Article Title: The relation between the application angle of spinal manipulative therapy (SMT) and resultant vertebral accelerations in an in situ porcine model Authors: Kawchuck GN and Perle SM Publication information: Manual Therapy 2009; 14: 480-483. BaCKgroUnd informaTion Spinal manipulative therapy (SMT) can be beneficial for spinal pain. As we know, SMT involves the application of a high-velocity, low-amplitude force to spinal tissues, with the goal of moving vertebrae, cavitating joints, influencing soft tissue function, neuro- logical input/output, and so on. Typically, SMT is applied at a specific angle under the assumption that the underlying vertebra moves in that same specific direction. Further, the application angle is most often described as being parallel to the articular space of the zygapophyseal joints so that vertebral displacement can be maximized. Recently, the long-held belief that SMT can create vertebral acceleration in specific direc- tions, determined by the practitioner, has come under scrutiny. This is in large part because of recent evidence suggesting that only the normal component of applied forces (that is, the portion applied perpendicular to the skin surface) is transmitted to spinal tissues, due to the frictionless interface between posterior spinal tissues and the underlying spinal structures (Bereznick, Ross and McGill 2002 – definitely recommended reading for those who have not yet read it). If this is the case, any force not applied at 90 degrees to the skin surface would not contribute to vertebral acceleration, and hence could be considered wasted. Since 2002, no studies have been able to support or refute this concept. Therefore, Dr. Shawn Thistle is the founder and president of Research Review Service Inc., an online, subscription- based service designed to help busy practitioners to integrate current, relevant scientific evidence into their practice. Shawn graduated from CMCC and holds an Hon- ours Degree in Kinesiology from McMaster University. He also holds a certificate in Contemporary Medi- cal Acupuncture from McMaster University, and is a Certified Active Release Techniques (ART®) Pro- vider. For more information about the Research Review Service, visit www.researchreviewservice.com. the goal of this study was to further investigate this by utilizing a porcine (pig) model. Of interest was the relation between the angle of SMT application and the acceleration response of the underlying vertebra. Often, the basis of medical techniques comes from research on animal models, and chiropractic is no different. The authors acknowledge that there are differences between pig and human posterior spinal tissues. However, the intent of this study was not to extrapolate directly to humans, but rather to test the previously published hypothesis and establish directions for future research in this area. ConClUsions and PraCTiCal aPPliCaTions The results of this study support previous literature1 , further strengthening the argu- ment that forces applied to the spine at non-normal angles do not influence vertebral acceleration, and that the interface between the vertebrae and posterior spinal tissues is frictionless. In fact, when force is applied at non-normal angles, the resultant accelera- tion diminishes in all planes (yet notably the highest values are still maintained in the posterior-anterior axis). These results were further strengthened by the authors’ ability to predict the reduction in acceleration at varying angles using simple trigonometry. A physicist may raise the question about why acceleration occurs in all axes when only the normal portion of the force is transmitted to the vertebra. An anatomist would remind us that vertebrae do not float freely in space, but rather are governed in their movement by adjacent soft tissues, articular boundaries, and so on. As chiropractors, we are adept at integrating concepts, and this is no different. We impart the force to the spine, and essentially the vertebra moves where it moves. The point of this research is to remind us that applying the force at a 90-degree angle maximizes efficiency, and (at least in theory) minimizes unnecessary joint stress on those delivering the manipulation. The next step is to perform similar studies on human cadaveric specimens…stay tuned. • For the review in full – including study methods and results and study strengths/weaknesses – please visit www.canadianchiropractor.ca. This brief review is a sample of the content posted weekly on Research Review Service and is one of over 60 reviews available per year of the latest scientific literature in chiropractic. RRS is approved for 4.0 CEUs/year by CMCC. 46 • CANADIAN CHIROPRACTOR | FEBRUARY 2010 www.canadianchiropractor.ca Shawn Thistle, BKin (Hons), DC, CSCS
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