Popping in the TMJ
Is it a click or is it a pop? And how do you know the difference?
TMJ (temporomandibular joint) disorders can involve a variety of symptoms, including clicking and popping sounds. Here’s how they differ:
Click – A click is typically a distinct, single sound that occurs when the jaw is moved, open or closed. It’s most often repeatable and occurs at the same level of opening.
Pop – A pop varies and may occur with different movements of the jaw and at various times.
Clicking is typically associated with disc displacement, whereas popping is related to gas bubbles popping.
Gas bubbles can form in the synovial fluid of the temporomandibular joint (TMJ) due to a phenomenon known as cavitation. This process can occur in any synovial joint in the body and involves the formation and collapse of gas-filled cavities or bubbles. The primary causes of gas bubble formation in the synovial fluid of the TMJ include:
- Pressure Changes: When the pressure within the joint changes rapidly, it can lead to the formation of gas bubbles. This can happen when the joint is suddenly stretched or moved in certain ways, creating a negative pressure that allows dissolved gases (mainly nitrogen, oxygen, and carbon dioxide) in the synovial fluid to come out of solution and form bubbles.
- Joint Movement: Specific movements or adjustments of the jaw can lead to rapid changes in joint space and pressure, causing gas bubbles to form and collapse. This is similar to the mechanism that causes the popping sound when cracking knuckles.
- Degenerative Changes: Degenerative joint diseases, such as osteoarthritis, can alter the structure and function of the TMJ. These changes can affect the pressure dynamics within the joint, making it more likely for gas bubbles to form.
- Trauma or Injury: Trauma or injury to the TMJ can disrupt the normal mechanics of the joint, potentially leading to the formation of gas bubbles in the synovial fluid.
- Inflammation: Inflammation within the joint can change the composition and behavior of the synovial fluid, potentially contributing to the formation of gas bubbles.
The mechanism of gas bubble formation in the synovial fluid, known as cavitation, involves several physical and physiological processes. Synovial fluid is a viscous liquid found in joint cavities, including the TMJ. It is composed mainly of hyaluronic acid and lubricin***, along with dissolved gases such as nitrogen, oxygen, and carbon dioxide.
Synovial fluid is a semi dilute hyaluronate (HA) polymer solution.
Lubricin – surface-active mucinous glycoprotein (HA-binding protein) is secreted in the synovial joint and plays an important role in cartilage integrity. Joint friction is elevated and accompanied by accelerated cartilage damage when there is deficiency in lubricin.
Under normal conditions, nitrogen, oxygen, and carbon dioxide are dissolved in the synovial fluid, similar to how carbon dioxide is dissolved in a carbonated beverage. When the jaw moves, particularly during activities like opening, closing, or grinding the teeth, it causes changes in the joint space and pressure within the TMJ. During certain movements, the joint space may increase, creating a negative pressure environment. This reduction in pressure lowers the solubility of gases in the synovial fluid. As the pressure decreases, the dissolved gases come out of solution and form gas bubbles. This is akin to opening a soda bottle, where the reduction in pressure allows carbon dioxide to form bubbles. Small imperfections or changes in the synovial fluid or joint surfaces can act as nucleation sites where gas bubbles begin to form. This is sometimes referred to as the ‘Cavitation Process’. Once nucleated, these gas bubbles can grow as more gas comes out of solution due to the sustained negative pressure. When the jaw movement changes again and the pressure in the joint returns to normal or positive, the gas bubbles collapse. This rapid collapse can create a sharp sound, commonly heard as a “pop”. The collapsing bubbles can also create a slight mechanical impact on the joint structures, contributing to the audible sound and sometimes the sensation of popping. Repeated cavitation events are common and generally harmless, but if they occur frequently, they may indicate underlying joint issues such as misalignment or degeneration.
Impact of Joint Pathologies:
Conditions like osteoarthritis, inflammation, or disc displacement can alter the normal pressure dynamics and structural integrity of the TMJ, making gas bubble formation more likely and more frequent.
Clinical Implications: While the formation and collapse of gas bubbles are generally benign, persistent symptoms or associated pain may warrant further investigation to rule out underlying TMJ disorders or other joint pathologies.