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Microstructural damage of arterial tissue due to repeated tensile strains
Letters to the Editor |
MICROSTRUCTURAL DAMAGE OF ARTERIAL TISSUE DUE TO REPEATED TENSILE STRAINS To the Editor: The study by Austin et al1 is very interesting and was well written in a cautious manner. They suggested that…“the results should not be translated to the human VA (vertebral artery) without due consideration.” Their opinion, that the application of 1000 repeat load cycles in a single session represents a “worst case scenario,” seems at face value to be reasonable. However, given the limitations of the two studies,1,2 some of which they discussed, there could be other “worst case scenarios” that need to be considered. Previous modeling studies3 indicated the potential of cervical rotation to cause much greater VA elongation at the contralateral C1/2 level than the 0.06 strain that was applied to the rabbit ascending aortas. With maximal atlantoaxial rotation while maintaining contact of the opposing facets, estimated possible strains 0.12 to 0.42 for this VA segment can be calculated assuming an absence of: the concomitant contralateral C1/2 lateral flexion that seems to occur frequently and/or the fully developed curves of the artery at this level.3 Therefore, repeated applications of cervical spinal manipulative therapy involving maximal head rotation4,5 applied to patients with these biomechanical anomalies could cause larger stresses to atlantoaxial VAs than 1000 load cycles of 0.06 strain, although, this is not to suggest that VA injury would be likely in these instances, without inherent weakness of the VA wall due to a connective tissue disorder. Austin et al1 explained that the human VA has a thinner tunica intima than rabbit ascending aorta, but that the intima was unlikely to affect the stress-strain relationship of these arteries because it provides little resistance to tissue strain. However, as an individual element of the arterial wall, the intima may be more prone to disruption, which seems to be evident in cases of VA dissection. The authors referred to the Sato et al6 paper regarding VA wall thickness, a study that also examined reductions in the intimal thickness of intracranial VAs that had arterial dissection. Hence, intimal thickness may be an important factor and, so, could be a major limitation for using rabbit ascending aorta as a model for human VAs. The effect of marked localized compression being repeatedly applied to perfused arteries that are human normotensive was not tested. This type of compression occurs with a small percentage of human VAs during contralateral cervical rotation, and there is some evidence that it precedes major stretch.7 Such a phenomenon is important to consider, given that popliteal arteries with an 480
anomalous course can be repeatedly compressed by the gastrocnemius muscles at their upper attachments as they contract during walking, leading to aneurysm formation and sometimes rupture of the popliteal arteries.8 Although the results Austin et al1 obtained may have limited clinical application now, the skills that have been learned in pursuing this research could be extremely valuable in the future. I wish their research team well! Michael J. Haynes, BSc, BApp.Sc.(Chiro), PhD Chiropractor, High Wycombe Chiropractic Clinic High Wycombe, Western Australia, Australia E-mail address: [email protected] 0161-4754/$36.00
Copyright © 2010 by National University of Health Sciences. doi:10.1016/j.jmpt.2010.06.011
REFERENCES 1. Austin N, DiFancesco LM, Herzog W. Microstructural damage of arterial tissue due to repeated tensile strains. J Manipulative Physiol Ther 2010;33:14-9. 2. Symons BP, Leonard T, Herzog W. Internal forces sustained by the vertebral artery during spinal manipulative therapy. J Manipulative Physiol Ther 2002;25:504-10. 3. Haynes MJ, Cala LA, Melsom A, Mastaglia FL, Milne N, McGeachie JK. Vertebral arteries and cervical rotation: modelling and magnetic resonance angiography studies. J Manipulative Physiol Ther 2002;25:370-83. 4. Haynes MJ. Cervical spine adjustments by Perth chiropractors and post-manipulation stroke: Has a change occurred? Chiropr J Aust 1996;26:43-6. 5. Good C. Letter to the editor. J Manipulative Physiol Ther 2003; 26:339-40. Re: Symons BP, Leonard T, Herzog W. Internal forces sustained by the vertebral artery during spinal manipulative therapy. J Manipulative Physiol Ther. 2002;25:504-510. 6. Dumas A, Salama J, Dreyfus P, Goldlust D, Chevrel JP. Magnetic resonance angiographic analysis of atlanto-axial rotation. Anatomical bases of compression of vertebral arteries. Surg Radiol Anat 1996;18:303-13. 7. Sato T, Sasaki T, Suzuki K, Matsumoto M, Kodama N, Hiraiwa K. Histological study of the normal vertebral artery. Etiology of dissecting aneurysms. Neurol Med Chir (Tokyo) 2004;44: 629-36. 8. Murray A, Halliday M, Croft RJ. Popliteal artery entrapment syndrome. Br J Surg 1991;78:1114-9.
IN REPLY To the Editor: 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.
MICROSTRUCTURAL DAMAGE OF ARTERIAL TISSUE DUE TO REPEATED TENSILE STRAINS To the Editor: The study by Austin et al1 is very interesting and was well written in a cautious manner. They suggested that…“the results should not be translated to the human VA (vertebral artery) without due consideration.” Their opinion, that the application of 1000 repeat load cycles in a single session represents a “worst case scenario,” seems at face value to be reasonable. However, given the limitations of the two studies,1,2 some of which they discussed, there could be other “worst case scenarios” that need to be considered. Previous modeling studies3 indicated the potential of cervical rotation to cause much greater VA elongation at the contralateral C1/2 level than the 0.06 strain that was applied to the rabbit ascending aortas. With maximal atlantoaxial rotation while maintaining contact of the opposing facets, estimated possible strains 0.12 to 0.42 for this VA segment can be calculated assuming an absence of: the concomitant contralateral C1/2 lateral flexion that seems to occur frequently and/or the fully developed curves of the artery at this level.3 Therefore, repeated applications of cervical spinal manipulative therapy involving maximal head rotation4,5 applied to patients with these biomechanical anomalies could cause larger stresses to atlantoaxial VAs than 1000 load cycles of 0.06 strain, although, this is not to suggest that VA injury would be likely in these instances, without inherent weakness of the VA wall due to a connective tissue disorder. Austin et al1 explained that the human VA has a thinner tunica intima than rabbit ascending aorta, but that the intima was unlikely to affect the stress-strain relationship of these arteries because it provides little resistance to tissue strain. However, as an individual element of the arterial wall, the intima may be more prone to disruption, which seems to be evident in cases of VA dissection. The authors referred to the Sato et al6 paper regarding VA wall thickness, a study that also examined reductions in the intimal thickness of intracranial VAs that had arterial dissection. Hence, intimal thickness may be an important factor and, so, could be a major limitation for using rabbit ascending aorta as a model for human VAs. The effect of marked localized compression being repeatedly applied to perfused arteries that are human normotensive was not tested. This type of compression occurs with a small percentage of human VAs during contralateral cervical rotation, and there is some evidence that it precedes major stretch.7 Such a phenomenon is important to consider, given that popliteal arteries with an 480
anomalous course can be repeatedly compressed by the gastrocnemius muscles at their upper attachments as they contract during walking, leading to aneurysm formation and sometimes rupture of the popliteal arteries.8 Although the results Austin et al1 obtained may have limited clinical application now, the skills that have been learned in pursuing this research could be extremely valuable in the future. I wish their research team well! Michael J. Haynes, BSc, BApp.Sc.(Chiro), PhD Chiropractor, High Wycombe Chiropractic Clinic High Wycombe, Western Australia, Australia E-mail address: [email protected] 0161-4754/$36.00
Copyright © 2010 by National University of Health Sciences. doi:10.1016/j.jmpt.2010.06.011
REFERENCES 1. Austin N, DiFancesco LM, Herzog W. Microstructural damage of arterial tissue due to repeated tensile strains. J Manipulative Physiol Ther 2010;33:14-9. 2. Symons BP, Leonard T, Herzog W. Internal forces sustained by the vertebral artery during spinal manipulative therapy. J Manipulative Physiol Ther 2002;25:504-10. 3. Haynes MJ, Cala LA, Melsom A, Mastaglia FL, Milne N, McGeachie JK. Vertebral arteries and cervical rotation: modelling and magnetic resonance angiography studies. J Manipulative Physiol Ther 2002;25:370-83. 4. Haynes MJ. Cervical spine adjustments by Perth chiropractors and post-manipulation stroke: Has a change occurred? Chiropr J Aust 1996;26:43-6. 5. Good C. Letter to the editor. J Manipulative Physiol Ther 2003; 26:339-40. Re: Symons BP, Leonard T, Herzog W. Internal forces sustained by the vertebral artery during spinal manipulative therapy. J Manipulative Physiol Ther. 2002;25:504-510. 6. Dumas A, Salama J, Dreyfus P, Goldlust D, Chevrel JP. Magnetic resonance angiographic analysis of atlanto-axial rotation. Anatomical bases of compression of vertebral arteries. Surg Radiol Anat 1996;18:303-13. 7. Sato T, Sasaki T, Suzuki K, Matsumoto M, Kodama N, Hiraiwa K. Histological study of the normal vertebral artery. Etiology of dissecting aneurysms. Neurol Med Chir (Tokyo) 2004;44: 629-36. 8. Murray A, Halliday M, Croft RJ. Popliteal artery entrapment syndrome. Br J Surg 1991;78:1114-9.
IN REPLY To the Editor: 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.