A few things I'd like to throw on the table for you here.
1 - Are you taking an EFA supplement? If the answer is no than start taking one, it will help with these types of aches.
2 - Are you locking out at the top of the movement? If you are stop, try to hold the weight at the top of the movement but don't lock your knees, keep constant tension on your thighs.
3 - Are you going low enough? The lower you go the easier it is on your joints believe it or not.