I am taking a guess here (with the limited information you gave about your injury) but maybe the knee was not completely healed when you returned to front squating. Knee health and recovery can fool anyone. When you think their ready for some serious work, they really may need a couple more weeks from any workout action. Might even have a slight tear/rupture around the area if a burning feeling starts. This will require much more rest.
As far as warm-ups go, you might consider using light knee wraps (nothing too tight) to keep warmth around the area. Or even a heating rub/lotion before and after training that area. If you are using a Smith machine, some people will find that the straight up and down locked in movement (as opposed to the natural "S" curve of the exercise with free weights) can cause knee/back problems. And others think the Smith is the greatest thing since sliced bread for leg work. Will depend on body mechanics for the most part. The Smith machine can be a good tool for workouts, just that it is not meant for every body type or every exercise. Good Luck.
