J Manipulative Physiol Ther 2010 (Jan); 33 (1): 14–19 ~ FULL TEXT
Neal Austin, BSc, Lisa M. DiFrancesco, MD, Walter Herzog, PhD
Walter Herzog, PhD, KNB 402 Human Performance Laboratory, The University of Calgary, Faculty of Kinesiology, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4
Objectives Vertebral artery (VA) damage has been anecdotally linked to high-speed, low-amplitude spinal manipulative treatments (SMTs) of the neck. Apart from a single study quantifying the maximum stresses and strains imposed on the VA during cervical SMT, there are no data on the mechanics of the VA for this treatment modality, and there is no information on the possible long-term effects of repeat exposure to cervical SMT. The purpose of this study was to quantify microstructural damage in arterial tissue exposed to repeat strain loading of a magnitude similar to the maximum strains measured in the VA during high-speed, low-amplitude cervical SMT.
Methods Twenty-four test specimens from cadaveric rabbit ascending aorta were divided into 2 control groups (n = 12) and 2 experimental groups (n = 6 each). Specimens were exposed to 1000 strain cycles of 0.06 and 0.30 of their in situ length. A pathologist, blinded to the experimental groups, assessed microstructural changes in the arteries using quantitative histology. Pearson X2 analysis (a = .05) was used to assess differences in tissue microstructure between groups.
Results Control and 0.06 strain tissues were statistically the same (P = .406), whereas the 0.30 strain group showed microstructural damage beyond that seen in the control group (P = .024).
Conclusions We conclude that cadaveric rabbit arterial tissue similar in size and mechanical properties of that of the human VA can withstand repeat strains of magnitudes and rates similar to those measured in the cadaveric VA during cervical SMT without incurring microstructural damage beyond control levels.
From the Full-Text Article:
Vertebral artery (VA) damage is commonly thought to occur as a result of everyday tasks such as turning the head while backing up a vehicle, lifting objects, coughing, sport-related injuries, and undergoing spinal manipulative therapy.  It is reported that the incidence of VA damage associated with cervical manipulation is between 1 and 1.5 cases per 100,000 people annually, and the population in which it is primarily identified in is middle-aged men and women. [2-3] Although the occurrence of VA damage is low, the consequences of injury are invariably serious. 
The anatomical orientation of the VA is thought to leave it prone to injury during cervical high-speed, low-amplitude manipulation because of neck rotation.  The VA runs from the subclavian artery through the transverse foramen of the cervical vertebrae before forming the vertebral loop between C1 and the foramen magnum.  The vertebral loop is thought to accommodate stretch of the VA during neck rotation and could theoretically be prone to injury during spinal manipulation. In an experiment involving 5 un-embalmed geriatric cadavers and 1 chiropractic clinician, the internal forces and associated strains acting on the VA during cervical spinal manipulation were quantified.  In this experiment, it was found that the VA is “slack” when the head is in a neutral position and is only unraveled or stretched toward the end of passive neck motion.  The head position during passive neck motion, which would cause a stretch in the VA, is not reached during normal spinal manipulative procedures; therefore, it has been suggested that a single neck treatment has no ill effect on the mechanical integrity of the VA.  However, a chiropractic patient may receive a dozen or more cervical spinal manipulations per year,  and the possible mechanical effects of repeat exposure to arterial strain remain unknown, because the possible microstructural changes due to such mechanical loading have never been quantified to our knowledge.
The relationship between the number of spinal manipulations a patient receives and the side effects resulting from repeat treatments has been studied and has been found to not exist.  Furthermore, investigation into the effects of cervical spinal manipulation on preexisting arterial lesions has been made, but no significant differences in arterial lesion dimensions (length, cross-sectional area, and volume) could be detected after 20 treatments.  Biomechanical research on cervical spinal manipulations has focused on the external forces applied by chiropractors on patients during cervical spinal manipulative treatment (SMT).
[7, 11, 12] Only a single study focused on the stresses and strains on the VA for a variety of high-speed, low-amplitude cervical SMTs.  The average peak forces applied externally to the neck region of subjects during spinal manipulation range from approximately 100 to 150 N and are achieved within approximately 100 milliseconds. For these external forces and neck manipulations, it was found that cervical spinal manipulation resulted in average peak strains of cadaveric VAs of 0.06 from their lengths with the head and neck in the neutral position. 
Experiments designed to investigate VA microstructural damage due to straining and/or repeat exposure to manipulative treatments have been unsuccessful in humans. Furthermore, questions pertaining to the relationship between repeat cervical spinal manipulation and its effect on VA microstructural damage have arisen in court, specifically the inquest into the death of Lana Dale Lewis.  The purpose of this study was to investigate the effects of repeat strain application on the microstructural integrity of arterial tissue.
Groups 1 and 2 control specimens showed some microstructural tissue damage. Specifically, both control groups contained 4 normal samples and 2 samples with mild damage. The mild tissue damage found in the control tissues is likely associated with the dissection and handling of the arterial tissues.
Group 3 specimens (0.06 strain) were statistically the same as the control samples (Table 1 and Fig 3), suggesting that the repeat straining of cadaveric rabbit arterial tissue similar to that experienced by the cadaveric VA during chiropractic neck manipulations does not cause microstructural damage.
Microstructural damage in group 4 specimens (0.3 strain) was significantly greater than those observed in the control group providing a positive control for our test procedures. This result illustrates that repeat strain exposure of arterial tissues might produce microdamage that is not observed in mechanical assessments of tissue integrity in a single loading cycle. 
Cerebrovascular accidents associated with neck manipulation likely have a mechanical origin. It has been speculated that chiropractic SMT might cause VA dissection possibly leading to stroke. Our previous work on the mechanics of the VA in human cadavers during high-speed, low-amplitude cervical spinal manipulative therapy suggests that this is not possible in a normal VA.  However, it is well established that biological tissues can fail because of microstructural damage when exposed to repeat mechanical loading that in itself is not damaging for a single loading bout. [30, 31] Thus, it has been argued that repeat exposure to chiropractic neck manipulations might predispose the VA to microstructural damage that might lead to stroke.  This study cannot refute this argument as we used an animal model rather than human vertebral artery and performed instrumented testing rather than performing repeated chiropractic manipulations. Nevertheless, we can conclude from this study that cadaveric rabbit arterial tissue of similar size, structure, and mechanical properties to the human VA does not incur microstructural damage when exposed to 1000 strain cycles of magnitude and speed corresponding to the maximal values observed in the human cadaveric VA during chiropractic SMT to the neck.
There are a number of limitations that need to be kept in mind when interpreting the results of this study. First, the strain magnitude (0.06) and the strain rate (0.6 strain/s) of the test specimens were based on a study using cadaveric specimens. This study had a limited number of independent observations (n = 6) that measured the surface strains in a nonperfused artery.  However, in the absence of any other data on the mechanical stresses and strains of VAs during cervical spinal manipulations, these remain the only data of relevance to this study and thus were used here as our starting point. Second, the rabbit ascending aorta is not the human VA, independent of the similarities in structure and mechanical behavior. Therefore, the results should not be translated to the human VA without due consideration. Third, the strain measurements of our experimental specimens were measured as the average change in length between the tissue clamps during the mechanical testing and were not done directly at the site of histologic analysis. Although not ideal, the assumption that the tissue strain was homogeneous across the experimental specimen was made and further ensured by preparing the arterial specimens into rectangular pieces. This method was chosen to achieve the necessary strain while minimizing the damage to the arterial specimen from any other factor than the mechanical strain applied. Fourth, the 1000 repeat loading cycles were performed in a single session, whereas a patient would receive maybe 2 or 3 repeat manipulations per session before receiving further neck manipulations at a later date. In the meantime, the arterial tissue in a patient would have the possibility to adapt to any imposed loading; thus, the protocol used here must be considered a worst-case scenario in producing microstructural damage to the VA that would likely not occur in real life. Finally, the histologic scale used to grade the experimental tissue was developed specifically for this study and has not previously been validated in the literature. Although the scale has not been validated, we are confident in its sensitivity. The pathologist, who was blinded to the purpose of the project, the mechanical testing, the experimental groups, and the number of experimental groups, identified significant differences in microstructural damage between the 0.30 strain group and the control and 0.06 strain groups.
Cadaveric arterial tissues of New Zealand white rabbit with similar size, structure, and mechanical properties of human vertebral artery did not exhibit histologically identifiable microdamage when exposed to repeated mechanical loading equivalent to the strains observed in human vertebral artery during chiropractic cervical spine manipulative therapy.
An animal model is necessary to study arterial tissue microdamage.
Strains similar to those occurring in the VA during SMT of the neck were reproduced mechanically.
One thousand repeat strain cycles mimicking SMT did not cause microdamage in arterial tissue.
One thousand repeat strain cycles of a magnitude corresponding to approximately 50% of ultimate failure strain (0.30) causes significant microdamage in arterial tissue.
Funding Sources and Potential Conflicts of Interest
No funding sources or conflicts of interest were reported for this study. The Canadian Chiropractic Association, the College of Chiropractors of Alberta, and the Canadian Chiropractic Protective Agency provided financial support.