Multiple channel recording of the articular crack associated with manipulation of the metacarpophalangeal joint. An observational study.

Background: The audible release or cracking sound associated with spinal manipulation is familiar to practitioners of spinal manipulative therapy. Furthermore, some authors believe the articular crack to be at least in part responsible for the therapeutic benefits derived from spinal manipulative therapy. Although some research has been directed towards the investigation of some aspects of this phenomenon, little research has be conducted in order to establish from which side and vertebral level the audible release occurs during the manipulative process.

Objective: To assess the reliability and accuracy of multiple surface mounted microphones to detect the audible release of the target joint during manipulation of the third metacarpophalangeal joint.

Design: Observational study.

Setting: Private practice of chiropractic, Ringwood, Victoria, Australia.

Participants: Twenty volunteers recruited from staff and patients of the private practice of chiropractic.

Method: Eight omnidirectional microphones were affixed to the palmar surface of the hand. Microphone No.1 was positioned directly over the third metacarpophalangeal joint while the remaining microphones were arranged in a uniform pattern over the palmar surface of the hand. Manipulation in the form of long axis traction was then applied to the third metacarpophalangeal joint. Where an audible release was associated with the manipulation the resultant signals were captured via computer and stored for later analysis.

Main outcome measure: A difference of greater than one volt in peak amplitude between the microphone positioned over the target joint and the other microphones. The student's t-test was then applied to the data in order to determine if the mean output of the target joint microphone was statistically different to the mean output of the other microphones.

Results: A total of eighteen manipulations resulted in nineteen audible release signals. The mean voltage of channel 1 was consistently greater than all the other channels in this group of subjects. This difference was statistically significant for all the channels.

Conclusion: This research suggests that multiple surface mounted microphones are capable of consistently detecting the audible release from the target joint, with manipulation directed to the third MCP joint. It is hoped that this method will be able to be applied to the audible release associated with spinal manipulative therapy and a better understanding of the manipulative process will ensue.