Cavitation involves the formation of microscopic gas bubbles within a liquid, followed by their rapid collapse due to sudden and significant changes in pressure. This process can have diverse effects on the human body, ranging from harmless everyday occurrences to therapeutic applications and, in some cases, unintended tissue damage.
The Science of Cavitation in Biological Systems
Cavitation occurs when pressure within a liquid drops below a critical threshold, allowing dissolved gases to form tiny bubbles. This process is similar to boiling, but induced by pressure reduction rather than heat. Within the body, fluids like blood or synovial fluid contain dissolved gases that can form these microbubbles under specific conditions. When the surrounding pressure quickly increases, these bubbles rapidly collapse, releasing energy as shock waves or microjets. These events exert localized mechanical forces on surrounding tissues.
Cavitation in Joint Movement
A common example of cavitation in the body is the sound produced when joints “crack,” such as knuckles, the back, or the neck. This popping occurs when a joint is stretched, creating negative pressure within the synovial fluid that lubricates the joint. This pressure drop causes dissolved gases, primarily nitrogen and carbon dioxide, to form a bubble within the joint space.
The audible pop is associated with the sudden formation of this cavity, rather than its collapse. After cracking, a “refractory period” follows, during which the gases re-dissolve into the fluid, preventing immediate re-cracking of the same joint. This form of cavitation is harmless and does not contribute to conditions like arthritis.
Therapeutic Uses of Cavitation
Medical professionals harness controlled cavitation for various therapeutic applications. High-Intensity Focused Ultrasound (HIFU) uses concentrated ultrasound waves to generate cavitation bubbles at a specific target. The mechanical effects of these bubbles, alongside thermal effects, can destroy target cells without invasive surgery, often used in treating tumors or uterine fibroids.
Lithotripsy is another use, a procedure which breaks down kidney stones or gallstones. Shock waves delivered from outside the body induce cavitation bubbles near the stones. The rapid collapse of these bubbles generates localized forces that fragment the stones into smaller pieces, allowing them to pass more easily from the body.
Sonoporation is a technique that uses cavitation to temporarily increase the permeability of cell membranes. This allows for enhanced delivery of medications or genetic material into cells for targeted drug delivery and gene therapy.
Potential Adverse Effects of Cavitation
While controlled cavitation offers therapeutic benefits, uncontrolled or excessive cavitation can harm the body. The mechanical stress from collapsing bubbles can damage surrounding tissues. This damage can manifest as cell lysis, where cell membranes rupture, or lead to localized heating and micro-hemorrhages, which are small bleeds.
Uncontrolled cavitation can occur under extreme pressure changes, like those experienced during traumatic injuries, or from uncontrolled exposure to high-intensity ultrasound. For example, cosmetic procedures involving ultrasonic cavitation for fat reduction can cause temporary side effects such as redness, swelling, mild bruising, or skin sensitivity. These scenarios emphasize the importance of precise control in medical applications, contrasting with the benign nature of joint cavitation.