Chiropractic Adjustment: Understanding Joint Motion, Cavitation, and Neurological Effects

What happens to your bones during chiropractic adjustment?

Chiropractic adjustments primarily involve restoring normal motion to specific joints, particularly in the spine, through a controlled, high-velocity, low-amplitude force, which can lead to a release of gas from the joint (cavitation) and subsequent neurological and muscular responses, rather than physically "moving bones back into place."

Understanding the Spinal Column and Joints

To comprehend the effects of a chiropractic adjustment, it's essential to first understand the anatomy of the spinal column. Your spine is a complex structure composed of 33 individual bones called vertebrae, stacked one upon another. Between most vertebrae are intervertebral discs, which act as shock absorbers and allow for flexibility.

The movement between adjacent vertebrae occurs at the facet joints, which are synovial joints. Like other synovial joints (e.g., knees, shoulders), facet joints are enclosed by a capsule and contain synovial fluid, which lubricates the joint and nourishes the cartilage. The entire spinal column is stabilized by a complex network of ligaments and muscles.

In exercise science and kinesiology, we understand that optimal movement relies on the proper function of these individual segments. When a joint loses its normal range of motion or exhibits restricted movement (often termed a "subluxation complex" in chiropractic, though the precise mechanism is debated in broader medical literature), it can impact local biomechanics and even broader nervous system function.

The Nature of a Chiropractic Adjustment

A chiropractic adjustment, or spinal manipulative therapy (SMT), is a highly specific, controlled procedure performed by a trained chiropractor. The most common technique is the high-velocity, low-amplitude (HVLA) thrust. This involves applying a quick, shallow thrust to a specific joint, typically within its physiological range of motion, but beyond its passive range. The goal is to restore normal joint function and motion, not to force bones into a new position.

What Happens to the Bones (and Joints) During an Adjustment?

While the term "bones" is used in the query, it's more accurate to focus on the joint complex surrounding the bones, as the primary effects occur within these articulations and the surrounding soft tissues.

Joint Cavitation and "The Pop"

One of the most noticeable phenomena during an adjustment is often a audible "pop" or "crack." This sound is known as cavitation. It occurs when the HVLA thrust rapidly separates the articular surfaces of a synovial joint, creating a sudden drop in pressure within the joint capsule. This pressure change causes gases (primarily carbon dioxide, nitrogen, and oxygen) dissolved in the synovial fluid to rapidly form bubbles, which then quickly collapse or "cavitate," producing the characteristic sound.

• Not Bones Grinding: It's crucial to understand that this sound is not bones grinding together or a bone "going back into place." It's a physiological event within the joint fluid.
• Not Indicative of Effectiveness: While common, cavitation is not universally required for an adjustment to be effective, nor does its absence mean the adjustment was unsuccessful. The therapeutic benefit is tied to the restoration of joint motion, not merely the sound.

Restoration of Joint Motion and "Joint Play"

The primary aim of an adjustment is to restore the normal arthrokinematic motion (the subtle, involuntary movements like glides and rolls that occur within a joint) and osteokinematic motion (the visible, voluntary movements like flexion and extension) of the affected segment.

• Addressing Hypomobility: When a joint becomes "stuck" or restricted (hypomobile), the small, accessory movements necessary for full range of motion are inhibited. The HVLA thrust aims to release these restrictions, improving the joint's ability to move freely and fully.
• Improved "Joint Play": This refers to the small amount of involuntary movement that exists within a healthy joint, which is crucial for pain-free motion. An adjustment seeks to restore this intrinsic "play."

Neurological Reflexes and Muscular Response

The impact of an adjustment extends beyond just the mechanics of the joint. The joint capsules are richly supplied with mechanoreceptors (sensory nerve endings that detect mechanical stimuli like pressure and stretch).

• Proprioceptive Input: The sudden stretch and movement during an adjustment stimulate these mechanoreceptors, sending a barrage of afferent (sensory) signals to the central nervous system. This increased proprioceptive input can help reset the nervous system's perception of the joint's position and movement.
• Muscle Relaxation: This neurological input can trigger reflexogenic effects, leading to a decrease in muscle hypertonicity (tightness or spasm) around the adjusted joint. This is thought to be a key mechanism by which adjustments reduce pain and improve mobility.
• Pain Modulation: The neurological input may also activate descending pain inhibitory pathways, helping to reduce the sensation of pain.

Ligament and Soft Tissue Response

While bones themselves don't change shape during an adjustment, the surrounding soft tissues, including ligaments and joint capsules, are stretched and stimulated. This can help to break up adhesions (scar tissue) that might form within the joint capsule or surrounding soft tissues, which can contribute to restricted movement and pain. The improved joint motion also promotes better fluid exchange within the joint, contributing to joint health.

Addressing Common Misconceptions

One prevalent misconception is that chiropractic adjustments "put bones back into place" or realign a "slipped disc." From an anatomical and biomechanical perspective, this is inaccurate.

• Bones Don't "Go Out of Place": Vertebrae are strongly connected by ligaments and muscles; they do not easily slip out of alignment in a way that can be manually "put back." Severe dislocations are medical emergencies, not the target of chiropractic care.
• Discs Don't "Slip": Intervertebral discs can bulge, herniate, or degenerate, but they do not "slip" in and out of place. Adjustments may indirectly help disc-related issues by improving spinal mechanics and reducing pressure on nerves, but they do not physically reposition a disc.

The focus is on restoring normal joint function, reducing nerve irritation, and improving the overall communication between the nervous system and the musculoskeletal system.

Conclusion

During a chiropractic adjustment, the primary action is a controlled, specific force applied to a joint to restore its normal range of motion and function. This often results in joint cavitation, which is the sound of gas bubbles forming and collapsing within the joint fluid. More importantly, the adjustment stimulates mechanoreceptors within the joint, leading to beneficial neurological reflexes that can relax muscles, modulate pain, and improve proprioception. While the term "bones" is often used, the true impact is on the intricate mechanics and neurological control of the entire joint complex, ultimately aiming to enhance overall musculoskeletal health and nervous system function.