Victoria Azarenka is treated for an injured right knee during her first round match of the French Open. (David Vincent/AP)

The Sounds of the Knee: Interpreting the Snap, Crackle and Pops

Pop, snap, grind and squish. Not exactly the sounds of music, but storytelling none the less. Here’s what your knees are saying—if you know how to listen.

Snaps: Snaps are most often the clicking of a scar tissue band over the edge of the upper thigh bone (the femur). These are usually not serious, though they can sometimes be painful. Scar bands can respond to physical therapy; soft tissue manipulation stimulates collagen cells to produce collagenase, which breaks down scar tissue and can also promote collagen formation in the healing process. Manual therapy is highly effective for many snaps.

Crackles: Muscles attach to bones through tendons. The tendons are covered in thin sheaths, which normally provide smooth gliding surfaces. When these sheaths get irritated they swell, and the thickened tissue increases the resistance for the tendon. The pulling and sliding of tendons against the swollen tissue of these inflames sheaths produces crackles, which can be both felt and heard. The patellar tendon and the pes anserinus tendons—the insertions of the hamstrings on the inner side of the knee—are most susceptible. Treatment is by ice, massage, injection of growth factors called PRP, and anti-inflammatory medications.

Pops: Popping when you pull your knuckle is a release of gas from the joint—so some believe. This is different from the knee pop. When a pop is loud enough, it’s usually caused by a big piece of tissue getting caught between big bones— often a torn piece of meniscus, or a loose body being pushed out from between the femur and the tibia. Pops can be serious. When they occur during a fall in skiing it is most often tearing of the ACL. When associated with pain, or reproducible during an exam, they often require surgery. So pay attention to the pops!

Grinds: A common cause is when the knee cap (patella) loses its smooth articular cartilage due to a previous injury or arthritis. The rough bone underneath, or the fragmented cartilage bits themselves, grind against the top of the femur, which is called the trochlea. Put your hand on your knee cap and do a squat. You can feel (and often hear) the grind. It can also happen between the femur and the tibia; usually after the meniscus cartilage has been torn or removed. Such grinding eventually leads to pain and often swelling., This is due to the small bits of cartilage being sprayed around the joint during the grind; they land in the joint capsule . Fluid is produced in response to this irritation. The knee swells, which hurts and limits the range of motion. Grinds are not good in coffee, or in knee joints.

So listen to your knees. Harmonies are smooth and pleasing. Cacophony is rough, painful and disabling. Tune the knee instrument as needed.

Dr. Kevin R. Stone is an orthopedic surgeon at The Stone Clinic and chairman of the Stone Research Foundation in San Francisco.

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