Research Review By Dr. Jeff Muir©


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Date Posted:

September 2015

Study Title:

Real-Time Visualization of Joint Cavitation


Kawchuk GN, Fryer J, Jaremko JL, et al.

Author's Affiliations:

Departments of: Physical Therapy, Radiology and Diagnostic Imaging, Chemical and Materials Engineering, Biomedical Engineering, University of Alberta, Edmonton, Alberta, Canada; Private Practice, Nanaimo, British Columbia, Canada; School of Medicine and Public Health, University of Newcastle, New South Wales, Australia.

Publication Information:

PLoS ONE 2015; 10(4): e0119470.

Background Information:

The cavitation associated with spinal manipulative therapy (SMT) is a classic component of chiropractic and manual medicine practice and an ongoing source of curiosity for both patients and clinicians. The prevailing theory regarding the source of the ‘cracking’ sound associated with synovial joint cavitation is that first proposed by Unsworth et al. (1), which described the rapid collapse of a bubble within the joint space. This explanation has generally been accepted by the current generation of clinicians and patients. Prior to Unsworth; however, Roston and Wheeler Haines (2) used serial radiography to visualize joint cracking under distraction forces and proposed that the sound was the result of the creation of a ‘clear space’, assumed to be a vapour cavity.

Other theories have suggested that the cracking sound is a product of ligamentous recoil (3), or tribonucleation (4, 5), a process that occurs when two closely opposed surfaces are separated by a thin film of viscous liquid. On distraction, viscous adhesion or tension resists the joint’s separation. As the distraction forces increase and overpower the adhesive force, the joint surfaces separate, rapidly creating a negative pressure that can create a vapour cavity within the fluid.

With no clear consensus on the exact mechanism at the root of the cracking sounds associated with joint cavitation, the authors sought to utilize modern technology in an attempt to provide evidence for a clear theory to explain the cracking sound from synovial joints.

Pertinent Results:

Static Imaging:
Prior to distraction, static imaging revealed normal metacarpophalangeal joints. Static imaging post-distraction demonstrated a dark intra-articular void.

Cine MRI (typically used to observe cerebrospinal fluid flow) demonstrated:
  • A slow increase in joint separation through approximately 6.2 seconds of distraction.
  • A drop in MRI signal-intensity within the intra-articular space to reveal a signal void as the joint expands.
  • Control regions of fluid within the joint and bone show relatively unchanging signal intensities over the course of the experiment.
  • A steady increase in signal intensity in the intra-articular space just prior to joint cracking.
Wilcoxon Signed-ranks tests demonstrated a significant difference in joint space in the cine frame prior to and following rapid surface separation and cracking.

The signal void associated with cracking was observed in all 10 MCP joints studies, varying only in shape, size and location.

Clinical Application & Conclusions:

The results of this study indicate that, despite popular belief, the initial theory offered by Roston and Wheeler Haines and that of tribonucleation are the most likely explanations of cavitation, and that the ‘cracking’ sound of cavitation is associated not with the collapse of a gas bubble, but in fact with the rapid creation of a clear space within the intra-articular cavity. As a result, clinicians should consider revising their standard response to the common question from patients: “What’s that cracking sound?”

Study Methods:

One adult male served as the study population, providing written consent for participation prior to study initiation.

Ten MCP joints were studied over two sessions. With the subject prone, the finger of interest was inserted into a tubular finger trap covering the finger from the apex to midway between the MCP and the proximal interphalangeal joint. The apex was connected in-series to a ¼ inch diameter cable and the finger centered over a radiofrequency coil designed for MRI imaging of digits.

Before and after MCP distraction, static magnetic resonance (MR) images were obtained of the MCP joint. During distraction of the MCP joint, cine MRI (typically used to observe cerebrospinal fluid flow) was acquired from the midline of the joint at a rate of 3.2 frames per second until the distraction force was removed following the cracking event.

The cable attached to the apex of the finger trap was threaded through a magnet, exiting on the side of the MRI opposite to the subject. A slowly increasing distraction force was applied manually through the cable until the subject reported joint cracking.

Study Strengths / Weaknesses:

  • The use of high-resolution, modern technology offers the most comprehensive imaging analysis of joint cavitation available; and
  • The use of one subject provides adequate internal controls and eliminates inter-subject variability.
  • The slice thickness used for cine MRI prevented the visualization the joint in its entirety.
  • The use of one subject may limit the ability to broadly apply the observations.
  • Some observers may question the ability to extrapolate findings in an MCP joint with that of a spinal facet joint.
  • This work does not explain the magnitude of the sound caused by cavity formation.

Additional References:

  1. Unsworth A, Dowson D, Wright V. ‘ Cracking joints ‘ A bioengineering study of cavitation in the metacarpophalangeal joint. Ann Rheum Dis 1971; 30: 348–358.
  2. Roston JB, Haines RW. Cracking in the metacarpo-phalangeal joint. J Anat 1947; 81: 165–73.
  3. Brodeur R. The audible release associated with joint manipulation. J Manip Physiol Ther 1995; 18: 155–164.
  4. Ikels K. Production of gas bubbles in fluids by tribonucleation. J Appl Physiol 1970; 28: 524–527.
  5. Campbell J. The tribonucleation of bubbles. J Phys D Appl Phys 1968; 1: 1085.