Joint manipulation, a common therapeutic technique involving passive joint movement, often produces an audible “pop” or “crack”
This sound has long been associated with therapeutic efficacy, leading to the belief that its presence signifies a successful treatment. However, recent research challenges this assumption, suggesting that the audible pop may be a misleading indicator of joint manipulation’s effectiveness.
Joint manipulation, a hands-on therapeutic technique, is used by various healthcare practitioners. Joint manipulation can be defined as a maneuver in which the joint is passively taken through various ranges. Joint manipulation varies in amplitude and speed, and it includes high-velocity thrust manipulation over a small range (HVLA). Joint manipulation is commonly used to improve joint mobility, relieve pain, and improve patient functionality.
Joint manipulation: A historical perspective
Joint manipulation has ancient roots that span thousands of years across multiple cultures. The earliest records of joint manipulation can be found in ancient civilizations, including Egypt, China, and Greece. Historical evidence suggests that joint manipulation was practiced as early as 3000 BCE, with ancient Egyptian hieroglyphs showing early forms of manual therapy. Hippocrates (460–370 BCE) is considered one of the first to document spinal manipulation techniques. Known as the “Father of Medicine,” Hippocrates observed that manual manipulation could relieve deformities, joint dysfunction, and nerve pain.
As the healthcare industry moved toward evidence-based practice in the late 19th century, HVLA techniques underwent rigorous scientific evaluation. While the therapeutic mechanisms of HVLA manipulation are still largely unknown, several theories of the mechanical, neurophysiological, and psychological effects have been proposed. Manual manipulation has been validated in the literature, and its efficacy has been reported to reduce the perception of pain, improve mobility, and reduce self-perceived disability in conditions such as low back pain, neck pain, and headaches. Joint manipulation is currently widely used by physical therapists, chiropractors, and osteopaths.
During HVLA manipulation, the periarticular structures are rapidly stretched without any lasting change in the articular separation. This separation often results in a popping sensation, sometimes just felt by the patient and clinician, and sometimes in an audible manipulation sound perceived by both. Audible sounds, often described as “crack,” “pop,” or “clicks,” historically, in both academia and the clinical arena, are often thought of as a necessary criterion when defining a successful HVLA manipulation.
The mechanics of joint manipulation
The exact mechanism causing the audible pop remains elusive. Because the audible pop only occurs during the HVLA manipulation of synovial joints in the human body, it infers a causative relationship between the joint capsule and synovia. Currently, the most accepted theories explaining the audible pop are cavitation collapse and tribonucleation theories.
In both therapies, there will be an overall stretched state of the joint capsule immediately following the HVTA, and this results in the depolarization of mechanoreceptors, resulting in relaxation and a subsequent increase in joint motion. The cavitation collapse theory hypothesizes that the rapid reduction in intraarticular joint pressure is due to joint separation caused by the manipulation force. The reduction in intra-articular pressure results in gas release (especially nitrogen) from the synovial fluid, resulting in an audible manipulation sound. Some similarities to the cavitation theory can be found in the tribonucleation theory.
Tribonucleation occurs because of a rapid joint capsule elongation causes a decrease in internal joint pressure. With a sufficient manipulation force, the synovial adhesion forces are overcome, allowing a drop in pressure inside the joint. During this process, gas dissolved in the synovial fluid will “come out of the solution,” creating a bubble (cavity) within the joint. The production of such a cavity would explain the audible joint sound. The critical difference between cavitation and tribonucleation is that the cavitation theory explains the creation of audible sounds through the collapse of gas bubbles within the joint fluid.
In contrast, the tribonucleation theory suggests that audible joint manipulation sounds arise from gas bubbles created by the rapid separation of joint surfaces, essentially a “pulling apart” mechanism that creates a low-pressure environment allowing dissolved gases to form bubbles.
Based on research evidence and observations seen with musculoskeletal ultrasound imaging of joints, the formed bubbles appear inside the joint without collapse for a prolonged period beyond the cessation of the audible manipulation sound, thus favoring the tribonucleation theory.
There is no clear evidence of whether the audible pop contributes to any positive clinical effects reported following an HVLA manipulation. Bialosky et al. demonstrated that any hypoalgesia effect following the joint manipulation producing audible pops was not correlated. This finding was supported by the lack of correlation between the audible pop and a decrease in post-manipulation pain levels reported by Flynn et al. This finding concurs with Sillevis and Cleland, who demonstrated that the audible pop during an HVLA thoracic manipulation did not cause any significant change in autonomic nervous system activity or a reduction in pain. Cleland et al. could not demonstrate any correlation between the audible pop and mobility changes or self-reported disability rating following an audible sound-producing manipulation. In summary, there is no research evidence that the audible pop as a manipulation byproduct impacts treatment outcomes, changes inflammatory markers, improves range of motion, or changes central nervous system activity or pain.
Despite this lack of evidence, subjects undergoing spinal manipulation and clinicians continue to correlate the success of the HVLA manipulation with the production of an audible pop. Research evidence demonstrates that anecdotally, patients believe that an HVLA manipulation producing an audible pop is more effective than a manipulation that does not produce an audible sound. Evidence supports the fact that patient expectations (positive and negative) and treatment outcomes are correlated.
Based on this relationship, exploring the possible effect of the audible manipulation sound and the central nervous system was necessary. For that reason, Dr. Sillevis and the team explored any impact of the audible manipulation sound in three spinal studies (lumbar, thoracic, and cervical thrust manipulation). A way to measure central nervous system activity is with the use of an electroencephalograph (EEG). Despite its inherent limitations, the EEG is considered one of the few real-time measures directly reflecting brain neural activity. The EEG measures electrical potential differences generated in the brain’s cortical layers through the scalp.
These potential differences are generated by inhibitory and excitatory synaptic potentials, which create electrical currents. Brainwave activity is reflected by the frequency typically classified into five waves: Delta, Theta, Alpha, Beta, and Gamma waves. Each type of wave represents different correlated brain activity. Activities such as stages 3 and 4 of the sleep cycle produce Delta waves ranging from 0.1 to 4 Hz and are typically abnormal in waking adults. That subconscious activity, often observed in relaxation, associated with the production of cortical hormones like serotonin and pain modulation, produces Theta waves and range from 4 to 8 Hz. Adults who are relaxed with their eyes closed produce Alpha waves with a frequency ranging from 8 to 13 Hz; Alpha waves are primarily present in the occipital and parietal regions of the brain Activities such as talking, decision making, or the use of judgment create Beta waves range from 13 to 30 Hz.
Additionally, Beta waves are related to our sensory intake, especially what we see, touch, hear, smell, and taste. Human perception, consciousness, and alertness produce Gamma waves, which range in frequency from 31 to 150 HZ. Beta waves are usually found bilaterally in the frontal and parietal lobes and have been associated with cortisol.
An EEG recording should reflect any change in human perception expressed by a change in neural activity in the brain, resulting in a frequency alteration or intensity alteration of the EEG signal. One could hypothesize that if the HVLA manipulations produce an audible pop, this should at least result in sensory integration, evident by increased brainwave activity in the brain’s auditory centers.
Identifying the changes in the EEG signaling and comparing this to the presence of audible manipulation sounds helps better understand the immediate effect of the audible pop.
For figures and further information on the latest research evidence based on spinal manipulation and audible sounds, read the full ebook.
