What’s up with the noise my knee makes?

Posted on July 14, 2014

(Last Updated On: November 8, 2017)

Crepitus -your joints making sounds- is part of a routine conversation I have with clients.

Client “My shoulder, knee, [enter joint here] makes this sound.”

Me “Is there any pain when it happens?”

Client “No.”

Me “Don’t worry about it.”

The sound is usually some popping or clicking, and joints making these sounds are just part of the joint being alive. I understand some get a little bent out of shape when they hear their body making noises, but it’s really nothing to worry about. When stuff moves around, sometimes you can hear it.

I believe this post will help with understanding why joints make some sounds, which will hopefully allow people to not care as about it as much. This is important because I know people can allow a sound to dictate how they feel. “My knee crunches so I must have some pain.” Or “My knee pops so I must be doing damage to it.” We’ll see why this isn’t true.

In some sense this has been adequately addressed elsewhere. Like with how gas can get caught and released within the joints (cracking your knuckles), tendons and ligaments moving over one another, etc.

This post will deal more specifically with the person who has had a significant injury, possibly surgery, and the consistent crepitus they encounter. The shoulder and knee seem to be the two most prevalent joints with this. I’m going to focus on the knee but keep in mind I see no reason this doesn’t extend to other joints.

A different level of symptoms

People who have had this happen will know exactly what I’m talking about. For this level of crepitus we are not talking every now and then my joint makes a sound. We are talking damn near every time you do a specific movement your joint makes a sound, and that sound is distinct. It is typically a significant level of “crunching,” some popping, and maybe “grating,” The crunching sound is particularly noteworthy.

Think of it this way: You pop or crack your knuckles during a certain movement, then you can’t do it again for a while. That’s your average person.

This type of crepitus would be, in the finger analogy, you bend your finger(s) a certain way and it crunches every single time. There may be some popping here and there, but that crunch is always present.

For the knee, this movement is deep bending, like a deep squat. Something where your foot is touching your thigh.

Toddler knee flexion lines

That said, we’re still talking almost exclusively pain free movement here. If there’s ever pain with this, it is atypical. The sound may be scary as hell, but it rarely accompanies pain. You may think to yourself, “Wow, that should hurt,” but when you step back, you realize it really doesn’t. When people around you hear the crunch sound they may gasp or quiver -the sound can be that noteworthy- but you’re actually alright.

Arthritis is an insufficient explanation

Immediately arthritis is given as the reason for this level of crepitus, but arthritis can’t always be right. I’ve had multiple people with symptoms like this who have confirmed X-Rays with no arthritis. This makes sense as some of these clients have been pretty damn young. I’ve had multiple under 25 years old. I actually was and am one of these people myself. My knee crepitus started at ~25 and it’s still going at 27.

Sure, young people can get arthritis, but again, X-Rays aren’t showing this. Could the X-Rays not be picking it up yet? Possible. But that doesn’t explain the next group.

The group who seems to have drawn the most attention for crepitus is those who’ve had a total knee replacement. Up to 18% of patients have this happen. Why do we care about this group?

In a total knee replacement there is no knee left to have arthritis in!

Yet these people are still getting crepitus. “Bone on bone” or “bones rubbing together” is impossible for this group. Those segments of bone don’t exist anymore.

Knee replacement before and after

Knee replacement procedure.

Could the materials used in the knee replacement be rubbing together? That’s one possibility, yes. But that doesn’t explain how someone like me, or those other clients I mentioned -crepitus, no arthritis, but no knee replacement- are having these symptoms. Something else must be causing the sound.

Some anatomy

I’m going to borrow from Histology for Pathologists: 3rd (third) Edition by Stacey Mills. I’m going to focus on the book’s discussion of the synovial membrane. This will get a bit technical, but bear with it:

“The synovial membrane lines the inner surface of the joint capsule and all other intra-articular structures, with the exception of articular cartilage and the meniscus. In addition to lining the joints, synovial membrane lines (a) the subcutaneous and subtendinous sacs known as bursae, which permit freedom of movement over a limited range for the structures adjacent to the bursae, and (b) the sheaths that form around tendons wherever they pass under ligamentous bands or through osseofibrous tunnels.”

Found here: http://apbrwww5.apsu.edu/thompsonj/Anatomy%20&%20Physiology/2010/2010%20Exam%20Reviews/Exam%202%20Review/Ch%208%20Synovial%20Joints.htm

Synovial membrane cellular view

“Electron microscopic studies reveal two principal types of synovial lining cells, designated by Barland as Types A and B. (Many cells have features of both types and have been called intermediate.) The less common cell (Type A) has many of the features of a macrophage, and there is good evidence that it is structurally adapted for phagocytic functions. The more common Type B cells are richly endowed with rough endoplasmic reticulum, contain Golgi systems, and often show pinocytotic vesicles. Normal synovial intima contains 25% Type A and 75% Type B cells.

Note from me: Phagocytic functions involve cleaning up cell debris.

“The synovial membrane has three principal functions: secretion of synovial fluid hyaluronate (Type B cells); phagocytosis of waste material derived from the various components of the joint (Type A cells); and regulation of the movement of solutes, electrolytes, and proteins from the capillaries into the synovial fluid, thus providing for the metabolic requirement of the joint chondrocytes and possibly also providing a regulatory mechanism for maintenance of the matrix through the role of various mediators.”

Long story short, the synovial membrane helps deliver nutrients to the joint, such as through synovial fluid. The membrane also helps clean things up around the joint.

From WebMd.

From WebMd.

During an injury the amount of nutrition a joint needs is increased. Hence, the joint swells as the body rushes in the cavalry. It makes sense part of this process is the body increasing the amount of synovial cells to help meet this demand.

“Injury and breakdown of cartilage and bone result in increased amounts of breakdown product and particulate debris within the joint cavity. This is removed from the synovial fluid by phagocytic cells (the Type A cells) of the synovial membrane. In consequence, the membrane becomes both hypertrophic and hyperplastic, and the breakdown products of the cartilage and bone matrix frequently evoke an inflammatory response.”

(This is why draining a joint is futile. The joint is inflamed for a reason. Draining it does not get rid of the reason. Get rid of what’s causing the swelling.)

“Extension of the hyperplastic synovium onto the articular surface of the joint (i.e., a pannus) is a common finding even in osteoarthritis, particularly in the hip.”

So, during an injury, the synovium can hypertrophy to meet the demands of an increased workload, and it can grow to the point it takes up space beyond where it normally would. It’s no different than muscle, tendon, bone, or the heart. Make these things work more and they get bigger.

These types of adaptations don’t only relate to acute events though. An acute response can manifest chronically.

During my college football days I dislocated two fingers. I had multiple teammates suffer the same. A common finding amongst us was the size of the dislocated finger, even years later.

Left ring finger

Never dislocated.

Right ring finger

Did dislocate.

Fingers side by side with outlines

I’ve dislocated the right finger but not the left.

The finger above is pretty much forever hypertrophied. I have full range of motion in my fingers (I have more than one like the above); not a single problem with them. But they’re still noticeably bigger. The body decided those particular injuries were so traumatic it would forever give an adaptation: Bigger fingers to better handle such an event should it happen in the future. To where hopefully my finger(s) wouldn’t dislocate next time.

It’s like weightlifting. Damage the body a bit and it comes back a bit stronger. With my fingers, they were significantly damaged, so the body gave a more significant adaptation.

I believe this is what happens to some people’s synovial fluid and membrane. Whether it’s after a significant injury, such as a meniscal or ACL tear, or whether it’s from a surgery to treat an injury -which then provokes an inflammatory response- I think some people’s synovium hypertrophies and hyperplasias. Not only acutely, but it can stay like that longterm. Just like my fingers.

  • Hypertrophy => Existing cells get bigger
  • Hyperplasia => New cells are made.

Overall the synovium is bigger and there’s more of it.

Synovial hyperplasia microscope with circle

Notice the bunching of synoviocytes.

How this causes the crunching sound

Sandpaper comes in different varieties composed of different “grits.” As the grit changes the particles on the paper either get bigger or smaller. The paper becomes more or less rough.


As the paper becomes more rough -as the particles get bigger- the paper makes more sound as you use it (as you rub it past another structure). Just like the synovium.

Synovial hyperplasia arthroscopic

Credit to paper: Patellofemoral crepitus after total knee arthroplasty: etiology and preventive measures (linked below)

A larger synovium is 1) Potentially in a better location to make sound:

“Extension of the hyperplastic synovium onto the articular surface of the joint (i.e., a pannus) is a common finding even in osteoarthritis, particularly in the hip.”

Synovial pannus with circle

And it’s 2) Large enough -rough enough- to make a sound.

It gets a bit thicker, sticks out a bit more, and may be more resistant to movement, causing things to move not quite as smoothly. Hence, sound is made on a much more consistent basis than your typical person whose joints make a sound here and there.

It’s like a scar on your skin. After a significant cut your skin grows back a bit thicker, and probably sticks out more compared to the rest of your skin. Just like a different type of sandpaper, things are a little more pronounced now. More bumps in the road and you make more noise when you’re driving through.

Sidewalk with rough spot with lines

But the skin can still work perfectly fine. You can still drive through without a problem. It just might not be the same exact type of ride as before. Whether it’s a beautiful paved road or a road with some gravel, the only difference is the sound you hear as you get there.


Takeaways so far

  • As long as there’s no swelling or pain associated with your joint(s) making noise, it’s almost assuredly nothing to worry about.
  • Joint(s) making noise doesn’t mean anything is wrong. In the context of this post, a joint making a sound indicates something was wrong.
  • If you really, really want to avoid the noise, your best bet is to avoid extreme ranges of motion. Most notably terminal flexion. (The point in which your joint bends most.) Although, as I’ll get to, you may be missing out on some benefits if you take this approach.
  • Besides that, unless you look at surgical intervention, the noise may forever be there. And getting surgery for this is a crazy decision in my opinion. For many, surgery is what caused the sound to begin with!

Some questions and theorizing

“I never had a traumatic injury to X joint, but I still suffer from crepitus. Does this all apply to me as well?”

Yes. Whether the joint adapted to an acute episode or something chronic, this all still applies. Lightly beating your shoulder up for years can cause some of the same adaptations that heavily beating your shoulder up one time can.

“Isn’t this all the same thing as saying scar tissue causes the noise?”

I mean, when you put it that way, I suppose it is. Hopefully this gives some insight as to what “scar tissue” means though. As that’s such a vague description to me.

“Why does something like the synovial membrane stay hypertrophied long after the injury?”

I mentioned why it makes sense the bone in my fingers would stay hypertrophied. The body doesn’t forget certain things. Burn your hand as a kid and your brain forever remembers to not touch a hot stove. Stress the body enough in a particular manner and the body seems to have mechanisms in place to forever adapt to that stress.

Tying this to the synovium, we could possibly think of this as the synovium stays thicker so it’s already in position to handle a potential future knee injury.

But why else would the body choose to keep that thicker synovium? Perhaps it does so as a form of increased protection. Namely that of added padding. I went into this with my fingers, but let’s use another example. When your body grows extra skin at a scarred site, you now have extra padding between the environment outside your skin and the environment inside your skin.

As I was writing this my girlfriend was asking me about things. I gave her some background, and before telling her this idea of mine, asked her to tell me what this picture looked like to her:

Synovial hyperplasia arthroscopic

Girlfriend “It looks like feathers. Almost like for padding.”

Me “Exactly!”

As I went over earlier, and quoted, this type of synovium response is typical when the joint’s cartilage is damaged. To our best understanding, the cartilage acts as shock absorbers. If the cartilage is damaged, if your shock absorbers aren’t as effective, then you need more shock absorption.

How do you get a softer pillow? Throw more feathers in it. If you’re the human body, then you grow more feathers / support / padding / however you want to think of it. If you have an injury to your cartilage or the inside of your knee, it’s conceivable for the body to respond with increasing the padding inside the knee. The cartilage doesn’t always have the best blood supply so perhaps the body uses another means.

I really think this is why surgery is so ineffective for knee arthritis, why surgery is so often worthless for a torn meniscuswhy ACL surgery is only worth it in very specific circumstanceswhy we have had so much trouble with cartilage implants, why our efforts to grow cartilage have been futile. A hurt knee, whether it’s a sprain, some type of tear, whatever, is such a common injury I can’t imagine the body doesn’t already have an ability to deal with it. An ability exceeding what our brains can conjure up. (At least so far.) You just need to give the body the environment to use its abilities.

“Is there really nothing you can do about the noise besides surgery?”

Going back to our bumps in the road analogy -perhaps it should be “rough feathers in the road” analogy- there does appear to be a possibility of this roughness smoothening out with time.

If you have a rough road, what do you do? You pave it. How do you pave a road? You move over it again and again, pounding it a bit as you go along. Anyone who’s been on this site probably knows where the answer to this is going…

If you are trying to pave things down within the knee joint, smoothen things out, you need to move that joint and pound on it a bit.

Are you ever going to make things normal again? Probably not. But I do think with enough time, with enough good movement, there is potential for the sound to diminish. (Good ole exercise giving us another benefit.)

If the sound doesn’t diminish that’s ok though. It may very well be that sound, while not helpful to the ears, is helpful to the joint.

If anyone would like to read further into this, particularly the intricacies of synovial fluid, I highly recommend Current Concepts in Synovial Tissue of The Knee JointThe paper discusses other poignant aspects, such as how a larger synovium can mimic the symptoms of a torn meniscus, which can also come with sounds. Or how the synovial fluid is crucial for repair processes, but it needs “joint forces” (movement) in order to do its job. And how joint lubrication is linked to joint use. If you want to oil up your joints (which often makes things less noisy!), you need to move them. 

Lastly, I want to give credit to the paper Patellofemoral Crepitus after Total Knee Arthroplasty: Etiology and Preventive Measures. While that paper only dealt with total knee replacements, it was the impetus for me looking into synovial hyperplasia and understanding how this could impact crepitus. They discuss how the surgical technique and materials can cause crepitus in a TKR, but also how the synovium can play a role. 

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