I agree with others that you don't need to max out the weight stand to do squats. My current routine is a super set that will have my legs quivering at the end. My continuous set goes like this: 15 x 16" box jumps, right to 10 x upside down bocu ball squats, right to 10 squats balancing on two medicine balls, right to failure to 10 reps of squats standing/balancing on a 55 cm stability ball.

That level of effort might not be for everyone but modifying it to an extent will provide a great and effective set that won't kill the knees.