i disagree with mashy..baby will know their fingers when the mittens is off for sometime. it's not a permanent thing..
i have checked with my PD and was worried we made my girl wear the mittens too long but my gynae reassure us it's not and it's ok and that my girl will know how to explore and use her fingers.. true enough, after sometime, her fingers open up. now, she is 17 months, she has no problem with her fingers at all! she can self feed when she was 10 mths?? can pick up gerber stars, etc..
if you're really worried, then take out the mittens when you're around for them to explore so that you're around to watch your baby from scratching his/her face. when you're away, then put it back..