Osteoarthritis, where the cartilage lining a joint gets worn down, is common (Australian data, UK data, US data). Most people will get it if they live long enough, and the knee joint is commonly affected. There is little that can be done to repair or reverse this process, and a related paper that covers many osteoarthritis treatments shows that most of the things we do (analgesics, anti-inflammatory medication, injections etc.) only provide temporary relief, and many of them hardly work at all. Treatment, if severe enough, often means a knee replacement.
Knee replacement surgery is major surgery so it is only reserved for those with severe osteoarthritis. So what do surgeons do with patients who have knee pain and mild or moderate arthritis? They often do an arthroscopy: a low risk, day-only procedure that pays well and seems to work some of the time. Hundreds of thousands are done in the US every year, and in my state the rate of arthroscopy is high and is rising.
The trouble is: it doesn’t work. Most patients still have pain, some get worse, and about 20% will end up having a knee replacement within 2 years anyway. Feel free to skip to the last paragraph for the Bottom Line, or read on for the details.