The common perception of a chiropractic adjustment, or Spinal Manipulative Therapy (SMT), often involves the expectation of an audible “pop” or “crack.” Many patients believe this noise proves the joint successfully moved back into place. This expectation is frequently misplaced and can cause unnecessary concern if the sound does not occur. The effectiveness of the therapy is not measured by its volume, and a soundless adjustment can be just as beneficial for the patient.
Understanding the Joint Sound
The sound produced during a high-velocity, low-amplitude (HVLA) adjustment is known as joint cavitation. This sound does not originate from bones grinding or snapping, but from a sudden change in pressure within the joint capsule. Spinal joints are surrounded by a capsule containing synovial fluid, which acts as a lubricant.
This fluid contains dissolved gases, such as nitrogen and oxygen. When the chiropractor applies a rapid, controlled thrust, the joint surfaces separate quickly, causing a sudden drop in pressure inside the joint. This pressure change forces the dissolved gases out of the solution, forming a temporary bubble or cavity. The audible “pop” is the sound of this bubble forming or collapsing.
Sound Does Not Equal Successful Treatment
The absence of an audible joint sound does not mean the adjustment was ineffective. The fundamental objective of SMT is to restore normal motion to a restricted joint and provide beneficial neurological input. These therapeutic goals can be fully achieved without any accompanying noise.
Scientific studies show that adjustments without cavitation produce the same positive neurological and mechanical changes as those with a sound. The restoration of movement, which is the primary mechanical goal, often occurs silently. The therapeutic outcome is tied to the joint’s movement and the corresponding neurological response, not the secondary auditory effect.
Factors Inhibiting Joint Cavitation
Several specific physical factors can prevent a joint from producing the audible cavitation sound, even when the adjustment is performed correctly.
Muscle Guarding and Tension
Excessive muscle guarding or tension is a common cause. The surrounding muscles reflexively tighten, making it difficult to achieve the rapid joint separation required for the gas bubble to form. If the patient is not fully relaxed, the practitioner’s force may not overcome this muscular resistance quickly enough.
Anatomical Limitations
Anatomical differences or joint hypermobility can also inhibit the sound. In a hypermobile joint, the capsule may already be stretched, meaning the rapid separation necessary to create the negative pressure drop does not occur. Conversely, if a joint is severely restricted, the force required to create separation may exceed what can be safely delivered.
Refractory Period
Another reason for a silent adjustment is the refractory period. Once a joint has cavitated, the dissolved gases require time, typically 15 to 30 minutes, to redissolve back into the synovial fluid. If the chiropractor attempts to adjust the same segment shortly after a previous adjustment, the lack of sound is a natural biological limitation.
Non-Audible Spinal Manipulation Methods
A chiropractor may intentionally choose non-audible techniques to achieve the same therapeutic benefits without the characteristic “pop.” These alternative methods are often preferred for patients who are apprehensive about the sound or for specific clinical conditions.
Activator Method
The Activator Method uses a small, hand-held, spring-loaded instrument to deliver a low-force, high-speed impulse to the joint. Because the speed of the impulse is so fast, the surrounding muscles do not have time to tense up in response, and the adjustment successfully restores motion without generating cavitation noise.
Drop-Table Technique
The use of a drop-table utilizes a specialized chiropractic table with segments that drop slightly upon a gentle thrust. This mechanical assistance allows the practitioner to use less force while still achieving joint mobilization, resulting in a silent adjustment.
Flexion-Distraction
The Flexion-Distraction technique is another common non-audible method that uses slow, gentle, pumping movements on a segmented table. This technique focuses on decompressing the spine and is particularly beneficial for disc-related issues.