Loose cartlige in knee vs arthritis in knee?

Same? This may be one and the same. Cartilage loosens as arthritis progresses. If you have locking and catching then an orthopedic surgeon may help.
See your doctor. You could have a host of issues that could cause your knee pain, could be bursitis, tendonitis, arthritis, ligament issues, meniscal injury, sciatica etc. So you may want to see an orthopedic pain specialist that can tell you what is going and help you with the latest treatments and exercise therapies available. Consider Stem Cell therapy to help as well. Regenexx.Com.
I agree. This is essentially the same, depending on your age. If this is causing pain and inhibiting your activities of daily living, you should see an orthopedist for treatment and eval.