2. Gail JM, Herzog W, et al. Forces and relative vertebral movements during SMT to embalmed post-rigor human cadavers: Peculiarities associated with joint cavitation. J Manipulative Physiol Ther 1995; 18(1):4-9.
3. Reggars JW, Pollard HP. Analysis of zygapophyseal joint cracking during chiropractic manipulation. J Manipulative Physiol Ther 1995; 18(2):65-71.
4. Brodeur R. The audible release associated with joint manipulation. J Manipulative Physiol Ther 1995; 18(3):155-164.
5. Herzog W, Conway PJ, Zhang YZ, Gal J, Guimaraes ACS. Reflex responses associated with manipulative treatments on the thoracic spine: a pilot study. J Manipulative Physiol Ther 1995; 18(4):233-236.
6. Herzog W. Mechanical and physiological responses to spinal manipulative treatments. J Neuromusculoskel Sys 1995; 3(1):1-9.
The first study looked at the immediate reflex responses of manipulations to the thoracic spine. The investigators measured surface EMG responses in paraspinal and more lateral (serratus posterior inferior) muscles after fast manipulations and slow manipulations and also measured the differences in responses between those in which cavitation took place and those in which it did not. They found that a brief increase in EMG activity was seen after fast manipulations regardless whether cavitation took place or not. A much smaller response was seen after slow manipulations, and again there was no difference in response between those with cavitation and those without. Responses were seen not only in the muscles at the same level as the adjustment but at other levels as well as in the more peripheral muscle measured. The response to fast manipulation was a quick burst of activity while that to slow developed more slowly.
It is interesting the difference in response related only to whether the manipulation was fast or slow, not to whether cavitation took place. In other words, an audible release was not necessary for the burst of muscle activity to be stimulated, but the adjustment did have to be fast to produce this response. It is important to remember that these findings relate to immediate responses, not those measured a period of time after the treatment.
Study number 2 represents the first experiment of manipulation on a cadaver. An examiner manipulated the T12 segment 5 times, achieving cavitation once, on the fourth trial. The only recorded difference between the manipulations was that there was a greater lateral translation of the vertebra against the adjacent vertebrae on the manipulation with cavitation. So when an audible release took place, there was a greater amount of movement created by the adjustment. This shows a distinct difference between cavitation and non-cavitation, but does not illuminate if this has any therapeutic import.
The third study looked at 51 asymptomatic subjects who were manipulated at C3-4 with a rotatory diversified adjustment. In most cases, there were 2-3 audible releases and in 94% of the cases the release was on the side to which the head was turned, not the contact side. In the three in the whom the crack was on the contact side, all had previous trauma. This is interesting because we often assume that the joint that we are contacting is the one that were are adjusting, but this study shows that this may not be the case. As the subjects in this study were asymptomatic, and there apparently was no attempt to determine whether the joint adjusted was determined to require an adjustment, we cannot necessarily associate this with a true clinical situation. This needs to be reproduced in a symptomatic population before firm conclusions can be drawn.
The fourth paper is a review of the literature on joint cavitation which revealed some new and interesting findings from recent research. It is explained that as a joint is being distracted, the capsule invaginates inward and as the stress on the capsule reaches a certain threshold, it suddenly snaps back from the synovial fluid, increasing the volume of the capsule (and decreasing pressure) and causing the audible sound. The sudden increase in the volume causes the tension to drop, allowing the joint to increase in movement. Eventually, the elastic limit of the capsule is reached and the process stops. The time elapsed during all this is shorter than that required for completion of the stretch reflex, so it can occur without muscular resistance. The sudden jerk on the capsule and the other periarticular tissues is theorized to cause firing of the high-threshold mechanoreceptors. This is an interesting update on our understanding of this phenomenon, and the paper includes some effective diagrams and load-separation curves to illustrate the current knowledge in this area.
This paper also discusses the commonly held belief in some circles that repeated knuckle cracking leads to the development of DJD. There is inconclusive evidence to determine whether this is true, but one study showed increased incidence of swelling of periarticular tissues and decreased grip strength among habitual knuckle crackers.
Paper number 5 reports on a study that is actually a pilot study to study number 1 reviewed here, although this pilot was published shortly after the main study. As with the other study, it showed that fast manipulations produced a burst of activity in muscles directly adjacent to the level manipulated and that slow manipulations did not. Also, this response was independent of the presence of an audible release. Some of the fast manipulations did not produce a release and some of the slow manipulations did, but still, all of the fast manipulations produced a response and none of the slow ones did.
The final paper by Dr. Herzog reviews the literature on his and other studies on the immediate responses that have been measured resulting from manipulation. It is an interesting review of all of this research and helps shed light on the physiological phenomena that are presently known to associated with spinal adjustments.
As we further investigate the exact events that are occurring during our treatments, we present ourselves with the opportunity to improve our methods and thus improve patient care. This should be the primary purpose of all research and all appraisal of research by chiropractic practitioners.