knee problems

I am a forty year old male who has under gone a fifth left knee arthroscopy. Each time it has been either a partial or full tear of the meniscal catilage. I am not over weight, but do much physical work in my job (retail). My last scope was on Aug. 8th of 2001. Have been going to physical therepy twice a week, and seems to be my knee is in worse shape than before the scope in august. I am experiencing severe pain when walking on the inside joint of my knee. Is this the sign of more problems or still in the healing process. The damage of my knee has always been the inside part of my knee, the doctor has assurred me that there was still cartilage in place. Why all the pain and discomfort?