Extended defects like stacking faults (SFs) can originate topologically protected metallic states in bulk topological insulators (TIs). These induced topological states are a response to the weakening of the inter-layer van der Waals interactions due to the SF defect. In TI thin films, the degeneracy of Dirac bands of opposite surfaces can be lifted upon the formation of SF defects. Such slab asymmetry can promote a net spin current, in the absence of backscattering processes, in thin film made of TIs. These results have been obtained by fully relativistic first principles calculations.