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The Mechanism of the Chiropractic

Spinal Adjustment/Manipulation:

Osseous Mechanisms

Part 1 of a 5 Part Series

By: Mark Studin

William J. Owens


Citation: Studin m., Owens W., (2017) The Mechanism of the Chiropractic Spinal Adjustment/Manipulation: Osseous Mechanisms, Part 1 of 5, American Chiropractor 39 (5), pgs. 30, 32, 34, 36-38


A report on the scientific literature



There have been many reports in the literature on chiropractic care and its efficacy. However, the reporting is often “muddled” based upon interchangeable terminology utilized to describe what we do. The etiology of the verbiage being used has apparently been part of a movement to gain acceptance within the healthcare community, but this attempt for a change in view by the healthcare community has cost us. Currently, the scientific community has lumped together manipulation performed by physical therapists or osteopaths with chiropractic spinal adjustments because all three professions perform “hands on” manual therapy to the spine. For example, Martínez-Segura, De-la-LLave-Rincón, Ortega-Santiago, Cleland, and Fernández-de-Las-Peñas (2012) discussed how physical therapists commonly use manual therapy interventions directed at the cervical or thoracic spine, and the effectiveness of cervical and thoracic spine thrust manipulation for the management of patients with mechanical, insidious neck pain. Herein lies the root of the confusion when “manipulation” is utilized as a “one-size-fits-all” category of treatment as different professions have different training and procedures to deliver the manipulation, usually applying different treatment methods and realizing different results and goals.

In addition, as discussed by Sung, Kang, and Pickar (2004), the terms “mobilization,” “manipulation” and “adjustment” also are used interchangeably when describing manual therapy to the spine. Some manipulation and virtually all chiropractic adjusting “…involves a high velocity thrust of small amplitude performed at the limit of available movement. However, mobilization involves repetitive passive movement of varying amplitudes at low velocity” (Sung, Kang, & Picker, 2004, p. 115).

To offset confusion between chiropractic and any other profession that involves the performance of some type of manipulation, for the purpose of clarity, we will be referring to any type of spinal therapy performed by a chiropractor as a chiropractic spinal adjustment (CSA) and reserve manipulation for other professions who have not been trained in the delivery of CSA. Until now, the literature has not directly supported the mechanism of the CSA. However, it has supported each component and the supporting literature, herein, will define the neuro-biomechanical process of the CSA and resultant changes. 

Components of the Adjustment or Thrust

Both human and animal studies have shown the tri-phasic process of the CSA and the time for the thrust duration of each phase.  In addition, the timing at each phase has been shown to be integral in understanding the neurological effect of the CSA. The forces are broken into 3 phases. These are the pre-load force, which takes the tissue close to its paraphysiological limit, the peak force or thrust stage and the resolution stage.

 Pickar and Bolton (2012) reported the following:

CSA, referred to in the literature as spinal manual therapy, “…in the cervical region has relatively little pre-load ranging from 0 to 39.5 N. In contrast, the average pre-load forces during [CSA] in the thoracic region (139 ± 46 N, ± SD) and sacroiliac region (mean 88 N ± 78 N) are substantially higher than in the cervical region and are potentially different from each other. From the beginning of the thrust to end of the resolution phase, [CSA] duration varies between 90 and 120 ms. (mean = 102 ms.). The time to peak force during the thrust phase ranges from 30 to 65 ms. (mean = 48 ms.). Peak applied forces range from 99 to 140 N (mean = 118 N, n = 6 treatments). In the same study with [CSA] directed at the thoracic (T4) region and applied to three different patients by the same practitioner, the mean (SD) time to peak force was 150 ± 77 ms. and mean peak force reached 399 ± 119 N. During the resolution phase, force returned to pre-[CSA] levels over durations up to two times longer than that of the thrust phase. When [CSA] was applied to the sacroiliac joint, mean applied peak forces reached 328 ± 78 N, with the thrust and resolution phases having similar durations (∼100ms.). The peak force during manipulation of the lumbar spine measured by Triano and Schultz (1997) tended to be higher than during the thoracic or sacroiliac manipulation measured by Herzog et al. (1994) and the force–time profiles resembled half-sine waves with the time to and from peak taking approximately 200 ms. Peak impulse forces during thoracic manipulation approximated the >400 N peak impulse force measured by Triano and Schultz (1997). (p. 786)



Note. “Spinal Manipulative Therapy and Somatosensory Activation,” by J. G. Pickar and P. S. Bolton, 2012, Journal of Electromyography and Kinesiology,22(5), 787. Copyright 2012 by Elsevier.

 Pickar and Bolton (2012) reported that the physical characteristics of an CSA may vary based upon the technique being used and the individual practitioner. However, the above scenario is an illustration and guide to the time and force for of a CSA.




Zygapophysial (Z) joints

Cramer et al. (2002) explained the following:

One component of spinal dysfunction treated by chiropractors has been described as the development of adhesions in the zygapophysial (Z) joints after hypomobility. This hypomobility may be the result of injury, inactivity, or repetitive asymmetrical movements…one beneficial effect of spinal manipulation may be the “breaking up” of putative fibrous adhesions that develop in hypomobile or “fixed” Z joints. Spinal adjusting of the lumbar region is thought to separate or gap the articular surfaces of the Z joints. Theoretically, gapping breaks up adhesions, thus helping the motion segment reestablish a physiologic range of motion. (p. 2459)


Control subject [left] before the CSA and after [right] a CSA. The red arrows depict the increase in the Z-Joint

Note. “The Effects of Side-Posture Positioning and Spinal Adjusting on the Lumbar Z Joints: A Randomized Controlled Trial with Sixty-Four Subjects,” by G. D.Cramer, D. M. Gregerson, J. T. Knudsen, B. B. Hubbard, L. M. Ustas, & J. A., 2002, Spine,27(22), 2462. Copyright 2002 byLippincott Williams & Wilkins.

 Cramer et al. (2002) found the following:

…significant differences between several groups in this study, with the group that received chiropractic adjustments and remained in the side-posture position showing the greatest increase in gapping. This finding is consistent with the hypothesis that chiropractic adjusting gaps the Z joints…The Z joints were found gap during side-posture positioning, although not as much as during side-posture adjusting…The flexion that occurs during the side-posture position and side-posture spinal adjustment may allow for greater gapping during axial rotation and may account for the difference in results between the studies. However, because both the side-posture positioning group and the group that had side-posture adjusting followed by continued side-posture positioning received equal amounts of flexion, the thrust given during the chiropractic procedure had the effect of increasing the gapping of the Z joints. (p. 2464)

 The average difference between the control subjects…and the subjects that received a chiropractic adjustment and remained in side-posture position was 1.33 mm…a difference of 0.71 mm was found between the side-posture group…and the group that received an adjustment and remained in the side-posture position…It will be recalled that the Z joints are very small [and this is a considerable gap in a joint as small as the Z joint]…Another important consideration is that the term “residual,” or “left-over” gapping, could be applied to the gapping measured in the adjustment group because it can be logically assumed that the Z joints gap a greater distance during the forceful loading of the manipulative procedure than recorded in this study. The tissues of the spine presumably bring the articular surfaces back toward the pre-adjustment (closed) position as the patient resumes a more typical side-posture position after the thrust of a manipulation. This “residual” gapping is what was seen during the 15- to 20-minute MRI scan taken immediately after the adjustment. (2464-2565)

 What makes this significant is the residual time that occurs after the CSA. During this period, and the time that follows is the foundation for biomechanical  changes in the adjacent discs and ancillary connective tissue attachments that will be discussed in the next article in the series. However, this is part of the foundation for bio-neuro-mechanical changes to the spine secondary to the CSA.

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