Everybody is familiar with the sound of "cracking knuckles." Although this sound can be annoying to others, it is not worrisome. Things are somewhat different in the knee where cracking sounds can be more than an unpleasant noise. These sounds have a number of possible causes.
These sounds are the result of so-called cavitation. This is a well-known phenomenon which occurs in any system of rapidly moving fluid. It is found everywhere from hydraulic turbines to boat propellers and human joint articulations. Pockets of air or gas form and then disappear with a potentially loud sound. Although this is not known to be harmful to joints, we do not recommend it on a habitual basis.
Cracking sounds can also come from other sources.
- Sound can be from a torn meniscus. The broken fragment snaps in and out of place as the knee is bent and straightened. The posterior aspect of the medial meniscus is the most common culprit.
- A snapping discoid meniscus is occasionally found in children. In this situation the (usually lateral) meniscus is abnormally shaped. Instead of being a fine crescent, it is a thick poker chip. Because of its abnormal size and shape, it can catch and snap as the knee is moved about.
- In some patients, the patella snaps into place as the knee is bent. This can be detected by placing ones hand on the patient's patella. This is not usually painful and requires no specific treatment.
A different type of cracking is crepitus, which sounds like the crackling of leaves we step on in the Fall. The sound usually emanates from around the kneecap and is said to be due the kneecap cartilage rubbing against the underlying cartilage of the femur. Crepitus can be soft or loud. Either way, it is not automatically a sign of pathology. When associated with pain, however, crepitus deserves a formal evaluation.
As the knee bends pressure can be placed on the inner or outer aspect of the knee. Certain noises are associated with deterioration of the cartilage within one compartment or the other. If the noise is not associated with pain and is found on the opposite, painless knee then it is not necessarily significant. On the other hand, if it is associated with pain, it is a potential concern and will help the doctor make a diagnosis.
Finally, there is the one time snapping sound that takes place during an accident. A snapping sound that occurs during a ski accident or an awkward landing from a jump is often worrisome. A significant injury to the knee can be assumed to have occurred. It is usually associated with sudden pain followed by rapid swelling of the knee. The snapping is due to rupture of a ligament such as the ACL (Anterior Cruciate Ligament) or MCL (Medial Collateral Ligament), and/or a significant (usually medial) meniscal tear. The doctor will perform a physical examination, which will be supplemented by imaging studies.
The various snapping and cracking noises, which occur about the knee, can be helpful to the doctor with regards to making a specific diagnosis. Most noises are of no consequence. However, on occasion, they can reflect pathology within the knee. An accurate diagnosis then leads to appropriate treatment.