You say this as though it's an established fact, however as I and others have pointed out, Nandrolone may well have direct (and almost certainly) indirect benefits to the joints themselves. It clearly has a benefit for muscle growth and general healing that is real, so a benefit for joints could well be occurring, as is supported by the logic i've posted before.
Also, on a related note, I just listened to the SuperhumanRadio episode (2943 I believe) on management of chronic pain and the doctor pointed out that even when someone is on pain meds that don't affect the underlying cause of the pain, there is a well-known rebound affect where the pain is perceived to be worse after the drug is stopped. So an increased perception of pain after stopping nandrolone is not in itself evidence that the Nandrolone did any harm.