synovial | andreacollo

Awesome people always pop their knuckles when they are with someone who clearly state they can’t stand it. The cracking sound that our knuckles (but also our fingers themselves, and sometimes our elbows and knees) produce when we “stretch” them is somehow addictive. I thought it would be interesting to understand what makes our joints pop in this strange way and I asked God Google for that.

First of all we have to focus on Diarthrodial Joints, better known (maybe) as Synovial Joints. We have synovial joints all over our body: in our hands, wrists, arms, but also in our shoulders and knees. Wikipedia actually says that a synovial joint is “the most common and most movable type of joint in the body of a mammal”.

As probably said in some previous post, in this kind of articulation two bones get in contact with each other through cartilage surfaces. A Joint Capsule acts as a connective tissue that folds the whole articulation and keeps everything at the right place, ensuring mobility and stability at the same time. This capsule is filled by Synovial Fluid, which has two main functions:

it continuously lubricates the articulation;
it is a source of nutrients for the cells that maintain the joint cartilage.

The synovial fluid contains dissolved gases, mainly oxygen (O2), nitrogen (N2) and carbon dioxide (CO2). These gases are responsible for the popping sound we are trying to explain! After this necessary introduction (yep, I’ve just spoiled the name of the murderer), let’s come back to the action of cracking our knuckles.

In order to pop our knuckles, we can stretch or bend our fingers. In any case, the bones of each knuckle joint (which is a synovial joint) pull apart. As we can easily figure out, by doing this the knuckle joint capsule gets stretched.

This causes the volume of the joint capsule to increase a bit (+ 15-20%). This slight change of volume is followed by a corresponding decrease of the synovial fluid pressure. As a consequence, the gases in the fluid suddenly become less soluble and they form small bubbles inside the tiny joint capsule. The process of “rapid pressure change -> formation of small cavities in the liquid -> formation of bubbles that immediately implode” is defined by fluid dynamics as cavitation.

The implosion of such small bubbles is thought to be the origin of the cracking sound that we hear while popping our knuckles. It normally takes 20-30 minutes for the gas to properly redissolve into the synovial fluid and reestablish the initial conditions. This means that after more or less half an hour of silence we’ll be able to start having fun again 🙂

Many people seem to be frightened by the idea that excessive knuckle popping may lead to unpleasant consequences, such as arthritis or sudden death (…). Luckily, a few studies confirm that apparently there is no correlation between knuckle cracking and osteoarthritis in the finger joints. Another study, however, showed that pathological addiction to knuckle popping may affect the joint capsule soft tissue (higher risk of damages and wear) and worsen hand grip strength conditions. On the positive side, there’s evidence of slightly increased mobility in joints right after popping (mainly thanks to muscle relaxation induced by this action).

In conclusion, we can crack our knuckles whenever we want, but we’d better do it in moderation.

sources: this website, this other one and wikipedia

The bursae of the knee can be defined in a very simple way: they are fluid sacs, or synovial pockets. This second definition comes from the sinovial fluid that fills them.

Synovial fluid is made of hyaluronic acid and lubricin, proteinases and collagenases. Its main functions are reducing friction by lubricating the joint, absorbing shocks and properly “feeding” joint cartilage. In the case of the knee, the Knee Capsule encloses the Knee Cavity which is filled with synovial fluid. Knee Bursae surround and sometimes communicate with the Knee Cavity, as we can see in the picture.

Usually Knee Bursae are thin-walled and represent the weak point of the joint. At the same time, their presence is really important since they enlarge the joint space. They can be grouped according to:

their characterization as communicating and non-communicating bursae. A communicating bursa is when a bursa is located adjacent to a joint, thus having the synovial membrane in communication with the joint itself.
their location (frontal, lateral, medial).

In pathological conditions, such as excessive local friction, infection, arthritides or direct trauma, fluid and debris collect within the bursa or fluid extends into the bursa from the adjacent joint. As a consequence, the walls of the bursa thicken as the bursal inflammation becomes longstanding. The term bursitis refers to pathological enlargement of the bursa. Clinically, bursitis mimics several peripheral joint and muscle abnormalities.