Crepitus of the Knee: Creaky Bones or Something More?
IT’S NOT unusual for people to hear a noise or feel a cracking, crunching, or popping sensation when they move their knee. This cracking or popping sensation, known as crepitus, is usually due to air bubbles being caught in body tissues. It can happen in the chest or the knee. In the knee, it can cause a sound when the knee is extended.
Knee crepitus can happen at any age, but it is more common as people get older. It can affect one or both knees. The sound may be audible to other people, or it may not. Crepitus is often harmless, but if it happens after a trauma or if there is pain and swelling, it may need medical attention. The three bones in the knee joint are the thighbone ( femur), the shinbone ( tibia), and the kneecap ( patella). The kneecap rests in a groove of the thighbone, called the trochlea. When a person bends or straightens their knee, the patella moves back and forth inside this groove.
Two wedge- shaped pieces of cartilage between the thighbone and the shinbone are called the meniscus. These enable the bones to glide smoothly against each other. The cartilage is tough and rubbery, and it helps to cushion the joint and keep it stable. There is also a thin layer of tissue called the synovial membrane that covers the joints and produces a small amount of synovial fluid, which helps to lubricate the cartilage.
The underside of the kneecap is lined with cartilage. This cartilage “rubs” against the end of the femur in the trochlear area and with ab- normal wear can cause grinding ( crepitus). In most cases, the popping sound comes from air seeping into the soft tissue, finding its way into the area around the joint and causing tiny bubbles in the synovial fluid. When a person bends or stretches their knees, the bubbles can burst with a popping or cracking sound. While it may sound alarming, this is harmless.
However, crepitus can also happen as cartilage rubs on the joint surface or other soft tissues around the knee when the joint moves, and when the cartilage becomes thin and wears away. If there is pain as the knee snaps or catches, it can be because scar tissue, a meniscus tear, or a tendon is moving over a protruding bone within the knee joint.
Pain or swelling can be a sign of a more serious problem, such as patellofemoral pain syndrome, torn cartilage or other soft tissue, or osteoarthritis ( OA). These issues may need medical attention. Let’s look at them now in more detail. When the pressure between the kneecap and the femur is greater than usual, the cartilage in the joint can start to soften and wear away, losing its smoothness and leading to a condition called patellofemoral pain syndrome ( PFS), or “runner’s knee.”
Rigorous exercise such as jogging, squatting, and climbing stairs can put strain on the area between the femur and the kneecap joint. A sudden increase in physical activity, such as exercising more frequently, or running further or on rougher terrain than usual, can also cause it.