Complex Regional Pain Syndrome (CRPS) was previously known as Reflex Sympathetic Dystrophy (RSD) and before that it was called 'Causalgia', 'Algodystrophy' or 'Sudeck's atrophy'.
This last three are very old-fashioned and not often used these days but doctors still tend to use the term RSD rather than the more modern and correct CRPS. The reason for this is probably that RSD suggests a mechanism for the condition - that it is a condition triggered by a 'reflex' that goes wrong with the 'sympathetic' nervous system, and which may in advanced cases lead to 'dystrophy' or wasting of the limb.
What is meant by this?
Well, let's firstly talk about something called the 'reflex arc'. Everybody is familiar with the reflex kick that occurs when the patellar tendon is tapped. When something is referred to as a 'reflex' it means that the nerve stimulus races to the spinal cord and initiates a response without really involving the brain in the decision. In this case the tap on the tendon is recorded by a sensory nerve (sensation) relaying the signal to the spinal cord and the kickback is triggered by a motor nerve (movement) relaying the signal back from the spinal cord.
Blood flow and skin temperature are also controlled by reflex arcs - in this case the arc involves 'sympathetic' nerves rather than motor ones. So if the room is cold the blood vessels in the skin will contract, for example. The brain is largely by-passed as with the patellar reflex.
Many of the symptoms of CRPS seem to be due to abnormal reflex responses of the sympathetic nerves - the temperature changes in the limb and the blotchy discoloration for example. The abnormal response seems to be aggravated by stress and there is no relation to the extent of the original knee problem.