Belgian researchers (if I remember correctly, I was the external examiner of the PhD of one of them) conducted a survey aimed at examining the beliefs about the cracking sounds often heard during high-velocity low-amplitude (HVLA) thrusts in individuals with and without personal experience of this technique.
The researchers included 100 individuals. Among them, 60 had no history of spinal manipulation, including 40 who were asymptomatic with or without a past history of spinal pain and 20 who had nonspecific spinal pain. The remaining 40 patients had a history of spinal manipulation; among them, 20 were asymptomatic and 20 had spinal pain. Participants attended a one-on-one interview during which they completed a questionnaire about their history of spinal manipulation and their beliefs regarding sounds heard during spinal manipulation.
Mean age was 43.5±15.4 years. The sounds were ascribed to vertebral repositioning by 49% of participants and to friction between two vertebras by 23% of participants; only 9% of participants correctly ascribed the sound to the release of gas. The sound was mistakenly considered to indicate successful spinal manipulation by 40% of participants. No differences in beliefs were found between the groups with and without a history of spinal manipulation.
The authors concluded that certain beliefs have documented adverse effects. This study showed a high prevalence of unfounded beliefs regarding spinal manipulation. These beliefs deserve greater attention from healthcare providers, particularly those who practice spinal manipulation.
So, what causes the sound often heard during spinal manipulation? This is how one chiropractor explains it: “In simple terms, the sound of cavitation means that the vertebrae are being gently and properly realigned. The sound of dissolved air bubbles in the fluid around the vertebrae releasing is what makes the pop.” And this is what another chiro states: “The actual pop is called a cavitation, and it’s the release of gas that makes the popping sound. The joints of the spine are called synovial joints (check out this simple and detailed description here) and they produce a fluid called synovial fluid. Synovial fluid lubricates the joint (for movement) and nourishes it. The byproducts formed in the production of synovial fluid are gasses – oxygen, nitrogen and CO2. When a joint is gapped, or opened up, the gas is released and you hear a distinctive popping sound. It’s very similar to the release of gas bubbles when you cork a champagne bottle, and equally pleasant in its after effects.” Finally NHS Choices tells us this: “During spinal manipulation, you may experience a popping sensation in your joints and hear a popping or cracking sound. It is thought this is caused by gas bubbles in the fluids that surround your joints – this is a normal part of spinal manipulation and other manual treatments.”
In reality the pop is much to do about nothing. If you pull hard on one of your fingers, chances are that you generate the same phenomenon and sound. As with your finger, the pop from the vertebral joints has no therapeutic value.