  We report ab initio calculations showing that a single one-dimensional
extended defect can originate topologically-protected metallic states in the
bulk of two-dimensional topological insulators. We find that a narrow extended
defect composed of periodic units consisting of one octogonal and two
pentagonal rings embedded in the hexagonal bulk of a bismuth bilayer introduces
two pairs of one-dimensional Dirac-fermion states with opposite spin-momentum
locking. Although both Dirac pairs are localized along the extended-defect
core, their interactions are screened due to the trivial topological nature of
the extended defect.
