Being of a curious mind, I am occasionally fascinated by simple things such as the phenomenon of cracking knuckles. In this respect, I have done some very basic research regarding this phenomenon and have been repeatedly disappointed by the explanations presented on nearly every web site I have seen that attempts to describe the process in question. There are many physiological reasons that our joints make noise, but the description of at least one of these processes is always, well…not what it is cracked up to be.
Specifically, virtually every web article mentions “cavitation” as the primary cause of the sound that accompanies knuckle cracking in our finger joints, but in almost every case they go on to describe cavitation incorrectly, or propose a scenario that has little, if nothing, to do with cavitation. Here, for example, is an excerpt from the article on knuckle cracking at HowStuffWorks.com (italics added):
“If you’ve ever laced your fingers together, turned your palms away from you and bent your fingers back, you know what knuckle popping sounds like. Joints produce that CRACK when bubbles burst in the fluid surrounding the joint.
Joints are the meeting points of two separate bones, held together and in place by connective tissues and ligaments. All of the joints in our bodies are surrounded by synovial fluid, a thick, clear liquid. When you stretch or bend your finger to pop the knuckle, you are causing the bones of the joint to pull apart. As they do, the connective tissue capsule that surrounds the joint is stretched. By stretching this capsule, you increase its volume. And as we know from chemistry class, with an increase in volume comes a decrease in pressure.
So as the pressure of the synovial fluid drops, gases dissolved in the fluid become less soluble, forming bubbles through a process called cavitation. When the joint is stretched far enough, the pressure in the capsule drops so low that these bubbles burst, producing the pop that we associate with knuckle cracking.”
The above excerpt is typical of the hundreds of articles about knuckle cracking, and the fact that no one questions this description of the cavitation process is tragic. In a closed system such as the synovial capsule of a joint, this description makes absolutely no sense. Why? Because these gas bubbles simply cannot burst! Gas bubbles created in a liquid under negative pressure do not pop like balloons, or soap bubbles in the air, no matter how low the fluids pressure becomes. To say they “burst” is to say that they somehow cease to be bubbles and become re-dissolved in the surrounding liquid. This defies the laws of physics, at least as I understand them. Balloons and soap bubbles burst because the material of the bubble/balloon eventually ruptures and fails. Gas bubbles in a liquid have no such container. They simply cannot “fail “ to be bubbles.
In such a system, the only things that individual gas bubbles can do is:
1.) Get bigger (take up more volume) when pressure is further reduced. (Note: This produces no sound whatsoever.)
2.) Combine with each other. (Ahh, now it can be argued that small bubbles “burst” in a fashion as they combine with larger bubbles, but this does not accurately describe the knuckle-cracking phenomenon. Think of opening a can of soda. As the small CO2 bubbles reach the surface, they “pop” as the surface tension of the liquid soda weakens and fails. But in this example we do not hear a single “pop.” Instead, we hear hundreds of “pops,” producing the typical fizzing sound.) By the way, this soda pop analogy is also often used incorrectly to describe cavitation in many knuckle-cracking articles. It is wrong for the same reason.
3.) Get smaller / collapse when the pressure is increased. (Aha! Now we are getting somewhere. The only thing that produces loud sound in such a system is the collapse of a gas bubble as a direct result of the sudden introduction of pressure. This is the very definition of cavitation!)
Cavitation is the sound that occurs when gas bubbles are partially or totally collapsed as result of a sudden and dramatic increase in surrounding pressure. The sides of the bubble clap together violently as it collapses, producing a surprisingly loud sound. Submariners hate cavitation because it occurs when the submarine’s propellers, or more properly “screws,” turn too quickly for the surrounding water conditions. The friction of the blade against the water produces a partial vacuum along its surface. Gas bubbles form as a result of this partial vacuum, and move to the edge of the blade. As these bubbles are pushed off of the blade, they are immediately no longer under the partial vacuum created by the blade friction and they collapse instantly, producing the telltale sound. This sound makes the submarine easily detectable at great distances, and hence, is extremely undesirable.
In a sealed system, like the synovial capsule of a finger joint, cavitation of a gas bubble formed under negative pressure can only occur if pressure is suddenly and quickly re-introduced. This is not discussed, described, or mentioned in any knuckle-cracking article I have ever read. This is not to say that cavitation is not at the heart of the sound that we hear, but rather that the common description of the knuckle cracking process is likely faulty.
Here, instead, I propose my own theory and description as to what happens when you crack your knuckles:
When you flex your knuckles beyond their normal range and/or direction of motion, you actually temporarily separate the synovial capsule of the joint into two or more distinct areas of high and low pressure. As you continue to pull/push/twist/flex the joint, dissolved gasses come out of solution in the synovial fluid and rapidly form bubbles in the area(s) of low pressure.
As you continue to pull/push/twist/flex, the synovial capsule stretches further, and at some point the physical seal produced between these high and low pressure areas can no longer be maintained. At this critical failure point, the synovial fluid under high-pressure rushes to the area of low pressure, rapidly collapsing the bubble(s) of gas, which causes the cavitation and the audible “crack.”
Since the synovial capsule has now been temporarily stretched in such a way that prevents adequate sealing of the joint capsule into the necessary areas of high and low pressure, you cannot immediately re-crack that joint (at least not in the exact same manner.) Only when the synovial capsule shrinks back to its normal size/shape, and can again support a temporary seal between potential areas of high and low pressure, do you regain the ability to “crack” the knuckle.
Admittedly, I do not have the requisite detailed knowledge of joint anatomy and physiology that is required to precisely explain how each joint capsule becomes temporarily compartmentalized in my theory of this process. As a result, I would love to hear from any rheumatologist, or orthopedic expert who is sufficiently knowledgeable on this subject and can either verify my theory, or offer a more accurate and technically valid description that doesn’t gloss over the actual process.