Audible Popping During Spinal Manipulation: Plausible Mechanisms and Clinical Relevance

The mechanisms and clinical relevance of the audible popping or cracking sounds during high-velocity low-amplitude (HVLA) thrust manipulations of the spine or extremity joints are still not well understood. Many practicing physiotherapists, chiropractors and osteopaths believe the audible popping during HVLA thrust manipulations to be an important part of the technique delivery and partly suggestive of positive outcomes for some conditions and some patients; however, academics have published a couple of papers that found the popping sound to have no relationship to outcomes in pain and disability.

Over the past 45 years, there have been several proposed theories regarding the underlying mechanisms of the audible popping during HVLA thrust manipulation. Perhaps the most widely known explanation was provided by Unsworth et al¹ in 1971. After taking radiographic images of MCP joints before and after traction manipulations, Unsworth et al described a rapid increase in joint volume, which they hypothesized dropped the intra-articular partial pressure to below that of carbon dioxide dissolved in the synovial fluid. This pressure change resulted in a phase change from liquid synovial fluid to gas, forming a bubble within the center of the joint space and causing an audible popping sound upon collapse. However in 1995, Brodeur² proposed that the cavitation process was generated by an elastic recoil of the synovial capsule as it “snaps back” from the capsule-synovial fluid interface. More specifically, Brodeur believed the capsular ligaments were drawn inward while the joint was being gapped to maintain a constant joint volume, but the capsular ligaments eventually reach a critical limit and snap back from the synovial fluid, thereby causing popping or cracking sounds. Recently in 2015, Kawchuck et al³ used real-time cine MRI to investigate the mechanism of the audible popping during MCP distraction manipulations. Contrary to what has traditionally been accepted, as the traction forces increased, MR images revealed the cavity inception occurred simultaneously with the joint separation and the popping sound production. These results offer experimental evidence that the audible popping or cracking sounds during HVLA thrust manipulation are associated with cavity inception rather than collapse of a pre-existing bubble. These observations are consistent with the tribonucleation theory, a known process where opposing surfaces resist separation until a critical point where they then separate rapidly, creating a phase change from liquid to gaseous cavities.

Although there has been conflict regarding the mechanisms responsible for the audible popping sounds during spinal manipulation, there is agreement that a 15-30 minute refractory period follows the cavitation of the MCP joint.^1-2 The refractory period is believed to be due to the presence of a collapsed gaseous nucleus or micro-bubbles in the synovial fluid that have yet to be reabsorbed in the joint, a phenomenon that is accompanied by an increase in resting post cavitation joint space (i.e. decreased intra-articular density), that prevents re-cavitation for at least 15 minutes.^1-2

CAVITATION IN SPINAL FACET JOINTS HAS NOT YET BEEN VISUALIZED

Notably, the majority of the studies that have attempted to explain the mechanism responsible for the audible popping during HVLA thrust manipulation have investigated the MCP joints, not the facet joints of the spine. In 2003, Cascioli et al⁴ used CT scans and X-rays to see if facet joint volume or density changed in 4 scenarios: premanipulation, premanipulation with traction, post manipulation, and post manipulation with traction. Importantly, they reported no changes in facet joint space and there was no evidence of intra-articular gas immediately after HVLA thrust manipulation. Therefore, Cascioli reintroduced the Capsular Detonization Theory that was originally proposed by Sandoz in 1969, whereby collagen fibers of the joint capsule are thought to be stretched beyond their threshold and rapidly lengthened without being torn.⁵ Subsequently, during lumbar HVLA thrust manipulation, Cramer et al^6,7 found increased gapping of the facet joints that was accompanied by audible popping which may support the cavitation theory; however, there was no investigation for the presence of intra-articular gaseous bubbles as has previously been reported in MCP joints.

IS THE AUDIBLE POPPING CLINICALLY NECESSARY?

Several studies have previously examined the mechanical,⁸ biochemical,⁹ neurophysiological¹⁰ and hypoalgesic¹¹ effects of HVLA thrust manipulation. After critically appraising the literature to determine the therapeutic benefit of audible popping, Reggars¹⁴ reported that there is a paucity of scientific evidence supporting the role of the audible popping during HVLA thrust manipulation techniques but conceded, “there is ample empirical evidence to support some therapeutic benefit from the audible release”. In contrast, Flynn et al^15,16 published two secondary analyses using data from a CPR study that has since been found to not be valid^17-19, and concluded that audible popping during spinal manipulation is not necessary for successful outcomes in patients with low back pain. Cleland et al²⁰ also reported no relationship between audible popping following thoracic HVLA manipulation and reduction in pain, disability, or improved AROM for patients with mechanical neck pain; likewise, this thoracic CPR was also later found to not be valid²¹. Additionally, and of concern, only 27% of patients in the Flynn study reported a dramatic improvement in pain following treatment,¹⁵ and the patients in the Cleland study did not exceed the minimal detectable change for the NPRS in patients with neck pain.^20,22 Thus, while both Flynn and Cleland downplayed the importance of audible popping during HVLA thrust manipulation, poor treatment strategy and/or even poor manipulative technique delivery may have actually marginalized their physiologic relevance.

HVLA thrust manipulation that is typically accompanied by audible popping has been associated with improved joint ROM,^23-25 decreased muscle hypertonicity,^26,27 brief electric silence,²⁸ reduced pain^24-26,30 and cellular changes.^31,14 Additionally, Bialosky et al³² found greater hypoalgesic effects (to temporal sensory summation or C-fiber mediated pain) in the lower extremity following lumbar HVLA thrust manipulation in those subjects that experienced audible popping when compared to those subjects without audible popping. Furthermore, Teodorczyk-Injeyan et al³³ observed a reduction of proinflammatory cytokine secretion in participants receiving HVLA thrust manipulation with audible popping in comparison to those without audible popping.

According to Dunning et al,^24-25,34 most practitioners anecdotally believe that the popping sounds are an indicator of the successful delivery of an HVLA thrust manipulation; furthermore, this may explain why researchers often perform repeat HVLA thrust manipulations if they do not feel or hear popping sounds on the first attempt.^24,35-38 Interestingly, many patients appear to want and/or expect popping sounds to accompany thrust manipulative procedures. As Sandoz opined in as early as 1969, “Any patient readily learns, even without being told, that the crack is a necessary condition for a successful manipulation, and conversely that a failure to obtain the crack means an unsuccessful manipulation. And we must frankly admit that for the manipulator, the crack represents also an important, although not an absolute nor a sufficient, criterion for a good manipulation.”⁵ Moreover, two recent studies published by Dunning et al^24-25 examined the effectiveness of HVLA thrust manipulation versus non-thrust mobilization in patients with mechanical neck pain and cervicogenic headache, respectively. Notably, upper cervical and upper thoracic HVLA thrust manipulation was found to be appreciably more effective than non-thrust mobilization in reducing short-term pain and disability for patients with mechanical neck pain²⁴ and at reducing headache intensity, duration, frequency, disability and medication intake at 3-month follow up for patients with cervicogenic headache.²⁵ While the presence of audible popping was required in the methods for each of these studies, no firm conclusions can be drawn about the clinical relevance of the audible pop without a direct comparison. Yes, more high quality research needs to be conducted to directly compare pain and disability outcomes in those with and without audible popping during spinal manipulation; however, the commonly taught assertion in entry-level physical therapy programs that the audible pop is not required for a successful HVLA thrust manipulation is not supported by Sackett’s 3-pillars of evidence-based practice;³⁹ that is, most patients and practicing clinicians alike (i.e. 2 of the 3 pillars of evidence-based practice) believe audible popping is a necessary part of correct technique delivery for a successful HVLA thrust manipulation.

AUTHORS

Alan Manning, PT, DPT, OCS, MTC, Cert DN
Physical Therapist & AAMT Fellow-in-Training
New River Wellness Institute, Okatie, SC

Raymond Butts, PhD, DPT, MSc (NeuroSci), Dip. Osteopractic
Senior Faculty, AAMT Fellowship in Orthopaedic Manual Physical Therapy
Senior Instructor, American Academy of Manipulative Therapy

James Dunning, DPT, MSc (Manip Ther), FAAOMPT, MMACP (UK)
Director, AAMT Fellowship in Orthopaedic Manual Physical Therapy
Senior Instructor, Spinal Manipulation Institute & Dry Needling Institute

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