In chapter 700 it is revealed that Sasuke still has his left arm missing yet Naruto has used what I theorize is white zetsu cells to regrow his right arm, much like how Obito was able to regrow his limbs with it back when Madara took him into his hideout after half of his body was crushed by boulders. My theory on Sausuke's left arm is that he refused to use white zetsu cells as they originate from Hashirama Senju's cells. Hashirama Senju being the fated rival of Madara Uchiha (Sasuke's ancestor), is inherently Sasuke's rival. This means Sasuke will not accept any aid from his enemy. My theory is that Sasuke is having a new left arm developed in a lab that is made up of black zetsu cells. In the manga we witness black zetsu take control of people's body and mind, imposing "Kaguya's will" onto them. I think Sasuke has Karin or Orochimaru, both ninja scientists, developing his new arm. He probably instructed them to remove the part of black zetsu that is responsible for taking control of the host. It would be faster for Sasuke to use Hashirama's cells, but he has too much Uchiha pride to regain his abilities at the courtesy of the Senju clan.