Articular cartilage serves to:

A. Bursa: Bursae are fluid-filled sacs that reduce friction between tissues and when inflamed (bursitis) cause pain and localized swelling but usually do not produce major mechanical limitation of knee motion.
B. Tendons: Tendons connect muscle to bone and injury can limit movement by impairing muscle action, but isolated tendon injury typically affects specific movements rather than general joint mobility.
C. Meniscus: The menisci are fibrocartilaginous pads that cushion and stabilize the knee; tears frequently cause joint locking, catching, pain, and significant limitation of knee mobility.
D. Ligaments: Ligaments stabilize the joint and when torn can produce instability and pain that limit function, though mechanical locking and restricted range are more characteristically produced by meniscal damage.

A. Normal joint function:Crepitus is not typically a sign of a normally functioning joint and usually indicates an underlying change in joint surfaces.
B. Inflammation of the synovial membrane:Synovial inflammation (synovitis) can cause pain and swelling but crepitus more specifically implies mechanical roughness rather than inflammation alone.
C. Presence of air in the joint space:Air in a joint (pneumarthrosis) is uncommon and would not be the usual explanation for the grinding or crackling sensation felt as crepitus.
D. Degeneration of joint structures:Crepitus often reflects irregular or roughened articular surfaces from wear or degeneration (for example osteoarthritis) producing a grating sensation during movement.