What's the significance of the scar that Amir develops as a result of the confrontation in The Kite Runner?

The physical scar that Amir gets in his fight with Assef
is significant for a couple of reasons.  The first is that it shows sacrifice.  Amir is
finally sacrificing for his friendship, the way he should have when he saw Ali being
violated in the alley after the kite festival.  The sacrifice he has to make now is the
result of years of repression and denial.  His scar is also significant because it moves
him closer to Ali, as his hare lip before the surgery defined him and Amir's scar now
will help to define him.  Amir's physicality cannot be removed, as it has become a part
of him, representation of the commitment both he denied and eventually embraced.  In the
notion of "becoming good again," the scar goes a long way to that point.  I would also
say that the scar is representative of the physical notion of taking action.  Part of
Amir's guilt and shame resides the fact that he did nothing when he saw Ali being
violated.  The physicality of action is something that is represented by the scar.  This
is beyond words, beyond thought, but rather pure action, representing a commitment
towards that which is good.