In The Kite Runner, what physical scars does Amir bear from his fight with Assef? Which is the most significant and why?

Amir is lucky to be alive after his encounter with Assef, and he has sustained some serious physical damage.  His ruptured spleen had to be removed, he has several broken ribs, and his lung has been punctured.  One of his eye socket bones has been fractured, and he has many cuts.  He has also lost a tooth.  The surgery to remove his spleen and to treat the punctured lung will leave scars, certainly.  But the major laceration was to his upper lip, in its middle, which had to be sewn back together. This is the scar that is of great significance in the story.

Early in the story, we learn that Hassan was born with what is called a "harelip."  This is a split lip, a birth defect.  He is made fun of by others because of this, and Baba finally pays for this to be fixed surgically, and Hassan is left with "just a pink jagged line running up from his lip" (47).  This "gift" to Hassan leaves Amir envious, thinking, "I wished I too had some kind of scar that would beget Baba's sympathy" (46).  He has always felt a sense of competition with Hassan for Baba's affection, without any realization of the undercurrents around him, since he does not know that Hassan is Baba's son, too, something he learns much later in the story, before he has his encounter with Assef. 

Now, Amir has a scar like Hassan's, and he realizes that he has gotten what he wished for those many years ago, thinking to himself, "Clean down the middle. Like a harelip" (297).  Now, although Baba and Hassan are gone, Amir has the same scar, providing Amir with a physical manifestation of his brotherhood with Hassan. So, not only is this scar a badge of honor, representing his fight to save Sohrab, Hassan's son, but also it is a badge of his brotherhood with Hassan.