The 'rule-of-three formula' was applied to calculate root length changes due to orthodontic treatment. It was assumed that during orthodontic treatment the crown length does not change (unless it is fractured). Therefore, the ratio between the initial crown length (C1) and the final crown length (C2) determined the enlargement factor. If no change occured in the root length during treatment, the ratio between the initial root length (R1) and the final root length (R2) should be equal to the C1/C2 ratio. If during treatment the root was shortened, the amount of OIIRR was R1-R2 (C1/ C2).