A growing rod adapted to be secured along a length of a spine for treating deformities of the spine. The growing rod comprises a base rod, an extendable rod having a proximal portion that is slidably coupled to the base rod and arranged with a gear rack, and a distraction unit. The distraction unit includes a housing attached to the base rod, a rotatable drive interface accessible by an external driver from outside of the housing, and a drive gear mechanism housed within the housing and coupled to the rotatable drive interface and the gear rack such that rotation of the rotatable drive interface causes linear movement of the extendable rod through the gear rack.