Let $FM_2=\langle a,b\rangle$ be the free monoid of rank 2. If we add a formal inverse to the word $aba$, we get the free group $F_2$ (because both $a$ and $b$ will have inverses). 

<b>Question:</b> For which other words $w=w(a,b)$, adding  a formal inverse to $w$ turns the free monoid into a free group? 

I need a complete description, not just examples.