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Biology Body Dive: Joint Capsules, Synovium and What Is That Cracking Anyway?

To understand pathology, when things go awry, we have to have a foundation in the biology and behavior of the tissues we are thinking about. Histology is the study of cells and tissues. This study is extremely important in medical and PT school to understand the physiology- the behavior of tissues and organs.

When you observe a joint structure, the interface between the ends of two bones, several tissues play important roles in the health of that joint.

Let’s begin with the most external structure, the joint capsule.

The Joint Capsule and Synovium:

Made up of dense irregular tissue (very similar to ligaments), the joint capsule creates an envelope around the end of two interfacing bones.

This envelope thickens in places, creating articular ligaments, and can have tendons attaching in to it (as is the case with the long head of the biceps in the shoulder).

Ligaments provide stability against tensile forces (forces that are distracting, pulling apart- lengthening). This is why you sprain your ligament in your ankle when you roll it- you over stretch one aspect of it.

The capsule is highly vascular and innervated by a healthy nerve supply. As such, when capsule fibers are stretched, nerves communicate to send information around proprioception- changes in the position of the joint- to the cortex.

But the nourishment the capsule supplies continues.

The capsule is lined by the synovium- a series of cells that produce and create a very small volume (only about a twentieth of a millimeter thick) of nutrient rich fluid material that bathes the articular cartilage called synovial fluid.

When we take a closer look at cartilage, we come to find it is aneural (without nerve supply) and avascular (without blood supply).

Synovium is able to feed and nourish articular cartilage surfaces. Sometimes, movement is required to assist with this process.

By the way, the cavitation or cracking of joints is thought to proceed at this level (which will be discussed further later on).

In addition to helping cartilage with nourishment, the synovial fluid assists healthy joint function by:

1. Separation and friction reduction between moving tissues (such as the acromium and rotator cuff within the shoulder)

2. A fluid/ mobile, compressible medium for joints to move more securely through

The joint capsule forms a pressurized seal around the joint, so that no fluid escapes. When we have surgery that involves opening the joint capsule, this pressurized ligament seal is lost, and sometimes does not repair completely.


Cartilage is an incredible connective tissue. It is perfectly designed to line our joint surfaces, making them more congruous with each other and dispersing load from weight bearing and muscle contractions.

Cartilage is made up of chondrocyte cells. These are puffy cells containing collagen and proteins.

Our entire skeleton is cartilage at one point in development, and gradually matures to bone through two different processes. Cartilage remains in several different places (famously at our ears, the end of our noses, end of ribs, and the ends of our long bones). Today we will focus on the type of cartilage that exists at the ends of our bones- hyaline cartilage.

Hyaline is derived from the latin Hyalinus- meaning glass. Like glass, this cartilage reduces friction and has a slippery feel to it that assists motion.

The shape of chondrocytes at borders (such as at the ends of bones) can get flattened and stretched out.

Unlike almost any other tissue in the body cartilage is avascular- it does not receive blood supply directly. Because of this, unlike other tissues (which stay nourished from a constant stream of blood bringing in new nutrients and removing old metabolic wastes) our cartilage relies on simple diffusion and more indirect diffusion during movement for the exchange of nutrients.

Cartilage is also aneural, meaning it does not have nerve supply. This means that it is not a generator of nociceptive information (pain). So if you have osteoarthritis and pain, it is coming from other tissues and not degenerative cartilage.

Because of the molecular structure of its membrane and cells, cartilage is hydrophilic- it draws water into it. In doing so, it draws in fresh nutrients in the form of synovium.

This is evidenced in the height we have when we first wake up in the morning, after the vertebral discs have had their share of puffing up with liquid during the non-weight bearing hours of sleep.

Once the cartilage has received new fluid and metabolized the nutrients available, it needs to be compressed (like a sponge wrung out) in order for waste products to move out of the cartilage and be processed by the body.

Every joint in the body that has hyaline cartilage has this amazing process going on at all times.

What is That Cracking in My Knee:

The medical community has proposed the theory of cavitation for a period of time, but it is not 100% confirmed.

Like all metabolic functions, cartilage uses nutrients in the synovial fluid to “breathe” and “breathes out” in the form of gases- carbon dioxide and nitrogen are the two gases most frequently mentioned as byproducts of cartilage metabolism. These gases sit in the synovial fluid and build up. When movement occurs, these gas bubbles can collapse in on themselves, releasing a “pop” and usually a sense of pressure release.

For a host of different reasons, it is not a good idea to crack your own joints (knuckles, neck, low back with twisting). This is because we tend to get movement in the same spot over and over instead of where there might be capsular adhesions. Restoring capsular mobility is not a DIY thing- go see a skillful provider (orthopedic physical therapists, osteopaths and athletic trainers are the only licensed providers of this type of care).

When Your Cartilage Gets Tuckered Out:

Similar to bone, cartilage can fatigue. This can occur when it is not compressed for a period of time, or compressed too often without nutrients moving back into it. These events can occur when people move too much such as with instability (creating excess motion at the joint called hypermobility), or when people move too little (such as when people have very limited capacity to extend the spine due to stenosis).

When this occurs, the cartilage cells die off. The dying off signals an inflammatory response within the synovial fluid. Recall that the synovium is highly innervated with blood and nerve and can be painful. Ask anyone who has experienced frozen shoulder.

The body is so smart- it has adapted to respond to this inflammation by making the joint capsule stiffer (so that the swelling is controlled).

Unfortunately, once you have had this cycle occur (and let’s face it- what adult hasn’t?!) you have stiffness in the joint capsule.

Freeing up a joint capsule is not an easy endeavor. This requires a specialized provider- such as an orthopedic PT or an osteopath to find out if stiffness does exist and where. The resulting stiffness in the capsule are called “articular adhesions” or “capsular adhesions”.

Providers can help you create more space by weeding out these capsular adhesions through specific joint mobilization (a technique we cannot do on our own) and get back on track to a return to your normal function.

If you think of all of the things that might impede this cycle of compression and imbibition that creates a pulsation of nutrients into and wastes out of the cartilage, it is overwhelming!

The most valuable practices for keeping your cartilage healthy is simple but it requires time to reflect and develop self-awareness:

· Keep the muscles across each joint balanced in strength and flexibility. Identify with a provider if you have more of one or the other qualities at your joint.

· Move your joints through their pain free full range of motion over the course of a day

· Practice core strengthening that maintains integrity of the center of our body so we can freely move our appendages

· Don’t spend too much time in any one static posture


Realize that the macroscopic motions and postural habits you assume over the course of your day guide and influence the microscopic environments and interactions of the cartilage tissue that line each joint in your body.

You can preserve your cartilage. But like preserving other tissue (such as gum tissue) it requires work.

Now that we have an understanding of cartilage and the joint capsule, let’s dive into the biomechanical breakdown of articular cartilage next week.

Happy Moving!


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