, that's awesome!
You should tell us more details about your knee problem.
If it's not something serious which requires medical treatment, it seems to me that you just need to either warm up your leg muscles before you meditate, strengthen your leg muscles and connective tissue by lifting light to moderately-heavy weights, or stretch more.
I'm not an expert. Those are just my amateur opinions. I have not yet gone beyond quarter lotus myself.
Warming up and stretching a little bit before meditating can make a big difference, although it might seem undignified to look like you're preparing for a sports event at some important Buddhist teaching. Try some body squats, lunges, marching in place, or jumping jacks to get the blood flowing. Then try stretching every major muscle group of your legs (hamstrings, quads, adductors, abductors, calves, tibialis).
Most people wouldn't try to do a split without warming up first, so it might make sense to warm up before trying to sit in the lotus position.