Associations Between Knee Effusion-synovitis and Joint Structural Changes in Patients with Knee Osteoarthritis
In OA, unlike RA, synovitis and effusion appear to be reactive to preceding bone and cartilage pathology.
While synovitis may be secondary to established bone and cartilage pathology in OA, once it sets in, it drives further inflammatory damage to the adjacent bone and cartilage, and is a major cause of pain and loss of function.
This still does not answer the question, “Which knee structure gives rise to the crepitus felt/heard clinically?”. We’ll find out for certain one day, just as we discovered that the sound from popping knuckles comes from air bubbles coming out of synovial fluid as the joint capsule gets stretched. The kind that is associated with OA and pain, I suspect arises from thickened and inflamed synovium and/or fat pad rubbing over each other as the joint moves.