth will be delayed.

Conversely, if the roots of the permanent tooth are more than ⅔ complete, the eruption of the permanent tooth will be accelerated.

Between ⅓ and ⅔, it is unknown exactly what will occur to the speed of eruption.