How are you holding the bar? There are three ways to hold the bar and that’s the Olympic clean grip where you hold the bar with your hands with writs bent back and bar resting on delts which can give you problems as weight get heaver, or the cross-grip where the bar rest on delts and you cross your arms and hold the bar. I prefer and use the stabilizer clean grip where you use wrist straps around the bar where your hands would go when doing a clean grip and pull wraps up as you lower into you squats see figure below. I you still having problems you could always do leg presses.

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ImageShack.usYou could do a 1x leg extensions 8-15 reps to failure then reduce weight by 20% when finished with leg extensions go straight into your back squats, for this to work well you should use a weight when squatting that you can bang out at least 8 to 10 good reps without any pause there’s no need to go complete full range on this so you keep the tension directly on the thigh you should be able to do this if you don’t go to heavy you do not want to be pausing like breathing squats this will put more stress and tier the back instead of killing those thighs keep weight reasonable legs will work just as hard.
Leg curls 8-12 reps to failure then reduce weight by 20% works well.