Arterial Strain During Cervical Adjustment

Most vascular accidents following spinal manipulation occur in the vertebrobasilar system, particularly the point at which the cephalad/distal loop of the vertebral artery (VA) leaves the foramen transversarium of C1 to enter the foramen magnum.

Research measuring the force of a typical spinal manipulation to the cervical region has been published, but the forces measured have related to the surface of the body, and not necessarily deeper tissues. It is not known how these forces are actually transmitted through tissues to deeper anatomical structures.

To determine the extent of strain on the VA in situ during spinal manipulative therapy (SMT), six VAs were utilized from five cadaveric specimens, with only the necessary arterial loops exposed. Strains on the arteries were measured during range of motion tests, vertebrobasilar insufficiency (VBI) tests and a number of SMT procedures. Next, each VA was removed and strained until failure on a materials testing machine, with the level of mechanical failure recorded.

*Strain From Diagnostic Testing: More strain to the VA was observed on the side contralateral to the side tested, such that during right lateral bending, greater strain was observed on the left VA, and the upper VA experienced more strain than the lower VA. The greatest strain observed during diagnostic procedures occurred in the VA contralateral to rotation movement, followed by contralateral vertebrobasilar insufficiency testing. The authors state that these data "support the contention that rotation may be a potential mechanism for causing VBI."

*Strain From Cervical SMT: In general, SMT on the contralateral cervical spine caused an average strain of 6.2% (over resting artery length) to the distal VA loop (C0-C1) and 2.1% to the proximal loop (C6-subclavian artery), similar to strains measured during range-of-motion tests. The greatest amount of strain to the contralateral distal VA occurred during the C6/C7 break SMT; the least strain was observed during a C3/C4 lateral SMT. The failure tests showed that the VA specimens could be stretched to 139-162% of their resting length prior to mechanical failure. Thus the strain required to damage the VAs was approximately nine times greater than strain produced by SMT. The average strains observed during SMT were lower than those observed during VBI screening tests and neck rotation, suggesting that SMT strains are within the normal range of strains experienced during normal motion of the cervical spine.

Conclusion: "... Under normal circumstances, a single typical (high-velocity/low-amplitude) SMT thrust is very unlikely to mechanically disrupt the VA."

Note: The authors clearly point out that these experiments were conducted on cadavers receiving a single SMT. The effects of straining the VA multiple times in a living subject are under current investigation.

Symons BP, Leonard T, Herzog W. Internal forces sustained by the vertebral artery during spinal manipulative therapy. Journal of Manipulative and Physiological Therapeutics 2002:25(8), pp. 504-510.