FROM:
J Manipulative Physiol Ther 2010 (Jan); 33 (1): 14–19
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 ?2 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:
Discussion
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. [7]
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. [7] 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. [13] 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.
Limitations
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. [7] 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.
Conclusion
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.
Practical Applications
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.
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