Internal forces sustained by the vertebral artery during spinal manipulative therapy

Background: Spinal manipulative therapy (SMT) has been established as a clinically effective modality for the management of several musculoskeletal disorders. One major issue with the use of SMT is its safety, especially with respect to neck manipulation and the risk of stroke in the vertebrobasilar system. Objectives: Our objectives were to quantify the strains and forces sustained by the vertebral artery (VA) in situ during SMT. Study Design: This was a cadaveric study. Methods: Six VAs were obtained from 5 unembalmed postrigor cadavers. The cephalad/distal (C0-C1) and caudad/proximal (C6-subclavian artery) loops of the VA were carefully exposed and instrumented with a pair of piezoelectric ultrasonographic crystals. The strains between each crystal pair were recorded during range of motion testing and diagnostic tests and during a variety of SMT procedures. The VA was then dissected free and strained on a materials testing machine until mechanical failure occurred. Results: SMT performed on the contralateral side of the cervical spine resulted in an average strain of 6.2% +/- 1.3% to the distal (C0-C1) loop of the VA and a 2.1% +/- 0.4% strain to the proximal (C6) loop. These values were similar to or lower than the strains recorded during diagnostic and range of motion testing. Failure testing demonstrated that the VAs could be stretched to 139% to 162% of their resting length before mechanical failure occurred. Therefore the strains sustained by the VA during SMT represent approximately one ninth of the strain at mechanical failure. Conclusions: SMT resulted in strains to the VA that were almost an order of magnitude lower than the strains required to mechanically disrupt it. We conclude that under normal circumstances, a single typical (high-velocity/low-amplitude) SMT thrust is very unlikely to mechanically disrupt the VA.