In response

Thank you for the opportunity to reply to Dr Carstensen's letter. We also wish to thank Dr Carstensen for his interest in our work and his insights into this area of research. We will attempt to address the issues raised by Dr Carstensen in point form: 1.We agree with Dr Carstensen that although spinal manipulative therapy (SMT) is not likely to directly cause vertebral artery (VA) damage, it may certainly precipitate a VA stroke if there is a preexisting lesion. The question is one of causation: can SMT cause a stroke directly or does it trigger a potential stroke that would have eventually occurred due to some other mechanical event? Our operational hypothesis is that SMT is very unlikely to cause a stroke directly. However, like any other mechanical event such as shoulder checking, blow-drying one's hair, etc., SMT can certainly precipitate a preexisting lesion into a VA stroke. This is one reason why we have emphasized the importance of a thorough history and examination prior to cervical SMT.12.Dr Carstensen's description of the endothelial layers of the VA is correct, and he has logically predicted our current research. We are now undertaking an in vitro, histological study of cadaveric VAs that have been subjected to repetitive strains in a manner to simulate cervical SMT. We hope to inform the chiropractic and scientific community regarding these results in the near future.3.Since these studies are performed on fresh cadaveric specimens, we are limited in our selection of subjects. At the Department of Anatomy of the University of Calgary, most of the specimens are from elderly persons who have donated their body to science. We will not obtain human VA specimens from other sources for ethical considerations, and this was 1 major limitation mentioned in the manuscript. Nevertheless, we are in the process of replicating these experiments and will report a larger sample in the near future. Overall, we feel that Dr Carstensen's letter implies VA dissection may be a side effect of cervical SMT. We should point out the fact that the underlying assumption, which presumes that cervical SMT exerts a force on the VA, was not observed in the present study. It is logical to assume that since 150 N to 200 N of force can be measured at the neck during a typical cervical SMT, some of this force must be transferred into the VA. However, at the segments of the cervical spine we instrumented, no detectable force was measured in the VA at lengths corresponding to those measured during SMT. During the materials testing procedure, the initial forces did not appear until the VA had been stretched to lengths corresponding to the end range of passive cervical rotation in situ. Therefore, we interpreted this result to mean that the VA was not stretched (ie, the structure of the VA did not increase in length) but was elongated without the tissues undergoing any stretch. A good analogy is a rubber band that is crumpled or slack and is elongated to its full length without stretching. Indeed, we observed in all of our dissections that with the cervical spine in neutral position, the upper loop of the VA was always slack (data not shown). Therefore, we would hypothesize that cervical SMT essentially takes up the slack in the system without causing any force; in other words, the VA elongates but does not stretch. In the absence of any detectable force in the VA, these results suggest that it is highly unlikely that cervical SMT can cause VA dissection under normal circumstances. Thank you for your time in considering this response to Dr Carstensen's letter.

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