This is going to sound like a very 'smart-ass' answer but if you are hitting your shins (or in your case your knees) then chances are you are standing too close to the bar on the initial lift. Move back just a couple of inches. Grasp the bar with a reversed grip, arms outside the stance of your legs, arms as straight as possible, knees bent slightly, back straight, head up, and butt sticking out. Don't allow your back to round or your shoulders to drift inwards during the movement. If necessary use an unweighted bar infront of the gym mirrors so you can watch your form and make corrections.