  Interactions between the surface and the bulk in a strong topological
insulator (TI) cause a finite lifetime of the topological surface states (TSS)
as shown by the recent experiments. On the other hand, interactions also induce
unitary processes, which, in the presence of anisotropy and the spin-orbit
coupling (SOC), can induce non-trivial effects on the spin texture. Recently,
such effects were observed experimentally in the $Bi_2X_3$ family, raising the
question that the hexagonal warping (HW) may be linked with new spin-related
anomalies. The most remarkable among which is the 6-fold periodic canting of
the in-plane spin vector. Here, we show that, this anomaly is the result of a
{\it triple cooperation between the interactions, the SOC and the HW}. To
demonstrate it, we formulate the spin-off-diagonal self energy. A unitary phase
with an even symmetry develops in the latter and modulates the spin-1/2 vortex
when the Fermi surface is anisotropic. When the anisotropy is provided by the
HW, a 6-fold in-plane spin-canting is observed. Our theory suggests that the
spin-canting anomaly in $Bi_2X_3$ is a strong evidence of the interactions.
High precision analysis of the spin-texture is a promising support for further
understanding of the interactions in TIs.
