INTRODUCTION

Spinal manipulation (high-velocity, low-amplitude) and spinal mobilization (low-velocity, low amplitude) are both popular manual therapy approaches used by chiropractors, osteopaths and physiotherapists. The most common assumed difference between these 2 techniques is the cavitation or “crack” that can occur during spinal manipulation. Through the influence of social media, many have begun to glamorize and prioritize the cavitation aspect of spinal manipulation, treating it as a key hallmark of what makes an effective adjustment or spinal correction. This has recently been highlighted on social media with an influx of trending “crack compilation” adjusting videos, which are heavily focussed exclusively on the sound of a cavitation.¹ Although hearing that “crack” and the process of tribonucleation and the disruption of synovial fluid almost always “feels” good,² research shows that the majority of changes elicited by spinal manipulation (HVLA) are mostly, if not all, mediated through neurological and physiological mechanisms³ and not by “clicking or pushing bones back in place” - a common and outdated misconception.^4–6 Some in the medical community have now begun to challenge the notion that a “good” manipulation requires cavitation - and rather suggest clinicians focus on proper pre-assessments, appropriate interventions, fantastic technique (finesse over force), and post-assessment changes.^7,8 The purpose of this commentary is to explore some of the key differences between spinal manipulation and spinal mobilization, while highlighting relevant factors such as joint cavitation, clinical efficacy, post assessment outcomes and contraindications.

METHODS

Various sources of literature were used to provide this commentary, including Google Scholar, ResearchGate, PubMed and ScienceDirect. Keywords used to locate the most appropriate and relevant research were spinal manipulation, chiropractic adjustments, high-velocity low-amplitude thrust, low-velocity low amplitude thrust, osteopathic articulation, spinal mobilization, cavitation and tribonucleation. 25 papers were used for this commentary. This included systematic reviews, meta-analyses, randomised control trials, experimental studies, case reports, case series, animal studies and medical textbooks.

DISCUSSION

What Really Makes that Sound? (Cavitation)

Synovial fluid, which contains gases such as nitrogen, oxygen, and carbon dioxide, lubricates the human body’s primary moveable joints. While the precise mechanism underlying the crack and pop sound is unknown, it is thought to be associated with the collapse of gas bubbles in the joint caused by rapid movement and separation of the joint articular surfaces.²

The Importance of Cavitation?

Spinal manipulation is often easily distinguished from other techniques by the audible “pop” or “crack” that can be heard as the fluid within the synovial membrane is redistributed and the gas reabsorbed; this occurs because the joint is swiftly taken beyond its active range of motion and into the paraphysiological space.⁹ For many years, it was taught that the cracking sound was, in fact, from the release of gas from a build-up over time; however, a new hypothesis regarding this joint cracking phenomenon was presented when new research (via rapid MRI frames taken at 3.2 fps) uncovered new information on the “crack” mystery. Kawchuk et al. proposed that rather than the “crack” being associated with the collapse of an already formed bubble within the synovial joint, it was more likely formed by cavity inception and consistent with an already understood process called tribonucleation.² This process is best described as a mechanism that instantly creates gas bubbles when 2 surfaces rapidly separate when in liquid. The key takeaway here is that an adjustment or spinal manipulation is not moving bones back into alignment,^3–6 and they are not releasing built-up gas in a joint – it is much more complicated than that and involves the spontaneous redistribution of pressure, creation of gas and absorption in an enclosed fluid-filled system.

Cavitation Vs. Post-Assessment

The extent to which you rate the success of your spinal manipulation should not be by the level of audible crack but by how effective the intervention was at eliciting the change you set out to make and the patient required. Many prioritize the audible cavitation as the leading hallmark of an effective manipulation; however, the “pop”, “crack”, or even “thud” is mostly clinically irrelevant.^9,10 In fact, Bakker and Miller concluded the perceived benefits from an audible release during a chiropractic adjustment are most likely all psychological and not physiological.⁸ A more appropriate model would be a comprehensive pre-assessment leading to the right intervention followed by a post-assessment check to identify the efficacy of your chosen application.

Spinal Manipulation (HVLA) Vs. Spinal Mobilization (LVLA)

HVLA stands for high-velocity low-amplitude thrust, a fast therapeutic gapping of a joint and separation of 2 articular surfaces with speed. It is more commonly known as spinal manipulation and/or adjustment. LVLA stands for “low-Velocity low-amplitude” thrust, a slow therapeutic rhythmical gapping movement of a joint. It is also referred to as spinal mobilization and/or spinal articulation.

A 2019 Meta-analysis looking at the benefits and harms associated with spinal manipulation found no significant difference between manipulation and mobilization for pain relief and improvement in function.⁷ In 2009, a randomised control trial found no difference between both interventions, leading the authors to conclude: “From an evidence-based care perspective, patient preference and clinical experience should drive how clinicians and patients make the SM procedure decision for this patient population.”¹¹ Additionally, Xia et al. found that both thrust and non-thrust spinal manipulative procedures demonstrated similar effects in short-term improvement, which were more significant than those of the control group.¹² Furthermore, Thomas et al. reported there was no significant difference in pain scores between spinal manipulation and spinal mobilization, which was further supported by a 2015 Cochrane review that concluded that manipulation and mobilization present similar results for every outcome at immediate/short/intermediate-term follow-up.^13,14 In fact, they discovered no significant difference between the effects of both interventions on neck pain.¹⁵ Coulter et al. concluded both methods had comparable results when treating chronic neck pain, with further research also supporting these findings, reporting no therapy was more effective than the other.^16,17

Mobilization Benefits

Jayabalan and Ihm found that mobilizing an osteoarthritic knee reduced pain levels immediately and improved function.¹⁸ Furthermore, animal studies show pain thresholds were found to be lower in the area of chronically inflamed joints and muscles after rhythmical articulation. Additionally, they discovered that this joint articulation reduced bilateral hyperalgesia, indicating that a central neural mediator had an impact on the analgesia of the affected regions.¹⁹ Furthermore, Courtney and colleagues reported that joint mobilization/articulation significantly reduced the flexor withdrawal responses in osteoarthritic knees when compared to control groups, with further research showing a reduction in post-treatment pain thresholds after cervical and lumbar spinal mobilization.^20,21 In terms of symptomatic and functional modification, the neurophysiological effects of LVLA follow the same pattern as HVLA (temporary).²² Joint articulation techniques can highlight mechanical effects that lead to increased extensibility of targeted joint tissues, increased mobility, and improved joint alignment. Pain and muscle spasms can thus be effectively modulated, while a joint range of motion and flexibility can be improved.³

Giles Gyer states “What I like about mobilization is how it’s done for a period of time, through repetitions, allowing the therapist to work on imbibition of joints and structures, so I do think it’s actually not only a gentler technique but one that provides more bang for your buck when applied to a patient.”

Contraindications

One must also consider the significant differences in clinical application between HVLA and LVLA. HVLA has significant risk factors, contraindications, and reduced suitability for selecting patient demographics. Certain arthritic conditions, fractures, dislocations, cancer, infection, osteoporosis, or vertebral artery stenosis are all common contraindications.^23,24 There is information suggesting there may be an association (NOT causation) between cervical thrust manipulation and vertebrobasilar stroke,^24,25

However, Cassidy et al., (2008) concluded there was no evidence of this.^26,27

“The increased risks of VBA stroke associated with chiropractic and PCP visits is likely due to patients with headache and neck pain from VBA dissection seeking care before their stroke. We found no evidence of excess risk of VBA stroke associated chiropractic care compared to primary care.”²⁶

The risk of HVLA changes how the technique is governed by health boards worldwide. However, both spinal manipulation and spinal mobilization are still recommended by numerous but not all clinical practice guidelines globally.²⁷

CONCLUSION

Research suggests that the only difference between mobilization and manipulation as therapeutic modalities is the speed with which they are applied. The priority of the clinician or therapist should not be to chase the cavitation nor to pursue the loudest audible possible, but rather to achieve effective post-intervention changes that are tangibly recorded during the post-assessment check. Spinal mobilization provides great utility and versatility across a wide patient demographic while remaining very effective at eliciting significant neurophysical changes. Similarly, spinal manipulation can absolutely be a helpful intervention but may not always be appropriate for the patient in front of you. A competent clinician/therapist will know when to use the right tool on a case-by-case basis for the right patient, resulting in the best outcome possible.

Conflicts of Interest

The authors report there are no competing interests to declare.