In Reply

We thank the author of the letter to the editor for their constructive criticism. The authors indicate that modeling studies revealed up to 42% strain of the vertebral artery (VA) on the C1/C2 level. If this was indeed the case, then our present study1Austin N DiFancesco LM Herzog W Microstructural damage of arterial tissue due to repeated tensile strains.J Manipulative Physiol Ther. 2010; 33: 14-19Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar and previous work2Symons BP Leonard T Herzog W Internal forces sustained by the vertebral artery during spinal manipulative therapy.J Manipulative Physiol Ther. 2002; 25: 504-510Abstract Full Text PDF PubMed Scopus (74) Google Scholar would suggest that cervical spinal manipulations needed to be stopped immediately, as repeat application of 30% strain produces significant micro-trauma to arterial tissues1Austin N DiFancesco LM Herzog W Microstructural damage of arterial tissue due to repeated tensile strains.J Manipulative Physiol Ther. 2010; 33: 14-19Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar and an average strain of 53% causes complete mechanical failure of the VA.2Symons BP Leonard T Herzog W Internal forces sustained by the vertebral artery during spinal manipulative therapy.J Manipulative Physiol Ther. 2002; 25: 504-510Abstract Full Text PDF PubMed Scopus (74) Google Scholar One limitation of our previous work on VA strains during neck manipulative therapy was that we could not measure the strains at the important C1/C2 level. However, new technologies made such measurements possible, and our newest findings suggest that elongations of VA at the C1/C2 level are less than 3% for all manipulative procedures tested.3Wuest S Symons B Leonard TR Herzog W Preliminary report: biomechanics of vertebral artery segments C1-C6 during cervical spinal manipulation.J Manipulative Physiol Ther. 2010; 33: 273-278Abstract Full Text Full Text PDF PubMed Scopus (29) Google Scholar These experimental values are substantially smaller than the 42% associated with the cited theoretical works. Another point that needs consideration is the definition of strain. We defined strain of VA as elongations from the neutral head and neck position. However, in subsequent stress experiments, we found that stress only appeared at strains of approximately 10% to 12%; therefore, we concluded that strains during manipulative therapy (<6% elongations from neutral) all occurred in the absence of stretching of the VA. Rather, the VA has some slack in its neutral configuration and it is “elongated” during manipulation and active head and neck movements, but not stretched or strained. The authors suggest that there might be problems with patients who have biomechanical VA anomalies. We acknowledge this point but cannot comment on it further as we have not investigated this topic. We also agree that the structure of the rabbit aorta is sufficiently different from the human VA; thus, comparison of our results1Austin N DiFancesco LM Herzog W Microstructural damage of arterial tissue due to repeated tensile strains.J Manipulative Physiol Ther. 2010; 33: 14-19Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar with the human VA need to be made with caution. The authors are also correct in stating that we did not measure the possible effects of repeated compression on the VA. There are several reasons for this. First and foremost, when we started this work 10 years ago, the consensus was that stretching of the VA during neck manipulation therapy was the primary biomechanical insult. A second reason is that if the VA is compressed at specific points during head movements, we would assume that the vessel wall at that compressive point would structurally differ from the places where no such compression occurs. Testing of a “compressive insult theory” thus would require that the site of compression is identified and testing is done precisely at that location. Finally, the fact that the medical profession happily applies compressive stresses to perfused arteries routinely (eg, when performing blood pressure measurements) appears to suggest that compressive stresses are well tolerated. The argument made relating popliteal aneurisms to muscle compression refers to an anomaly in the course of the popliteal artery, and we would not feel comfortable commenting on what might happen in arteries with abnormal structure or anatomical course.

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