Not a doctor, but I fought patellar tendonitis for 25 years (age 17-42). Finally at age 42, while playing semi-pro football, one night at practice--SNAP! My left patellar tendon snapped away from the kneecap, thus completely detatching my quad from the lower leg. I'm about 3 weeks post-surgery now.
I will tell you that the characteristic pain (for me) was specifically right at the place where the patellar tendon fuses into the kneecap, and this is a sure alarm for you to CHANGE whatever you're doing until that pain subsides.
I had been battling severe patellar tendonitis this season in my left knee, but took painkillers and played through it. That pain was telling me, "I'm about to pop!", and it did.
Having said that, if I were you, I would search in this area:http://www.kneeguru.co.uk/KNEEtalk/index.php?PHPSESSID=c1a7828834ada132a6478b91d84a4725&topic=6793.0
It might be hard to copy and past the whole link, but this is how I found it:
in the search box, just type in patellar tendonitis.
The 4th topic that comes up is: Pain and rehab - Patellar Tendonitis. There are a lot of good things in there about the topic.
I would not suggest attempting radical therapy or knee injections for this condition.
If you have an abnormal growth on the kneecap or tendon--that might be the time to do something outside the norm -- like having a bone spur removed. A bone spur around the kneecap-tendon junction can cause pain and even a tendon failure, and may have been a big contributor to my tendon failure.
Rest. Ice. Ibuprofen. Excellent shoes. A knee strap and knee sleeve. Don't jump high and land on concrete when your knee is sore. Choose your quad exercises wisely. Squats--no. Front squats--no. The standing leg machine that is essentially a front squat machine--no.
Inclined leg press--yes--but do NOT complete the leg press all the way into the lockout, fully extended knee position--stop short of complete extension. Leg extensions--yes. Running--if tolerable only. Biking--yes. Hamstring curls--yes. Calf exercises--yes. Leg stretching--yes.
Build those leg muscles up, and lay off them when they are mad at you. Don't be a hero. Time, patience, wisdom, and consistency.
If an extraordinary coarse of action becomes necessary (surgery, injections of some sort, etc.), it should be absolutely crystal clear that it HAS to be done.
I treated my tendonitis fairly well most of the time, and enjoyed a lot of activity and success. There at the end, it was hurting really badly, and I pushed and snapped it, and at 42, I can live with that. I'm expected a 90-100% recovery. But I'm glad I never resorted to anything radical or risky. Just discipline, ice, rest, Ibuprofen, moderate training, and knowing when to lay off for a few days. (Except there at the end, when I ruptured it)