A chiropractic adjustment is a therapeutic procedure that involves the precise application of a high-velocity, low-amplitude thrust to a specific joint. This controlled, quick movement is used to restore normal motion and improve function in the spine or other joints of the body. The audible release that often accompanies this procedure, commonly referred to as the joint manipulation sound (JMS) or simply the “pop,” has been the subject of much curiosity and scientific investigation. Understanding its mechanism offers insight into the effects of the treatment.
The Structures Responsible for the Sound
The sound originates in a specific type of connection found throughout the body, known as a synovial joint, which includes the small facet joints of the spine targeted during a typical adjustment. Each of these joints is encased by a sealed structure called the joint capsule, creating a small, fluid-filled space between the two bone surfaces.
This space is filled with synovial fluid, a thick, viscous liquid that acts as a natural lubricant to reduce friction during movement. The synovial fluid contains various dissolved gases, primarily nitrogen, oxygen, and carbon dioxide, held in solution under normal atmospheric pressure. These dissolved gases also provide cushioning for the cartilage surfaces.
The Process of Joint Cavitation
The popping noise itself is the result of a physical process called cavitation. When a chiropractor applies a quick, controlled thrust to the joint, the action causes a rapid and momentary separation of the two joint surfaces. This sudden gapping of the joint capsule leads to an immediate and significant drop in pressure within the synovial fluid. This reduction in pressure forces the dissolved gases to rapidly come out of solution, forming a gas bubble within the joint space. The characteristic popping sound is created by the swift formation of this gas-filled cavity, a process which some researchers refer to as tribonucleation.
What the Popping Sound Is Not
The audible release is a fluid and gas phenomenon, which means the sound is not caused by bones grinding or cracking against one another. It is also not the sound of a joint “snapping back into place,” nor is it the sound of ligaments or tendons physically snapping or breaking. The presence of the sound is not an absolute measure of the adjustment’s effectiveness. Once the gas bubble has formed, the joint cannot be manipulated again to produce the same sound immediately. This period, known as the refractory period, is required for the gas to redissolve back into the synovial fluid, typically taking 20 to 60 minutes.