Complex Regional Pain Syndrome (CRPS) seems to be unrelated to the extent or seriousness of the original insult to the knee.
Often the original problem is, in the grand scheme of things, relatively trivial.
There is an injury, or surgery (often minor), and almost invariably the person has spent a period with the joint immobilised. The problems tend to become apparent when the person goes off to rehabilitation. Rehabilitation does not progress smoothly. Range of movement is limited by pain. The person insists on continuing with crutches despite the physiotherapists wanting them stopped. The pain becomes illogically severe, and has a particular unremitting characteristic. Even touching the limb can be painful, and the sensation is unpleasant and burning in nature. Other symptoms include boggy swelling around the knee, where the skin can be poked down and takes a while to come back to normal. The skin can be shiny red and very warm compared to the other side. The leg can feel quite sweaty, and the sweatiness can actually be measured and compared to the other limb.
This stage is called Stage I CRPS. The patient may stay in this phase and not progress to any worsening symptoms. Stage I is more amenable to treatment than later stages.